Total Phosphorous Removal Research Articles

Comparative Study of the Treatment Performance and Microbial Diversity of an Innovative Upflow Microaerobic Sludge Bed Bioreactor and an Aerobic Control System

The treatment performance and microbial diversity of two parallel single-stage activated sludge bioreactors were investigated and compared. An innovative upflow microaerobic sludge bed (UMSB) bioreactor and an aerobic control system (ACS) were operated at dissolved oxygen levels of 0.5–0.9 and 2.5–4 mg/L, respectively. Under similar conditions, at a hydraulic retention time of 25 h, an organic loading rate of 1.4±0.1 kg COD/m3·d, and an influent total phosphate (TP) of 43±4 mg/L, the chemical oxygen demand (COD) removal efficiencies of the UMSB and the ACS were 94%±1% and 95%±1%, respectively, and no significant difference in performance was noted between the two systems (p=0.498). It was, however, noted that TP removal efficiency of the UMSB (57%±5%) was significantly higher compared with ACS (34%±6%) (p=0.00). Phylogenetic analysis indicated that bacteria in the UMSB and ACS were highly diverse and obviously different. It is likely that facultative anaerobes, microaerophiles, and aerobes were able to coexist in the UMSB. This suggests the possibility of constructing microecosystems that integrate anaerobic and aerobic niches in one reactor. In this study, we demonstrate that under oxygen-limited conditions, it is possible to construct a single-stage activated sludge bioreactor for removing organic substances and phosphorous simultaneously. These findings have important consequences for recognizing and supplementing the enhanced biological phosphorus removal process and for developing new energy-saving treatment technologies.

Performance of simultaneous organic and nutrient removal in a pilot scale anaerobic–anoxic–oxic membrane bioreactor system treating municipal wastewater with a high nutrient mass ratio

A pilot-scale anaerobic–anoxic–oxic membrane bioreactor (A2O-MBR) was applied to treat Shiraz municipal wastewater with high ratio of total phosphorous (TP): total nitrogen (TN): chemical oxygen demand (COD) (3.4: 21.4: 100) and remarkable nitrate concentration. Various operating conditions were tested to investigate nutrient and organic matter removal as well as membrane capacity and biomass development. The pilot system was operated at a constant sludge retention time of 60 d. During different runs, the biomass concentration increased with a reduction of HRT while the F/M ratio was stable around an average of 0.087 gCOD g−1MLSS d−1. Results also showed that the treatment performance of A2O-MBR system was extremely stable and was not affected by aerobic hydraulic retention time (HRT) changes (12, 10, 8, 6 h). COD reduction reached over 95% and nitrification was completed. The maximum efficiency of TN and TP removal was 86.2% and 55.9%, respectively, which occurred with an overall HRT of 12 h, external and internal mixed liquor recycle rate of 200%. Membrane capacity measurements showed an average flux of 5.5 L m−2h−1 with a mean transmembrane pressure of 30 kPa under periodical air backwashing without any chemical cleaning over three months.

Evaluation of Sequential Batch Reactor (SBR) Cycle Design to Observe the Advantages of Selector Phase Biology to Achieve Maximum Nutrient Removal

Pilot plant studies of sequential batch reactor (SBR) cycles revealed the role of selector phase biology (SPB) involved in COD removal, nitrification, denitrification and total phosphate removal. The phases namely anoxic, anaerobic and aerobic in single tank reactor design of SBR were showed effective biological nutrient removal (BNR) > 90% on average in 4 h designed cycle time with 12 h of hydraulic retention time (HRT). The phases were 2 h fill which includes 1 h simultaneous fill and aeration, 1 h settle and 1 h decant. This cycle design found effective to create selective environment for microbes to carry biological organic and nutrient removal reactions with improvement in sludge volume index (SVI).

