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Best Management Practices for Chemical Treatment Systems for Construction Stormwater and Dewatering

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3. Design for CTS

• 3.1 Approvals
• 3.2 Criteria for Chemical Treatment Product Use
• 3.3 Treatment System Design Elements
• 3.4 Sizing Criteria
• 3.5 Location of CTS

3.1 Approvals

1. The use of chemical treatment must have the advanced, written approval of the appropriate permitting authority in your particular state. For Federally authorized projects, this will be the Environmental Protection Agency (EPA) or an EPA designated agency.
2. The intention to use chemical treatment shall be indicated on the notice of intent for coverage under the construction general permit. Chemical treatment systems should be designed as part of the construction Stormwater Pollution Prevention Plan (SWPPP) or non-National Pollutant Discharge Elimination System (NPDES) equivalent, not after the fact.
3. Chemical treatment may be used to correct problem sites (i.e., CTS were not originally planned). The contractor must submit a plan to use CTS and receive formal written approval from the appropriate permitting authority prior to use.

3.2 Criteria for Chemical Treatment Product Use

Chemically treated stormwater discharged from construction sites must be non-toxic to aquatic organisms. The following protocol should be used to evaluate chemicals proposed for stormwater treatment at construction sites. Authorization to use a chemical in the fi eld based on this protocol does not relieve the applicant from responsibility for meeting all discharge standards and receiving water quality criteria.

1. Treatment chemicals must be approved for use by the local or state permitting authority. Petroleum-based emulsions or carriers are prohibited.
2. Treatment chemicals must have already passed aquatic toxicity testing protocols, and so do not need to be reevaluated. Contact the appropriate permitting authority for a list of treatment chemicals that have been, or may be approved for use.
3. Prior to authorization for field use, jar tests shall be conducted in order to demonstrate that turbidity reduction necessary to meet the receiving water quality criteria can be achieved. Test conditions, particularly the dosage, should be indicative of fi eld conditions. Although these small-scale tests cannot be expected to reproduce actual performance under all fi eld conditions, they are indicative of treatment capability and indicative of various chemical dose rates required for effective treatment.

3.3 Treatment System Design Elements

Good erosion and sediment control practices should always be part of the overall stormwater management plan and treatment system design in order to minimize erosion and sediment loading. The design and operation of a chemical treatment system should take into consideration the factors that determine optimum, cost-effective performance. It is important to recognize the following:

1. The right chemical must be used at the right dosage. There is an optimum dosage rate for every combination of sediment and chemical. Dosing at lower or higher rates will result in reduced performance of the system. This is a situation where the adage "adding more is always better" is not the case. As stated previously, it is important to match a specific chemical to specific soil types. When mixing a dry concentrated flocculant solution for metering into the pumped stormwater during treatment, be sure to add the flocculant granules or powder slowly, providing equal and uniform wetting to an agitated or circulating tank of water. Adding the powder too quickly may produce gelatinous masses or "fish eyes" that will not dissolve. Flocculants may be provided as a concentrated solution or an emulsion, negating this concern. Various chemicals require different mixing times resulting in various floc sizes and settling rates, so it is important to follow the manufacturer's recommendations regarding handling, mixing, storage and dispersion of all chemicals.
2. A pH-adjusting chemical may be needed in the CTS to bring the pH into the chemical flocculant manufacturer's recommended operating range. The pH adjusting chemical or gas must be mixed thoroughly into the stormwater to insure proper dispersion and contact. This can be achieved in various ways after the chemical is introduced to the flow of the turbid water, including metering the chemical into the pump intake or routing the treated flow through corrugated pipe, static mixers, and/or tanks with baffles.
3. There must be a post-treatment settling or filtration system to remove the flocculated sediment. Chemically treated stormwater should never be directly discharged from the construction site. Settling basins designed to allow sufficient settling time are commonly used, preferably with surface discharge and porous baffles. Where space is limited, mechanical sand filters can remove the flocs and pump the sediment backwash sludge to isolated contained storage areas for decanting and final stabilization or disposal. The treated stormwater can be pumped into geotextile sediment bags. These will usually remove the flocs, but in doing so they will often clog relatively quickly and may need to be replaced frequently. This must be considered when determining the costs and the maintenance requirements of the CTS.
4. The settling basin will require periodic maintenance, so access should be readily available to clean accumulated sediment. Where baffles are used, most of the accumulation will occur in the inlet area of the basin and this can be the main access point. Mechanical sand filter backwash should be isolated from stormwater and must be prevented from discharging into surface waters.

