Water Online

September 2014

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BNR 101: Tutorial wateronline.com ■ Water Online The Magazine 14 performance without chemical addition. How can an EBPR system consistently remove phosphorus to a very low concentration with a low or variable supply of influent VFA? The anaerobic zone needed for EBPR performs multiple functions. The primary function is the uptake of VFA by PAOs, but PAOs constitute only a small subset of the bacterial population in the mixed liquor suspended solids (MLSS) in the anaerobic zone. The rest of the bacteria are switching gears to ferment organic compounds to obtain food and energy (Figure 1). These facultative bacteria do not consume VFA; they break down complex soluble organic chemicals to form VFA, allowing the PAOs to take up VFA and release phosphorus. Therefore, the anaerobic zone in an EBPR plant simultaneously conditions PAOs and provides an environment for additional fermentation of soluble organics to VFA. The analytical tool used to predict the amount of VFA that can be formed is the soluble readily biodegradable COD fraction or rbCOD. The rbCOD concentration is derived from a special flocculation and filtration treatment procedure — called filtered flocculated COD (ffCOD) — to pretreat the sample prior to COD analysis. Subtracting the effluent ffCOD (nonbiodegradable COD) from the influent ffCOD (total soluble COD) results in the rbCOD concentration. The ratio of rbCOD to P is a better indication of the performance of the EBPR process than the total COD:P ratio referenced in textbooks because only the soluble rbCOD will be fermented into VFA in the anaerobic zone. There is not enough time for suspended COD to be fermented to VFA in the anaerobic zone. Figure 2 illustrates the variable relationship of rbCOD to P. As the fraction of influent rbCOD that is VFA decreases, the necessary rbCOD:P ratio for successful EBPR increases. If fermentation occurs in the collection system due to anaerobic activity in the collection system (odor issues at the WWTP headworks is good indication of this), the anaerobic zone does not have to provide much additional fermentation for EBPR to work well. Conversely, if fermentation does not occur in the collection system, it has to occur in the anaerobic zone for EBPR to work well. The discussion thus far has focused mainly on the phosphorus release portion of the EBPR process because phosphorus uptake is covered through good aeration design. Keeping >2 mg/L DO residual at the head end of the plug flow aeration basin is the goal. The phosphorus release depletes the PAO of energy, which stresses the microbe. This stress causes the PAO to take up excess phosphorus in the oxic zone; if DO is limited at the front end of the oxic zone, the PAOs fail to uptake as much excess phosphorus. Fermentation at a WWTP can take many forms. A dedicated primary sludge fermenter is common for plants that have primary clarifiers. Primary clarifiers can also be "activated" to enhance fermentation. In two-stage anaerobic digestion the acid digestion phase is essentially fermentation and can be used as a VFA source. MLSS fermentation is another viable option for carbon augmentation and PAO conditioning. If the influent is short on rbCOD, a simple strategy of cycling mixers on and off in the anaerobic zone allows MLSS to settle and increase the solids retention time in that zone, which results in formation of additional rbCOD/VFA. If more soluble material is needed, a separate return activated sludge (RAS) or MLSS fermenter will provide more efficient fermentation. Compared with RAS fermentation, MLSS fermentation ferments a higher fraction of primary effluent volatile solids and colloidal material absorbed by the mixed liquor, which increases carbon available for fermentation. Fermentation in the collection system is desirable for operation of an EBPR system, but can present problems for collection system staff due to undesirable side

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