Biological excess sludge reduction in adsorption/bio-oxidation process by enhancing predators' growth
Somayeh Kheiri1, Ensiyeh Taheri2, Nasim Rafiei3, Ali Fatehizadeh2, Mohammad Ghasemian1, Mohammad Mehdi Amin2, Mohammad Koushafar1, Seyed Mohammad Mousavi4
1 Department of Agriculture Sciences, Islamic Azad University, Ardestan Branch, Ardestan, Iran
2 Department of Environmental Health Engineering, School of Health; Environment Research Center, Research Institute for Primordial Prevention of Noncommunicable Disease, Isfahan University of Medical Sciences, Isfahan, Iran
3 Environment Research Center, Research Institute for Primordial Prevention of Noncommunicable Disease; Student Research Committee, School of Health, Isfahan University of Medical Sciences, Isfahan, Iran
4 Esfahan province Water and Wastewater Company, Isfahan, Iran
|Date of Submission||03-Dec-2019|
|Date of Acceptance||23-Dec-2019|
|Date of Web Publication||31-May-2020|
Dr. Mohammad Mehdi Amin
Environment Research Center, Research Institute for Primordial Prevention of Noncommunicable Disease, Isfahan University of Medical Sciences, Isfahan
Source of Support: None, Conflict of Interest: None
Aims: This study was carried out to investigate the effect of predators' growth on biological excess sludge reduction of adsorption/bio-oxidation process (A/B process) as a modification of activated sludge system. Materials and Methods: The real municipal wastewater after screening and gritting was pumped into A/B pilot plant which consists of two aeration and sedimentation tanks in series. The hydraulic residence time for A and B stages was set at 1 and 4 h, respectively, at an average flow rate of 32 L/h. During operation, the mixed liquor suspended solids (MLSS) in A and B stages was gradually increased. In this period, the operational parameters including dissolved oxygen, pH, volatile suspended solids (VSS), MLSS, alkalinity, biochemical oxygen demand (BOD5), chemical oxygen demand (COD), phosphorus, nitrogen, and sludge volume index were monitored. Results: The results showed that with increasing solid retention time (SRT) from 0.6 to 56.8 days in Stage A, the biomass yield (Y) decreased from 1.29 to 0.23 g VSS/g COD. Similar results were observed in Stage B and correspondence to 67% reduction of Y as SRT increased from 1.6 to 123.8 days. During the A/B operation, overall tBOD5and tCOD removal was 70% ± 20% and 57% ± 24%, respectively. Conclusion: Based on the results, A/B process operation with high SRT led to predator growth enhancement and lower biological excess sludge production.
Keywords: Aquatic worm, predators, sludge reduction, solid retention time, two-stage activated sludge
|How to cite this article:|
Kheiri S, Taheri E, Rafiei N, Fatehizadeh A, Ghasemian M, Amin MM, Koushafar M, Mousavi SM. Biological excess sludge reduction in adsorption/bio-oxidation process by enhancing predators' growth. Int J Env Health Eng 2020;9:2
|How to cite this URL:|
Kheiri S, Taheri E, Rafiei N, Fatehizadeh A, Ghasemian M, Amin MM, Koushafar M, Mousavi SM. Biological excess sludge reduction in adsorption/bio-oxidation process by enhancing predators' growth. Int J Env Health Eng [serial online] 2020 [cited 2021 Apr 17];9:2. Available from: https://www.ijehe.org/text.asp?2020/9/1/2/285521
| Introduction|| |
Activated sludge (AS) process is a frequent wastewater treatment plant (WWTP) for both industrial and municipal wastewater treatments. In an AS process, the Y coefficient is about 0.4–0.6 volatile suspended solids (kg VSS)/chemical oxygen demand (kg COD), thus a large amount of sludge is produced during organic matter oxidizing. On the other hand, legislations for sludge discharge to environment have been strengthened. The sludge mainly consists of water, and its conventional treatment process includes thickening, stabilization, and dewatering, which impose 50%–60% of the total costs on the WWTP operation costs. A feasible and environmental-friendly method is highly desired to reduce excess sludge. In previous studies, different approaches to reduce excess sludge were investigated. Mainly, sludge reduction processes have been divided into two categories: (a) in-situ sludge reduction processes and (b) sludge post treatment. Comparing to posttreatment process, in-situ sludge reduction process has some advantages. In case of in-situ sludge reduction process, the minimization occurs by decreasing the yield of the sludge production. The in-situ reduction treatment includes chemical and biological uncoupling metabolism (such as oxic-settling-anaerobic), maintenance metabolism, and predation.,
Predation is one of the in-situ, biological approaches to reduce biological excess sludge. The primary consumers in wastewater treatment systems are bacteria, which themselves are consumed by higher organisms in food chain such as metazoa and protozoa. The worm predation is described as using microfauna to feed on microorganisms. Hence, to reduce biological excess sludge, inducing such organisms' growth can help.