Cultivation of Chlorella sp. using raw dairy wastewater for nutrient removal and biodiesel production: Characteristics comparison of indoor bench-scale and outdoor pilot-scale cultures

The biomass productivity and nutrient removal capacity of simultaneous Chlorella sp. cultivation for biodiesel production and nutrient removal in raw dairy wastewater (RDW) in indoor bench-scale and outdoor pilot-scale photobioreactors were compared. Results from the current work show that maximum biomass productivity in indoor bench-scale cultures can reach 260mgL−1day−1, compared to that of 110mgL−1day−1 in outdoor pilot-scale cultures. Maximum chemical oxygen demand (COD), total nitrogen (TN), and total phosphorous (TP) removal rate obtained in indoor conditions was 88.38, 38.34, and 2.03mgL−1day−1, respectively, this compared to 41.31, 6.58, and 2.74mgL−1day−1, respectively, for outdoor conditions. Finally, dominant fatty acids determined to be C16/C18 in outdoor pilot-scale cultures indicated great potential for scale up of Chlorella sp. cultivation in RDW for high quality biodiesel production coupling with RDW treatment.

Nutrient and greenhouse gas dynamics through a range of wastewater-loaded carbonate sand treatments

Most countries in the South Pacific rely on traditional soak pits which transfers wastewater down through the unsaturated carbonate sand sediment profile with little nutrient attenuation. With surface nutrients having an almost direct path to groundwater, this may have consequences for the sustainability of subsurface water reserves. This study examined the long-term removal and production of wastewater nutrients (including greenhouse gases) in tropical sediments and materials under different saturation regimes. The study showed that unsaturated carbonate sands alone are not an effective means of reducing nutrient loads to groundwater. There was no significant difference in removal rates of total dissolved nitrogen (TN) and phosphorus (TP) in unsaturated carbonate sand columns and columns amended with coconut husk and basalt sediment. Doubling of the effluent flow rate resulted in no significant decrease in the TN removal rate, but did halve the TP removal rate. The addition of a biochar filter to the unsaturated carbonate sand columns increased TN removal from ∼8% to ∼42%. The TN removal rate in the wastewater-saturated treatments (662.1±49.4mgm−3day−1) was approximately six times higher than the unsaturated treatments (110.2±27.2mgm−3day−1). TN removal in the fully saturated columns increased from 63% to 95% after the addition of an external carbon source (glucose). Higher nitrous oxide (N2O) and methane (CH4) concentrations in partially saturated columns and at the surface of the fully saturated columns were likely due to partial coupled nitrifier denitrification. The addition of basalt sediments to carbonate sands increased the total phosphorous removal from 72% to 95%. Sampling of groundwater below an in situ wastewater treatment system showed an almost complete removal of TN and TP within two meters of the aquifer surface, even with limited bioavailable carbon to facilitate denitrification. Modelling of NO3− and N2 concentrations for an idealized treatment system indicated reduced NO3− concentrations may be due to a combination of dilution with high volumes of groundwater (∼45% of removal at the farthest sampling point from the system) and denitrification.

Development of anaerobic osmotic membrane bioreactor for low-strength wastewater treatment at mesophilic condition

A novel anaerobic osmotic membrane bioreactor (AnOMBR) was developed for treating low-strength wastewater. The AnOMBR utilizes a forward osmosis (FO) membrane to retain influent organic waste, which facilitates anaerobic wastewater treatment and energy recovery in the form of methane gas. The feasibility of AnOMBR for treating low-strength wastewater at mesophilic temperature was evaluated and membrane fouling was investigated. Permeate flux declined under the combined effects of both salt accumulation and membrane fouling. Although flux reduction was dominated by the effect of salt accumulation in the reactor, the presence of organic fouling and inorganic scaling could be clearly identified. Bulk pH could be maintained within neutral to slightly alkaline due to the retention of alkalinity by the FO membrane. The AnOMBR shows good and stable removal of soluble chemical oxygen demand (sCOD) and nearly complete removal of total phosphorous. However, only partial removal of total nitrogen and ammonia was observed. The elevated salt environment appeared to have little effect on bioactivity of methanogens, and stable methane production of 0.3L/g sCOD digested was obtained.

Performance evaluation and modeling of a submerged membrane bioreactor treating combined municipal and industrial wastewater using radial basis function artificial neural networks.

