Journal of Hazardous Materials 283 (2015) 608–616
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Preparation of ceramic filler from reusing sewage sludge and application in biological aerated filter for soy protein secondary wastewater treatment Suqing Wu a,b,1 , Yuanfeng Qi a,b,1 , Qinyan Yue b,∗ , Baoyu Gao b , Yue Gao c , Chunzhen Fan a , Shengbing He a a
School of Environmental Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China c School of Environment, Tsinghua University, Beijing 100084, PR China b
h i g h l i g h t s • • • • •
Sewage sludge and clay utilized as raw materials for preparing ceramic filler. The optimum addition of DSS for preparing the filler was about 25.0 wt%. The filler employed in BAF system for soy protein secondary wastewater treatment. 91.02% of CODcr and 90.48% of NH4 + -N was removed by the BAF system. The wastewater treatment effect was helpful for application on practical project.
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 23 September 2014 Accepted 13 October 2014 Available online 23 October 2014 Keywords: Ceramic filler Sewage sludge Biological aerated filter Soy protein processing wastewater Secondary treatment
a b s t r a c t Dehydrated sewage sludge (DSS) and clay used as raw materials for preparation of novel media-sludge ceramic filler (SCF) and SCF employed in a lab-scale up-flow biological aerated filter (BAF) were investigated for soy protein secondary wastewater treatment. Single factor experiments were designed to investigate the preparation of SCF, and the characteristics (microstructure properties, toxic metal leaching property and other physical properties) of SCF prepared under the optimum conditions were examined. The influences of media height, hydraulic retention time (HRT) and air–liquid ratio (A/L) on chemical oxygen demand (CODcr) and ammonia nitrogen (NH4 + -N) removal rate were studied. The results showed that the optimum addition of DSS was approximately 25.0 wt% according to the physical properties of SCF (expansion ratio of 53.0%, v/v, water absorption of 8.24 wt%, bulk density of 350.4 kg m−3 and grain density of 931.5 kg m−3 ), and the optimum conditions of BAF system were media height of 75.0 cm, HRT of 10.0 h and A/L of 15:1 in terms of CODcr and NH4 + -N removal rate (91.02% and 90.48%, respectively). Additionally, CODcr and NH4 + -N (81.6 and 15.3 mg L−1 , respectively) in the final effluent of BAF system met the national standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 , GB 18918-2002, secondary standard). © 2014 Elsevier B.V. All rights reserved.
1. Introduction As a by-product from wastewater treatment process, sewage sludge is mainly generated during the primary (physical and/or chemical), secondary (biological) and tertiary treatments [1].
∗ Corresponding author. Tel.: +86 531 88365258; fax: +86 531 88364513. E-mail addresses: [email protected], [email protected] (Q. Yue). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jhazmat.2014.10.013 0304-3894/© 2014 Elsevier B.V. All rights reserved.
However, sewage sludge may contain pathogenic organisms and pollutants, various toxic metals and large amounts of soluble salts, which are harmful to human health and environment [2]. Moreover, a great quantity of sludge is produced, which poses a notable hazard. Hence, the treatment and disposal of sludge from wastewater treatment has been one of the most critical environmental issues [3]. Conventional sewage sludge treatments mainly involve incineration, landfilling, composting, land application, etc. [4]. In recent years, some sustainable methods have been utilized to handle sewage sludge, converting it into useful materials (e.g. preparation
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Nomenclature DSS SCF A/L SRT SS A/O BAF HRT COD NH4 + -N BOD DO
dehydrated sewage sludge sludge ceramic filler air–liquid ratio sludge retention time (h) suspended solid (mg L−1 ) anoxic/oxic process biological aerated filter hydraulic retention time (h) chemical oxygen demand (mg L−1 ) ammonia nitrogen (mg L−1 ) biological oxygen demand (mg L−1 ) dissolved oxygen (mg L−1 )
of adsorbents) [5]. Therefore, eco-friendly and economical methods for safe handling, disposal and recycling of sewage sludge are of great significance. As a kind of lightweight aggregate, ceramics have been widely used in construction industry (e.g. as cement mortars, concrete mixtures, bricks, fine aggregate for mortars and ceramic materials) [6,7], and also in other fields such as lightweight geotechnical fill, insulation products, soil engineering, hydro-culture, drainage, roof gardens and filters [8]. In recent years, many researchers have paid attention to ceramic sintering from sewage sludge, in order to decrease consumption of natural resources and immobilize hazardous waste [9]. Moreover, the research can meet the needs of sustainable development. Biological aerated filters (BAF) were developed on the basis of biological filters in Europe in the late 1980s and then widely applied all over the world as novel, flexible and effective bioreactors [10]. BAF was originally used in tertiary or advanced wastewater treatment, and subsequently applied in secondary wastewater treatment as this technology became mature. BAF presents several advantages over other fixed-film reactors (e.g. rotating biological contactors or trickling filters), including high loading rates [11,12], high active biomass concentration and sludge retention time (SRT), good pollutant efficiency (carbon, nitrogen, phosphorus and pathogen) removal combined with a high filtering capacity [13–15]. Furthermore, the space required for BAF construction is quite small compared to other aerobic process [16,17]. Therefore, BAF is an effective and economical method for municipal and industrial wastewater treatment. Soy protein, as a major foodstuff additive, is produced from soybean. However, a large quantity of industrial water is consumed and a great deal of wastewater is generated during soy protein manufacturing process. The wastewater usually contains organic substances (e.g. protein, sugar) and inorganic substances (e.g. salt, Cl− , and SO4 2− ) [18], characterized by high COD, BOD [19,20] and SS due to the high protein content, which inevitably causes serious environment pollution. Currently, the studies on treating soy protein processing wastewater are mainly focused on biological technologies. Zhu et al. utilized anaerobic baffled reactor to treat soybean protein processing wastewater (COD of 10,000 mg L−1 ), and 92–97% COD was removed at HRT of 39.5 h and temperature of 35.0 ◦ C. Cassini et al. used ultrafiltration membranes to treat wastewater from isolated soy protein production (COD of 16,000 mg L−1 ), and COD removal rate could reach approximately 34%. Li et al. applied conventional biofilters in soybean protein wastewater tertiary treatment (COD of 120–150 mg L−1 ), and final effluent COD was less than 60 mg L−1 [18,21–24]. However, with single treatment, soy protein processing wastewater could not meet the discharge standards. Therefore, the primary, secondary and tertiary treatments are all crucial to the
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wastewater treatment. It is necessary to develop effective, economical and integrated methods for soy protein wastewater treatment. In our previous studies, ceramic fillers prepared from different constituents were investigated for different wastewater treatment processes. Ceramic-corrosion-cell fillers prepared from clay, dried sewage sludge and scrap iron (sintered at 400 ◦ C) were applied in micro-electrolysis reactor for cyclohexanone industry wastewater pretreatment, where COD and cyclohexanone content were about 64,000 mg L−1 and 4.5 wt%, respectively, and approximately 90% of COD and cyclohexanone were removed. Ultra-lightweight sludge ceramics (ULSC) prepared from clay and dried sewage sludge (preheated at 400 ◦ C and sintered at 1150 ◦ C) were employed in a lab-scale up-flow BAF for pharmaceutical advanced wastewater treatment (influent CODcr and NH4 + -N were 198–260 and 34.8–39.7 mg L−1 , respectively). The mean effluent NH4 + -N was 6.2 mg L−1 and the maximum CODcr in the effluent was 96 mg L−1 . In this research, there were three objectives as follows: (1) DSS and clay were used as raw materials for SCF preparation, and the optimum DSS addition was determined by the properties of SCF. (2) The possibility of SCF used as fillers in a lab-scale up-flow BAF was investigated for soy protein processing wastewater secondary treatment, whereby the wastewater treatment process could be simplified and the wastewater treatment cost could be reduced. (3) The optimum conditions (including media height, HRT and A/L) for the wastewater treatment were determined by the removal efficiency of contaminants and the limits as specified by the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + N ≤ 25.0 mg L−1 ,GB 18918-2002, secondary standard) [25]. 