The present study was aimed to evaluate the presence and growth of predators on sludge reduction of adsorption/bio-oxidation (A/B process) fed by real wastewater from the North Isfahan WWTP.
| Materials and Methods|| |
In this study, a two-stage AS (A/B process) with similar pattern as the North Isfahan WWTP (Isfahan, Iran) was implemented. The schematic of the studied A/B process is shown in [Figure 1], which was made by stainless steel.
The influent was collected from real municipal wastewater after passing screening and gritting and then was pumped into the A/S process pilot at a flow rate of 30 L/h. The characteristics of the influent wastewater are summarized in [Table 1]. Before A/B process operation, in order to providing microbial consortium, the AS was extracted from Stages A and B of North Isfahan WWTP with mixed liquid suspended solid (MLSS) of 5264 and 4142 mg/L, respectively. After that, aeration was started, and the dissolved oxygen (DO) concentration was kept in the range of 0.5–1 mg/L in Tank A and 1–2 mg/L in Tank B.
The pilot was operated for 183 days. After 22 days of A/B process operation, the MLSS concentration reached to 3132 and 3500 mg/L in Tanks A and B, respectively. In order to enhance predator's growth and also to reduce the excess biological sludge, the solid retention time (SRT) was increased to higher than the North Isfahan WWTP. To SRT increment, the sludge withdrawal was avoided unless it was necessary.
For A/B process performance monitoring, the influent and effluent wastewater were analyzed with respect to the operation period of DO, pH, temperature, VSS, MLSS, mixed liquor volatile suspended solids (MLVSS), alkalinity, biochemical oxygen demand (BOD5), COD, phosphorus, nitrogen, and sludge volume index (SVI). The analyses were accomplished according to the standard methods for water and wastewater examination.
The Y coefficient in A/B process was calculated by using equations (1) and (2):
where Yobs is observed biomass yield coefficient (gVSS/gCOD), Y is absolute biomass yield coefficient (g VSS/g COD), kd is death rate constant (d-1), and fd is residue mass (d-1).
where SRT is sludge retention time (days), MLVSS is mixed liquor volatile suspended solids of the reactor (mg/L), Vaeration is aeration tank volume (L), Xw is VSS of waste sludge (mg/L), Xe is VSS of effluent (mg/L), Qw is the flow of waste sludge (m3/day), and Qe is the flow rate of effluent (m3/day).
where Fw is waste sludge flow rate (m3/day), VSSw is VSS in waste sludge, Fef is the flow rate of effluent, VSSef is VSS in effluent, Fin is influent flow rate, and CODin and CODout are COD of influent and effluent wastewater, respectively.
Equations 1 and 3 were used to calculate sludge reduction.
| Results|| |
Variation of Y coefficient during adsorption/bio-oxidation process operation
Variation of Yobs, as a function of SRT, is shown in [Figure 2]. As depicted in [Figure 2], the operation A/B process with high SRT led to lower Yobs. With SRT increasing from 0.6 to 56.8 days in Tank A, the Y coefficient decreased from 1.29 to 0.23 g VSS/g COD and also reduced from 0.57 to 0.19 g VSS/g COD; in Stage B, as the SRT increased from 1.64 to 123.78 days, the biomass yield coefficient decreased. [Figure 3] illustrates the effect of SRT on excess sludge reduction during A/B process operation.
|Figure 2: Variation of observed biomass yield in Tanks A and B during the pilot operation|
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|Figure 3: Effect of solid retention time enhancement on excess sludge reduction|
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The A/B operation with higher SRT led to more predators' growth in sludge. In Stage A, as SRT increased up to 4 days, the Paramecium spp. was observed and maintained during the A/B process operation. By increasing SRT from 10 to 123 days in Stage B, the rotifers and Lumbriculus variegatus worms with 1 cm of length were seen. Paramecium and worm observed in Stage B of the A/B process are depicted in [Figure 4].
|Figure 4: The paramecium and worm observed in the two-stage activated sludge pilot|
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Food-to-microorganism ratio variation
Variation of food to microorganism (F/M) ratio in A and B stages during the A/B process operation is depicted in [Figure 5]. The mean F/M in Tanks A and B was 0.62 ± 0.36 and 0.16 ± 0.2/day, respectively. As illustrated in [Figure 5], the steepness trend of F/M ratio as a function of SRT was observed in A and B stages.
