Engineering Design of a Modern Soil Treatment Unit.

Onsite and decentralized systems will continue to play a critical role in providing cost-effective wastewater infrastructure while enabling water reuse and resource recovery in the U.S. and abroad. For these systems to realize their full potential, the design process needs to rational and made more uniform across practitioners and regulatory jurisdictions. In contrast to an empirical design process based largely on local experiences embodied in guidance prescribed in regulatory codes that have persisted for decades, rational design guidance should be built on clear and compelling science and engineering underpinnings.  Today, the vast majority of onsite and decentralized systems in the U.S. include a unit operation involving effluent infiltration into soil to achieve tertiary treatment with natural disinfection. Similar to a confined treatment unit (e.g., septic tank, packed bed filter, constructed wetland), effluent infiltration into an unconfined soil profile can be conceptualized as a wastewater treatment unit operation – a soil treatment unit - that is designed to: 1) hydraulically process and purify an effluent within the soil profile to the extent needed to protect public health and water quality; 2) provide a long service life with low operation and maintenance requirements; 3) enable resource recovery and reuse; and 4) have an affordable cost.  This presentation provides a summary of the comprehensive research carried out concerning effluent infiltration design and performance that provides an underpinning for science-based design. It also describes a rational design process to implement a soil treatment unit as a unit operation within an onsite wastewater treatment and water reclamation system. The presentation will also highlight how such a process has been adopted into regulations for onsite system design.