A Comparison of Biosolids Odorants and Odors Emitted from Belt Filter Press and Centrifuge Dewatering

Water resource recovery facilities (WRRFs) invariably question the timing and the advantages of modifying their biosolids dewatering systems from belt filter presses to centrifuges, as this change can reduce the water content from about 80% to 70%. The upgrade makes perfect sense keeping in mind the ever-increasing distances of transporting the biosolids to end users. However, a factor of concern that needs to be considered, besides the investment capital, is the alleged sharp increase of emitted odors by centrifuge dewatering, causing a negative public impact. This paper compares the chemical and sensory odor results from a large WRRF that originally dewatered its anaerobically digested biosolids through belt filter presses and then replaced this process with dewatering centrifuges. No major changes in the wastewater characteristics or in the unit operations occurred in the interval, which provides a unique opportunity to observe the changes in odor emissions. The results show that there is a change in the predominant chemical odorants emitted as well as a sharp increase in the variety of odor characters and intensities corresponding with the change in dewatering technology. Worst-case odorant concentrations (the maximum value observed) from belt filter press to centrifuge dewatering significantly increased from 52 to 4980 ppb for hydrogen sulfide, increased from 390 to 1100 ppb for methyl mercaptan, increased from 520 to 3200 ppb for dimethyl sulfide, and increased from 27 to 270 ppb for dimethyl disulfide. At the same time, whereas the worst-case rotten vegetable odor intensity remained the same, and earthy/musty decreased by one order of magnitude, the fecal and manure odor intensities increased by two orders of magnitude. Keeping in mind that odor intensity is reported in a log scale, these changes are significant, especially as they peak at the top end of the odor intensity scale.