Simultaneous nitrification and denitrification in membrane bioreactor: Effect of dissolved oxygen.

Membrane bioreactor with the floc activated sludge (mixed liquor suspended solids (MLSS) = 7500 mg/L) was constructed in this work for simultaneously nitrification and denitrification (SND). The effect of dissolved oxygen (DO) on SND process and the nitrogen pathways were investigated. The average TN removal efficiencies were 63.05%, 91.17%, 87.04% and 70.02% for DO 0.5, 1, 2 and 3 mg/L systems, respectively. The effluent ammonia concentration was continuously lower than 5.0 mg/L when the DO was higher than 1 mg/L. Nitrogen in DO 1 and DO 2 mg/L systems was mainly removed via the SND process. The rise of DO concentration increased the abundance of nitrite oxidizing bacteria (NOB) and Nitrospira was the predominant NOB in all the four MBRs. Dechloromonas and Azoarcus were the dominant denitrifying bacteria (DNB) in DO 1 systems responsible for nitrite denitrification. The dominant aerobic DNB Pseudomonas also contributed SND via nitrate denitrification and was little affected by DO changes. Nitrate reductase was the main enzyme for the reduction of NO3--N to NO2--N, and narG was the main responsible gene. Nitrite oxidoreductase was the main enzyme for the oxidation of NO2--N to NO3--N, and nxrA was the main responsible gene in all the four MBR systems.

Promoting lignocellulose degradation during green waste composting by maintaining a specific temperature through heap size control.

Owing to its high lignocellulose content, the recalcitrance of green waste is a technical challenge obstructing the composting process. This study aimed to identify a temperature that could facilitate efficient lignin and cellulose degradation during green waste composting, and maintain this temperature by controlling the heap size to enhance the degradation. The optimum temperature was determined by conducting a laboratory-scale cultivation experiment under controlled temperatures, and a pilot-scale experiment was conducted to explore heap size control and its influence on green waste composting. The results showed that efficient lignin and cellulose degradation was achieved when the temperature was between 45 and 60 ℃, and maintaining this temperature for at least 150 days maximized the lignin and cellulose degradation rates. This was achieved by constraining the heap size at 0.8 m3 at the beginning of composting; 1.56, 2.60, and 4.00 m3 on days 15, 39, and 96; and then enlarging the heap as much as possible on day 156. Following this approach, the duration of the target temperature was extended by over six times, the lignin and cellulose degradation rates were increased by 18.82-21.38 % and 9.54-11.55 %, and nitrification and humification were enhanced. Correlation analysis showed that lignocellulose degradation, nitrification, and humification were positively and significantly correlated with the duration of the target temperature. Generally, heap size control is an ecological and economic method of enhancing the efficiency and quality of green waste composting and compost, respectively.