Odor generation patterns during different operational composting stages of anaerobically digested sewage sludge

This study aimed to evaluate the global patterns of odor generation and odorant composition for different operational stages of anaerobically digested sewage sludge (ADS) composting at pilot scale. To this end, gas emissions were sampled and analyzed during storage, forced aeration treatment (active phase), turning process and curing. For each operational stage, odors were monitored by measuring the odor emission rates (OER in OU_E h^-1 kg^-1ADS) through dynamic olfactometry and computing the odor activity values (OAVs) of compounds quantified by analytical methods (i.e., GC/MS). Ammonia and volatile sulfur compounds (VSCs) were the most abundant air pollutants, representing 55.5% and 20.6% of the cumulative mass emitted, respectively. The first eight days of aerobic treatment and the first turning of the compostable mixture were the critical steps for odor generation with OER ranging from 30 to 317 OU_E h^-1 kg^-1ADS. Particularly, the first turning process was responsible for strong odor episodes that were emitted in a short process time (295 OU_E h^-1 kg^-1ADS). Based on the OAVs approach, dimethyl disulfide, dimethyl sulfide, and methanethiol were the predominant odorants along these early operational stages. Odor potential and composition shifted for the middle and later active phase, second turning, and curing stage where OER fluctuated from 0.18 to 12.6 OU_E h^-1 kg^-1ADS, and hydrogen sulfide showed the most substantial odor contribution. A principal component analysis explaining 77% of the variability in odor concentration and OAVs datasets eased the recognition of these odor patterns.