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future growth within the system and to infiltration and inflow from unidentifi- 
able sources. Such tax revenues offset 47 percent of the total capital require- 
ments and four percent of the operational expenses; the four percent representing 
the portion of plant operations attributed to infiltration and inflow. In de- 
veloping this system, East Bay reasoned that using the ad valorem tax base was 
the proper way to charge for services that benefit property and that treatment 
for infiltration and inflow, and additional capacity for future use were the 
two aspects of a POTW's operations that benefited the community generally. 
Pretreatment charges were based on the results of East Bay's sampling pro- 
gram. The sampling showed that customers could be assigned a waste strength 
based on their business category. Once the survey was completed, the 20,000 
non-residential customers of the District were divided into the 87 classification 
groups created. Data from the survey was then used to assign a strength to each 
pollutant in each business class. For an individual discharger, meter readings 
and sampling data were used to calculate the total amount of pollutants and 
the volume of discharge. Unit rates were calculated for each parameter based 
on East Bay's knowledge of the amount of each parameter accounted for in the 
overall system, and the cost to the District of handling each parameter. Using 
the BCC system, East Bay pretreatment officials calculated the separate con- 
tributions of each user in each category, and thendetermined the limits to 
place on such users directly from the POTW's discharge requirements or effluent 
limitations. 
Once unit rates were calculated for each parameter within each BCC class, 
a list of charges was adopted for each individual class. Such charges were 
rounded to the nearest five cents. Designers of the pretreatment system decided 
that the five cent increment represented the degree of accuracy that they 
could achieve with charges based on volume and quality, given the spread of 
each user's wastewater strength within a business classification group. East 
Bay officials tested the accuracy of this general procedure by comparing the 
mass emmission for each parameter computed for all categories with the respective 
amounts of pollutant entering the treatment plant. Data collected during this 
verification process proved the validity of assigning the strength to each 
individual class of user. Further, the sampling procedure used by the 
District in its general survey showed that wastewater strength was independent 
of wastewater volume, as mentioned previously. 
Issuing Effective Permits and Credits 
The strength of East Bay's pretreatment program rests on its permit system. 
That system involves issuing specific sets of effluent limitations to industrial 
dischargers within each of the BCC classes established, based on the POTW's own 
limitations as set forth in East Bay's NPDES permit. Certain permits are 
issued automatically to “critical industries" specified in the District's waste- 
water control ordinance. A critical industry classification includes all 
manufacturing and processing industries and industries discharging other than 
Sanitary wastewater. 
In addition to a set of mandatory permits, East Bay also issues two types 
of optional permits. The first type is available for customers classified into 
