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Why use a thermophilic aerobic membrane reactor for the treatment of industrial wastewater/liquid waste? a
a
Maria Cristina Collivignarelli , Alessandro Abbà & Giorgio Bertanza
b
a
Department of Civil and Architectural Engineering, University of Pavia, via Ferrata 1, 27100 Pavia, Italy b
Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, via Branze 43, 25123 Brescia, Italy Accepted author version posted online: 23 Feb 2015.Published online: 23 Mar 2015.
Click for updates To cite this article: Maria Cristina Collivignarelli, Alessandro Abbà & Giorgio Bertanza (2015) Why use a thermophilic aerobic membrane reactor for the treatment of industrial wastewater/liquid waste?, Environmental Technology, 36:16, 2115-2124, DOI: 10.1080/09593330.2015.1021860 To link to this article: http://dx.doi.org/10.1080/09593330.2015.1021860
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Environmental Technology, 2015 Vol. 36, No. 16, 2115–2124, http://dx.doi.org/10.1080/09593330.2015.1021860
Why use a thermophilic aerobic membrane reactor for the treatment of industrial wastewater/liquid waste? Maria Cristina Collivignarellia , Alessandro Abbàa∗ and Giorgio Bertanzab a Department
of Civil and Architectural Engineering, University of Pavia, via Ferrata 1, 27100 Pavia, Italy; b Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, via Branze 43, 25123 Brescia, Italy
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(Received 28 November 2014; accepted 14 February 2015 ) This paper describes the advantages of thermophilic aerobic membrane reactor (TAMR) for the treatment of high strength wastewaters. The results were obtained from the monitoring of an industrial and a pilot scale plant. The average chemical oxygen demand (COD) removal yield was equal to 78% with an organic loading rate (OLR) up to 8–10 kgCOD m−3 d−1 despite significant scattering of the influent wastewater composition. Total phosphorus (TP) was removed with a rate of 90%, the most important removal mechanism being chemical precipitation (as hydroxyapatite, especially), which is improved by the continuous aeration that promotes phosphorus crystallization. Moreover, surfactants were removed with efficiency between 93% and 97%. Finally, the experimental work showed that thermophilic processes (TPPs) are complementary with respect to mesophilic treatments. Keywords: biological treatments; thermophilic aerobic membrane reactor (TAMR); ultrafiltration; phosphorus removal; surfactants removal
1. Introduction In the recent years, researchers have developed technological innovations in the treatment of industrial wastewaters that require intensive processes, due to their qualitative characteristics, in order to ensure the discharge in water bodies or their acceptability in municipal wastewater treatment plants (WWTPs).[1] As reported in Best Available Techniques Reference Document for wastewater treatment (e.g. in the Chemical Sector), the most appropriate process for inorganic/non-biodegradable/poorly degradable soluble content are based on physical and/or chemical operation such as: precipitation/sedimentation/filtration, chemical oxidation, membrane filtration, adsorption, ion exchange, extraction, distillation/rectification, evaporation, stripping and incineration. After adequate treatment, the wastewater can either be discharged into a receiving water body or into a subsequent municipal WWTP.[2] In order to obtain the performance required, a combination of different treatments is needed. However, the choice of more appropriate processes and their performance mainly depend on WWTP features, influent wastewater quality and receiving water body characteristics. Moreover, the chemical oxidation processes are more expensive than biological treatments.[3] Chemical oxidation is mainly used for the treatment of wastewaters containing soluble organic matter, nonbiodegradable and/or inhibitory/toxic substances (for the
*Corresponding author. Email: [email protected] © 2015 Taylor & Francis
biological process), hazardous organic and inorganic compounds, in order to decrease the hazard of these substrates. In chemical oxidation processes, there is a ‘change’ of pollutant substance characteristics both in chemical structure and chemical/biological reactivity: in particular, as concerns the organic matter, chemical oxidation ‘breaks’ long molecules into smaller intermediate compounds and increases the oxygen percentage in these compounds (e.g. in the form of alcohol groups, acids, etc.). So, chemical oxidation is suitable as a pre-treatment upstream a biological process, as it improves the biodegradability of wastewaters [3–7]; in effect, short chains of organic acids (deriving from the conversion of complex organic substances by means of chemical oxidation) are difficult to further degrade by chemical treatments, but they are easily biodegradable.[8–10] Biological treatments are really cheap and reliable; however, several substances are not treatable with this kind of processes. A combination of chemical and biological treatments would mean a cheaper option.[11] Among biological treatments, thermophilic processes (TPPs) are very interesting; they show some advantages compared with the traditional mesophilic treatments: • high removal rate of biodegradable substrates (up to 10 times higher than those measured in mesophilic conditions [12]), mainly due to fast hydrolysis of the
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M.C. Collivignarelli et al. organic substance (at thermophilic temperature), that allows the substrate solubilization and its availability for the oxidation; considerable reduction of retention times and, therefore, the decrease in volume and plant footprint; high OLR – Organic Loading Rate (between 5 and 30 kgCOD m−3 d−1 ), according to [13–15]; good removal yields for synthetic molecules, that are very difficult to treat in a conventional biological process [14]; low specific production of biomass, assessed between 0.05 and 0.3 kgVSS produced per kgCOD removed [16,17]; the authors, in a previous experimental work,[18] obtained a value of 0.02 kgVSS produced per kgCOD removed; inhibition of the pathogens due to high process temperature; this advantage is very important for the treatment of some kinds of liquid wastes [12,19,20]; ability to treat wastewaters with high concentrations of salts,[18] or with the presence of hazardous compounds [21] even succeeding to reduce the metals (particularly iron) [22]; reduction of gas flow and thus the stripping phenomenon decrease (that is relevant in the conventional processes) due to the use of pure oxygen; in this case, the concentrations of dissolved oxygen are higher than those achievable with air. The use of pure oxygen allows the development of high gradients of concentration that operate as ‘driving force’ for the oxygen transfer between the liquid medium and the inner parts of the sludge flocs (generally poor in oxygen); so, oxygen can penetrate into the mass of sludge flocs more efficiently than using air systems; process stability [12]; moreover a previous work [18] showed that a change in operative conditions involves a rapid performance reinstatement; reduction of gaseous/odorous emissions with an easier management, due to undersized aeration tank; heat recovery [12]; compatibility of the TPPs effluent with a biological mesophilic treatment [23].