Treatment process models are efficient tools to assure proper operation and better control of wastewater treatment systems. The current research was an effort to evaluate performance of a submerged membrane bioreactor (SMBR) treating combined municipal and industrial wastewater and to simulate effluent quality parameters of the SMBR using a radial basis function artificial neural network (RBFANN). The results showed that the treatment efficiencies increase and hydraulic retention time (HRT) decreases for combined wastewater compared with municipal and industrial wastewaters. The BOD, COD, {mathrm{NH}}_4^{+}-mathrm{N} and total phosphorous (TP) removal efficiencies for combined wastewater at HRT of 7 hours were 96.9%, 96%, 96.7% and 92%, respectively. As desirable criteria for treating wastewater, the TBOD/TP ratio increased, the BOD and COD concentrations decreased to 700 and 1000 mg/L, respectively and the BOD/COD ratio was about 0.5 for combined wastewater. The training procedures of the RBFANN models were successful for all predicted components. The train and test models showed an almost perfect match between the experimental and predicted values of effluent BOD, COD, {mathrm{NH}}_4^{+}-mathrm{N} and TP. The coefficient of determination (R2) values were higher than 0.98 and root mean squared error (RMSE) values did not exceed 7% for train and test models.

Nutrient and organics removal from swine slurry with simultaneous electricity generation in an alum sludge-based constructed wetland incorporating microbial fuel cell technology

This study investigates the ability of four alum sludge-based constructed wetlands, incorporating microbial fuel cell technology, to achieve high organic and nutrient removal from swine slurry while simultaneously producing electricity. As a cross-comparison the effects of electrode spacing and flow pattern are investigated. By providing a simultaneous upflow–downflow regime the maximum power density is boosted by 70% to 0.268W/m3 and an ammonium removal efficiency of 75% is achieved. However, the COD removal efficiency falls to 64% compared with 80%, 79% and 81% achieved by operating with a continuous upflow regime in the other three systems of the study. The alum sludge wetland medium showed an enhanced capacity to immobilise phosphorous with total phosphorous and reactive phosphorous removal rates of 85–86% and 89–90%, respectively. Accordingly, multi-stage hybrid CW-MFC systems and alternative operational strategies are discussed and recommended for full nutrient and organic removal.

The contribution of plant uptake to nutrient removal by floating treatment wetlands

Floating treatment wetlands (FTWs) may provide an appealing alternative to the more conventional (sub) surface flow wetlands to solve problems associated with eutrophication in urban surface waters, because they do not claim additional land area. This study examined the contribution of plant uptake to overall removal capacity of FTWs. A batch mesocosm experiment was performed during the growing season using thirty 84L polyethylene tanks covered with 0.28m2 floating Styrofoam mats. Ten tanks served as a control (only Styrofoam cover), 10 tanks were planted with Iris pseudacorus, and 10 with Typha angustifolia. Nutrients were added weekly to keep total nitrogen (TN) and total phosphorous (TP) concentrations at approximately 4mgNL−1 and 0.25mgPL−1. Total removal of TN an TP from the treatment with Typha was relatively low, resulting from the limited increase in plant biomass during the experiment. Total removal of TN and TP from the tanks planted with Iris was 277mgNm−2d−1 and 9.32mgPm−2d−1 during the experiment. These values were significantly higher than the values for total removal from the control tanks, i.e., 54 times higher for TN removal and 10 times higher for TP removal. Plant uptake played a major role in the removal of nitrogen and phosphorous from the water by FTWs, i.e., 74% of TN removal and 60% of TP removal resulted from Iris uptake. These results suggest that FTWs planted with Iris can be applied in a temperate climate to overcome problems with excessive algae growth in surface waters.

Biodegradation of bisphenol A by the freshwater microalgae Chlamydomonas mexicana and Chlorella vulgaris

The endocrine-disrupting chemical, bisphenol A (BPA) has attracted much attention due to its estrogenic activity and widespread environmental distribution. The toxicity and cellular stresses of BPA to Chlamydomonas mexicana and Chlorella vulgaris and its biodegradation/bioaccumulation by both microalgae were investigated. The 120-h EC50 of BPA for C. mexicana and C. vulgaris were 44.8 and 39.8mgL−1, respectively. The dry cell weight and chlorophyll a content of both microalgae decreased with increasing BPA concentration higher than 10mgL−1. Growth of C. vulgaris was significantly inhibited at 50mgL−1 BPA compared to C. mexicana. Total nitrogen (TN) and total phosphorous (TP) removal was higher in C. mexicana than in C. vulgaris. Microalgae performed the bioaccumulation and biodegradation of BPA to varying extents at different initial BPA concentrations. The highest rates of BPA biodegradation, 24 and 23% by C. mexicana and C. vulgaris, respectively, were achieved at 1mgL−1 BPA. Both the total fatty acid and carbohydrate contents increased with increasing BPA concentration. This study demonstrated that C. mexicana was more tolerant to BPA and could be used for treatment of BPA contaminated aqueous systems.

