Gravity Sewer Excavations Disrupt Communities
Effluent Sewer Shallow Trenching Saves Headaches
See Video of Effluent Sewer's Low Impact Construction (5 min)
"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
Analyzing the True Costs of Community Wastewater Systems
Sewering small communities has always been a problem, and for decades, engineers have had difficulty identifying affordable wastewater solutions that work for them. Even during the construction grant era following the passage of the Clean Water Act in 1972 — when grants dwarfed loans — state and federal agencies were unable to identify and implement cost-effective solutions for small communities.
Pressure sewers (effluent and grinder) were designed to overcome these obstacles, and the EPA has been advocating the use of pressure sewers in small communities for decades, most notably in its 1997 “Response to Congress on Use of Decentralized Wastewater Treatment Systems.” Effluent sewers are generally more cost-effective than gravity sewers in … (1) sparsely populated or urban areas, (2) hilly or flat terrain, (3) poor soil conditions or areas with rock, (4) areas with high groundwater, (5) wherever lift stations or creek/river crossings are required, and (6) wherever O&M capabilities are minimal.
Terry Bounds, P.E. one of the founders of Orenco Systems, supervised the installation of an effluent sewer for Glide, Oregon, in 1980. At the time, it was one of the largest effluent sewers ever constructed. Orenco has been tracking real-world construction and O&M costs for effluent sewers ever since. In 2006, the company created an “Asset Management” Division to formalize these activities and support the customers of the company’s hundreds of effluent sewer collection systems and thousands of treatment systems.
Sewer Construction Costs
Recently, Orenco’s Asset Management Division has collected and analyzed constructed costs from more than forty publicly funded and bid collection systems (effluent sewer, gravity, and grinder) serving small communities. In addition, the Division has researched and analyzed O&M costs for both effluent and grinder sewers.
The table below presents this constructed costs data and its conclusions: on average, the constructed costs of effluent sewers are 41% less than gravity sewers and about the same as constructed costs for grinder sewers.
WERF’s cost estimating tools for decentralized systems (Wastewater Planning Model) estimate an even greater savings in capital costs for effluent sewers compared to gravity sewers, and grinder sewers.
Sewer O&M Costs
While capital costs are important, most sewage collection systems and virtually all wastewater treatment plants will expend more fiscal resources for operation, maintenance, and repair over the lifetime of a given facility than will be invested in the initial capital costs. The tables below present Orenco’s data on O&M costs for two pressure sewer technologies: effluent sewers and grinder sewers. These data sets show that the operational costs of Orenco effluent sewers are about 60% less than they are for grinder sewers … very similar to the conclusions in WERF’s Wastewater Planning Model.
Gravity sewer O&M costs aren’t negligible, and they are often underfunded. Gravity sewer O&M budgets must fund the costs of materials and supplies, machinery and equipment, and personnel. An effective gravity sewer maintenance program includes timely sewer cleaning and inspection, manhole inspections and rehabilitation, and maintenance/repairs of pipelines and lift stations. In addition, the impact of I & I on conventional gravity collection systems often creates the need for major treatment plant expansions.
It is commonly believed that the present worth costs of effluent sewers are greater than those of gravity sewers, despite the major savings in capital costs with effluent sewers. This belief stems from a lack of information about effluent sewer O&M costs, as well as a tendency to underestimate gravity sewer O&M costs, such as those cited above.
Historical capital and O&M costs for the various collection system options provide some insight into the present worth costs of the various sewering options. For example, based on the differential in constructed costs shown above, the average difference in costs between an effluent sewer ($9,702/connection) and a gravity sewer ($16,394/connection) is $6,692/connection. If the average difference in costs between the two technologies were financed over 30 years at 3% interest, the monthly debt retirement cost per connection would be $28.44 — well above the estimated $7.05 monthly on-lot O&M costs for effluent sewers.
For decades, engineers have had limited access to detailed costs — constructed costs and O&M costs — for all sewer technologies. As we know, costs for decentralized sewers are still not included in most college curricula or other educational venues. So conservatively estimating costs is a prudent and common practice in the absence of reliable and proven cost and performance data.
However, engineers, regulators, owners, and funding agencies now have access to decades worth of capital and O&M cost data, from Orenco as well as from third-party organizations such as WERF. Dozens of actual bid tabulations are available for reference, as well as actual O&M costs from systems that were constructed in the 1980’s. Orenco also has dozens of case studies.
The community of Bethel Heights, Arkansas, offers a good example of low life-cycle costs with Orenco's effluent sewers followed by treatment. Bethel Heights' system was installed in 2003 and serves nearly 500 homes and businesses. Residential rates are $35/month and the community just paid off its sewer bond, 20 years ahead of schedule.
One of the reason Bethel Heights has such low life-cycle costs is because its new effluent sewer also saves on the cost of sewage treatment. Because effluent sewers incorporate primary treatment into the collection system, they typically provide a deduct at the treatment facility due to the elimination of primary clarifiers and influent screens. Effluent sewers also reduce long-term costs associated with electrical usage, which constitutes between 25 to 40 percent of a typical wastewater treatment plant's operating budget. As a result of the treatment system capital deducts and the O&M deducts associated with effluent sewers, engineers and communities should evaluate various collection and treatment systems together, not separately or independently.
Small communities have struggled for years to economically construct and operate wastewater systems. Affordable, well-documented solutions are available. It’s time to reconsider our options and reference actual, real-world costs and performance data from alternative systems.