|Figure 5: Effect of solid retention time enhancement on food-to–microorganism ratio|
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Adsorption/bio-oxidation process performance
In biological wastewater treatment, the organic compounds were qualified with BOD5 and COD as indicators. In this study, the performance of A/B process with respect to BOD5 was monitored, and BOD5 variation during the A/B process operation is presented in [Figure 6]. As shown in [Figure 6], during 180 days of operation, the tBOD5 concentration ranged from 221 ± 118 to 114 ± 42 mg/L and also from 114 ± 42 to 61 ± 35 mg/L in Stages A and B, respectively. The removal efficiency of tBOD5 in Stages A and B was 45% ± 15% and 46% ± 23%, respectively. In addition, the overall tBOD5 removal efficiency A/B process was 70% ± 20%. Similarly, the same trend was observed in case of sBOD5. The average of effluents sBOD5 in A and B stages were 103 ± 53, 45 ± 26, and 22 ± 13 mg/L, respectively. The average of sBOD5 removal efficiency in Tanks A and B and the overall A/B process was 53% ± 19%, 47% ± 20%, and 76% ± 13%, respectively.
|Figure 6: Pilot efficiency for tBOD5and sBOD5 removal. BOD: Biochemical oxygen demand|
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For the prediction of sludge-settling behavior, SVI was monitored. The SVI during the A/B process did not drastically change. The mean SVI in A and B stages was 62 ± 17 and 85 ± 30 mL/g, respectively. In order to calculate waste excess sludge during the A/B process, the quantification of suspended solids (SS) and volatile suspended solid (VSS) in returned AS is critical. During A/B process operation, the average of SS and VS in returned AS in Stage A was 16,000 ± 8900 and 10,635 ± 6200 mg/L and also in Stage B was 10,500 ± 7000 and 6700 ± 5200 mg/L, respectively. The VS/SS ratio is a vital parameter to evaluate AS quality. As monitored in the present study, the VS/SS ratio was 0.68 ± 0.12 and 0.66 ± 0.16 in A and B stages, respectively. Previous studies demonstrated that the VS/SS ratio for high-quality AS was higher than 0.75. To assess the nutrient removal of A/B process during long-term operation, the total Kjeldahl nitrogen (TKN) and ammonia were selected. Overall, the average of TKN and ammonia removal was 92% and 88%, respectively.
| Discussion|| |
For predators' growth enhancement and proliferation and also to reduce excess achieved sludge, the SRT was increased to much more than A/B process design criteria. As a result, the predators developed and Yobs decreased. The maximum excess sludge reduction in A and B stages of A/B process was 76% and 65%, respectively.
The operation of A/B process with higher SRT leads to lower Yobs, which means that sludge production is decreased., The obtained results showed that with increasing SRT, the predators' growth was intensified. Predators consume sludge flocs as food source and as a result, the dry mass of sludge is reduced and also a clarified effluent and a biomass full of protein are produced.
Elissen reported that the operation of a reactor with fixed media led to Lytechinusvariegatus growth promotion and found 75% of the total suspended solid (TSS) removal efficiency for influent wastewater with 4000 mg/L of TSS.
As depicted in [Figure 5], diminishing of the F/M was related to SRT increment. This behavior may be attributed to the accumulation of activated biomass in aeration tank and also the constant concentration of inlet substrate. When F/M ratio is <0.2, the bacteria's growth shifts to endogenous phase and leads to better settlement of sludge.
The average of SVI in A and B stages was 62 ± 17 and 85 ± 30 mL/g, respectively. The result of SVI monitoring reflected improvement of sludge sedimentation properties with increasing SRT and growth promotion of predators. The direct relationship between SVI and sludge dewaterability was demonstrated in a previous study, which indicates that better settlement rate leads to a better dewatering rate.,
The lower SVI in A stage presumably related to a small population of filamentous bacteria. In comparison to a conventional AS, the higher F/M ratios and lower DO concentration in A stage led to the act of aeration tank as a selector. As a result, the conditions are more suitable for floc forming organism growth than filamentous organisms.
| Conclusion|| |
In present study, the effect of predators' growth on biological excess sludge reduction of A/B process was studied. A/B process was consisted of two aeration and sedimentation tanks in series and feed with real municipal wastewater after screening and gritting. During A/B process operation, MLSS in aeration tanks was gradually increased and effluent wastewater subjected to DO, pH, VSS, MLSS, alkalinity, BOD5, COD, nutrient analysis. Based on the results, the following conclusion could be drowned.
- By increasing SRT from 0.6 to 56.8 d in stage A, the Y coefficient was decreased from 1.29 to 0.23 g VSS/g COD.
- In stage B, the SRT increasing from 1.6 to 123.8 d lead to 67% reduction of Y coefficient.
- During the A/B operation, overall tBOD5 and tCOD removal was 70% ± 20% and 57% ± 24%, respectively.
Financial support and sponsorship
The present publication has been made possible through the financial, technical, administrative, and logistic support from Isfahan province Water and Wastewater Company under the grant No. 500/92/7208.
Conflicts of interest
There are no conflicts of interest.
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