Oxic-settling-anaerobic Process Research Articles

Unravelling the impact of oxic-settling-anaerobic cycle implementation and solid retention time on sludge generation, membrane operation, and contaminant removal in membrane bioreactors

Sewage sludge production represents a critical issue nowadays. The oxic-settling-anaerobic (OSA) system is a promising strategy for reducing sludge production during biological wastewater treatment. However, the distinct effect of OSA process and solid retention time (SRT) on sludge production has not been clarified yet. Therefore, during this study, the impacts of OSA system implementation at different SRT on sludge generation, membrane operation, and contaminant removal were elucidated in two lab-scale membrane bioreactors working at 20 d (MBR20) and 60 d (MBR60). After OSA implementation, the MBR-OSA20 and MBR-OSA60 were operated at a hydraulic retention time in the sludge holding tank (HRTSHT) of 6–12 h and of 6–24 h, respectively. The estimated observed sludge yield (Yobs) showed a reduction of 38–48 % for the MBR-OSA20, but no beneficial effect was observed for MBR-OSA60 at 6 h of HRTSHT. At the HRTSHT of 24 h, Yobs decreased by 17 % due to hydrolysis and acidogenic biomass fermentation. OSA implementation and long SRT had no negative effects on organic carbon and ammonium removal (96(±3) % and 100 %, respectively) and improved total nitrogen (TN) reduction by 19–41 %. Membrane fouling and sludge filterability were not significantly affected, whereas sludge settleability worsened after the implementation of the OSA cycle. This activity provided additional insights about the distinct impact of the OSA process and SRT on sludge production and characteristics, confirming the OSA process as a valid solution for reducing sludge production under a moderate SRT, but revealing a negligible effect on Yobs under a prolonged SRT and a short HRTSHT.

Sludge reduction, nitrous oxide emissions, and phosphorus removal by oxic-settling-anaerobic (OSA) process: the effect of hydraulic retention time

This paper presents a study on reducing sewage sludge by an oxic-settling-anaerobic (OSA) pilot plant compared to the conventional activated sludge (CAS) process in view of resource recovery and moving towards plant carbon neutrality. The OSA plant was supplied with real wastewater and the anaerobic reactor was operated under two hydraulic retention times (HRT) (4 and 6 h). Greenhouse gas (GHG) emissions were monitored for the first time to determine the OSA process’s production mechanism. The results highlighted that under the lowest HRT (4 h), the removal efficiencies of COD and PO4−P, increased from 75 to 89% and from 39 to 50% for CAS and OSA configurations, respectively. The observed yield coefficient was reduced from 0.58 gTSS gCOD−1 (CAS period) to 0.31 gTSS gCOD−1 (OSA period). A remarkable deterioration of nitrification efficiency under OSA configuration was obtained from 79% (CAS) to 27% (OSA with HRT of 6 h). The huge deterioration of nitrification significantly affected the GHG emissions, with the N2O-N fraction increasing from 1% (CAS) to 1.55% (OSA 4 h HRT) and 3.54% (OSA 6 h HRT) of the overall effluent nitrogen, thus suggesting a relevant environmental implication due to the high global warming potential (GWP) of N2O.Graphical abstract

Reduction of sewage sludge and N2O emissions by an Oxic Settling Anaerobic (OSA) process: The case study of Corleone (Italy) wastewater treatment plant

Biosolid management is becoming one of the most crucial issues for wastewater treatment plant (WWTP) operators. The application of the Oxic Settling Anaerobic (OSA) process allows the minimisation of excess sludge production. This study compares conventional activated sludge (CAS) and OSA layouts in a full-scale WWTP (namely, Corleone - Italy). Extensive monitoring campaigns were conducted to assess treatment performances regarding carbon and nutrient removal, greenhouse gas (GHG) emissions, excess sludge production, and biomass activity (by means of respirometric analysis). Results showed that the effluent quality consistently met the Italian discharge limits. However, with the implementation of the OSA process, there was a decrease in ammonium removal efficiency, which could be attributed to reduced nitrifier activity related to reduced biomass production and extended anaerobic conditions affecting the nitrification process. On the other hand, the OSA configuration significantly increased phosphorus removal, indicating a high phosphorus content in the resulting waste sludge. A worsening of the sludge settling properties was observed with the OSA configuration likely due to decreased EPS concentrations. The sludge production in the OSA configuration decreased by 17.3 % compared to CAS. Nitrous-oxide measurements did not show a variation between CAS and OSA configurations, confirming that the OSA process can be a suitable solution for reducing WWTP's carbon footprint.