2. Materials and methods 2.1. Preparation of SCF 2.1.1. Pretreatment of raw materials DSS and clay were obtained from local municipal wastewater treatment plant (Jinan City, Shandong Province, China) and mountainous area (in Zibo City, Shandong Province, China) respectively. Firstly, the raw materials (DSS and clay) were dried in stove at 105 ◦ C for 4.0 h. Secondly, they were crushed in a ball mill and sieved (the diameter of the sieve mesh was 0.154 mm). Finally, dry powder from raw materials was obtained and preserved in polyethylene vessels to avoid humidity before it was used. The wastewater treatment system utilized in the wastewater treatment plant mainly contained flocculation, oxidation ditch and anaerobic–anoxic–oxic process (A2 O). The sewage sludge was generated during these processes featuring 85.7 wt% of moisture content, 36.5 wt% of VSS/SS, 56.8 wt% of ignition loss and 13.2 MJ kg−1 of calorific value (in dry basis). Besides, the chemical components of clay and DSS are shown in Table 1. 2.1.2. Preparation and sintering of SCF Five mass ratios of clay to DSS (9.5:0.5, 8.5:1.5, 7.5:2.5, 6.5:3.5 and 5.5:4.5) were selected in the preparation process. Preparation and sintering treatment of SCF according to our previous study are shown in Fig. 1 and the three steps were as follows [26]: Step 1: Dosage, mixing and drying. DSS and clay were stirred in a dry powder stirrer (B10-20B, made in China) for about 10.0 min, then the mixture was poured into a pelletizer (DZ20, made in China) to produce pellets (about 7.00 wt% of water was added in this process). Two sieves (the diameters of meshes were 5.00 mm and 6.00 mm, respectively) were
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Table 1 The chemical components of DSS and clay (wt%). Materials
SiO2
Al2 O3
Total Fe
MgO
CaO
K2 O
Total S
Total P
TiO2
Na2 O
Other
Clay DSS
67.70 26.90
16.81 24.10
5.27 4.86
2.20 2.71
5.23 22.65
0.54 3.11
– 5.08
– 9.62
1.02 0.95
1.21 –
0.02 0.02
were compared with the national standard (GB 5085.3-2007, China) [28]. Chemical components of SCF before and after heating treatment were determined by energy dispersive X-ray fluorescence spectrometer (EDX), and DZ10-100 equipment was used during this part. 2.2. Soy protein secondary wastewater treatment Fig. 1. Flow chart for SCF preparation.
used to sift the pellets, and a range of particle sizes were obtained by sieving. Before thermal treatment, raw pellets were stored in draught cupboard at room temperature (22 ◦ C) for about 24.0 h. Step 2: Preheating and sintering treatment. The dried raw pellets placed in porcelain crucibles were settled in muffle and preheated at 400 ◦ C for 20.0 min. After preheating treatment, the pellets were rapidly transferred into electric tube rotary furnace (KSY-4D-16, made in China) and sintered at 1150 ◦ C for 10.0 min in anoxic conditions. The pellets were equally heated as the equipment rotated. Step 3: Cooling treatment. After the sintering process, the pellets were kept in draught cupboard until they cooled down to room temperature (22 ◦ C). 2.1.3. Characterization of SCF The properties of SCF (bulk density, grain density, water absorption and expansion ratio) were used to characterize the sintered pellets. Water absorption and bulk density were determined according to the national standard (GB/T 17431.2-2010, China) [27], and individual grain density was calculated according to the Archimedes’ principle. The above-mentioned physical properties were obtained as follows: Bulk density =
mass of ceramic bodies (kg m−3 ) bulk volume of ceramic bodies
Grain density =
mass of ceramic bodies (kg m−3 ) volume of ceramic bodies
(1) (2)
Water absorption (%) =
1 h saturated mass of ceramic bodies − mass of dry ceramic bodies × 100 mass of dry ceramic bodies
(3)
Expansion ratio (%) =
volume of sintered ceramic bodies − volume of raw pellets bodies × 100 volume of raw pellets bodies
(4)
SCF prepared in the optimum conditions were examined by scanning electron microscopy (Hitachi S-520, made in Japan) both in the surface and in the cross-section (Au coated). 1000.00 g of SCF prepared in the optimum conditions was soaked in 1.00 L of hydrochloric acid (0.20 mol L−1 ; HCl: = 1.19 g mL−1 Guaranteed Reagent (GR)) for 24 h. 1.00 mL of leach solution obtained from the supernatant was collected for leaching test of the toxic metal elements. Toxic metal concentrations (Cu, Zn, Pb, Cr, Cd, Hg, Ba, Ni, and As) of 1000.00 g of SCF were determined by ICP-AES (IRIS Intrepid II XSP equipment) and the results
2.2.1. The quality of influent The soy protein processing wastewater utilized in this research was obtained from a soy protein manufacturer (Weifang City, Shandong Province, China). In our previous studies, integrated treatment and advanced treatment for soy protein processing wastewater had been investigated and the results were feasible [18,22]. Consequently, secondary treatment for the wastewater would be studied in this research on the basis of the previous results. The influent of the BAF system in this research was obtained from sedimentation basin, where the effluent of preliminary treatments (flocculation and up-flow anaerobic sludge blanket) was stored. The wastewater quality including CODcr, BOD5 , NH4 + -N, SS and pH for this study and the national standard are shown in Table 2 [25]. 2.2.2. Reactor for BAF system A lab-scale up-flow BAF system setup was schematically illustrated in Fig. 2. The cylindrical reactor made from polymethyl methacrylate had an effective volume of 14.13 L (20.0 cm in diameter and 1.40 m in height). The reactor was filled with cobble stone at the bottom as supporting layer (20.0 cm), and with 90.0 cm filler layer of SCF (prepared in the optimum conditions as determined in above experiments) on the top, leaving a headspace of 15.0 cm to retain fillers during backwashing operation. Aerating apparatus was installed at the bottom layer, to supply oxygen for treatment process and backwashing operation. 2.2.3. Starting and running of BAF The BAF system was inoculated with concentrated activated sludge obtained from the anoxic/oxic (A/O) process at a local soy protein processing enterprise. The reactor was fed with the wastewater as shown in Table 2 throughout the entire experiment. At the start-up, the reactor was operated in a batch mode for about one week and then switched to continuous operation. Basic operation parameters including media height, HRT and A/L in the acclimatization period were 90.0 cm, 12.0 h and 15:1, respectively. The system was operated at room temperature and organic loading rates were varied according to the removal efficiency of CODcr and NH4 + -N. The column was backwashed according to the effluent quality. After reaching steady state, operation parameters including media height, HRT and A/L were varied and investigated to determine the optimum conditions. Firstly, six media heights (15.0, 30.0, 45.0, 60.0, 75.0 and 90.0 cm) were selected, and other conditions including HRT and A/L were 12.0 h and 15:1. Secondly, according to the effective volume of the column, seven HRTs (14.0, 12.0, 10.0, 8.0, 6.0, 4.0, and 2.0 h) were selected, the media height was determined in the first experiment and A/L was 15:1. Thirdly, five A/Ls (1:1, 5:1, 10:1, 15:1 and 20:1) were selected, and the media height and HRT were determined in the above experiments. Under each condition, there was a two-day acclimatization period and all samples were drawn and measured on the third day. Then the optimum
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Table 2 The quality of the wastewater and the national standard. Indicant
CODcr (mg L−1 )
BOD5 (mg L−1 )
NH4 + -N (mg L−1 )
SS (mg L−1 )
Raw wastewater GB18918-2002, secondary standard
812–1023 ≤100
216.2–305.4 ≤30.0
87.6–115.2 ≤25.0
65.5–76.8 ≤30.0
pH 6.7–8.2 6.0–9.0
3. Results and discussion 3.1. Preparation of SCF
conditions were determined by the effluent quality according to the national standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ) [25].
