Onsite Wastewater Treatment Systems:  A Technical Review

On November 1, 2007, Paladino and Company met with representatives from Seattle Public Utilities (SPU), Department of Planning and Development (DPD), and other City of Seattle departments to discuss research into the application of onsite wastewater treatment systems. The issues discussed at this meeting served as a basis for this document. The objective was to provide a technical overview and address the specific concerns of both the utility and the project owner when considering the installation of an onsite wastewater treatment system. As such, this document is intended to serve as an internal resource for the Seattle Public Utilities (SPU) to use as a foundation for future research and collaboration with customers interested in installing onsite wastewater systems in the Seattle region.

The relevance of this research is particularly evident due to recent storm events. Over five inches of rain fell on western Washington on December 3, 2007, during which “more than 5.8 billion gallons of record rainfall, the equivalent of six Green Lakes full of water, [caused] neighborhood flooding and landslides throughout the city of Seattle.” This storm overwhelmed local sewage treatment plants, resulting in raw sewage discharge to the Puget Sound. Thus, as centralized facilities are increasingly taxed by storm events, investigation of new strategies to supplement or reduce our municipal treatment demands is critical to preserving our local natural resources.

Overview

The following report presents an overview of the benefits, challenges, and opportunities of onsite wastewater treatment systems as a sustainable design strategy.

It is important to note that this document is not intended as a comprehensive overview of all onsite treatment technologies. The information provided is focused specifically on systems that have the potential to be installed within urban areas. Installations intended for large scale agricultural applications or remote residential applications are not included in this study. Within this context, the key points of this research include:

• Onsite wastewater treatment may reduce demands, mitigate peak conditions and save cost.
• Wastewater reuse has potential for non-potable applications in numerous building sectors.
• Onsite wastewater treatment systems provide preliminary, primary, secondary and tertiary treatment. Most technologies differ in their means of secondary treatment.
• Onsite wastewater treatment systems are designed to be reliable and self-sufficient. Labor is the largest expense, but is often integrated into standard facilities operation procedures.
• Health, safety, and liability issues can be resolved through early collaboration and design.
• Onsite treatment scales more directly with demand, which significantly reduces investment risk.
• Utilities and owners must balance the shifting of capitol costs, expenses, and revenue to validate onsite treatment.
• Project funding is available at the local, state and federal level for projects that align to water resource initiatives.

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