Stormwater Solids Reduction for NPDES Permits

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Finally obtaining a National Pollution Discharge Elimination System (NPDES) Permit is often a relief but is by far not the end of the story. Meeting the requirements of that permit often proves to be quite difficult and stiff fines are freely doled out to those who fail to meet regulatory limits.

For one Midwest jet propulsion manufacturing facility, the issue of total daily solids discharge limitations has been a major concern. Parking lots, roof tops and other impermeable surfaces result in a lot of stormwater runoff for any large manufacturing facility. In addition, this plant adds blow-down water from a series of large cooling towers as well as blow-down water from several boilers to the stormwater mix. Stormwater is carried underground through a large pipeline for about 300 meters (approximately 1000 feet) before terminating in a one-acre stormwater detention pond. Water is then discharged from this pond through a 91.5 cm (36 inch) manual valve into an underground concrete conduit for nearly 400 meters (approximately ¼ mile) before discharging to a natural stream. The Indiana Department of Environmental Management (IDEM) is responsible for administration and enforcement of most federal Environmental Protection Agency (EPA) permits within the state. The facility has a real-time nephelometer with chart recorder installed in the concrete conduit at the detention pond discharge site along with a recording flowmeter and flow-proportional automatic sampler.

IDEM personnel empirically converted nephelometric turbidity unit (NTU) values to total suspended solids (TSS) concentrations in parts per million (ppm) then devised a table that combines flow rate readings with turbidity readings to get the approximate daily solids discharge. As turbidity values (therefore TSS concentrations) increased, personnel had to manually close down the discharge valve to decrease the flow, thereby decreasing the total solids discharge. This labor-intensive method was far from foolproof.

Facility personnel began an investigation to see if an automatic self-cleaning filter system could improve total solids concentrations in the discharge. A pilot installation was started in the summer months of 2002 to test performance during the peak algae season. Previous laboratory tests had shown the solids in question to be predominantly algae and sand. Other than problems with keeping the portable pump primed during intermittent use, the pilot test was quite successful in reducing the amount of total solids in the final discharge. The following spring a skid-mounted filtration system was purchased and installed consisting of a pump, 6” Amiad SAF-6000 automatic self-cleaning filter, isolation valves, flow meter, pressure sustaining valve, flow control valve and a fully integrated PLC controller. Thoughts were put into interfacing the controls of two variable frequency drive (VFD) pumps with the flow meter and nephelometer outputs. This interface could be used to automatically increase the flow rate when the turbidity was low downstream of the filter system and decrease the flow rate when the turbidity was high.

This type of feedback control is quite valid but the cost was prohibitive. Therefore, the installed system is operated automatically at a manually set discharge rate.

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