Stoichiometry of Industrial Biological
Wastewater Treatment
JOSEPH H. SHERRARD. Associate Professor
Department of Civil Engineering
Virginia Polytechnic Institute and State University
Blacksburg. Virginia 24061
EDWARD D. SCHROEDER. Associate Professor
Department of Civil Engineering
University of California
Davis. California 95616
INTRODUCTION
Many industrial wastewaters containing organic material are deficient in nitrogen
and or phosphorus. In such cases these nutrients must be added to provide a balanced
growth media for efficient biological wastewater treatment. Generally, it is believed that a
wastewater must be balanced stoichiometrically at a BOD,:\:P ratio of approximately
100:5:1 for a significant fraction of the organic matter to be removed (I). If nutrients are not
added to the wastewater, two resulting observations have been widely noted: I) takeover or
predominance of the mixed liquor suspended solids with filamentous bacteria, as in the
activated sludge process; and 2) incomplete conversion of the organic materials to end
products, i.e.. inefficient BOD, or COD removal. If excess nutrients are added to the
wastewater, however, a large fraction of these nutrients may not be incorporated into the
mixed liquor suspended solids and may pass into an effluent receiving stream. As a result,
algal growth may be stimulated in the receiving water. Thus, determination of the
quantities of nutrients which must be added to a nutrient deficient wastewater is an
important factor in the successful design and operation of a treatment facility.
Addition of nutrients also involves both capital costs for storage and chemical feeders
and operating costs for the purchase of chemicals. Minimizing these costs requires an
understanding of treatment process stoichiometry.
The objective of this paper is to provide a procedure for determining the quantity of
nutrients that must be added to a given wastewater as a function of operating conditions.
This information can be used by the treatment plant operator to accurately predict nutrient
requirements needed on a daily basis and for the design engineer in selecting the optimal
treatment process modification for the specified wastewater.
PROCEDURE DEVELOPMENT
The procedure for determining quantities of nutrients that must be added to a given
wastewater is based on principles of microbial continuous culture theory and chemical
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