|
Food Service/Food Preparation Wastewater Discharge Information...
Glossary of Terms for Wastewater Discharge
Biochemical Oxygen Demand (BOD)
Biochemical oxygen demand, generally referred to as BOD, is a measure
of the oxygen utilized by bacteria to reduce the organic material
contained in wastewater. 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 while
increasing the costs as well.
Domestic Wastewater
Domestic wastewater is a term used to describe wastewater discharged
from a residential customer. This discharge is composed of all wastewater
exiting a house or apartment. Hanover County Department of Public
Utilities has established the strength of domestic wastewater to
be 250 mg/l BOD and 250 mg/l TSS. Any non-domestic wastewater discharge
in excess of these levels will be considered strong waste and surcharged
accordingly.
Grease Trap / Grease Interceptor
Grease traps/grease interceptors are typically reservoirs, which
vary in size, built into a discharge piping system a short distance
from the grease producing area. Baffles in the traps slow the wastewater
flow and reduce turbulence allowing the grease and water to separate.
Grease naturally rises to the surface, and the trap retains the
grease while allowing grease-free wastewater to flow into the sewer
system. Grease must be removed from the trap periodically.
Oil and Grease (O&G)/Animal-Vegetable
Animal-vegetable derived O&G is more biodegradable than petroleum
based O&G, however, the impact on the sewerage system is greater.
It is not uncommon for large amounts of O&G to be released from
food preparation facilities and accumulate in the down stream pipes.
Consequently, this problem increases sewer maintenance costs.
Oil and Grease/Petroleum
Petroleum based O&G is not readily biodegradable and is generally
toxic in larger quantities. Service stations, garages, and car dealerships
are the main sources of this type O&G.
Strong Waste
Strong waste is a term applied to the relative strength of a commercial,
industrial, or institutional discharge into the County's sewer system.
If the discharge exceeds 250 mg/l BOD or 250 mg/l TSS it is considered
to be a strong waste. By definition, any commercial or industrial
customer exceeding these threshold values is a "strong waste
customer."
Strong Waste Surcharge
A surcharge, or charge in addition to the normal use fees, is charged
for strong waste as provided for in County Code section 20-47 and
shall be applied as follows: (1) Twenty-four dollars and twenty-four
cents ($24.24) per hundred weight for BOD concentrations in excess
of 250 mg/L, and (2) Fifteen dollars and eighty-three cents ($15.83)
per hundred weight for TSS concentrations in excess of 250 mg/L.
Flow (Q) is measured in millions of gallons.
Surcharge Calculation
Q x (BOD - 250) x 8.34 x (1/100) x $24.24 = BOD surcharge
Q x (TSS - 250) x 8.34 x (1/100) x $15.83 = TSS surcharge
BOD surcharge + TSS surcharge = Total surcharge
Total Suspended Solids (TSS)
Total Suspended Solids, generally referred to as TSS, is a measure
of the settleable solids and non-settleable solids in wastewater.
TSS, like BOD, is an indicator of the relative strength of the liquid;
accordingly, the higher the TSS concentration, the greater the strength
of the wastewater. As the strength of wastewater increases, greater
amounts of energy are required to clean the wastewater while increasing
the costs as well.
|
|
