Cluster Systems acting as catalysts

April 2, 2018

About the author: Dennis Hallahan is technical director for Infiltrator Systems. He can be reached at 800/221-4436 or by e-mail at [email protected]. Ray Kimball is project manager and design engineer for Inland Northwest Consultants. He can be reached at 208/773-8370 or by e-mail at [email protected].


The increasing popularity nationwide of cluster orcommunity onsite septic systems is good news for sustainable development. It presents opportunities to provide environmentally sound solutions for wastewater. These community septic systems generally serve multiple residential dwellings or commercial establishments and often use technologically advanced collection and treatment systems, monitoring capability and engineered infiltration chambers to provide a higher level of treatment. By definition, these systems treat wastewater and return it to the ground very close to where the wastewater was generated, rather than transporting it long distances to a centralized sewer facility.

What is driving this trend is the high cost of sewers and lack of available quality land nationwide. This allows developers and builders to consider developing sites that may have previously been deemed financially unavailable due to the high costs of centralized systems. Often these projects feature difficult soils and tough terrain, and new onsite wastewater strategies and alternative methods of treatment are often the only way to make these sites work and to get them approved.

Poor soil areas

In remote areas that may also have poor soil, cluster systems are being installed in conjunction with advanced treatment systems. These systems extend the ability of the soil to absorb effluent, thus ensuring its safe return into the soil environment. Because of their level of performance and reduced impact on soil, chambers are often the technology of choice in cluster system pretreatment and leachfield designs.

In many cases, the economies of scale of today’s cluster systems combined with the low cost of small-diameter sewers brings the cost per unit of more advanced cluster systems below that of regionalized sewers and very close to a common septic system. This is especially true with the new breed of installation and engineering (design-build) companies arriving on the scene that can “package” systems.

Project profile

The Crossings at Willow Bay near Priest River, Idaho, is a high-end residential development with 76 home sites; however, many of the sites did not have suitable soil for drainfield disposal and were situated on steep waterfront lots, which complicated the design of the wastewater treatment system.

Luckily there were several large tracts of open space nearby with excellent soil and mild slopes. Infiltrator Quick4 chambers were selected for this large soil absorption systems (LSAS) project because of their ability to follow the contours of the land and meet the standards dictated by the Panhandle Health District.

The desire to maintain the open space in a natural state, as well as economic concerns, drove the design toward a gravel-less chamber system. State regulations allow for a 40% reduction in drainfield size with open surface architecture provided by chamber systems. This reduced the site impact to 2.3 acres, which could be accommodated in an existing meadow.

Daily loading was based on 300 gal per day (gpd) per home and 1,320 gpd for the marina facility. Total daily loading was figured to be 9,720 gpd with A to 2B soil groups. This placed the system in the LSAS category. The constraints of the LSAS regulations require that two complete drainfields be constructed, each sized to be able to handle the total daily loading. In addition, area for a third drainfield must be kept in reserve. In all, 1,700 Quick4 Standard chambers were installed in the system. The system covers more than 3.5 acres including setbacks to accommodate the soil conditions and the requirement for area for three drainfields.

The system was designed so that each home would pump from an individual septic tank to a common 4-in. force main, which led to the LSAS site. The force main discharges to a 1,000-gal holding/dosing tank equipped with a float-activated duplex pump system. The pump system alternates between the constructed drainfields, dosing approximately 200 gal per cycle to 3,600 linear ft of trench. Lateral spacing is at 9 ft center-to-center. Each drainfield consists of six modules, with each module comprised of six laterals. A distributing valve cycles the dosing through each module to maintain even distribution throughout the system.

The cluster system trend

Cluster systems offer many benefits to the builder, homeowner, regulatory agency and most importantly, the environment. When given a true review at the project feasibility stage, these systems can prove to be the most cost-effective option for wastewater service. Due to the simplicity and robust treatment offered by a septic tank, they will continue to be a core unit in these systems. Cluster systems enjoy the economies of scale, and therefore reap the cost benefits. This cluster system trend will also be a catalyst for the wastewater industry to develop quality, low-cost collection systems and other technologies that can be applied in these community systems.

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