This article was downloaded by: [New York University] On: 17 June 2015, At: 20:49 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Co-composting of horticultural waste with fruit peels, food waste, and soybean residues a
ab
a
a
a
Sing Ying Choy , Ke Wang , Wei Qi , Ben Wang , Chia-Lung Chen & Jing-Yuan Wang
ac
a
Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore b
School of Municipal and Environmental Engineering, Harbin Institute of Technology, People's Republic of China c
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Published online: 03 Feb 2015.
Click for updates To cite this article: Sing Ying Choy, Ke Wang, Wei Qi, Ben Wang, Chia-Lung Chen & Jing-Yuan Wang (2015) Co-composting of horticultural waste with fruit peels, food waste, and soybean residues, Environmental Technology, 36:11, 1448-1456, DOI: 10.1080/09593330.2014.993728 To link to this article: http://dx.doi.org/10.1080/09593330.2014.993728
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2015 Vol. 36, No. 11, 1448–1456, http://dx.doi.org/10.1080/09593330.2014.993728
Co-composting of horticultural waste with fruit peels, food waste, and soybean residues Sing Ying Choya , Ke Wanga,b , Wei Qia , Ben Wanga , Chia-Lung Chena and Jing-Yuan Wanga,c∗ a Residues
and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore; b School of Municipal and Environmental Engineering, Harbin Institute of Technology, People’s Republic of China; c School of Civil and Environmental Engineering, Nanyang Technological University, Singapore
Downloaded by [New York University] at 20:49 17 June 2015
(Received 2 January 2014; accepted 26 November 2014 ) Horticultural waste was co-composted with fruit peels, food waste, and soybean residues individually to evaluate the effects of these easily available organic wastes in Singapore on the composting process and product quality. Each co-composting material was mixed with horticultural waste in the wet weight ratio of 1:1 and composted for 46 days. Results showed that all co-composting materials accelerated the degradation of total carbon and resulted in higher nutrients of nitrogen (N), phosphorous (P), and potassium (K) in the final product compared with horticultural waste alone. Mixture with fruit peels achieved the fastest total carbon loss; however, did not reach the minimum required temperature for pathogen destruction. The end product was found to be the best source for K and had a higher pH that could be used for the remediation of acidic soil. Food waste resulted in the highest available nitrate (NO3 -N) content in the end product, but caused high salt content, total coliforms, and slower total carbon loss initially. Soybean residues were found to be the best co-composting material to produce compost with high N, P, and K when compared with other materials due to the highest temperature, fastest total carbon loss, fastest reduction in C/N ratio, and best conservation of nutrients. Keywords: composting; horticultural waste; food waste; soybean residues; fertilizer quality
1. Introduction Composting is an environmental-friendly method to treat organic waste and to obtain organic fertilizer. Organic waste such as horticultural waste is recalcitrant and requires long composting time. Various materials have been co-composted with horticultural waste to shorten the composting time and to improve product quality along with other benefits. For instance, slaughterhouse wastes accelerated the compost stabilization and resulted in end product with high nitrogen availability [1]; household refuses and market waste containing mainly vegetables and fruit waste reduced water requirement during composting [1,2]; chicken faeces were effective to be cocomposted with wood chips; and [3] market waste and yard waste resulted in a higher quality end product compared with their mixture with grass.[2] On the other hand, horticultural waste is also used to complement the composting of other organic waste as the bulking agent or for waste containing high nitrogen due to its high carbon content in order to obtain a desired initial C/N ratio.[4,5] Though many different co-composting have been researched, the practice depends on the availability of cocomposting materials in the local environment. Organic waste such as fruit peels, soybean residues, and food waste are easily available and contain high nutrients such as
*Corresponding author. Email: [email protected] © 2015 Taylor & Francis
nitrogen (N), phosphorus (P), and potassium (K). But most of the time, these waste are not utilized effectively and disposed through incineration in the case of Singapore. The wasted nutrients could actually be used to complement the low nutrient of composting product from the local horticultural waste generated. Hence, it is worth to study the effect of these materials on the process of local horticultural waste composting and on the product quality. Despite extensive co-composting research being carried out as mentioned above, limited information is available in the literature about the effects of fruit peels and soybean residues. Kalemelawa et al. researched on enhancing the composting of banana peels by co-composting with animal manure, poultry litter, or earthworm.[6] But composting with horticultural waste was not covered in the study. Wong et al. studied the optimal turning frequency for co-composting of soybean residues with leaves and sawdust.[7] However, main nutrients such as phosphorus and potassium were not reported. Food waste is easily available and has been researched widely including the study of different bulking agents,[5] operating system,[8] and pH control.[9] Hence, it is meaningful to compare the effect of it with the rarely studied fruit peels and soybean residues. In this study, three co-composting materials, namely fruit peels, food waste, and soybean residues were
Environmental Technology
Downloaded by [New York University] at 20:49 17 June 2015
compared for their effect on local horticultural waste composting. The effect studied on the composting process included temperature, pH, electrical conductivity, total carbon, and C/N ratio. The quality of the end product from each mixture was also compared in terms of germination, respiration, total coliforms, and NPK content. The results are expected to fill the knowledge gap of co-composting and serve as a reference for research of producing organic fertilizer.
2. Materials and methods 2.1. Materials for composting Horticultural waste and the mixture of it with fruit peels, food waste, and soybean residues individually were used for composting in this study. Horticultural waste consisting of tree prunings and leaves was collected from a composting facility in Singapore and shredded to particle size of < 30 mm before use. Fruit peels that contain peels from bananas, papayas, water melons, dragon fruits, guavas, mangoes, and honey dews were obtained from fruit stalls in a local university. The fruit peels were cut into < 5 cm long before use. Food waste from plate leftover food consisting mainly rice, cooked vegetables, cooked meats was collected from canteen of a local university. Bones and food with length more than about 4 cm were removed from the collected food waste. Soybean residues were collected from soya bean food and beverage retailers. It is the residues in pulp form after pureed soybeans are filtered in the production of soya milk and soya bean curd. The residues were used directly in the study without any pre-treatment. All the materials used for composting were analysed for their properties and the results are summarized in Table 1.
Table 1. Characteristics of horticultural waste, fruit peels, food waste, and soybean residues prior to composting. Parameter Water content (%) Volatile solid (%, wet basis) pH Electrical conductivity (dS/m) C (%, dry basis) H (%, dry basis) N (%, dry basis) S (%, dry basis) P (%, dry basis) K (%, dry basis) C/N
Horticultural waste
Fruit peels
Food waste
Soybean residues
65.77
91.91
71.62
75.96
26.15
7.71
24.27
20.43
5.20 0.79
4.00 0.77
4.50 1.55
5.00 0.88
44.81 6.24 0.70 2.99 0.04 0.38 65.26
40.49 5.77 1.68 3.13 0.33 3.35 24.15
47.12 6.94 5.06 3.98 0.18 0.34 9.35
46.89 7.19 4.55 3.86 0.29 0.84 10.32
1449
2.2. Experimental set-up Co-composting materials were added to the horticultural waste in equal fresh weight for comparison as follows for composting: 100% horticultural waste (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3). The total weight of each mixture was 24 kg. Two 270 L domestic compost reactors (KompostKu, JK270, USA), each with two compartments inside were used to place the composting mixtures separately. The reactors were designed with few holes at one side of the wall for air ventilation and could be turned 360° to provide mixing for the materials present inside. The reactors were turned three times per week in the first three weeks and one time per week when the temperatures of all mixtures reached to near ambient temperature starting from the fourth week. Water was added to the piles when necessary according to the grab test (i.e. the materials are too wet if water can be squeezed out of a handful and too dry if the handful does not feel moist to the touch) during the active composting period in the first three weeks.[10] The composting was continued until the temperature and the carbon to nitrogen (C/N) ratio were stable for three consecutive data which was 46 days.