Influence of the Oxic-Settling-Anaerobic (OSA) Process on Methane Production by Anaerobic Digestion of Sewage Sludge

The present study evaluated different sludge-reduction mechanisms in the oxic-settling-anaerobic (OSA) process in terms of their effects on methane productivity by anaerobic digestion of sewage sludge. Two different layouts were investigated for the sludge return from an anaerobic side-stream reactor (ASSR) to the anoxic (scheme A) or the aerobic (scheme B) reactor of a pre-denitrification plant. Biochemical methane-potential (BMP) assays performed on the excess sludge revealed that scheme A promoted an overall increase of methane production in the OSA (20 mLCH4 gVSS−1d−1, +19%), although compared with a control CAS plant a significant decrease in the excess sludge production (31%) was obtained. Operating conditions in scheme A caused the occurrence of cell lysis and EPS hydrolysis, thereby increasing the biodegradability of sludge. In contrast, scheme B favoured the occurrence of uncoupling and a maintenance metabolism that did not involve sludge hydrolysis. Consequently, despite a higher reduction of excess sludge (82%), a significant decrease in methane productivity in the OSA (4 mLCH4 gVSS−1d−1, −41%) was observed. Based on the results, implementing the OSA process may allow high levels of methane production by anaerobic digestion to be maintained if specific sludge-reduction mechanisms are triggered in the waterline, also raising the possibility of co-digestion with other feedstocks.

Insights on mechanisms of excess sludge minimization in an oxic-settling-anaerobic process under different operating conditions and plant configurations

In the present research, insights about the mechanisms of excess sludge minimization occurring in an oxic-settling-anaerobic (OSA) were provided. The investigation involved two systems operating in parallel. In particular, a conventional activated sludge (CAS) system as control and a system implementing the OSA process both having a pre-denitrification scheme were considered. Five periods (P1–P5) were studied, during which several operating conditions and configurations were tested. Specifically, the hydraulic retention time (HRT) in the anaerobic reactor of the OSA system (P1 8 h, P2–P3 12 h, P4 8 h, P5 12 h) and the return sludge from the anaerobic to the anoxic (scheme A) (P1–P2) or aerobic (scheme B) mainstream reactors (P3–P5) were investigated. The results highlighted that the excess sludge production in the OSA was lower in all the configurations (12–41%). In more detail, the observed yield (Yobs) was reduced from 0.50-0.89 gTSS gCOD−1 (control) to 0.22 -0.34 gTSS gCOD−1 in the OSA process. The highest excess sludge reduction (40%) was achieved when the OSA was operated according to scheme B and HRT of 12 h in the anaerobic reactor (P3). Generally, scheme A enabled the establishment of cell lysis and extracellular polymeric substances (EPS) destructuration, leading to a worsening of process performances when high anaerobic HRT (>8 h) was imposed. In contrast, scheme B enabled the establishment of maintenance metabolism in addition to the uncoupling metabolism, while cell lysis and EPS destruction were minimized. This allowed obtaining higher sludge reduction yield without compromising the effluent quality.

