Aerobic Hydraulic Retention Time Research Articles

Attaining superior nitrogen removal from integrated mature landfill leachate and kitchen waste digestion liquid via a two-stage partial nitrification/anammox (PN/A) process

The advanced nitrogen removal of mature landfill leachate (MLL) with low carbon to nitrogen ratio (C/N) is a topic worthy of further research. A two-stage partial nitrification/anammox (PN/A) system for integrated MLL and kitchen waste digestion liquid (KWDL) treatment was established in this study. The nitrogen removal efficiency (NRE) of this process was 98.1 ± 1.3 %, with an effluent TIN concentration of 15.2 ± 1.8 mg/L. In the partial nitrification-sequencing batch reactor (PN-SBR), aerobic hydraulic retention time (HRT) accounted for just 36.4 % of the whole process with dissolved oxygen (DO) < 0.3 mg/L, which significantly reduced energy consumption. Furthermore, the introduction of KWDL heightened anoxic denitrification and aerobic simultaneous nitrification and denitrification (SND) processes, contributing to 74.2 ± 4.3 % and 25.8 ± 4.3 % of the total NRE, respectively. This greatly strengthened the nitrogen removal capacity of PN-SBR. Batch tests indicated that the capacity of KWDL as a carbon source to drive the process of partial denitrification coupled with anammox (PDA) was equal to that of sodium acetate. Adding KWDL in the later phase of each cycle in the anammox-sequencing batch reactor (A-SBR) facilitated achieving stable PDA and advanced nitrogen removal. The nitrate accumulation problem in the traditional two-stage PN/A process has been resolved without the addition of carbon sources. Nitrosomonas (3.7 %→7.9 %), Thauera (0.04 %→40.7 %) and a salt-tolerant AnAOB microbe, unclassified_f_Brocadiaceae (0 %→5.3 %) were dominant in the combined system. This study revealed a pioneering avenue for efficient high-strength wastewater treatment.

Enhancing nitrogen removal performance through intermittent aeration in continuous plug-flow anaerobic/aerobic/anoxic process treating low-strength municipal sewage

Advanced nitrogen removal cannot be achieved through the conventional biological nitrogen removal process, which requires higher carbon sources and aeration energy. The proposal of intermittent aeration in the aerobic chambers offered an innovative approach to enhance nitrogen removal in low carbon-to-nitrogen ratio (C/N) municipal sewage, using a plug-flow reactor with anaerobic/aerobic/anoxic (AOA) process. Due to the effective utilization of internal carbon sources through the intermittent aeration, the total inorganic nitrogen removal efficiency (NRE) increased to 77.9 ± 3.2 % with the mean aerobic hydraulic retention time of only 3.2 h and a low C/N of 3.3 during the operation of 210 days. Polyhydroxyalkanoates dominated the nitrogen removal in this AOA system, accounting for 48.0 %, primarily occurring in the alternant aerobic/anoxic chambers. Moreover, the microbial community structure remained unchanged while the NRE increased to 77.9 %. This study provided an efficient and economic strategy for the continuous plug-flow AOA process.

Evaluating the Effect of Nonaerobic Hydraulic Retention Time on Nutrients Removal in Nonaerobic and Aerobic SBR

The nonaerobic stage is critical for the processes of denitrification and phosphorus release in biological wastewater treatment. A lab-scale sequencing batch reactor (SBR) operating as nonaerobic/aerobic was used to treat synthetic wastewater, and the nutrients removal performance was investigated under different nonaerobic hydraulic retention times (HRTs). The results showed when the nonaerobic HRT changed from 0 to 4 h with constant aerobic HRT of 8 h [dissolved oxygen (DO) 0.50–1.50 mg·L−1], the removal efficiencies of PO43−─P and NH4+─N were all above 90.00%, respectively. The nonaerobic HRT could influence carbon utilization between denitrification and phosphorus release, which showed a close relationship with the ratio of consumed chemical oxygen demand (COD)phosphorus release to CODdenitrification (0.94). Specifically, the consumed CODphosphorus release increased with the increase of nonaerobic HRT (0.90), whereas the relationship between consumed CODdenitrification and nonaerobic HRT was limited (−0.60). Furthermore, internal carbon source produced through phosphorus release could promote simultaneous nitrification denitrification. This control strategy could be conveniently applied in intermittent-flow sewage treatment plant through adjusting nonaerobic HRT.