2.3. Physicochemical analysis The temperature of the centre point of the composting materials was measured before every turning using a digital hygrometer/psychrometer (TPI, model TPI597, USA). Samples were taken from different parts of the composting materials before every turning. The collected sample was then divided for the following analysis. For pH and electrical conductivity measurement, samples were mixed with deionized water at weight ratios of 1:5 and 1:10, respectively, according to Singapore CUGE Standards [11] and shaken for 30 min before measuring in a pH/conductivity meter (Mettler Toledo, model SG23, Switzerland). Nitrate content in the sample was analysed by the ion chromatography (IC) system (Dionex, model ICS-1100, USA). Before IC analysis, the samples were mixed with deionized water in weight ratio between 1:10 and 1:100 to suit the detection limit of the equipment, and shaken for 30 min. The solution of the mixture was used for IC analysis. Duplicate samples were analysed for the following tests. The number of total coliforms in the sample was determined using the plate count method. The sample was mixed with sterilized water and shaken for 30 min. The solution of the mixture was further mixed with sterilized water into different dilutions, inoculated on ® ChromoCult Coliform Agar (Merck, Germany), and incubated for 24 h at 35–37°C. Moisture content was calculated by taking the loss of sample weight after drying in an oven under 105°C for 24 h. Part of the oven-dried sample
S.Y. Choy et al.
Downloaded by [New York University] at 20:49 17 June 2015
was further heated at 550°C between 1 and 2 h until no change of weight, and the loss of weight was calculated as volatile solids content (modified from [12]). The other part of the oven-dried sample was grinded into less than 0.5 mm powder with a rotor mill (RETSCH ZM200, Germany) for the following two analyses. The percentages of carbon, hydrogen, nitrogen, sulphur were analysed using an elemental analyzer (Elementar, vario EL cube, Germany). Phosphorus and potassium contents were analysed by digesting the solid samples with a microwave digestion system (ETHOS One, Milestone, USA) followed by inductively coupled plasma optical emission spectrometer analysis (Perkin Elmer, Optima 2000DV, USA). 2.4. Stability analysis The stability of the sample was determined by the respiration study using carbon dioxide (CO2 ) evolution. The sample was first adjusted to moisture content between 65% and 80% via the ‘vacuum suction filter method’ with deionized water and pre-incubated at room temperature for 24 h. Five grams of the incubated sample was transferred into an incubation vessel containing 20 ml of 1 M NaOH. The vessel was sealed and kept at 37°C for four days. By back titration of the residual NaOH with 1 M HCl, the amount of CO2 adsorbed by NaOH was determined daily. Stability index [13] was used to evaluate the CO2 evolution rate (Q) in the unit of CO2 -C (mg) per organic matter (g) per day. The mass of organic matter refers to the volatile solid content mentioned before. Mass of CO2 -C was calculated with the following equation: 1 mol HCl 1000 ml 1 mol NaOH 1 mol C − CO2 × × 1 mol HCl 2 mol NaOH 12g C − CO2 × 1 mol C − CO2 1000 mg C − CO2 × , (1) 1g C − CO2
CO2 − C(mg) = HClb − HCls ×
where HClb is the HCl used in titration of blank (ml), HCls is the HCl used in titration of sample (ml) and CO2 -C the mass of CO2 -carbon generated (mg). 2.5. Germination index test The maturity of compost was determined by germination index (GI). To the sample deionized water was added in the ratio of 1: 1 (weight/volume) and shaken for 30 min. Twenty milliliter of the liquid extract was added to a Phytotestkit (MicroBioTests Inc., Belgium), which contained a foam pad at the bottom and a filter paper on the top. Ten cress seeds (Lepidium sativum L.) were evenly placed on the filter paper and incubated at room temperature (25°C)
for 72 h after which the number of germinated seed and the length of roots were measured. Duplicates were done for each sample. GI was determined according to the following equation [14]:
GI% =
Seed germinaiton (%) × root length of treatment (cm) Seed germination (%) × root length of control (cm)
× 100%.
(2)
3. Results and discussion 3.1. Temperature Temperature variations of all the composting mixtures are shown in Figure 1. In general, the temperature of all composting mixtures increased to the thermophilic range ( > 41°C) [10] in the first three to five days followed by a gradual decrease to a constant state near ambient temperature with some fluctuations. This trend was in agreement with the temperature profile of composting described in most of the literature.[2,4–7] The increased temperature is due to intense microbial activity that metabolizes the easily degradable compounds and produces bioheat. The decrease in temperature to near ambient is the shift of the decomposition process to a slower degradation process due to a rapid decrease in the easily degradable organic components in the substrate. The materials probably tend to become stable. The rate of initial increase, maximum temperature, and duration of thermophilic range were different among the composting materials. The rate of increase and temperature peak is proportional to the microbial consumption of the available organic material. Higher temperature peak also indicates higher microbial activity according to Adhikari et al.[15] Control and mixture with soybean 70 Control Mixture 1 Mixture 2 Mixture 3
60 Temperture (o C)
1450
50 40 30 20 10 0
10
20
30
40
50
Time (days)
Figure 1. Temperature variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
1451
Downloaded by [New York University] at 20:49 17 June 2015
Environmental Technology
3.2. pH The changes in pH are illustrated in Figure 2. In general, all mixtures increased from low pH to near neutral pH in three to five days and then remained in the weakly alkaline range thereafter except for the mixture with food waste. The pH of the mixture with food waste varied in the acidic range initially and increased to the neutral range only after eight days. Mixture with food waste remained in the acidic pH range for a longer time in the early stage compared with other mixtures that could be due to the formation of organic acids from microbial degradation of food waste. Similar results of food waste composting remaining in low pH for a week or more in early stage were also reported by other researchers.[9,17–19] As temperature was increased, pH increased rapidly. The increase in pH as observed in all composting materials could be explained by two factors. The first factor would be the decomposition of nitrogencontaining organic matter leading to the accumulation of ammonia that dissolves in water fraction to form alkaline NH4 + .[20] The second factor would be the combination of
10
8
pH
residues reached a maximum temperature on the third day (Figure 1). The temperature of the mixture with fruit peels and mixture with food waste continues to rise and reach a maximum temperature on the fifth day. The temperature peak was in the order of mixture 3 with soybean residues (63°C) > mixture 2 with food waste (57°C) > control and mixture 1 with fruit peels (46°C), suggesting that cocomposting horticultural waste with soybean residues or with food waste promoted temperature build-up as compared with horticultural waste alone and its mixture with fruit peels in a mass ratio of 1:1. This could be related to several reasons such as a higher proportion of readily degradable substances, more diverse population of microorganism, or higher mineral nutrients (e.g. N and P) in the materials that are essential for the biodegradation of organic compounds.[6] The availability of organic material inside soybean residues was higher than fruit peels and food waste as the rate of increase to maximum temperature was faster. The pulp form of soybean residues may facilitate this availability in comparison with the piece form of fruit peels and the irregular form of food waste. Temperature peaks of both mixture with food waste and mixture with soybean residues exceeded the minimum temperature of 55°C required for the destruction of pathogens. The temperature peak of mixture with soybean residues also exceeded a minimum temperature of 60°C required for deactivation of weed seeds and plant parasites.[16] Mixture with food waste and mixture with soybean residues had a longer active composting period (32 days) before dropping to the near ambient temperature, while the active composting period of control and mixture with fruit peels was 22 days. Longer active composting period or higher temperature could also assist decomposition of oil and grease which are commonly found in food waste.
6 Control Mixture 1 Mixture 2 Mixture 3
4
2 0
10
20
30
40
50
Time (days)
Figure 2. pH variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
available K+ in water-soluble form with bi-carbonic acids (HCO3 − ) produced during organic matter mineralization leading to the generation of potassium hydroxide.[6] In this study, the ranking of pH value during the first half period of composting was exactly the same as the ranking of K content in the raw materials. For instance, fruit peels contained the highest K content among all and the pH of it was also the highest. While during the last half period of composting which was after day 22, the pH of the mixture with food waste and mixture with soybean residues declined, possibly because of their higher nitrogen content that increased ammonium volatilization compared to others. The final pH of control, mixture with food waste, and mixture with soybean residues conformed to the requirement of pH 5.5–8 from Singapore CUGE Standards.[11] The final pH of the mixture with fruit peels was 8.8 and was the highest among all, suggesting that the studied fruit peels would increase the pH in composting and result in a high pH product. 3.3. Electrical conductivity Electrical conductivity (EC) is a measure of the soluble salt content which can greatly affect germination and plant growth. The changes in EC of all the composting piles are illustrated in Figure 3. The initial EC of all piles was close to each other in the range between 0.8 and 1.0 dS/m. After composting, EC of control and mixture with fruit peels decreased slightly. The decrease might be due to little volatilization of ammonia or precipitation of mineral salts.[21] Mixture with food waste and mixture with soybean residues increased EC significantly by 2.6 times and 1.5 times, respectively. The increase may be caused by the release of large quantities of mineral salts (e.g. phosphate ion) and greater concentration effect of mass loss [6,21] in food waste (mixture 2) and soybean residues
1452
S.Y. Choy et al. 50
5
46
Control Mixture 1 Mixture 2 Mixture 3
3
C (%)
EC (dS/m)
48
Standard limit
4
2
44 42 40
Control Mixture 1 Mixture 2 Mixture 3
38
1
36 0 0
10
20
30
40
50
0
10
30
40
50
Time (days)
Time (days)
Downloaded by [New York University] at 20:49 17 June 2015
20
Figure 3. EC variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
Figure 4. Variations of total carbon (dry basis) of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
(mixture 3) after biological decomposition. Mixture 2 containing food waste had the highest level of soluble salt. Based on the guidelines related to crop response with EC in composts,[22] control, mixture with fruit peels, and mixture with soybean residues had negligible effects on crops, and mixture with food waste could affect yields of only sensitive crops. Final EC values of all piles were below the maximum value of 4 dS/m required by Singapore CUGE Standards.[11]
Table 2. Nutrients content in final composting mixture (dry basis).
