As a category, water pollution from stormwater runoff has long been identified as the greatest source of contamination to our nation’s waters. The expansion of stormwater regulations in recent years has been driven by a number of factors, including the Clean Water Act (CWA), Endangered Species Act (ESA), and litigation by environmental groups. Stringent industrial benchmarks for heavy metals, suspended solids, phosphorus, and other contaminants have required the rapid and continuing evolution of stormwater management and treatment technology to meet regulatory limits. This article describes a number of recent innovations in stormwater treatment technology used for industrial sources.
Grassy swales, rain gardens, detention ponds, mobile sedimentation tanks and other traditional stormwater treatment tools are well-established and accepted approaches, but generally require favorable site conditions, extensive collection systems and space availability. As the focus of this post is on recent innovations, and more traditional approaches are covered only as they are used in combination with other treatment technologies under discussion.
In the last few years, there has been significant evolution of both complex stormwater treatment systems, and ‘home-grown’ systems utilizing off-the-shelf products, chemical additives, electrical energy or other means. These have helped many facilities meet regulatory limits. These treatment technologies can be characterized in terms of both the pollutants and their approach to treatment. Regulated pollutants include heavy metals, suspended particles, phosphorus, oils and organic compounds. Innovative approaches to treatment include media filtration, adsorption technologies, ion-exchange and electrocoagulation. These are often used in combination with other mechanical forms, including sedimentation, various forms of mechanical filtration and dissolved air flotation.
Media filtration and adsorbtion are able to treat a wide spectrum of pollutants, are relatively cost-effective, but flow rate can be limited by volumes needed to achieve adequate treatment. Filters often need large surface area or high pressures for operation. Media filters use a tank or vault filled with sand, organic media, minerals or coal to provide a three-dimensional matrix to trap particles that flow in water through the filter system. Filter systems only treat suspended particles, and do not treat pollutants that are dissolved in the stormwater.
Adsorption is similar to filtration, in that some form of tank or vault is used, but instead of physical retention by a filter, pollutants are chemically attracted and attach directly to the surfaces of the treatment media. These media can be organic or inorganic; natural or manufactured. Adsorption media is placed in tanks, beds or cartridges, and can often perform filtration and adsorption functions. Unlike a simple filter, these products can treat both suspended and dissolved pollutants.
While more capital intensive than media filtration or adsorption, electrocoagulation (EC) is an attractive option for heavy pollutant loads in industrial applications. EC can simultaneously treat suspended particles, oils and heavy metals. Conversely, ion exchange (IX) is used in applications with relatively dilute dissolved metals or as the final step in a treatment train to remove ions that remain after primary treatment steps. Traditional IX processes can be expensive to operate and subject to fouling, but new organic forms are proving to be lower cost alternatives. Ultimately, the selection of the treatment technology depends on flow rate, pollutant loading, and the numerical limits that must be achieved. The trade-off between capital expense and operational costs must also be considered.
Stormwater Catch Basin Inserts (CBIs) are another common approach undergoing rapid innovation. These typically use a non-woven geotextile material placed within a parking lot catch basin ( or storm drain), which filter particles and/or adsorb oils in a single pass-through of stormwater effluent. These may be effective in achieving compliance where loading is intermittent or low concentration, when the facility is close to meeting benchmark levels, or when the contaminant is well suited to retention by an insert material. Specialty-insert manufacturers (e.g. Cleanway, Gullywasher) have augmented the filtration with adsorbent and/or ion exchange media, but have not seen broad acceptance in the marketplace as stand-alone treatment. The advancements in the augmented CBIs rely on the addition of chemical reagents or adsorbents in the catch basin, which have a limited amount of volume and contact time with the pollutants. For this reason, augmented CBIs may only be effective in a low flow environment, for intermittent contamination or for a limited time.
Pressurized filter technology is evolving. This technology is a long-accepted the treatment method, especially for turbidity and suspended solids. Sand filters or pressurized mixed media filters have become a standard approach for particulates, and can be used as a stand-alone technology for influent streams with the majority of the particles in excess of 25-50 microns. Deep bed systems (> 3 feet deep) are now designed for particles as small as 15 microns.
When the influent has a higher population of smaller particles, a depth filter can be augmented by the use of shrimp shell extract (Chitosan) or other flocculent aids. These have proven to be highly reliable when used in combination with sedimentation or detention facilities. Provision for back-flushing the depth filters must also be included in the design and footprint, and pumping requirements typically include a minimum of 50 PSI or greater at full water quality design flow to be effective.
A recent innovation in the arena of media filters is the development of high efficiency disk filters. Amiad® Water Systems has produced a grooved disk filtration system that, when compressed, provides a highly efficient graduated filter media. The system is comprised of a stack of spirally-grooved interlocking disks. The influent is passed to the center, and flows radially through the progressively smaller interlocking slots in the disc surface. When the media has become loaded or the pressure drop exceeds a pre-determined set-point, the disks are de-compressed and allowed to expand axially, allowing the trapped particles to be released into a process analogous to a back-flush. A relatively small footprint and low energy requirements are advantages of the system.
