A pre- and post-denitrification system treating a very high ammonia landfill leachate: effects of pH change on...

The prime objective of this research was to investigate the nitrogen removal capabilities of a pre- and post-denitrification process, when treating landfill leachate containing an ammonia concentration of over 2,200 mg N l(-1). The treatment system, also known as a 4-Stage Bardenpho process, was operated with an external recycle ratio of 3:1 and an internal recycle ratio of 4:1. The first anoxic reactor actual hydraulic retention time was 1.5 hours, while the first aerobic retention time was 3 hours. The very high ammonia concentration was simulated by pumping ammonium chloride into the first anoxic reactor of the system. Methanol was used as the organic carbon source for denitrification. At an influent ammonia concentration of about 2,200 mg N l(-1), the anoxic pH levels stabilized at about 8.6 within the first reactor, and at about 9.8 within the second reactor. These high anoxic pH levels were partially responsible for decreased denitrification and, hence, residual NO concentrations in the effluent. By decreasing the pH in the anoxic basins, the overall performance of the system immediately improved; the effluent NOx concentration decreased rapidly over a period of about six days, from an average 80 mg N l(-1) to about 60 mg N l(-1), with some samples as low as 40 mg N l(-1). Subsequently, increased leachate toxicity resulted in an unexpected system failure; although the treatment system eventually recovered and stabilized, denitrification in each anoxic reactor remained at only about 55%, with final effluent NOx concentrations of over 100 mg N l(-1). Despite this reduced level of performance, the decrease in anoxic pH resulted in enhanced nitrification performance, complete ammonia removal in the first aerobic reactor, and a more stable, overall performance in the 4-Stage Bardenpho process.