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Considerations for Alternative Supplemental Carbon Sources in Deep Bed Denitrification Filters to Achieve Low Effluent Nutrient Concentrations

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The City of Baltimore has investigated various alternatives to meet the future effluent total nitrogen goal of 3 mg/L and current stringent total phosphorous limit of 0.2 mg/L at its Back River Wastewater Treatment Plant. The selected technology, deep-bed downflow denitrification filter, allows for both nitrogen reduction and partial solids removal. In an effort to confirm the design criteria, a small-scale filter pilot study was undertaken between January and July, 2009. As a result of comprehensive nutrient reduction initiatives for the Chesapeake Bay watershed, the demand for supplemental carbon sources such as methanol is growing and there has been a great deal of interest in the practicability of utilizing alternative carbon sources. Hence, it was also important to evaluate use of alternative carbon sources in order to (1) establish the carbon requirements specific to each substrate (for design considerations) and (2) evaluate operational conditions. This pilot study was segmented into three phases: methanol (as control), ethanol and MicroC™-glycerin. The results of this study suggest that, under warm temperature periods (∼20°C), the effluent nitrate (NOx-N) concentration of approximately 0.5 mg/L was achieved with all the carbon sources at or below the average nitrate loading rate of 38 lb/1000 ft3/day (0.6 kg/m3/d). Even with higher loading rates, effluent quality did not degrade significantly and remained < 1 mg/L most of the time. Observed COD/NOx-N ratios were ∼5.5 (ethanol) and ∼7.0 (glycerin). Some operational differences were noted between methanol, ethanol and glycerin. Glycerin appeared to promote the growth of an unusual biofilm on all of the piping surfaces with which it came in contact, and had noticeably higher yield that led to an increase in backwash frequency when compared to ethanol. In summary, denitrification performance with both of the tested alternative carbon sources met removal requirements. This testing also confirmed that excess available carbon was needed in the filter effluent to maintain performance.

Keywords: Alternative Carbon Sources; Deep Bed Denitrification Filters (DBDF); Enhanced Nutrient Removal (ENR); Limit of Technology (LOT); Post Denitrification

Document Type: Research Article


Publication date: 2010-01-01

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