Abstract

Anaerobic digestion of wastewater sludges poses the benefits of sludge minimization/ stabilization, energy recovery, pathogen reduction, etc. However, a negative impact of anaerobic digestion on the wastewater treatment process is the production of unpolished return water streams following downstream biosolids dewatering (e.g. high nutrient and soluble organic loads). Post-treatment of anaerobic digested biosolids by short-term (5-6 day SRT) aerobic digestion has been explored for its ability to further minimize residual solids and remove nitrogen and soluble organics. Results show that as much as 90% removal of total ammonia nitrogen (TAN) and 10% additional volatile solids destruction can be readily achieved with such a process (Novak et al., 2009). These removal rates (especially with respect to TAN removal) were found to be particularly sensitive to an appropriate level of oxygen supply to meet the demand imposed by both organic and nitrogenous species.Initially, oxygen demand was defined based on mass balances on solids, chemical oxygen demand (COD) and TAN around the aerobic digester, but these values were found to be extremely sensitive to both the nature of the waste being treated and ancillary factors such as ammonia stripping. For example, following anaerobic digestion of thermally hydrolyzed sludge, residual volatile fatty acids (VFA) tend to persist at levels between 3200-6200 mg/L as COD, representing as much as 10% of residual volatile solids after anaerobic digestion. While these dissolved solids are readily degraded during aerobic digestion, a mass balance on solids is complicated by the loss of these volatile organic solids during solids drying at 104°C. The loss of TAN due to the decomposition of ammonium-bicarbonate during solids drying, and its effect on solids analyses, is similarly explained by Beall et al. (1998).Efforts were then made to measure the oxygen uptake (i.e. depletion) rate (OUR) within the aerobic digester. Both oxygen-sensitive membrane galvanic electrodes and optodes were used in order to measure OUR, however neither probe was able to respond sufficiently rapidly to the high in-situ OUR. Ex-situ tests were also unreliable as the dissolved oxygen measurement range was physiologically different than in-situ conditions (i.e. DO_EX-SITU>DO_IN-SITU, DO_EX-SITU>>K_DO).Though this hypothesis is still under investigation, it currently appears thatparticulate solidsdestruction during post-aerobic digestion was initially overestimated, and that VFAremoval waslargely responsible for measured COD/solids destruction across this digester. As a result of thisfinding, the nitrogenouscomponent of the overall oxygen demand was also modified to reflectthe effect of reduced ammonification of particulate volatile solids. Measures that allowed us to assess particulateand dissolved solids distinctly (i.e. VSS vs. VS reduction, and soluble vs. totalCOD) were also usefulfor characterizing degradation during aerobic digestion. Additionalreactor operation and analysis is being performed in order to better predict the oxygen supplyrequirements for the post-aerobic digestion process.The short SRT of the proposed aerobicdigesterand high OUR of the digested biosolids results in a small margin of error for the operation of a conventional diffused aeration system, generallycapable of supplying 100 ppm O₂/hr. Results from this study will be usedto determine whetherthe use of diffused air is feasible for this application, or whether pure oxygenation is necessary. This study reveals the criticality of understandingthe analytical components of an oxygen massbalance(e.g. solids destruction/loss, ammonia removal/loss, residual soluble organics)and thesensitivity of such a toolto specific waste characteristics (e.g. soluble COD, particulate COD,VFA, TAN) and measurement techniques.

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