By Dennis F. Hallahan, PE


Good onsite wastewater treatment combines proper soils evaluation, siting, selection of the most effective product, and installation best practices regardless of the size of the system. Add to that proper operation and maintenance and decentralized wastewater treatment system will serve an individual dwelling or commercial development effectively for the long haul. Whether a conventional septic system or an advanced treatment and dispersal system, the soil is the workhorse. It is the soil – and not the drainfield media – that does the treatment. The media, no matter what kind, is there simply to help distribute the water to the soil and to provide storage for those times of higher usage.

It is very common for decentralized systems to utilize a subsurface discharge of treated effluent. This beneficial because it returns water close to the point of origin and recharges the aquifer. Large decentralized treatment systems for community and commercial use have become common as the cost to install centralized sewers has risen and new decentralized system options can offer treatment on par with centralized systems. Engineers, developers, and municipal regulators are embracing these systems to sustain responsible development and protect public health and the environment. In each case, these systems are designed, sited, and sized based on the soils in the treatment area. As construction progresses, it is vital to protect those soils from construction traffic and other activities to avoid compaction and retain the soil’s permeability.

Subsurface disposal systems for large decentralized systems can discharge and treat large facility flows. The soils infiltrate the treated water and must be protected during the construction process.

Minimizing Soil Compaction and Disruption

Unlike sewering, which takes water away from the site requiring much energy for pumping and treatment, onsite treatment relies on the soil’s surface area, oxygen-holding pore spaces, and abundant microbial life to process and treat the effluent and return it to the groundwater. The day-to-day onsite wastewater residential system installer realizes the importance of the soil and has the knowledge and best practices and equipment to ensure the soil is protected during the construction process. However, in the case of large decentralized systems, the project may go to bid and be awarded to a large utility contractor. The utility contractor has heavy construction equipment and understands how to move large volumes of soil fast and efficiently, however this may come at the expense of damaging the soil so important to the wastewater treatment process. One example is a large drainfield that is very wide and long, and two-feet deep. The utility contractor may propose to use scrapers to excavate the bed in minimal time. The wheeled scrapers will severely compact the soils and compromise the performance of the system.

If the plans and specifications do not clearly state the means and methods of construction or the intent to protect the soils, then the owner may be in for an expensive change order. The intent should be clear in the project documents to protect the soils thereby leaving open to the contractor to be creative with the means and methods.

Large disposal fields can be configured to conform to the slope at differing elevations and to accommodate ease of backfilling by minimizing the width of the bed.

How Soils Play the Leading Role

Effluent, which has received primary treatment from the septic tank(s) and possibly advanced treatment is discharged to the leachfield through a distribution piping network. The leachfield consists of individual trenches or a bed, depending on the site, the size of the system, and state or local code. The dispersal field may also be known as a drainfield or leachfield, and to reflect its true function could be referred to as the “Soil Treatment Unit.” Research has demonstrated that the soil treatment processes include physical (filtration), chemical, and biological processes. Disease causing pathogens and virus can be starved or poisoned by soil bacteria, fungi, and other microbes that produce antibiotics. There is also predation. Soil is a hostile environment for pathogens outside of the human host where significantly different conditions exist such as temperature, moisture, and acidity. Because of these processes the Soil Treatment Unit archives tertiary treatment and provides natural disinfection.

Whereas onsite systems of the past have mainly been concerned with hydraulics (e.g. make the water go away), today’s large decentralized systems are designed for hydraulics and treatment. Not only must the primary treated effluent be adequately dispersed and absorbed, but sufficient treatment must also be provided to protect public health and the environment.

Preparing and Excavating the Wastewater Treatment Site

The main goal is to protect to soil, therefore approach each phase of construction with that in mind.

  1. The soil plays an important role in disposing and treating effluent, the project plans and specifications should define methods to protect the soils during construction.