However, the poor sludge settleability is the main problem regarding the TPPs [12,24–27]; in order to overcome this disadvantages, a proper solution is the implementation of a Membrane Bioreactor (MBR) system. In this case, the advantages are: • the feasibility to maintain (in the system) the biomass suitable for the purification process: MBR improves the microbial community (than using a traditional gravity settler) because the separation of biomass is independent by morphology and their ability to aggregation and sedimentation;
• the ability to retain, within the system, non (or slowly) biodegradable compounds (generally high molecular weight molecules), in order to ensure their biodegradation; • the improvement of stability in the effluent quality; • the reduction of the biological reactor volume. Although the TPPs present many advantages compared to the traditional mesophilic treatments, the full-scale applications of this technology are rare. A thermophilic aerobic membrane reactor (TAMR) at industrial scale was studied for about 10 years (approximately 5000 analysis for each parameter). Moreover, in order to study the potential uses of TPPs, several tests were carried out in different field applications. So, the experimental works with best results (that could be more useful for industrial applications) were examined in depth. In particular, the use of TPPs for phosphorus and surfactants removal were studied by means of a TAMR at pilot scale. Furthermore, the integration between mesophilic process and TPP was studied with the use of a conventional activated sludge (CAS) system at pilot scale, which treats the effluent of TPP (at industrial scale) coupled with a final clarifier. The study of different features of TPPs coupled with an ultrafiltration membrane was performed by the discussion of the results obtained from different experimental researches carried out in some years, both at pilot and industrial scale. The main disadvantages of TPPs were also investigated by means of laboratory equipment and pilot plants.
2. Materials and methods 2.1. Plants description 2.1.1. TAMR at industrial scale TAMR at industrial scale is located in a plant of Northern Italy, for the treatment of industrial liquid wastes containing readily biodegradable or biodegradable pollutants with organic loads too high for the treatment in municipal WWTPs. The core of the plant is the thermophilic MBR process with pure oxygen (Figure 1(a)). The biological reactor (volume of 1000 m3 ) is combined with an ultrafiltration system. Additional information are reported in [28]. 2.1.2. TAMR at pilot scale TAMR at pilot scale (similar to the plant at industrial scale) was designed and built with dimensions comparable to a semi-industrial unit. It consists of a biological reactor (1 m3 volume), thermally insulated and an ultrafiltration system (7 tubular membranes, with 23 channels, with cut-off equal to 300 kDa) (Figure 1(b)). More details are described in [18].
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(b)
(c)
Figure 1. TAMR at industrial scale (a) and at pilot scale (b); CAS at pilot scale (c).
2.1.3. CAS process at pilot scale The CAS system at pilot scale consists of two compartments (Figure 1(c)): • a biological oxidation reactor (with a capacity of 630 litres); • a final clarifier with a volume of 80 litres; • a substrate feed system, which consists of a tank (volume 1 m3 ) equipped with a piston pump (maximum flow rate: 10 L h−1 ); • a piston pump (maximum flow rate: 34 L h−1 ) for sludge recirculation (from final clarifier to biological tank); • an air supply system, which consists of an air blower and six spreading ceramic plates (diameter equal to 30 cm) at the bottom of biological reactor. 2.2. Liquid wastes characteristics The main characteristics of liquid waste treated in TAMR at industrial scale are reported in Table 1 (the values shown represent the 10th and 90th percentiles); the chemical
Table 1. Concentrations of pollutants for the liquid waste fed to TAMR (industrial scale). Parameter COD (mg L−1 ) −1 N − NH+ 4 (mg L ) −1 ) (mg L N − NO− x Cu (mg L−1 ) Cr (mg L−1 ) Ni (mg L−1 ) Zn (mg L−1 )
Value 18,000–60,000 100–400 600–1500 0.01–0.2 0.001–0.01 2–7 0.3–1.5
oxygen demand (COD) concentrations are between 18,000 and 60,000 mg L−1 . As concerns the experimental activities carried out on TAMR at pilot scale, two different types of liquid wastes were tested, in order to study different aspects (Table 2 reports the variation of pollutants concentration). Liquid waste 1 (L.W. 1) shows high concentration of total phosphorus (TP); moreover, the concentration of total nitrogen (TN) is significant. In liquid waste 2 (L.W. 2), the content of surfactants (anionic and non-ionic) is very high.
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Table 2. Concentrations of pollutants for the liquid wastes fed to TAMR (pilot scale). Value Parameter pH COD (mg L−1 ) TN (mg L−1 ) −1 N − NH+ 4 (mg L ) − N − NOx (mg L−1 ) TP (mg L−1 ) TAS (mg L−1 ) MBAS (mg L−1 )
L.W. 1 (phosphorus L.W. 2 (surfactants removal) removal) 7.5–8 8000–62,000 200–4150 30–810 2–42 329–2353 n.a. n.a.
5.5–8.3 4000–24,300 11–82 1.2–4.3 6.3–23 n.a. 120–2470 680–5520
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n.a. = not available.
2.3. Analytical methods During the experimental research, the physical–chemical parameters were monitored. COD, BOD5 , NH+ 4, N− − , N − NO , TN, anionic surfactants (methylene blue NO− 2 3 active substances – MBAS) and non-ionic surfactants (tetrakis active substances – TAS), total suspended solid (TSS) and volatile suspended solid (VSS) were measured according to the standard methods for water and wastewater.[29] Heavy metals and TP were analysed by an inductivity coupled plasma mass spectrometry (ICP-MS, PerkinElmer Optima 8300). The settling characteristics of biomass were evaluated by means of settling test with 2 L glass cylinder. The test consists of the measurement of TSS concentration in the supernatant after 5, 15 and 30 minutes. In order to reduce the influence of sludge concentration, before the test, the biomass was diluted with process effluent in order to obtain similar TSS concentrations. Ammonia Utilization Rate (AUR) respirometric tests were carried out in thermophilic conditions. These tests aimed to evaluate the nitrification activity in biomass. Thermophilic AUR tests were performed at 45–48 °C using thermophilic biomass (extracted from TAMR at industrial scale) added with a nutrient solution (casein peptone). Before the tests, biomass was washed in order to reduce the interfering substances. The washing procedure consists of two sequential phases of centrifugation (each step at 4500 rpm for 5 minutes) with the aim to remove the liquid fraction that contains the interfering substances; moreover, a solution of salts (5 g L−1 K2 HPO4 ; 3 g L−1 NaCl; 0.28 g L−1 KH2 PO4 ; 0.14 g L−1 (NH4 )2 SO4 ) was added in order to revitalize the biomass.
3. Results and discussion In the later text, the results of TAMR (at industrial scale) monitoring are reported in order to evaluate the COD removal efficiency. Moreover, the results obtained from
different experimental researches with TAMR at pilot scale aimed to study the phosphorus and surfactants removal are described. Furthermore, the combination of thermophilic and mesophilic treatments was analysed with the use of TAMR at industrial scale and CAS process.
3.1. Advantages 3.1.1. High COD removal Since 2006, TAMR at industrial scale (daily monitored) treated liquid waste with chemical characteristics according to Table 1. The average value of OLR was equal to 4.5 kgCOD m−3 d−1 ; the average COD output concentration was equal to 5000 mg L−1 with few peaks higher than 10,000 mg L−1 . In Figure 2, the OLR is compared with the Organic Removal Rate (ORR). The data obtained were close to the removal yield equal to 78%, regardless from the OLR values. The low scattering was influenced by the high variability of liquid wastes fed to TAMR (see Table 1). The other process conditions were kept as constant as possible: pH from 6.5 to 7 and temperature from 47°C to 49°C. Moreover, even with high values of OLR, the performance was very good. It can be noted that the variability of feeding, typical for a full-scale plant, has involved a modest influence on the removal yields of COD. Even with significant variations of COD load ingoing to the plant (coefficient of variation higher than 30%), average COD removal yield did not decrease below 74%. The plant performance were enhanced over the time due to the installation of a heat exchanger for the temperature control (between 45 and 48 °C). In fact, when the
Figure 2.