The effect of vermifiltration height and wet:dry time ratio on nutrient removal performance and biological features, and their influence on nutrient removal efficiencies

We investigated the effects of vermifiltration (VF) height and the wet:dry time (WD) ratio on nutrient removal from synthetic domestic wastewater, earthworm population characteristics, and microbial numbers in the substrate. We also evaluated the key factors influencing nutrient removal. Results showed that variation in VF height had a significant effect on chemical oxygen demand (COD) and total phosphate (TP) removal rates, earthworm population, and actinomycetes numbers, but had no effect on ammonia nitrogen (NH3-N) and total nitrogen (TN) removal rates, and bacterial and fungi numbers. All parameters, except TP removal and fungi numbers, were significantly affected by variation in the WD ratio. Furthermore, when combined, the VF height and the WD ratio significantly influenced the earthworm reproduction rate and actinomycetes numbers. In particular, good nutrient removal efficiencies, vigorous earthworm activity and high microbial numbers were observed when the WD ratio was 1/3 and the VF height was 60cm. Results suggest that earthworm growth and reproduction rates, fungi, and actinomycetes numbers are good indicators of the COD and TP removal efficiencies by VF. These factors, along with the bacterial numbers, are good indicators of NH3-N removal efficiency; however, only the earthworm growth rate and actinomycetes numbers are useful indicators for the TN removal efficiency from wastewater by VF.

Performance and Microbial Community of Aerobic Granular Sludge Bioreactor

Aerobic granular sludge was cultivated from activated-sludge in sequencing batch reactor. The change of physical properties and the effect of treatment of organic substance and microbial community were studied in the process of the different influent organic loading rate. The results showed that the formation process of aerobic granulation was rapid but the granular sludge was not stable and existed a disintegration-reunion dynamic balance system. The value of MLSS descended from 5.12g/L to 1.03g/L. The removal efficiency of NH4+-N decreased to 74.17% and total phosphorous (TP) removal efficiency maintained beyond 90% all the while. The CODCr removal was over 85% which higher than that of after adding in methanol, then reduced but had trend of rise to 31.89% at last. Microbial species in granulation were speculated by detecting polyhydroxyalkan- oates between granular sludge and activated sludge in the reactor. The results showed that microbial species of activated sludge are more diverse.

Nutrient removal (nitrogen and phosphorous) in secondary effluent from a wastewater treatment plant by microalgae

Microalgae as a feasible option to remove nutrients (phosphorous and nitrogen) from domestic wastewater treatment plant discharge is demonstrated. Laboratory-scale experiments are described, characterizing nutrient removal of total phosphorous and ammonia by three cultured microalgae strains: Chlorella vulgaris, Spirulina maxima, and mixed cultures of naturally growing algae found in wastewater from the Collingwood Wastewater Treatment Plant in Ontario, Canada containing Synechocystis sp. (dominant), Chlorella sp. (common), and a few cells of Scenedesmus sp. Removal of phosphates strongly positively relates to solution pH. Volatilization of ammonia due to increase in pH is not a dominant contributor to overall removal efficiency. Total phosphorous removal rates reached 95.8% and 90.4% for untreated and autoclaved secondary effluent, respectively. Ammonia removal rates reached 94.6% and 86.2% for untreated and autoclaved secondary effluent, respectively. These results demonstrate that use of microalgae represents a sustainable approach to improve removal efficiencies of nutrients in wastewater treatment.

Production of PHA from Cassava Starch Wastewater in Sequencing Batch Reactor Treatment System

The project attempt to investigate the potential of cassava starch wastewater for producing polyhydroxyalkanoate (PHA) from sequencing batch reactor (SBR) treatment system seeded with Bacillus tequilensis MSU 112, a PHA-producing bacterial strain. A total sequence of 24h with a reaction phase (22h) comprised anoxic/aerobic steps of 4/18h was employed. The effect of varies chemical oxygen demand (COD) concentration of the synthetic cassava starch wastewater (CSW) (3,000 4,000 and 5,000mg/L) on PHA production and treatment efficiency was determined. The CSW with COD at a concentration of 4,000mg/L produced the highest PHA concentration, PHA yield, and total kjeldahl nitrogen and total phosphate removal efficiencies at 3,346mg/L, 79.2% as dry sludge weight, 20.6% and 27.7%, respectively, while the CSW with COD concentration of 5,000mg/L provided the highest COD removal efficiency at 94.8%. In addition, the activated sludges obtained from an anoxic phase provided higher PHA production than an aerobic phase. The results revealed that the SBR system treating CSW with the presence of B. tequilensis MSU 112 offers a promising approach for PHA production.

