By Arthur H. Benedict, Guy Oliver, Robert Franklin,
and Ron Devitt

Since 1995, Redmond, WA, has been a leader in requiring high levels of treatment to reduce turbidity levels in construction stormwater. Located in the Puget Sound region of western Washington (Figure 1), the city has a strong stormwater management program to ensure that local salmon spawning streams and other environmentally sensitive water bodies are protected from stormwater runoff. Management of stormwater from construction activities has been, and remains, one of the cornerstones of the city's program.

Puget Sound's wet season typically is from October through April. Although precipitation patterns vary, long-duration events (three to seven days) that can generate large quantities of construction runoff are not uncommon. Best management practices (BMPs) that prevent mobilization of sediment during construction are often the most effective and least costly site stormwater management measures. Where such measures are not practical, however, some means of sediment control is required.

Once mobilized, coarse sediments can be removed by sediment control BMPs, such as detention ponds and sediment traps. Fine sediments and clay-size particles cannot be readily controlled using these conventional measures, however. Thus the City of Redmond, working with the Washington State Department of Ecology, local contractors, site owners, and technology suppliers over the last eight years, has provided guidance for applying three innovative treatment technologies - polymer clarification, electrocoagulation, and chitosan-enhanced sand filtration (CSF) - to remove fine sediments and clay-size particulates from construction runoff at locations within the city (EES Consulting Inc., 2001, 2003; Economic and Engineering Services Inc., 2003).

Regulatory Environment

Stormwater management at construction sites is regulated under the Clean Water Act and provisions of the Pollution Prevention Act of 1990. Washington Administrative Code (WAC 173-201A) states that for receiving waters, such as lakes and other high-quality waters, discharges must not raise turbidity by more than 5 NTU (nephelometric turbidity units) over background when the background turbidity is 50 NTU or less or increase turbidity by more than 10% above background when background turbidity is greater than 50 NTU. Many water bodies in the state are subject to these stringent standards.

The Department of Ecology implements the state regulations through a statewide general permit for construction activities (general permit) issued under the National Pollutant Discharge Elimination System (NPDES). The City of Redmond developed a technical manual that defines specific policies and procedures applicable to stormwater management and erosion control (City of Redmond, 1999). A key feature of the city's stormwater management and control program - identification of the need to provide high-level treatment of construction runoff - is triggered when an applicant submits a stormwater pollution prevention plan and preliminary construction documents for review under the general permit. For the construction sites described in this article, provisions for stringent turbidity control requirements were added to site permits under state-issued administrative orders. The Department of Ecology formed an advisory committee to develop a protocol by which new construction stormwater treatment technologies can be approved for general statewide use. Approved technologies are posted on a Web site, thus eliminating the need for such administrative orders with these technologies.

Why Raise the Bar?

The technologies described in this article - polymer clarification, electrocoagulation, and CSF - are capable of reliably achieving turbidity levels in treated construction stormwater equal to 10 NTU or less. What are the benefits of achieving such high-quality treated stormwater?

As previously noted, stringent standards for turbidity must be met for many state water bodies. Thus the primary benefit of requiring high-level treatment is achieving water-quality and habitat protection. Another benefit is improved overall site stormwater management. Contractors involved with the technologies described herein reported that planning for stormwater treatment fostered better scheduling of construction activities and better overall site stormwater management. Contractors also noted that through this early planning, opportunities to optimize use of conventional BMPs were identified. Therefore the awareness of stormwater management personnel was enhanced and personnel training needs were identified.

Implementation of high-level construction stormwater treatment also provides an opportunity for construction to occur during the wet season. Construction otherwise might be curtailed during the wet season in environmentally sensitive areas because of concerns over the capability of conventional BMPs to provide the necessary protection. The innovative technologies employ fail-safe design and operating features to prevent inadvertent violations of discharge limitations. Hence they provide a barrier to turbidity release, allowing for wet-season construction in the city.

How Is Treatment Accomplished?