COD removal yields (TAMR at industrial scale).
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Table 3. Concentrations of TP (minimum, average and maximum values) input and output from TAMR and removal yield obtained (TAMR at pilot scale). Concentration of TP (mg L−1 ) IN Period
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Start-up Regime
HRT (d) 18 10
OLR (kgCOD
m−3
0.5–13 0.4–13
d−1 )
OUT
Min
Average
Max
Min
Average
Max
Removal yield (%)
1056 329
1624 1108
2303 2353
21 20
45 81
95 220
93–99 80–99
temperature is higher than 50°C, significant reduction in COD removal yields were observed.[19, 26] Moreover, the authors evaluated the feasibility of heat recovery from the process, especially in warm periods. The full-scale plant treats many kinds of liquid waste; so it is very difficult to correlate the performance with the feeding quality. Furthermore, the thermophilic biomass does not seem to be negatively influenced: the liquid wastes are mixed (the capacity of storage tanks is 30 m3 and 50 m3 ), there is a physical–chemical treatment before TAMR and hydraulic retention time (HRT) is equal to 5 days. The stability of the process was also confirmed by [18]. 3.1.2. High phosphorus removal The TPPs are very useful for the treatment of liquid waste containing specific pollutants. This has been studied by the monitoring of the industrial scale plant that normally treats liquid waste characterized by high concentrations of TP, surfactants, solvents, chlorides, etc. In order to quantify the ability to reduce these pollutants, specific experimental researches were carried out by using TAMR at pilot scale, because at industrial scale it is not easy to obtain accurate mass balance. As concerns the evaluation of TP removal, TAMR at pilot scale was monitored for six months. The removal of phosphorus has a double meaning: the first concerns the protection of water bodies quality (according to the European Water Framework Directive in force – [30] – the control of phosphorus discharged from WWTPs is a key factor in order to reduce the water body eutrophication); moreover, phosphorus is a depleting resource, so that in the last decade the attention has been focused on the issue of recovery.[31–33] The pilot plant was fed with liquid waste 1 (see Table 2). For the whole experimental research, the average value of OLR was equal to 2.6 kgCOD m−3 d−1 , with a minimum value of 0.4 kgCOD m−3 d−1 and a maximum equal to 13 kgCOD m−3 d−1 . Moreover, HRT in the start-up phase was 18 days and HRT in the other step is 10 days. According to this operative conditions, the average removal yields of COD were 90% in the start-up phase and 80% in the next phase. The comparison between the input and output TP concentrations is shown in Table 3. It can be noted that the TP
removal yields (with the exception of start-up phase) varied from 80% to 99%; the average value was equal to 90%. The good result is due to chemical precipitation: this phenomena could be due to the dosage of lime (that, in some case, has occurred in a chemical–physical treatment before the TPP), but the most important aspect is related to the aeration of reactor that promoted the phosphorus crystallization.[34] In effect, remarkable concentrations of Ca5 (PO4 )3 OH (hydroxyapatite) were detected in the sludge by means of X-ray diffraction analysis (data not shown).
3.1.3. High surfactants removal As is known, the surfactants can be classified in different groups depending on the electrostatic charge of its hydrophilic group: anionic (MBAS), non-ionic (TAS), cationic and amphoteric surfactants (the last compounds behave as acids or bases, depending on the solution acidity). The first two groups are the most common and account for over 80% of the total usage in detergents. Cationic surfactants are used mainly in fabric conditioners to give a pleasant soft feel to fabric. Amphoteric surfactants are used mainly for their skin mildness properties.[35] The European Detergent Regulation [36] (that entered in force on 2005) requires that the surfactants used in household detergents must be biodegradable, while a derogation may be requested for surfactants in detergents used in special industrial or institutional sectors. Likewise, the TP, a specific experimental research was carried out (about for one month) by using TAMR at pilot scale, in order to study the ability to reduce these compounds. During the experimentation, industrial liquid waste (L.W. 2) with high contents of COD, MBAS and TAS was tested (Table 2). In this case, the advantage of TPPs is the ability to treat surfactants with a low biodegradability; moreover, the process involves low formation of toxic metabolites. During the experimental research, the liquid waste was fed with an increase in the flow rate (and the decrease in HRT); the average input concentrations (calculated for each period of experimental research) varied, for TAS, from 267 to 2470 mg L−1 and, for MBAS, from 2270 to 5520 mg L−1 (Table 4). In summary, the thermophilic aerobic treatment has shown good performance even with very different OLR
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Table 4. Concentrations of surfactants (average values) input and output from TAMR and removal yield obtained (TAMR at pilot scale). COD
Period A
B
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Total
HRT (d)
OLR (kgCOD m−3 d−1 )
10 7 5 4 3 4 3
1.1 2.2 3.0 2.2 5.1 6.1 7.0
Removal yield (%) 61 80 80 78 91 90 89 84
TAS
MBAS
Average concentration (mg L−1 )
Average concentration (mg L−1 )
IN
OUT
387 278 267 332 1395 2470 2470
22 22 20 18 17 27 39
Removal yield (%) 94 92 92 94 99 99 98 98
IN
OUT
2270 2145 2347 2543 3676 5520 5520
19 38 62 76 81 114 134
Removal yield (%) 99 98 97 97 98 98 98 98
Figure 3. Concentrations of COD input and output CAS treatment (at pilot scale).
(period A); the average surfactant removal yields were included between 97% and 99% for MBAS; for TAS between 92% and 99%. In the final step of experimental research (period B), with a steady feeding load, the removal yields were equal to 98% for both anionic and non-ionic surfactants.
3.1.4.
Treatability of thermophilic effluent in mesophilic conditions Another interesting aspect of thermophilic treatment is the compatibility with a mesophilic biological process. This feature allows a more extensive use of this technology and, in particular, its application as ‘pre-treatment’ before a traditional biological process with a mesophilic biomass.
