Water treatment systems treat stormwater surface runoff or other polluted water. Stormwater surface runoff is of concern for two main reasons: 1) because of the effects of its volume and flow rate, and 2) because of the pollution and contamination it can carry. The volume and flow rate of stormwater is problematic because high volumes and high flow rates can cause erosion and flooding. Pollution and contamination are problematic because stormwater is carried into rivers and streams, lakes and wetlands, and eventually the oceans. Pollution and contamination that are carried by stormwater have adverse affects on the health and ecological balance of the environment.
Stormwater is characterized by large concentrations of various pollutants including trash, debris and sediments. Reports have shown that for urbanized area an average of 7.6 cubic feet of trash and 2.4 cubic yards of sediment are generated per acre of impervious surface per year. In many areas, where proper erosion control measures are not taken, which is common, the loading of sediment is much higher.
The Clean Water Act of 1972 set the stage for vast improvements to water infrastructure and quality. Water pollution has been divided into two categories: point source and non-point source. Point sources include wastewater and industrial waste. Point sources are more easily identifiable, and therefore direct measures can be taken to control them. The other category, non-point source, is more difficult to identify. Stormwater runoff is the major contributor to non-point source pollution in rivers, lakes, steams and oceans. Studies have suggested and confirmed the leading cause of pollution to our waterways is from contaminated stormwater runoff. As houses, buildings, parking lots, roads and other impervious areas are built, the amount of water that runs off these structures and into our stormwater drainage systems increases. As more land becomes impervious, less of the rain seeps back into the ground. This leads to less groundwater recharge and higher velocity flows in streams, which cause erosion and increased loads of contaminants into these waterways.
There are numerous sources of pollutants that are present in stormwater runoff. Sediments come from hillsides and other natural areas that are disturbed during construction and other human activities. When land is stripped of vegetation, the soil more easily erodes and finds its way to storm drains. Trash and other unnatural debris are dropped on the ground every day and find their way into the drainage system and waterways. Leaves from trees and grass clippings from landscape activities that land on hardscape areas no longer decompose back into the ground, but instead flow to storm drains and collect in huge concentrations in lakes and streams. These organic substances leach out huge loads of nutrients, decompose and cause large algae blooms which deplete the dissolved oxygen levels, killing fish and other organisms. Other unnatural sources of nutrients including nitrogen, phosphorus, and ammonia come from residential and agricultural fertilizers that are used in excess and find their way to storm drains. Nutrients are one of the top pollutants of concern.
Other major pollutants of concern include heavy metals, which come from numerous sources and are harmful to fish and other organisms including humans. Heavy metals include but are not limited to zinc, copper, lead, mercury, cadmium and selenium. Many waterways are no longer safe to swim or fish in and therefore no longer have any beneficial use. These metals come from car tires, and brake pads, paints, galvanized roofs and fences, industrial activities, mining, recycling centers, any metal materials left uncovered. Other major pollutants of concern are hydrocarbons, including oils & grease. These pollutants come from leaky cars and other heavy equipment and include hydraulic fluid, brake fluid, diesel fuel, gasoline, motor oils, cooking oils and other industrial activities. These particulates cause media filtration beds to clog, which decreases their treatment flow capacity and increases the maintenance and replacement requirements of the granular media within the media filtration bed.
Bacteria, pesticides and organic compounds are a few other categories of pollutants which are also harmful to waterways, wildlife and humans. Over the last 20 years the EPA (Environmental Protection Agency) has been monitoring the pollutant concentrations in most of the streams, rivers and lakes throughout the country. Over 50% of waterways are impaired by one or more of the above mentioned pollutants. As part of the Phase 1 and Phase 2 NPDES (National Pollutant Discharge Elimination System) permits which control industrial and non-industrial development, activities to control these sources of pollutants is now mandated. Phase 1 was initiated in 1997, and Phase 2 was initiated in 2003. While there are many requirements for these permits, the three main focuses are on source control, pollution control during construction and post-construction pollution control. Post-construction control mandates that any new land development or redevelopment activities are required to incorporate methods and solutions that both control increased flows of rain water and decrease (filter out) the concentration of pollutants off of the development site. These are commonly known as quantity and quality control. Another part of the these requirements is for existing publicly-owned developed areas to retrofit the existing drainage infrastructure with quality and quantity control methods and technologies to decrease the existing amount of rain water runoff and pollutant concentrations.
