THE IMPACT OF DEWATERING EQUIPMENT ON ODORANT PRODUCTION FROM ANAEROBICALLY DIGESTED BIOSOLIDS

This study was conducted to evaluate factors influencing odorant production from high-solids dewatering equipment. In an initial experiment, the headspace of a high-solids centrifuge dewatered cake sample was analyzed for methanethiol, dimethyl sulfide, indole, skatole and p-cresol. It was found that the concentration of the volatile sulfur compounds were two orders of magnitude greater than the other odorants. The odorant emission profiles were very different with the VSC emissions mainly occurring within the first two weeks of storage and the indole, skatole and p-cresol emissions mainly occurring after two-weeks of storage. In an additional experiment, three types of dewatering processes were evaluated specifically for VSC production characteristics. Biosolids cakes were obtained from full-scale and laboratory-scale dewatering trials and were anaerobically stored for several days. The type of dewatering equipment (for similar cake solids) impacted VSC production characteristics considerably. The cakes obtained from a high-solids centrifuge produced the highest amount of VSC. The cakes obtained from a low-solids centrifuge produced the next highest amount of VSC. The cakes obtained from a belt filter press (BFP) simulator produced no detectable VSC. Additional BFP simulation tests showed that the desiccation of cakes (using a desiccant) followed by storage did not increase VSC emissions. Air-drying of cakes to 35% dry solids content and subsequent anaerobic storage also did not increase VSC emissions. The shearing of BFP cake with subsequent anaerobic storage resulted in VSC production and emission (dimethyl sulfide). It is therefore hypothesized that the cause of the VSC production and emission at least in part may be from shear during dewatering. The role of high- and low-solids centrifuges in VSC production was further evaluated by analyzing for labile protein, protein removal rates and methane emission rates. Protein lability was correlated with methanethiol and DMS emission. In addition, headspace methane emission rates was also related with a decrease in peak methanethiol and DMS emission. In a final experiment, protein removal and methane emission were related to peak methanethiol emission. It is proposed that an odor cycle exists for production and degradation of odorants in biosolids and balancing of these production and degradation pathways may be required for successful odor mitigation.