In this work, the results of an experimental research (for 1 year), with a sequence of thermophilic and mesophilic processes, are summarized. The mixed liquor derived from TAMR at industrial scale was submitted to sedimentation (in a clarifier) and the supernatant obtained was fed to CAS process at pilot scale. The operative conditions varied during the experimental activity: HRT between 1 and 4.5 days and Food/Microorganism (F/M) ratio between 0.15 and 0.5 kgCOD kgVSS −1 d−1 . As concerns the COD, a significant scattering of the removal yields was observed (Figure 3); considering the whole experimental period, the average value is equal to 30%. This performance could seem too low, but when the COD load increased, the removal yields were more stable and higher with respect to the other period: in fact,
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Figure 4. COD concentrations measured in influent and effluent of CAS (at pilot scale) after filtration.
considering the phases with a flow rate higher than 500 L d−1 , the COD removal yields have reached a mean value of 42%. The mesophilic process is able to remove the colloidal COD present in the effluent of TPP. This aspect was studied by means of COD measuring (in raw sample, after filtration at different pore size) both in the influent of CAS (at pilot scale) and in the effluent. The filtration tests were performed both on 2.5 μm and 0.45 μm. It can be noted that there is an important difference between the concentrations of COD measured in the different fractions of CAS influent (Figure 4): the effluent of TPP shows a considerable amount of colloidal material not settleable. After mesophilic treatment this difference is less noticeable. Therefore, in the biological reactor of CAS, an important reduction of colloidal material has occurred. The effluent of CAS shows a higher quality (compared to TPP) due to low turbidity. As regards BOD5 , the mesophilic process showed an average removal yield equalto 90% (Figure 5). In summary, a mesophilic treatment downstream a TPP is very useful in order to increase the overall organic matter removal and to reduce the colloidal COD that the TPP was not able to remove efficiently. The authors, in a previous work,[18] have also proven (with the analysis of Oxygen Uptake Rate results) that a pre-treatment with TAMR is very well ‘harmonized’ with a mesophilic process, because thermophilic effluent (from TAMR) is characterized by a percentage of organic matter,
Figure 5.
BOD5 removal yields (CAS at pilot scale).
which is not degradable by thermophilic biomass, but it is well biodegradable in a mesophilic treatment. 3.2. Disadvantages 3.2.1. Poor sludge settleability The main disadvantage of TPPs is related to the poor biomass settleability. Figure 6 shows the results of settling
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(a)
(b)
Figure 6. Settling tests: TSS/TSS0 (a) and COD/COD0 (b) in supernatant.
tests carried out both on mesophilic biomass (derived from CAS at pilot scale) and thermophilic biomass (taken from TAMR at industrial scale). In particular, Figure 6(a) shows the concentrations of TSS with respect to TSS0 (initial value) measured in the supernatant as a function of time. Mesophilic biomass shows a higher settling rate with respect to thermophilic one: already after 5 minutes, mesophilic biomass showed TSS removal equal to the value obtained at the end of the test. Moreover, the supernatants of thermophilic biomass showed high turbidity due to the presence of colloidal matter. This aspect led to worse quality (especially in terms of COD concentration) of thermophilic effluent compared to mesophilic one (Figure 6(b)). In order to reduce the poor sludge settleability of thermophilic biomass, a proper solution is the implementation of a filtration system equipment.
3.2.2.
Low nitrification activity
The results of several AUR tests carried out on thermophilic biomass derived from TAMR at industrial scale
showed nitrification rate very low: the value obtained var−1 −1 h . So, the ied from 0.002 to 0.015 mg N − NO− 3 gVSS nitrifying bacteria are not present in thermophilic biomass or, at least, they don’t act to achieve a nitrification process. The results obtained were confirmed by other works.[17,37,38] 4. Conclusions The results obtained in this work allow the following conclusions: • The TAMR allows to obtain COD removal yield of 78% with an OLR between 3 and 6 kgCOD m−3 d−1 ; no reduction in removal yields was observed also increasing OLR up to 8–10 kgCOD m−3 d−1 . Moreover, average removal yield of phosphorus was 90%; chemical precipitation is the predominant mechanism. • The disadvantages of TPPs (mainly concerning the poor sludge settleability) could be reduced both with the implementation of a MBR and with a subsequent conventional treatment in mesophilic conditions.
Environmental Technology • Thermophilic and mesophilic biomasses were complementary to each other in the overall removal of organic matter; the mesophilic treatment allows to remove the colloidal COD present after the TPP. Acknowledgements Authors wish to thank Idroclean S.p.A. (Casirate d’Adda – Bergamo, Italy) for giving technical support to the experimental research.
Funding
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Authors wish to thank Idroclean S.p.A. (Casirate d’Adda – Bergamo, Italy) for giving financial support to the experimental research.
Disclosure statement No potential conflict of interest was reported by the authors
ORCID G. Bertanza
http://orcid.org/0000-0002-2965-023X
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[10] de Morais JL, Zamora PP. Use of advanced oxidation processes to improve the biodegradability of mature landfill leachates. J Hazard Mater. 2005;123(1–3):181–186. [11] Marco A, Esplugas S, Saum G. How and why combine chemical and biological processes for wastewater treatment. Water Sci Technol. 1997;35(4):321–327. [12] Lapara TA, Alleman JE. Thermophilic aerobic biological wastewater treatment. Water Res. 1999;33(4):895–908. [13] Becker P, Koster D, Popov AN, Markossian S, Antranikian G, Markl H. The biodegradation of olive oil and the treatment of lipid-rich wool scouring wastewater under aerobic thermophilic conditions. Water Res. 1999;33:653–660. [14] LaPara TA, Konopka A, Nakatsu CH, Alleman JE. Thermophilic aerobic treatment of a synthetic wastewater in a membrane-coupled bioreactor. J Ind Microbiol Biotechnol. 2001;26:203–209. [15] Visvanathan C, Choudhary AK, Montalbo AT, Jegatheesan V. Landfill leachate treatment using thermophilic membrane bioreactor. Desalination. 2007;204(1–3):8–16. [16] Suvilampi J, Rintala J. Thermophilic aerobic wastewater treatment, process performance, biomass characteristics, and effluent quality. Rev Environ Sci Biotechnol. 2003;2:35–51. [17] Kurian R, Acharya C, Nakhla G, Bassi A. Conventional and thermophilic aerobic treatability of high strength oily pet food wastewater using membrane coupled bioreactors. Water Res. 2005;39:4299–4308. [18] Collivignarelli MC, Abbà A, Bertanza G. Treatment of high strength pharmaceutical wastewaters in a thermophilic aerobic membrane reactor (TAMR). Water Res. 2014;63:190– 198. [19] Juteau P. Review of the use of aerobic thermophilic bioprocesses for the treatment of swine waste. Livestock Sci. 2006;102:187–196. [20] Mohaibes M, Vuorinen H, Heinonen-Tanski H. Effect of temperature on microbial population and performance of an aerobic thermophilic reactor treating cattle slurry and waste food. Environ Technol. 2011;32(11):1223–1232. [21] Rozich AF, Colvin RJ. Design and operational considerations for thermophilic aerobic reactors treating high strength wastes and sludges. In: Proceeding of the 52nd Industrial Waste Conference, Purdue University, West Lafayette, Indiana; 1997 May 5–7; Chelsea, MI: Ann Arbor Press; 1998. [22] Slobodkin AI. Thermophilic microbial metal reduction. Microbiology. 2005;74(5):501–514. [Translated from Mikrobiologiya. 2005;74(5):581–595]. [23] Suvilampi J. Aerobic wastewater treatment under high and varying temperatures – thermophilic process performance and effluent quality. Jyvaskyla: Department of Biological and Environmental Science, University of Jyväskylä; 2003. [24] Çetin FD, Sürücü G. Effects of temperature and pH on the settleability of activated sludge flocs. Water Sci Technol. 1990;22(9):249–254. [25] Barr TA, Taylor JA, Duff SJB. Effect of HRT, SRT and temperature on the performance of activated sludge reactors treating bleached kraft mill effluent. Water Res. 1996;30(4):799–810. [26] Tripathi CS, Allen DG. Comparison of mesophilic and thermophilic aerobic biological treatment in sequencing batch reactors treating bleached kraft pulp mill effluent. Water Res. 1999;33(3):836–846. [27] Suvilampi J, Lehtomäki A, Rintala J. Biomass characterization of laboratory-scalethermophilic-mesophilic wastewater treatment processes. Environ Technol. 2006;27(1): 41–51.