3.4. Total carbon The total carbon variations of all composting materials on dry basis are shown in Figure 4. Total carbon in all composting materials decreased significantly during first 10 days when the temperature was high, and changed very little during the later period of composting. Reduction in total carbon could be a measure of biodegradability rate for the compostable part of material. All mixtures degraded faster and degraded more than the control. Mixture with fruit peels and mixture with soybean residues degraded faster than mixture with food waste indicating that the degradation of compostable part for mixtures with fruit peels and soybean residues was faster than food waste in the initial stage. Comparing the initial and final total carbon, loss was the highest for mixture containing food waste and soybean residues, followed by fruit peels and control. This indicated that the degree of degradation of compostable part in mixtures with food waste and soybean residues was more than the mixture with fruit peels. 3.5. Nitrogen, phosphorus, and potassium The NPK contents in co-composting materials and in final composting product are shown in Tables 1 and 2,
Item
N (%)
P (%)
K (%)
Control Mixture 1 Mixture 2 Mixture 3
1.032 1.664 2.199 3.362
0.078 0.178 0.205 0.240
0.524 1.411 0.786 1.411
respectively. Food waste contained the highest N, while fruit peels contained the highest P and K. Soybean residues contained moderate N and P. After co-composting with horticultural waste in the same mass ratio, mixture 3 with soybean residues had the highest NPK contents, and mixture 1 with fruit peels had a comparable high K content. These results indicate that in order to obtain a final product with high nutrient from co-composting, selecting material with high nutrient may not guarantee higher nutrients in the final product compared with selecting a material with low nutrient. Other than nutrient concentration in the initial material, there are other contributing factors affecting the nutrient concentration in the final product. Figure 5 shows the changes of N, P, K, and nitrate (NO3 -N) in all the composting materials. The graphs of N and K show more fluctuation compared with the graph of P. This tendency of fluctuation is in agreement with other literatures and the possible reasons could be explained as follows. N has a higher chance of loss through different ways via leaching, ammonia gas emission, or denitrification.[23] K salts have high mobility that are easily dissolved in water and could be lost through leaching. P is less mobile as it forms strong bonds with organic matter.[20] In this study, no significant leaching in the liquid form was observed throughout the composting process except the few drops
1453
Environmental Technology (b)
(a) 4
0.3 Total P (%)
Total N (%)
3
0.4
2
1
0.2
0.1
0
0.0 0
10
20
30
40
50
0
10
Time (days) (d)
40
50
1.0
0.8 NO 3 -N (%)
1.5 Total K (%)
30
Time (days)
(c) 2.0
Downloaded by [New York University] at 20:49 17 June 2015
20
1.0
0.6
0.4
0.5 0.2 0.0
0.0 0
10
20
30
40
50
0
Time (days)
Control
10
20
30
40
50
Time (days)
Mixture 1
Mixture 2
Mixture 3
Figure 5. Variations of (a) nitrogen, (b) phosphorus, (c) potassium, and (d) nitrate on dry basis in composting mixtures with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
of water coming out from the reactor during the addition of water in the first three weeks. Overall, the control did not change much in the content of NPK, mixture 3 with soybean residues shows a significant increment in NPK, while mixture 2 with food waste shows a moderate increment in NPK. The N of mixture 1 with fruit peels increased slightly, but P and K did not change much. All the increment was the highest during the first 10–20 days which is also when the microbial degradation is intensive at thermophilic temperature. The reason of increment has been mentioned in other research that the increase was the concentration effect due to a higher rate of carbon loss compared with that of N, P, and K when the organic carbon in the materials decomposed into carbon dioxide.[6,21] The lower NPK increment of mixture 1 with fruit peels was due to a lower organic degradation rate which was proven by the lower
loss of total carbon and the lower peak temperature compared with others. The increment of NPK for mixture 3 with soybean residues was higher than mixture 2 with food waste, but the loss of total carbon was lower than mixture 2 with food waste. This result indicates that more NPK loss for mixture with food waste and soybean residues is better in conserving nutrients especially K. In order for organic N in the composting materials to be available to plants, it has to be mineralized by microorganism to soluble inorganic forms such as ammonium and nitrate. The changes in NO3 -N shown in Figure 5 indicate the part of the available N from the composting materials for plant growth. During the initial part of the composting period, NO3 -N for all materials was low due to the high temperature that inhibited the activity and growth of nitrifying bacteria responsible for nitrate formation.[21]
1454
S.Y. Choy et al.
80 Control Mixture 1 Mixture 2 Mixture 3
C/N ratio
60
40
20
0 0
10
20
30
40
50
Downloaded by [New York University] at 20:49 17 June 2015
Time (days)
Figure 6. C/N ratio variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
The concentration of NO3 -N was found to increase sharply after about 22 days when the temperature was in the mesophilic range. At the end of composting, mixture 2 with food waste contained higher NO3 -N than mixture 3 with soybean residues. It can be deduced from the above discussion that the levels of NPK in final composting product were affected by initial nutrient concentration of co-composting materials, degradation properties, and nutrient loss through emission or leaching. 3.6. C/N ratio According to Singapore CUGE Standards,[11] ratios between 12 and 24 are assumed to be indicative of stable compost. It can be seen from Figure 6 that replacing half of the material with the studied co-composting materials generally decreased the initial C/N ratio. The final C/N ratio of control was 43 and did not meet the standard mentioned above. The C/N ratio standard was met within 17 days for mixture 1 with fruit peels, 10 days for mixture 2 with food waste, and 8 days for mixture 3 with soybean residues. The decrease in C/N ratio was due to the concentration effect as carbon is biodegraded during composting. The stability of C/N ratio achieved was the fastest in mixture 3 with soybean residues, followed by mixture 1 with fruit peels, mixture 2 with food waste, and control. The results indicate that co-composting horticultural waste with any of the three organic wastes could significantly shorten the composting time to reach the standard C/N ratio. 3.7. Stability, GI, and total coliforms With the purpose of determining the stability of final composting materials, respiration study using CO2 evolution
rate was conducted. Stability index was calculated in CO2 – C (mg) per organic matter (g) per day, as being very stable (Q < 2); stable (2 ≤ Q ≤ 5); or moderately stable (5 ≤ Q ≤ 10).[13] The indexes obtained were 1.07, 2.31, 1.36, and 1.36 for control, mixture with fruit peels, food waste, and soybean residues, respectively, indicating that all the composting materials were either very stable or stable at the end of the composting. GI is used as an indicator of the maturity of compost. It combines the measure of relative seed germination and relative root elongation. GI higher than 50% can be considered as an acceptable level of phytotoxicity as suggested by Zucconi.