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Fixed-film fermentation of wastewater and its effect on BNR plant performance Doucette, Dwayne Pius

Abstract

Biological nutrient removal (BNR) at wastewater treatment plants, with strict effluent discharge limits, often require supplementary fermentation by-products to meet permit requirements without chemical addition. However, some wastewaters entering BNR plants, especially those located in warmer climates, have sufficient fermentation by-products, or volatile fatty acids (VFAs) present in the influent for efficient nutrient removal without supplementary VFAs or chemical addition. It is believed that fermentation within the collection systems is responsible for contributing necessary VFA to such wastewaters. This theory, and efforts to simulate and measure the effects of such fermentation in a wastewater treatment plant unit operation, forms the basis for this research. The main objective of this study was to investigate fixed-film fermentation of wastewater and measure its effects on biological nutrient removal at a pilot-scale wastewater treatment plant. Research to support this objective was conducted over a nine month period, beginning in summer of 1997 and ending in the spring of 1998. Control and Experiment fixed-film fermenters, which were designed and constructed by a previous researcher (Dumitrescu, 1998), were attached to the front end of a 3-Stage Bardenpho process. It was demonstrated in this study that fixed-film fermentation of primary effluent was a feasible means of producing VFAs at a rate between 2 mg/L/hr and 9 mg/L/hr. Furthermore, when solids, present in the primary effluent, were allowed to settle and accumulate in the fermenters, VFA production was enhanced considerably as a result of fixed-film and solids fermentation. The effects of prefermentation on twin 3-Stage Bardenpho processes running in parallel were observed during three experimental runs. Process parameters of interest included ammonia, nitrates, total Kjeldahl nitrogen, phosphates, total phosphorus, carbon, solids, and mixed liquor suspended solids. During Experimental Run #1, there was no significant difference in process performance between the Control Process, which had a fermenter containing no media, and the Experiment Process which had a fermenter containing Ringlace. Both processes performed exceedingly well, reducing effluent phosphorus to less than 0.3 mg/L and effluent nitrogen to less than 5 mg/L. During Run #2, it was decided to eliminate fermentation from the Control Process and continue to run the Experimental side as per Run #1. Again, there was no significant difference between Control and Experiment Process performance, even though the fixed-film fermenter was contributing additional VFAs to the Experiment Process. It was concluded that sufficient VFA (and fermentation by-products) were already present in the wastewater for good nutrient removal, nullifying any improvements attributable to the fermenters. During experimental Run #3, it was decided to add phosphorus to the anaerobic zone to reveal any process performance improvements which might be attributable to VFAs produced in the fixed-film fermenters. Once again, there were no significant differences between Control or Experiment Processes with both sides performing equally well. Three possible explanations for the failure to show improved BNR performance with the addition of a fixed-film fermenter are discussed in this report: • The existence of sufficient fermentation by-products inherent to the raw wastewater masked any process performance improvements that might have otherwise been observed. • Nitrates present in the anaerobic zone hindered phosphorus removal. Microbes, responsible for denitrification in the anaerobic zone, utilized VFAs from the subject fermenters which would have been available for phosphorus accumulating organisms. • A combination of the above.

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