A pre- and post-denitrification system treating a very high ammonia landfill leachate: Effects of pH change on process performance

The prime objective of this research was to investigate the nitrogen remova l capabilities of a pre and post-denitrification process, when treating lan dfill leachate containing an ammonia concentration of over 2200 mg N l(-1). The treatment system, also known as a 4-Stage Bardenpho process, was opera ted 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 an oxic reactor of the system. Methanol was used as the organic carbon source for denitrification. At an influent ammonia concentration of about 2200 mg N l(-1), the anoxic pH levels stabilized at about 8.6 within the first reac tor, and at about 9.8 within the second reactor. These high anoxic pH level s were partially responsible for decreased denitrification and, hence, resi dual NO concentrations in the effluent. By decreasing the pH in the anoxic basins, the overall performance of the system immediately improved; the eff luent NV, 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 rec overed and stabilized, denitrification in each anoxic reactor remained at o nly 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 resu lted in enhanced nitrification performance, complete ammonia removal in the first aerobic reactor, and a more stable, overall performance in the 4-Sta ge Bardenpho process.