3.1.1. The effect of dosage on properties of SCF The effect of dosage on physical properties of SCF is shown in Fig. 3. The results indicated that expansion ratio and water absorption of SCF increased gradually as the addition ratio of DSS (by mass) was enhanced from 5.0% to 25.0%. However, when the ratio was increased from 25.0% to 45.0%, expansion ratio and water absorption decreased gradually. Compared to the expansion ratio and water absorption, the bulk density and grain density of SCF showed an opposite trend as the addition ratio of DSS was increased. When the ratio was about 25.0%, SCF presented the highest expansion ratio (53.0%) and water absorption (8.24%), and the lowest bulk density (350.4 kg m−3 ) and grain density (931.5 kg m−3 ). It is well known that the chemical components of ceramics can be classified into three groups: (1) the glassy phases (SiO2 and Al2 O3 ), where the framework and surfaces of ceramics were formed and the mass ratio of the glassy phases was about 75.0%; (2) flux, which lowered the melting point, mainly containing alkali metal oxide and alkaline earth metal (CaO, Na2 O, K2 O, MgO, etc.); (3) the gaseous components, mainly containing carbon and Fe2 O3 , which generated gases (CO and CO2 ) and bloated the ceramic bodies in the sintering process at high temperature (about 1150 ◦ C) [26]. It could be deduced that the gaseous components of ceramics mainly came from two kinds of materials, including expansion materials of DSS (organic matter, which played a main role in the bloating process) and expansion components of clay (Fe2 O3 , generating oxygen at high temperature (>1100 ◦ C)) [30,31]. More gases and expansion force would be generated by increasing DSS addition ratio (ranging from 5.0% to 25.0%). Firstly, apertures inside ceramics increased as the expansion force enhanced, resulting in the increase of ceramics volume and the decrease of bulk and grain density. Secondly, the shell of the ceramics surface in the molten state would bloat due to the generated gases inside, decreasing the thickness of the shell, resulting in porous structure of ceramics and increasing water absorption of ceramics [26]. Then when DSS addition ratio was too high (>25.0%), the rapid increase of gases generated inside ceramics would break through the shell of ceramics due to the gases generation inside and expansion force. The ceramics volume decreased accordingly, decreasing the expansion ratio and water absorption and increasing the bulk and grain density. Overall, the addition ratio of DSS should be approximately 25.0 wt% according to the physical properties of SCF.
2.2.4. Analytical methods The concentrations of CODcr, NH4 + -N, BOD5 and SS in the influent and effluent were measured according to national standard methods (State Environmental Protection Administration of China, 2002) [29]. Other parameters such as pH, temperature and dissolved oxygen (DO) were monitored regularly, with pH meter (PHS-3C-01, made in China), thermometer (SWM1-1, made in China), and DO meter (HQ 30d 5330300 LDOTM HACH, made in USA). All measurements of each sample were conducted in five replicates.
3.1.2. Microstructure of SCF The appearance and microstructure of SCF (the addition ratio of DSS was approximately 25.0 wt%) are shown in Fig. 4. The results revealed that the surface of ceramics was rough due to apertures (about 30–60 m in diameter), which can possibly be attributed to the inside gases broking through the shell of ceramics at high temperature. These apertures connecting the surface with the interior of ceramics could enhance the absorption ability of ceramics. Microorganism could easily enter the ceramics, probably leading to anaerobic and anoxic reaction inside the ceramics. Moreover, the apertures inside were hypertrophic and there were some large apertures (about 100–300 m in diameter) in the frameworks and
Fig. 2. Schematic diagram of experiment (dimension in mm).
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Fig. 3. The properties of SCF (A: bulk density; B: grain density; C: expansion ratio; D: water absorption).
some small apertures (about 20–60 m in diameter) inside the frameworks, increasing the specific surface area of the ceramics, providing space for the growth of anaerobic and anoxic microorganisms and enhancing water mass transfer. 3.1.3. Toxic metal leaching properties of SCF The results of toxic metal leaching test of SCF before and after heating treatment (25.0 wt% of DSS addition ratio) are shown in
Table 3. It revealed that all the nine metal (Cu, Zn, Pb, Cr, Cd, Hg, Ba, Ni, and As) contents in lixivium were much lower than the limits of the national standard (GB 5085.3-2007, China) [28]. Although the nine metals contents in lixivium from raw particles were lower than the limits of the national standard, the contents of Cu (9.93 mg kg−1 ), Zn (5.81 mg kg−1 ), Ba (2.97 mg kg−1 ), Pb (1.53 mg kg−1 ) and Cr (0.65 mg kg−1 ) in lixivium were high, which was harmful to the environment when it stayed in
Fig. 4. The appearance and microstructure of SCF (SEM): A – surface, B – fracture surface.
S. Wu et al. / Journal of Hazardous Materials 283 (2015) 608–616 Table 3 Toxic metal leaching test of SCF. Toxic metal
Contents (mg kg−1 of raw particles)
Contents (mg kg−1 of SCF)
Threshold (mg kg−1 of hazardous waste)
Total Cu Total Zn Total Cd Total Pb Total Cr Total Hg Total Ba Total Ni Total As
9.93 5.81 0.02 1.53 0.65 0.01 2.97 0.08 0.06
0.06 0.01 0.02 0.09 0.01 0.01 0.03 – 0.02
100.00 100.00 1.00 5.00 15.00 0.10 100.00 5.00 5.00
environment for a long time. Moreover, the contents of the toxic metals (mainly from DSS) in SCF after the heat treatment was much lower than that in the raw particles before the heating treatment, indicating that the toxic metals in DSS was involved and immobilized in SCF. Thus, DSS utilized as raw materials for sintering SCF could turn hazardous solid waste (DSS) into safe and useful materials (SCF). When applied as fillers for BAF system, SCF was nontoxic and would not cause secondary pollution to water environment. 3.1.4. Chemical components of SCF The chemical components of SCF before and after heating treatment are shown in Table 4. It could be seen that the main contents of raw particles and sintered particles were SiO2 (57.40% and 57.82%), Al2 O3 (18.72% and 19.10%) and CaO (9.62% and 10.03%), and the contents of these components were much higher than other components. SiO2 was mainly from clay and CaO was mainly derived from DSS, resulting from pH adjustment in the wastewater treatment by adding CaO. Al2 O3 was partly from DSS, possibly due to poly aluminum chloride addition in the flocculation process. Besides, total S and P in SCF were not detected after the heating treatment, which revealed that S and P in the raw particles might be released in other phase. The comparisons of chemical components before and after heating treatment suggested that the inorganic compounds of the particles increased slightly (except S and P) during the heating treatment, possibly due to the reduction of S, P and other matters. 3.2. Soy protein secondary wastewater treatment 3.2.1. Influence of media height on CODcr and NH4 + -N removal The influence of media height on CODcr and NH4 + -N removal rate is presented in Fig. 5. CODcr removal rate increased rapidly when media height increased from 0 to 45.0 cm. It is likely that abundant nutrient substance and dissolved oxygen (DO) at the entrance of the BAF reactor promoted the growth and reproduction of heterotrophic bacteria. Subsequently, CODcr removal rate increased slowly (45.0–75.0 cm in media height) and varied slightly (75.0–90.0 cm in media height), possibly due to the deficient nutrient substance and DO on the upper layer of the reactor, restricting the growth and reproduction of heterotrophic bacteria. Additionally, NH4 + -N removal rate increased gradually as media height increased. Firstly, NH4 + -N removal rate increased slowly when media height increased from 0 to 45.0 cm, possibly due to fast growing heterotrophic bacteria at the entrance of the
