Wastewater Surcharge BOD and TSS Calculator
Many utilities charge industrial users extra when wastewater strength exceeds a base level, usually for BOD and TSS. This calculator turns your concentrations and flow into daily loads and annual surcharge, and tests the value of pretreatment.
Your numbers
Level above which your utility applies a surcharge. See your permit.
Your results
Surcharge formulas differ by utility. Some bill on pounds, some on tiered concentration bands. Use the rate sheet in your discharge permit.
Annual surcharge vs BOD concentration
Sweeps BOD5 concentration from 100 to 2000, holding your other inputs fixed.
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How to use the calculator
Enter average daily flow and the average concentrations from your compliance sampling. Then enter the base limits and the per kilogram surcharge rates from your utility's rate schedule or permit. Set the number of discharge days per year and the share of the excess you expect pretreatment to remove. The results show loads, the surcharge per day and per year, and the saving. Run it again with the highest sample results to see worst case exposure.
- Use flow-weighted averages, not a single grab sample
- Check whether the surcharge applies to the excess only or to the whole load
- Include only the parameters your utility charges for
The math
Because 1 mg/L equals 1 g/m3, the load in kg per day is flow in m3 per day times concentration divided by 1000. The excess load uses the concentration above the base limit, floored at zero. The surcharge is excess kg times the rate for each parameter, summed for BOD and TSS, then multiplied by discharge days. The chart sweeps BOD and shows that annual surcharge rises linearly once you pass the limit and stays flat below it.
Reading the result
At defaults, the plant discharges 270 kg of BOD per day with 180 kg above the limit and 90 kg of TSS above the limit. That is roughly 108 dollars per day or about 32,000 dollars per year. Pretreatment such as equalization, dissolved air flotation or screening that removes 60% of the excess would save around 19,000 dollars per year, before capital and operating cost. Compare that to the equipment payback to decide.
Common mistakes
Mixing mg/L with lb per 1000 gallons, or using gallons with a metric rate, is the top source of error. Another is using the annual average when the utility bills monthly on peak samples. Surcharges may also have minimum fees. Keep sampling results, flow meter readings and invoice dates in one record so you can challenge disputed bills and prove compliance.
- Match units to the rate sheet
- Check the sampling method in your permit
- Validate flow with meter data
Frequently Asked Questions
What is BOD5?
Biochemical oxygen demand over five days is the oxygen microorganisms consume breaking down organic matter in a water sample at 20 C. It measures how much organic load your wastewater puts on the treatment plant.
How do I convert mg/L to kg per day?
Multiply the concentration in mg/L by the flow in m3 per day, then divide by 1000. For US units, multiply mg/L by flow in million gallons per day by 8.34 to get pounds per day.
Does every utility charge the same way?
No. Some charge only the excess above a base strength, some bill tiered rates, and others charge a flat strength factor. Use the formula in your utility's ordinance or rate schedule and adjust the inputs accordingly.
What reduces BOD and TSS cheaply?
Good housekeeping, dry cleanup before washdown, spill containment, screening and equalization often cut load before any capital-intensive process. Dissolved air flotation or biological pretreatment follows if the savings justify it.
Netray can connect lab results, flow meters and utility invoices from your ERP and files into a searchable environmental record, kept private on your servers.
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