Tyler Marshall, P.E., is a principal environmental engineer in the Iowa City office of Stanley Consultants, performing civil and environmental engineering projects for major industrial companies since 1998. He can be reached at [email protected].
Industrial facility operators face stricter storm water regulations with the final U.S. Environmental Protection Agency’s (EPA) 2021 Multi-Sector General Permit (MSGP). The best operators will save money and time by attending to simple housekeeping, regular testing and mediation steps, but even the best owners may find themselves with standard exceedances.
If pollutant storm water sources have been identified and best practices and source reduction are not enough, what can a facility do to avoid millions of dollars of investment with a conventional treatment system? Whether it is suspended solids, granulated dust from a stack, cooling tower chemicals from downdrafts, rats living in a sewer, nitrates or other fertilizer runoff, many facilities will be challenged to comply. Operators cannot bury their heads and wait two or three years. Time flies by. When the federal standards are inevitably adapted by state and local governments, and the data comes in high, operators face expensive engineered solutions.
Why Storm Water Treatment is Such a Challenge
Some might think that storm water treatment should be a simple problem to address. Rain only happens occasionally and it is relatively clean to begin with. So, what is the problem? While storm water may seem clean compared to industrial process or sanitary wastewater, the MSGP benchmarks are still quite low and will be difficult to achieve for many facilities.
First point of order: distinguish between storm and wastewater treatment. Three characteristics of storm water discharges make it especially challenging to treat:
- Heavy downpours. Traditional wastewater treatment systems depend on predictable inflows of concentrated wastes that respond to conventional biological and physical treatment. On the other hand, intense precipitation events can generate a large amount of water in a short period of time. You might have 3 inches of rain in an hour, or you might have 8 inches of rain over seven days. Large amounts of precipitation can cause even a small facility to have a very large amount of runoff, especially for a site with a lot of paved surfaces and roofs. If your facility is at the bottom of a large watershed draining into it, a quarter inch of rain could generate millions of gallons of regulated storm water. Traditional physical and chemical treatment, such as settling, requires a very large volume to get the flow velocity low enough to be effective.
- Storm runoff is very peaky. Runoff can go from zero flow to maximum flow in the space of hours or even minutes. Treatment facilities must be sized to handle peak flow rates without losing function and handle long periods of no flow. Almost all biological systems simply cannot function without a steady influx of water and nutrients.
- Storm water is dilute. Conventional biological treatment is ineffective because there is not a rich enough environment for the bacteria to grow. Conventional physical systems may see greatly reduced removal efficiency if starting with an already low concentration of pollutants.
The best way to manage storm water is to avoid managing it at all. If one has narrowed the source of pollutants down to specific areas, and process changes and housekeeping are not sufficient, then the first step should be to look for ways to reduce the volume of storm water flow.
- Get rid of non-industrial storm water. Not all storm water is created equal. Facility benchmarks are applied only to storm water that interacts with industrial processes. Runoff from non-industrial activity — such as run-on from off-site, lawns, unused areas, office buildings, employee parking — may be able to be diverted so that it is not blending with industrial storm water. This may be a matter of re-grading a ditch or installing a new storm sewer or outfall.
- Turn industrial storm water into non-industrial storm water. Add a roof or canopy over loading docks or chemical tanks. Cover stockpiles when not in use. Eliminate unnecessary boneyards and scrap storage. If there is no contact, then storm water from industrial operation areas may still be classified as non-industrial, thereby avoiding the benchmarks if discharged separately.
- Consolidate dirty processes. One may notice they only have one or two sources of low-quality storm water in certain portions of their site. Look for ways to physically relocate those activities closer to other pollutant sources. This could also include looking at the travel paths that materials take as they move through the plant. Can paths be shortened, or can loading operations be combined? Reducing the footprint of these activities directly reduces the volume of regulated storm water.
Consider Existing Wastewater Treatment Systems
People often say, “The solution to pollution is dilution.” So why go to all this work to get rid of the clean storm water? Remember that storm water is extremely difficult to treat. Most treatment systems cannot handle a sudden influx of low-strength water. Reducing the volume and increasing the concentration of pollutants by putting all the dirty processes together, the storm water becomes much more amenable to treatment.
With some added equalization volume, the dirty storm water might be appropriate to send to existing on-site pretreatment, or maybe discharge directly to the local sanitary sewer system. While there is some cost for storage and pumping, the long-term economics might show this to be the cheapest option, even with the increase in volume-based user fees. A facility would avoid the cost of installing and maintaining dedicated storm water treatment units and would have the regulatory certainty of eliminating the storm water direct discharge entirely.
If source reduction is not going to cut it, and existing treatment systems are not an option, then one must look for ways to treat storm water to meet benchmarks. If one can condense storm water to a relatively manageable volume, consider targeted storm water treatment units. These units are designed to target the pollutants being introduced from individual sources areas, such as material stockpiles, loading docks, or other material transfer or loading areas. Many have their origins in the treatment of urban storm water and are designed for retrofit into existing storm water infrastructure.