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reactor, inhibiting nitrifying bacteria due to the competition between heterotrophic and nitrifying bacteria for nutrient substance, space and DO [10,32]. Subsequently, NH4 + -N removal rate increased rapidly when media height increased from 45.0 to 75.0 cm. A possible explanation was that deficient nutrient substance on the upper layer of the reactor inhibited heterotrophic bacteria, deceasing competition between heterotrophic and nitrifying bacteria. As media height increased from 75.0 to 90.0 cm, NH4 + -N removal rate varied slightly due to the lack of oxygen and ammonia at the exit of the BAF reactor. As shown above, the optimum media height should be 75.0 cm in order to achieve high CODcr and NH4 + -N removal rate (90.26% and 91.23%, respectively), and the effluent in this media height met the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + N ≤ 25.0 mg L−1 ) [25]. 3.2.2. Influence of HRT on CODcr and NH4 + -N removal HRT was a crucial parameter in biological wastewater treatment. The effect of HRT (2.0–14.0 h) on CODcr and NH4 + -N removal rate is shown in Fig. 6. Firstly, CODcr removal rate increased rapidly when HRT increased from 2.0 h to 10.0 h. It was likely that heterotrophic microorganisms in exterior layer of the biofilm and suspension were highly shocked by the fast flow rate of air and water, strong shear press with short HRT, and the outer layer of the biofilm were easily detached and heterotrophic microorganisms in suspension was easily washed out from the BAF reactor due to the high water velocity and strong shear press. Thus the organic substance in the wastewater could not be degraded effectively. However, water velocity and shear press reduced gradually as HRT increased, increasing heterotrophic microorganisms in the outer layer of the biofilm and suspension Therefore, the organic substance in the wastewater could be degraded effectively and CODcr removal rate increased gradually. Secondly, CODcr removal rate varied slightly with long HRT (10.0–14.0 h), possibly due to little biodegradable organics residue in the wastewater, which slowed down reproduction of heterotrophic microorganisms in outer layer of the biofilm and suspension. Thereby, CODcr removal rate was stable as HRT increased from 10.0 h to 14.0 h. Based on these results, HRT should not be less than 10.0 h, and CODcr removal rate could reach 91.26% and the effluent CODcr did not exceed the limits of the national discharge standard (100 mg L−1 ) at HRT of 10.0 h [25]. Moreover, NH4 + -N removal rate increased rapidly as HRT increased from 2.0 h to 6.0 h. Subsequently, NH4 + -N removal rate varied slightly when HRT increased continually. It was well known that fast growing heterotrophic bacteria stayed in the outer layer of the biofilm and slowly growing nitrifying bacteria stayed deeper inside the biofilm [33]. When HRT was too short (2.0 h), the biofilm attached in outer and inner layer were mostly detached by the strong shear press from the water flow, thus both CODcr and NH4 + -N removal rates were reduced. When HRT was increased from 2.0 h to 6.0 h, NH4 + -N removal rate increased rapidly, possibly due to the decrease in the shear press and thicker biofilm including nitrifying bacteria with longer HRT. Meanwhile heterotrophic bacteria in outer layer of the biofilm could not be formed above the nitrifying bacteria in the biofilm due to the shear press from the water flow. Therefore, nitrifying bacteria was dominant microorganism in the biofilm of the BAF reactor at HRT of ≤6.0 h. However,
Table 4 The chemical components of SCF before and after heat treatment (wt%). Materials
SiO2
Al2 O3
Total Fe
MgO
CaO
K2 O
Total S
Total P
TiO2
Na2 O
Other
Raw particles Sintered particles (SCF)
57.40 57.82
18.72 19.10
5.17 5.53
2.42 2.85
9.62 10.03
1.12 1.49
1.32 –
2.28 –
1.00 1.41
0.93 1.33
0.02 0.44
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Fig. 5. Influence of media height on CODcr and NH4 + -N removal rate.
NH4 + -N removal rate decreased slightly as HRT increased from 8.0 to 14.0 h. It could be deduced that heterotrophic bacteria could be easily formed above the nitrifying bacteria and stay in outer layer of the biofilm especially at the entrance of the BAF reactor with longer HRT. Meanwhile the increasing liquid phase viscosity of the wastewater increased with longer HRT, reducing the driving force for the circulation and ultimately the shear stress for the biofilm [33]. Consequently, a thick layer of heterotrophic bacteria was formed above the nitrifying bacteria, increasing the resistance of oxygen mass transfer from water to inner layer of the biofilm [34]. The nitrifying bacteria in the biofilm were restrained due to the competition between heterotrophic and nitrifying bacteria. To sum up, 6.0–12.0 h should be a suitable HRT to achieve high NH4 + -N removal rate. Meanwhile HRT should not be shorter than 10.0 h in order to attain feasible CODcr removal rate. Therefore, the optimum HRT should be 10.0 h to obtain high CODcr and NH4 + -N removal rate, and the final effluent could meet the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ) [25].
3.2.3. Influence of A/L on CODcr and NH4 + -N removal The influence of A/L on CODcr and NH4 + -N removal rate is shown in Fig. 7. It revealed that CODcr removal rate increased rapidly as A/L increased from 1:1 to 10:1, possibly due to the increasing DO in the wastewater (especially at the entrance of the BAF reactor) as A/L enhanced, promoting organic substance degradation by aerobic heterotrophic bacteria. Then CODcr removal rate decreased slightly (3.64%) when A/L increased from 10:1 to 20:1. It is likely that the increasing water velocity with higher A/L generated strong shear press from the water flow, heterotrophic bacteria in the outer layer of the biofilm was easily detached and washed out from the BAF reactor and CODcr removal rate decreased. Therefore, 10:1 was a better choice for A/L than other ratios to get high CODcr removal rate. Additionally, NH4 + -N removal rate increased rapidly as A/L increased from 1:1 to 15:1. It was known that nitrifying bacteria grew in the inner layer of the biofilm especially at the entrance of the BAF reactor and dominated in the upper layer of the reactor.
Fig. 6. Influence of HRT on CODcr and NH4 + -N removal rate.
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Fig. 7. Influence of A/L on CODcr and NH4 + -N removal rate.
DO was mainly consumed by heterotrophic bacteria in the outer layer of the biofilm with low A/L, restricting nitrifying bacteria. As A/L enhanced, increasing DO in the upper space of the reactor promoted the growth of nitrifying bacteria in the upper layer of the reactor, decreasing NH4 + -N accumulation and increasing NH4 + -N removal rate [35]. Additionally, when A/L increased from 15:1 to 20:1, the increasing shear press from the water flow led to the detachment of heterotrophic bacteria in outer layer of the biofilm, promoting the oxygen transfer to nitrifying bacteria in the inner layer of the biofilm and slightly increasing NH4 + -N removal rate. Consequently, high NH4 + -N removal rate could be attained at A/L of ≥15:1. In general, A/L of 15:1 could yield high CODcr and NH4 + -N removal rate (91.02% and 90.48%, respectively). Moreover, effluent CODcr and NH4 + -N (81.6 mg L−1 and 15.3 mg L−1 , respectively) could meet the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ) [25].
4. Conclusion Preparation and application of novel media-SCF in a lab-scale up-flow BAF was investigated for soy protein secondary wastewater treatment. The results were as follows:
(1) DSS and clay used as raw materials for sintering SCF were feasible with the optimum DSS addition of approximately 25.0 wt% according to the properties of SCF. SCF with rough surface and porous structure was nontoxic to microorganisms. (2) SCF utilized as fillers in BAF for soy protein secondary wastewater treatment was verified to be satisfactory in terms of the effluent quality. The optimum conditions for the BAF system were media height of 75.0 cm, HRT of 10.0 h and A/L of 15:1 according to high CODcr and NH4 + -N removal rate (91.02% and 90.48%, respectively). (3) The final effluent of the BAF system under optimum conditions (CODcr of 81.6 mg L−1 and NH4 + -N of 15.3 mg L−1 ) met the national discharge standards (GB 18918-2002, secondary standard, CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ), which was beneficial for SCF application in practical projects.
Acknowledgement This research is supported by Technology Foresight Program of Shandong Province (No. 2012GGE27011).
