“Given the diversity of the new technology that is now being developed…the continued use of conventional gravity flow systems will be a thing of the past.”
~Dr. George Tchobanoglous, UC Davis, Author of Wastewater Engineering: Treatment, Disposal, Reuse and Small and Decentralized Wastewater Management Systems
With an Orenco effluent sewer, raw sewage flows from the house or business to a watertight underground tank. Only the filtered liquid portion is discharged (by either pump or gravity) to shallow, small-diameter collection lines that follow the contour of the land. Solids remain in the underground tank, for passive, natural treatment, and need be pumped only every 10 to 12 years.
Effluent sewers are also known as STEP (Septic Tank Effluent Pumping) or STEG (Septic Tank Effluent Gravity) systems.
Installation time is reduced by one-half or more, compared to conventional sewers. Inexpensive, small diameter collection lines are shallowly buried, just below the frost line, reducing material and excavation costs.
This ease of installation causes less disruption to communities, allowing businesses to operate normally during construction. Installation ease also makes effluent sewer systems well-suited for community “self-help” programs, as in Starbuck, Washington (65 KB, PDF).
Conventional gravity sewer is an up-front capital expense, requiring total installation just to get the project started. However, this is not the case with decentralized sewer. The on-lot equipment — the largest portion of the total cost — is only installed after each home is built allowing the expense to be included in the price of each home.
In the case of existing homes, the on-lost cost is only incurred when a home is added to the system. Therefore, the majority of the cost of decentralized sewer is a deferred capital expense that is spread out over the lifetime build-out of the project, as opposed to the large, up-front expense required by gravity sewer.
Downstream treatment costs are significantly reduced because only low-strength effluent is collected as solids stay behind to decompose in watertight tanks. A pressurized, closed system means expensive manholes and lift stations are eliminated. And because effluent sewers are designed as watertight, there’s virtually no infiltration and inflow, making oversizing of the system unnecessary, and lowering the capacity requirements of the treatment plant.
It’s also critically important to look beyond upfront costs to evaluate long-term life cycle costs when choosing a wastewater collection method. Costs for repair and replacement, operation and maintenance, and debt financing vary greatly among effluent sewer, gravity, vacuum, and grinder collection. We can help you sort through the considerations.
From an environmental perspective (57 KB, PDF), effluent sewers are hard to beat. Passive primary treatment, energy-efficient fractional-horsepower effluent pumps, and watertight construction are features that help minimize environmental impact. Designers can appreciate not compromising between technical design and environmental stewardship.
Orenco engineers stand ready to help with reference materials for Environmental Impact Report (EIR) investigations.