Abstract
Sludge retention time (SRT) is an important factor affecting not only the performance of the nutrient removal and sludge characteristics, but also the production of secondary pollutants such as nitrous oxide (N2O) in biological nutrient removal (BNR) processes. Four laboratory-scale sequencing batch reactors (SBRs), namely, SBR5, SBR10, SBR20 and SBR40 with the SRT of 5 d, 10 d, 20 d and 40 d, respectively, were operated to examine effects of SRT on nutrient removal, activated sludge characteristics and N2O emissions. The removal of chemical oxygen demand or total phosphorus was similar under SRTs of 5–40 d, SRT mainly affected the nitrogen removal and the optimal SRT for BNR was 20 d. The molecular weight distribution of the effluent organic matters was in the range of 500–3,000 Da under SRTs of 5–40 d. The lowest concentration of the effluent soluble microbial products concentration was obtained at the SRT of 5 d. Nitrifier growth was limited at a short SRT and nitrite existed in the effluent of SBR5. With increasing SRTs, mixed liquor suspended solids concentration increased while the excess sludge production was reduced due to the high endogenous decay rate at high SRTs. Endogenous decay coefficients were 0.020 d−1, 0.036 d−1, 0.037 d−1 and 0.039 d−1 under SRTs of 5–40 d, respectively. In BNR, the N2O emission occurred mainly during the aerobic phase and its emission ratio decreased with increasing SRTs. The ratio between the N2O-N emission and the removed ammonium nitrogen in the aerobic phase was 5%, 3%, 1.8% and 0.8% at the SRT of 5 d, 10 d, 20 d and 40 d, respectively. With low concentrations of dissolved oxygen and high concentrations of oxidized nitrogen, the N2O emission was significantly accelerated due to heterotrophic denitrification activities.
Highlights
The increasing occurrence of eutrophication has become a serious environmental problem.Nutrients such as nitrogen and phosphorus have been recognized as the major factors inducing eutrophication
Shan et al [26] examined the N2O emission of Biological nutrient removal (BNR) under Sludge retention time (SRT) of 9 d and 15 d, and showed that most of the N2O emission came from the aerobic phase and there was more N2O emission at low SRTs, high nitrification efficiency was achieved in both systems
The specific mass phosphorus release rates were in the range of 1.7–6.7 mg P/g volatile suspended solids (VSS), and these were much lower than the reported values between 8.4–22.6 mg P/g VSS from other lab-scale studies [40], showing that the bioactivity of polyphosphate accumulating organisms (PAOs) was relatively low in the present study
Summary
The increasing occurrence of eutrophication has become a serious environmental problem. Sludge retention time (SRT) is an important factor which affects the performance of nutrient removal and sludge characteristics and the production of secondary pollutants such as nitrous oxide (N2O) in the BNR. Shan et al [26] examined the N2O emission of BNR under SRTs of 9 d and 15 d, and showed that most of the N2O emission came from the aerobic phase and there was more N2O emission at low SRTs, high nitrification efficiency was achieved in both systems. Previous studies do not explain why more N2O is produced under low SRTs. In this study, four lab-scale BNR process were operated to examine the operating performance at different SRTs, including nutrient removal, sludge characteristics and the N2O emission, with the aim of providing valuable information for operation of WWTPs
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