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Preparation of ceramic filler from reusing sewage sludge and application in biological aerated filter for soy protein secondary wastewater treatment Suqing Wu a,b,1 , Yuanfeng Qi a,b,1 , Qinyan Yue b,∗ , Baoyu Gao b , Yue Gao c , Chunzhen Fan a , Shengbing He a a
School of Environmental Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, PR China Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, PR China c School of Environment, Tsinghua University, Beijing 100084, PR China b
h i g h l i g h t s • • • • •
Sewage sludge and clay utilized as raw materials for preparing ceramic filler. The optimum addition of DSS for preparing the filler was about 25.0 wt%. The filler employed in BAF system for soy protein secondary wastewater treatment. 91.02% of CODcr and 90.48% of NH4 + -N was removed by the BAF system. The wastewater treatment effect was helpful for application on practical project.
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 23 September 2014 Accepted 13 October 2014 Available online 23 October 2014 Keywords: Ceramic filler Sewage sludge Biological aerated filter Soy protein processing wastewater Secondary treatment
a b s t r a c t Dehydrated sewage sludge (DSS) and clay used as raw materials for preparation of novel media-sludge ceramic filler (SCF) and SCF employed in a lab-scale up-flow biological aerated filter (BAF) were investigated for soy protein secondary wastewater treatment. Single factor experiments were designed to investigate the preparation of SCF, and the characteristics (microstructure properties, toxic metal leaching property and other physical properties) of SCF prepared under the optimum conditions were examined. The influences of media height, hydraulic retention time (HRT) and air–liquid ratio (A/L) on chemical oxygen demand (CODcr) and ammonia nitrogen (NH4 + -N) removal rate were studied. The results showed that the optimum addition of DSS was approximately 25.0 wt% according to the physical properties of SCF (expansion ratio of 53.0%, v/v, water absorption of 8.24 wt%, bulk density of 350.4 kg m−3 and grain density of 931.5 kg m−3 ), and the optimum conditions of BAF system were media height of 75.0 cm, HRT of 10.0 h and A/L of 15:1 in terms of CODcr and NH4 + -N removal rate (91.02% and 90.48%, respectively). Additionally, CODcr and NH4 + -N (81.6 and 15.3 mg L−1 , respectively) in the final effluent of BAF system met the national standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 , GB 18918-2002, secondary standard). © 2014 Elsevier B.V. All rights reserved.
1. Introduction As a by-product from wastewater treatment process, sewage sludge is mainly generated during the primary (physical and/or chemical), secondary (biological) and tertiary treatments [1].
∗ Corresponding author. Tel.: +86 531 88365258; fax: +86 531 88364513. E-mail addresses: [email protected], [email protected] (Q. Yue). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jhazmat.2014.10.013 0304-3894/© 2014 Elsevier B.V. All rights reserved.
However, sewage sludge may contain pathogenic organisms and pollutants, various toxic metals and large amounts of soluble salts, which are harmful to human health and environment [2]. Moreover, a great quantity of sludge is produced, which poses a notable hazard. Hence, the treatment and disposal of sludge from wastewater treatment has been one of the most critical environmental issues [3]. Conventional sewage sludge treatments mainly involve incineration, landfilling, composting, land application, etc. [4]. In recent years, some sustainable methods have been utilized to handle sewage sludge, converting it into useful materials (e.g. preparation
S. Wu et al. / Journal of Hazardous Materials 283 (2015) 608–616
Nomenclature DSS SCF A/L SRT SS A/O BAF HRT COD NH4 + -N BOD DO
dehydrated sewage sludge sludge ceramic filler air–liquid ratio sludge retention time (h) suspended solid (mg L−1 ) anoxic/oxic process biological aerated filter hydraulic retention time (h) chemical oxygen demand (mg L−1 ) ammonia nitrogen (mg L−1 ) biological oxygen demand (mg L−1 ) dissolved oxygen (mg L−1 )
of adsorbents) [5]. Therefore, eco-friendly and economical methods for safe handling, disposal and recycling of sewage sludge are of great significance. As a kind of lightweight aggregate, ceramics have been widely used in construction industry (e.g. as cement mortars, concrete mixtures, bricks, fine aggregate for mortars and ceramic materials) [6,7], and also in other fields such as lightweight geotechnical fill, insulation products, soil engineering, hydro-culture, drainage, roof gardens and filters [8]. In recent years, many researchers have paid attention to ceramic sintering from sewage sludge, in order to decrease consumption of natural resources and immobilize hazardous waste [9]. Moreover, the research can meet the needs of sustainable development. Biological aerated filters (BAF) were developed on the basis of biological filters in Europe in the late 1980s and then widely applied all over the world as novel, flexible and effective bioreactors [10]. BAF was originally used in tertiary or advanced wastewater treatment, and subsequently applied in secondary wastewater treatment as this technology became mature. BAF presents several advantages over other fixed-film reactors (e.g. rotating biological contactors or trickling filters), including high loading rates [11,12], high active biomass concentration and sludge retention time (SRT), good pollutant efficiency (carbon, nitrogen, phosphorus and pathogen) removal combined with a high filtering capacity [13–15]. Furthermore, the space required for BAF construction is quite small compared to other aerobic process [16,17]. Therefore, BAF is an effective and economical method for municipal and industrial wastewater treatment. Soy protein, as a major foodstuff additive, is produced from soybean. However, a large quantity of industrial water is consumed and a great deal of wastewater is generated during soy protein manufacturing process. The wastewater usually contains organic substances (e.g. protein, sugar) and inorganic substances (e.g. salt, Cl− , and SO4 2− ) [18], characterized by high COD, BOD [19,20] and SS due to the high protein content, which inevitably causes serious environment pollution. Currently, the studies on treating soy protein processing wastewater are mainly focused on biological technologies. Zhu et al. utilized anaerobic baffled reactor to treat soybean protein processing wastewater (COD of 10,000 mg L−1 ), and 92–97% COD was removed at HRT of 39.5 h and temperature of 35.0 ◦ C. Cassini et al. used ultrafiltration membranes to treat wastewater from isolated soy protein production (COD of 16,000 mg L−1 ), and COD removal rate could reach approximately 34%. Li et al. applied conventional biofilters in soybean protein wastewater tertiary treatment (COD of 120–150 mg L−1 ), and final effluent COD was less than 60 mg L−1 [18,21–24]. However, with single treatment, soy protein processing wastewater could not meet the discharge standards. Therefore, the primary, secondary and tertiary treatments are all crucial to the
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wastewater treatment. It is necessary to develop effective, economical and integrated methods for soy protein wastewater treatment. In our previous studies, ceramic fillers prepared from different constituents were investigated for different wastewater treatment processes. Ceramic-corrosion-cell fillers prepared from clay, dried sewage sludge and scrap iron (sintered at 400 ◦ C) were applied in micro-electrolysis reactor for cyclohexanone industry wastewater pretreatment, where COD and cyclohexanone content were about 64,000 mg L−1 and 4.5 wt%, respectively, and approximately 90% of COD and cyclohexanone were removed. Ultra-lightweight sludge ceramics (ULSC) prepared from clay and dried sewage sludge (preheated at 400 ◦ C and sintered at 1150 ◦ C) were employed in a lab-scale up-flow BAF for pharmaceutical advanced wastewater treatment (influent CODcr and NH4 + -N were 198–260 and 34.8–39.7 mg L−1 , respectively). The mean effluent NH4 + -N was 6.2 mg L−1 and the maximum CODcr in the effluent was 96 mg L−1 . In this research, there were three objectives as follows: (1) DSS and clay were used as raw materials for SCF preparation, and the optimum DSS addition was determined by the properties of SCF. (2) The possibility of SCF used as fillers in a lab-scale up-flow BAF was investigated for soy protein processing wastewater secondary treatment, whereby the wastewater treatment process could be simplified and the wastewater treatment cost could be reduced. (3) The optimum conditions (including media height, HRT and A/L) for the wastewater treatment were determined by the removal efficiency of contaminants and the limits as specified by the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + N ≤ 25.0 mg L−1 ,GB 18918-2002, secondary standard) [25]. 