Stormwater Treatment:

Mohr Separations Research can help with high efficiency Stormwater Treatment solutions. Each year, large quantities of sediments and other contaminants enter surface waters due to rainwater runoff and snowmelt. Bennett, et al, (1981) note that "Contributions of organic pollutants and suspended matter from stormwater are significant and are quite important for highly developed areas. Storm waters produce instantaneous shock loadings on a stream and result in short-term very high pollutant concentrations."

In the United States, much of the water that falls during rainstorms goes directly to surface bodies of water by dedicated storm sewers. Some rain flows directly into the surface water by streams and culverts, and some of the water enters the surface water by Combined Sewer Overflows (CSOs).

Oil and grease, heavy metals, and other contaminants found in rain water and snowmelt can be very toxic to aquatic life and detract from the pleasurable use of streams, lakes, and bays.

Regulatory and Legal Considerations:

In 1987, amendments to the Clean Water Act (CWA) prohibited non-stormwater discharges to storm sewers and dry weather discharges from industrial sites without an NPDES permit. (Buchholz, 1994).

The CWA also required the EPA to promulgate stormwater regulations. These regulations, when issued required treatment for all stormwater issuing from industrial sites where "raw materials or finished goods" are stored outside.

Contaminants in Stormwater:

Hydrocarbons and heavy metals are often found in the sediments in stormwater, so it is difficult to separate discussions of these contaminants, but the following is an attempt to discuss them as if they were separate issues:

Hydrocarbons

Most of us have seen a small oil slick "rainbow" on the water runoff in a parking lot during a rainstorm. This constitutes a small but measurable amount of oil, and when multiplied by the hundreds of parking lots in a city can be a large amount of oil. Estimates indicate that as much as 1,200 tons per year of oil and grease enter the San Francisco Bay estuary every year, and other bodies of water receive as much or more.

In addition to runoff from parking lots, rainwater runoff from service stations, highways and bridges, and industrial sites contribute to the hydrocarbon content of the rainwater.

Many studies have been performed using gravimetric methods for determining hydrocarbon content, which may not be as accurate as late studies using infrared spectrophotometers for hydrocarbon analysis. It is possible that the gravimetric type studies simply missed the small droplets which are difficult to separate gravimetrically.

It is also recognized that contaminants exist in snowmelt runoff, but not to as great an extent as are found in rainwater (Bennett, et al, 1981).

Sediment

Buchholz (1994) notes that among the most frequently observed results of (stormwater) monitoring program: "Sediments were the most critical and frequently observed pollutant in stormwater flows."

Since researchers have found that hydrocarbons tend to partition to the solids in a stormwater stream, and many of the particles are automotive exhaust particles, (Eaganhouse, 1981), removal of sediments should also tend to help remove the hydrocarbon content of the stormwater. Conversely, failure to remove the particulates from the stormwater should allow some of the hydrocarbons to pass through a separator.

Heavy Metals

Heavy metals of concern are Copper, Lead, and Zinc. Heavy metal concentrations seem to be related to pH and hardness of the stormwater. (EPA, NURP, 1983). A recent report by the EPA (EPA, 1992) on the San Francisco Estuary Project indicated that most pollutants in the estuary were due to agricultural and forestry activities, but that urban runoff was the most significant contributor for lead and hydrocarbons.

Contaminant Removal:

One important point is that the EPA found that "the impacts of stormwater discharges varied widely and were mostly affected by upstream pollution sources and the assimilation capacity of specific receiving waters" (EPA, 1983). Note: Italics added by author. This argues for two conclusions: 1) Stormwater best management practices should be locally determined. 2) In areas where the receiving waters are especially sensitive such as Puget Sound and other coastal waters, greater efforts must be made to remove contaminants.

Treatment systems designed by Mohr Separations Research, Inc., can provide treatment of stormwater to meet the requirements of any regulatory body for removal of hydrocarbons or particulate matter.

Advantages include:

High efficiency – removals down to 5 mg/l (note the Clean Water Act requires that the discharge water not have a sheen, and the common definition of sheen is 15 mg/l or less)

Low operating cost – Mohr Separations Research designs systems that operate by gravity and require no electrical or other utility input for operations.

Low maintenance cost – no moving parts and no parts that wear out or otherwise need replacement lead to very low maintenance costs.

Simplicity of operations – gravity is the simplest of all operating principles and the systems require almost no operator attention.

Recovery of oil – the oil that is recovered by a system of this type is readily recyclable and contains little water

Metals concentrations – any metals present as particles in the input will be concentrated in the sludge at the bottom of the separator. This allows for the smallest possible volume of contaminants to be disposed as not chemicals are added.

Precision design – design by Kirby Mohr, an engineer of 30 years experience who has written many of the definitive articles about stormwater treatment for oil and solids particle removal.

The simplest system that can be guaranteed to work is always the best.

 BIBLIOGRAPHY

Bennett, E.R., Linstedt, K.D., Nilsgard, V., Battaglia, G.M., Pontius, F.W., "Urban Snowmelt - Characteristics and Treatment", Journal WPCF, Vol. 53, No. 1, January, 1981.

Buchholz, G.M., Defining the Urban Stormwater Runoff Problem, a report prepared for the National League of Cities and The National Realty Committee, Montgomery Watson Co., Walnut Creek, CA, 1994.

Eaganhouse, R.P., and Kaplan, I.R., "Extractable Organic Matter in Urban Stormwater Runoff," Environmental Science and Technology, Vol. 15, No. 3, March, 1981.

U.S. Environmental Protection Agency, Results of National Urban Runoff Program (NURP), 1983.