Impact of dissolved oxygen concentration on membrane filtering resistance and soluble organic matter characteristics in membrane bioreactors

This study investigated the impact of dissolved oxygen (DO) concentration on membrane filtering resistance, soluble organic matter (SOM) and extracellular polymeric substance (EPS) characteristics in a membrane bioreactor (MBR). A laboratory-scale MBR was operated under DO limited (0.2 mg L(-1) DO) and fully aerobic (3.7 and 5.4 mg L(-1) DO) conditions. Membrane filtering resistance was determined for the mixed liquor suspended solids (MLSS) and for resuspended microbial biomass after removing SOM. Regardless of the DO concentration, the cake resistance (Rc) was approximately 95 percent of the total resistance (Rt). The membrane cake resistance was found to decrease significantly after removing the SOM. The total resistance caused by the resuspended biomass was 29 percent of that caused by the MLSS under DO limited conditions, while the total resistance caused by resuspended biomass was 41 to 48 percent of that caused by the MLSS under fully aerobic conditions. Under DO limited conditions, SOM in the MLSS contained a larger amount of high molecular weight compounds, leading to higher cake resistance than under fully aerobic conditions. There was significant variation in the molecular weight fractions of the EPS, with no clear relationship with DO concentration. There was also no distinct relationship between membrane filtering resistance and molecular weight fraction of the EPS.     

Achieving and maintaining of short-cut nitrification in a cyclic activated sludge system

A lab-scale Cyclic Activated Sludge Technology (CAST) system was operated more than 5 months to evaluate the effects of the operation mode on nitrogen removal performance and investigate a feasible method for achieving short-cut nitrification in the system. Results showed that nitrogen was removed by conventional biological nitrification and denitrification in traditional operation mode (fill/aeration 2 h, settle 1 h, decant 1 h), whereas short-cut nitrification and denitrification was the main nitrogen removal pathway in modified operation mode and the nitrogen removal performance was enhanced. Short-cut nitrification was successfully achieved in CAST system at 17 ± 1 °C by adjusting operation conditions and the average total nitrogen removal efficiency increased by 11.4% compared to traditional mode. It was assumed that low dissolved oxygen (<1.0 mg/L) limitation combined with free ammonia (0.28-0.34 mg/L) inhibition on nitrite-oxidizing bacteria caused nitrite accumulation in modified mode. During maintaining period of short-cut nitrification, preset aeration time enhanced ammonium-oxidizing bacteria dominance. It was also found that low DO could result in overgrowth of filamentous microorganisms and poor sludge settleability. The pH variation could provide effective information for controlling aeration duration in modified mode. However, no evident breakpoint appeared on pH and DO profiles in traditional mode.     

A novel wastewater treatment process: simultaneous nitrification, denitrification and phosphorus removal.

Simultaneous nitrification and denitrification (SND) via the nitrite pathway and anaerobic-anoxic enhanced biological phosphorus removal (EBPR) are two processes that can significantly reduce the COD demand for nitrogen and phosphorus removal. The combination of these two processes has the potential of achieving simultaneous nitrogen and phosphorus removal with a minimal requirement for COD. A lab-scale sequencing batch reactor (SBR) was operated in alternating anaerobic-aerobic mode with a low dissolved oxygen concentration (DO, 0.5 mg/L) during the aerobic period, and was demonstrated to accomplish nitrification, denitrification and phosphorus removal. Under anaerobic conditions, COD was taken up and converted to polyhydroxyalkanoates (PHA), accompanied with phosphorus release. In the subsequent aerobic stage, PHA was oxidized and phosphorus was taken up to less than 0.5 mg/L at the end of the cycle. Ammonia was also oxidised during the aerobic period, but without accumulation of nitrite or nitrate in the system, indicating the occurrence of simultaneous nitrification and denitrification. However, off-gas analysis found that the final denitrification product was mainly nitrous oxide (N2O) not N2. Further experimental results demonstrated that nitrogen removal was via nitrite, not nitrate. These experiments also showed that denitrifying glycogen-accumulating organisms rather than denitrifying polyphosphate-accumulating organisms were responsible for the denitrification activity.     

