Industrial and commercial customers, which include food service establishments, nursing homes and manufacturing facilities, generate wastewater streams that are higher in strength than the average residential customer. The indicators of “strength” are Biochemical Oxygen Demand (BOD5) and Total Suspended Solids (SS). 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. Many localities in Virginia, and in the nation, have developed similar programs. 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).

 

In Henrico County the BOD5 threshold is 250 milligrams per liter (mg/L) and the SS threshold is 275 mg/L. Above these numbers, a customer is subject to receiving a surcharge on the water/sewer bill. The volume of wastewater discharged to the sanitary sewerage system is also an important factor in a strong waste bill. An increase in volume causes an increase in the surcharge. The formula for calculating a strong waste charge is:

 

[(Volume of wastewater) x ((Measured BOD5 – Measured SBOD5) – BOD5 Threshold)) x (Rate)]

and

[(Volume of wastewater) x (Measured SS – SS Threshold) x (Rate)]

= $ amount of surcharge.

 

Volume is measured in CCF (hundred cubic feet). The strength (mg/L) of the indicators is determined by laboratory analysis of samples taken from a customer’s wastewater discharge.

 

Strong waste monitoring samples are periodically obtained and reported during the year. For each customer sampled, the values for BOD5 and SS are entered into their account and used to calculate strong waste surcharges on their next bills. These values remain active on the account until new values are reported. Most bills are generated every 60 days.

 

Examples of charges for 220 CCF, 1,759 mg/L BOD5, 1,500 mg/L SBOD5, 799 mg/L SS,

and rates effective 7/1/15:

 

BOD5: (220 CCF) X ((1,759 mg/L – 1,500 mg/L) – 250 mg/L) X (0.001907) = $3.78

SS: (220 CCF) X (799 mg/L – 275 mg/L) X (0.001376) = $158.63

 

The dollar amounts represent the strong waste surcharges that appear on a customer’s bill below the water and sewer charges.

 


Definitions

 

 

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

 

Sewerage System

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.

 

Strong Waste

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 installing 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.

 

Total Suspended Solids (SS)

All solids that either float on the surface or are in suspension in water, sewage, wastewater or other liquids and which are removable by laboratory filtering. SS 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