Advanced nitrogen removal via nitrite from municipal landfill leachate using a two-stage UASB–A/O system

A system consisting of a two-stage up-flow anaerobic sludge blanket （UASB） reactor and an anoxic/aerobic （A/O） reactor was used to treat municipal landfill leachate. Denitrification took place in the first stage of the UASB reactor （UASB1）. The chemical oxygen demand of the UASB1 effluent was further decreased in the second stage （UASB2）. Nitrification was accomplished in the A/O reactor. When diluted with tap water at a ratio of 1：1, the ammonia nitrogen concentration of the influent leachate was approximately 1200 mg· L^-1, whereas that of the system effluent was approximately 8-11 mg· L^-1, and the corresponding removal efficiency is about 99.08%. Stable partial nitrification was achieved in the A/O reactor with 88.61%-91.58% of the nitrite accumulation ratio, even at comparatively low temperature （ 16℃）. The results demonstrate that free ammonia （FA） concentrations within a suitable range exhibit a positive effect on partial nitrification. In this experiment when FA was within the 1-30 mgmg· L^-1 range, partial nitrification could be achieved, whereas when FA exceeded 280 mgmg· L^-1, the nitrification process was entirely inhibited. Temperature was not the key factor leading to partial nitrification within the 16-29 ℃ range. The inhibitory influence of free nitrous acid （FNA） on nitrification was also minimal when pH was greater than 8.5. Thus, FA concentration was a major factor in achieving partial nitrification.

Achieving nitritation at low temperatures using free ammonia inhibition on Nitrobacter and real-time control in an SBR treating landfill leachate

Free ammonia（FA） inhibition on nitrite-oxidized bacteria（NOB） and real-time control are used to achieve nitrogen removal from landfill leachate via nitrite pathway at low temperatures in sequencing batch reactor. The inhibition of FA on NOB activity during the aerobic period was prolonged using real-time control. The degree of nitrite accumulation was monitored along with variations of the ammonia-oxidizing bacteria and NOB population using fluorescence in situ hybridization techniques. It is demonstrated that the end-point of ammonia oxidization is detected from the on-line measured dissolved oxygen,oxidization–reduction potential, and p H signals, which could avoid the loss the FA inhibition on NOB caused by excess aeration. At low temperature（13.0–17.6°C）, the level of nitrite pathway rapidly increased from 19.8% to 90%, suggesting that nitritation was successfully started up at low temperature by applying syntrophic association of the FA inhibition and real-time control, and then this high level of nitrite pathway was stably maintained for as long as 233 days. Mechanism analysis shows that the establishment of nitritation was primarily the result of predominant ammonia-oxidizing bacteria developed in the nitrifying bacteria population compared to NOB. This was mainly due to a gradual reduction of nitrite amount that is available to provide energy for the growth of NOB,eventually leading to the elimination of NOB from the bacterial clusters in sequencing batch reactor sludge system.