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[28] Bertanza G, Collivignarelli MC, Crotti BM, Pedrazzani R. Integration between chemical oxidation and membrane thermophilic biological process. Water Sci Technol. 2010;61(1):227–234. [29] APHA, AWWA, WEF. Standard methods for the examination of water and wastewater. 22nd ed. Washington, DC: American Public Health Association; 2012. [30] European Commission. Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for community action in the field of water policy. Official Journal of the European Union; 2000. [31] de-Bashan LE, Bashan Y. Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997–2003). Water Res. 2004;38:4222–4246. [32] Shu L, Schneider P, Jegatheesan V, Johnson J. An economic evaluation of phosphorus recovery as struvite from digester supernatant. Bioresour Technol. 2006;97:2211–2216. [33] Vaccari DA, Strigul N. Extrapolating phosphorus production to estimate resource reserves. Chemosphere. 2011;84:792–797.
[34] Suzuki K, Tanaka Y, Osada T, Waki A. Removal of phosphate, magnesium and calcium from swine wastewater through crystallization enhanced by aeration. Water Res. 2002;36:2991–2998. [35] AISE. The new detergents regulation: fact sheet on aerobic biodegradation of surfactants. Available from: http://www.aise.eu/downloads/05_Fact%20sheet%20bio degradability-updated%2027082012.pdf [36] European Commission. Regulation (EC) No 648/2004 of the European Parliament and of the council of 31 March 2004 on detergents. Official Journal of the European Union; 2004. [37] Abeynayaka A, Visvanathan C. Mesophilic and thermophilic aerobic batch biodegradation, utilization of carbon and nitrogen sources in high-strength wastewater. Bioresour Technol. 2011;102(3):2358–2366. [38] Abeynayaka A, Visvanathan C. Performance comparison of mesophilic and thermophilic aerobic sidestream membrane bioreactors treating high strength wastewater. Bioresour Technol. 2011;102(9):5345–5352.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Why use a thermophilic aerobic membrane reactor for the treatment of industrial wastewater/liquid waste? a
a
Maria Cristina Collivignarelli , Alessandro Abbà & Giorgio Bertanza
b
a
Department of Civil and Architectural Engineering, University of Pavia, via Ferrata 1, 27100 Pavia, Italy b
Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, via Branze 43, 25123 Brescia, Italy Accepted author version posted online: 23 Feb 2015.Published online: 23 Mar 2015.
Click for updates To cite this article: Maria Cristina Collivignarelli, Alessandro Abbà & Giorgio Bertanza (2015) Why use a thermophilic aerobic membrane reactor for the treatment of industrial wastewater/liquid waste?, Environmental Technology, 36:16, 2115-2124, DOI: 10.1080/09593330.2015.1021860 To link to this article: http://dx.doi.org/10.1080/09593330.2015.1021860
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Environmental Technology, 2015 Vol. 36, No. 16, 2115–2124, http://dx.doi.org/10.1080/09593330.2015.1021860
Why use a thermophilic aerobic membrane reactor for the treatment of industrial wastewater/liquid waste? Maria Cristina Collivignarellia , Alessandro Abbàa∗ and Giorgio Bertanzab a Department
of Civil and Architectural Engineering, University of Pavia, via Ferrata 1, 27100 Pavia, Italy; b Department of Civil, Environmental, Architectural Engineering and Mathematics, University of Brescia, via Branze 43, 25123 Brescia, Italy
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(Received 28 November 2014; accepted 14 February 2015 ) This paper describes the advantages of thermophilic aerobic membrane reactor (TAMR) for the treatment of high strength wastewaters. The results were obtained from the monitoring of an industrial and a pilot scale plant. The average chemical oxygen demand (COD) removal yield was equal to 78% with an organic loading rate (OLR) up to 8–10 kgCOD m−3 d−1 despite significant scattering of the influent wastewater composition. Total phosphorus (TP) was removed with a rate of 90%, the most important removal mechanism being chemical precipitation (as hydroxyapatite, especially), which is improved by the continuous aeration that promotes phosphorus crystallization. Moreover, surfactants were removed with efficiency between 93% and 97%. Finally, the experimental work showed that thermophilic processes (TPPs) are complementary with respect to mesophilic treatments. Keywords: biological treatments; thermophilic aerobic membrane reactor (TAMR); ultrafiltration; phosphorus removal; surfactants removal
1. Introduction In the recent years, researchers have developed technological innovations in the treatment of industrial wastewaters that require intensive processes, due to their qualitative characteristics, in order to ensure the discharge in water bodies or their acceptability in municipal wastewater treatment plants (WWTPs).[1] As reported in Best Available Techniques Reference Document for wastewater treatment (e.g. in the Chemical Sector), the most appropriate process for inorganic/non-biodegradable/poorly degradable soluble content are based on physical and/or chemical operation such as: precipitation/sedimentation/filtration, chemical oxidation, membrane filtration, adsorption, ion exchange, extraction, distillation/rectification, evaporation, stripping and incineration. After adequate treatment, the wastewater can either be discharged into a receiving water body or into a subsequent municipal WWTP.[2] In order to obtain the performance required, a combination of different treatments is needed. However, the choice of more appropriate processes and their performance mainly depend on WWTP features, influent wastewater quality and receiving water body characteristics. Moreover, the chemical oxidation processes are more expensive than biological treatments.[3] Chemical oxidation is mainly used for the treatment of wastewaters containing soluble organic matter, nonbiodegradable and/or inhibitory/toxic substances (for the
*Corresponding author. Email: [email protected] © 2015 Taylor & Francis
biological process), hazardous organic and inorganic compounds, in order to decrease the hazard of these substrates. In chemical oxidation processes, there is a ‘change’ of pollutant substance characteristics both in chemical structure and chemical/biological reactivity: in particular, as concerns the organic matter, chemical oxidation ‘breaks’ long molecules into smaller intermediate compounds and increases the oxygen percentage in these compounds (e.g. in the form of alcohol groups, acids, etc.). So, chemical oxidation is suitable as a pre-treatment upstream a biological process, as it improves the biodegradability of wastewaters [3–7]; in effect, short chains of organic acids (deriving from the conversion of complex organic substances by means of chemical oxidation) are difficult to further degrade by chemical treatments, but they are easily biodegradable.[8–10] Biological treatments are really cheap and reliable; however, several substances are not treatable with this kind of processes. A combination of chemical and biological treatments would mean a cheaper option.[11] Among biological treatments, thermophilic processes (TPPs) are very interesting; they show some advantages compared with the traditional mesophilic treatments: • high removal rate of biodegradable substrates (up to 10 times higher than those measured in mesophilic conditions [12]), mainly due to fast hydrolysis of the
