StormTreat Systems, Inc.
September 1997
Technology Proponent. . . . . . . . . . . . . . . . . 4
Technology Description. . . . . . . . . . . . . . . . 4
Technical Feasibility . . . . . . . . . . . . . . . . 6
Competing Technologies. . . . . . . . . . . . . . . . 7
Data Supporting Claims. . . . . . . . . . . . . . . . 7
Test Installations . . . . . . . . . . . . . . . 8
Performance Summary. . . . . . . . . . . . . . . 9
Site Suitability. . . . . . . . . . . . . . . . . . .10
Sizing . . . . . . . . . . . . . . . . . . . . .10
Inspection and Maintenance . . . . . . . . . . .11
Regulatory Issues . . . . . . . . . . . . . . . . . 12
Cross Media Impacts . . . . . . . . . . . . . . . . 12
Energy Issues . . . . . . . . . . . . . . . . . . . 12
Need for Additional Research, Demonstration, and STEP Support. . . 12
Summary Recommendation. . . . . . . . . . . . . . . .13
References. . . . . . . . . . . . . . . . . . . . . .14
Appendix. . . . . . . . . . . . . . . . . . . . . . .16
(Pages numbers reflect the printed copy - they are irrelevant
here. Click on the link to go to the section.)
The StormTreat system is a unique stormwater treatment technology that combines several treatment processes into a unitary system. The system includes sedimentation chambers and a biological filter capable of sustaining wetland plants. The system is installed at a depth of approximately 4 feet on a bed of washed stone. The system can be configured for recharge into the subsurface soils below the excavation required for installation or can also be configured for discharge to surface water or to a stormwater conveyance system. The system requires a pretreatment unit, such as a catch basin. There are several competing technologies with varying degrees of similarity. Few technologies exist, which combine the treatment process offered by the StormTreat system. The StormTreat system has the potential to provide enhanced treatment of stormwater compared to conventional stormwater BMPs such as sand filters and infiltration basins. The system requires no energy inputs and may be constructed out of recycled plastic materials. An assessment of the performance capabilities was prepared based on data submitted by the proponent from two installation in Massachusetts. Performance verification beyond the scope of available test data was made from comparable conventional BMPs described within the DEP Stormwater Management Handbooks. It is the conclusion of this assessment that the system, when sized according to recommended criteria, with proper operation and maintenance, can provide levels of treatment required under Standards 4 and 6, as specified by the DEP Stormwater Management Handbook. Under special circumstances, the system may provide as much as 98% removal of TSS when sized according to design criteria. The system, when configured for recharge can meet Standard 3. The closed mode system is also likely to meet Standard 5, for land uses with higher potential pollutant loads, when sized according to design criteria. Higher performance capabilities may be possible, but are currently undocumented. It is the recommendation of this assessment that when additional data becomes available, that the performance ratings be amended accordingly. STEP will provide additional assistance at the request of STS and the STEP partners.
The system is a prefabricated unitary structure which provides sedimentation, oil and grease separation, sand filtration, and biological filtration. In the system, a chambered sedimentation unit and oil and grease separator is combined with a containerized biofilter. The system is designed as a recharge unit or with controlled discharge to surface water or a stormwater conveyance system (Closed Mode). The system is 9.5 feet in diameter and 4 feet in depth. Depending on the area to be treated, any number of units could be utilized in parallel. The chamber is manufactured using rotational molded recycled polyethylene. Other components are made of PVC, gravel (in the biofilter), native wetland plants (in the biofilter), metal closures, and various fittings constructed of plastic or other durable materials. All installation of the StormTtreat system require basic pretreatment in the form of a separate stormwater inlet or catch basin. The unit, designed for recharge, is installed in a 12' x 12' excavation, with a minimum of 12" below the unit and stone surrounding the unit's sides. The closed mode unit is installed in the same sized excavation with 6" of stone below the unit. Treated stormwater effluent discharges from the closed mode unit through a 1 to 2" PVC pipe to a surface discharge or stormwater conveyance system. The STS unit is shown in Figure 1. Discharge components number 8 and 9 (Figure 1), illustrate closed mode discharge and recharge, respectively. Under typical installations, only one mode would be used per site.