垃圾渗滤液中好氧反硝化生物脱氮的最佳条件

对垃圾渗滤液间歇曝气(曝气时DO为5.5～7 mg/L;停止时DO为1.1～5.5 mg/L),在仅有有机碳、无机氮的条件下进行好氧反硝化作用.通过正交试验确定了好氧反硝化的最佳条件为水力停留时间168 h,DO为5.5～7 mg/L,有机碳源为乙醇,当有机碳源为乙醇时C/N为10.并得出影响好氧反硝化作用因素的重要性排序依次为C/N＞水力停留时间＞有机碳源＞DO.C/N是影响好氧反硝化的主要因素,对TN去除率有高度显著的影响;水力停留时间对TN去除率有显著影响;有机碳源对TN去除率有一定影响;DO对TN去除率没有显著影响.     

Treatment of municipal wastewater in a hybrid process using a new suspended carrier with large surface area.

An activated sludge/biofilm hybrid process treating municipal wastewater was studied in pilot plant trials. A new type of suspended carrier, with large effective surface area, was employed in the process with the aim of enhancing nitrification. The pilot plant was operated for 1.5 years in five different configurations including pre-denitrification in all five and enhanced biological phosphorus removal in the final two. The wastewater temperature ranged between 11 degrees C and 20 degrees C, and the nominal dissolved oxygen (DO) level was 5-6 mg/L. The nitrification rate obtained on the new carrier within the hybrid stage was in the range of 0.9-1.2 g NH4-N/m2/d corresponding to a volumetric rate of 19-23 g NH4-N/m3/h (total nitrification including nitrification in the suspended solids). More than 80% of the total nitrification took place on the carrier (and the remainder in the suspended solids). The nitrification rate was shown to correlate with DO, decreasing when the DO was decreased. The results supported the idea of using the new carrier as a tool to upgrade plants not having nitrification today or improve nitrification in activated sludge processes not reaching necessary discharge levels. The large surface area present for nitrification makes it possible to obtain high nitrification rates within limited volumes. The possibility to keep the total suspended solid content low (< 3 g/L) and avoiding problems with the filament Microthrix parvicella, are other beneficial properties of the hybrid process.     

Continuous and sequencing membrane bioreactors applied to food industry effluent treatment.

This work focuses on the performances of two immersed membrane bioreactors used for the treatment of easily biodegradable organic matter present in food industry effluents, for the purpose of water reuse. Two reactor functioning modes (continuous and sequencing) were compared in terms of organic carbon removal and of membrane permeability. For each working mode, pollutant removal was very high, treated water quality presented a low COD concentration (< 125 mg x L(-1)), no solids in suspension and low turbidity (< 0.5 NTU). The quality of the treated water (including germ removal) enabled its reuse on site. Moreover, by developing high biomass concentrations in the reactor, excess sludge production remained very low (< 0.1 gVSS x gCOD(-1)). The performances appeared slightly better for the continuous system (lower COD concentration in the effluent, < 50 mg x L(-1), and lower sludge production). In terms of filtration, a distinct difference was observed between continuous and sequencing systems; transmembrane pressure showed a small and constant evolution rate in continuous membrane bioreactor (CMBR) although it appeared more difficult to control in sequencing membrane bioreactor (SMBR) probably due to punctually higher permeate flow rate and modified suspension properties. The rapid evolution of membrane permeability observed in SMBR was such that more frequent chemical cleaning of the membrane system was required.     

Boosting nitrification by membrane-attached biofilm.

Nitrification is a key step for reliable biological nitrogen removal. In order to enhance nitrification in the activated sludge (AS) process, membrane-attached biofilm (MAB) was incorporated in a conventional activated sludge tank. Simultaneous organic carbon removal and nitrification of the MAB incorporated activated sludge (AS + MAB) process was investigated with continuous wastewater treatment. The effluent TOC concentration of AS and the AS + MAB processes were about 6.3 mg/L and 7.9 mg/L, respectively. The TOC removal efficiency of both AS and AS + MAB were above 95% during the wastewater treatment, indicating excellent organic carbon removal performance in both processes. Little nitrification occurred in the AS process. On the contrary, successful nitrification was obtained with the AS + MAB process with nitrification efficiency of about 90%. The volumetric and surface nitrification rates were about 0.14 g/Ld and 6.5 g/m2d, respectively. The results clearly demonstrated that nitrification in the conventional AS process was boosted by MAB. Furthermore, the microfaunal population in the AS + MAB process was different from that in the AS process. The high concentration of rotifers in the AS + MAB process was expected to decrease the generation of excess sludge in the process.     

Nitrification of high-strength ammonium wastewater by a fluidized-bed reactor.