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M.C. Collivignarelli et al. organic substance (at thermophilic temperature), that allows the substrate solubilization and its availability for the oxidation; considerable reduction of retention times and, therefore, the decrease in volume and plant footprint; high OLR – Organic Loading Rate (between 5 and 30 kgCOD m−3 d−1 ), according to [13–15]; good removal yields for synthetic molecules, that are very difficult to treat in a conventional biological process [14]; low specific production of biomass, assessed between 0.05 and 0.3 kgVSS produced per kgCOD removed [16,17]; the authors, in a previous experimental work,[18] obtained a value of 0.02 kgVSS produced per kgCOD removed; inhibition of the pathogens due to high process temperature; this advantage is very important for the treatment of some kinds of liquid wastes [12,19,20]; ability to treat wastewaters with high concentrations of salts,[18] or with the presence of hazardous compounds [21] even succeeding to reduce the metals (particularly iron) [22]; reduction of gas flow and thus the stripping phenomenon decrease (that is relevant in the conventional processes) due to the use of pure oxygen; in this case, the concentrations of dissolved oxygen are higher than those achievable with air. The use of pure oxygen allows the development of high gradients of concentration that operate as ‘driving force’ for the oxygen transfer between the liquid medium and the inner parts of the sludge flocs (generally poor in oxygen); so, oxygen can penetrate into the mass of sludge flocs more efficiently than using air systems; process stability [12]; moreover a previous work [18] showed that a change in operative conditions involves a rapid performance reinstatement; reduction of gaseous/odorous emissions with an easier management, due to undersized aeration tank; heat recovery [12]; compatibility of the TPPs effluent with a biological mesophilic treatment [23].
However, the poor sludge settleability is the main problem regarding the TPPs [12,24–27]; in order to overcome this disadvantages, a proper solution is the implementation of a Membrane Bioreactor (MBR) system. In this case, the advantages are: • the feasibility to maintain (in the system) the biomass suitable for the purification process: MBR improves the microbial community (than using a traditional gravity settler) because the separation of biomass is independent by morphology and their ability to aggregation and sedimentation;
• the ability to retain, within the system, non (or slowly) biodegradable compounds (generally high molecular weight molecules), in order to ensure their biodegradation; • the improvement of stability in the effluent quality; • the reduction of the biological reactor volume. Although the TPPs present many advantages compared to the traditional mesophilic treatments, the full-scale applications of this technology are rare. A thermophilic aerobic membrane reactor (TAMR) at industrial scale was studied for about 10 years (approximately 5000 analysis for each parameter). Moreover, in order to study the potential uses of TPPs, several tests were carried out in different field applications. So, the experimental works with best results (that could be more useful for industrial applications) were examined in depth. In particular, the use of TPPs for phosphorus and surfactants removal were studied by means of a TAMR at pilot scale. Furthermore, the integration between mesophilic process and TPP was studied with the use of a conventional activated sludge (CAS) system at pilot scale, which treats the effluent of TPP (at industrial scale) coupled with a final clarifier. The study of different features of TPPs coupled with an ultrafiltration membrane was performed by the discussion of the results obtained from different experimental researches carried out in some years, both at pilot and industrial scale. The main disadvantages of TPPs were also investigated by means of laboratory equipment and pilot plants.
2. Materials and methods 2.1. Plants description 2.1.1. TAMR at industrial scale TAMR at industrial scale is located in a plant of Northern Italy, for the treatment of industrial liquid wastes containing readily biodegradable or biodegradable pollutants with organic loads too high for the treatment in municipal WWTPs. The core of the plant is the thermophilic MBR process with pure oxygen (Figure 1(a)). The biological reactor (volume of 1000 m3 ) is combined with an ultrafiltration system. Additional information are reported in [28]. 2.1.2. TAMR at pilot scale TAMR at pilot scale (similar to the plant at industrial scale) was designed and built with dimensions comparable to a semi-industrial unit. It consists of a biological reactor (1 m3 volume), thermally insulated and an ultrafiltration system (7 tubular membranes, with 23 channels, with cut-off equal to 300 kDa) (Figure 1(b)). More details are described in [18].
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(a)
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(b)
(c)
Figure 1. TAMR at industrial scale (a) and at pilot scale (b); CAS at pilot scale (c).
2.1.3. CAS process at pilot scale The CAS system at pilot scale consists of two compartments (Figure 1(c)): • a biological oxidation reactor (with a capacity of 630 litres); • a final clarifier with a volume of 80 litres; • a substrate feed system, which consists of a tank (volume 1 m3 ) equipped with a piston pump (maximum flow rate: 10 L h−1 ); • a piston pump (maximum flow rate: 34 L h−1 ) for sludge recirculation (from final clarifier to biological tank); • an air supply system, which consists of an air blower and six spreading ceramic plates (diameter equal to 30 cm) at the bottom of biological reactor. 2.2. Liquid wastes characteristics The main characteristics of liquid waste treated in TAMR at industrial scale are reported in Table 1 (the values shown represent the 10th and 90th percentiles); the chemical
Table 1. Concentrations of pollutants for the liquid waste fed to TAMR (industrial scale). Parameter COD (mg L−1 ) −1 N − NH+ 4 (mg L ) −1 ) (mg L N − NO− x Cu (mg L−1 ) Cr (mg L−1 ) Ni (mg L−1 ) Zn (mg L−1 )
Value 18,000–60,000 100–400 600–1500 0.01–0.2 0.001–0.01 2–7 0.3–1.5
oxygen demand (COD) concentrations are between 18,000 and 60,000 mg L−1 . As concerns the experimental activities carried out on TAMR at pilot scale, two different types of liquid wastes were tested, in order to study different aspects (Table 2 reports the variation of pollutants concentration). Liquid waste 1 (L.W. 1) shows high concentration of total phosphorus (TP); moreover, the concentration of total nitrogen (TN) is significant. In liquid waste 2 (L.W. 2), the content of surfactants (anionic and non-ionic) is very high.
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Table 2. Concentrations of pollutants for the liquid wastes fed to TAMR (pilot scale). Value Parameter pH COD (mg L−1 ) TN (mg L−1 ) −1 N − NH+ 4 (mg L ) − N − NOx (mg L−1 ) TP (mg L−1 ) TAS (mg L−1 ) MBAS (mg L−1 )
L.W. 1 (phosphorus L.W. 2 (surfactants removal) removal) 7.5–8 8000–62,000 200–4150 30–810 2–42 329–2353 n.a. n.a.
