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

Back River Wastewater Treatment Plant
Baltimore, Maryland

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
MicroCâ„¢-glycerin. The results of this study suggest that, under warm temperature periods
(~20C), 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.

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Posted: May 20th, 2011 | Filed under: 500K-1M, Waste Water Treatment | Tags: , , , | No Comments »