2. Materials and methods 2.1. Preparation of SCF 2.1.1. Pretreatment of raw materials DSS and clay were obtained from local municipal wastewater treatment plant (Jinan City, Shandong Province, China) and mountainous area (in Zibo City, Shandong Province, China) respectively. Firstly, the raw materials (DSS and clay) were dried in stove at 105 ◦ C for 4.0 h. Secondly, they were crushed in a ball mill and sieved (the diameter of the sieve mesh was 0.154 mm). Finally, dry powder from raw materials was obtained and preserved in polyethylene vessels to avoid humidity before it was used. The wastewater treatment system utilized in the wastewater treatment plant mainly contained flocculation, oxidation ditch and anaerobic–anoxic–oxic process (A2 O). The sewage sludge was generated during these processes featuring 85.7 wt% of moisture content, 36.5 wt% of VSS/SS, 56.8 wt% of ignition loss and 13.2 MJ kg−1 of calorific value (in dry basis). Besides, the chemical components of clay and DSS are shown in Table 1. 2.1.2. Preparation and sintering of SCF Five mass ratios of clay to DSS (9.5:0.5, 8.5:1.5, 7.5:2.5, 6.5:3.5 and 5.5:4.5) were selected in the preparation process. Preparation and sintering treatment of SCF according to our previous study are shown in Fig. 1 and the three steps were as follows [26]: Step 1: Dosage, mixing and drying. DSS and clay were stirred in a dry powder stirrer (B10-20B, made in China) for about 10.0 min, then the mixture was poured into a pelletizer (DZ20, made in China) to produce pellets (about 7.00 wt% of water was added in this process). Two sieves (the diameters of meshes were 5.00 mm and 6.00 mm, respectively) were
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Table 1 The chemical components of DSS and clay (wt%). Materials
SiO2
Al2 O3
Total Fe
MgO
CaO
K2 O
Total S
Total P
TiO2
Na2 O
Other
Clay DSS
67.70 26.90
16.81 24.10
5.27 4.86
2.20 2.71
5.23 22.65
0.54 3.11
– 5.08
– 9.62
1.02 0.95
1.21 –
0.02 0.02
were compared with the national standard (GB 5085.3-2007, China) [28]. Chemical components of SCF before and after heating treatment were determined by energy dispersive X-ray fluorescence spectrometer (EDX), and DZ10-100 equipment was used during this part. 2.2. Soy protein secondary wastewater treatment Fig. 1. Flow chart for SCF preparation.
used to sift the pellets, and a range of particle sizes were obtained by sieving. Before thermal treatment, raw pellets were stored in draught cupboard at room temperature (22 ◦ C) for about 24.0 h. Step 2: Preheating and sintering treatment. The dried raw pellets placed in porcelain crucibles were settled in muffle and preheated at 400 ◦ C for 20.0 min. After preheating treatment, the pellets were rapidly transferred into electric tube rotary furnace (KSY-4D-16, made in China) and sintered at 1150 ◦ C for 10.0 min in anoxic conditions. The pellets were equally heated as the equipment rotated. Step 3: Cooling treatment. After the sintering process, the pellets were kept in draught cupboard until they cooled down to room temperature (22 ◦ C). 2.1.3. Characterization of SCF The properties of SCF (bulk density, grain density, water absorption and expansion ratio) were used to characterize the sintered pellets. Water absorption and bulk density were determined according to the national standard (GB/T 17431.2-2010, China) [27], and individual grain density was calculated according to the Archimedes’ principle. The above-mentioned physical properties were obtained as follows: Bulk density =
mass of ceramic bodies (kg m−3 ) bulk volume of ceramic bodies
Grain density =
mass of ceramic bodies (kg m−3 ) volume of ceramic bodies
(1) (2)
Water absorption (%) =
1 h saturated mass of ceramic bodies − mass of dry ceramic bodies × 100 mass of dry ceramic bodies
(3)
Expansion ratio (%) =
volume of sintered ceramic bodies − volume of raw pellets bodies × 100 volume of raw pellets bodies
(4)
SCF prepared in the optimum conditions were examined by scanning electron microscopy (Hitachi S-520, made in Japan) both in the surface and in the cross-section (Au coated). 1000.00 g of SCF prepared in the optimum conditions was soaked in 1.00 L of hydrochloric acid (0.20 mol L−1 ; HCl: = 1.19 g mL−1 Guaranteed Reagent (GR)) for 24 h. 1.00 mL of leach solution obtained from the supernatant was collected for leaching test of the toxic metal elements. Toxic metal concentrations (Cu, Zn, Pb, Cr, Cd, Hg, Ba, Ni, and As) of 1000.00 g of SCF were determined by ICP-AES (IRIS Intrepid II XSP equipment) and the results
2.2.1. The quality of influent The soy protein processing wastewater utilized in this research was obtained from a soy protein manufacturer (Weifang City, Shandong Province, China). In our previous studies, integrated treatment and advanced treatment for soy protein processing wastewater had been investigated and the results were feasible [18,22]. Consequently, secondary treatment for the wastewater would be studied in this research on the basis of the previous results. The influent of the BAF system in this research was obtained from sedimentation basin, where the effluent of preliminary treatments (flocculation and up-flow anaerobic sludge blanket) was stored. The wastewater quality including CODcr, BOD5 , NH4 + -N, SS and pH for this study and the national standard are shown in Table 2 [25]. 2.2.2. Reactor for BAF system A lab-scale up-flow BAF system setup was schematically illustrated in Fig. 2. The cylindrical reactor made from polymethyl methacrylate had an effective volume of 14.13 L (20.0 cm in diameter and 1.40 m in height). The reactor was filled with cobble stone at the bottom as supporting layer (20.0 cm), and with 90.0 cm filler layer of SCF (prepared in the optimum conditions as determined in above experiments) on the top, leaving a headspace of 15.0 cm to retain fillers during backwashing operation. Aerating apparatus was installed at the bottom layer, to supply oxygen for treatment process and backwashing operation. 2.2.3. Starting and running of BAF The BAF system was inoculated with concentrated activated sludge obtained from the anoxic/oxic (A/O) process at a local soy protein processing enterprise. The reactor was fed with the wastewater as shown in Table 2 throughout the entire experiment. At the start-up, the reactor was operated in a batch mode for about one week and then switched to continuous operation. Basic operation parameters including media height, HRT and A/L in the acclimatization period were 90.0 cm, 12.0 h and 15:1, respectively. The system was operated at room temperature and organic loading rates were varied according to the removal efficiency of CODcr and NH4 + -N. The column was backwashed according to the effluent quality. After reaching steady state, operation parameters including media height, HRT and A/L were varied and investigated to determine the optimum conditions. Firstly, six media heights (15.0, 30.0, 45.0, 60.0, 75.0 and 90.0 cm) were selected, and other conditions including HRT and A/L were 12.0 h and 15:1. Secondly, according to the effective volume of the column, seven HRTs (14.0, 12.0, 10.0, 8.0, 6.0, 4.0, and 2.0 h) were selected, the media height was determined in the first experiment and A/L was 15:1. Thirdly, five A/Ls (1:1, 5:1, 10:1, 15:1 and 20:1) were selected, and the media height and HRT were determined in the above experiments. Under each condition, there was a two-day acclimatization period and all samples were drawn and measured on the third day. Then the optimum
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Table 2 The quality of the wastewater and the national standard. Indicant
CODcr (mg L−1 )
BOD5 (mg L−1 )
NH4 + -N (mg L−1 )
SS (mg L−1 )
Raw wastewater GB18918-2002, secondary standard
812–1023 ≤100
216.2–305.4 ≤30.0
87.6–115.2 ≤25.0
65.5–76.8 ≤30.0
pH 6.7–8.2 6.0–9.0
3. Results and discussion 3.1. Preparation of SCF
conditions were determined by the effluent quality according to the national standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ) [25].
