Impact of Stream Assimilation Capacity on
Waste Treatment Requirements
RUSSELL C. MT. PLEASANT, Senior Sanitary Engineer
IRVING  GROSSMAN,  Associate  Sanitary  Engineer
Basin  Development  Section
New York State Department of Health
Albany, New York
INTRODUCTION
The baseline for water quality management in New York State has been
established by the legal adoption of classifications and quality standards for all
waters. These standards acknowledge existence of multiple water usages and recognized best usages (Table I). Although treated waste assimUation is a water usage, it
is no longer accepted as a best usage in New York. The Health Department's philosophy of minimum secondary treatment i.e. removal of 85 per cent of BOD and suspended solids plus disinfection) of municipal waste waters is a reasonable objective
consistent not only with the technology within the State but also responsive to the
public support for the Governor's Pure Waters Program. To this end, secondary
treatment or equivalent is required of organic, industrial waste with appropriate
credit for in-plant improvements.
The concept of stream assimilation capacity plays a major role in determining
the extent of waste treatment based on an appraisal of the integrated influences of
self-purification, stream usage and water quality standards. It may also provide a
basis for allocation and low flow augmentation requirements.
This paper wUl briefly describe the mechanisms of stream assimUation capacity,
methodology, and some experiences with industrial wastes and treatment requirements.
CONCEPTS OF ASSIMILATION CAPACITY
The water quality standards mentioned previously are invoked to abate existing
pollution and to prevent future pollution by providing design guidelines for waste
treatment facilities. In the latter application, they are interpreted to apply during a
critical drought stream flow equivalent to the minimum average seven consecutive
day flow with a probabUity of occurring once in ten years (MA7CD/10 yr).
With this ground rule, the waste assimilation capacity of a stream is defined as
the maximum amount of treated wastes that can be safely stabilized during critical
stream conditions while maintaining assigned water quality standards without impairing legitimate water usages.   Quality standards for pH, temperature, DO and NH3
are of major concern to oxygen balances and will be considered herein.
The assimilation capacity is a characteristic peculiar to the stream in question
and is a function of the basin hydrology, channel hydraulics and geometry, and stream
biological reactions to the imposed waste loadings. These factors quantify the mechanisms of deoxygenation and reaeration described by modifications to the classical
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