Integrating electrochemical pretreatment (EPT) and side-stream sulfidogenesis with conventional activated sludge process: Performance, microbial community and sludge reduction mechanisms

• Coupling EPT and sulfidogenic side-stream reactor (ESSR) with CAS process was studied. • EPT at 15 V/5min with 125 mg S/L of SO 4 2− dosage improved sludge reduction. • The ESSR could be designed with a very short SRT of 2.5 days. • The integrated process (ESSR-CAS) was operated over 200 d with good performance. • Possible reasons for fast sludge hydrolysis in ESSR-CAS were discussed. Recently, inserting an anaerobic side-stream reactor into a conventional activated sludge (CAS) process for upgrading biological wastewater treatment plants with in-situ sludge reduction function has been extensively studied. A typical example is an oxic-settling anaerobic (OSA) process, which can achieve efficient sludge reduction, but requires ample space, limiting its application potential. Optimizing the conventional OSA process towards compact and energy-efficient is necessary. This paper studied a new method by integrating electrochemical pretreatment (EPT) and sulfidogenesis-enhanced anaerobic treatment with a conventional OSA process to achieve considerable in-situ sludge reduction with a limited side-stream size requirement (i.e. short sludge retention time (SRT)). The proposed idea was verified experimentally – first, batch experiments were conducted to determine the effective sulfate concentration for the EPT and sulfidogenic side-stream reactor (ESSR); then, the ESSR-CAS process (upgrading CAS with a new ESSR) and a stand-alone CAS process (control) were operated in parallel for 200 days. The experimental results showed that: The new ESSR, with approximately 125 mg S/L of sulfate dosed and 5 min electrochemical sludge pretreatment at 15 V, can successfully accelerate anaerobic reactions, resulting in a short SRT of 2.5 days. The ESSR-CAS process can reduce sludge production by 61%, with high soluble chemical oxygen demand (SCOD) and total nitrogen (TN) removal efficiencies of 96% and 70% respectively. The possible mechanisms of ESSR for the augment of sludge hydrolysis (sludge floc disintegration, extracellular polymer destruction, and cell structure breakage) as well as the economic considerations of this new sulfur-cycle bioprocess are discussed.

Assessment of sludge reduction and microbial dynamics in an OSA process with short anaerobic retention time

Abstract This study operated for 160 days and evaluated sludge minimization using an oxic-settling-anaerobic (OSA) system fed with real wastewater. At first, a pilot-scale conventional activated sludge (CAS) system was used to perform conventional wastewater treatment over a course of 60 days. Next, the CAS system was coupled with a sludge holding tank (SHT) in the sludge recirculation line to employ the OSA system, using a short 12h-hydraulic retention time (HRT) over a course of 100 days. The observed sludge yield ( Y obs ) dropped from 0.33 (CAS) to 0.19 (OSA) kg TSS kg−1 COD, ensuring a reduction of 43% of sludge in the OSA system. Better wastewater treatment performance was achieved using the OSA system (BOD5: 87%, TKN: 92%, NH 4 + -N: 94%) when compared with the CAS system (BOD5: 76%, TKN: 74%, NH 4 + -N: 78%), considering the organic matter and nitrogen removal rates. Moreover, the enrichment of anaerobic and hydrolyzing communities in the SHT (mainly Pedobacter and Acidovorax) enhanced the COD and nitrogen hydrolysis, which therefore improved the autotrophic metabolism in the aeration tank.

Combination of the OSA process with thermal treatment at moderate temperature for excess sludge minimization

This study investigated the chance to couple the conventional Oxic Settling Anaerobic (OSA) process with a thermic treatment at moderate temperature (35 °C). The maximum excess sludge reduction rate (80%) was achieved when the plant was operated under 3 h of hydraulic retention time (HRT). Compared with the conventional OSA system, the thermic treatment enabled a further improvement in excess sludge minimization of 35%. The observed yield coefficient decreased from 0.25 gTSS gCOD−1 to 0.10 gTSS gCOD−1 when the temperature in the anaerobic reactor was increased to 35 °C, despite the lower HRT (3 h vs 6 h). Moreover, the thermic treatment enabled the decrease of filamentous bacteria, thereby improving the sludge settling properties. The thermic treatment enhanced the destruction of extracellular polymeric substances and the increase of endogenous decay rate (from 0.64 d−1 to 1.16 d−1) that reduced the biomass active fraction (from 22% to 4%).