One of the main technologies that help to obtain these goals is referred to as structural best management practices, or BMPs. Structural BMPs are proprietary and non-proprietary technologies that are developed to store and/or remove pollutants from stormwater. Methods such as detention ponds and regional wetlands are used to control the volume of runoff while providing some pollutant reduction capabilities. Over the past ten years numerous technologies have been invented to effectively store water underground and free up buildable land above them. Various treatment technologies such as catch basin filters, hydrodynamic separators, media filters are used to remove pollutants. These technologies commonly work by using the following unit processes: screening, separation, physical filtration and chemical filtration. The maintenance of all stormwater BMPs can be very expensive and a burden to property owners.
Other technologies such as bio swales, infiltration trenches, and bioretention areas (commonly known as low impact development (LID) or green technologies) have recently been implemented to both control flow volumes and remove pollutants on a micro level. These LID technologies have proven successful at removing difficult pollutants, such as bacteria, dissolved nutrients and metals. LID technologies provide physical, chemical and biological filtration processes by incorporating a living vegetation element to create a living microbial community within the media. The plants' root systems assist in pollutant removal. Biological filtration processes have proven to be excellent at removing many of the pollutants that physical and chemical filtration systems alone cannot. While these technologies are effective, they take up substantial amounts of space that are not always available on various construction projects. As such, a need has arisen for compact LID technologies that offer the same advantages as their larger and space expensive counterparts.
Recent technology advancements in the field have focused on taking the traditional bioretention concept which is focused around vertical downward flow media filtration beds that pond water on top of the bed and making them up to ten times smaller by using high flow rate filtration media. As with traditional large bioretention systems, these new compact bioretention systems accept stormwater runoff directly without pre-treatment and therefore receive large amounts of particulates that can clog the media filtration bed. This clogging has been exacerbated with these compact systems as the surface area of the media is only one tenth that of the traditional large bioretention systems. These downward flow systems are notorious for clogging as sediments accumulate on top of the media filtration beds surface. Also, the traditional downward vertical flow path through a media bed is the most problematic for clogging, as gravity allows inflow particulates to quickly and easily accumulate on top of the media bed.
Also, with changing stormwater regulations, a move is being made from flow-based design to volume-based design. Volume-based design requires treatment along with volume control. Volume-based design requires not only a treatment system but a storage system.
Some systems include a wetlands chamber having a vegetative submerged bed, one or more walls, a floor, one or more inlet water transfer pipes and one or more outlet water transfer pipes. Examples of related systems are described in U.S. Pat. No. 7,425,262 B1, U.S. Pat. No. 7,470,362 B2 and U.S. Pat. No. 7,674,378, the contents of each of which are incorporated herein by reference in their entirety. In other systems, each of the walls and floor have an inner and outer metal mesh wall, with a space between the inner and outer walls to house stonewool filtration media slabs. Having a catch basin or chamber also includes one or more inflow pipes in one or more of the four walls to allow influent to pass into the catch basin. The system is configured so that the sediments and associated pollutants settle out of the influent and accumulate on the floor of the catch basin or chamber. A filtration panel comprising four or more walls enclosing an open space housing a filtration media, the walls being water permeable in structure to allow passage of water in either direction, the filtration media filling the entire inner chamber of the filtration panel and being water permeable.
With the ever changing stormwater regulations a system that provides features that lowers maintenance costs, increases performance and pollutant removal and can be integrated with storages systems and placed downstream are in great need and demand.