Development of a high rate biological system (Anammox + phosporous) for the treatment of low strength wastewaters

Since their detection and abundance are disrupted by a number of unidentified conditions, anaerobic ammonium oxidising bacteria (anammox) play a vital role in wastewater. Low Strength Wastewaters (LSW) are those that have a chemical oxygen demand (COD) < 1000 ppm. Nitrogen and phosphorus are the major nutrients present in LSW in addition to the organic carbon. The biological nitrogen removal was studied through using a recent process of anaerobic ammonium oxidation (Anammox) process. A thorough explanation of the anammox substrate removal mechanism and denitrification phosphorus removal is provided. Denitrification was studied in a high rate anoxic hybrid reactor (HR) which yielded NO3−-N removal in the range of 92–100% till a Hydraulic Retention Time (HRT) of 0.3 d and 79% at 0.24 d respectively for an inlet NO3−-N concentration of 500 ppm. Total kjedahl nitrogen (TKN) removal of 91%, 77%, 69% and 61% were obtained in the HR containing anammox and denitrifying granules at a HRT of 2.14, 0.94, 0.56 and 0.4 per day respectively for an inlet TKN concentration of 500 ppm. Phosphorus removal of 100, 83 and 72% was obtained for an inlet PO43−- P concentration of 50, 75 and 100 ppm, respectively at an optimum anaerobic HRT of 0.06 d (1.5 h) and aerobic HRT of 0.94 d. It is evident that the phosphorus removal efficiency is better in continuous mode than in the batch mode.

Stable nitrogen removal in the novel continuous flow anammox system under deteriorated partial nitrification: Significance and superiority of the anaerobic-oxic-anoxic–oxic operation mode

The collapse of mainstream anammox system caused by deterioration of partial nitrification (PN) is easy to occur and it is vital to quickly restore the stable nitrogen elimination performance. Herein, a novel continuous push-flow anaerobic-oxic-anoxic–oxic (AOAO) process treating sewage was used to restore the nitrogen elimination performance rapidly under deteriorated PN. The increased abundances of Nitrospira and Candidatus Nitrotoga was responsible for the deterioration of PN. Effluent total inorganic nitrogen of 8.7 mg N/L and a stable nitrogen removal rate of 0.083 kg N/m3·d were obtained with the aerobic hydraulic retention time (HRT) of 3.75 h even PN deteriorated. Endogenous partial denitrification coupled anammox in the anoxic zone was essential to maintain stable nitrogen removal under the deterioration of PN and the anammox contribution increased from 17.2 % to 23.6 %. The AOAO system shows robustness on nitrogen removal even PN deteriorated under the decrease of HRT from 16 to 12 h.

A novel control strategy to strengthen nitrogen removal from domestic wastewater through eliminating nitrite oxidizing bacteria in a plug-flow process

In this study, intermittent aeration strategy was investigated in a plug-flow reactor on real municipal wastewater. Over 200 days of operation, the total inorganic nitrogen (TIN) removal efficiency of 72.43 ± 7.56% was achieved with a total aerobic hydraulic retention time in the range 2.4–3.0 h under a low C/N ratio of 3.19. The batch tests showed that the activity of nitrite oxidizing bacteria (NOB) was effectively inhibited, and simultaneous nitrification and denitrification via nitrite were observed under double intermittent aeration mode. The Illumina MiSep sequencing revealed that the relative abundance of the Nitrospira as the only detected NOB, decreased from 2.22% (day 0) to 0.91% (day 207) at the genus level. Overall, this study provides a new strategy for NOB suppression to strengthen nitrogen removal from low C/N domestic wastewater through the continuous process.

