How to filter stormwater

Stormwater treatment can look very different depending on the level of treatment a project, development or municipality are trying to obtain. Removing matter before the stormwater is treated is an essential part of the process and the various ways to do that also play into the pumping system and design.

The treatment of stormwater can take a lot of different forms but there is a time and place for each, depending on the use.  

Large material filtering  

Large matter is important to filter out prior to reaching the pumping system. Large matter (like towels, rags, trash, trash bags) are all things that can cause even the largest pumps to clog. Filtering these objects out will help to increase the success of your system. 

How to filter out the large stuff: 

• Upstream catch basin “first flush vault” or bar screen to filter large matter with physical boundaries. This is an upstream vault that has a weir wall inside or a screen of some sort. As the water comes in, the trash is filtered out. 
• Upstream straining. All the water passes through a structure like a grate or basket that catches and removes large debris.  
• Hydrodynamic separator. This is a large structure that can be either upstream or downstream of the system where the water enters a cylindrical container that spins around, and with the water spinning the solids fall out of suspension. Once the solids fall out then someone can physically go in and scoop everything out. 
• Trash basket: This is similar to a “first flush vault” except there is not an additional structure. You have a porous basket that is mounted to the inlet of the wet well. As water drains in, you can catch all the solids to remove. 

Fine particulate filtering 

Fine matter includes “Volatile Organic Compounds (VOC),” as well as other organic compounds that may drain off parking lots, industrial sites or any other location with stormwater runoff. Fine matter could also be harsh chemicals or leachate.  

Forms of treatments for this can include:  

• pH neutralization tanks — These are for highly acidic or high volumes of alkaline wastewater. A pH neutralizer system raises the pH of the water to a more neutral pH value. Some systems use calcium and magnesium media, which increase water alkalinity on contact.  

Other neutralization systems use small pumps to inject tiny amounts of sodium carbonate into the water. Either way the outcome is the same and results in water with a more neutral pH value. 

• Media filters — Companies have products that are filled with a “media” or “media cartridge” that will filter out organic compounds and require that they be replaced every so often. These filters treat stormwater by filtering out nutrients, and metals and provide hydrocarbon removal.  
• Bioswale — A “bioswale” is a natural approach to the filters discussed above. Think of a planter box, designed with a variety of layers that filter out the runoff. The stormwater is collected at the system and then pumped a short distance to an area filled with organic media and plants. Typically, all are installed with some drain rock. The bioswale is designed to slow down stormwater through a curved or linear path.  

Bioswale examples 

For the Westfield Mall project, in Santa Clara, California, Romtec Utilities designed and supplied a stormwater runoff pump station to handle low flows of runoff from a parking lot located within a bioswale. The bioswale helps filter out concentrated stormwater that is conveyed into it which helps remove debris and pollutants like oil, gasoline, or other groundwater toxins.

This stormwater pump system utilized an economic design with a 4-foot ID precast concrete wet well, schedule 80 PVC discharge piping, and small ½ hp Weil pumps in a duplex configuration. This system included NOLTA floats for liquid level sensing, and a UL-listed control panel was provided with a weather-rate NEMA 4 enclosure. The entire in-well electrical system and the control panel were wired for intrinsically safe applications.

For another project, Romtec Utilities designed six stormwater pump stations to handle low flows of stormwater for a casino in San Jose, California. The casino is developing new hotels around it and each system will serve the new hotels as it expands. The systems handle stormwater runoff from the parking lots, which lead into the bioswales where the systems are located.

These pump stations needed to meet the low flow requirements while maintaining consistent head conditions. To achieve this, Romtec Utilities engineered four of the pump stations with 1/3 hp Goulds pumps in a simplex configuration. Another pump station was designed with a 1/2 hp pump, and the final pump station included a 1.5 hp submersible pump.

The electrical controls operate on single phase power, which can be appropriate in stormwater applications. The controls are housed in Adalet enclosures and use NOLTA floats for liquid level sensing. 

Large and fine matter removal

The Port of Tacoma project involved taking untreated stormwater to be pumped into the hydrodynamic separator that then feeds into a large bed of media filters. These three powerful pump systems were installed in drainage basins around the site.

The systems each discharge directly into an array of modular wetland systems by Contech to remove any pollutants from the stormwater. Flygt submersible pumps with the required horsepower were installed in duplex pumping configurations to meet the varying flow rates in each system and to provide redundancy.

The volume of water and requirements for these pumping systems called for rectangular wet wells with baffle walls and flap gates. During extreme weather and when the onsite water treatment systems are exceeding capacity, stormwater will be discharged directly into the bay to prevent system overflow. Discharge valves were installed in separate vaults adjacent to the wet wells for easy access. 

Prior to shipping the system, some mechanical assemblies were prefabricated and installed to expedite the installation and decrease interruption of Port Operations. Meeting the unique requirements of this application required expertise with baffles, flap gates and modular wetlands systems on a large scale. 

Upstream straining project

The Santa Ana Delhi diversion pump system sits adjacent to the Santa Ana Delhi channel in Orange County, Southern California. The water from the Santa Ana Delhi Channel is diverted to a Romtec Utilities’ pumping system which then pumps to an Orange County treatment facility.

However, “large matter” treatment occurs right upstream of the Romtec Utilities system. All the floating debris is diverted from the channel and physically removed via the “mesh bag strainer” assembly. Once the strainer assembly is filled, then the maintenance team services the site and empties the strainer. If for some reason the strainer is “overran” then the metal screen shown behind the strainer assembly will also act as a strainer and filter out the larger materials.

Romtec Utilities designs, manufactures and supplies stormwater pump stations for a wide variety of applications all over the country.  

Stormwater treatment can mean a variety of things and Romtec Utilities has the experience to accommodate each solution seamlessly. The type of pumping system required depends on the type of treatment and how best to support the overall stormwater management system.