[14] From the results obtained, GI of control, mixture with fruit peels, food waste, and soybean residues were 126%, 125%, 63%, and 81%, respectively. Hence, the products from all the materials achieved acceptable levels in terms of phytotoxicity to plants. The percentage of fruit peels was higher than other co-composting materials suggesting that it achieved better maturity at the end of composting. Food waste may need longer time for better maturity. For root elongation, the length of root in the case of deionized water was 5.11 cm, while the lengths of root in the case of control, mixture with fruit peels, food waste, and soybean residues were 7.71, 6.41, 4.42, and 4.39 cm, respectively. Root growth was better with product co-composted with fruit peels. The quantity of total coliforms is a useful indicator for contamination and the potential presence of pathogens associated with compost. The variations of total coliforms during the composting period are listed in Table 3. In the beginning, control, and mixtures with fruit peels and soybean residues were detected with about same amount levels of total coliforms (e.g. 107 CFU/g); while mixture 2 with food waste showed a higher value of 108 CFU/g. The source of coliforms could be mainly from horticultural waste. Normally, the waste collector placed tonnes of horticultural waste on the ground and in outdoor environment during the storage period before shredded by an equipment that was not sanitized. After three days of composting, total coliforms in mixture 2 with food waste and mixture 3 with soybean residues achieved at least 96% reduction, while mixture 1 with fruit peels achieved the same reduction after 8 days. Control had a slower reduction rate where 87% reduction was achieved only on day 10. The rapid reduction in mixture with food waste and mixture with soybean residues was caused by the higher temperature peak reached within three to five days compared with the rest. It can be seen from the table that in all the mixtures, the amount of total coliforms was once below the detection limit. This may be caused by temperature inactivation as mentioned before. The total coliforms were detected again thereafter and remained till the end of the composting process. It seems that total coliforms still can survive during the composting process and could not be destroyed totally. The survival of pathogens during the composting process has also been reported by other researchers.[24] A higher
1455
Environmental Technology Table 3. Variations of total coliforms in each composting mixture during the composting process. Total coliforms (CFU/g) Day of Composting
Downloaded by [New York University] at 20:49 17 June 2015
1 3 5 8 10 12 15 17 22 26 32 39 46
Control
Mixture 1
Mixture 2
Mixture 3
× × × × × × × × × × × × ×
1.50 × 107 4.00 × 107 4.65 × 107 5.00 × 105 2.94 × 105 6.98 × 104 1.40 × 106 4.20 × 105 4.00 × 104 4.00 × 105 N.D 3.80 × 104 3.40 × 103
5.10 × 108 1.85 × 107 2.10 × 107 N.D N.D 3.30 × 104 1.64 × 105 2.95 × 105 3.77 × 106 3.30 × 106 3.47 × 106 3.10 × 106 1.64 × 106
6.35 × 107 1.00 × 105 1.00 × 105 N.D N.D 3.86 × 104 3.55 × 106 2.01 × 106 3.97 × 105 2.35 × 106 8.89 × 106 3.22 × 107 1.42 × 106
4.43 3.77 5.80 5.75 5.63 1.52 1.20 4.10 3.05 4.81 1.00 3.12 1.84
107 109 109 107 106 105 106 106 105 106 104 104 104
Note: ND = Not detected.
amount of total coliforms found in mixture with food waste may indicate that the characteristics of food waste were conducive for the growth of coliforms. At the end of composting, total coliforms were reduced to at least 98% for all the composting materials.
4. Conclusion Compared with composting horticultural waste alone, co-composting of horticultural waste with either fruit peels, food waste, or soybean residues showed a higher loss of total carbon, higher NPK levels in final product, and shorter composting time to reach the standard C/N ratio. This study also shows that the final product with the highest nutrient (e.g. N, P, K) did not come from adding co-material with the highest nutrient. Other than nutrient concentration in the initial composition, there are other factors that can alter the nutrient concentration in the final composting product such as degradation properties and loss of nutrient during the process. Fruit peels were the best resource for K and the final product had a higher pH. Food waste contained available N in NO3 -N form more than mixture with soybean residues. But it had a higher salt content, total coliforms, and slower biological degradation. Soybean residues were found to be the best co-composting material for all N, P, and K. It also achieved the highest temperature for destruction of pathogens and weed seeds, highest rate of total carbon loss, shortest period to reach standard C/N ratio, and best conservation of nutrients.
Acknowledgements The authors would like to thank GreenBack Pte Ltd for providing shredded horticultural waste, NEWRI-R3C/NTU family for their contributions to this research, and Ms. Mo Yu for her assistance in part of the sample collection.
Disclosure statement No potential conflict of interest was reported by the authors.
References [1] Kaboré TW, Houot S, Hien E, Zombré P, Hien V, Masse D. Effect of the raw materials and mixing ratio of composted wastes on the dynamic of organic matter stabilization and nitrogen availability in composts of Sub-Saharan Africa. Bioresour Technol. 2010;101:1002–1013. [2] Aydn GA, Kocasoy G. Investigation of appropriate initial composition and aeration method for co-composting of yard waste and market wastes. J Environ Sci Health Part B. 2003;38:221–231. [3] Suzuki T, Ikumi Y, Okamoto S, Watanabe I, Fujitake N, Otsuka H. Aerobic composting of chips from clear-cut trees with various co-composting materials. Bioresour Technol. 2004;95:121–128. [4] Huang GF, Fang M, Wu QT, Zhou LX, Liao XD, Wong JWC. Co-composting of pig manure with leaves. Environ Technol. 2001;22:1203–1212. [5] Jolanun B, Towprayoon S. Novel bulking agent from clay residue for food waste composting. Bioresour Technol. 2010;101:4484–4490. [6] Kalemelawa F, Nishihara E, Endo T, Ahmad Z, Yeasmin R, Tenywa MM, Yamamoto S. An evaluation of aerobic and anaerobic composting of banana peels treated with different inoculums for soil nutrient replenishment. Bioresour Technol. 2012;126:375–382. [7] Wong JWC, Mak KF, Chan NW, Lam A, Fang M, Zhou LX, Wu QT, Liao XD. Co-composting of soybean residues and leaves in Hong Kong. Bioresour Technol. 2001;76: 99–106. [8] Lin C. A negative-pressure aeration system for composting food wastes. Bioresour Technol. 2008;99:7651–7656. [9] Li S, Huang GH, An CJ, Yu H. Effect of different buffer agents on in-vessel composting of food waste: performance analysis and comparative study. J Environ Sci Health Part A. 2013;48:772–780. [10] Pace MG, Miller BE, Farrell-Poe KL. The composting process, All archived publications (1995), Paper 48. Available from: http://digitalcommons.usu.edu/extension_histall/48
Downloaded by [New York University] at 20:49 17 June 2015
1456
S.Y. Choy et al.
[11] Specifications for composts & mulches, CUGE Standards, CS A02, Centre for Urban Greenery & Ecology, National Parks Board, Singapore, 2009. [12] Test Methods for the Examination of Composting and Compost (TMECC), The US. Composting Council, 2001. [13] Epstein E. The science of composting. Lancaster: Technomic Pub. Co.; 1997. [14] Zucconi F, Forte M, Monac A, Beritodi M. Biological evaluation of compost maturity. Biocycle. 1981;22: 27–29. [15] Adhikari BK, Barrington S, Martinez J, King S. Effectiveness of three bulking agents for food waste composting. Waste Manage. 2009;29:197–203. [16] Chiumenti A. Modern composting technologies. Emmaus: J G Press; 2005. [17] Zhou Y, Selvam A, Wong JWC. Evaluation of humic substances during co-composting of food waste, sawdust and Chinese medicinal herbal residues. Bioresour Technol. 2014;168:229–234. [18] Wong JWC, Fung SO, Selvam A. Coal fly ash and lime addition enhances the rate and efficiency of decomposition
[19]
[20] [21] [22] [23] [24]
of food waste during composting. Bioresour Technol. 2009;100:3324–3331. Wang X, Selvam A, Chan M, Wong JWC. Nitrogen conservation and acidity control during food wastes composting through struvite formation. Bioresour Technol. 2013;147:17–22. Tumuhairwe JB, Tenywa JS, Otabbong E, Ledin S. Comparison of four low-technology composting methods for market crop wastes. Waste Manage. 2009;29:2274–2281. Kumar M, Ou Y-L, Lin J-G. Co-composting of green waste and food waste at low C/N ratio. Waste Manage. 2010;30:602–609. Epstein E. Industrial composting: environmental engineering and facilities management. Boca Raton, FL: CRC Press; 2011. Sommer SG. Effect of composting on nutrient loss and nitrogen availability of cattle deep litter. Eur J Agron. 2001;14:123–133. Ivanov VN, Wang JY, Stabnikova OV, Tay STL, Tay JH. Microbiological monitoring in the biodegradation of sewage sludge and food waste. J Appl Microbiol. 2004;96:641–647.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Co-composting of horticultural waste with fruit peels, food waste, and soybean residues a
ab
a
a
a
Sing Ying Choy , Ke Wang , Wei Qi , Ben Wang , Chia-Lung Chen & Jing-Yuan Wang
ac
a
Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore b
School of Municipal and Environmental Engineering, Harbin Institute of Technology, People's Republic of China c
School of Civil and Environmental Engineering, Nanyang Technological University, Singapore Published online: 03 Feb 2015.