Strengthening anoxic glycogen consumption in SNEDPR-CW as a strategy to control PAO–GAO competition under carbon limited condition

Cooperation between Phosphate and Glycogen Accumulating Organisms (PAOs and GAOs) plays a pivotal role in nutrients removal in simultaneous nitrification endogenous denitrification and phosphorous removal (SNEDPR) systems. Recent findings have expanded the application of SNEDPR from activated sludge system to constructed wetland (CW). However, how to regulate competition between PAOs and GAOs in SNEDPR-based CW is still unclear. Here we showed that, GAOs could easily gain dominance over PAOs in SNEDPR-CW under alternating anaerobic/aerobic (A/O) operational mode. Shortening aerobic hydraulic retention time (HRT) at low oxygen concentration was benefit for simultaneous nitrification endogenous denitrification (SNED) and denitrifying dephosphatation but would reduce the overall phosphorus uptake rate and lead to high phosphorus effluent concentrations. Extended aerobic HRT promoted the proliferation of aerobic GAOs over PAOs, decreasing both enhanced biological phosphorus removal (EBPR) and SNED performance. Surprisingly, by switching the operation of system to alternating anaerobic/aerobic/anoxic (A/O/A) mode, an extraordinary nutrients removal performance with mean nitrogen and phosphorus removal efficiency of 84.57% and 89.37% was achieved under carbon sources limited condition. Stoichiometric analysis demonstrated that adding anoxic stage strengthened the intracellular glycogen oxidization of GAOs for denitrification which compromised its subsequent anaerobic carbon sources uptake and PHA storage and provided sufficient carbon sources for PAOs. Microbial community analysis showed that numerical ratio of GAOs to PAOs decreased from 6.67 under A/O to 4.89 under A/O/A mode, which further indicated strengthening glycogen denitrification of GAOs should be an effective way to regulate microbial competition in order to obtain a desired nutrients removal performance in SNEDPR-CW.

Nutrients removal from low C/N actual municipal wastewater by partial nitritation/anammox (PN/A) coupling with a step-feed anaerobic-anoxic-oxic (A/A/O) system

In this study, a novel combined strategy was successfully established by partial nitritation/anammox (PN/A) within a step-feed A/A/O process integrated with fixed-biocarriers to treat municipal sewage for 200 days. The excellent nutrients removal performance of this system compared with national level of discharging standard were achieved: low total inorganic nitrogen (7.1 mg/L) and phosphorus (0.3 mg/L) in the effluent with the influent (51.1 and 4.2 mg/L) at C/N ratios of 3.4 ± 0.5, mainly attributed to the stable PN (oxic zone) and subsequently anammox effect (anoxic zone). Nitrogen mass balance indicated that anammox contribution in anoxic zones to nitrogen loss could be up to 42% at stable phase. Therefore, aeration and carbon cost could be greatly reduced under low DO, low C/N and aerobic hydraulic retention time (HRT) of 7.4 h condition. The low DO and anammox bacteria retention in anoxic chambers promoted the washout of NOB and combination of anammox and partial nitritation process. During long-term operation, the activity of AOB effectively maintained while that of NOB drastically reduced to 0.1 mg N / g MLSS / h resulting in high and stable nitrite accumulation ratios (about 90%). The achievement of partial nitritation was mainly due to low DO (0.4–0.5 mg/L) and effective retention of anammox bacteria even with a low temperature (14.5 °C). Notedly, anammox activity gradually increased both on the biocarriers and in the flocs while a higher anammox abundance was observed on the biocarriers (2.48%) than that in suspend flocs (0.03%). As above, this study indicated that the novel combined strategies could be applicable to mainstream anammox, and a pilot-scale reactor will be established to verify and promote the industrial application of mainstream anammox.

A continuous plug-flow anaerobic/aerobic/anoxic/aerobic (AOAO) process treating low COD/TIN domestic sewage: Realization of partial nitrification and extremely advanced nitrogen removal

