N. NPDES Narratives

Appendix N. NPDES Permit narratives

Excerpts from Ohio EPA TSD – Point Source Loadings Part B.1

B.1 Point Source Pollutant Loadings

There are 51 active permitted point source discharges in the Big Darby Creek watershed.

Many (48) of the facilities are small wastewater treatment plants (WWTPs) serving small cities, villages, schools, businesses, and mobile home parks, and discharging from 2,000 to 100,000 gallons per day (gpd), which are generically called “package plants.” The term “package plant” is used to describe a wastewater treatment system consisting of a prefabricated steel or concrete tank that is divided into an aeration chamber and settling chamber. Most package plants are preceded by some type of pretreatment, which removes untreatable matter, including plastics, rags, rocks, wood, etc. Occasionally, flow equalization tanks are also included. The package treatment system is generally followed with effluent polishing and disinfection either by chlorination followed by dechlorination or ultraviolet light. Sometimes the plant will have a sludge holding tank.

The pretreatment devices in a package plant are generally one or two of the following: a trash trap, which is a tank that has an outlet opening that is smaller than the inlet and is in the lower 1/3 of the tank; a bar screen, which is a rack of bars in front of the package plant inlet usually spaced 1-2" apart; and/or a comminutor which grinds the trash into small pieces.

A flow equalization tank assists in providing the package plant with a steady flow, hopefully over an entire day since water use and subsequent wastewater generation varies within a day. A constant flow of sewage facilitates the effective breakdown of organic material in the sewage by providing the microbiological fauna providing the treatment a “steady state” of nutrients. Consistent flow yields improved effluent quality.

When working properly, the aeration chamber facilitates the breakdown of up to 90% of the organic material in the wastewater by injecting large volumes of air into the water. This provides the oxygen required to facilitate the aerobic breakdown of the wastes by the microfauna. Carbon-based oxygen demanding substances, as measured by cBOD testing, are broken down by the activity of bacteria and protozoans. If enough detention time is provided in the aeration tank, breakdown of nitrogenous wastes (i.e., ammonia) will also occur. As the waste is “eaten” by the different microfauna, the microfauna multiply.

The following chamber in the tank is the settling chamber, also termed a clarifier. Its function is to permit settling of the microfauna, also known as sludge, by gravity. Some of the settled organisms are then pumped back to the aeration chamber to resume consumption of the raw wastewater components. Clear water is decanted over the top of a barrier or weir and moves onto the next stage of treatment, usually polishing and/or disinfection. If the system has been upgraded, a sludge holding tank will be present to store excess microfauna. When this tank fills, the “sludge” is dried or dewatered and taken to a land fill or directly removed to larger WWTP for digestion. Polishing treatment generally consists of sand filtration or a polishing pond. Both function to remove light weight “sludge” that did not settle in the clarifier. The treated wastewater then flows to a small disinfection tank. Disinfection, destruction of remaining microfauna in the wastewater, is generally achieved by chlorination by tablet or liquid bleach and followed at the outlet of the tank with dechlorination which is accomplished by tablets or liquid.. Occasionally, ultraviolet light will be used to provide disinfection; however, this is uncommon in package plants.

These “package plants” combine to account for approximately 4.5% of the wastewater flow, 9.4% of the cBOD₅ loading, 35.3% of the ammonia loading, and 6.8% of the suspended solids loading discharged to the Big Darby watershed. Many of these package plants are not properly maintained and operated, hence the disproportionate amounts of ammonia, cBOD₅, and suspended solids loadings documented as discharged from these facilities.

Individual NPDES Permits (Alphabetical Order):

Alton Campground Mobile Home Park

Battelle Memorial Institute

B&B Motel

Canaan Community Mobile Home Park

Canaan Elementary

Clarks Lake Subdivision

Community Gardens Mobile Home Park

COJV School Dis trict , aka Tolles Technical School

Cypress Wesleyan School

Darby Creek Golf Course

Darby Dan Farms LLP