[Reactivation Performance of Nitrosation Flocculent Sludge After Long-term Storage]

In order to explore the nitrogen removal performance of nitrosation flocculent sludge subject to long-term storage, nitrosation flocculent sludge which had been stored at 4℃ for ten months was inoculated in a continuous stirred tank reactor (CSTR) to investigate its activity recovery performance. MiSeq high throughput sequencing technology was also employed to analyze the structure of the microbial community before and after reactivation. Results showed that, under DO control of DO 0.4-0.8 mg·L^-1, pH value of about 8 and temperature of (30±1)℃, the sludge could be reactivated in 15 days. Both the removal efficiency of ammonia nitrogen and the accumulation ratio of nitrite nitrogen were higher than 90%. The color of the sludge quickly changed from grey at early inoculation to brown, while the sludge volume index (SVI) quickly decreased and both mixed liquor volatile suspended solids (MLVSS)/mixed liquor suspended solids (MLSS) and extracellular polymer substances (EPS) significantly increased. With the recovery of nitrosation performance, anaerobic and heterotrophic bacteria were washed out, the relative abundance of ammomia-oxidizing bacteria such as Nitrosomonas significantly increased, while the growth of nitrite-oxidizing bacteria such as Nitrospria was selectively inhibited. Nitrosation flocculent sludge could be inoculated after long-term storage to achieve rapid start-up of partial nitrification and allow practical application of partial nitrification processes.

[Fast Start-up of Shortcut Nitrification in a CSTR and an MBR]

In order to achieve fast start-up of shortcut nitrification, a continuous stirred tank reactor (CSTR) and a membrane bioreactor (MBR) were selected for a comparative study about the start-up characteristic of shortcut nitrification. The differences in three aspects of the two reactors were investigated, namely start-up time, nitrogen transformation, and sludge performance. The results showed that the start-up of shortcut nitrification was successfully achieved in the CSTR and MBR after 44 and 56 days of operation, respectively, with influent C/N=1, temperature around 30℃±1℃, pH of 7.5-8.0, and dissolved oxygen (DO) of 0.6-1.0 mg·L^-1, as well as combining anoxic/aerobic ratio 1:3 (15 min:45 min) with shortened hydraulic retention time (HRT). The start-up period of the MBR was shorter. On the 14th, 28th, and 56th day, the average nitrite accumulation efficiencies in the CSTR were 51%, 66%, and 89%, respectively, and 50%, 71%, and 93% in the MBR, and nitrate formation rates of the CSTR was successively 7.4, 4.0, and 1.7 mg·(g·h)^-1 (NO₃^--N/MLVSS), respectively, and 7.6, 3.5, and 1.0 mg·(g·h)^-1 (NO₃^--N/MLVSS) for the MBR. A higher nitrite accumulation efficiency and a lower nitrate formation rate were detected in the MBR on the 28th and 56th day, respectively, which could be beneficial to the fast start-up of shortcut nitrification. During the operation period, the shortcut nitrification sludge in the two reactors was yellow, the sludge volume index (SVI) was between 55 and 110 mL·g^-1, and the mixed liquor volatile suspended solids (MLVSS)/mixed liquor suspended solids (MLSS) ratio was around 0.6-0.8. The good sludge performance created favorable conditions for the fast start-up of shortcut nitrification in both the CSTR and MBR. In conclusion, the MBR exhibited better performance in the fast start-up of shortcut nitrification.

Oxidative degradation of tetramethylammonium hydroxide (TMAH) by UV/persulfate and associated acute toxicity assessment

Tetramethylammonium hydroxide (TMAH) is widely used in high-tech industries as a developing agent. Ultraviolet (UV) light-activated persulfate (PS, S₂O₈^2-) can be used to generate strongly oxidative sulfate radicals, and it also exhibits the potential to treat TMAH-containing wastewater. This study initially investigated the effect of S₂O₈^2- concentration and UV strength on the UV/S₂O₈^2- process for the degradation of TMAH in a batch reactor. The results suggested that 15 watts (W) of UV-activated S₂O₈^2- at concentrations of 10 or 50 mM resulted in pseudo-first-order TMAH degradation rate constants of 3.1-4.2 × 10^-2 min^-1, which was adopted for determining the hydraulic retention time (HRT) in a continuous stirred tank reactor (CSTR). The operating conditions (15 W UV/10 mM S₂O₈^2-) with a HRT of 129 min resulted in stable residual concentrations of S₂O₈^2- and TMAH at approximately 2.6 mM and 20 mg L^-1 in effluent, respectively. Several TMAH degradation intermediates including trimethylamine, dimethylamine, and methylamine were also detected. The effluent was adjusted to a neutral pH and evaluated for its biological acute toxicity using Cyprinus carpio as a bioassay organism. The "bio-acute toxicity unit" (TU_a) was determined to be 1.41, which indicated that the effluent was acceptable for being discharged into an aquatic ecosystem.