Grantee Research Project Results
Final Report: Gray Water Treatment System: For Sustainable Home Irrigation
EPA Grant Number: SU839265Title: Gray Water Treatment System: For Sustainable Home Irrigation
Investigators: McCreanor, Philip , Rexhausen, Victoria , Kim, Jin Young , Murphy, Griffin , Hart Jr., Deryl
Institution: Mercer University
EPA Project Officer: Page, Angela
Phase: I
Project Period: January 1, 2018 through December 31, 2018 (Extended to August 31, 2019)
Project Amount: $14,986
RFA: P3 Awards: A National Student Design Competition for Sustainability Focusing on People, Prosperity and the Planet (2017) RFA Text | Recipients Lists
Research Category: Sustainable and Healthy Communities , P3 Awards , P3 Challenge Area - Safe and Sustainable Water Resources
Objective:
The goals of this project were (1) monitor an existing in-ground concrete-vessel treatment system for residential gray water, (2) install a second in-ground treatment system for residential gray water that used light weight plastic materials for the vessel, and (3) monitor the new, in-ground plastic vessel treatment system for residential gray water.
Summary/Accomplishments (Outputs/Outcomes):
A 1500-gallon concrete septic tank was used as the vessel for the first residential gray water treatment system. The treatment processes in this system consisted of settling, coarse filtration, an AGBTU, and fine filtration (150-mesh, 100-micron), Figure 2. The first unit is a 500-gallon settling tank. Gravity flow pushes water from the settling tank through a bristle-type effluent filter and into perforated pipes. The perforated pipes discharge over the AGBTU which consists of Eljen® GSF modules (3 modules total, 24 ft2footprint) and a 16” layer of #20 filter sand. Atlantis D-RaintankTMmodules were used to create a sump beneath the AGBTU. A transition layer of coarse 6/10 sand and a high-strength, single-strand woven geotextile were used to separate the #20 filter sand and the Atlantis D-RaintankTMmodules. This system has been operated in both constant forward flush (return flow to the settling tank) and single pass mode. Results from these operation modes are presented in Figures 2 and 3, respectively. The system was operated in recirculating or constant forward flush mode from August 2016 to September 2017. The average gray water production rate during this time period was 142 gpd, the recirculation rate was 57%, and the loading rate to the AGBTU was 5.9 gpd/ft2based on gray water production not total flow. The settling tank had an average BOD5 and SS of 87.5 and 12.6 mg/l, respectively, while the average BOD5 and SS in the sump were 13.5 and 2.2 mg/l, respectively. It should be noted that measurements from the settling tank do not represent raw gray water characteristics as the return flow from the drip zones is routed to the settling tank. The system was transitioned to single-pass operation in September 2017 and has been left in this operational mode to the date of this memorandum. The average gray water production rate during single-pass operation was 147 gpd and the loading rate to the AGBTU was 6.1 gpd/ft2. The settling tank had an average BOD5 and SS of 161.6 and 35.6 mg/l, respectively, while the average BOD5 and SS in the sump were 21.8 and 3.5 mg/l, respectively. This system has required no maintenance and the SS-mesh filter has yet to have an increase in the pressure drop across it.
The results of the in-ground concrete-vessel treatment system indicate that residential gray water can be successfully treated to regulatory standards with a small footprint system that consists of settling, coarse filtration, and an attached growth biological treatment unit that consists of a layer of Eljen® GSF modules over sand. A shortcoming of this system was the use of the concrete vessel which is not only heavy (~6,000 lb) but also required modifications to the barrier wall and casting of a special lid unit. These factors combine to significantly limit the number of contractors that would be able to install a gray water treatment system that uses a concrete vessel. The use of lightweight plastic materials to construct the settling tank and containment unit was therefore proposed as an alternative. In addition, the concrete vessel system used #20 filter sand under the Eljen® GSF modules. Filter sand is expensive and not readily available in all communities, the new system therefore used an ASTM C33 sand which is less expensive and readily available due its use in construction projects. The new system was installed as an upgrade to a project site at that had used above ground tanks with discharge to a sub-surface drip irrigation system to study the characteristics of raw gray water.
A significant challenge to installation of the AGBTU/sump unit was compliance with OSHA regulations regarding human presence in excavations. These regulations effectively prohibit human presence in excavations greater than 30-inches deep without terracing back the side walls. Terracing of the sides slopes would make filling the vessel with granular materials extremely difficult. Therefore, a plan was developed to lower the entire sump and liner materials into the excavation. This consisted of building a support frame of 2-in. SCH40 PVC pipe and fittings around the sump component. The sump component and frame were then wrapped with non-woven hole punched geotextile, 30-mil PVC liner, and non-woven hole punched geotextile. This whole unit could then be lifted, moved, and lowered into the excavation. The geotextile/liner layers would then be unwrapped and the installers could stand on top of the sump unit placing them at safe position relative to the walls of the excavation while the granular materials were placed. Installation of the IM-540 plastic septic tank, AGBTU, and gravity drainage pipe was completed on May 10, 2019. The effluent discharge pump was connected to the drip irrigation system and electrical connections were completed on May 17, 2019, Finally, the control system was connected and programmed on May 21, 2019, and gray water flows from the home were routed to the treatment system on this date.
While it was possible to install the sump, granular materials, and Eljen® GSF modules in a plastic lined system, the installation process for this portion of the system required approximately eight hours of manual labor by four people plus a back hoe operator. While the support cage allowed for ‘unwrapping’ of the geo-textile and liner layers, placement of the granular materials acted to pull these layers down into the excavation. In order to prevent this, three people held these layers up while the back hoe operator deposited granular materials inside the lined unit. A fourth person then manually pushed the granular materials out to the side of the lined unit. This process was repeated until the necessary depth of granular material was achieved. The number of people and time commitment required would prohibit a commercial contractor from being able to cost-effectively install this system. While a ‘skilled homeowner’ working with unskilled labor could complete filling and assembly of the lined treatment unit, a septic contractor would be needed to install the IM-540 and to create the excavation for the lined unit as this excavation must have a level bottom and depth of this excavation is dependent on final position of the IM540.
Conclusions:
The new, plastic component system has been operating in single pass mode for approximately six months. The results presented here representing monitoring from May 21 through October 1, 2019. The home consumed 98,672 liters (2,035 gal.) of potable water and produced 49,175 liters (12,975 gal.) of gray water during this time period indicating that 50% of the potable water consumed is used for shower, bathing, and laundry activities. Potable water consumption and gray water production rates averaged 741.9 and 369.7 lpd (196 and 98 gpd), respectively over this time period. The primary water quality characteristics of concern in on-site sewage management system are BOD5 and TSS. Regulations typically stipulate that BOD5 and TSS are not to exceed 25 and 30 mg/l, respectively. BOD5 of the settled, bio-treated, wye-filtered samples averaged 124.6, 11.8, and 10.8 mg/l while average TSS levels in these samples were 37.9, 4.2, and 3.9 mg/l, respectively. These results indicate that the much lower cost and more readily available ASTM C33 sand when used in conjunction with Eljen® GSF modules can produce and effluent that meets the regulatory requirements of BOD5<25 mg/l and SS<30 mg/l. In addition, the Infiltrator IM-540, 500- gallon plastic septic tank, is an effective component to use for the settling component of this system.
Journal Articles:
No journal articles submitted with this report: View all 6 publications for this projectSupplemental Keywords:
gray water, residential, on-site treatment, irrigation, attached growth biological treatment, intermittent sand filterProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.