3.1.1. The effect of dosage on properties of SCF The effect of dosage on physical properties of SCF is shown in Fig. 3. The results indicated that expansion ratio and water absorption of SCF increased gradually as the addition ratio of DSS (by mass) was enhanced from 5.0% to 25.0%. However, when the ratio was increased from 25.0% to 45.0%, expansion ratio and water absorption decreased gradually. Compared to the expansion ratio and water absorption, the bulk density and grain density of SCF showed an opposite trend as the addition ratio of DSS was increased. When the ratio was about 25.0%, SCF presented the highest expansion ratio (53.0%) and water absorption (8.24%), and the lowest bulk density (350.4 kg m−3 ) and grain density (931.5 kg m−3 ). It is well known that the chemical components of ceramics can be classified into three groups: (1) the glassy phases (SiO2 and Al2 O3 ), where the framework and surfaces of ceramics were formed and the mass ratio of the glassy phases was about 75.0%; (2) flux, which lowered the melting point, mainly containing alkali metal oxide and alkaline earth metal (CaO, Na2 O, K2 O, MgO, etc.); (3) the gaseous components, mainly containing carbon and Fe2 O3 , which generated gases (CO and CO2 ) and bloated the ceramic bodies in the sintering process at high temperature (about 1150 ◦ C) [26]. It could be deduced that the gaseous components of ceramics mainly came from two kinds of materials, including expansion materials of DSS (organic matter, which played a main role in the bloating process) and expansion components of clay (Fe2 O3 , generating oxygen at high temperature (>1100 ◦ C)) [30,31]. More gases and expansion force would be generated by increasing DSS addition ratio (ranging from 5.0% to 25.0%). Firstly, apertures inside ceramics increased as the expansion force enhanced, resulting in the increase of ceramics volume and the decrease of bulk and grain density. Secondly, the shell of the ceramics surface in the molten state would bloat due to the generated gases inside, decreasing the thickness of the shell, resulting in porous structure of ceramics and increasing water absorption of ceramics [26]. Then when DSS addition ratio was too high (>25.0%), the rapid increase of gases generated inside ceramics would break through the shell of ceramics due to the gases generation inside and expansion force. The ceramics volume decreased accordingly, decreasing the expansion ratio and water absorption and increasing the bulk and grain density. Overall, the addition ratio of DSS should be approximately 25.0 wt% according to the physical properties of SCF.
2.2.4. Analytical methods The concentrations of CODcr, NH4 + -N, BOD5 and SS in the influent and effluent were measured according to national standard methods (State Environmental Protection Administration of China, 2002) [29]. Other parameters such as pH, temperature and dissolved oxygen (DO) were monitored regularly, with pH meter (PHS-3C-01, made in China), thermometer (SWM1-1, made in China), and DO meter (HQ 30d 5330300 LDOTM HACH, made in USA). All measurements of each sample were conducted in five replicates.
3.1.2. Microstructure of SCF The appearance and microstructure of SCF (the addition ratio of DSS was approximately 25.0 wt%) are shown in Fig. 4. The results revealed that the surface of ceramics was rough due to apertures (about 30–60 m in diameter), which can possibly be attributed to the inside gases broking through the shell of ceramics at high temperature. These apertures connecting the surface with the interior of ceramics could enhance the absorption ability of ceramics. Microorganism could easily enter the ceramics, probably leading to anaerobic and anoxic reaction inside the ceramics. Moreover, the apertures inside were hypertrophic and there were some large apertures (about 100–300 m in diameter) in the frameworks and
Fig. 2. Schematic diagram of experiment (dimension in mm).
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Fig. 3. The properties of SCF (A: bulk density; B: grain density; C: expansion ratio; D: water absorption).
some small apertures (about 20–60 m in diameter) inside the frameworks, increasing the specific surface area of the ceramics, providing space for the growth of anaerobic and anoxic microorganisms and enhancing water mass transfer. 3.1.3. Toxic metal leaching properties of SCF The results of toxic metal leaching test of SCF before and after heating treatment (25.0 wt% of DSS addition ratio) are shown in
Table 3. It revealed that all the nine metal (Cu, Zn, Pb, Cr, Cd, Hg, Ba, Ni, and As) contents in lixivium were much lower than the limits of the national standard (GB 5085.3-2007, China) [28]. Although the nine metals contents in lixivium from raw particles were lower than the limits of the national standard, the contents of Cu (9.93 mg kg−1 ), Zn (5.81 mg kg−1 ), Ba (2.97 mg kg−1 ), Pb (1.53 mg kg−1 ) and Cr (0.65 mg kg−1 ) in lixivium were high, which was harmful to the environment when it stayed in
Fig. 4. The appearance and microstructure of SCF (SEM): A – surface, B – fracture surface.
S. Wu et al. / Journal of Hazardous Materials 283 (2015) 608–616 Table 3 Toxic metal leaching test of SCF. Toxic metal
Contents (mg kg−1 of raw particles)
Contents (mg kg−1 of SCF)
Threshold (mg kg−1 of hazardous waste)
Total Cu Total Zn Total Cd Total Pb Total Cr Total Hg Total Ba Total Ni Total As
9.93 5.81 0.02 1.53 0.65 0.01 2.97 0.08 0.06
0.06 0.01 0.02 0.09 0.01 0.01 0.03 – 0.02
100.00 100.00 1.00 5.00 15.00 0.10 100.00 5.00 5.00
environment for a long time. Moreover, the contents of the toxic metals (mainly from DSS) in SCF after the heat treatment was much lower than that in the raw particles before the heating treatment, indicating that the toxic metals in DSS was involved and immobilized in SCF. Thus, DSS utilized as raw materials for sintering SCF could turn hazardous solid waste (DSS) into safe and useful materials (SCF). When applied as fillers for BAF system, SCF was nontoxic and would not cause secondary pollution to water environment. 3.1.4. Chemical components of SCF The chemical components of SCF before and after heating treatment are shown in Table 4. It could be seen that the main contents of raw particles and sintered particles were SiO2 (57.40% and 57.82%), Al2 O3 (18.72% and 19.10%) and CaO (9.62% and 10.03%), and the contents of these components were much higher than other components. SiO2 was mainly from clay and CaO was mainly derived from DSS, resulting from pH adjustment in the wastewater treatment by adding CaO. Al2 O3 was partly from DSS, possibly due to poly aluminum chloride addition in the flocculation process. Besides, total S and P in SCF were not detected after the heating treatment, which revealed that S and P in the raw particles might be released in other phase. The comparisons of chemical components before and after heating treatment suggested that the inorganic compounds of the particles increased slightly (except S and P) during the heating treatment, possibly due to the reduction of S, P and other matters. 3.2. Soy protein secondary wastewater treatment 3.2.1. Influence of media height on CODcr and NH4 + -N removal The influence of media height on CODcr and NH4 + -N removal rate is presented in Fig. 5. CODcr removal rate increased rapidly when media height increased from 0 to 45.0 cm. It is likely that abundant nutrient substance and dissolved oxygen (DO) at the entrance of the BAF reactor promoted the growth and reproduction of heterotrophic bacteria. Subsequently, CODcr removal rate increased slowly (45.0–75.0 cm in media height) and varied slightly (75.0–90.0 cm in media height), possibly due to the deficient nutrient substance and DO on the upper layer of the reactor, restricting the growth and reproduction of heterotrophic bacteria. Additionally, NH4 + -N removal rate increased gradually as media height increased. Firstly, NH4 + -N removal rate increased slowly when media height increased from 0 to 45.0 cm, possibly due to fast growing heterotrophic bacteria at the entrance of the
