Co-composting of sewage sludge and municipal solid waste

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Courtesy of ORBIT e.V.

Israel production of sewage sludge and MSW grows each year due to the increasing number of modern Wastewater plants that are using activated sludge process and the increasing population. Non-stabilized Sewage sludge is an environmental nuisance due to high level of pathogens, offensive odours, insect attraction and potential water sources pollution.

The Haifa region itself, where the study took place, generates 200 tons of anaerobic digested sludge and 800 tons of MSW each day containing 50% organic matter.

The general policy of the ministry of the environment is to reduce land and sea disposal of sludge and to encourage recycling for agricultural use.

Most Sewage plants in Israel utilise anaerobic stabilisation that is not sufficient for pathogen reduction and volatile solid reduction.

In order to recycle sewage sludge in an environmentally safe manner, the sludge must undergo a stabilization process that will make the product safe for the environment and attractive for agricultural uses.

Effective stabilisation process must include:

1. Long term thermal treatment to reduce pathogens and destroy seeds.
2. Massive reduction in volatile solids concentration to reduce bad odours, vector attractions and eliminating
re-growth of pathogens.
3. Aerobic conditions to degrade phytotoxic organic acids.

Studies done in the past showed that combined windrow composting of sludge with MSW (co-composting) can provide a low cost solution to stabilisation problems. The solid waste particles function as bulking agent for aerobic digestion of the sludge and the high temperature resulting from the composting process acts as a further treatment to reduce pathogens.

Among the problems that came up from studies in the past was the slow rate of pathogen elimination in the turning phase, which meant that effective reduction of pathogens was only achieved after a long curing phase. This and other factors lead to the difficulty of determining standard Time/Temperature protocols for windrow composting. Studies performed at our laboratory (Robinzon, 1997) on Israeli MSW composting had shown that the pile temperatures are relatively high and that VS reductions of 45% were achieved. Drying rates of the piles were high and the addition of water at 300-400 litre /Ton wet waste was needed to maintain constant moisture. 

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