Käytetyn koagulanttityypin vaikutus nitrifikaatio- ja denitrifikaatioprosesseihin yhdyskuntajätevesien käsittelyssä

Abstract

Biological processes particularly nitrification and denitrification are the most employed methods for removing nitrogen-containing pollutants from wastewater due to its effectiveness and low maintenance cost. The conversion of ammonium to nitrite and ultimately to nitrate (nitrification), as well as the conversion of nitrate to nitrogen gas (denitrification)- which is regarded innocuous to the environment is a key element of these biological approaches. Nitrogen and BOD concentrations, alkalinity, temperature, and the presence of hazardous compounds coming from coagulants used to treat the wastewater are all factors that can affect or be affected by nitrification and denitrification processes. Furthermore, purification activities (i.e., coagulant treatment and residual concentrations in the effluent water) that occur before nitrification and denitrification can have a variety of effects on these processes.

The goal of this master’s thesis study was to investigate the impact of residual coagulant concentrations on the nitrification and denitrification processes. Batch test laboratory studies were set up to imitate the nitrification and denitrification processes independently under controlled conditions to attain this goal. Polyaluminium chloride (pre-hydrolyzed, inorganic product), chitosan (semi-natural organic biopolymer), polyacrylamide and polyamine (synthetic organic polymers) were utilized as coagulants.

Laboratory experiments were conducted in 4 phases: Phase 1- Determination of optimum dosage range of coagulants; Phase 2- Pre-incubation tests of coagulants; Phase 3- Batch incubations for nitrification; Phase 4- Batch incubations for denitrification. Samples of wastewater were taken from the Taskila wastewater treatment plant (Oulu, Finland). The dosage requirement of four different coagulants for the treatment of sewage water were evaluated during the preliminary testing. The influence of the residual coagulants on the progress and effectiveness of the nitrification and denitrification processes was assessed by applying these doses to actual wastewater samples in low, medium, and high dosages.

Overall, a clear pattern in all samples showed an increase in nitrate concentration and a decrease in ammonium concentration during the batch nitrification process regardless of the coagulant type. In addition, there was a clear nitrate reduction in the batch denitrification process. The rates of low dosages of all coagulants were close to that of the control while nitrification and denitrification rates dropped when the coagulant dosage was increased. Changes on the physico-chemical parameters, organic matter, and metals content indicated that the processes occurred accordingly and showed the different effects between coagulant types.