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reactor, inhibiting nitrifying bacteria due to the competition between heterotrophic and nitrifying bacteria for nutrient substance, space and DO [10,32]. Subsequently, NH4 + -N removal rate increased rapidly when media height increased from 45.0 to 75.0 cm. A possible explanation was that deficient nutrient substance on the upper layer of the reactor inhibited heterotrophic bacteria, deceasing competition between heterotrophic and nitrifying bacteria. As media height increased from 75.0 to 90.0 cm, NH4 + -N removal rate varied slightly due to the lack of oxygen and ammonia at the exit of the BAF reactor. As shown above, the optimum media height should be 75.0 cm in order to achieve high CODcr and NH4 + -N removal rate (90.26% and 91.23%, respectively), and the effluent in this media height met the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + N ≤ 25.0 mg L−1 ) [25]. 3.2.2. Influence of HRT on CODcr and NH4 + -N removal HRT was a crucial parameter in biological wastewater treatment. The effect of HRT (2.0–14.0 h) on CODcr and NH4 + -N removal rate is shown in Fig. 6. Firstly, CODcr removal rate increased rapidly when HRT increased from 2.0 h to 10.0 h. It was likely that heterotrophic microorganisms in exterior layer of the biofilm and suspension were highly shocked by the fast flow rate of air and water, strong shear press with short HRT, and the outer layer of the biofilm were easily detached and heterotrophic microorganisms in suspension was easily washed out from the BAF reactor due to the high water velocity and strong shear press. Thus the organic substance in the wastewater could not be degraded effectively. However, water velocity and shear press reduced gradually as HRT increased, increasing heterotrophic microorganisms in the outer layer of the biofilm and suspension Therefore, the organic substance in the wastewater could be degraded effectively and CODcr removal rate increased gradually. Secondly, CODcr removal rate varied slightly with long HRT (10.0–14.0 h), possibly due to little biodegradable organics residue in the wastewater, which slowed down reproduction of heterotrophic microorganisms in outer layer of the biofilm and suspension. Thereby, CODcr removal rate was stable as HRT increased from 10.0 h to 14.0 h. Based on these results, HRT should not be less than 10.0 h, and CODcr removal rate could reach 91.26% and the effluent CODcr did not exceed the limits of the national discharge standard (100 mg L−1 ) at HRT of 10.0 h [25]. Moreover, NH4 + -N removal rate increased rapidly as HRT increased from 2.0 h to 6.0 h. Subsequently, NH4 + -N removal rate varied slightly when HRT increased continually. It was well known that fast growing heterotrophic bacteria stayed in the outer layer of the biofilm and slowly growing nitrifying bacteria stayed deeper inside the biofilm [33]. When HRT was too short (2.0 h), the biofilm attached in outer and inner layer were mostly detached by the strong shear press from the water flow, thus both CODcr and NH4 + -N removal rates were reduced. When HRT was increased from 2.0 h to 6.0 h, NH4 + -N removal rate increased rapidly, possibly due to the decrease in the shear press and thicker biofilm including nitrifying bacteria with longer HRT. Meanwhile heterotrophic bacteria in outer layer of the biofilm could not be formed above the nitrifying bacteria in the biofilm due to the shear press from the water flow. Therefore, nitrifying bacteria was dominant microorganism in the biofilm of the BAF reactor at HRT of ≤6.0 h. However,
Table 4 The chemical components of SCF before and after heat treatment (wt%). Materials
SiO2
Al2 O3
Total Fe
MgO
CaO
K2 O
Total S
Total P
TiO2
Na2 O
Other
Raw particles Sintered particles (SCF)
57.40 57.82
18.72 19.10
5.17 5.53
2.42 2.85
9.62 10.03
1.12 1.49
1.32 –
2.28 –
1.00 1.41
0.93 1.33
0.02 0.44
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S. Wu et al. / Journal of Hazardous Materials 283 (2015) 608–616
Fig. 5. Influence of media height on CODcr and NH4 + -N removal rate.
NH4 + -N removal rate decreased slightly as HRT increased from 8.0 to 14.0 h. It could be deduced that heterotrophic bacteria could be easily formed above the nitrifying bacteria and stay in outer layer of the biofilm especially at the entrance of the BAF reactor with longer HRT. Meanwhile the increasing liquid phase viscosity of the wastewater increased with longer HRT, reducing the driving force for the circulation and ultimately the shear stress for the biofilm [33]. Consequently, a thick layer of heterotrophic bacteria was formed above the nitrifying bacteria, increasing the resistance of oxygen mass transfer from water to inner layer of the biofilm [34]. The nitrifying bacteria in the biofilm were restrained due to the competition between heterotrophic and nitrifying bacteria. To sum up, 6.0–12.0 h should be a suitable HRT to achieve high NH4 + -N removal rate. Meanwhile HRT should not be shorter than 10.0 h in order to attain feasible CODcr removal rate. Therefore, the optimum HRT should be 10.0 h to obtain high CODcr and NH4 + -N removal rate, and the final effluent could meet the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ) [25].
3.2.3. Influence of A/L on CODcr and NH4 + -N removal The influence of A/L on CODcr and NH4 + -N removal rate is shown in Fig. 7. It revealed that CODcr removal rate increased rapidly as A/L increased from 1:1 to 10:1, possibly due to the increasing DO in the wastewater (especially at the entrance of the BAF reactor) as A/L enhanced, promoting organic substance degradation by aerobic heterotrophic bacteria. Then CODcr removal rate decreased slightly (3.64%) when A/L increased from 10:1 to 20:1. It is likely that the increasing water velocity with higher A/L generated strong shear press from the water flow, heterotrophic bacteria in the outer layer of the biofilm was easily detached and washed out from the BAF reactor and CODcr removal rate decreased. Therefore, 10:1 was a better choice for A/L than other ratios to get high CODcr removal rate. Additionally, NH4 + -N removal rate increased rapidly as A/L increased from 1:1 to 15:1. It was known that nitrifying bacteria grew in the inner layer of the biofilm especially at the entrance of the BAF reactor and dominated in the upper layer of the reactor.
Fig. 6. Influence of HRT on CODcr and NH4 + -N removal rate.
S. Wu et al. / Journal of Hazardous Materials 283 (2015) 608–616
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Fig. 7. Influence of A/L on CODcr and NH4 + -N removal rate.
DO was mainly consumed by heterotrophic bacteria in the outer layer of the biofilm with low A/L, restricting nitrifying bacteria. As A/L enhanced, increasing DO in the upper space of the reactor promoted the growth of nitrifying bacteria in the upper layer of the reactor, decreasing NH4 + -N accumulation and increasing NH4 + -N removal rate [35]. Additionally, when A/L increased from 15:1 to 20:1, the increasing shear press from the water flow led to the detachment of heterotrophic bacteria in outer layer of the biofilm, promoting the oxygen transfer to nitrifying bacteria in the inner layer of the biofilm and slightly increasing NH4 + -N removal rate. Consequently, high NH4 + -N removal rate could be attained at A/L of ≥15:1. In general, A/L of 15:1 could yield high CODcr and NH4 + -N removal rate (91.02% and 90.48%, respectively). Moreover, effluent CODcr and NH4 + -N (81.6 mg L−1 and 15.3 mg L−1 , respectively) could meet the national discharge standard (CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ) [25].
4. Conclusion Preparation and application of novel media-SCF in a lab-scale up-flow BAF was investigated for soy protein secondary wastewater treatment. The results were as follows:
(1) DSS and clay used as raw materials for sintering SCF were feasible with the optimum DSS addition of approximately 25.0 wt% according to the properties of SCF. SCF with rough surface and porous structure was nontoxic to microorganisms. (2) SCF utilized as fillers in BAF for soy protein secondary wastewater treatment was verified to be satisfactory in terms of the effluent quality. The optimum conditions for the BAF system were media height of 75.0 cm, HRT of 10.0 h and A/L of 15:1 according to high CODcr and NH4 + -N removal rate (91.02% and 90.48%, respectively). (3) The final effluent of the BAF system under optimum conditions (CODcr of 81.6 mg L−1 and NH4 + -N of 15.3 mg L−1 ) met the national discharge standards (GB 18918-2002, secondary standard, CODcr ≤ 100 mg L−1 , NH4 + -N ≤ 25.0 mg L−1 ), which was beneficial for SCF application in practical projects.
Acknowledgement This research is supported by Technology Foresight Program of Shandong Province (No. 2012GGE27011).
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