Successful application of polymer clarification, electrocoagulation, and CSF was accomplished through careful planning, design, and operation of the treatment facilities to ensure that they were integrated into overall site stormwater management. As illustrated in Figure 2, construction stormwater treatment involved five basic steps: interception and collection of onsite runoff, primary detention of the untreated runoff, treatment to effect turbidity reduction, secondary detention of treated stormwater, and monitoring and testing prior to release to the receiving environment. In addition, all sites were required to install and maintain appropriate conventional BMPs to prevent or minimize erosion during construction.

Primary detention facilities required at each site served two purposes. First, by accumulating and temporarily storing stormwater until treatment was needed, treated flows were attenuated, and treatment facility sizing could be optimized. Second, they served as sediment control BMPs to remove coarse particulates such that subsequent treatment facilities were not overloaded with sediment. Stormwater released from the primary detention facilities still contained fine sediment and stable, clay-size particulate suspensions that needed further treatment to meet the stringent state water-quality standards.

In some cases, the primary detention facilities were sized as permanent facilities to meet postconstruction criteria. In other cases, detention capacity was provided only for the construction phase. In the latter situations, the primary detention facilities were either modified for postconstruction use or decommissioned once construction was complete. Both lined and unlined primary detention facilities were used at the Redmond sites.

Figures 3, 4, and 5 present flow schematics illustrating the basic treatment steps involved in each technology. Regardless of the technology, the quantity of runoff that accumulated in the primary detention facility at each site was routinely monitored, as were forecasts of expected weather events. Treatment was initiated based on this information and site-specific criteria. Thus, although treatment facilities were maintained on-site at all times during the period when generation of sediment-laden construction runoff was expected, active stormwater treatment during this operating period was intermittent.

Polymer Clarification

Figure 3 illustrates a typical polymer clarification system employing two lined treatment cells that treat stormwater in batches. Batch treatment begins by filling Cell No. 1 with stormwater from the primary detention facility (Cell No. 2 is empty). As Cell No. 1 is filling, an appropriate dose of synthetic polymer is metered into the cell. The polymer causes fine sediment and clay-size particles to form large particles called "flocs" that can be gravity settled. All polymer clarification systems in Redmond used the synthetic polymer Catfloc 2953. This is the only polymer approved for such use in Washington.

When Cell No. 1 is full, inflow to the cell and mixing of cell contents is stopped. A settling period follows. During the settling step for Cell No. 1, Cell No. 2 is filled and dosed with polymer. Once the settling step in Cell No. 1 is complete, testing for residual turbidity, pH, and acute toxicity (bioassays) occurs (see the section on monitoring and testing). If testing is satisfactory, the contents of Cell No. 1 are released to the environment and the fill-treat-settle-test-release cycle is repeated. If the tests are unsatisfactory, stormwater is re-treated. For the polymer clarification systems used in Redmond, the treatment cells themselves served as secondary detention facilities; hence separate secondary detention facilities were not required (see Figure 2).

Electrocoagulation

Figure 4 illustrates a typical electrocoagulation system for construction stormwater treatment. Following a decision to treat, stormwater from the primary detention facility is pumped at a continuous and preset rate to the electrocoagulation cells where fine sediment and clay-size suspensions are subjected to an electrical field. The electrical field generates flocs that subsequently can be removed by gravity settling. At the Redmond sites, facilities referred to as tube settlers were used for this purpose. Granular media filtration followed to remove fine floc particles. Filtered stormwater then was pumped to the secondary detention facility for temporary storage, monitoring and testing prior to release to the environment through 10-micron cartridge filters.

Chitosan-Enhanced Sand Filtration

Figure 5 illustrates the CSF system. Chitosan is a biodegradable polymer produced from the chitin in crab shells. In its liquid form, it treats stormwater in a manner similar to the synthetic polymer Catfloc 2953; that is, it treats stormwater by forming flocs from fine sediments and clay-size suspensions that then can be separated from the liquid stream. The liquid chitosan used at the Redmond construction site was produced by Vanson Inc. and marketed under the trade name Storm Klear Liqui-Floc by Natural Site Solutions LLC.

