Surcharges - COD and TSS
Surcharges are assessed on two parameters: COD and TSS.
COD means "Chemical Oxygen Demand" which is a measure of the ability of a waste to pollute. That is, if a waste source were to be discharged into a body of water, it would consume a measurable amount of oxygen depriving aquatic life of that oxygen. If the oxygen is depleted enough, that body of water will be come putrid.
TSS means "Total Suspended Solids" and is the measure of the solid waste coming into the wastewater plant as opposed to the liquid and dissolved matter.
COD, along with calculated flows are the two main parameters used by the engineers to design and size wastewater treatment plants. They have determined that the average COD is 450 mg/l and the average TSS is 250 mg/l. They have also calculated the cost to treat a pound of COD and TSS.
The COD test is performed by taking a representative sample and mixing it in a blender for two minutes to homogenize it. A small sample is drawn and added to a tube that contains a pre-measured amount of reagent. The tube is then placed in a reactor where it is heated for two hours. After two hours, it is cooled and placed into a spectrophotometer where the instrument reads it and gives a direct numeric readout as mg/l COD.
TSS is performed by taking a measured portion of the homogenized sample and filtering it through a pre-weighed filter. The filter is dried and weighed. It is then dried and weighed again to assure complete dryness. The difference in the filter weight before and after filtering gives the TSS in mg/l.
As was mentioned, the engineers have determined that the average COD is 450 mg/l, and the average TSS is 250 mg/l. Therefore, those have been established as the allowable limits and a surcharge is levied for waste contributions above those limits.
Basic sewer use is charged on 90% of the customer's water usage. The surcharge begins when the COD value goes over 450 mg/l. The charge is $1.08 per pound of COD over 450 mg/l and $0.31 per pound of TSS over 250 mg/l. These charges are what the engineers have determined it costs the city to treat the wastes in the treatment plant.
Surcharging allows the city to recover expenses and to allocate funds for future treatment plant expansion.
Our treatment plant is designed with a capacity of 3,550,000 gallons per day (3.55 MGD) at an average loading of 450 mg/l COD. If a contributor discharges a waste at a higher concentration, then that is capacity that is taken up and unavailable for a new connection. Plant capacity can be exceeded by hydraulic or organic (COD) overload or both.
At this time, the city is moving toward plant expansion due to increasing hydraulic and organic loading. This is an expensive project and the costs have to be born equitably by all contributors based upon their usage. A contributor discharging a waste of 350 mg/l would not like paying as much as a contributor discharging 1,000 mg/l. It's like using the Turnpike; the more you use it, the more it costs you.
For any questions you have related to COD and TSS, contact Wayne Friesen , IW Technician at the Wildwood Wastewater Treatment Plant at (352) 330-1349, Monday through Friday between the hours of 7:00 a.m. and 4:00 p.m.
