Application Corner: Calculating billing costs for sewage districts

Previous articles described the sewage collection systems for two adjacent sewage districts and how the costs associated with operating the sewage treatment plant were allocated each year. These allocations were then used to calculate the billing rates that were used on a daily basis for the next year. The following examples illustrate the wide variation in billing the first sewage district that can occur on different days.

If the billing rate is $3 per 1,000 gallons and the surcharge is $10 for flow in excess of 500,000 gallons during a given day, the cost associated with a dry weather flow of (say) 300,000 gallons on a given day would be $900 (3 x 300).

However, wet weather events, such as rain and snow melts, can significantly increase the amount of liquid that needs to be treated by the water treatment plant. For example, the cost associated with a daily flow of 700,000 gallons on a given day would be $2100 (3 x 700) plus a surcharge of $2000 (10 x 200) or $4100, which is more than four times the amount billed on a typical dry day.

This would not be an issue in desert locations such as Las Vegas where few wet weather events occur. However, this did present a significant issue in the northeastern U.S. where these flowmeters were located.

Read more next month about how the first sewage district discovered a high sewage flow.

David W. Spitzer is a principal at Spitzer and Boyes, LLC, which offers engineering, focused market research, writing/editing white papers, strategic marketing consulting, distribution consulting, seminars and expert witness services for manufacturing and automation companies. Spitzer has written more than 400 technical articles and 10 books about flow measurement, instrumentation and process control. He can be reached at 845-623-1830 or via spitzerandboyes.com.