The realization of stable partial nitrification and advanced nitrogen removal are not acquired effectively in conventional pre-denitrification biological nitrogen removal processes treating domestic sewage. Herein, a novel anaerobic/aerobic/anoxic/aerobic (AOAO) continuous plug-flow reactor, characterized with double sludge reflux and a bypass of anaerobic mixed liquor conveyed to anoxic zone, was first constructed to realize stable partial nitrification in treating domestic sewage. The alternating anoxic/aerobic conditions and longer anoxic sludge retention time might be responsible for the partial nitrification. Nitrite accumulation ratio reached 89.3 ± 3.3% with the maximum activity ratio of AOB to NOB increasing from 0.72 to 8.17. A content total inorganic nitrogen (TIN) removal efficiency (93.7 ± 2.2%) and effluent TIN concentration (2.9 ± 0.9 mg N/L) were obtained after 238 days' operation. Specifically, nitrogen balance of the typical cycle showed that about 30.1% of TIN was removed through simultaneous partial nitrification and denitrification (SND) in aerobic zone and 48.2% by endogenous denitrification in anoxic zone. The AOAO process is an economic treatment for domestic sewage with aerobic hydraulic retention time (HRT) of 4 h.

Understanding microbial shift of Enhanced Biological Phosphorus Removal process (EBPR) under different Dissolved Oxygen (DO) concentrations and Hydraulic Retention Time (HRTs)

Abstract Enhanced biological phosphorus removal effectiveness is specifically conditional to systems ecology and microbial community structure. Understanding process operation aids in comprehension of microbial communities and their underlying mechanisms to improve the Enhanced biological phosphorus removal process control and troubleshooting. The performance of biological phosphorus removal (BPR) in sequential batch reactors under various aeration strategies was investigated for different dissolved oxygen (DO) concentrations and hydraulic retention times (HRT). The results showed that DO as an operational factor, highly influences P-removal performance and phosphorus accumulating organism’s (PAO) dominance where high P-removal is achieved at lower DO levels. The results in this study showed, 0.8 mg/L DO concentration could achieve successful BPR with higher than 90 % removal, due to a shift in bacterial population towards PAOs. The considerable higher P-release and uptake performance have also been observed, due to a substantial decrease in glycogen accumulating organisms (GAO) abundance at low DO levels. Thus, controlled aeration at low DO concentrations, provided a selective advantage for PAOs over GAOs in microbial competition, by minimizing the putative GAOs abundance. Further investigation on high aerobic retention time, showed a decline in PAOs population and increase in GAO’s activity. In addition, combination of high HRT and high DO level indicated limited P-removal and further process failure with low anaerobic P-release. Therefore, controlling DO at low levels and aerobic HRT at optimal duration promotes BPR accompanied with lower operational, controlling the system in reduction of undesirable (GAO) microbial community proliferation.

Long-term simulation of a full-scale EBPR plant with a novel metabolic-ASM model and its use as a diagnostic tool

This study evaluates the predictive capacity of the META-ASM model, a new integrated metabolic activated sludge model, in describing the long-term performance of a full-scale enhanced biological phosphorus removal (EBPR) system that suffers from inconsistent performance. In order to elucidate the causes of EBPR upsets and troubleshoot the process accordingly, the META-ASM model was tested as an operational diagnostic tool in a 1336-day long-term dynamic simulation, while its performance was compared with the ASM-inCTRL model, a version based on the Barker & Dold model. Overall, the predictions obtained with the META-ASM without changing default parameters were more reliable and effective at describing the active biomass of polyphosphate accumulating organisms (PAOs) and the dynamics of their storage polymers. The primary causes of the EBPR upsets were the high aerobic hydraulic retention times (HRTs) and low organic loading rates (OLRs) of the plant, which led to periods of starvation. The impact of these factors on EBPR performance were only identified with the META-ASM model. Furthermore, the first signs of process upsets were predicted by variations in the aerobic PAO maintenance rates, suggesting that the META-ASM model has potential to provide an early warning of process upset. The simulation of a new viable operational strategy indicated that troubleshooting the process could be achieved by reducing the aerated volume by switching off air in the first half of the aeration tank. In this new strategy, the META-ASM model predicted a simultaneous improvement in the biological phosphorus (P) and nitrogen (N) removal due to the enhancement of the hydrolysis and fermentation of the mixed liquor sludge in the new unaerated zone, which increased the availability of volatile fatty acids (VFAs) for PAOs. This study demonstrates that the META-ASM model is a powerful operational diagnostic tool for EBPR systems, capable of predicting and mitigating upsets, optimising performance and evaluating new process designs.