A laboratory-scale fluidized-bed reactor with an external aeration loop was used for nitrification of high-strength ammonium wastewater (up to 500 mg NH4-N/L). The results demonstrated that the system is capable of handling ammonium removal rates of up to 2.5 kg NH4-N/m3 x d, while removal efficiencies were as high as 98% and independent of the applied ammonium loading rates. Ammonium loading rates higher than 2.5 kg NH4-N/m3 x d resulted in decreased ammonium removal efficiency. The data show that near complete ammonium removal occurred at DO concentrations as low as 0.3-0.5 mg/L. However, the nitrite-nitrogen fraction in the effluent increased from 3.5% to 23.2% when the DO dropped from 1.0 mg/L to approximately 0.4 mg/L, respectively. The high specific removal rates in this system are one order of magnitude higher than that of suspended-growth systems. This can reduce the supplementary reactor volumes required for nitrification to less than 10% of that needed in conventional activated sludge systems. These results clearly indicate the potential economic gains that could be achieved through implementation of this technology.     

DO对短程同步硝化反硝化除磷工艺的影响

针对碳源偏低的城市污水,采用序批式活性污泥法研究D0对短程同步硝化反硝化除磷工艺的影响,同时对短程同步硝化反硝化和反硝化除磷的机理进行探讨。试验表明：控制DO浓度可在同一个反应器内既实现短程同步硝化反硝化反应又达到反硝化除磷的效果。综合考虑COD、NHg—N、TN、TP的出水浓度达到一级A排放标准,得出最佳的D0控制范围。当D0浓度在0．5～1．0mg／LU时．COD的去除率达到93％～94％,Nil,＋一N的去除率为97％～98％,TN的去除率达到85％一96％,TP的去除率为91％～93％。     

于桥水库沉积物内源污染特性研究

通过对天津市于桥水库沉积物中污染物的释放实验研究,找出了于桥水库底泥中磷、有机质、氮、铁、锰等污染物释放的影响因素,得出了溶解氧、pH值、水温等影响因素对底泥污染物释放的影响程度,并对各污染物的释放机理进行了探讨。磷的释放条件:pH值≥8时,好氧条件下会缓慢释放,当ρ(DO)<2 mg/L时快速释放;在中酸性条件下,当ρ(DO)<1 mg/L时发生较快速释放;低溶解氧和高pH值协同作用强烈。有机物和氮的释放主要受溶解氧浓度影响,与pH值关系不大,在ρ(DO)<2 mg/L时发生了快速释放。溶解性铁、锰的释放取决于水体pH值,在酸性条件下,ρ(DO)<1 mg/L时释放。     

High strength nitrogen removal from nightsoil and piggery wastes.

Nightsoil and piggery wastes generally present high strength organics and nitrogen. This study evaluated the nitrogen removal characteristics with the existing and modified nightsoil and piggery waste treatment plants. The existing conventional plants showed 20 to 40% nitrogen removal, but the modification with SBR or MLE process could remove effectively both nitrogen and organics with the minimum COD/TN and alkalinity/TN ratios of 6 and 3.6, respectively. Nitrite nitrification and denitrification rates obtainable at higher nitrogen loads were faster than the rates of nitrate nitrification and denitrification resulting in less reactor volume requirement. However, the higher nitrogen loads increased the organic loads resulting in the reactor temperature inhibiting nitrification. Thus, a combined treatment with anaerobic digestion with the adjustment of influent bypass rates was proposed to reduce the reactor temperature and the external carbon requirement. The biological treatment could discharge about 1,100 mg/L soluble COD and 50 mg/L soluble nitrogen, respectively.     

溶解氧对同步硝化反硝化脱氮影响研究

以实际生活污水为处理对象,利用生物膜内所具有的A/O环境,针对DO浓度对生物膜法同步脱氮效果影响进行试验研究.研究结果表明,在DO为2.5 mg/L时SND脱氮效果达最佳,TN去除率近70%;DO浓度过高或过低都不利于生物膜内部DO浓度梯度的形成,合理控制DO浓度,对生物膜法同步脱氮尤为重要.     

Granular biomass capable of partial nitritation and anammox

A novel and efficient way of removing nitrogen from wastewater poor in biodegradable organic carbon, is the combination of partial nitritation and anoxic ammonium oxidation (anammox), as in the one-stage oxygen-limited autotrophic nitrification/denitrification (OLAND) process. Since anoxic ammonium-oxidizing bacteria grow very slowly, maximum biomass retention in the reactor is required. In this study, a lab-scale sequencing batch reactor (SBR) was used to develop granular, rapidly settling biomass. With SBR cycles of one hour and a minimum biomass settling velocity of 0.7 m/h, OLAND granules were formed in 1.5 months and the nitrogen removal rate increased from 50 to 450 mg N L(-1) d(-1) in 2 months. The granules had a mean diameter of 1.8 mm and their aerobic and anoxic ammonium-oxidizing activities were well equilibrated to perform the OLAND reaction. Fluorescent in-situ hybridization (FISH) demonstrated the presence of both beta-proteobacterial aerobic ammonium oxidizers and planctomycetes (among which anoxic ammonium oxidizers) in the granules. The presented results show the applicability of rapidly settling granular biomass for one-stage partial nitritation and anammox.     

