1.
Side-stream sludge treatment using free nitrous acid selectively eliminates nitrite oxidizing bacteria and achieves the nitrite pathway.
Wang, Q, Ye, L, Jiang, G, Hu, S, Yuan, Z
Water research. 2014;:245-55
Abstract
Nitrogen removal via nitrite (i.e. the nitrite pathway) is beneficial for carbon-limited biological wastewater treatment plants. This study presents a novel strategy for achieving the nitrite pathway, which involves recirculating a portion of the activated sludge through a side-stream sludge treatment unit, where the sludge is subject to treatment with free nitrous acid (FNA i.e. HNO2). The strategy is proposed based on a novel discovery reported in this work that in the concentration range of 0.24-1.35 mg HNO2(-)-N/L, FNA is substantially more biocidal to nitrite oxidizing bacteria (NOB) than to ammonium oxidizing bacteria (AOB). Two sequencing batch reactors (SBR) treating synthetic domestic wastewater were used to demonstrate the concept, with one serving as an experimental reactor and the other as a control. In the experimental system, 22% of the sludge from the SBR was transferred to the side-stream treatment unit each day, and was subject to FNA treatment at 1.35 mg N/L for 24 h and then returned to the SBR. The nitrite pathway was rapidly (in 15 d) established in the experimental reactor with an average nitrite accumulation ratio (NO2(-)-N/(NO2(-)-N + NO3(-)-N) × 100%) of above 80%. Fluorescence in-situ hybridization demonstrated that the NOB population in the experimental reactor was 80% lower than that in the control reactor, indicating that the majority of NOB were eliminated from the experimental reactor. The FNA-based strategy for establishing the nitrite pathway substantially improved total nitrogen removal, and did not increase N2O emission or deteriorate sludge settleability. The strategy can be easily integrated with a previously demonstrated strategy, which enhances methane production through pre-treatment of secondary activated sludge, to enable maximum energy recovery while achieving improved nitrogen removal.
2.
Experimental evaluation of decrease in bacterial activity due to cell death and activity decay in activated sludge.
Hao, X, Wang, Q, Zhang, X, Cao, Y, van Mark Loosdrecht, CM
Water research. 2009;(14):3604-12
Abstract
Decrease in bacterial activity (cell decay) in activated sludge can be attributed to cell death (reduction in the amount of active bacteria) and activity decay (reduction in the specific activity of active bacteria). The aim of this study was to experimentally differentiate between cell death and activity decay as a source of decrease in microbial activity. By means of measuring maximal oxygen uptake rates, verifying membrane integrity by live/dead staining and verifying presence of 16S rRNA with fluorescence in-situ hybridization, the decay rates and the death rates of ammonium oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB) and ordinary heterotrophic organisms (OHOs) were determined respectively in a nitrifying sequencing batch reactor (SBR) and a heterotrophic SBR. The experiments revealed that in the nitrifying system activity decay contributed 47% and 82% to the decreased activities of AOB and NOB and that cell death was responsible for 53% and 18% of decreases in their respective activities. In the heterotrophic system, activity decay took a share of 78% in the decreased activity of OHOs, and cell death was only responsible for 22% of decrease in their activity. The difference between the importance of cell death on the decreased activities of AOB and OHOs might be caused by the mechanisms of substrate storage and/or cryptic growth/death-regeneration of OHOs. The different nutrient sources for AOB and NOB might be the reason for a relatively smaller fraction of cell death in NOB.
3.
Effect of solids retention time and temperature on waste activated sludge hydrolysis and short-chain fatty acids accumulation under alkaline conditions in continuous-flow reactors.
Feng, L, Wang, H, Chen, Y, Wang, Q
Bioresource technology. 2009;(1):44-9
Abstract
The effects of solids retention time (SRT) and temperature on waste activated sludge (WAS) hydrolysis and short-chain fatty acids (SCFAs) accumulation were investigated in a series of continuous-flow reactors at pH 10. The experimental results showed that the increase of either SRT or temperature benefited the hydrolysis of WAS and the production of SCFAs. The changes in SRT gave also impact on the percentage of acetic and propionic acids in the fermentative SCFAs, but little influence on that of the slightly long-chain SCFAs, such as n-butyric, iso-butyric, n-valeric and iso-valeric acids. Compared with the control (pH unadjusted) experiment, at SRT of 12d and temperature of 20 degrees C the concentration of SCFAs produced at pH 10 increased from 261.2 to 933.5mg COD/L, and the propionic acid percentage improved from 11.7 to 16.0%. It can be concluded from this investigation that the efficient continuous production of SCFAs at pH 10 is feasible.