A new sulfidogenic oxic-settling anaerobic (SOSA) process: The effects of sulfur-cycle bioaugmentation on the operational performance, sludge properties and microbial communities

In-situ sludge reduction can be achieved by inserting an anaerobic side-stream reactor in the sludge return line of the conventional activated sludge (CAS) process. This modified oxic-settling-anaerobic (OSA) process can reduce sludge production by 30–50% through feast-fast alternating conditions. This paper proposes a new bioprocess called the sulfidogenic oxic-settling anaerobic (SOSA) process with OSA configuration and the addition of sulfate in side-stream reactor. The new bioprocess augments the conventional anaerobic/anoxic/aerobic feast-fast bioconversions with sulfur biochemical transformations (i.e. sulfate reduction and sulfur-oxidizing autotrophic denitrification). A lab-scale SOSA process was operated for 260 days in parallel with the anoxic/oxic (AO) CAS process and the conventional OSA process as control systems. Based on the experimental results, the feasibility of the new SOSA process was evaluated, and the effects of sulfur bioaugmentation on the effluent quality, sludge reduction, sludge physico-chemical properties and microbial communities were examined. The SOSA process i) removed 98% of the organics (chemical oxygen demand, COD) and 99% of the ammonia present with a lower observed sludge yield (0.204 g TSS/g CODremoved) than those of the OSA and AO processes (0.292 and 0.473 g TSS/g CODremoved respectively), ii) denitrified 18% and 6% more nitrogen to dinitrogen gas than did the CAS and OSA processes respectively, iii) produced sludge with improved settleability and dewaterability, iv) encouraged sludge decomposition with greater destruction of extracellular polymeric substances and v) enriched sulfur-cycle related and hydrolytic/fermentative bacteria. The possible mechanisms of sulfur augmentation and limitations of the present study are also discussed.

Incorporating the oxic‐settling‐anaerobic (OSA) process into an anoxic–oxic system for sewage sludge reduction and nutrient removal

The production of sludge in biological wastewater treatments is a growing problem worldwide as it entails economic, environmental, and health problems. The oxic‐settling‐anaerobic (OSA) process is one of the techniques that have been developed to reduce the sludge produced in the activated sludge process (ASP). This article reports the feasibility of the application of the OSA process in a nutrient removal system, specifically, in an anoxic–oxic (A) process. The results reveal TN removal in the A + OSA system of 68.84% and 69.91% (for 7 and 11 h of sludge anaerobic exposure time, SAET, respectively) versus 38.02% and 32.47% in the OSA system with similar SAET. The percentage sludge reductions obtained were, respectively, 22.41% and 31.93% in the OSA system and 23.53% and 32.49% in the A + OSA system compared to the sludge produced in the conventional ASP. The study confirms the suitability of the application of the OSA process in a nutrient removal system, as nutrient removal performances are not affected, while at the same time achieving higher sludge reduction. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 1068–1074, 2018

Behavior of activated sludge systems with an active heterotrophic biomass inflow – a novel perspective for sludge minimization

AbstractBACKGROUNDThis study evaluated the microbial behavior of activated sludge systems operated with an active heterotrophic biomass inflow. The evaluation focused on the possibility of sludge minimization. Thus, related process stoichiometry was derived on the basis of mass balance for essential parameters.RESULTSStoichiometric relationships showed that the reactor holds a larger amount of biomass without the controlling impact of the sludge age, which decreases the magnitude of microbial growth compared with endogenous decay. Consequently, the observed yield, YOBS becomes much lower than what may be observed in conventional activated sludge systems. This stoichiometry exactly defines the microbial basis of the OSA (oxic/settling/anaerobic) system and identifies the interchange stream connecting the anaerobic reactor to the aerobic main reactor as the source of the active biomass inflow. Therefore, biomass inflow, largely overlooked in related studies, offers a novel microbial perspective for the merit of the OSA process in minimizing sludge generation.CONCLUSIONThe impact of lower YOBS, while explaining lower active biomass production, cannot be extrapolated to cover inert particulate COD fractions, namely a similar reduction in the initial inert COD and the residual particulate microbial products. Future studies should be directed to explore the fate of residual particulate compounds. © 2017 Society of Chemical Industry

Effect of aeration rates on the performance of an OSA-based sludge reduction process: limitations and implications

