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IntroductionComposting is one of the important methods of controlling biological waste in developing country like Sri Lanka. Passive windrow composting with turning is a widely used strategy to manage the organic fraction of MSW in the country because; MSW of Sri Lanka contains more than 80% (dry weight basis) of organic matter and has moisture content about 60-70% [1]. But there are many problems associated with the composting plants in Sri Lanka. The major problem is bad odour of the compost piles during the bio-oxidative phase. It is really a social problem of those areas where closer to the composting plants. Main reason is that, unskilled and uneducated labourers, who often find the addition of water results insufficient or excess moisture contents inside the piles, maintain composting plants. If water is accumulated faster than it is eliminated via either aeration or evaporation (driven by high temperatures), then oxygen flow is impeded and anaerobic conditions result [2]. This usually occurs at a moisture level of about 65 percent or higher. If the moisture level drops below about 40 to 45 percent, the nutrients are no longer in an aqueous medium and easily available to the microorganisms. Their microbial activity decreases and the composting process slow. Below 20 percent moisture, very little microbial activity occurs [3].Another reason is that, literature based moisture contents, which have been prescribed for composting are heavily dependent on the composition of MSW, climatic factors of the regions and the turning frequency of windrows. In this study, determine the optimum moisture range in the composting pile was addressed. To achieve it, the performance of the composting process is checked by employing several physic-chemical parameters proposed in literature through which the metabolic activity of the micro-organisms involved in the composting process was evaluated. Methodology Preparation of windrowThe study was carried out in the composting plant at Meethotamulla, which is located in a suburb of the capital city of Colombo, Sri Lanka. Passive windrow composting with manual turning for aeration was the technique used in the study. The organic fraction was segregated from mixed MSW at the site. Four rectangular shaped composting piles were prepared with 1m high × 2 m width × 3 m length. Moisture ranges were adjusted to 60% ± 10% at first 05 weeks and then reduced to 40% ± 10% in pile Control, 40% ± 10% moisture maintain in pile A while 60% ± 10% moisture maintained in pile B and 80% ± 10% moisture maintain in pile C by manual moisture addition throughout the 08 weeks of bio oxidative period. Moisture was added to all the piles once a week to ensure that the moisture content was within the experimental range. The moisture range of pile Control (60% ± 10%) was maintained for first five weeks and then reduced to 40% ± 10% for the remaining three weeks of the bio-oxidative phase which is the practice of some composting plants in Sri Lanka. Piles were turned manually once a week to aerate the compostin...