Reduction of Ammonia Emission and Waste Gas Volume by Composting at Low Oxygen Pressure

Abstract

This chapter reports first results on the performance of a composting reactor with off-gas oxygen levels ≤10%. The low oxygen level is the result of uncoupling the two functions of the airflow: heat removal and oxygen supply. Uncoupling these functions is achieved by cooling and recirculating the gas, in this way reducing off-gas flow and emission of gaseous ammonia. The objective of the research was to identify a control strategy ensuring reactor performance stability, and a sufficient compost quality.

In an 80-li bench-scale composting unit three runs were made with oxygen concentrations of 10% (v/v) (run A), 5% (B) and 1% (C), respectively. Run A was considered the control object; this level is known to operate smoothly. The feed was a mixture of pig faeces and wheat straw 19:1(w/w). Temperature was regulated at 58 °C. It was not possible to reach the set temperature at oxygen levels below 10%, an initial level of 10% was necessary. Once the set temperature was reached, the oxygen level could be lowered.

The loss of ammonia and N_tot, (g-N g⁻¹DM) from the solids was comparable for the three runs. At lower oxygen concentrations however, more ammonia was trapped in the condense: 53% for run A, 60% for B and 77% for C. Compared to run A, the reduction in off-gas was 33% for B and 50% for C. The loss of organic matter was 25% less for B and 33% less for C, but composting time may not have to be increased. The cumulative oxygen consumption after 450 h of composting was comparable for all runs. Apart from small differences in dry matter content, the composts were comparable for all three runs.