Enhanced Biological Phosphorus Removal In Alternating Aerated Biofilter System

This study is a part of Project (2004BA604A01) supported by the National Tenth 5-year Key Task Project of Science and Technology.It is well known that submerged fixed-film biofilter has proved to be reliable for organic removal in wastewater treatment. For enhanced biological phosphorus removal (EBPR), biofilter should be subjected to alternating anaerobic/aerobic conditions to accumulate phosphorus-accumulating organisms (PAOs). According to literatures, biofilter is not common for EBPR. Backwashing of filters must take place for removal of phosphorus from biofilter, which is the only means for removal of P-rich biomass.Regular and intensive backwashing is necessary for the efficient operation of EBPR in biofilter, while frequent backwashing may increase energy cost in the operation. The removal of P-rich biomass is a key limitation in biofilm EBPR system.In this study, a combined fixed-film system developed for EBPR consists of anaerobic biofilter, intermittent aerated biofilter (IABF), and aerobic biofilter in treatment of domestic wastewater. IABF is the main body for P removal in the system, which is also the emphasis in this study. An aerobic continuous feeding (ACF) regime is applied to IABF, which includes the discharge of P-rich water at the end of anaerobic phase in cycle duration (CD), and continuous feeding in the subsequent aerobic phase. The innovative method for removal of phosphorus from IABF makes an efficient P removal and longer operation duration without wasting in the reactor, where performance of P removal is no longer dependent on backwashing intervals, in contrast to traditional biofilm EBPR system. The main conclusions in this study are presented as follows:①At a hydraulic loading of 1.68～8.39 m3m-2d-1, the effluent concentration of COD and TP are 8～84mg/L, 35 mg/L in average and 0.15～0.84mg/L, 0.48 mg/L in average, respectively. C/N ratio in the sewage is 3.3～6.5, and 4.3 in average. The average effluent concentration of ammonium and TN are 4.2 mg/L and 40.6 mg/L, respectively. With recirculation of the effluent of the combined system to anaerobic reactor, TN removal efficiency increases from 36.8 % without the recirculation to 51.9 % with recirculation ratio of 1:1. The result indicates that the system has a good adaptability of fluctuations in aerobic loadings, and achieves a stable performance during the experimental period. Anaerobic biofilter is used as a fermentation reactor and fed with the sewage. The complex organic substances in influent is hydrolyzed and decomposed under anaerobic condition, as well as the organic components in particles that is hold up by filter bed in the reactor. It is shown that volatile fatty acids (VFAs) concentration increase after the pre-fermentation and VFAs production ratio grows with hydraulic loading.②Two parallel sets of IABF, named A-IABF and B-IABF, were operated under different loadings to investigate the effects of loadings on performance of P removal in the reactor. At a condition of hydraulic loading of 1.68～8.39m3m-2d-1 and organic loading of 0.96～4.10gCOD L-1d-1, the average effluent concentration of COD and TP in A-IABF are 64 mg/L and 0.47 mg/L, respectively, that of B-IABF operating under a controlled loadings are 69 mg/L and 0.36 mg/L, respectively. The average removal efficiency of COD and TP in A-IABF are 73.6 % and 84.7 %, respectively, that of B-IABF are 73.4 % and 86.1 %, respectively. Performance of B-IABF is stable during the experimental period. While P Removal efficiency in A-IABF is decreasing under low concentration feeding.③Under the ACF regime, anaerobic and aerobic phase in IABF are two separate stages. Reduction rate of organic or P is directly proportional to organic or P loading in aerobic phase. Performance of organic and P removal are similar at the two different CD conditions. It is observed that the organic is mainly reduced at the height of 0~0.06m in the column of IABF, while P is at the height of 0.06~0.56m with the continuous feeding, which implies the distribution of ordinary heterotrophic organisms and PAOs along the flow direction in the reactor.④Comparing with the sequence batch regime, P release rate is not clearly related to initial VFAs concentration in anaerobic phase except for low concentration condition, but directly related to aerobic P uptake rate under the ACF regime. Moreover, aerobic P transformation in biofilm is related to P loading in aerobic phase and a correlation between P transformation in biofilm under anaerobic condition and that of aerobic is observed. The result indicates that aerobic P uptake exerts a strong influence on anaerobic P release, as well as P available in aerobic phase. It is possible that glycogen-accumulating organisms exist in IABF according to the analysis on intracellular polymers content in biofilm of IABF. Moreover, the higher content and transformation of glycogen observed in B-IABF may imply a higher proportion of this kind of group in the reactor.⑤According to P distribution in the bulk and the biofilm during a long backwashing interval, P accumulation in biofilm is related to influent condition, loading rates and operation duration etc. The accumulation could be in different extent at different part of the column of IABF with longer duration or higher loading rate. It demonstrates that the regime of long backwashing intervals is practicable in IABF under the ACF regime. It achieves a lower frequency of backwashing and less energy cost by using the ACF regime, in contrast to the traditional biofilm EBPR system.⑥The performance of P removal in A-IABF and B-IABF is different under a low concentration condition. The decrease of P uptake rate is observed in A-IABF under a higher loading rate, while performance of P removal in B-IABF is stable under a controlled and lower loading rate. It is shown that PHB and glycogen content in biofilm of A-IABF are apparently decreasing during the operation. Moreover, higher P accumulation in A-IABF results in reduction of P storage capacity. Therefore, performance of P removal in A-IABF becomes worse at the conditions.⑦According to P and flow balance of the combined system, 47.5 % of P input of the system can be stepped into further chemical treatment at a condition of hydraulic loading of 4.65 m3m-2d-1 and CD length of 6h. At original pH condition of P-rich water, ferrous sulphate as precipitator, low P content in the liquor is achieved with ferrous/orthophosphate molar ratio of 1.3～1.7:1, as well as low cost of the precipitator.