The main objective of this thesis is to evaluate the risk of clogging in the leachate recirculation system and to study the methods to reduce the risk. Furthermore, the aim is to study the feasibility of biological anoxic-aerobic processes to pre-treatment of landfill leachate at low-temperature. Importance is placed on the effect of activated sludge process and biofilm process. The experimental part is conducted by using laboratory-scale facilities and leachates from Ämmässuo landfill.

The removal of both NH4-N and NOx-N are the major parameters of the effects of the treatment processes. Finally, it is expected to discover an economical and efficient solution on the leachate pre-treatment for leachate recirculation system in Ämmässuo. This solution can supply the leachate recirculation system with safe recirculated leachate and help it to achieve the optimal results.

Moreover, the experimental work can also provide the experiences for the future leachate treatment in Ämmässuo, which can reduce landfill leachate impacts on environment and the costs to treat the landfill leachate. Even the leachate pre-treatment for leachate recirculation system and the future leachate treatment in Ämmässuo can be made an overall plan together, because the only difference between these are the requirements for their effluents.