Click for updates To cite this article: Sing Ying Choy, Ke Wang, Wei Qi, Ben Wang, Chia-Lung Chen & Jing-Yuan Wang (2015) Co-composting of horticultural waste with fruit peels, food waste, and soybean residues, Environmental Technology, 36:11, 1448-1456, DOI: 10.1080/09593330.2014.993728 To link to this article: http://dx.doi.org/10.1080/09593330.2014.993728
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2015 Vol. 36, No. 11, 1448–1456, http://dx.doi.org/10.1080/09593330.2014.993728
Co-composting of horticultural waste with fruit peels, food waste, and soybean residues Sing Ying Choya , Ke Wanga,b , Wei Qia , Ben Wanga , Chia-Lung Chena and Jing-Yuan Wanga,c∗ a Residues
and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, Singapore; b School of Municipal and Environmental Engineering, Harbin Institute of Technology, People’s Republic of China; c School of Civil and Environmental Engineering, Nanyang Technological University, Singapore
Downloaded by [New York University] at 20:49 17 June 2015
(Received 2 January 2014; accepted 26 November 2014 ) Horticultural waste was co-composted with fruit peels, food waste, and soybean residues individually to evaluate the effects of these easily available organic wastes in Singapore on the composting process and product quality. Each co-composting material was mixed with horticultural waste in the wet weight ratio of 1:1 and composted for 46 days. Results showed that all co-composting materials accelerated the degradation of total carbon and resulted in higher nutrients of nitrogen (N), phosphorous (P), and potassium (K) in the final product compared with horticultural waste alone. Mixture with fruit peels achieved the fastest total carbon loss; however, did not reach the minimum required temperature for pathogen destruction. The end product was found to be the best source for K and had a higher pH that could be used for the remediation of acidic soil. Food waste resulted in the highest available nitrate (NO3 -N) content in the end product, but caused high salt content, total coliforms, and slower total carbon loss initially. Soybean residues were found to be the best co-composting material to produce compost with high N, P, and K when compared with other materials due to the highest temperature, fastest total carbon loss, fastest reduction in C/N ratio, and best conservation of nutrients. Keywords: composting; horticultural waste; food waste; soybean residues; fertilizer quality
1. Introduction Composting is an environmental-friendly method to treat organic waste and to obtain organic fertilizer. Organic waste such as horticultural waste is recalcitrant and requires long composting time. Various materials have been co-composted with horticultural waste to shorten the composting time and to improve product quality along with other benefits. For instance, slaughterhouse wastes accelerated the compost stabilization and resulted in end product with high nitrogen availability [1]; household refuses and market waste containing mainly vegetables and fruit waste reduced water requirement during composting [1,2]; chicken faeces were effective to be cocomposted with wood chips; and [3] market waste and yard waste resulted in a higher quality end product compared with their mixture with grass.[2] On the other hand, horticultural waste is also used to complement the composting of other organic waste as the bulking agent or for waste containing high nitrogen due to its high carbon content in order to obtain a desired initial C/N ratio.[4,5] Though many different co-composting have been researched, the practice depends on the availability of cocomposting materials in the local environment. Organic waste such as fruit peels, soybean residues, and food waste are easily available and contain high nutrients such as
*Corresponding author. Email: [email protected] © 2015 Taylor & Francis
nitrogen (N), phosphorus (P), and potassium (K). But most of the time, these waste are not utilized effectively and disposed through incineration in the case of Singapore. The wasted nutrients could actually be used to complement the low nutrient of composting product from the local horticultural waste generated. Hence, it is worth to study the effect of these materials on the process of local horticultural waste composting and on the product quality. Despite extensive co-composting research being carried out as mentioned above, limited information is available in the literature about the effects of fruit peels and soybean residues. Kalemelawa et al. researched on enhancing the composting of banana peels by co-composting with animal manure, poultry litter, or earthworm.[6] But composting with horticultural waste was not covered in the study. Wong et al. studied the optimal turning frequency for co-composting of soybean residues with leaves and sawdust.[7] However, main nutrients such as phosphorus and potassium were not reported. Food waste is easily available and has been researched widely including the study of different bulking agents,[5] operating system,[8] and pH control.[9] Hence, it is meaningful to compare the effect of it with the rarely studied fruit peels and soybean residues. In this study, three co-composting materials, namely fruit peels, food waste, and soybean residues were
Environmental Technology
Downloaded by [New York University] at 20:49 17 June 2015
compared for their effect on local horticultural waste composting. The effect studied on the composting process included temperature, pH, electrical conductivity, total carbon, and C/N ratio. The quality of the end product from each mixture was also compared in terms of germination, respiration, total coliforms, and NPK content. The results are expected to fill the knowledge gap of co-composting and serve as a reference for research of producing organic fertilizer.
2. Materials and methods 2.1. Materials for composting Horticultural waste and the mixture of it with fruit peels, food waste, and soybean residues individually were used for composting in this study. Horticultural waste consisting of tree prunings and leaves was collected from a composting facility in Singapore and shredded to particle size of < 30 mm before use. Fruit peels that contain peels from bananas, papayas, water melons, dragon fruits, guavas, mangoes, and honey dews were obtained from fruit stalls in a local university. The fruit peels were cut into < 5 cm long before use. Food waste from plate leftover food consisting mainly rice, cooked vegetables, cooked meats was collected from canteen of a local university. Bones and food with length more than about 4 cm were removed from the collected food waste. Soybean residues were collected from soya bean food and beverage retailers. It is the residues in pulp form after pureed soybeans are filtered in the production of soya milk and soya bean curd. The residues were used directly in the study without any pre-treatment. All the materials used for composting were analysed for their properties and the results are summarized in Table 1.
Table 1. Characteristics of horticultural waste, fruit peels, food waste, and soybean residues prior to composting. Parameter Water content (%) Volatile solid (%, wet basis) pH Electrical conductivity (dS/m) C (%, dry basis) H (%, dry basis) N (%, dry basis) S (%, dry basis) P (%, dry basis) K (%, dry basis) C/N
Horticultural waste
Fruit peels
Food waste
Soybean residues
65.77
91.91
71.62
75.96
26.15
7.71
24.27
20.43
5.20 0.79
4.00 0.77
4.50 1.55
5.00 0.88
44.81 6.24 0.70 2.99 0.04 0.38 65.26
40.49 5.77 1.68 3.13 0.33 3.35 24.15
47.12 6.94 5.06 3.98 0.18 0.34 9.35
46.89 7.19 4.55 3.86 0.29 0.84 10.32
1449
2.2. Experimental set-up Co-composting materials were added to the horticultural waste in equal fresh weight for comparison as follows for composting: 100% horticultural waste (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3). The total weight of each mixture was 24 kg. Two 270 L domestic compost reactors (KompostKu, JK270, USA), each with two compartments inside were used to place the composting mixtures separately. The reactors were designed with few holes at one side of the wall for air ventilation and could be turned 360° to provide mixing for the materials present inside. The reactors were turned three times per week in the first three weeks and one time per week when the temperatures of all mixtures reached to near ambient temperature starting from the fourth week. Water was added to the piles when necessary according to the grab test (i.e. the materials are too wet if water can be squeezed out of a handful and too dry if the handful does not feel moist to the touch) during the active composting period in the first three weeks.[10] The composting was continued until the temperature and the carbon to nitrogen (C/N) ratio were stable for three consecutive data which was 46 days.
2.3. Physicochemical analysis The temperature of the centre point of the composting materials was measured before every turning using a digital hygrometer/psychrometer (TPI, model TPI597, USA). Samples were taken from different parts of the composting materials before every turning. The collected sample was then divided for the following analysis. For pH and electrical conductivity measurement, samples were mixed with deionized water at weight ratios of 1:5 and 1:10, respectively, according to Singapore CUGE Standards [11] and shaken for 30 min before measuring in a pH/conductivity meter (Mettler Toledo, model SG23, Switzerland). Nitrate content in the sample was analysed by the ion chromatography (IC) system (Dionex, model ICS-1100, USA). Before IC analysis, the samples were mixed with deionized water in weight ratio between 1:10 and 1:100 to suit the detection limit of the equipment, and shaken for 30 min. The solution of the mixture was used for IC analysis. Duplicate samples were analysed for the following tests. The number of total coliforms in the sample was determined using the plate count method. The sample was mixed with sterilized water and shaken for 30 min. The solution of the mixture was further mixed with sterilized water into different dilutions, inoculated on ® ChromoCult Coliform Agar (Merck, Germany), and incubated for 24 h at 35–37°C. Moisture content was calculated by taking the loss of sample weight after drying in an oven under 105°C for 24 h. Part of the oven-dried sample
S.Y. Choy et al.