The oxic-settling-anaerobic (OSA) process can decrease the amount of sludge generated during wastewater treatment. In this study, the effect of aeration rates on the performance of an OSA-based sludge reduction process was investigated. Aeration rates did not significantly impact on sludge reduction in the main reactor (44.5% reduction at the lower aeration versus 45.4% reduction at the higher aeration rate). Integrating a side reactor for hydrolysis/acidification did not affect effluent quality, but resulted in slightly greater dominance of Proteobacteria and Dechloromona in the main wastewater treatment reactor, especially under the low aeration. Recycling anaerobic hydrolysate improved denitrification, while nitrification was unaffected. However, recycling the anaerobic hydrolysate decreased sludge settle ability under the low aeration. These results have implications for operating and optimizing an OSA process for sludge reduction.

Evaluation of a biological wastewater treatment system combining an OSA process with ultrasound for sludge reduction

Sludge production is an undesirable by-product of biological wastewater treatment. The oxic-settling-anaerobic (OSA) process constitutes one of the most promising techniques for reducing the sludge produced at the treatment plant without negative consequences for its overall performance. In the present study, the OSA process is applied in combination with ultrasound treatment, a lysis technique, in a lab-scale wastewater treatment plant to assess whether sludge reduction is enhanced as a result of mechanical treatment. Reported sludge reductions of 45.72% and 78.56% were obtained for the two regimes of combined treatment tested in this study during two respective stages: UO1 and UO2. During the UO1 stage, the general performance and nutrient removal improved, obtaining 47.28% TN removal versus 21.95% in the conventional stage. However, the performance of the system was seriously damaged during the UO2 stage. Increases in dehydrogenase and protease activities were observed during both stages. The advantages of the combined process are not necessarily economic, but operational, as US treatment acts as contributing factor in the OSA process, inducing mechanisms that lead to sludge reduction in the OSA process and improving performance parameters.

The investigation of the sludge reduction efficiency and mechanisms in oxic-settling-anaerobic (OSA) process.

This paper aims to provide a full understanding of the sludge reduction mechanisms in the oxic-settling-anaerobic (OSA) process and presents an evaluation of the sludge reduction efficiencies and sludge characteristics in this process compared to the conventional activated sludge process. Fifty-eight percent reduction in observed yield in the OSA process was achieved compared to the control system at the end of the operational period with no deterioration of effluent quality. The settleability of sludge in the OSA process was also found to be better than that of the control system in terms of sludge volume index. In long-term operation, capillary suction time and specific resistance to filtration values confirmed that the OSA process showed good filterability characteristics. The results of batch experiments showed that higher endogenous respiration in the systems might lead to lower sludge production and that energy uncoupling had only a limited impact on sludge reduction.

Study of the diversity of microbial communities in a sequencing batch reactor oxic-settling-anaerobic process and its modified process.

To further reveal the mechanism of sludge reduction in the oxic-settling-anaerobic (OSA) process, the polymerase chain reaction - denaturing gradient gel electrophoresis protocol was used to study the possible difference in the microbial communities between a sequencing batch reactor (SBR)-OSA process and its modified process, by analyzing the change in the diversity of the microbial communities in each reactor of both systems. The results indicated that the structure of the microbial communities in aerobic reactors of the 2 processes was very different, but the predominant microbial populations in anaerobic reactors were similar. The predominant microbial population in the aerobic reactor of the SBR-OSA belonged to Burkholderia cepacia, class Betaproteobacteria, while those of the modified process belonged to the classes Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. These 3 types of microbes had a cryptic growth characteristic, which was the main cause of a greater sludge reduction efficiency achieved by the modified process.

