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The Philadelphia Water Department (PWD) is uniquely situated to place the recent findings of two major WERF-sponsored biosolids studies into the context of detailed performance of a single wastewater facility. That is because Philadelphia's Southwest Water Pollution Control Plant (SW WPCP) stands alone as the one facility used in both WERF reports as a case study for linking performance of single-stage mesophilic anaerobic digesters with characteristics of biosolids odor generation and of fecal coliform reactivation and regrowth. Philadelphia has a monitoring program for the operation of its digesters and dewatering equipment that goes well beyond minimum requirements; for example, it has continuously monitored its digested and dewatered biosolids for fecal coliform since 1994, even though Philadelphia demonstrates compliance for pathogen reduction through time-temperature record keeping, as required for this Process to Significantly Reduce Pathogens. What is more, PWD has maintained a comprehensive record of operational parameters for its digesters, including volatile solids loading rates, volatile acids, alkalinity, and pH, in addition to digester temperature, sludge retention time, and volatile solids reduction based on intensive daily monitoring that mirrors the intensive “snap shot” created for each of the 11 WWTP plants in the WERF odor study and the 7 WWTP plants in the fecal coliform study, so that a direct correspondence to the WERF research can be observed. Additionally, the procedure used in the WERF odor study to characterize odorant emissions from biosolids cake – the headspace chemical analysis for odorous compounds – has been applied to monthly samples of biosolids cake from the SW WPCP for the period May 2004 to the present, the only such long-term odor characterization database of its kind.

This paper reports on the statistical treatment of the operational records of the digesters and centrifuges at its SW WPCP and the adjoining centralized dewatering facility, the Biosolids Recycling Center (BRC), against the corresponding fecal coliform and odor databases to test for associations between digester and dewatering equipment performance and these parameters. This review first shows that the WERF studies engaged in sampling events at the SW WPCP that were representative of typical operations at the SW WPCP, and that the consequent fecal coliform densities and odorant intensities were within levels measured as part of Philadelphia's long-term monitoring for fecal coliform and odors, which for both parameters exhibited a wide variation, typically two orders of magnitude over the 2 ½ year monitoring period. The broad findings of this review confirm the WERF study report conclusions. That is: 1) no apparent correlation is seen between variation at the SW WPCP in volatile solids destruction and the variation in odorant intensity of the biosolids or the variation in geometric mean of fecal coliform over the 2-½ year period 2004 to 2006; 2) no apparent correlation is seen between the variation in time-temperature performance at the SW WPCP digesters over the study period and variations in odor intensity and fecal coliform density. Additionally, various operational parameters beyond those reported for compliance with VAR and PR standards were examined for correlation with odor intensity and fecal coliform densities. These include percent volatile solids in digester effluent, volatile acid concentrations in the digesters, iron concentrations in the biosolids, and polymer dosage rate in dewatering. Likewise, none of these showed notable linear relationships with odor and pathogen measures. A graphical treatment of the data bases revealed suggestive non linear relationships between odor and SRT and temperature. Overall, the lack of association between conventional measures of digester and dewatering equipment performance and the principal quality outcomes – odors and pathogens – argues for more research by the profession into underlying biological and engineering processes resulting in odor and pathogen reduction.
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Document Type: Research Article

Publication date: 2007-01-01

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