Effluent Sewer Collection
"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
How it works
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. Far fewer construction headachesInstallation 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. Because only liquid is being pumped, system designers do not need to worry about minimum velocity and associated grade.
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. Cost advantagesConventional 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. Environmental responsibilityFrom an environmental perspective, 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. Even the Sierra Club and the Surfrider Foundation endorsed an Orenco effluent sewer over the gravity sewer option proposed for a coastal California project, stating that a STEP effluent sewer would provide "significantly greater protections to the groundwater, sensitive ecosystems, and culturally significant sites in the area." Orenco engineers stand ready to help with reference materials for Environmental Impact Report (EIR) investigations. Covered by one or more Orenco patents. |
Resources
Video Introduction to Effluent Sewer (5 min) Environmental Science & Engineering article "We ran almost all our collection lines down alleys and across fields. When the state's Rural Development Director came to town for our dedication, he pulled me aside and asked, 'When are you going to get this project finished?' I said, 'It is finished.' He said, 'But when are you going to tear up the streets?' He couldn't believe we didn't have to!" Engineer Bill Walker on Effluent Sewer in New Minden, Illinois |
