Industrial and commercial customers, including 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 BOD5 (biochemical oxygen demand) and SS (total suspended solids).  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 commercial/industrial and residential customers more equitable, a strong waste surcharge program has been established.  Many jurisdictions in the metropolitan area, and in the nation, have developed similar programs.

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 – 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 two indicators is determined by laboratory analysis of samples taken from a designated monitoring manhole.  The rate is determined and established by County Code in the same manner as other water and sewer rates.

Strong waste monitoring samples are periodically obtained and reported during the year.  Monitoring occurs on average every 6 months.  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. 

 Examples of charges for 675 CCF, 759 mg/L BOD5, 799 mg/L SS, and rates effective 7/1/13:

BOD5:  (675 CCF)  X  (759 mg/L – 250 mg/L) X (0.001763) = $605.72

SS:  (675 CCF)  X  (799 mg/L – 275 mg/L) X (0.001273) = $450.26

The dollar amounts are the strong waste surcharges that appear on a customer’s bill.

 

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).

Strong Waste Monitoring

The testing is accomplished by installing an automatic sampling device to sample only that waste discharged by the facility.  The facility is sampled for one day (24 hours) at a collection rate of every 15 minutes.

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.

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.”

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