Nutrient removal and microbial community in a two-stage process: Simultaneous enhanced biological phosphorus removal and semi-nitritation (EBPR-SN) followed by anammox

This study developed a two-stage process, including simultaneous enhanced biological phosphorus-removal and semi-nitritation (EBPR-SN) sequencing batch reactor (SBR), followed by Anammox SBR, to achieve advanced nitrogen (N) and phosphorus (P) removal from real sewage with low carbon/nitrogen (2.82). The long-term operation suggested that removal efficiencies for TIN (86.2 ± 3.5%) and P (95.0 ± 5.5%) were stably obtained, with nitrite accumulation ratio of 98.7% in EBPR-SN SBR. Mechanism analysis indicated contribution of anammox to N-removal being 57.3%–73.7% and superior P-removal due to the majority of removed organics (~74.5%) being stored by polyphosphate-accumulating organisms (PAOs). In EBPR-SN SBR, high-throughput sequencing showed ammonium-oxidizing bacteria was 0.03% while nitrite-oxidizing bacteria was not detected, and PAOs accounted for 30.07%. In Anammox SBR, Candidatus Brocadia (9.75%) was the only anammox bacteria. Remarkably, short aerobic hydraulic retention time (4.29 h) with low DO (0.3–1.2 mg/L) during the whole process provided desirable energy-saving.

A new modified anoxic-anaerobic-membrane bioreactor for treatment of real wastewater with a low carbon/nutrient ratio and high nitrate

A new modified configuration of pilot-scale anoxic-anaerobic-membrane bioreactor (MAA-MBR) was designed for excellent organic and nutrient removal from real urban wastewater with a low carbon/nutrient ratio and notable nitrate concentration. Eight different runs of mixed liquor recycle (MLR) ratio of 100–600 % and hydraulic retention time (HRT) of 2–12 h were used to identify the optimal operating conditions of the MAA-MBR system. Statistical analysis (ANOVA) of the results showed that the food to microorganism ratio and degradation performance of the system were significantly independent of the aerobic HRT of 3–12 h. However, chemical oxygen demand (COD) and total nitrogen (TN) removal dropped by 7 % at 2 h aerobic HRT. The results also highlighted that TN and total phosphorus (TP) removal were significantly affected by the MLR ratio. Additionally, the MAA-MBR system was able to improve TP removal by 21.6 % comparing to the A2O-MBR system at the same operating conditions without any chemical addition. The maximum five-day biochemical oxygen demand, COD, TN, and TP removal were 99.2 %, 96.9 %, 88.1 %, and 75.8 %, respectively, at total HRT of 8.1 h and MLR ratio of 400 %. Moreover, TP removal improved to 92.0 % when methanol was added to the anaerobic reactor. In addition, more than 87 % of the maximum TP and TN removal occurred via the enhanced biological nutrient removal (EBNR) and the remaining removal happened via the assimilation mechanism. In conclusion, the proposed MAA-MBR system is highly efficient for EBNR of wastewater with a high nitrate concentration.

Enhancement of Biological Nutrient Removal in an Alternating Anaerobic-Aerobic Sequencing Batch Reactor: Optimization of Anaerobic and Aerobic Hydraulic Retention Times

Biological phosphorus removal (BPR), possesses the significant advantages of low operational costs and little environmental impacts, is an economical and sustainable process to remove P by recycling the activated sludge through anaerobic and aerobic processes. For the BPR system, the hydraulic retention times (HRT) in aerobic and anaerobic processes are the most significant controlling parameters which can directly affect the effluent quality, especially the effluent phosphorus removal efficiencies. In this study, six lab-scale sequencing batch reactors (SBRs) were operated to conduct the single-factor experiments. 13 experimental runs designed by a 2-factor and 5-level response surface methodology (RSM) using Central composite design (CCD) were used to optimize the relationship between anaerobic HRT (X1, h) and aerobic HRT (X2, h) and two most important responses, COD removal efficiency (CRE, Y1, %) and P removal efficiency (PRE, Y2, %). High squared regression coefficients R2 (> 0.99) and adjusted R2 (> 0.99) indicated a high degree of correlation between the predicted and actual responses, which means that the model could fit the response well. Experimental validation by operating under the optimal combination of the two operational HRTs were conducted. Good correlation between the predicted and experiments values provides confidence in the obtained models.

