A study was undertaken to examine the feasibility of biologically treating a combined waste stream of landfill leachate and municipal sewage. The ratio of sewage to leachate was 9 to 1 by volume. The combined waste had an average BOD5 430 mg/l, COD 1090 mg/l, and TKN 133 mg/l (80% of which was in the form of ammonia). A laboratory-scale sequencing batch activated sludge reactor was used to carry comparative performance evaluations of biological treatment, including nitrification and denitrification. The SBR reactor was operating in daily time cycles employing the following sequential operation phases: filling phase, anoxic phase, aeration reaction phase, settling phase, and drain phase. In particular, the anoxic and aeration periods were tailored in order to develop conditions conducive to desired nitrification and denitrification. During the reaction period, the process was operated under an extended aeration mode with the MLSS concentration being around 3500 mg/l. The results indicated that successful biotreatment of combined leachate and sewage was possible, with the trealed effluent being low in BOD5 and COD. The system was capable of BODS removal efficiencies exceeding 95%. Furthermore, nitrate removal during the anoxic phase was approximately 99% due to denitrification. However, the overall nitrogen removal during a full cycle was about 50%. The inclusion of an anoxic period right after the aeration phase enhanced the nitrogen removal efficiency, yet this phase required the addition of an external carbon source to the reactor due to the low concentration of biodegradable carbon, and at the same time the process became less efficient in BOD removal.