Downloaded by [New York University] at 20:49 17 June 2015
was further heated at 550°C between 1 and 2 h until no change of weight, and the loss of weight was calculated as volatile solids content (modified from [12]). The other part of the oven-dried sample was grinded into less than 0.5 mm powder with a rotor mill (RETSCH ZM200, Germany) for the following two analyses. The percentages of carbon, hydrogen, nitrogen, sulphur were analysed using an elemental analyzer (Elementar, vario EL cube, Germany). Phosphorus and potassium contents were analysed by digesting the solid samples with a microwave digestion system (ETHOS One, Milestone, USA) followed by inductively coupled plasma optical emission spectrometer analysis (Perkin Elmer, Optima 2000DV, USA). 2.4. Stability analysis The stability of the sample was determined by the respiration study using carbon dioxide (CO2 ) evolution. The sample was first adjusted to moisture content between 65% and 80% via the ‘vacuum suction filter method’ with deionized water and pre-incubated at room temperature for 24 h. Five grams of the incubated sample was transferred into an incubation vessel containing 20 ml of 1 M NaOH. The vessel was sealed and kept at 37°C for four days. By back titration of the residual NaOH with 1 M HCl, the amount of CO2 adsorbed by NaOH was determined daily. Stability index [13] was used to evaluate the CO2 evolution rate (Q) in the unit of CO2 -C (mg) per organic matter (g) per day. The mass of organic matter refers to the volatile solid content mentioned before. Mass of CO2 -C was calculated with the following equation: 1 mol HCl 1000 ml 1 mol NaOH 1 mol C − CO2 × × 1 mol HCl 2 mol NaOH 12g C − CO2 × 1 mol C − CO2 1000 mg C − CO2 × , (1) 1g C − CO2
CO2 − C(mg) = HClb − HCls ×
where HClb is the HCl used in titration of blank (ml), HCls is the HCl used in titration of sample (ml) and CO2 -C the mass of CO2 -carbon generated (mg). 2.5. Germination index test The maturity of compost was determined by germination index (GI). To the sample deionized water was added in the ratio of 1: 1 (weight/volume) and shaken for 30 min. Twenty milliliter of the liquid extract was added to a Phytotestkit (MicroBioTests Inc., Belgium), which contained a foam pad at the bottom and a filter paper on the top. Ten cress seeds (Lepidium sativum L.) were evenly placed on the filter paper and incubated at room temperature (25°C)
for 72 h after which the number of germinated seed and the length of roots were measured. Duplicates were done for each sample. GI was determined according to the following equation [14]:
GI% =
Seed germinaiton (%) × root length of treatment (cm) Seed germination (%) × root length of control (cm)
× 100%.
(2)
3. Results and discussion 3.1. Temperature Temperature variations of all the composting mixtures are shown in Figure 1. In general, the temperature of all composting mixtures increased to the thermophilic range ( > 41°C) [10] in the first three to five days followed by a gradual decrease to a constant state near ambient temperature with some fluctuations. This trend was in agreement with the temperature profile of composting described in most of the literature.[2,4–7] The increased temperature is due to intense microbial activity that metabolizes the easily degradable compounds and produces bioheat. The decrease in temperature to near ambient is the shift of the decomposition process to a slower degradation process due to a rapid decrease in the easily degradable organic components in the substrate. The materials probably tend to become stable. The rate of initial increase, maximum temperature, and duration of thermophilic range were different among the composting materials. The rate of increase and temperature peak is proportional to the microbial consumption of the available organic material. Higher temperature peak also indicates higher microbial activity according to Adhikari et al.[15] Control and mixture with soybean 70 Control Mixture 1 Mixture 2 Mixture 3
60 Temperture (o C)
1450
50 40 30 20 10 0
10
20
30
40
50
Time (days)
Figure 1. Temperature variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
1451
Downloaded by [New York University] at 20:49 17 June 2015
Environmental Technology
3.2. pH The changes in pH are illustrated in Figure 2. In general, all mixtures increased from low pH to near neutral pH in three to five days and then remained in the weakly alkaline range thereafter except for the mixture with food waste. The pH of the mixture with food waste varied in the acidic range initially and increased to the neutral range only after eight days. Mixture with food waste remained in the acidic pH range for a longer time in the early stage compared with other mixtures that could be due to the formation of organic acids from microbial degradation of food waste. Similar results of food waste composting remaining in low pH for a week or more in early stage were also reported by other researchers.[9,17–19] As temperature was increased, pH increased rapidly. The increase in pH as observed in all composting materials could be explained by two factors. The first factor would be the decomposition of nitrogencontaining organic matter leading to the accumulation of ammonia that dissolves in water fraction to form alkaline NH4 + .[20] The second factor would be the combination of
10
8
pH
residues reached a maximum temperature on the third day (Figure 1). The temperature of the mixture with fruit peels and mixture with food waste continues to rise and reach a maximum temperature on the fifth day. The temperature peak was in the order of mixture 3 with soybean residues (63°C) > mixture 2 with food waste (57°C) > control and mixture 1 with fruit peels (46°C), suggesting that cocomposting horticultural waste with soybean residues or with food waste promoted temperature build-up as compared with horticultural waste alone and its mixture with fruit peels in a mass ratio of 1:1. This could be related to several reasons such as a higher proportion of readily degradable substances, more diverse population of microorganism, or higher mineral nutrients (e.g. N and P) in the materials that are essential for the biodegradation of organic compounds.[6] The availability of organic material inside soybean residues was higher than fruit peels and food waste as the rate of increase to maximum temperature was faster. The pulp form of soybean residues may facilitate this availability in comparison with the piece form of fruit peels and the irregular form of food waste. Temperature peaks of both mixture with food waste and mixture with soybean residues exceeded the minimum temperature of 55°C required for the destruction of pathogens. The temperature peak of mixture with soybean residues also exceeded a minimum temperature of 60°C required for deactivation of weed seeds and plant parasites.[16] Mixture with food waste and mixture with soybean residues had a longer active composting period (32 days) before dropping to the near ambient temperature, while the active composting period of control and mixture with fruit peels was 22 days. Longer active composting period or higher temperature could also assist decomposition of oil and grease which are commonly found in food waste.
6 Control Mixture 1 Mixture 2 Mixture 3
4
2 0
10
20
30
40
50
Time (days)
Figure 2. pH variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
available K+ in water-soluble form with bi-carbonic acids (HCO3 − ) produced during organic matter mineralization leading to the generation of potassium hydroxide.[6] In this study, the ranking of pH value during the first half period of composting was exactly the same as the ranking of K content in the raw materials. For instance, fruit peels contained the highest K content among all and the pH of it was also the highest. While during the last half period of composting which was after day 22, the pH of the mixture with food waste and mixture with soybean residues declined, possibly because of their higher nitrogen content that increased ammonium volatilization compared to others. The final pH of control, mixture with food waste, and mixture with soybean residues conformed to the requirement of pH 5.5–8 from Singapore CUGE Standards.[11] The final pH of the mixture with fruit peels was 8.8 and was the highest among all, suggesting that the studied fruit peels would increase the pH in composting and result in a high pH product. 3.3. Electrical conductivity Electrical conductivity (EC) is a measure of the soluble salt content which can greatly affect germination and plant growth. The changes in EC of all the composting piles are illustrated in Figure 3. The initial EC of all piles was close to each other in the range between 0.8 and 1.0 dS/m. After composting, EC of control and mixture with fruit peels decreased slightly. The decrease might be due to little volatilization of ammonia or precipitation of mineral salts.[21] Mixture with food waste and mixture with soybean residues increased EC significantly by 2.6 times and 1.5 times, respectively. The increase may be caused by the release of large quantities of mineral salts (e.g. phosphate ion) and greater concentration effect of mass loss [6,21] in food waste (mixture 2) and soybean residues
1452
S.Y. Choy et al. 50
5
46
Control Mixture 1 Mixture 2 Mixture 3
3
C (%)
EC (dS/m)
48
Standard limit
4
2
44 42 40
Control Mixture 1 Mixture 2 Mixture 3
38
1
36 0 0
10
20
30
40
50
0
10
30
40
50
Time (days)
Time (days)
Downloaded by [New York University] at 20:49 17 June 2015
20
Figure 3. EC variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
Figure 4. Variations of total carbon (dry basis) of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
(mixture 3) after biological decomposition. Mixture 2 containing food waste had the highest level of soluble salt. Based on the guidelines related to crop response with EC in composts,[22] control, mixture with fruit peels, and mixture with soybean residues had negligible effects on crops, and mixture with food waste could affect yields of only sensitive crops. Final EC values of all piles were below the maximum value of 4 dS/m required by Singapore CUGE Standards.[11]
Table 2. Nutrients content in final composting mixture (dry basis).