During active treatment, stormwater that accumulated in the primary detention facility was pumped at a continuous, preset rate to a mixing chamber where liquid chitosan was metered and rapidly dispersed into the stormwater at an appropriate dose rate. Flow from the mixing chamber then was directed to sand filtration equipment where flocs were separated from treated stormwater. One-micron cartridge filters followed the sand filtration equipment. They acted as a protective barrier to prevent noncompliant stormwater from being discharged to the receiving environment. As with the other systems, monitoring and testing were performed prior to release of treated stormwater from the secondary detention facilities.

Monitoring and Testing

Monitoring and testing requirements were site-specific. At a minimum, however, the turbidity and pH of both untreated and treated stormwater were measured prior to each release of treated stormwater. Also, bioassays typically were required on the first five batches of treated stormwater and then on every 10­20 batches. Acute toxicity tests involved 24-hour static bioassays using Daphnia magna (water flea) and 96-hour static bioassays using Onchorynchus mykiss (rainbow trout).

Site Characteristics

The described treatment technologies have been used at 12 construction sites in Redmond. Table 1 summarizes site characteristics. Nine sites employed polymer clarification, two used electrocoagulation, and one used CSF. Site drainage areas were between 5 and 65 ac., and average slopes varied from essentially zero to about 10%. All sites except one were for new commercial developments; that one was for a new public transit facility.

Click here for larger view

The operating periods reported in Table 1 (1.75­20 months) reflect the length of time that the treatment facility remained on-site, whether or not active treatment occurred. In some cases, treatment facilities were on-site during more than one wet season. The treatment rates reported represent the total volume of stormwater treated by the system (expressed as gallons of runoff treated per acre of drainage area) divided by the operating period, expressed in months. Thus they represent the average treated runoff rate for each site (after detention).

Allowable release rates to the receiving environment varied from 60 to 884 gpm. Higher values were allowed only under extreme circumstances. Allowable release rates typically were between 60 and 200 gpm. Allowable release rate was an important site and treatment criterion because the release rate determined in large measure the size of the detention facilities required relative to site area, expected rainfall-runoff relationships, and similar site-specific factors.

Treatment Performance

Turbidity Reduction

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Table 2 illustrates the turbidity reduction capability of each stormwater treatment technology based on experience in Redmond. All technologies were able to reduce construction stormwater turbidity to median levels below 10 NTU, with the electrocoagulation and CSF systems yielding values at or below 5 NTU. The maximum treated stormwater turbidities reported for both the electrocoagulation and CSF systems were below 10, whereas maximum values for the polymer systems were generally higher.

Demonstrated performance of the treatment systems covers a wide range of untreated stormwater turbidity concentrations. Median values of untreated (presettled) stormwater were between 117 and 14,000 NTU for the polymer systems, whereas lower values were experienced with the electrocoagulation and CSF systems. There are more sites, a larger quantity of stormwater treated, and a longer experience record (six years) for the polymer systems than for either of the other technologies. The electrocoagulation performance record includes experience at two sites over a six- to nine-month period, and the chitosan system represents performance over two and a half months. It is expected that the high performance demonstrated by the electrocoagulation and CSF systems would be maintained even with a longer period of record.

Control of pH

NPDES permit conditions for the Redmond sites required that the pH of stormwater discharged to the receiving environments be maintained between 6.5 and 8.5, with a target of matching receiving-water pH. Although rainfall tends to be acidic (Minton, 2002), contact with site soils frequently resulted in construction runoff at the Redmond locations that tended to be neutral. The main exception to this generality was during periods when concrete pours for foundations and similar structural components were part of construction activities. During these periods, the pH of construction runoff frequently reached values of 11.0 or more. Control of pH was maintained either through the addition of a neutralizer or by carbon dioxide dispersion. With only a few exceptions, the pH of treated stormwater was maintained at all times at all sites for the entire eight-year period of record.