There are a huge variety of small-scale engineered treatment options, such as centrifugal in-ground units, modular cartridge filters and oil-water-grit separators. Storm water pollutants are larger, denser particles that are carried off in storm water flows. These physical properties mean that they can be removed through an appropriately selected physical barrier or process.
Some manufacturers of storm water treatment systems utilize filtration as their primary treatment mechanism. This could be as simple as drop-in filters inside of storm inlets, or as sophisticated systems of buried vaults with banks of cartridge filters selected for a specific pollutant size distribution.
Other systems will utilize the density of storm water pollutants and the velocity of the flows to cause pollutant removal by centrifugal forces. There is a dizzying array of new systems and technologies coming onto the market every month, so it is important to really understand one’s pollutant sources and characteristics to find the system that might work for one’s facility.
These systems have the advantage of a smaller footprint but tend to have higher capital and maintenance costs. Prices range from a couple hundred dollars for a single catch basin filer retrofit to up to hundreds of thousands of dollars or more for large quantity systems. Technologies can work well especially if they are targeting suspended solids or particulates, but maintenance is imperative otherwise more issues will arise.
Maintenance of Targeted Treatment Systems is Key.
It is easy to forget that targeted treatment systems are there. However, just like many other types of pollution control technologies, they will not work if they are not taken care of. Lack of maintenance can cause units to cease to function, resulting in facility flooding, regulatory noncompliance or potentially even a lowering of storm water quality. If solids accumulate in certain types of units, they can get blown out if a particularly heavy rain event passes through the unit.
Also, if the accumulated solids are heavy in organics, those materials can decay during dry weather and cause pH and chemistry of the first flush of runoff from the next storm event to be way out of permit compliance. Decomposing corn might be great when running a distillery, but not so great if it is in a storm water sample.
If source reduction and targeted treatment units are not a good fit for one’s facility, and if the facility has some unused land, then some low-tech, lower cost options can be effective for treating the entire volume of industrial storm water runoff. These systems, while tending to require much more real estate, can provide the same level of treatment as purpose-built systems for pennies on the dollar.
Retention basins can be effective at reducing suspended solids and inorganics, if they are bound up in the solids. These basins are often required by local drainage ordinance for the purpose of reducing downstream drainage issues due to peak runoff from heavily developed areas.
These basins, even though intended to satisfy a drainage ordinance, can serve double duty as storm water treatment basins, if designed to provide an appropriate hydraulic retention time for the settling of solids that are being transported in rainwater. They can even serve triple duty as a last line of defense against oil or chemical spills if the outlet controls are designed to retain floatable materials or be valved off in the event of a spill.
However, these basins need to be engineered to provide the right settling time to match the characteristics of storm water. If the main source of storm water pollutants is dissolved, which is commonly the case for organics or nutrients, they may not provide much of an improvement in runoff quality.
Alternatively, there are engineered wetlands. Engineered wetlands can be one of the very few systems that excel at cost-effectively addressing dissolved or organic pollutants in storm water as well as sediments.
Wetlands can serve in much the same manner as a retention basin for particulate pollutants. Solids and metals drop out as runoff slows down and flows through the wetland, and then are bound in place by plant roots. The wetland plant structure also serves as a growth media for bacteria and other aquatic life, which also tend to absorb pollutants from the water, similar to the way that a fixed media filter might work in a sanitary wastewater treatment plant.
Wetlands are masters at the removal of nutrients such as nitrogen and phosphorus, as they are taken up and metabolized by plants and bacteria. In fact, these systems are so reliable for dealing with dilute nutrients that they are used by many municipalities as a final polishing step for their sanitary sewage treatment.
The drawback to these types of systems is the comparatively large footprint, which may not be available for many facilities. One possible workaround to the required footprint is to work collaboratively with a municipality or an adjacent landowner. There is a growing movement towards regionalized water quality initiatives, and public-private ventures are increasingly being viewed as possible compliance options by regulators.
They not only provide water quality improvements, engineered wetlands can also provide opportunities for habitat enhancement, trails and recreation, peak flood mitigation and stream stability improvements. All of these are desirable features in their own right, and stacking multiple benefits into a single project can help get buy-in from many different stakeholders.
Public participation as stakeholders in these facilities can also open a large array of potential local, state, and federal funding sources, including grants and low interest loans. Recent pushes for public water quality and infrastructure investment at the state and federal level are pumping millions into these types of projects and loosening eligibility requirements in some cases as well.
With proper planning and creative thinking, facilities can turn a storm water quality problem into improvements that enhance sustainable design and community relations, while doubling as a cost-effective regulatory compliance solution.
Now to recap what has been covered in this three-part series for Industrial Water & Waste Digest:
The first part of the series explained that numerical standards are coming, and that state and local authorities will inevitably follow EPA guidance on storm water benchmarks. It is much easier and cheaper to adopt simple housekeeping methods to keep a facility in compliance.
The second part of this series showed how important proper storm water testing is and how it can help you make intelligent decisions about how to minimize the impact of the new benchmarks of your facility.
Also explained is how facilities can take non-traditional measures to address their storm water footprint, divert drainage channels and implement targeted storm water treatment units. Together these steps can ensure industrial operations run smoothly without expensive treatment systems or worse, such as fines and penalties that could endanger one’s operation.