5.5–8.3 4000–24,300 11–82 1.2–4.3 6.3–23 n.a. 120–2470 680–5520
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n.a. = not available.
2.3. Analytical methods During the experimental research, the physical–chemical parameters were monitored. COD, BOD5 , NH+ 4, N− − , N − NO , TN, anionic surfactants (methylene blue NO− 2 3 active substances – MBAS) and non-ionic surfactants (tetrakis active substances – TAS), total suspended solid (TSS) and volatile suspended solid (VSS) were measured according to the standard methods for water and wastewater.[29] Heavy metals and TP were analysed by an inductivity coupled plasma mass spectrometry (ICP-MS, PerkinElmer Optima 8300). The settling characteristics of biomass were evaluated by means of settling test with 2 L glass cylinder. The test consists of the measurement of TSS concentration in the supernatant after 5, 15 and 30 minutes. In order to reduce the influence of sludge concentration, before the test, the biomass was diluted with process effluent in order to obtain similar TSS concentrations. Ammonia Utilization Rate (AUR) respirometric tests were carried out in thermophilic conditions. These tests aimed to evaluate the nitrification activity in biomass. Thermophilic AUR tests were performed at 45–48 °C using thermophilic biomass (extracted from TAMR at industrial scale) added with a nutrient solution (casein peptone). Before the tests, biomass was washed in order to reduce the interfering substances. The washing procedure consists of two sequential phases of centrifugation (each step at 4500 rpm for 5 minutes) with the aim to remove the liquid fraction that contains the interfering substances; moreover, a solution of salts (5 g L−1 K2 HPO4 ; 3 g L−1 NaCl; 0.28 g L−1 KH2 PO4 ; 0.14 g L−1 (NH4 )2 SO4 ) was added in order to revitalize the biomass.
3. Results and discussion In the later text, the results of TAMR (at industrial scale) monitoring are reported in order to evaluate the COD removal efficiency. Moreover, the results obtained from
different experimental researches with TAMR at pilot scale aimed to study the phosphorus and surfactants removal are described. Furthermore, the combination of thermophilic and mesophilic treatments was analysed with the use of TAMR at industrial scale and CAS process.
3.1. Advantages 3.1.1. High COD removal Since 2006, TAMR at industrial scale (daily monitored) treated liquid waste with chemical characteristics according to Table 1. The average value of OLR was equal to 4.5 kgCOD m−3 d−1 ; the average COD output concentration was equal to 5000 mg L−1 with few peaks higher than 10,000 mg L−1 . In Figure 2, the OLR is compared with the Organic Removal Rate (ORR). The data obtained were close to the removal yield equal to 78%, regardless from the OLR values. The low scattering was influenced by the high variability of liquid wastes fed to TAMR (see Table 1). The other process conditions were kept as constant as possible: pH from 6.5 to 7 and temperature from 47°C to 49°C. Moreover, even with high values of OLR, the performance was very good. It can be noted that the variability of feeding, typical for a full-scale plant, has involved a modest influence on the removal yields of COD. Even with significant variations of COD load ingoing to the plant (coefficient of variation higher than 30%), average COD removal yield did not decrease below 74%. The plant performance were enhanced over the time due to the installation of a heat exchanger for the temperature control (between 45 and 48 °C). In fact, when the
Figure 2.
COD removal yields (TAMR at industrial scale).
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Table 3. Concentrations of TP (minimum, average and maximum values) input and output from TAMR and removal yield obtained (TAMR at pilot scale). Concentration of TP (mg L−1 ) IN Period
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Start-up Regime
HRT (d) 18 10
OLR (kgCOD
m−3
0.5–13 0.4–13
d−1 )
OUT
Min
Average
Max
Min
Average
Max
Removal yield (%)
1056 329
1624 1108
2303 2353
21 20
45 81
95 220
93–99 80–99
temperature is higher than 50°C, significant reduction in COD removal yields were observed.[19, 26] Moreover, the authors evaluated the feasibility of heat recovery from the process, especially in warm periods. The full-scale plant treats many kinds of liquid waste; so it is very difficult to correlate the performance with the feeding quality. Furthermore, the thermophilic biomass does not seem to be negatively influenced: the liquid wastes are mixed (the capacity of storage tanks is 30 m3 and 50 m3 ), there is a physical–chemical treatment before TAMR and hydraulic retention time (HRT) is equal to 5 days. The stability of the process was also confirmed by [18]. 3.1.2. High phosphorus removal The TPPs are very useful for the treatment of liquid waste containing specific pollutants. This has been studied by the monitoring of the industrial scale plant that normally treats liquid waste characterized by high concentrations of TP, surfactants, solvents, chlorides, etc. In order to quantify the ability to reduce these pollutants, specific experimental researches were carried out by using TAMR at pilot scale, because at industrial scale it is not easy to obtain accurate mass balance. As concerns the evaluation of TP removal, TAMR at pilot scale was monitored for six months. The removal of phosphorus has a double meaning: the first concerns the protection of water bodies quality (according to the European Water Framework Directive in force – [30] – the control of phosphorus discharged from WWTPs is a key factor in order to reduce the water body eutrophication); moreover, phosphorus is a depleting resource, so that in the last decade the attention has been focused on the issue of recovery.[31–33] The pilot plant was fed with liquid waste 1 (see Table 2). For the whole experimental research, the average value of OLR was equal to 2.6 kgCOD m−3 d−1 , with a minimum value of 0.4 kgCOD m−3 d−1 and a maximum equal to 13 kgCOD m−3 d−1 . Moreover, HRT in the start-up phase was 18 days and HRT in the other step is 10 days. According to this operative conditions, the average removal yields of COD were 90% in the start-up phase and 80% in the next phase. The comparison between the input and output TP concentrations is shown in Table 3. It can be noted that the TP
removal yields (with the exception of start-up phase) varied from 80% to 99%; the average value was equal to 90%. The good result is due to chemical precipitation: this phenomena could be due to the dosage of lime (that, in some case, has occurred in a chemical–physical treatment before the TPP), but the most important aspect is related to the aeration of reactor that promoted the phosphorus crystallization.[34] In effect, remarkable concentrations of Ca5 (PO4 )3 OH (hydroxyapatite) were detected in the sludge by means of X-ray diffraction analysis (data not shown).