3.4. Total carbon The total carbon variations of all composting materials on dry basis are shown in Figure 4. Total carbon in all composting materials decreased significantly during first 10 days when the temperature was high, and changed very little during the later period of composting. Reduction in total carbon could be a measure of biodegradability rate for the compostable part of material. All mixtures degraded faster and degraded more than the control. Mixture with fruit peels and mixture with soybean residues degraded faster than mixture with food waste indicating that the degradation of compostable part for mixtures with fruit peels and soybean residues was faster than food waste in the initial stage. Comparing the initial and final total carbon, loss was the highest for mixture containing food waste and soybean residues, followed by fruit peels and control. This indicated that the degree of degradation of compostable part in mixtures with food waste and soybean residues was more than the mixture with fruit peels. 3.5. Nitrogen, phosphorus, and potassium The NPK contents in co-composting materials and in final composting product are shown in Tables 1 and 2,
Item
N (%)
P (%)
K (%)
Control Mixture 1 Mixture 2 Mixture 3
1.032 1.664 2.199 3.362
0.078 0.178 0.205 0.240
0.524 1.411 0.786 1.411
respectively. Food waste contained the highest N, while fruit peels contained the highest P and K. Soybean residues contained moderate N and P. After co-composting with horticultural waste in the same mass ratio, mixture 3 with soybean residues had the highest NPK contents, and mixture 1 with fruit peels had a comparable high K content. These results indicate that in order to obtain a final product with high nutrient from co-composting, selecting material with high nutrient may not guarantee higher nutrients in the final product compared with selecting a material with low nutrient. Other than nutrient concentration in the initial material, there are other contributing factors affecting the nutrient concentration in the final product. Figure 5 shows the changes of N, P, K, and nitrate (NO3 -N) in all the composting materials. The graphs of N and K show more fluctuation compared with the graph of P. This tendency of fluctuation is in agreement with other literatures and the possible reasons could be explained as follows. N has a higher chance of loss through different ways via leaching, ammonia gas emission, or denitrification.[23] K salts have high mobility that are easily dissolved in water and could be lost through leaching. P is less mobile as it forms strong bonds with organic matter.[20] In this study, no significant leaching in the liquid form was observed throughout the composting process except the few drops
1453
Environmental Technology (b)
(a) 4
0.3 Total P (%)
Total N (%)
3
0.4
2
1
0.2
0.1
0
0.0 0
10
20
30
40
50
0
10
Time (days) (d)
40
50
1.0
0.8 NO 3 -N (%)
1.5 Total K (%)
30
Time (days)
(c) 2.0
Downloaded by [New York University] at 20:49 17 June 2015
20
1.0
0.6
0.4
0.5 0.2 0.0
0.0 0
10
20
30
40
50
0
Time (days)
Control
10
20
30
40
50
Time (days)
Mixture 1
Mixture 2
Mixture 3
Figure 5. Variations of (a) nitrogen, (b) phosphorus, (c) potassium, and (d) nitrate on dry basis in composting mixtures with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
of water coming out from the reactor during the addition of water in the first three weeks. Overall, the control did not change much in the content of NPK, mixture 3 with soybean residues shows a significant increment in NPK, while mixture 2 with food waste shows a moderate increment in NPK. The N of mixture 1 with fruit peels increased slightly, but P and K did not change much. All the increment was the highest during the first 10–20 days which is also when the microbial degradation is intensive at thermophilic temperature. The reason of increment has been mentioned in other research that the increase was the concentration effect due to a higher rate of carbon loss compared with that of N, P, and K when the organic carbon in the materials decomposed into carbon dioxide.[6,21] The lower NPK increment of mixture 1 with fruit peels was due to a lower organic degradation rate which was proven by the lower
loss of total carbon and the lower peak temperature compared with others. The increment of NPK for mixture 3 with soybean residues was higher than mixture 2 with food waste, but the loss of total carbon was lower than mixture 2 with food waste. This result indicates that more NPK loss for mixture with food waste and soybean residues is better in conserving nutrients especially K. In order for organic N in the composting materials to be available to plants, it has to be mineralized by microorganism to soluble inorganic forms such as ammonium and nitrate. The changes in NO3 -N shown in Figure 5 indicate the part of the available N from the composting materials for plant growth. During the initial part of the composting period, NO3 -N for all materials was low due to the high temperature that inhibited the activity and growth of nitrifying bacteria responsible for nitrate formation.[21]
1454
S.Y. Choy et al.
80 Control Mixture 1 Mixture 2 Mixture 3
C/N ratio
60
40
20
0 0
10
20
30
40
50
Downloaded by [New York University] at 20:49 17 June 2015
Time (days)
Figure 6. C/N ratio variations of composting piles with time for horticultural waste alone (control), 50% horticultural waste and 50% fruit peels (mixture 1), 50% horticultural waste and 50% food waste (mixture 2), and 50% horticultural waste and 50% soybean residues (mixture 3).