Performance and Kinetics Evaluation of Integrated Suspended Growth Bioreactor Treating Beverage Industry Wastewater

In this research, feasibility of using a continuous flow integrated suspended growth bioreactor (i-SGBR) pilot plant was explored to treat beverage industry wastewater. The bioreactor treatment units comprise of three sequentially arranged suspended growth bioreactors with anoxic (ANX-C), aerobic (AER-C), and aerobic digester chambers (AD-C). Clarifier (CLR) was installed as last chamber to settle sludge. Parameters such as total chemical oxygen demand (TCOD) were monitored and validated with biochemical oxygen demand (BOD5). Other parameters measured include soluble COD (sCOD), mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), total suspended solids (TSS), and pH. Transformational behavior of aerobic metabolic performance for extended aeration process was investigated by operating regimes of variable aerobic hydraulic retention time (HRT) and organic loading rate (OLR) between 20 and 30 h and 0.49–0.79 kg COD/m3 day, respectively. Solids retention time (SRT) between 20 and 40 days was operated. The aim was to generate data for bacterial growth and substrate utilization kinetics from modified Monod’s model. Removal of TCOD, BOD5, and TSS were achieved in the range of 95.2–97.9% (Influent 995 ± 21–1028 ± 25 mg/L and Effluent 21 ± 2–4.9 ± 3 mg/L), 98–98.7% (Influent 489 ± 19–507 ± 7 mg/L and Effluent 27 ± 2–41 ± 1.8 mg/L), and 91.2–94.6% (Influent 500 ± 23–653 ± 11 mg/L and Effluent 6.3 ± 7–41 ± 1.8 mg/L), respectively. The maximum substrate utilization rate (k), half velocity constant (Ks), growth yield co-efficient (Y), and decay coefficients (kd) were determined as 2.81 days−1, 979 mg sCOD/L, 0.72 mg VSS/mg sCOD, and − 0.0172 day−1, respectively. Maximum specific growth rate (μmax) was found as 2.03 days−1. Treatment efficiencies declined with reduction of HRT and with increased OLR applied to the bioreactor. The aerobic digester (AD) achieved between 9.8% (Influent 15,021 mg/L) and 18.6% (10,893 mg/L) MLVSS reduction, where performance decreased with additional solids concentration from influent aerobic digester (IAD). i-SGBR has accomplished effective removal of pollutants and simultaneous sludge degradation of beverage industry wastewater. Kinetic parameters obtained could be useful for design and modeling of aerobic treatment unit to improve effluent quality.

Long-term effects of sulphide on the enhanced biological removal of phosphorus: The symbiotic role of Thiothrix caldifontis

Thiothrix caldifontis was the dominant microorganism (with an estimated bio-volume of 65 ± 3%) in a lab-scale enhanced biological phosphorus removal (EBPR) system containing 100 mg of sulphide per litre in the influent. After a gradual exposure to the presence of sulphide, the EBPR system initially dominated by Candidatus Accumulibacter phosphatis Clade I (98 ± 3% bio-volume) (a known polyphosphate accumulating organism, PAO) became enriched with T. caldifontis. Throughout the different operating conditions studied, practically 100% phosphate removal was always achieved. The gradual increase of the sulphide content in the medium (added to the anaerobic stage of the alternating anaerobic-aerobic sequencing batch reactor) and the adjustment of the aerobic hydraulic retention time played a major role in the enrichment of T. caldifontis. T. caldifontis exhibited a mixotrophic metabolism by storing carbon anaerobically as poly-β-hydroxy-alkanoates (PHA) and generating the required energy through the hydrolysis of polyphosphate. PHA was used in the aerobic period as carbon and energy source for growth, polyphosphate, and glycogen formation. Apparently, extra energy was obtained by the initial accumulation of sulphide as an intracellular sulphur, followed by its gradual oxidation to sulphate. The culture enriched with T. caldifontis was able to store approximately 100 mg P/g VSS. This research suggests that T. caldifontis could behave like PAO with a mixotrophic metabolism for phosphorus removal using an intracellular sulphur pool as energy source. These findings can be of major interest for the biological removal of phosphorus from wastewaters with low organic carbon concentrations containing reduced S-compounds like those (pre-)treated in anaerobic systems or from anaerobic sewers.

