Complete nitrification–denitrification of swine manure in a full-scale, non-conventional composting system

Abstract

A full-scale composting plant (track type, aerated by screws), treating liquid swine manure (94.8% on mass basis) with straw (<0.8%) and sawdust (4.4%), was monitored. The main objectives were testing the performance of the process and assessing its environmental sustainability. Particular attention was dedicated to verify the possibility that this process could determine significant mass reduction, along with Nitrogen reduction, mainly by denitrification. Emissions were evaluated by measuring NH3, N2O and CH4 (by static chamber), H2S and odor emissions (by dynamic olfactometry). Quality and quantity of inputs and outputs and process parameters (redox, oxygen, and temperature) were monitored. The process produced a mature, highly humified (Humification Index=0.27), solid product with 92.8% mass reduction (mainly evaporation), and nitrogen reduction (85.8% referred to input TN). The process was revealed to be environmentally sustainable: emissions of odors and H2S resulted negligible; emissions of N–N2O represented 0.18% of TN input, while emissions of N–NH3 represented 0.87% of input TN. Microbiological analyses determined the presence of 107 CFU/g of bacteria related to N cycle and real time PCR demonstrated the presence in the final product of 4.77⋅107 couples of genes of Bacterial amoA/gTS and 2.46⋅107 couples NosZ/gTS, indicating nitrification and complete denitrification. These results exhibit that nitrification and complete denitrification can efficiently occur in a composting process effectively transforming N2O into N2 as consequence of the optimized alternation of aerated and anoxic phases in the feedstock.