The concentration of NO3 -N was found to increase sharply after about 22 days when the temperature was in the mesophilic range. At the end of composting, mixture 2 with food waste contained higher NO3 -N than mixture 3 with soybean residues. It can be deduced from the above discussion that the levels of NPK in final composting product were affected by initial nutrient concentration of co-composting materials, degradation properties, and nutrient loss through emission or leaching. 3.6. C/N ratio According to Singapore CUGE Standards,[11] ratios between 12 and 24 are assumed to be indicative of stable compost. It can be seen from Figure 6 that replacing half of the material with the studied co-composting materials generally decreased the initial C/N ratio. The final C/N ratio of control was 43 and did not meet the standard mentioned above. The C/N ratio standard was met within 17 days for mixture 1 with fruit peels, 10 days for mixture 2 with food waste, and 8 days for mixture 3 with soybean residues. The decrease in C/N ratio was due to the concentration effect as carbon is biodegraded during composting. The stability of C/N ratio achieved was the fastest in mixture 3 with soybean residues, followed by mixture 1 with fruit peels, mixture 2 with food waste, and control. The results indicate that co-composting horticultural waste with any of the three organic wastes could significantly shorten the composting time to reach the standard C/N ratio. 3.7. Stability, GI, and total coliforms With the purpose of determining the stability of final composting materials, respiration study using CO2 evolution
rate was conducted. Stability index was calculated in CO2 – C (mg) per organic matter (g) per day, as being very stable (Q < 2); stable (2 ≤ Q ≤ 5); or moderately stable (5 ≤ Q ≤ 10).[13] The indexes obtained were 1.07, 2.31, 1.36, and 1.36 for control, mixture with fruit peels, food waste, and soybean residues, respectively, indicating that all the composting materials were either very stable or stable at the end of the composting. GI is used as an indicator of the maturity of compost. It combines the measure of relative seed germination and relative root elongation. GI higher than 50% can be considered as an acceptable level of phytotoxicity as suggested by Zucconi.[14] From the results obtained, GI of control, mixture with fruit peels, food waste, and soybean residues were 126%, 125%, 63%, and 81%, respectively. Hence, the products from all the materials achieved acceptable levels in terms of phytotoxicity to plants. The percentage of fruit peels was higher than other co-composting materials suggesting that it achieved better maturity at the end of composting. Food waste may need longer time for better maturity. For root elongation, the length of root in the case of deionized water was 5.11 cm, while the lengths of root in the case of control, mixture with fruit peels, food waste, and soybean residues were 7.71, 6.41, 4.42, and 4.39 cm, respectively. Root growth was better with product co-composted with fruit peels. The quantity of total coliforms is a useful indicator for contamination and the potential presence of pathogens associated with compost. The variations of total coliforms during the composting period are listed in Table 3. In the beginning, control, and mixtures with fruit peels and soybean residues were detected with about same amount levels of total coliforms (e.g. 107 CFU/g); while mixture 2 with food waste showed a higher value of 108 CFU/g. The source of coliforms could be mainly from horticultural waste. Normally, the waste collector placed tonnes of horticultural waste on the ground and in outdoor environment during the storage period before shredded by an equipment that was not sanitized. After three days of composting, total coliforms in mixture 2 with food waste and mixture 3 with soybean residues achieved at least 96% reduction, while mixture 1 with fruit peels achieved the same reduction after 8 days. Control had a slower reduction rate where 87% reduction was achieved only on day 10. The rapid reduction in mixture with food waste and mixture with soybean residues was caused by the higher temperature peak reached within three to five days compared with the rest. It can be seen from the table that in all the mixtures, the amount of total coliforms was once below the detection limit. This may be caused by temperature inactivation as mentioned before. The total coliforms were detected again thereafter and remained till the end of the composting process. It seems that total coliforms still can survive during the composting process and could not be destroyed totally. The survival of pathogens during the composting process has also been reported by other researchers.[24] A higher
1455
Environmental Technology Table 3. Variations of total coliforms in each composting mixture during the composting process. Total coliforms (CFU/g) Day of Composting
Downloaded by [New York University] at 20:49 17 June 2015
1 3 5 8 10 12 15 17 22 26 32 39 46
Control
Mixture 1
Mixture 2
Mixture 3
× × × × × × × × × × × × ×
1.50 × 107 4.00 × 107 4.65 × 107 5.00 × 105 2.94 × 105 6.98 × 104 1.40 × 106 4.20 × 105 4.00 × 104 4.00 × 105 N.D 3.80 × 104 3.40 × 103
5.10 × 108 1.85 × 107 2.10 × 107 N.D N.D 3.30 × 104 1.64 × 105 2.95 × 105 3.77 × 106 3.30 × 106 3.47 × 106 3.10 × 106 1.64 × 106
6.35 × 107 1.00 × 105 1.00 × 105 N.D N.D 3.86 × 104 3.55 × 106 2.01 × 106 3.97 × 105 2.35 × 106 8.89 × 106 3.22 × 107 1.42 × 106
4.43 3.77 5.80 5.75 5.63 1.52 1.20 4.10 3.05 4.81 1.00 3.12 1.84
107 109 109 107 106 105 106 106 105 106 104 104 104
Note: ND = Not detected.
amount of total coliforms found in mixture with food waste may indicate that the characteristics of food waste were conducive for the growth of coliforms. At the end of composting, total coliforms were reduced to at least 98% for all the composting materials.
4. Conclusion Compared with composting horticultural waste alone, co-composting of horticultural waste with either fruit peels, food waste, or soybean residues showed a higher loss of total carbon, higher NPK levels in final product, and shorter composting time to reach the standard C/N ratio. This study also shows that the final product with the highest nutrient (e.g. N, P, K) did not come from adding co-material with the highest nutrient. Other than nutrient concentration in the initial composition, there are other factors that can alter the nutrient concentration in the final composting product such as degradation properties and loss of nutrient during the process. Fruit peels were the best resource for K and the final product had a higher pH. Food waste contained available N in NO3 -N form more than mixture with soybean residues. But it had a higher salt content, total coliforms, and slower biological degradation. Soybean residues were found to be the best co-composting material for all N, P, and K. It also achieved the highest temperature for destruction of pathogens and weed seeds, highest rate of total carbon loss, shortest period to reach standard C/N ratio, and best conservation of nutrients.
Acknowledgements The authors would like to thank GreenBack Pte Ltd for providing shredded horticultural waste, NEWRI-R3C/NTU family for their contributions to this research, and Ms. Mo Yu for her assistance in part of the sample collection.
Disclosure statement No potential conflict of interest was reported by the authors.
References [1] Kaboré TW, Houot S, Hien E, Zombré P, Hien V, Masse D. Effect of the raw materials and mixing ratio of composted wastes on the dynamic of organic matter stabilization and nitrogen availability in composts of Sub-Saharan Africa. Bioresour Technol. 2010;101:1002–1013. [2] Aydn GA, Kocasoy G. Investigation of appropriate initial composition and aeration method for co-composting of yard waste and market wastes. J Environ Sci Health Part B. 2003;38:221–231. [3] Suzuki T, Ikumi Y, Okamoto S, Watanabe I, Fujitake N, Otsuka H. Aerobic composting of chips from clear-cut trees with various co-composting materials. Bioresour Technol. 2004;95:121–128. [4] Huang GF, Fang M, Wu QT, Zhou LX, Liao XD, Wong JWC. Co-composting of pig manure with leaves. Environ Technol. 2001;22:1203–1212. [5] Jolanun B, Towprayoon S. Novel bulking agent from clay residue for food waste composting. Bioresour Technol. 2010;101:4484–4490. [6] Kalemelawa F, Nishihara E, Endo T, Ahmad Z, Yeasmin R, Tenywa MM, Yamamoto S. An evaluation of aerobic and anaerobic composting of banana peels treated with different inoculums for soil nutrient replenishment. Bioresour Technol. 2012;126:375–382. [7] Wong JWC, Mak KF, Chan NW, Lam A, Fang M, Zhou LX, Wu QT, Liao XD. Co-composting of soybean residues and leaves in Hong Kong. Bioresour Technol. 2001;76: 99–106. [8] Lin C. A negative-pressure aeration system for composting food wastes. Bioresour Technol. 2008;99:7651–7656. [9] Li S, Huang GH, An CJ, Yu H. Effect of different buffer agents on in-vessel composting of food waste: performance analysis and comparative study. J Environ Sci Health Part A. 2013;48:772–780. [10] Pace MG, Miller BE, Farrell-Poe KL. The composting process, All archived publications (1995), Paper 48. Available from: http://digitalcommons.usu.edu/extension_histall/48
Downloaded by [New York University] at 20:49 17 June 2015
1456
S.Y. Choy et al.
[11] Specifications for composts & mulches, CUGE Standards, CS A02, Centre for Urban Greenery & Ecology, National Parks Board, Singapore, 2009. [12] Test Methods for the Examination of Composting and Compost (TMECC), The US. Composting Council, 2001. [13] Epstein E. The science of composting. Lancaster: Technomic Pub. Co.; 1997. [14] Zucconi F, Forte M, Monac A, Beritodi M. Biological evaluation of compost maturity. Biocycle. 1981;22: 27–29. [15] Adhikari BK, Barrington S, Martinez J, King S. Effectiveness of three bulking agents for food waste composting. Waste Manage. 2009;29:197–203. [16] Chiumenti A. Modern composting technologies. Emmaus: J G Press; 2005. [17] Zhou Y, Selvam A, Wong JWC. Evaluation of humic substances during co-composting of food waste, sawdust and Chinese medicinal herbal residues. Bioresour Technol. 2014;168:229–234. [18] Wong JWC, Fung SO, Selvam A. Coal fly ash and lime addition enhances the rate and efficiency of decomposition
[19]
[20] [21] [22] [23] [24]
of food waste during composting. Bioresour Technol. 2009;100:3324–3331. Wang X, Selvam A, Chan M, Wong JWC. Nitrogen conservation and acidity control during food wastes composting through struvite formation. Bioresour Technol. 2013;147:17–22. Tumuhairwe JB, Tenywa JS, Otabbong E, Ledin S. Comparison of four low-technology composting methods for market crop wastes. Waste Manage. 2009;29:2274–2281. Kumar M, Ou Y-L, Lin J-G. Co-composting of green waste and food waste at low C/N ratio. Waste Manage. 2010;30:602–609. Epstein E. Industrial composting: environmental engineering and facilities management. Boca Raton, FL: CRC Press; 2011. Sommer SG. Effect of composting on nutrient loss and nitrogen availability of cattle deep litter. Eur J Agron. 2001;14:123–133. Ivanov VN, Wang JY, Stabnikova OV, Tay STL, Tay JH. Microbiological monitoring in the biodegradation of sewage sludge and food waste. J Appl Microbiol. 2004;96:641–647.