OSA 공정의 세포 내 ATP, NAD(H), NADP(H) 농도

OSA (oxic-settling-anaerobic)공정은 종래 활성슬러지법(conventional activated sludge)의 잉여슬러지 감량을 목적으로 슬러지 반송라인에 혐기조가 추가된 공정이다. 본 연구에서는 호기조(1)-혐기조-호기조(2)가 순차적으로 구성된 OSA 모의공정의 슬러지 감량원리를 조사하기 위해 각 반응조의 세포 내 에너지 전달물질(ATP, NAD(H), NADP(H)) 변화를 관찰하였다. 시간이 경과함에 따라 호기조 및 혐기조 모두 에너지전달물질이 급격히 감소하였다. 혐기조의 경우 호기조(1)과 (2)보다 낮은 에너지를 함유하는 것으로 나타났으며, 혐기조를 거친 호기조(2)의 경우 호기조(1) 보다 에너지 전달물질이 낮은 수준으로 관찰되었다. 또한, OSA 공정에서 내생호흡을 유도하여 슬러지를 감량하는 혐기조의 화학적 산소요구량 (SCOD), 체류시간 및 온도변화에 따른 호기조(1)과 (2)의 에너지량 차이를 확인한 결과 낮은 농도의 SCOD, 긴 체류시간 및 높은 온도는 호기조(2)의 세포 내 에너지량을 감소시켰다. 혐기조 및 호기조(2)의 세포 내 에너지 수준은 각각 호기조(1)의 57.73% 및 39.12% 수준으로, 두 단계의 호기조 사이에 추가된 혐기조는 OSA 공정의 세포 내 에너지 수준을 낮출 뿐 아니라, CAS 공정보다 적은 양의 슬러지가 생산하였다. 본 연구결과를 토대로, 호기조 사이에 추가된 혐기조의 운전조건에 따라 각 반응조의 세포 내 에너지 수준의 조절뿐만 아니라 잉여슬러지의 생성을 제어할 수 있을 것이라 생각된다. In order to investigate why OSA (oxic-settling-anaerobic) process produces less sludge than CAS (conventional activated sludge) process, sequential cultivation through 1^st aerobic-anaerobic-2^nd aerobic conditions, were carried out. Then, the intracellular concentrations of adenosine triphosphate (ATP), nicotinamide adenine dinucleotide (NAD and NADH), and nicotinamide adenine dinucleotide phosphate (NADP and NADPH) were monitored for these three stages. Results showed that the concentrations of these energy substances rapidly decreased through time in both aerobic and anaerobic conditions but the anaerobic culture contained the lower energy level than aerobic culture. The 2^nd aerobic culture that experienced anaerobic condition showed lower concentration of these energy substances than those of the 1^st aerobic culture. Meanwhile, the anaerobic culture corresponding to the sludge holding stage of OSA was subjected to different soluble chemical oxygen demand (SCOD) levels, detention time, and temperature to evaluate the effects of these variations on the energy level difference between the 1^st and 2^nd aerobic stages. The lower the SCOD concentration, the longer detention time; and the higher temperature in the anaerobic stage tended to further reduce the intracellular level of the 2^nd aerobic culture. On the average, the intracellular energy level of the anaerobic and 2^nd aerobic stage were 57.73% and 39.12% of the 1st aerobic culture, respectively. These indicated that the insertion of an anaerobic stage between two aerobic stages could lower the intracellular energy levels, hence the lower the sludge in OSA than CAS process. Moreover, manipulation of the operating conditions of the intervening anaerobic stage can change intracellular energy levels thereby controlling sludge production.

Specific oxygen uptake rate gradient – Another possible cause of excess sludge reduction in oxic-settling-anaerobic (OSA) process

Oxic-settling-anaerobic process (OSA) has established that anaerobic followed by aerobic process in a cyclic reaction reduces the biological sludge growth. However, the mechanism behind the process of reduction is still undefined. The recirculation of aerobic sludge from a sequencing batch reactor (SBR) to an anaerobic side reactor at the rate of 3.8–9.8gVSSrecycled/gVSS resulted in sludge reduction from 3% to 51% (Average=14.6–39.8%) in comparison to absence of OSA process in conventional SBR. The observed sludge reduction was in proportion to specific oxygen uptake rate (SOUR) gradient between sludge recycle and no recycle condition. The SOUR gradient is the ratio of its difference between fasting when sludge came out of anaerobic reactor and feasting conditions during aeration, in comparison to SOUR difference between aeration and termination of settling in the absence of any sludge recycle. SOUR kept on increasing with increasing sludge recycle during feasting (in aeration) in comparison to fasting (in anaerobic). This gradient resulted from high substrate oxidation during aeration (feasting), which may be induced by increased energy uptake stimulated by prolonged fasting. Increased cell maintenance also validated the effect of SOUR gradient in direct proportion.