The system is designed to treat the first flush of stormwater
events, consistent with Standards 4 and 6, of the Stormwater Management
Handbook (DEP and CZM, 1997). In the recharge mode, the system may be designed
to meet Standard 3 for recharge. The system operates during the initial
flush of storm events. Influent loading above the capacity of the unit
is directed to an overflow pipe in the catch basin or other pretreatment
device. Stormwater enters at the bottom of the unit through a PVC pipe
connected to the central sedimentation chambers, configured like a pie
with six sections. Influent first passes through a bag filter (component
2, Figure 1) collecting grit and large debris. The influent is directed
around the sedimentation chambers, separated by solid bulkheads, through
floating skimmers (component 4, Figure 1). The skimmers retain oil and
grease by transferring clarified water from 3-4" below the surface of the
water to the next chamber. The final sedimentation chamber effluent outlets
to a slotted PVC pipe extending into the biofilter (component 5, Figure
1). The biofilter surrounds the sedimentation chambers on the perimeter
of the unit occupying roughly « the radius of the unit (component
6, Figure 1). The biofilter contains a gravel matrix, with an effective
aggregate size diameter between 3 and 5 mm. The biofilter is designed to
support facultative wetland plants such as, bullrush. Effluent is directed
in a counter clockwise direction through the biofilter to slotted PVC pipe
(component 7, Figure 1) to an outlet pipe with an adjustable valve for
closed mode discharge (component 8, Figure 1). Lineal distance of the biofilter
is approximately 25 feet. The flow rate on the outlet is adjustable. STS
recommends the outlet flow rate be set at 0.25 gallons per minute, resulting
in a 5 day residency time at full capacity. The outlet valve can be closed
for containment purposes in the event of a hazardous waste spill. The unitary
system provides an approximate storage capacity of 1400 gallons. The alternative
recharge system design, allows for effluent recharge to permeable soils
around and below the unit. The capacity of the recharge design is determined
by the soil water permeability of the most limiting soil horizon. Treated
effluent discharges from the biofilter via slotted PVC pipe located at
the top (component 9, Figure 1) and bottom (not shown) of the unit to slotted
PVC pipe located outside the unit in the surrounding gravel. Additional
distribution pipe in the gravel below the unit is recommended by this author
for uniform distribution to the soils below. The outlet of the biofilter
is six inches above the bottom of the unit leaving sufficient water for
plant growth between storm events.
Hydraulic properties of the system suggest that residency time (5 days) within the separation chambers and the biofilter will have a direct relationship on treatment potential. Removal rates of pathogens may be much lower at shorter retention times. The biofilter, like sand filters, should be capable of high bacterial removal based on filtration and retention in small pores. Total N reduction will result from assimilation by microbes and plants and transformations via biochemical oxidation processes. Total N may also be reduced through biochemical reduction (denitrification) of oxidized forms of N. Denitrification will occur if oxidized N and soluble carbon are present in an anoxic environment such as a wetland. Phosphorus can be fixed on soil minerals or precipitated with iron and aluminum oxides. Fixed P will equilibrate with the solution, resulting in a dynamic removal process that has an finite capacity. Phosphorus removal will not be possible once the filter capacity is reached. STS recommends changing the biofilter medium at some interval to assure continued P removal, if P removal is an objective of the project. There is no information regarding performance characteristics as a function of flow rate. Solution equilibration time may be strongly affected by flow rate.
The biological component of the system is based on well documented technologies for reduction of carbon, TSS, and petroleum hydrocarbons. Residency time and operating conditions, such as: moisture, temperature, oxygen, nutrients, and light will most likely affect performance. Seasonal variation of performance is likely in the biofilter, as vegetative growth will be low or dormant for as many as 8 months per year. Additionally, the plants in the filter may not be at their maximum in spring, when runoff is highest, resulting in lower removal rates for biologically controlled processes (nitrogen and bacteria). Seasonal variation of TSS removal is not likely to occur as a result of the biofilter system performance, since removal is primarily based on sedimentation/filtration. TSS removal is more likely to vary as a function of sediment loading.
The system, when designed as a recharge unit should provide equivalent or better treatment to conventional infiltration BMPs when the unit is sized according to standard practices. The water permeability (cm/sec) of the surrounding soils will be the limiting design factor. The StormTreat system is untested as a recharge system. Total flow through the system as a recharge unit is not likely to be limited by the hydraulic properties of the internal structures of the system. Typical flow rates through the system will range from approximately 2 to 50 gallons per minute, based on a 0.5 inch precipitation event over a 5.8 hour duration (Wanielista and Yousef, 1993).
STS suggests that minimal maintenance is required, with major suction or vacuum pumping of solids once every 3 to 5 years. Shorter periods of time between pumping may be required with higher sediment loadings. The sedimentation bulkheads can be removed and washed or repaired through the central access. Sediment should be removed by qualified maintenance personnel, such as a DPW, with a vacuum truck. The biofilter medium should also be inspected as needed to assess solids buildup. Seasonal changes may require plant maintenance which could occur along with normal cleaning of catch basins and connecting pipes.
