Removal of nitrogen and phosphorus in a combined A^2/O-BAF system with a short aerobic SRT

A bench-scale anaerobic/anoxic/aerobic process-biological aerated filter （A^2/O-BAF） combined system was carded out to treat wastewater with lower C/N and C/P ratios. The A^2/O process was operated in a short aerobic sludge retention time （SRT） for organic pollutants and phosphorus removal, and denitrification. The subsequent BAF process was mainly used for nitrification. The BAF effluent was partially returned to anoxic zone of the A^2/O process to provide electron acceptors for denitrification and anoxic P uptake. This unique system formed an environment for reproducing the denitdfying phosphate-accumulating organisms （DPAOs）. The ratio of DPAOs to phosphorus accumulating organisms （PAOs） could be maintained at 28% by optimizing the organic loads in the anaerobic zone and the nitrate loads into the anoxic zone in the A^2/O process. The aerobic phosphorus over-uptake and discharge of excess activated sludge was the main mechanism of phosphorus removal in the combined system. The aerobic SRT of the A^2/O process should meet the demands for the development of aerobic PAOs and the restraint on the nitrifiers growth, and the contact time in the aerobic zone of the A^2/O process should be longer than 30 min, which ensured efficient phosphorus removal in the combined system. The adequate BAF effluent return rates should be controlled with 1--4 mg/L nitrate nitrogen in the anoxic zone effluent of A^2/O process to achieve the optimal nitrogen and phosphorus removal efficiencies.

Anoxic Biological Phosphorus Uptake in A^2O Process

A lab-scale anaerobic-anoxic-oxic (A2O) process used to treat a synthetic brewage wastewater was investigated. The objectives of the study were to identify the existence of denitrifying phosphorus removing bacteria (DPB), evaluate the contribution of DPB to biological nutrient removal and enhance the denitrifying phosphorus removal in A2O bioreactors. Sludge analysis confirmed that the average anoxic P uptake accounted for approximately 70% the total amount of P uptake, and the ratio of anoxic P uptake rate to aerobic P uptake rate was 69%. In addition, nitrate concentration in the anoxic phase and different organic substrate introduced into the anaerobic phase had significant effect on the anoxic P uptake. Compared with conventional A2O processes, good removal efficiencies of COD, phosphorus, ammonia and total nitrogen (92.3%, 95.5%, 96% and 79.5%, respectively) could be achieved in the anoxic P uptake system, and aeration energy consumption was saved 25%. By controlling the nitrate recirculation flow in the anoxic zone, anoxic P uptake could be enhanced, which solved the competition for organic substrates among poly-P organisms and denitrifiers successfully under the COD limiting conditions. Therefore, in wastewater treatment plants the control system should be applied according to the practical situation to optimize the operation.

Nitrous oxide emission by denitrifying phosphorus removal culture using polyhydroxyalkanoates as carbon source

Nitrous oxide （N2O） emission has been reported to be enhanced during denitrification when internally-stored compounds are used as carbon sources. However, negligible N2O emissions have been detected in the few studies where polyhydroxyalkanoates （PHA） were specifically used. This study investigated and compared the potential enhancement of N2O production, based on utilization of an internally-stored polymer and external carbon （acetate） by a denitrifying phosphorus removal culture. Results indicated that at relatively low chemical oxygen demand-to-nitrogen （COD/N） ratios, more nitrite was reduced to N2O in the presence of an external carbon source as compared to an internal carbon source （PHA）. At relatively higher COD/N ratios, similar N2O reduction rates were obtained in all cases regardless of the type of carbon source available. N2O reduction rates were, however, generally higher in the presence of an internal carbon source. Results from the study imply that when the presence of an external carbon source is not sufficient to support denitrification, it is likely competitively utilized by different metabolic pathways of denitrifying polyphosphate accumulating organisms （DPAOs） and other ordinary denitfifiers. This study also reveals that the consumption of PHA is potentially the rate-limiting step for N2O reduction during denitrification.

