Wastewater Surcharge Cost Calculator for BOD and TSS
Many municipal utilities bill industrial dischargers extra when BOD or suspended solids exceed a base strength. This calculator turns your flow and sample results into pounds per day over the limit and a yearly surcharge estimate.
Your numbers
Use the metered or billed discharge volume.
From the latest composite sample.
Set by your sewer use ordinance or permit.
Your results
Surcharge formulas differ by utility. Some bill on strength ratios, minimum charges or banded rates. Use your sewer use ordinance or rate schedule to confirm the formula and rates before relying on the result.
Annual surcharge vs BOD concentration
Sweeps BOD concentration from 100 to 2000, holding your other inputs fixed.
Track surcharge exposure from your own data
Optional: discuss pulling flow, sample results and billing into one monthly view.
Optional. The tool and every export work without this. No spam.
How to use the calculator
Enter your average daily discharge, the latest BOD and TSS results, the allowances in your ordinance and the surcharge rates from your rate schedule. The output shows pounds per day above the allowance and the annual cost. Move the BOD value to see how a pretreatment project that lowers strength changes your bill, and use the chart to read savings at other concentrations.
- Use average concentrations over a representative period, not one peak sample
- Check that the allowance is in mg/L and the rate is per pound
- Add days of actual discharge, not calendar days, if you shut down
The formula
Load in pounds per day equals concentration in mg/L, multiplied by flow in million gallons per day, multiplied by 8.34. The surcharged load uses only the concentration above the allowance. Cost equals surcharged pounds multiplied by the rate for that parameter. BOD and TSS are calculated separately and added.
Worked example
With 60,000 gal/day, BOD at 620 mg/L and an allowance of 250 mg/L, the excess is 370 mg/L. Flow is 0.06 MGD, so surcharged BOD is 370 times 0.06 times 8.34, about 185 lb/day, at 0.35 dollars per pound is about 65 dollars per day. TSS excess of 160 mg/L adds about 80 lb/day, or 20 dollars per day. Over 300 days the total is roughly 25,000 dollars.
Common mistakes
Using total load instead of excess load, mixing gallons per day with million gallons per day, and assuming a utility uses the same formula as another city. Some tariffs apply strength ratios, flat minimums or charges for fats, oil and grease and nitrogen, which this tool does not include.
Frequently Asked Questions
Why is the factor 8.34?
One gallon of water weighs about 8.34 pounds, so 1 mg/L in 1 million gallons equals 8.34 pounds. This is the standard conversion for loading calculations in wastewater work.
What is BOD and TSS?
BOD is biochemical oxygen demand, the oxygen consumed by microorganisms breaking down organic matter in the sample. TSS is total suspended solids. Both indicate how much treatment effort the sewer plant needs.
Does this match my utility bill?
It will only match if your utility uses this simple excess load method. Request the ordinance and rate schedule, and compare the calculation to a recent bill before using the numbers in a business case.
How can I reduce a surcharge?
Common options include segregating high strength streams, dissolved air flotation, equalization tanks, screening, solids recovery and process changes. Estimate each against the annual surcharge you calculate here to judge payback.
If your discharge data and sampling results are stored in ERP, maintenance or LIMS records, we can help build a read-only view that tracks surcharge exposure monthly.
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