EVALUATION OF PERCENT REMOVAL VARIABILITY
FOR PRIORITY POLLUTANTS IN POTWs
Michael T. Unger, Associate Scientist
Laboratories for Environmental Research
Indiana University Northwest
Gary, Indiana 46408
Roger E. Claff, Environmental Engineer
JRB Associates
Schaumburg, Illinois 60195
INTRODUCTION
A statistical approach was used to evaluate the percent removal of priority pollutants in publicly
owned treatment works (POTWs). Percent removal data were obtained from POTWs monitored as
part of an EPA study [1] of the occurrence and fate of priority pollutants in 40 POTWs.
The removal credits section of the General Pretreatment Regulations [2] states that POTWs must
demonstrate consistent removal of pollutants in order to grant industrial users a credit for removal
of pollutants by the POTW. The data show that percent removals vary greatly among POTWs and
among pollutants. Consequently, percent removal data should be interpreted on a plant-by-plant,
pollutant-by-pollutant basis. The statistical treatment used generated information which can be used
to: 1) provide quantitative measures of how consistent pollutant removal efficiencies in POTWs really
are; and 2) establish realistic margins of safety when determining revised discharge limits for industrial users.
INTRODUCTION AND BACKGROUND
The final removal credit rule of the General Pretreatment Regulations was published in the Federal
Register on August 3, 1984. Section 403.7 of the General Pretreatment Regulations was promulgated
to implement a 1977 amendment to section 307(b)(1) of the Clean Water Act which allows a POTW
to grant industrial users a "credit" (in the form of reduced pretreatment requirements) for removal
of pollutants by the POTW. Industrial users which qualify for removal credits may thus discharge
larger quantities of regulated pollutants to the POTW than would otherwise be allowed by the applicable categorical pretreatment standard.
One of the most significant changes to the proposed removal credits section was the elimination
of provisions for "national removal rates" which would have permitted a qualified POTW to rely
on national removal rates developed by EPA rather than on rates established through the collection
of data by the POTW which demonstrate its actual removal performance. Under the final regulations,
the following prerequisites are necessary for a POTW to obtain authorization to grant removal credits:
• The POTW must apply for and receive authorization to grant removal credits.
• The POTW must demonstrate consistent removal.
• The POTW must have an approved local pretreatment program or qualify for an exemption.
• The POTW must meet all applicable sludge criteria.
• The POTW must be consistently in compliance with all of its NPDES permit limits and conditions.
The objective of this paper is to offer a statistical basis which supports a case-by-case approach
for demonstrating consistent percent removals of priority pollutants by POTWs.
915