On-farm treatment of swine manure based on solid–liquid separation and biological nitrification–denitrification of the liquid fraction

In some regions, intensive pig farming has led to soil and water pollution due to the over-application of manure as an organic fertilizer, thereby necessitating alternative treatment technologies to help manage the large amounts of manure generated. The present study seeks to determine the effectiveness of an on-farm swine manure treatment plant consisting of a solid–liquid separation phase using screw pressing followed by a coagulation–flocculation process, and nitrification–denitrification of the liquid fraction. Each treatment unit was evaluated for its contribution towards reducing the raw manure concentration of solids, organic matter, nutrients (nitrogen and phosphorous), metals, and pathogens. The overall system presented high removal efficiencies of up to 71% of TS (total solids) and 97% of TCOD (total chemical oxygen demand). Approximately 97% TKN (total Kjeldahl nitrogen) and 89% TP (total phosphorous) removal was achieved. Metals (copper and zinc) diminished in the liquid fraction to non-detectable concentrations (<1.0 mg L−1). As regards microbial removal, total concentration reductions of 3.6 log10 for Escherichia coli and 1.8 log10 for Salmonella were achieved. Finally, the system was evaluated from a financial standpoint. Results indicate that screw pressing and coagulation–flocculation for solid–liquid separation and nitrification–denitrification of the liquid fraction is a technological alternative for reducing the environmental impact of intensive pig farming in a given area.

English

The treatment performance of two parallel continuous single stage activated sludge bioreactor: an aerobic bioreactor (ABR) operated at dissolved oxygen (DO) levels of 3-4 mg/L, and an innovative micro-aerobic bioreactor (MABR) operated at DO levels of 0.4-0.8 mg/L and the microbial diversity of MABR was investigated and compared. Under the similar conditions, at a hydraulic time (HRT) of 25 h, an organic loading rate (OLR) of 3.4&plusmn;0.1 kg COD /m3&middot;d, the influent total phosphate (TP) was 44&plusmn;2 mg/L. The COD removal efficiencies of the MABR and the ABR were 93&plusmn;3 and 90&plusmn;3%, respectively, and no significant difference in performance was noted between the two systems (P=0.087&gt;0.05). It was, however, noted that TP removal efficiency of the MABR (62&plusmn;4%) was significantly higher than that of ABR (43&plusmn;2%) (P=0.00). Phylogenetic analysis indicated that bacteria in the MABR was highly diverse. It is likely that facultative anaerobes, microaerophile and aerobes were able to coexist in the MABR. These findings might be helpful in developing an economic treatment system, which can be a better alternative for treatment of many pollutants requiring aerobic/anaerobic sequential treatment. &nbsp; Key words: Biodiversity, micro-aerobic bioreactor, phosphorous removal, phylogenetic analysis.

Treatment of reverse osmosis concentrate by biological aerated filter

The aim of this study is to employ the biological aerated filter (BAF) in the treatment of reverse osmosis (RO) concentrate received from reuse of wastewater treatment plant. BAF is known as economic and efficient treatment method for the effluent standards. The result of the treatment of RO concentrate by BAF process was found to be efficient for biological oxygen demand (BOD) removal (95.86%). However, it was relatively less efficient in the chemical oxygen demand (COD) (88.95%) and suspended solids (SS) (81.12%) removal. A high BOD removal efficiency is due to the complete oxidation of organic matter which comes from low food to micro-organisms (F/M) ratio (0.049 kg BOD/kg MLSS day) of the influent. The TN (total nitrogen) and NO3-N removal percent were found to be 81.42% and 76.70%, respectively. However, total phosphorous (TP) and PO4-P removal percent were obtained low with 67.66 and 61.42%, respectively. It is observed that decreasing the COD/N ratio caused to decrease the TP and PO4-P removal efficiency. However, the denitrification and nitrification rates were increased from 211.8 to 301.0 mg/L day and 87.7 to 109.4 mg/L day, respectively, for a change in COD/N ratio from 8.19 to 7.64. Therefore, in order to reuse the RO concentrate, BAF process could effectively treat the RO concentrate.