3.4 Sizing Criteria

The combination of the sediment basin or other stormwater detention area and treatment capacity should be large enough to treat stormwater during multiple day storm events. Local permitting authorities may have flow control requirements regulating the volume of discharge during storm events and establishing sediment basin size, volume, and drawdown design criteria. Bypass should be provided around the chemical treatment system - into a settling pond - to accommodate extreme storm events. Primary settling should be encouraged in the sediment basin/storage pond. A forebay with access may be beneficial for maintenance and operational function.

There are two opposing considerations in sizing batch 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 required to empty the cell. A larger cell may also be less effective at flocculation, therefore requiring a longer settling time.

The simplest approach to sizing the treatment cell is to multiply the allowable discharge flow rate by the desired drawdown time. A 4-hour drawdown time allows one batch per cell per 8-hour work period, given 1 hour each for operations and for flocculation followed by 2 hours of settling. The permissible discharge rate governed by potential downstream effects can be used to calculate the recommended size of the treatment cells. Flow through and mechanical treatment systems are limited by pump and filter size as well as backwash duration. When sizing a mechanical system, filter maintenance and backwash duration must be considered to properly size the operational 10 volume of the CTS. The following discharge flow rate limits apply, absent any local requirements:

1. If the discharge is direct or indirect to a stream, the discharge flow rate should not exceed 50 percent of the peak flow rate for all events between the 2-year and the 10-year, 24-hour event.
2. If discharge is occurring during a storm event equal to or greater than the 10-year storm, the allowable discharge rate is the peak flow rate of the 10-year, 24-hour event.
3. Discharge to a stream should not increase the stream flow rate by more than 10 percent.
4. If the discharge is directly to a lake or major receiving water, or to an infiltration system, there is no discharge flow limit.
5. If the discharge is to a municipal storm drainage system, the allowable discharge rate may be limited by the capacity of the public system. It may be necessary to clean the municipal storm drainage system prior to the start of the discharge to prevent scouring solids from the drainage system.
6. Runoff rates may be calculated using the Rational Method, unless another method is required by the local flood control agency, NPDES permitting authority, or agency that issued the grading permit.

Costs

The costs of a system and its operation will be highly variable depending on a wide range of factors. Some of these are listed below:

• Volume and pumping rate of water being treated.
• Turbidity levels of the source water and reduction in turbidity needed.
• Types of systems allowed by the permitting authority.
• Water chemistry, especially pH.

To provide some estimates of potential costs for a CTS, a detailed analysis of three different pumping rates is provided in Appendix B. This was done for a system that involved a flocculant (chitosan) and sand filtration. As the pumping rate increases, the cost per gallon drops considerably. At a little less than a million gallons per month, the cost is $0.021 per gallon. Th is drops to $0.009 per gallon as the rate approaches 5 million gallons per month. A system of this type is likely the most expensive due to its complexity and the cost of the chitosan. Substituting another chemical additive or polymer, such as polyacrylamide (PAM), in the same system may reduce costs, assuming they have similar performance. Again, remember that the right chemical at the right dosage must be used. Soil analysis with jar testing along with regulatory guidance will determine the correct polymer/additive that will meet the necessary requirements.

An alternative system involves only a settling basin after the chemical is introduced to the pumped water. This system is much simpler and has been shown to work well in many circumstances. The flocculant is pumped directly into the intake of the water pump and metered in at a rate to achieve the desired concentration. The water pump and hoses provide initial mixing and contact before discharge into the settling basin. By eliminating the sand filtration and controller modules and switching to a lower cost polymer/additive, the treatment cost per gallon is reduced by about 40%. Th is assumes there is room for a settling basin and does not include the basin construction cost. If the turbid water is in a holding pond, the treated water could be returned to the pond until the turbidity was reduced to the desired level, then released.

For small quantities of water, a variation on the above system is to pump the water into a geotextile bag (sediment bag). Again, the flocculant would be introduced into the pump intake. The resulting flocs have been found to stay inside typical sediment bags. However, the bags tend to clog, and the project budget would have to take into account the use and disposal of these. It is not possible to estimate accurately how many bags would be used because it depends on the amount of pumping and the turbidity of the pumped water. Nevertheless, many more bags would be used if there was no chemical treatment of the pumped water.

3.5 Location of CTS

In most cases, the sediment basin collecting stormwater will be at the lowest point on the project. CTS should be placed adjacent to the basin to allow convenient pumping. Usually they are placed close to the site perimeter to keep out of the way of construction activities. An access road that is reasonably all-weather needs to be installed to provide access to the system due to the need for frequent maintenance of these systems. The discharge from the CTS needs to be stabilized so that no erosion or scour occurs at the discharge point. A generalized layout is shown in Appendix B.

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