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Technology of compost production from sewage sludge with reduction of ammonia emission and heavy metal content

Final Activity Report Summary - CLEANCOMPOST (Technology of compost production from sewage sludge with reduction of ammonia emission and heavy metal content)

At present, sewage sludge management is not considered to be an important source of ammonia emissions. Current data in national and European ammonia emissions inventories is based on too small qualitative information and is not relevant to modern sludge treatment processes. However, the results obtained during the realisation of this project clearly showed that typical way of agricultural usage of sewage sludge can be the source of important ammonia and methane emission. This happens during the stage of composting of sewage sludge and is related with too small carbon / nitrogen (C/N) ratio in composted mass. This low C/N ratio (in real conditions usually under 12) is caused by high price of structural materials (like straw, sawdust or wooden chips). That is why in order to economise the technology costs, the composting companies use insufficient quantities of material with high carbon content which produces the huge ammonia losses.

Additionally, the lack of structural material during composting process causes the anaerobic zones inside the composting heaps. This fact influence on huge methane emission during the whole composting process. Because methane is 20 - 60 times more responsible for greenhouse effect than CO2, the results obtained during CLEANCOMPOST project show the potential big negative environmental impact of sewage sludge composting if the usual practices of sewage sludge management will not be changed.

The experiments with electroremediation of heavy metals from sewage sludge, the second topic of the research, showed positive effect of this technology on heavy metal content. However, it was obtained not directly in sewage sludge matrix but on earlier step, in the phase of liquid wastewater, before addition of flocculants. Looking on large variability of urban wastewater this technology still needs more research for its optimisation. However, the initial results with reduction of heavy metal content seem to be very promising for the future implementation of electroremediation technology on full-scale level.