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.

The impact of aeration on the competition between polyphosphate accumulating organisms and glycogen accumulating organisms

In wastewater treatment plants (WWTPs), aeration is the major energetic cost, thus its minimisation will improve the cost-effectiveness of the process. This study shows that both the dissolved oxygen (DO) concentration and aerobic hydraulic retention time (HRT) affect the competition between polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs). At low DO levels, Accumulibacter PAOs were shown to have an advantage over Competibacter GAOs, as PAOs had a higher oxygen affinity and thus largely maintained their aerobic activity at low DO levels, while GAO activity decreased. Bioreactor operation at low DO levels was found to increase the PAO fraction of the sludge. Furthermore, an increase in aerobic HRT (at a DO level of 2 mg O2/L), promoted the proliferation of GAOs over PAOs, decreasing the EBPR efficiency. Overall, this study shows that low aeration can be beneficial for EBPR performance through selecting for PAOs over GAOs, which should be incorporated into WWTP models in order to minimise energetic costs and improve WWTP sustainability.

Model-based operational guidelines of a bioprocess for biological nitrogen removal and complete stabilisation of anaerobically digested sewage sludge

The concept of one-stage reactor system for biological nitrogen removal over nitrite of ammonium high loaded sidestreams is going to be applied to remove nitrogen from anaerobically digested sewage sludge and to achieve its complete stabilisation. Dealing with sludge, the organic matter needed to denitrify is present in the inflow as particulate substrate, which requires a hydrolysis step. The latter implies high anoxic hydraulic retention time (HRT). During both aerobic and anoxic phases, ammonium is released which implies the need to enlarge aerobic HRT. Both effects lead to a total HRT higher than those for nitrification-denitrification of wastewater with soluble substrate. The purpose of this paper is to define, by computer simulation, a set of theoretical criteria, which will be applied later to the operation of a pilot-scale post-aeration reactor to be located in a Spanish WWTP. These criteria will be defined by simulating the reactor performance under different operating conditions. As a conclusion, some operation guidelines have been established for the above-mentioned scenario in terms of aerobic and anoxic retention time, dissolved oxygen concentration and effluent requirements (NH(4)(+), NO(2)(-) and NO(3)(-)).

Biological phosphorus removal from dairy wastewater by alternating anaerobic and aerobic conditions

In this study, the possibility of applying the enhanced biological phosphorus removal (EBPR) process for Algiers dairy wastewater which can have phosphorus contents up to 130 mg/L was examined. EBPR is conventionally performed by an anaerobic-aerobic process. The objectives of this work were to determine an optimal hydraulic retention time (HRT) in aerobic conditions and to study the effect of short chain fatty acids (SCFA) (acetic and propionic acids) addition on the phosphorus release in anaerobic conditions. The tests were performed in a batch reactor operating with an aerobic/anaerobic sequence of phases. Batch tests have been carried out at 3 HRTs in aerobic conditions (1, 2 and 3 h) while the anaerobic retention time was fixed at 4 h, to examine the effect of stress related to changes of aerobic HRT. Main results show that the most favorable aerobic retention time was found to be 2 h. The amount of P released in anaerobic phase increases from 2.25 to 2.48 mgP/gVSS with increasing aerobic HRT from 1 to 2 h and decreases to 1.28 mgP/g VSS for a time of 3 h using acetic acid. Similarly, this amount increases from 1.62 to 4.38 mgP/gVSS for 1 to 2 h and decreases to 1.41 mgP/gVSS for a time of 3 h using propionic acid. The initial release rate was directly proportional to the amount of added substrate. Propionate may be a more effective carbon source for biological phosphorus removal than acetate. Based on the results presented herein, we can confirm the possibility of phosphorus removal from dairy wastewater in the aerobic-anaerobic biological process. Keywords: Dairy wastewater, biological phosphorus removal, aerobic, anaerobic, release, acetic acid, propionic acid