3.1.3. High surfactants removal As is known, the surfactants can be classified in different groups depending on the electrostatic charge of its hydrophilic group: anionic (MBAS), non-ionic (TAS), cationic and amphoteric surfactants (the last compounds behave as acids or bases, depending on the solution acidity). The first two groups are the most common and account for over 80% of the total usage in detergents. Cationic surfactants are used mainly in fabric conditioners to give a pleasant soft feel to fabric. Amphoteric surfactants are used mainly for their skin mildness properties.[35] The European Detergent Regulation [36] (that entered in force on 2005) requires that the surfactants used in household detergents must be biodegradable, while a derogation may be requested for surfactants in detergents used in special industrial or institutional sectors. Likewise, the TP, a specific experimental research was carried out (about for one month) by using TAMR at pilot scale, in order to study the ability to reduce these compounds. During the experimentation, industrial liquid waste (L.W. 2) with high contents of COD, MBAS and TAS was tested (Table 2). In this case, the advantage of TPPs is the ability to treat surfactants with a low biodegradability; moreover, the process involves low formation of toxic metabolites. During the experimental research, the liquid waste was fed with an increase in the flow rate (and the decrease in HRT); the average input concentrations (calculated for each period of experimental research) varied, for TAS, from 267 to 2470 mg L−1 and, for MBAS, from 2270 to 5520 mg L−1 (Table 4). In summary, the thermophilic aerobic treatment has shown good performance even with very different OLR
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Table 4. Concentrations of surfactants (average values) input and output from TAMR and removal yield obtained (TAMR at pilot scale). COD
Period A
B
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Total
HRT (d)
OLR (kgCOD m−3 d−1 )
10 7 5 4 3 4 3
1.1 2.2 3.0 2.2 5.1 6.1 7.0
Removal yield (%) 61 80 80 78 91 90 89 84
TAS
MBAS
Average concentration (mg L−1 )
Average concentration (mg L−1 )
IN
OUT
387 278 267 332 1395 2470 2470
22 22 20 18 17 27 39
Removal yield (%) 94 92 92 94 99 99 98 98
IN
OUT
2270 2145 2347 2543 3676 5520 5520
19 38 62 76 81 114 134
Removal yield (%) 99 98 97 97 98 98 98 98
Figure 3. Concentrations of COD input and output CAS treatment (at pilot scale).
(period A); the average surfactant removal yields were included between 97% and 99% for MBAS; for TAS between 92% and 99%. In the final step of experimental research (period B), with a steady feeding load, the removal yields were equal to 98% for both anionic and non-ionic surfactants.
3.1.4.
Treatability of thermophilic effluent in mesophilic conditions Another interesting aspect of thermophilic treatment is the compatibility with a mesophilic biological process. This feature allows a more extensive use of this technology and, in particular, its application as ‘pre-treatment’ before a traditional biological process with a mesophilic biomass.
In this work, the results of an experimental research (for 1 year), with a sequence of thermophilic and mesophilic processes, are summarized. The mixed liquor derived from TAMR at industrial scale was submitted to sedimentation (in a clarifier) and the supernatant obtained was fed to CAS process at pilot scale. The operative conditions varied during the experimental activity: HRT between 1 and 4.5 days and Food/Microorganism (F/M) ratio between 0.15 and 0.5 kgCOD kgVSS −1 d−1 . As concerns the COD, a significant scattering of the removal yields was observed (Figure 3); considering the whole experimental period, the average value is equal to 30%. This performance could seem too low, but when the COD load increased, the removal yields were more stable and higher with respect to the other period: in fact,
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Figure 4. COD concentrations measured in influent and effluent of CAS (at pilot scale) after filtration.
considering the phases with a flow rate higher than 500 L d−1 , the COD removal yields have reached a mean value of 42%. The mesophilic process is able to remove the colloidal COD present in the effluent of TPP. This aspect was studied by means of COD measuring (in raw sample, after filtration at different pore size) both in the influent of CAS (at pilot scale) and in the effluent. The filtration tests were performed both on 2.5 μm and 0.45 μm. It can be noted that there is an important difference between the concentrations of COD measured in the different fractions of CAS influent (Figure 4): the effluent of TPP shows a considerable amount of colloidal material not settleable. After mesophilic treatment this difference is less noticeable. Therefore, in the biological reactor of CAS, an important reduction of colloidal material has occurred. The effluent of CAS shows a higher quality (compared to TPP) due to low turbidity. As regards BOD5 , the mesophilic process showed an average removal yield equalto 90% (Figure 5). In summary, a mesophilic treatment downstream a TPP is very useful in order to increase the overall organic matter removal and to reduce the colloidal COD that the TPP was not able to remove efficiently. The authors, in a previous work,[18] have also proven (with the analysis of Oxygen Uptake Rate results) that a pre-treatment with TAMR is very well ‘harmonized’ with a mesophilic process, because thermophilic effluent (from TAMR) is characterized by a percentage of organic matter,
Figure 5.
BOD5 removal yields (CAS at pilot scale).
which is not degradable by thermophilic biomass, but it is well biodegradable in a mesophilic treatment. 3.2. Disadvantages 3.2.1. Poor sludge settleability The main disadvantage of TPPs is related to the poor biomass settleability. Figure 6 shows the results of settling
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(a)
(b)
Figure 6. Settling tests: TSS/TSS0 (a) and COD/COD0 (b) in supernatant.
tests carried out both on mesophilic biomass (derived from CAS at pilot scale) and thermophilic biomass (taken from TAMR at industrial scale). In particular, Figure 6(a) shows the concentrations of TSS with respect to TSS0 (initial value) measured in the supernatant as a function of time. Mesophilic biomass shows a higher settling rate with respect to thermophilic one: already after 5 minutes, mesophilic biomass showed TSS removal equal to the value obtained at the end of the test. Moreover, the supernatants of thermophilic biomass showed high turbidity due to the presence of colloidal matter. This aspect led to worse quality (especially in terms of COD concentration) of thermophilic effluent compared to mesophilic one (Figure 6(b)). In order to reduce the poor sludge settleability of thermophilic biomass, a proper solution is the implementation of a filtration system equipment.
3.2.2.
Low nitrification activity
The results of several AUR tests carried out on thermophilic biomass derived from TAMR at industrial scale
showed nitrification rate very low: the value obtained var−1 −1 h . So, the ied from 0.002 to 0.015 mg N − NO− 3 gVSS nitrifying bacteria are not present in thermophilic biomass or, at least, they don’t act to achieve a nitrification process. The results obtained were confirmed by other works.[17,37,38] 4. Conclusions The results obtained in this work allow the following conclusions: • The TAMR allows to obtain COD removal yield of 78% with an OLR between 3 and 6 kgCOD m−3 d−1 ; no reduction in removal yields was observed also increasing OLR up to 8–10 kgCOD m−3 d−1 . Moreover, average removal yield of phosphorus was 90%; chemical precipitation is the predominant mechanism. • The disadvantages of TPPs (mainly concerning the poor sludge settleability) could be reduced both with the implementation of a MBR and with a subsequent conventional treatment in mesophilic conditions.
Environmental Technology • Thermophilic and mesophilic biomasses were complementary to each other in the overall removal of organic matter; the mesophilic treatment allows to remove the colloidal COD present after the TPP. Acknowledgements Authors wish to thank Idroclean S.p.A. (Casirate d’Adda – Bergamo, Italy) for giving technical support to the experimental research.
Funding
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Authors wish to thank Idroclean S.p.A. (Casirate d’Adda – Bergamo, Italy) for giving financial support to the experimental research.
Disclosure statement No potential conflict of interest was reported by the authors
ORCID G. Bertanza
http://orcid.org/0000-0002-2965-023X
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