Treating dissolved heavy metals has proven to be especially challenging aspect of achieving compliance. Adsorption, electrocoagulation and ion-exchange have been used successfully, both as free-standing systems and in combination with other methods.
Adsorption is one of the most common methods to treat metals. StormwateRx Aquip system combines multiple layers of media in an above ground tank to remove particles organics, phosphorus and metals by mechanical filtration and adsorption. If additional metals treatment is needed, StormwateRx will add a Purus® ion-exchange system for polishing. Enpurion® Water Systems use an organically grown, chemically-activated agricultural media as the key element of an adsorption/ion-exchange technology. The media is placed in a series of modules up to four columns to provide progressively higher levels of metals removal, with the advantage that only the first module is replaced at each maintenance interval. The Aquip, Purus and Enpurion Systems all require periodic media replacement which must be considered in the overall costs.
Enpurion(R) Metals Treatment (EMT) was recently approved by the Washington State Department of Ecology’s TAPE program for metals and suspended solids. EMT has a number of advantages. First is that the systems are modular and can be configured an infinite number of ways. Small distributed systems can replace extensive collection infrastructure. EMT is also renewable, and made of food-grade organic materials and naturally occurring minerals. The EMT costs significantly less to purchase and to operate.
Enpurion Metals Treatment System
Electrocoagulation is a process where an electrical current is applied using electrodes in a water stream to de-stabilize the electrical charges of dissolved and suspended pollutants. Changing the electrical charge on the contaminants causes larger particles to form, which can either settle to the bottom or be filtered out of solution. Electrocoagulation and electrochemical reduction systems (Water Tectonics Wave Ionics, Oil Trap, Enpurion EB) treat an array of pollutants simultaneously. The reaction to the electrical current simultaneously achieves high levels of treatment for particulate, metals and oils. Hydrogen gas forms on the surface of the cathodes inside the reactor, which acts as a separation medium for lower density suspended particles, while larger, heavier particles are removed by sedimentation or filtration. These systems have proven highly effective for complex streams with multiple contaminants. The disadvantages of electrocoagulation are a relatively high capital cost, high energy requirements, and the need for filtration and/or detention tanks in combination with the electrolytic components.
Ion exchange uses a chemical reaction between a solid resin and liquid water to chemically treat pollutants. The solid phase is comprised of resins, which are held in tanks or containers. Chemicals with an electrical charge, called ions, react with the resin surface and are removed from solution. Metal ions, such as zinc or copper, literally exchange places with non-hazardous ions from the resin to render the treated water safer. Often, the chemical reaction from the exchanged ions forms water molecules, so the resulting water stream is extremely pure. Unfortunately, ion exchange is an expensive alternative.
In-ground systems such as Storm-Filter, Filterra, Bay filter and others are well-established media filters constructed in vaults to remove particles and a variety of other contaminants. Some have shown the ability to treat metals and oils, but may not have adequate removal rates for industrial applications. Some of these structures have more recently been approved to use specialized adsorption media to treat a variety of pollutants, including metals and phosphorus.
‘Home-grown’ technologies are enjoying success in some applications, with a growing number of facilities able to achieve regulatory limits by constructing their own systems on-site. The Port of Vancouver’s GRATTIX system is presented as a “rain-garden-in-a-box” and is used effectively as a roof run-off treatment unit to remove zinc and other metals. The system is designed to sit below building downspouts and filter water from building roofs before it is discharged into creeks and rivers. The GRATTIX system uses compost, soils, oyster shells and selected plants to filter and remove pollutants as stormwater falls from a roof. The Port of Seattle is using oyster shells shoveled directly in stormwater catch basins to adsorb heavy metals, with the goal of increasing water hardness and decreasing metals. The oyster shells help neutralize the pH of the stormwater, and reduce the solubility of pollutants through neutralization and surface adsorption. The Port of Seattle claims to reduce copper up to 50% by this method.
Other home-grown technologies include compost-amended sand filters and compost boxes. The Port of Tacoma is using compost and ‘blooming-boxes’ to treat zinc from facility roofs at the port. These systems also capture runoff from building roofs and filter the water prior to discharge. Some concern has arisen from the treatment processes becoming a significant source of other pollutants, such as phosphorus and nitrates, which can cause environmental issues like algae growth and low oxygen levels in lakes and streams.
The adoption of proactive stormwater regulations in the Western States and territories will drive further innovation and reduction in cost. As the industry matures and technology improves, more facilities are expected to adopt stormwater treatment technology as a means of achieving compliance. At the same time, it is expected that regulatory limits will continue to decrease, and the available technology will likely continue to struggle to reach limits that, even now, can be on the edge of being attainable.