Conditions and technologies of biological wastewater treatment in Hungary

A survey has been carried out involving 55 Hungarian wastewater treatment plants in order to evaluate the wastewater quality, the applied technologies and the resultant problems. Characteristically the treatment temperature is very wide-ranging from less than 10 °C to higher than 26 °C. Influent quality proved to be very variable regarding both the organic matter (typical COD concentration range 600-1,200 mg l(-1)) and the nitrogen content (typical NH(4)-N concentration range 40-80 mg l(-1)). As a consequence, significant differences have been found in the carbon availability for denitrification from site to site. Forty two percent of the influents proved to lack an appropriate carbon source. As a consequence of carbon deficiency as well as technologies designed and/or operated with non-efficient denitrification, rising sludge in the secondary clarifiers typically occurs especially in summer. In case studies, application of intermittent aeration, low DO reactors, biofilters and anammox processes have been evaluated, as different biological nitrogen removal technologies. With low carbon source availability, favoring denitrification over enhanced biological phosphorus removal has led to an improved nitrogen removal.     

Analysis of nitrifying bacterial communities in aerobic biofilm reactors with different DO conditions using molecular techniques

In order to assess the relationship between the dissolved oxygen (DO) concentration and the characteristics of nitrifying bacterial communities in an aerobic biofilm reactor, molecular techniques including denaturing gradient gel electrophoresis (DGGE)/cloning based on PCR targeting 16S rRNA and the amoA gene and fluorescence in situ hybridisation (FISH) were conducted. The D-1, D-2, D-3 and D-4 reactors with different DO concentrations (1, 3, 5 and 7 mg/L, respectively) were set up in the thermostat and acclimated. The optimal DO concentration with stable nitrification efficiency was above 5.0 mg/L. As was shown by the results of DGGE and cloning, the community of ammonia-oxidising bacteria (AOB) and the ratio of Nitrosomonas sp. changed only slightly despite their differing nitrification efficiencies. The results of FISH indicated that higher DO concentrations resulted in an increase in AOB and nitrite-oxidising bacteria (NOB), and a reduction in heterotrophic microorganisms. The INT-dehydrogenase activity (DHA) test demonstrated that the activity of AOB decreased with reductions in the DO concentration. This means that the DO concentration does not influence the community of AOB, but rather the activity of AOB. In the relationship between the attached biomass and the nitrification efficiency, only the active biomass affected the nitrification efficiencies.     

Enhancement of oxygen transfer and nitrogen removal in a membrane separation bioreactor for domestic wastewater treatment.

Biological nitrogen removal in a membrane separation bioreactor developed for on-site domestic wastewater treatment was investigated. The bioreactor employed hollow fiber membrane modules for solid-liquid separation so that the biomass could be completely retained within the system. Intermittent aeration was supplied with 90 minutes on and off cycle to achieve nitrification and denitrification reaction for nitrogen removal. High COD and nitrogen removal of more than 90% were achieved under a moderate temperature of 25 degrees C. As the temperature was stepwise decreased from 25 to 5 degrees C, COD removal in the system could be constantly maintained while nitrogen removal was deteriorated. Nevertheless, increasing aeration supply could enhance nitrification at low temperature with benefit from complete retention of nitrifying bacteria within the system by membrane separation. At low operating temperature range of 5 degrees C, nitrogen removal could be recovered to more than 85%. A mathematical model considering diffusion resistance of limiting substrate into the bio-particle is applied to describe nitrogen removal in a membrane separation bioreactor. The simulation suggested that limitation of the oxygen supply was the major cause of inhibition of nitrification during temperature decrease. Nevertheless, increasing aeration could promote oxygen diffusion into the bio-particle. Sufficient oxygen was supplied to the nitrifying bacteria and the nitrification could proceed. In the membrane separation bioreactor, biomass concentration under low temperature operation was allowed to increase by 2-3 times of that of moderate temperature to compensate for the loss of bacterial activities so that the temperature effect was masked.     