Anoxic phosphorus removal in a pilot scale anaerobic-anoxic oxidation ditch process

The anaerobic-anoxic oxidation ditch(A^(2)/O OD)process is popularly used to eliminate nutrients from domestic wastewater.In order to identify the existence of denitrifying phosphorus removing bacteria(DPB),evaluate the contribution of DPB to biological nutrient removal,and enhance the denitrifying phosphorus removal in the A^(2)/O OD process,a pilot-scale A^(2)/O OD plant(375 L)was conducted.At the same time batch tests using sequence batch reactors(12 L and 4 L)were operated to reveal the significance of anoxic phosphorus removal.The results indicated that:The average removal efficiency of COD,NH^(+)_(4),PO^(3–)_(4),and TN were 88.2%,92.6%,87.8%,and 73.1%,respectively,when the steady state of the pilotscale A^(2)/O OD plant was reached during 31–73 d,demonstrating a good denitrifying phosphorus removal performance.Phosphorus uptake took place in the anoxic zone by poly-phosphorus accumulating organisms NO^(-)_(2) could be used as electron receptors in denitrifying phosphorus removal,and the phosphorus uptake rate with NO^(-)_(2) as the electron receptor was higher than that with NO^(–)_(3) when the initial concentration of either NO^(-)_(2) or NO^(–)_(3) was 40 mg/L.

Optimization of phosphorus removal in uniFed SBR system for domestic wastewater treatment

The characteristic of phosphorus removal andappropriate change of the traditional operation modes wereinvestigated in UniFed sequencing batch reactor (SBR)laboratory-scale apparatus (40 L), treating actual domesticwastewater with low ratios of C/N (2.57) and C/P (30.18),providing theoretical basis for actual application ofwastewater treatment plant. UniFed SBR system with itsunique operation mode had the distinct superiority ofphosphorus removal. On this occasion, the effect ofvolumetric exchange ratio (VER) and the method ofinfluent introduction for phosphorus removal were studied.When the carbon source became the limiting factor tophosphorus release, the higher the VER, the lower thephosphorus concentration in the effluent. Three differentinfluent patterns, including one-time filling, four-timefilling, and continuous filling with the same quantity ofwastewater could increase the release rate of anaerobicphosphorus from 0.082 to 0.143 mg·P·(L·min)-1. Appropriatechange of the traditional operation modes couldoptimize the efficiency of phosphorus removal. When thefeed/ decant time was extended from 2 h to 4 h, thephosphorous removal efficiency increased from 59.93% to88.45% without any external carbon source. In the mode ofalternation of anoxic-aerobic (A/O) condition, phosphorousremoval efficiency increased from 55.07% to 72.27%clearly. The carbon source in the influent can be usedadequately, and denitrifying phosphorus removal wascarried out in anoxic stage 2 (A2). This mode was optimalfor the treatment of actual domestic wastewater with lowC/N and C/P ratios.

Effect of temperature on anoxic metabolism of nitrites to nitrous oxide by polyphosphate accumulating organisms

Temperature is an important physical factor, which strongly influences biomass and metabolic activity. In this study, the effects of temperature on the anoxic metabolism of nitrite （NO2） to nitrous oxide （N2O） by polyphosphate accumulating organisms, and the process of the accumulation of N2O （during nitrite reduction）, which acts as an electron acceptor, were investigated using 91% ：e 4% Candidatus Accumulibacterphosphatis sludge. The results showed that N2O is accumulated when Accumulibacter first utilize nitrite instead of oxygen as the sole electron acceptor during the denitrifying phosphorus removal process. Properties such as nitrite reduction rate, phosphorus uptake rate, N2O reduction rate, and polyhydroxyalkanoate degradation rate were all influenced by temperature variation （over the range from 10 to 30℃ reaching maximum values at 25℃）. The reduction rate of N2O by N2O reductase was more sensitive to temperature when N2O was utilized as the sole electron acceptor instead of NO2, and the N2O reduction rates, ranging from 0.48 to 3.53 N2O-N/（hr.g VSS）, increased to 1.45 to 8.60 mg N2O-N/（hr·g VSS）. The kinetics processes for temperature variation of 10 to 30℃ were （01 = 1.140-1.216 and θ2 = 1.139-1.167）. In the range of 10℃ to 30℃, almost all of the anoxic stoichiometry was sensitive to temperature changes. In addition, a rise in N2O reduction activity leading to a decrease in N2O accumulation in long term operations at the optimal temperature （27℃ calculated by the Arrhenius model）.