1.44 BMP C250 - Construction Stormwater Chemical Treatment
This BMP applies when using stormwater chemicals in batch treatment or flow-through treatment.
Turbidity is difficult to control once fine particles are suspended in stormwater from a construction site. Sedimentation ponds are effective at removing larger particulate matter by gravity settling, but are ineffective at removing smaller particulates such as clay and fine silt. Traditional erosion and sediment control BMPs may not be adequate to ensure compliance with the water quality standards for turbidity in the receiving water.
Formal written approval from Ecology and the City is required for the use of chemical treatment regardless of site size. When approved, include the chemical treatment system in the Stormwater Pollution Prevention Plan (SWPPP).
Chemically treated stormwater discharged from construction site must be nontoxic to acquatic organisms. The Chemical Technology Assessment Protocol - Ecology (CTAPE) must be used to evaluate chemicals proposed for stormwater treatment. Only chemicals approved by Ecology undetr the CTAPE may be used for stormwater treatment. The approved chemicals, their applciation rates, and conditions of use can be found at the Department of Ecology Emerging Technologies website: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Emerging-stormwater-treatment-technologies
1.44.3 Design and Installation Specifications
Treatment System Design Considerations
Chemically treated stormwater must be nontoxic to aquatic organism.
Only chemicals approved by Ecology through the CTAPE process may be used in system design.
Care must be taken in the design of the withdrawal system to minimize outflow velocities and to prevent floc discharge. Stormwater that has been chemically treated must be filtered per BMP C251 - Construction Stormwater Filtration prior to discharge.
System discharge rates must take into account downstream conveyance integrity.
The following equipment should be located onsite in a lockable shed:
Secondary containment for acid, caustic, buffering compound, and treatment chemical.
Monitoring equipment which consists of a pH meter and a turbidimeter.
There are two types of systems for applying chemical treatment to stormwater: batch chemical treatment systems and flow-through chemical treatment system.
Design and Installation of Batch Chemical Treatment Systems
A batch chemical treatment system consists of the stormwater collection system (either temporary diversion or the permanent site stormwater system), an untreated stormwater storage pond, pumps, a chemical feed system, treatment cells, and interconnecting piping.
The batch treatment system shall use a storage pond for untreated stormwater followed by minimum of two lined treatment cells. Multiple treatment cells allow for clarification of treated water while other cells are being filled or emptied. Treatment cells may be ponds or tanks. Ponds with constructed earthen embankments greater than six feet high or which impound more than 10 acre-feet are subject to the Washington Dam Safety Regulations (Chapter 173-175 WAC).
Stormwater is collected at interception point(s) on the site and is diverted by gravity or by pumping to an untreated stormwater storage pond or other untreated stormwater holding area. The stormwater is stored until treatment occurs. It is important that the holding pond be large enough to provide adequate storage.
The first step in the treatment sequence is to check the pH of the stormwater in the untreated stormwater storage pond. The pH is adjusted by the application of carbon dioxide or a base until the stormwater in the storage pond is within the desired pH range, 6.5 to 8.5. When used, carbon dioxide is added immediately downstream of the transfer pump. Typically sodium bicarbonate (baking soda) is used as a base, although other bases may be used. When needed, base is added directly to the untreated stormwater storage pond. The stormwater is recirculated with the treatment pump to provide mixing in the storage pond. Initial pH adjustments should be based on daily bench tests. Further pH adjustments can be made at any point in the process. See BMP C252 - Treating and Disposing of High pH Water for more information on pH adjustment.
Once the stormwater is within the desired pH range (dependant on coagulant being used), the stormwater is pumped from the untreated stormwater storage pond to a lined treatment cell as coagulant is added. The coagulant is added upstream of the pump to facilitate rapid mixing.
After coagulant addition, the water is kept in a lined treatment cell for clarification. In a batch mode process, clarification typically takes from 30 minutes to several hours. Prior to discharge, samples are withdrawn for analysis of pH, coagulant concentration,and turbidity. If levels are acceptable, the treated water is withdrawn, filtered, and discharged.
Several configurations have been developed to withdraw treated water from the treatment cell. The original configuration is a device that withdraws the treated water from just beneath the water surface using a float with adjustable struts that prevent the float from settling on the cell bottom. This reduces the possibility of picking up sediment-floc from the bottom of the pond. The struts are usually set at a minimum clearance of about 12 inches; that is, the float will come within 12 inches of the bottom of the cell. Other systems have used vertical guides or cables which constrain the float, allowing it to drift up and down with the water level. More recent designs have an H-shaped array of pipes, set on the horizontal.
This scheme provides for withdrawal from four points rather than one. This configuration reduces the likelihood of sucking settled solids from the bottom. It also reduces the tendency for a vortex to form. Inlet diffusers - long floating or fixed pipes with many small holes in them - are also an option.
Safety is a primary concern. Design should consider the hazards associated with operations, such as sampling. Facilities should be designed to reduce slip hazards and drowning. Tanks and ponds should have life rings, ladder, or steps extending from the bottom to the top.
Sizing Criteria for Batch Chemical Treatment Systems
Chemical treatment systems must be designed to control the velocity and peak volumetric flowrate discharged from the system and project site per Element #3: Control Flow Rates.
