The effect of the composting time on the gaseous emissions and the compost stability in a full-scale sewage sludge composting plant

Abstract

Volatile organic compounds (VOCs) and ammonia are some of the compounds present in gaseous emissions from waste treatment facilities that contribute to odour pollution. In the present work, the effect of the residence time on the biological stability of raw sludge (RS) composted in dynamic windrows and the gaseous emissions generated were studied at a full-scale composting plant, aiming to provide specific pollutant emission factors and to determine their variability depending on the composting time. Waste stability and emissions analysis considered both a first phase where mixed RS and vegetal fraction (RS - VF) is actively composted in dynamic windrows and a second standard curing phase in turned piles, which lasted 31 days. Two windrows were operated at 4 days of composting time while two other windrows were operated simultaneously at 14 days composting time. Increasing the residence time leads to a better waste stabilization in the first composting phase, providing a 50% reduction of the Dynamic Respiration Index. A decrease of the ammonia emission factor was achieved when increasing the composting time (from 168.5 g NH₃·Mg^-1RS - VF d^-1 to 114.3 g NH₃·Mg^-1RS - VF d^-1), whereas the VOCs emission factor was maintained for the same process conditions (between 26.0 and 28.0 g C-VOC·Mg^-1RS - VF d^-1). However, an increase of the emission masses of both pollutants was observed (from 0.16 to 0.39 kg tVOCs·Mg^-1RS - VF and from 1.21 to 1.60 kg NH₃·Mg^-1RS - VF). Finally, ammonia and VOCs emissions generated at the curing piles were nearly avoided when increasing the composting time of the first phase.

Keywords: Ammonia; Biological stability; Composting; Gaseous emissions; VOC; Wastewater sludge.