    Site clearing and grubbing: If there are trees to be cut and stumps to be removed then evaluate methods for the activity to occur without impacting the soil. For example, if installing a shallow dispersal system in sensitive soils (low permeability silt and clay soils), it may be best to specify tracked equipment only for stumping and dragging out logs. Or it may be practical to cut shrubs and trees flush and leave the stumps.

  2. Soil moisture content: It is not recommended to install a system in wet conditions or in overly moist soils, as this causes machinery to smear the soil interface, which can affect system performance. It is recommended that the contractor meet with the design engineer prior to the installation to review soil conditions and determine if the conditions are appropriate for installation. Plans should be detailed to excavate in phases that can be completed daily to minimize exposure to rain events.
  3. Construction staging in the drainfield area: The drainfield area should not be utilized as a staging area for construction equipment or materials before or after construction of the wastewater treatment system. Temporary fencing, warning tape, barriers and/or appropriately located signs are recommended to prevent unauthorized traffic from damaging the drainfield area.
  4. Construction traffic: Construction traffic should never be directed over the system area before or following installation. This includes dump trucks and wheeled vehicles. Prior to installation of the wastewater treatment system, the contractor should review the plans and devise an installation process to minimize construction traffic over the disposal areas.
  5. Erosion and Sedimentation Control: Silt and clay can be carried as sediment during storm events. The small particles can plug up soil infiltration systems. Protect the system excavation from stormwater runoff by constructing berms, swales, or other erosion control structures as necessary to divert surface water away from the excavation.
  6. The specified product for disposal can influence the performance of the system. Here the dust and fines of an aggregate system can be seen, which will limit the native soils’ permeability. And the placement will cause compaction and embedment.

    Drainfield product selection: It is important to note that regardless of the drainfield product specified, it does not infiltrate nor treat the effluent, the soil does both of those functions. Although, the choice of product can have a significant impact upon the soils. Aggregate has many disadvantages including soil compaction, the presence of fines, and embedment of the stone; all of which are detrimental to system performance. The specification of a gravelless product will avoid these pitfalls and allow for a much quicker installation timeframe. The faster the system can be installed, the less time the soils are exposed to the elements. The gravelless products are also absent of fines and are lightweight thus avoiding compaction and embedment.

  7. Utilizing tracked equipment: Only tracked equipment should be used to excavate the wastewater treatment system. For trench systems, it is recommended to excavate individual trenches with tracked equipment with a bucket width that matches the specified trench width. Bed system excavation may require additional planning; large bed widths should be excavated without machinery in the base of the excavation. The bed or trench should be level and the bottom and sidewalls should be scarified. Construction foot traffic within the disposal system should be minimized
Wheeled vehicles and construction laydown areas should not be allowed over the disposal fields.

Sites with Sensitive Soils and Sloping Terrain

Very sensitive soils and/or sloping terrain on a site require even further precaution and preparation. Each manufacturer typically provides installation instructions for specialized conditions. Project contractors and design engineers can contact the system manufacturer to review the best method of installation when in doubt.


Due to the natural treatment capacity of the soil, decentralized systems have been and will continue to be the mainstays of wastewater treatment. Available to the public and developers where there are no other options, and in some cases as an alternative to cost prohibitive sanitary sewers, decentralized systems effectively protect community health. Better science, better understanding of processes, higher standards for installers, updated codes, operation and maintenance programs, and more thorough site evaluations will help to continue to raise the bar in this ever-growing field. But whatever else may change, the soil will still be working on our behalf.

Dennis F. Hallahan, PE has more than 30 years of experience with onsite wastewater treatment systems’ design and construction.  Currently Technical Director at Infiltrator Water Technologies, he is responsible for technology transfer between Infiltrator and the regulatory and design communities and consults on product research and testing for universities and private consultants. Hallahan received his MS in civil engineering from the University of Connecticut and his BS in civil engineering from the University of Vermont. He is a registered professional engineer in Connecticut and holds several patents for on-site wastewater products. He can be reached at