Potential of water hyacinth for phytoremediation in low temperature environment

Water hyacinth, i.e., Eichhornia crassipes Solms can remove a large variety of different pollutants from water and are considered an excellent candidate for phytoremediation of contaminated waters. Nevertheless, exploitation of water hyacinth for phytoremediation has so far been limited to tropical or subtropical regions. In this study, the influence of temperature on the morphology, growth, and nutrient removal of water hyacinths was studied in a relatively low temperature range, 12–25°C, in the form of three night/day temperature cycles, i.e., 12/15°C, 15/20°C, and 15/25°C. It was shown that water hyacinth is very sensitive to temperature in the tested range and 15/25°C is the most favorable condition among the tested. A biomass productivity of 10.7 tons DW/ha/yr, total nitrogen removal rate of 400 kg N/ha/yr and total phosphorous removal rate of 65 kg P/ha/yr were estimated according to the experimental results on the basis of five operational months per year. No visible morphological changes were observed when the plants were cultivated at 15/20°C or 15/25°C but significant morphological changes were observed when the plants were transferred from a rooftop greenhouse to a growth chamber controlled at 12/15°C with plant size shrinking significantly. Nevertheless, substantial plant growth and nutrient removal were observed at 12/15°C, although with a prolonged adaptation phase of 2 weeks. Results of this research suggest good potential of water hyacinths for water phytoremediation in relatively cold regions such as Canada. © 2013 American Institute of Chemical Engineers Environ Prog, 32: 976–981, 2013

Removal of phosphate from landscape water using an electrocoagulation process powered directly by photovoltaic solar modules

In this paper, removal of phosphate from landscape water using an electrocoagulation process with aluminum electrodes powered directly by photovoltaic solar modules was investigated. The results showed that total phosphate (TP) residual concentration in landscape water decreases with an increasing electrolysis time. A first order kinetics about TP removal is concluded. An optimum electrode gap of 25mm is determined. Conductivity of landscape water was adjusted by adding NaCl, and an optimum initial conductivity range of 765–1135μS/cm can be obtained. In an overcast day, a higher TP removal efficiency can be obtained by extending time. TP removal efficiency increases with an increasing number of serial photovoltaic modules during early reaction. The optimal condition, order and significance of the factors on TP removal were determined by an orthogonal test with four factors and three levels, and the predictive value for TP removal efficiency is (97.77±2.13)%. Photoelectricity conversion efficiency for an overcast day is the highest and 12.2%. However, energy for removing a unit mass of TP and energy for removing TP per volume unit are all the lowest, and values are 15.75kWh/kg and 0.71kWh/m3, respectively. The process used for removal of phosphate from landscape water is feasible.

Study on a novel reactor of sludge process reduction for domestic sewage treatment

A laboratory-scale novel Sludge Reduction Reactor with Arc Guide Plate (SRR) for sludge process reduction was developed in this study. Pollutant removal efficiency and biomass yield for domestic sewage treatment in the Anaerobic/Anoxic/Oxic-SRR (A2/O-SRR) process were compared with performances in a control A2/O process. One of the competitive advantages in the SRR was that part of the inert suspended solids (ISS) could be separated and discharged out of system with flux at the bottom of the SRR. Mixed liquid volatile suspended solids (MLVSS) in the A2/O-SRR system also could be steadily kept at a good level under a relatively long solid retention time. The average MLVSS/mixed liquor suspended solids (MLSS) ratio of 77.5% in the A2/O-SRR was higher than that in the A2/O process. Average removal rates of chemical oxygen demand (COD), total nitrogen (TN) and showed little difference, while total phosphorous (TP) removal efficiency in the A2/O-SRR decreased slightly (81.89% in the A2/O-SRR and 86.50% in the A2/O process) due to the reduction of sludge discharge. The A2/O-SRR system demonstrated a considerable sludge reduction effect, with the sludge reduction ratio of 43.8%, lower solid volume index and higher dehydrogenase activity (DHA) value in comparison to the control A2/O system. The mainly mechanisms of sludge reduction in the SRR have been proved to be the uncoupling effect under the condition of anaerobic/oxic circulation and the sludge lysis with the lack of substrate.