The total volume of the untreated stormwater storage pond and treatment ponds or tanks must be large enough to treat the volume of stormwater that is produced during multiple day storm events. At a minimum, size the untreated storage pond to hold 1.5 times the runoff volume of the 10-year, 24-hour storm event assuming a Type 1A rainfall distribution (3.0-inches). Provide bypass around the chemical treatment system to accommodate extreme storm events. Use worst-case land cover conditions (i.e., producing the most runoff) for analyses (in most cases, this would be the land cover conditions just prior to final landscaping).
Primary settling should be encouraged in the untreated stormwater storage pond. A forebay with access for maintenance is beneficial.
There are two opposing considerations in sizing the treatment cells. A larger cell is able to treat a larger volume of water each time a batch is processed. However, the larger the cell the longer the time is required to empty the cell. A larger cell may also be less effective at flocculation and therefore require a longer settling time. The simplest approach to sizing the treatment cell is to multiply the allowable discharge flowrate times the desired drawdown time. A 4-hour drawdown time allows one batch per cell per 8-hour work period, given 1 hour of flocculation followed by 2 hours of settling.
If the chemical treatment system does not allow you to discharge at the rates as required by Element #3: Control Flow Rates, and if the site has a permanent Flow Control BMP that will serve the project, the discharge from the chemical treatment system may be directed to the permanent Flow Control BMP to comply with Element #3: Control Flow Rates. In this case, all discharge (including water passing through the treatment system and stormwater bypassing the treatment system) will be directed into the permanent Flow Control BMP. If site constraints make locating the untreated stormwater storage pond difficult, the permanent Flow Control BMP may be divided to serve as the untreated stormwater storage pond and the post-treatment temporary flow control pond. A berm or barrier must be used in this case so the untreated water does not mix with the treated water. Both untreated stormwater storage requirements and adequate post-treatment flow control must be achieved. The designer must document in the Construction SWPPP how the permanent Flow Control BMP is able to attenuate the discharge from the site to meet the requirements of Element #3: Control Flow Rates. If the design of the permanent Flow Control BMP was modified for temporary construction flow control purposes, the construction of the permanent Flow Control BMP must be finalized, as designed for its permanent function, at project completion.
Design and Installation of Flow-Through Chemical Treatment Systems
At a minimum, a flow-through chemical treatment system consists of a stormwater collection system (either a temporary diversion or the permanent stormwater system), an untreated stormwater storage pond, and a chemically enhanced sand filtration system.
Stormwater is collected at interception point(s) on the project site and is diverted by gravity or by pumping to an untreated stormwater storage pond or other untreated stormwater holding area. The stormwater is stored until treatment occurs. It is important that the holding pond be large enough to provide adequate storage.
Stormwater is then pumped from the untreated stormwater storage pond to the chemically enhanced sand filtration system where a coagulant is added. Adjustments to pH may be necessary before coagulant addition. The sand filtration system continually monitors the stormwater effluent for turbidity and pH. If the discharge water is ever out of an acceptable range for turbidity or pH, the water is returned to the untreated stormwater pond where it will begin the treatment process again.
Sizing Criteria for Flow-Through Chemical Treatment Systems:
Refer to BMP C251 - Construction Stormwater Filtration for sizing requirements of flow-through chemical treatment systems.
Conduct the following monitoring. Record test results on a daily log kept on site. Additional testing may be required by the NPDES permit based on site conditions.
Total volume treated and discharged
Flow must be continuously monitored and recorded at not greater than 15-minute intervals
Type and amount of chemical used for pH adjustment, if any
Type and amount of coagulant used for treatment
Influent and effluent pH, flocculant chemical concentration, and turbidity must be continuously monitored and recorded at not greater than 15-minute intervals.
pH and turbidity of the receiving water
Prior to discharge, treated stormwater must be sampled and tested for compliance with pH, flocculant chemical concentrations, and turbidity limits. These limits may be established by the Construction Stormwater General Permit, or a site-specific discharge permit. Sampling and testing for other pollutants may also be necessary at some sites. pH must be within the range of 6.5 to 8.5 standard units and not cause a change in the pH of the receiving water of more than 0.2 standard units.
Treated stormwater samples and measurements shall be taken from the discharge pipe or another location representative of the nature of the treated stormwater discharge. Samples used for determining compliance with the water quality standards in the receiving waterbody shall not be taken from the treatment pond prior to discharge. Compliance with the water quality standards is determined in the receiving water.
Treated stormwater must be non-toxic to aquatic organisms. Treated stormwater must be tested for aquatic toxicity or residual chemical content. Frequency of biomonitoring will be determined by Ecology.
Residual chemical tests must be approved by Ecology prior to their use.
If testing treated stormwater for aquatic toxicity, you must test for acute (lethal) toxicity. Bioassays shall be conducted by a laboratory accredited by Ecology, unless otherwise approved by Ecology. Acute toxicity tests shall be conducted per the CTAPE protocol.
Each project using chemical treatment must have a trained operator who is certified for operation of an Enhanced Chemcial Treatment System. The operator must be trained and certified by an organization approved by Ecology. Organizations approved for operator training can be found here: https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Contaminated-water-on-construction-sites
Remove sediment from the storage or treatment cells as necessary. Typically, sediment removal is required at least once during a wet season and at the decommissioning of the cells. Sediment remaining in the cells between batches may enhance the settling process and reduce the required chemical dosage.
Sediment that is known to be non-toxic may be incorporated into the site away from the stormwater system.