THE FATE OF NITRATE IN A SINGLE STAGE
NITRIFICATION ACTIVATED SLUDGE SYSTEM
Richard T. Donahue, Jr., Senior Engineer
Merck and Company, Inc.
Rahway, New Jersey 07065
Jeffrey L. Pintenich, Group Manager
AWARE, Inc.
Nashville, Tennessee 37204
INTRODUCTION
In 1980 a preliminary engineering study was initiated at the Merck and Co., Inc. Stonewall Plant
in Elkton, Virginia, to upgrade and expand the pharmaceutical wastewater treatment system.
Laboratory studies employing continuous-flow bench-scale bioreactors were conducted to develop
design criteria for a single stage nitrification activated sludge system. Performance data revealed that
the nitrogen material balances around the reactors would not close. Denitrification was not evident in
the clarifier sections of the reactors to account for the nitrogen imbalance.
Batch reactor experiments were designed and conducted to examine this phenomenon in more
detail. Experimental results demonstrated that substantial denitrification was occurring in the aeration compartments of the reactors, even at elevated mixed liquor dissolved oxygen levels. Subsequent
pilot plant and full-scale plant nitrogen performance data also support this hypothesis.
WASTEWATER PRETREATMENT
The process wastewaters produced at the Stonewall Plant are variable in both quantity and composition and generally contain significant concentrations of organics, suspended matter, and both
organic and ammonia nitrogen. Pretreatment unit operations upstream of the activated sludge process
include equalization, neutralization, sedimentation, and biological degradation in a "roughing"
trickling filter. This roughing filter is operated with effluent recycle at approximately 70 mil gal/acre-
day and 725 lb BOD5/1,000 cu ft/day to achieve a BOD5 load reduction of 25%. Roughing filter
effluent wastewater was the subject of the investigation described herein.
BENCH-SCALE STUDIES
Continuous-flow treatability investigations were performed with five Plexiglas? 20-liter bench-
scale activated sludge units, as shown in Figure 1. Data reported in this presentation were collected
after an acclimation period of two months and represent operating periods of one to two months.
Chemical analyses of influent and effluent samples conformed to established U.S. EPA protocols.
Nitrogen series analyses were conducted twice weekly. Total Kjeldahl Nitrogen (TKN) analyses were
finished with the titrimetric method after ammonia distillation. Nitrate (N03-N) concentrations were
determined with the cadmium reduction method, which quantifies both nitrite and nitrate species.
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