TSS performance claims are based on 6 storm events from the Kingston site. Mean reduction of TSS at the Kingston site by storm event was 95% with a 4% coefficient of variation. Raw data, uncorrected for storm event, resulted in a mean removal rate of 98% as shown in Table 1. Mean percent removal rates for Fecal Coliform, total N, P, and TPH were 97%, 77%, 90%, and 90%, respectively. TPH removal was based on one sample and is not statistically significant. The system also showed better than 77% removal of heavy metals.
Greenfield
Limited information is available on the StormTreat system installation
in Greenfield. The site receives stormwater from a parking area and roadway
associated with a municipal sewage treatment plant. Higher pollutant loadings
may have occurred on this system due to periodic contamination from septage
haulers, although this is unconfirmed. Data from a single storm event at
the Greenfield site had a TSS removal rate of 85% with influent and effluent
concentrations at 6.0 and 0.4 mg/L, respectively. Additional data from
this installation may become available.
| Pollutant | Influent Concentration | Effluent Concentration | Removal Rate
(percent) |
Storm Events | |||
|---|---|---|---|---|---|---|---|
| Mean | Range | Mean | Range | ||||
| Fecal Coliform (no/100ml) | 690 | 40-60000 | 20 | 10-462 | 97 |
|
|
| TSS (mg/L) | 76.6 | 4-344 | 1.75 | 0.5-12 | 98 | 6 | |
| COD (mg/L) | 95 | 41-232 | 17 | 1-30 | 82 | 4 | |
| Total N (mg/L) | 5.6 | 2.5-13 | 1.5 | 0.7-4.2 | 77 | 3 | |
| TPH (mg/L) | 3.4 |
|
0.3 | 0.1-0.6 | 90 | 1 | |
| Lead (ug/L) | 6.5 | 4-115 | 1.5 | 1-2 | 77 | 5 | |
| Chromium (ug/L) | 18 | 16-60 | 1 | 0.5-1 | 98 | 3 | |
| Phosphorus (ug/L) | 300 | 121-590 | 26 | 22-60 | 90 | 3 | |
| Zinc (ug/L) | 184 | 70-590 | 28 | 2.5-84 | 90 | 3 |
Predicted performance capability of the unit, designed with recharge, is based on performance characteristics of conventional infiltration BMPs. Conventional infiltration BMPs are reportedly capable of reducing TSS by 80%, if designed in accordance with the Stormwater Management Handbook (DEP and CZM, 1997). This suggests that the StormTreat system, as a recharge system, will be capable of removing at least 80% of the TSS and is likely to be capable of removing up to 96% of the TSS, when sized on the water permeability of the surrounding soils. Massachusetts Stormwater Policy prohibits recharge BMPs for land uses with higher pollution potential (Standard 5). Therefore the StormTreat unit must be used without recharge in these locations.
Removal rates of other standard parameters not specified in the Stormwater Management Handbooks (DEP and CZM, 1997) were obtained from the system installed at the Kingston site. The system has demonstrated relatively high removal rates for fecal coliform, petroleum hydrocarbons, nitrogen, phosphorus, and some heavy metals (Table 1). While these removal rates are not required for meeting Standards 4, 5, and 6, they demonstrate innovative and effective treatment potential compared to conventional BMPs, especially for Standard 5. Under conditions similar to the Kingston Site and in environmentally sensitive areas, such as coastal shellfish areas, the StormTreat system may provide superior treatment over conventional BMPs.
In high ground water conditions or flood plains, the system is designed to withstand the hydrostatic pressures created by the saturated soil conditions around the unit. Buoyancy control of the unit has been engineered into the system design. Installations that have a discharge to tidally influenced areas include a check valve on the outlet to allow discharge during low and mid tide conditions.
The closed mode system, with a surface discharge, was shown to be capable of removing as much as 98% TSS from an area of 6050 ft2 of impervious surface per unit, calculated as the area receiving 0.5 inches of rainfall totaling 1885 gallons (see Appendix for explanation of sizing). When the system is installed with a catch basin, capable of 25% TSS removal, the StormTreat system must be capable of removing 73.3% of the influent TSS to meet Standard 4. (This is the difference between the 75% remaining and the goal of 80% total removal). Since the system has been demonstrated to provide 98% TSS removal, the area for Standard 4 can be calculated as the measured removal rate (98%) divided by the required removal rate (73.3%) multiplied by the known area of treatment (6050 ft2). This calculation results in 8,080 ft2 of impervious surface per unit, assuming pretreatment of 25%.
