Production of odorous indole, skatole, p-cresol, toluene, styrene, and ethylbenzene in biosolids

Six odorous volatile aromatic compounds (OVACs) were identified through GC-MS analysis of headspace samples from stored biosolids, including toluene, ethylbenzene, styrene, p-cresol, indole, and skatole. Based on the chemical structures of these OVACs and past research, they are likely to be the degradation products of protein, more specifically the aromatic amino acids tryptophan, tyrosine, and phenylalanine. The objectives of this research were to determine if the pathways for production of OVACs from protein or amino acid degradation existed in biosolids. If so, the results could be used to develop odor control strategies. Tryptophan addition was tested and an elevated concentration of indole was observed immediately after 1 day of storage. Unlike the immediate formation of indole, skatole appeared to have a lag-period before accumulation. This suggests the importance of available tryptophan in biosolids to indole and skatole production. In addition, p-cresol accumulated Immediately after the addition of tyrosine. Phenylalanine, which was expected to be the precursor for toluene, ethylbenzene, and styrene formation, did not show any influence on their production except at day 1. Previous research indicated that methanogens are responsible for the degradation of VOSCs, and VOSCs accumulate when methanogens are inhibited. However, no apparent concentration change of OVACs was observed when 2-bromoethanesulfonic acid (BES) was incorporated into biosolids for methanogen inhibition. The observed OVAC profiles indicated a slow and low level production of OVACs, which lasted for at least six months during biosolids storage. With the low odor detection threshold of these six odorous compounds, they may be the major causes for the malodor in biosolids during long-term storage.