Introduction 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 opera- tion and maintenance and decentralized wastewater treatment system will serve an individual dwelling or commercial development effec- tively 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 dis- charge of treated effluent. This beneficial because it returns water close to the point of origin and recharges the aquifer. Large decentralized treat- ment 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 activi- ties to avoid compaction and retain the soil’s permeability. 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 pro- cess. 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 Protecting the Soil – Best Construction Practices for Large Decentralized Wastewater Treatment Systems By Dennis F. Hallahan, PE
The soil with its available texture, surface area, permeability, and porosity for air movement make it an ideal medium for wastewater treatment, if the soil is not protected during construction then many of the benefits will be lost.
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.
scrapers to excavate the bed in minimal time. The wheeled scrapers will severely compact the soils and compromise the performance of the system. 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.
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csengineermag.com
november 2020
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