Stormwater Harvesting Using Retention and In-Line Pipes for Treatment Consistent with the new Statewide Stormwater Rule
Wet detention ponds collecting runoff from highways were examined as possible candidates for harvesting (reuse). Media mixes and methods to filter water for reuse were demonstrated.
Filter media mixes categorized as Biosorption Activated Media (BAM) were laboratory tested for pollution removal and filtration rates. The media mixes with highest removal effectiveness were then demonstrated in pipes placed in operation at existing wet detention ponds. The use of a media mix was also demonstrated in a pilot operation to simulate retention areas in swales along highways.
Down-flow and up-flow media filters were installed to improve water quality from wet detention ponds. Three of the filters were installed with provisions for removing debris and with mechanisms to backwash the filter media. A mobile pipe-in-pipe system was also demonstrated at a high rate of filtration. A disc filter operation was also demonstrated and water quality effectiveness documented.
A swale filter system was demonstrated to document pollutant removal. The water quality of the percolate was improved with BAM. The removal was especially significant when new sod was used over the filter media as the new sod added nutrients to the water before entering the BAM. In a field location, a pipe was used to collect the filtrate from the BAM filter, and the water was of a quality sufficient for reuse. For a BAM augmented swale, calculations for removal and cost were presented.
One potential obstacle to reuse from a highway wet detention pond was the ability to document possible effects on the surrounding groundwater. Thus, a computer program, called SHARP for Stormwater Harvesting and Assessment for Reduction of Pollution, was developed to help predict a safe yield, and an associated reuse rate that was constrained by adjacent groundwater levels. The use of the model was demonstrated at an Interstate wet detention pond in Miramar, Florida
Stormwater rules and regulations are evolving. Thus, there is a need for research that supports alternative methods for water quality treatment of runoff water. The information in this report supports the use of filtration media called Biosorption Activated Media (BAM) that improves runoff water quality. Runoff to impaired waters may need additional treatment or reduced volume of discharge to meet a mass discharge limitation. In addition, some nutrients in runoff waters may need to be removed before they percolate to nutrient sensitive areas such as aquifers with discharge to springs or estuaries. Thus, stormwater harvesting or reuse is another best management practice that can be used to reduce the mass of pollutants in runoff discharged to surface waters.
Harvesting of stormwater for a single user is typically done by direct use of the water from a pond provided there is no cross connection and that a screen filter is used. When contact with the general public is expected, irrigation quality water is needed. The water in a stormwater pond has to be treated by some form of filtration to provide irrigation quality water. Treatment methods considered within this report are those resulting from biosorption filtration media, commonly called BAM, and from disc technologies. When using BAM, the media can be placed in a pipe or other suitable containment and the runoff water or wet detention pond water passes through the filter in either a down-flow or up-flow configuration. Another option is to place BAM in a pipe within a pipe in a wet detention pond and draft the water through the pipe. This BAM pipe-in-pipe can then be moved from one location to another, and thus is considered to be a mobile treatment method. Some options for the use of BAM are called pipe treatment systems because of their practical installation configurations.
Harvesting can also occur after runoff water has infiltrated into the ground, such as from shoulder or swale areas adjacent to roadways. This infiltrated water can either be collected by compartments (pipes are common) or be allowed to further percolate into the ground until they reach a point of discharge.
A concern resulting from harvesting water from a wet detention pond is the potential effect on the surrounding wetlands when the water in a wet detention pond is lowered. Thus, a computer model was developed and tested to determine the safe yield of a wet detention pond as controlled by the harvesting schedule and the minimum ground water level at select points in the study area. This integrated surface and ground water model was used for Stormwater Harvesting and Assessment for Reduction of Pollution and is thus called the SHARP model. The model was tested at an interstate highway wet detention pond in Miramar, Florida.
BAM filtration media mixes were laboratory tested for pollution removal and filtration rates. The laboratory work was conducted in six inch diameter columns, and the media mix depth was equal to what was expected in a full-scale operating filter (2 feet depth is common). The media mixes were then installed in pipes placed in operation at existing wet detention ponds, and effectiveness in the removal of nutrients was documented.
A wet detention pond in Tampa receiving runoff from an urban watershed composed of highways, parking lots, and buildings was the site of the down-flow filter. The down-flow media filter for water from this wet detention pond was successful in removing pollution. Another wet detention pond in Sarasota County was used as a demonstration for an up-flow filter. This pond collects both highway and residential runoff. The up-flow filter operation was demonstrated to include a backwashing operation and at a filtration rate of up to 2 million gallons per day. Both ponds require installation of provisions for removing debris and with mechanisms to backwash the filter media. A reliable and redundant operation was demonstrated since the water quality in the wet detention ponds did not meet a majority of the irrigation water quality standards.
A mobile pipe-in-pipe system was also demonstrated, but application at a high rate of filtration provided marginal improvement in water quality. Due to this, a lower filtration rate was recommended. This system can also be used in emergency situations.
The water quality effectiveness and continual operation of disc filtration using water from an interstate highway wet detention pond in Miramar, Florida was also documented. A disc filter was an alternative to filtration using BAM. It provided reliability and redundancy in meeting irrigation quality standards.
A swale filter system using BAM was also demonstrated and water quality effectiveness documented. The BAM filter removed more pollutants relative to the use of parent soils documented as Type A-3 soils. The removal was especially significant when new sod was used on top of the BAM filter. Runoff not collected in the slope of the swale can be collected in the bottom of the swale if not transported. This collection can be enhanced with the use of exfiltration or French drains. Also, since filtration is assumed using at least two feet of media, the collected water can be reused. Example calculations for a BAM filter with a swale were presented.
Every time runoff water is not discharged, pollution removal can be expected. This pollution removal can be quantified and the total maximum daily load reduction estimated. Limited cost and removal information for these systems are presented.