Bioassays

As previously stated, bioassays were required as part of routine monitoring and testing at all sites. None of the stormwater treated and discharged failed the acute toxicity criteria.

Sediment Disposal

All of the treatment technologies generate sediment that must be disposed. Quantities were small and varied depending on site-specific factors; accumulated sediment typically remained in the treatment systems to enhance floc formation and was removed when treatment facilities were decommissioned. Sediments were tested to document that they were nonhazardous and disposed either on-site or at a local landfill.

Treatment Costs

The cost of construction stormwater treatment is site-specific. Actual costs associated with polymer clarification were not readily available from site owners because of the sensitivity to release of this information. Based on the information provided, however, polymer clarification costs varied from 0.8 to 1.5% of project construction costs. These values include costs associated with construction of the batch treatment cells and for labor, chemicals, sediment disposal, and monitoring and laboratory activities. They also include the costs of primary detention facilities at those sites where temporary, construction-phase installations were used. The costs of primary detention facilities that were sized for postconstruction use are not included.

Table 3 shows a breakdown of polymer system costs. The breakdown is based on information from four sites and represents the general cost levels associated with each cost category. The largest single cost is associated with construction of the treatment cells (and primary detention facilities if developed for temporary construction-phase use).

Monthly unit costs, expressed as actual cost per 1,000 gal. of stormwater treated per month of operation ($/1,000 gal./mo.), were developed for the electrocoagulation and CSF systems. For the two electrocoagulation sites reported (Table 1), monthly unit costs were $5.83/1,000 gal./mo. and $8.00/1,000 gal./mo. Equipment lease costs accounted for 60-75% of the total costs. The values reported here do not include primary detention, secondary detention, or site mobilization/demobilization costs.

Monthly unit cost for the CSF site was $10.22/1,000 gal./mo. Approximately 30% of the total cost was for system setup (sand filter materials, pipe, materials, metering pump installation, labor, and equipment). Monthly operating cost (equipment rentals, power, chitosan, and monitoring) accounted for the remaining 70% of the total cost.

The monthly unit costs reported here should not be indiscriminately applied to other construction sites. These unit costs are sensitive to site-specific factors, such as the quantity of stormwater actually treated, the length of time treatment facilities and operations are required at the site, and the treatment capacity provided at the site. This is illustrated in the cost-sensitivity analysis for electrocoagulation found in Table 4.

The analysis was prepared for a 10-ac. site operating over an eight-month period that generated treated runoff rates (see Table 1) between 20,000 and 60,000 gal./ac./mo. The analysis shows that monthly unit cost will vary - in this case from $4.08/1,000 gal./mo. to $8.25/1,000 gal./mo. - in response to the volume of stormwater actually treated. A similar analysis for the CSF system, which treated runoff at an actual rate of 10,100 gal./ac./mo., shows that the monthly unit cost would be $5.11/1,000 gal./mo. rather than the $10.22/1,000 gal./mo. actually reported if the treated runoff rate had been 20,000 gal./ac./mo.

References

City of Redmond. Stormwater Management and Erosion Control Technical Notebook, Issue Number 3. Stormwater Engineering Services Division. April 1, 1999.

Economic and Engineering Services Inc. City of Redmond, Field Experience with Alternative Construction Stormwater Treatment Technologies. December 2003.

EES Consulting Inc. City of Redmond, Washington - Polymer Treatment, Review of Field Experience, Phase 3. July 2001.

EES Consulting Inc. City of Redmond, Electrocoagulation Treatment of Construction Stormwater. February 2003.

Minton, G.R. Stormwater Treatment. Resource Planning Associates, Seattle, WA. 2002.

Arthur H. Benedict, Ph.D., P.E., is an associate with Economic and Engineering Services Inc. in Bellevue, WA. Guy Oliver is lead construction inspector and Robert Franklin is one of the Development Services Division managers in the City of Redmond Public Works Department. Ron Devitt is a facilities manager for general stormwater permits at Washington State Department of Ecology.

SW May/June 2004

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