System sizing for the recharge design is based on soil water permeability. An example of the sizing requirements using soil water permeability is presented in Table 2. Calculations used to establish the number of units per area of impervious surface is based on Standards 4 and 6 (0.5 and 1.0 inch of rainfall), mean storm duration of 5.8 hours (Wanielista and Yousef, 1993), and an infiltrative surface area of 144 ft2/unit. No credit for sidewall area of the excavation is given, due the fact that a restrictive mat is not likely to form under normal operating conditions. All flow is assumed to be vertical. Sizing, based on soil water permeability does not account for reduced permeability as a function of solids loading. The long term acceptance rate of soils receiving solids is known to decrease with time. The sizing data presented below do not include pretreatment removal efficiency or reduced loading due to rainfall intensity. Therefore, impervious area treated per unit will be slightly higher depending on the level of pretreatment and rainfall intensity.
Footenote: The sizing information presented by STS in the initial
submission lacked adequate justification based on existing experimental
data. In particular, the sizing data for 89% and 90% removal efficiency,
based on soil type, did not consider details such as: storm duration, soil
water permeability, and potential for reduced efficiency of the StormTreat
system at higher flow rates. The STS claimed that the closed mode installation
should be capable of treating the first 0.5 inch of storm water frpm, 8,920
ft2 of impervoius surface. Our analysis of the treatment potential
for a given drainage area suggests that the sizing should be based on the
holding capacity of the unit and any conveyance pipes that are full at
the end of the storm event. It is important to note that the mean interval
between storm events in the Northeast is 73 hours (Wanielista and Yousef,
1993). This period is shorter than the claimed system process time of 120
hours and only 40% of the process time for the volume of runoff from 8,080
ft2. This suggests that the total volume treated by the system, on an annual
basis, may be lower than predicted. Shorter process times may be achieved
by adjusting the outflow rate; however, shortening the process time may
potentially reduce performance. Without additional field data, the performance
capability a higher flow rates cannot be verified.
| Impervious Area Treated per Unit | |||
|---|---|---|---|
| Soil Water
Permeability (in/min) |
Standard 4
80% TSS Removal (0.5 inch event) |
Standard 6
80% TSS Removal (1.0 inch event) |
|
| 0.50 | 50,000 ft2 | 25,000 ft2 | |
| 0.25 | 20,000 ft2 | 10,000 ft2 | |
| 0.13 | 12,000 ft2 | 6,000 ft2 | |
| 0.06 | 6,000 ft2 | 3,000 ft2 |
Unites States Environmental Protection Agency, 1974. Water Quality Management Planning for Urban Runoff. EPA 440/9-75-004.
Wanielista, M. P. and Y. A. Yousef, 1993. Stormwater Management. John Wiley & Sons, Inc. New York.
| Length (ft) | Width (ft) | Area (ft2) | ||
|---|---|---|---|---|
| Elm Street | 900 | 24 | 21600 | |
| Water Dept. lot | 90 | 50 | 4500 | |
| 6 diveways @ 500 sq. ft. | 3000 | |||
| Rte. 106 | 1050 | 38 | 39900 | |
| 8 driveways @ 500 sq. ft. | 4000 | |||
| TOTAL AREA | 73000 |
| Quantity | Unit Volume or
Flow Rate |
Volume (gal) | ||
|---|---|---|---|---|
| STS Units | 4 | 1400 | 5600 | |
| feed pipes 6" and 4" | varied | 1217 | ||
| STS discharge | 348 min | 1 gpm | 348 | |
| catch basins | 1 | 376 | 376 | |
| free draining catch basins | 14 | 376 | 5264 | |
| Total Volume | 12805 |
The free draining sumps do not contribute to storage, so the real treated
volume is the total minus the free draining sumps.
| Unit | Volume (gal) | ||
|---|---|---|---|
| old total | 12805 | ||
| catch basins | 5264 | ||
| new total | 7541 | ||
| divide by 4 units | 1885 | Volume per unit |
Calculate the area covered by 0.5 inches of precipitation that produces
a volume of 1885 gallons.
1885 gal / {7.48 gal/ft2 x (0.5 in/ (12 ft/in))} = 6048 ft2
Given that the STS unit is preceded by a 25% removal efficiency device and the requirement is 80%, calculate the required STS removal efficiency to reduce the 75% remaining to 55% (the difference between 80% and 25%).
Take the ratio of 55%/75% = 0.733 removal efficiency
Calculate the ratio of the known performance and required performance. 98%/ 73.3% = 1.34 Factor
Multiply this factor by the area treated in the study 6050 ft2.
This is the area that can be treated with the STS unit and catch basin,
achieving 80% 8080 ft 2 area/unit