Industrial and commercial customers, which include food service establishments, nursing homes, users with discharge greater than 25,000 gallons per day or other manufacturing facilities, generate wastewater streams that are higher in strength than the average residential customer. Indicators of “strength” are Biochemical Oxygen Demand (BOD5), Total Suspended Solids (SS), Total Phosphorus (TP), and Total Kjeldahl Nitrogen (TKN). Higher strength wastes cost more to treat and are often more troublesome for maintenance of the sanitary sewerage system. To recover costs and to make the difference between industrial/commercial and residential customers more equitable, a strong waste surcharge program was established around 1990. These facilities are monitored by Henrico County to determine if a high strength waste surcharge is warranted. The rate is determined and established by Henrico County Code in the same manner as other water and sewer rates (Chapter 23, Section 23-362. – Sewer service charges and rates).
Strong Waste Surcharge Formulas based on rates effective 7/1/16
The formula for calculating a strong waste charge is as follows:
[(Volume of wastewater) x ((BOD5 – SBOD5) – BOD5 Threshold)) x (Rate)]
[(Volume of wastewater) x (SS – SS Threshold) x (Rate)]
= $ amount of surcharge.
For each customer sampled, the values for BOD5 and SS are entered into our customer information system on their account and used to calculate strong waste surcharges on their water and sewer bill. These values remain active on the account until new values are reported. Most bills are generated bi-monthly (every 60 days).
Example of charges:
Volume: 220 CCF,
BOD5: 1,759 mg/L,
SBOD5: 1,500 mg/L,
SS: 799 mg/L.
BOD5: (220 CCF) X ((1,759 mg/L – 1,500 mg/L) – 250 mg/L) X (0.002003) = $3.97
SS: (220 CCF) X (799 mg/L – 275 mg/L) X (0.001445) = $166.58
The dollar amounts represent the strong waste surcharges that would appear on a customer’s bill below the water and sewer charges.
Link to Water & Sewer Rates:
Biochemical Oxygen Demand (BOD5)
The biochemical oxygen demand, generally referred to as BOD5, is a measure of the oxygen utilized in the biochemical oxidation of organic matter under standard laboratory procedures for five days at 20 degrees Celsius. It is simply an indicator of the organic strength of wastewater. As the strength of wastewater increases greater amounts of energy are required to clean the wastewater. As the strength increases so do the costs to treat it. BOD5 is expressed in terms of weight and concentration (milligrams per liter).
CCF = 100 cubic feet = 100 X 7.48 gallons per cubic foot = 748 gallons
Food service establishments make up a large percentage of facilities that may discharge high strength wastes into the sewerage system. Discharge of food solids, animal and vegetable oils and greases and cleaning chemicals can all increase insoluble BOD5 and SS concentrations. Improper maintenance of grease control devices or lack of a grease control device may also result in higher strength waste discharges. In addition, the use of garbage grinders may increase wastewater strength by introducing additional solids into the waste stream. The strength is determined by laboratory analysis of samples taken from a customer’s wastewater discharge and reported in milligrams per liter (mg/L).
Insoluble Biochemical Oxygen Demand
The resultant value in mg/L from subtracting SBOD5 from total BOD5.
List of Average Insoluble BOD5 Percentages by Customer Use Type
- County Offices – 57%
- Convenience Store – 30%
- Hospital – 67%
- Multiple Listings – 48%
- Medical Treatment – 67%
- Nursing Home – 51%
- Service Station – 91%
- Food Service Establishment – 39%
- Motel/Hotel – 56%
- Industrial Manufacturing – 39%
- Schools – 61%
The monitoring for strong waste is accomplished by deploying an automatic sampling device to sample only the isolated wastewater discharged by one customer account. In some instances, a series of grab samples are collected and composited together to make one sample. Whatever the sampling method performed, the account is monitored for a full business day to gather a representative sample. Monitoring is performed by trained Henrico County personnel or can be performed by a supplemental self-monitoring consultant hired by the customer.
A sewerage system is the network of pipes and pumping stations leading to a treatment facility. Sewerage and sewage are often used interchangeably, however this is not correct. Sewage is what passes through the sewerage system.
Soluble Biochemical Oxygen Demand (SBOD5)
The result measured by the BOD5 test procedure specified in 40 CFR 136 after the sample is filtered (vacuum of 25 mmHg) through a 0.45 μm pore size filter. The soluble BOD5 is a measure of food for microorganisms that is dissolved in the water being treated.
This is a term applied to the relative strength of an industrial or commercial discharge into the County’s sewerage system. If the discharge exceeds 250 mg/L BOD5 or 275 mg/L SS it is considered to be a strong waste. By definition, if any commercial or industrial customer exceeds the threshold values that customer is automatically a “strong waste customer.”
Strong Waste Monitoring
The composite sample testing is accomplished by deploying an automatic sampling device to sample only that waste discharged by the facility. A composite sample consists of a series of individual, proportional grab samples taken every 15 minutes or more apart over a period of one full business day during normal operating conditions.
Use of a private environmental consultant to perform strong waste monitoring at a customer location.
In Henrico County, the BOD5 threshold is 250 milligrams per liter (mg/L) and the SS threshold is 275 mg/L. Above these values, a customer is subject to receiving a surcharge on the water/sewer bill. In 2016, Henrico County began the practice of separating the BOD5 laboratory results into components of soluble vs. insoluble. The value that is used for billing purposes is the insoluble component (higher in strength).