Application of a sponge media (BioCube) process for upgrading and expansion of existing caprolactam wastewater treatment plant for nitrogen removal.

For the upgrade and expansion of an existing caprolactam wastewater treatment plant, a freely floating sponge media (BioCube) process was selected based on extensive pilot-plant tests, due to extreme space constraints. In order to protect nitrifier inhibition caused by high strength organics in caprolactam wastewater, the pilot plant consisted of an organics removal reactor, which functioned as a pretreatment for nitrification, and followed the nitrogen removal reactor. The suspended MLSS was 1,800-4,000 and the media attached MLSS was maintained at 22,000-26,000 mg/L. The final effluent COD was noticeably low, around 20.4-37 mg/L, even with fairly large fluctuations in the feed levels, between 1,400-6,770 mg/L. The removal of total nitrogen with the system, when denitrification was close to completion, was approximately 97.6%. For the entire run, complete nitrification of 99.6% was achieved, which might have been due to well-acclimatized nitrifiers attached in the BioCube media. Specifically, after adaptation, the nitrification continuously increased in the organics removal reactor, even under high residual organics conditions. From the numerous experimental results, the BioCube process seemed to be an effective method for the upgrading and expansion of the existing wastewater treatment plant, with minimum reactor enlargement.     

Simultaneous nitrification/denitrification in an aerobic biofilm system

In the biological pretreatment of landfill leachate in Mechemich (Germany) a loss of inorganic nitrogen of up to 90% was observed in the nitrification step (rotating biological contactor) under low DO conditions. Ammonia was removed but only small amounts of nitrate were produced. Nitrite accumulation did not occur. In aerobic batch tests nitrogen loss was confirmed without any addition of organic substrate, even when homogenizing the biofilm mechanically to destroy possible anoxic microzones. N₂ was measured as the gaseous end product of the process. From results presented it can be assumed that a largely autotrophic microorganism population performed this aerobic nitrification/denitrification.     

Pre-ozonation in the activated sludge process: fate of nitrogen species

The objective of this research was to include ozonation prior to an activated sludge treatment and investigate the effect on the nitrogen species, their fate and the consequences of this oxidation upon the biomass. Three parallel treatment systems were used: the base system, where feed went directly to the activated sludge reactor, and two others, where the influent was ozonated at two different dosages, 15 and 25 mg/L of influent, prior to the biological reactors. The results from the ozonation chamber show a high oxidation capacity of the entering ammonia and organic nitrogen, proportional to the ozone dose. The oxidation product was nitrate. No de-nitrification was expected because a high oxygen concentration (4 mg/L) was maintained in the reactors. The reactors receiving ozonated influent showed a lower assimilation of nitrogen by the biomass. The sludge nitrogen content resulted in 11, 9.3 and 7.4% dry-weight corresponding to no-ozone, low ozone and high ozone dosages, respectively. In spite of the lower ammonia available in the ozonated flows, the corresponding reactors showed a higher specific nitrification rate. The ozonated system also performed better in terms of chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) removals, besides showing a higher true biomass yield coefficient.     

In-situ characterization of microbial community in an A/O submerged membrane bioreactor with nitrogen removal.

The bacterial community involved in removing nitrogen from sewage and their preferred DO environment within an anoxic/oxic membrane bioreactor (A/O MBR) was investigated. A continuously operated laboratory-scale A/O MBR was maintained for 360 d. At a sludge age of 150 d and a C/N ratio of 3.5, the system was capable of removing 88% of the influent nitrogen from raw wastewater through typical nitrogen removal transformations (i.e. aerobic ammonia oxidation and anoxic nitrate reduction). Characterization of the A/O MBR bacterial community was carried out using fluorescence in situ hybridization (FISH) techniques. FISH results further showed that Nitrosospira spp. and Nitrospira spp. were the predominant groups of ammonia and nitrite oxidizing group, respectively. They constituted up to 11% and 6% of eubacteria at DO below 0.05 mg/l (low DO), respectively, and about 14% and 9% of eubacteria at DO between 2-5 mg/l (sufficient DO), respectively, indicating preference of nitrifiers for a higher DO environment. Generally low counts of the genus Paracoccus were detected while negative results were observed for Paracoccus denitrificans, Alcaligenes spp, and Pseudomonas stutzeri under the low and sufficient DO environments. The overall results indicate that Nitrosospira spp., Nitrospira spp. and members of Paracoccus spp. can be metabolically functional in nitrogen removal in the laboratory-scale A/O MBR system.