Changes in enzymatic and microbiological activity during adaptation of a conventional activated sludge (CAS) to a CAS-oxic-settling-anaerobic (OSA) adapted process

The production of excess sludge in the conventional activated sludge (CAS) process is an important aspect of the operation of wastewater treatment plants. The oxic-settling-anaerobic (OSA) process is one of the most promising strategies among those to achieve a reduction in the excess sludge produced. Cell decay occurs at a low oxidation–reduction potential and subsequent degradation reactions seem to be the major causes of sludge reduction in the OSA process. As a consequence of previous exposure of the sludge to anaerobic conditions in the sludge holding tank, an increase of enzymatic and microbiological activity in the aeration reactor could enhance the degradation of the released material. In this study, the results of a pilot plant running sequentially as a CAS and a CAS-OSA system are shown not only in the terms of the crucial aspect of excess sludge reduction, but also in terms of enzymatic and microbiological activities in the sludge from the aeration tank. Specifically, the application of the OSA strategy with a sludge anaerobic exposure time (SAET) of 5.5 h achieved a 33.6% reduction of the produced sludge in comparison with the CAS approach. Similarly, this system showed an average increase of 23.2, 22.6 and 7.59% in the specific oxygen uptake rate (SOUR), dehydrogenase, and protease activity, respectively, and a negligible change in glucosidase activity compared to the conventional process. At the same way, the OSA process with a SAET of 8 h led to a reduction of 38.9% of produced sludge and an average increase of 27.9, 11.31, 28.7 and 26.57% in SOUR, dehydrogenase, protease, and glucosidase activity, respectively, compared to control unit.

Study of the sludge reduction in an oxic-settling-anaerobic activated sludge process based on UNITANK.

An oxic-settling-anaerobic process (OSA) can effectively reduce sludge production, but most of the research studies on the OSA process have been either under laboratory test conditions or based on synthetic wastewater, which cannot fully reflect the performance and sludge reduction efficiency in existing OSA process. Thus, aiming at examining the sludge reduction efficiency and the stability of the OSA process, UNITANK and UNITANK-OSA processes were performed in a 120 m(3)/d pilot-scale system using actual sewage. The results indicate that UNITANK-OSA achieved a 48% reduction of the sludge compared to the reduction due to UNITANK, not considering the accumulation of the effluent-suspended solids. The effluent quality was not found to change significantly, except that the total phosphorus concentration increased slightly. The extracellular polymeric substances metal floc theory may, to some extent, explain this reduction in this study. The OSA process could be used to reform the classic wastewater treatment process to get lower sludge mass.

Effects of oxidation reduction potential in the bypass micro-aerobic sludge zone on sludge reduction for a modified oxic–settling–anaerobic process

Batch experiments were conducted to determine the effect of oxidation reduction potential (ORP) on sludge reduction in a bypass micro-aerobic sludge reduction system. The system was composed of a modified oxic-settling-anaerobic process with a sludge holding tank in the sludge recycle loop. The ORPs in the micro-aerobic tanks were set at approximately +350, -90, -150, -200 and -250 mV, by varying the length of aeration time for the tanks. The results show that lower ORP result in greater sludge volume reduction, and the sludge production was reduced by 60% at the lowest ORP. In addition, low ORP caused extracellular polymer substances dissociation and slightly reduced sludge activity. Comparing the sludge backflow characteristics of the micro-aerobic tank's ORP controlled at -250 mV with that of +350 mV, the average soluble chemical oxygen (SCOD), TN and TP increased by 7, 0.4 and 2 times, median particle diameter decreased by 8.5 μm and the specific oxygen uptake rate (SOUR) decreased by 0.0043 milligram O2 per gram suspended solids per minute. For the effluent, SCOD and TN and TP fluctuated around 30, 8.7 and 0.66 mg/L, respectively. Therefore, the effective assignment of ORP in the micro-aerobic tank can remarkably reduce sludge volume and does not affect final effluent quality.