Onsite wastewater treatment challenge

December 2013 » Features » PROJECT CASE STUDY
Recirculating sand filter and low-pressure pipe effluent disposal system handles large flows.
Dennis Hallahan, P.E.
Design wastewater flows from Stockton State Park's campsites, cabins, marina, and snack bar average 6,300 gpd with peak flows approaching 12,000 gpd.

Located on a peninsula separating the two major river arms of Stockton Lake, Stockton State Park is a popular recreation and boating area in southwest Missouri. Thousands of visitors come to the park each year for daytrips, to camp, and to enjoy the lake. Guest facilities at the park are extensive, including 76 campsites, nine cabins and duplexes, two restroom/shower houses, and a 300-slip marina. A dump station is provided at the east campground loop for recreational vehicle (RV) use. The marina includes a restroom, 22-seat snack bar, and watercraft pump-out station.

Project
Stockton State Park Wastewater Treatment System, Cedar County, Mo.

Participants
Missouri Department of Natural Resources
White River Engineering

Product application
Onsite wastewater treatment uses a septic tank, recirculating pea gravel filter, ultraviolet light disinfection, and PVC low-pressure pipe disposal system with EZflow by Infiltrator geosynthetic aggregate bundles.

Design wastewater flows from the campsites, cabins, and marina at an average of 6,300 gallons per day (gpd) with peak daily flows approaching 12,000 gpd, and karst activity at the site impacted the number of available options for a wastewater treatment system design. Park and marina closure during the off-season months of November through March also impacted treatment options. In addition, the Missouri Department of Natural Resources (MDNR) Division of Environmental Quality (DEQ) imposed stringent effluent limitations prior to subsurface discharge. To address the challenge, the MDNR Division of State Parks (DSP) engaged White River Engineering in Springfield, Mo.

The existing wastewater treatment system consisted of a single-cell lagoon and slow-rate land application system. There also was an inactive, dried-up lagoon on the site. Both lagoons had a design capacity of less than 150 persons, well below the design capacity needed for existing or future visitation and both were more than 15 years old.

The treatment system, tanks, and pump vaults will treat and buffer out the flows prior to disposal.

Discovery of karst activity in close proximity to the existing active lagoon and application site caused the DSP to initiate efforts to construct new wastewater treatment facilities and properly close the existing active and inactive lagoons. Karst is a terrain with distinctive landforms and hydrology characterized by springs, caves, sinkholes, and a unique hydrogeology that results in aquifers that are highly productive but extremely vulnerable to contamination.

Treatment site details
The treatment site is located 1,200 feet or more from existing park and water supply facilities and was heavily vegetated with sericea lespedeza, native grasses, scattered trees, and brush. Based on USDA-NRCS soil survey data, soils on the proposed site were well-drained, gravelly silt loam with slopes ranging from 3 to 8 percent and with depth to bedrock greater than 80 inches. This made the site highly suitable for subsurface wastewater disposal.

The initial wastewater treatment system design included a new facultative lagoon and land application system at the site. However, a soil collapse (sink hole) opened up in the bottom of the new lagoon during construction and state regulators from the MDNR would not allow the lagoon to be completed because of the potential for future collapses.

White River Engineering revised the system design to include a recirculating pea gravel filter system preceded by a septic tank with recycle of a portion of the filter effluent back to the septic tank to achieve denitrification, followed by ultraviolet light disinfection and subsurface disposal of the treated effluent via a low-pressure pipe (LPP) system (see Figure 1).

Figure 1: System design includes a septic tank, recirculating pea gravel filter, and ultraviolet light disinfection prior to subsurface disposal via a low-pressure pipe system.

The literature on recirculating sand filter (RSF) technology is extensive. During the last several years, a number of works have explored the use of modified recirculating sand filter systems for nitrogen removal. It has been demonstrated that almost complete nitrification occurs in recirculating sand filters provided the filter remains aerobic. It also has been observed that some degree of denitrification occurs in conventional recirculating sand filters when sand filter effluent mixes with septic tank effluent in the dosing tank. Recent works indicate that recycle of recirculating sand filter effluent back to the first compartment of the septic tank provides a better anoxic environment for denitrification to occur with 60- to 90-percent reduction in total nitrogen. Plant piping is configured to include this design feature. In addition, piping and flow control valves provide a closed-loop recycle system to keep the pea gravel filter in operation during the off-season period.

Critical factors that affect the performance of an LPP system are dosing and distribution of the effluent. The dosing and resting periods help maintain aerobic conditions in the soil and around the distribution trench. Because an LPP system cycles between aerobic and anaerobic conditions, the distribution of wastewater must be uniform to prevent hydraulic overload. As is the case with all systems, LPP systems can also be susceptible to hydraulic overloading due to excess infiltration of rainwater or groundwater leaking into the septic and pump tanks. Therefore, it was critical that the wastewater treatment and LPP system tanks be watertight.

MDNR regulations required the LPP system consist of two independent disposal fields, each sized for 75 percent of average daily design flow. Geotechnical engineering work performed at the site recommended a conservative soil loading rate of 0.2 gallons per square foot per day, which required 23,625 square feet of area for each field. The large size of the LPP system dictated that each field be divided into multiple zones to minimize dosing pump size (see Figure 2).

Figure 2: Distribution laterals within each of the two independent disposal fields consist of a network of pipes divided into six equally sized zones.

