Understanding Wastewater Surcharges
A POTW charges standard sewer rates based on the volume of water discharged. However, volume alone doesn't dictate treatment cost. Pollutants require oxygen, chemicals, and sludge handling to remove.
When a facility discharges concentrations of pollutants above the domestic baseline (usually defined by the municipality, e.g., 250 mg/L), a surcharge is applied to the excess mass of that pollutant.
Common Surcharge Parameters: BOD, TSS, and FOG
Surcharges are almost universally based on three main parameters.
Biochemical Oxygen Demand (BOD) measures the amount of oxygen required by bacteria to break down organic matter. Total Suspended Solids (TSS) measures particles trapped by a filter. Fats, Oils, and Grease (FOG) measures lipid content, which can foul sewer lines and treatment equipment.
How to Calculate Excess Mass Loading
To calculate a surcharge, you must first convert the excess concentration (measured in milligrams per liter, mg/L) and flow rate into total pounds of pollutant. This is done using the standard 8.34 conversion factor.
The Formula: Flow (in Millions of Gallons per Day, MGD) x Excess Concentration (mg/L) x 8.34 lbs/gal = Excess Pounds.
Example: A food processor discharges 0.1 MGD. Their BOD is 600 mg/L, and the POTW threshold is 250 mg/L. Excess BOD is 350 mg/L. Calculation: 0.1 MGD x 350 mg/L x 8.34 = 291.9 lbs of excess BOD per day. If the POTW charges $0.50 per pound of BOD, the daily surcharge is roughly $145.95.
Strategies to Reduce Surcharge Costs
Surcharges can run into tens of thousands of dollars annually. Facilities can mitigate these costs through source reduction—preventing waste from entering the drain in the first place by dry-cleaning floors or capturing solid waste.
If source reduction isn't enough, installing physical pretreatment systems like Dissolved Air Flotation (DAF) units or equalization tanks can strip out TSS and FOG before the effluent hits the municipal sewer.
Frequently asked questions
Why is the constant 8.34 used in the wastewater formula?
The number 8.34 represents the weight of one gallon of water in pounds. It acts as the unit conversion factor to translate mg/L and Millions of Gallons into total pounds.
Is Chemical Oxygen Demand (COD) the same as BOD?
No, COD measures all oxidizable matter chemically, while BOD only measures biologically degradable matter. Some municipalities use COD for surcharges because the test takes hours instead of five days, but they are technically different values.
Can pH fluctuations result in a surcharge?
Typically, pH violations result in fines or notices of violation rather than surcharges. Surcharges are usually reserved for treatable, compatible pollutants, whereas extreme pH can damage the sewer infrastructure itself.
Do I pay surcharges on my entire pollutant load?
Usually no. You generally only pay for the 'excess' concentration above the domestic baseline defined by your local POTW, as the cost of treating the baseline amount is covered in your standard volumetric sewer bill.