BENEFITS OF USING NITRATE AS NUTRIENT IN ACTIVATED SLUDGE TREATMENT SYSTEM
Nitrogen is added as a nutrient in many activated sludge treatment systems. Without this addition, the biomass becomes nutrient deficient. This nutrient deficiency causes suppression in BOD removal rates and often causes poor bio-flocculation and corresponding elevated TSS concentrations.
This deficiency routinely causes poor sludge settleability due to filamentous bulking. Work by Eckenfelder and others have indicated that these deficiencies can be avoided by maintaining an adequate residual nitrogen concentration in the effluent. For most activated sludge systems a minimum
effluent concentration of 1 mg/L N has been required to prevent these deficiencies.
Historically, ammonium or urea has been added as the nutrient source. Many have been taught that only nitrogen in these forms can be used as nutrients. For plants that do not have an ammonia-nitrogen
effluent limit, this addition has not jeopardized effluent compliance. However, many plants discharge to either low flow streams or water quality limited streams. For these plants, it is common to have effluent ammonia-nitrogen limits of 0.5 mg/L or total nitrogen limits of 3 mg/L.
The number of plants with these restrictive limits will increase as TMDL based limits are more fully applied. These limits require plants that add ammonium and urea as a nutrient to operate nutrient limited. This, in turn, causes these plants to typically suffer from sludge bulking and/or
elevated effluent TSS concentrations.
Contrary to popular belief, nitrate is used as a nutrient by activated sludge. Use of nitrate as a nutrient not only allows reduction in effluent ammonia-nitrogen but also total nitrogen when factoring in denitrification that will occur in the secondary
clarifiers. In addition, nitrate when used as a nutrient may result in a lower cell yield (e.g., sludge production) than that exhibited by ammonium addition. Results of parallel batch treatability tests of a volatile fatty acid wastewater, presented below in Table 1, indicated this phenomenon.
This phenomenon is consistent with work by Pipes in 1963.
This initial treatability work demonstrated a greater than anticipated reduction in biomass production while not slowing the COD removal rate. This promising result prompted a full-scale demonstration at a chemical industry and confirmatory
treatability testing. Results of the fullscale demonstration testing and confirmatory testing are presented below.
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