To achieve this, the distribution laterals within each field consist of a network of pipes divided into six equally sized zones, which are dosed sequentially with one zone in both fields being dosed simultaneously. Each distribution zone consists of a 2-1/2-inch PVC manifold line and six, 1-1/2-inch PVC lateral lines approximately 72 feet long on opposite sides of the center manifold line with 5-foot spacing between laterals. Each lateral line has 13, 1/8-inch-diameter holes drilled in the bottom of the pipe at 5-foot intervals that allow flow to enter the trench. In lieu of gravel, the laterals were placed inside a geosynthetic aggregate pipe system, which is a 12-inch-diameter cylinder comprised of a 4-inch perforated flexible pipe surrounded by polystyrene beads and enclosed in a high-strength polypropylene netting with geosynthetic filter fabric positioned in the top half of the cylinder between the netting and aggregate.

The LPP system dosing pump control system incorporates a repeat cycle timer with high- and low-level overrides to control pump cycles. The dosing pumps move effluent through the supply and manifold lines to the distribution laterals under low pressure – approximately 3-feet of pressure head. Pressure dosing disperses the effluent uniformly throughout the entire drain field area. Two, six-outlet indexing valves installed in vaults are used to direct flow to each distribution zone within each field. The valves cycle to the next zone each time the dosing pumps stop and start. Each manifold line leaving the indexing valve includes a PVC gate valve to allow balancing of flow to each distribution zone. Transparent piping is used on the supply side of the gate valves to provide visual indication of flow.

Each lateral line within a distribution zone includes a PVC ball valve to allow balancing of flow to each lateral within a zone. Earth dams are used at the beginning of each lateral trench to prevent redistribution of effluent from higher trenches to those lower within the landscape. The distal ends of each lateral pipe are turned upward at a 90-degree angle and provided with a PVC ball valve and threaded plug to facilitate above ground access for cleaning and flushing of lines. Transparent pipe tubes with threaded adaptors were supplied and can be connected to the distal ends of the laterals to provide a visual aid when balancing flow to each lateral within a zone.

The revised project design also included a pretreatment system for the RV Dump Station to address concerns related to preservative chemicals (Formaldehyde) contained in RV waste, which is toxic to microorganisms that carry out wastewater treatment. This pretreatment system consisted of an aerobic treatment unit (ATU) preceded by a 1,500 gallon settling (trash) tank.

Products incorporated
The LPP system includes 9,000 linear feet of EZflow by Infiltrator (1201 LPP-GEO) expanded polystyrene (EPS) geosynthetic aggregate bundles. The system design treats the wastewater to a very high degree prior to subsurface disposal. Well-drained soils at the site are an additional advantage for treatment. Pressure pipe installed inside the bundles provides easy installation and long-term performance.

A header manifold feeds the laterals in the low-pressure pipe disposal fields. Valves can isolate each lateral for operation and maintenance purposes.
The EZflow bundles contain a perforated pipe that allows the smaller-diameter laterals to be installed easily. Trenches are shallow to place the system in the root zone, which will maximize evapotranspiration.

EZflow by Infiltrator is an environmentally friendly gravel replacement using an engineered EPS geosynthetic aggregate modular design. Perfect for LPP systems, shallow installations, sloping sites, or areas where contouring around trees and landscaping is needed, the EZflow system improves infiltrative performance. The preassembled units feature a 3-inch or 4-inch perforated pipe surrounded by polystyrene aggregate and will not crush, degrade, or breakdown over time. The aggregate is held in place with durable, high-strength netting.

The EPS geosynthetic aggregate bundles installed in the Stockton State Park LPP system were selected for ease of installation, saving on labor costs, and to minimize construction traffic on the fields, thus protecting the sensitive soils, which is a common concern when using granular bedding material.

The LPP system also includes two Hydro-Tek (6006-RCW) indexing valves. These valves were selected for their ease of maintenance and ability to provide a reliable and economical method to automate the multiple-zoned LPP system.

The dosing pumps are Myers ME Series (ME75) effluent pumps installed on lift-out rail systems in a 12,000-gallon fiberglass reinforced plastic (FRP) dosing tank. The 3/4-HP pumps were designed to deliver 50 gpm at 48 feet total dynamic head.

The RV Dump Station pretreatment system design was based on the High Strength FAST System (HSF 3.0) manufactured by Bio-Microbics, Inc. However, due to funding limitations, the proposed RV dump station pretreatment system has not yet been constructed. DSP wants to determine if RV waste adversely impacts the septic tank/pea gravel filter system before deciding whether to construct the pretreatment system. It is expected that dilution resulting from combining RV waste with other waste stream sources in the park will mitigate the potential adverse impact of preservative chemicals contained in RV waste.

Conclusion
The new $800,000 treatment system was placed in continuous operation in July 2013 and provides Stockton State Park a reliable and environmentally friendly wastewater treatment method. The recirculating pea gravel filter system and LPP subsurface dispersion system were selected based on their ability to achieve a very high degree of treatment to protect shallow groundwater and avoid a surface discharge of treated wastewater effluent directly into Stockton Lake.

Dennis F. Hallahan, P.E., technical director at Infiltrator Systems, has more than 20 years of experience with onsite wastewater treatment system design and construction. He is responsible for government relations and technology transfer between Infiltrator Systems and regulatory and design communities. He can be contacted at dhallahan@infiltratorsystems.net.

Richard McMillian, P.E., president of White River Engineering, Inc., has more than 37 years of professional engineering experience, including planning, design, and construction of numerous public infrastructure projects throughout Southwest Missouri. He can be contacted at richard@whiterivereng.com.


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