Small Diameter Collection
Construction and O&M Costs for Small Diameter Collection Systems
Septic Tank Effluent Pump and Gravity (STEP/STEG) systems are becoming more popular and accepted as low-cost, low-maintenance alternatives to traditional sewers. In addition, STEP/STEG systems can operate over variable grades, making them a logical choice for sites with undulating terrain or where deep excavation is undesirable. Because STEP/STEG systems have been in existence since the early 1970s, there is sufficient research available to estimate capital and annual costs associated with construction, operations and maintenance. This report summarizes the available research from published sources.
Overview of STEP Installations
In the last two decades, more than 30 STEP systems have been built in Michigan, mostly for lake-side communities with high groundwater tables. More than 100 systems have been constructed in California, Oregon, and Washington. The large majority of STEP systems have operated well, providing significant savings to their communities. Where STEP systems have encountered problems is in places where the operators have either a) not been adequately trained or provided with an Operations and Maintenance Manual, or b) ignored their training in order to make more money on service calls and overtime. The bulk of this report is based on studies of systems that utilize well-trained operators.
Screens and Pumps
The Glide, Oregon system (described below) introduced the idea of using an in-tank pump vault screen. The use of screens has been documented to reduce total suspended solids in the collection and subsequent treatment steps by as much as 50%. Screens also permit the use of turbine-effluent pumps, which usually allow a single model of ½ horsepower pump to be used throughout the entire system, greatly reducing maintenance and replacement costs. The lower flows made possible by the use of low HP turbine pumps also permit the use of smaller-diameter mains: another source of savings. Screens do clog, such as when an illicit drug-manufacturing lab is discharging to the tank, or when a tank should be pumped and has not been due to operator oversight. Clogging prevents material that would cause difficulties in the treatment system from reaching the collection lines. This enables the operator to solve the problem as inexpensively as possible by pumping out the tank and cleaning the screen, rather than, for example, replacing the front end of a wetland. Clogging also alerts the operator to unsafe practices on the part of individual homeowners, thus ensuring that responsibility for repeat problems rests not with the entire community, but with the source of the issue.
Overview of STEG Installations
Where septic tanks can be installed above the hydraulic grade line of the collection system, no pumps are necessary. Like STEP systems, Septic Tank Effluent Gravity installations have a screen, which filters solids and reduces treatment and maintenance costs. Like the pump-vault screens, STEG screens are removable for cleaning, but rarely need to be.
Glide, Oregon (700 homes) and Elkton, Oregon (135 homes)
A STEP system was finished in 1980 to serve the communities of Glide and Idleyld Park in Douglas County, Oregon. At the time it was the largest STEP system in North America. The community decided to go with a STEP system after the engineering firm CH2M determined that even with the maximum available grant monies, a conventional treatment system was not feasible. By 1992, more than 500 residential and commercial connections were on-line, totaling more than 700 Equivalent Dwelling Units (EDUs). The Glide system is maintained and operated by two full-time operators and a supervisor who splits his time between Glide and another regional wastewater treatment facility (WWTF). System components are quickly replaceable: the average service call lasts no more than 15 minutes.
Cost of Septage Pumping
The Douglas County engineering staff began monitoring sludge and scum accumulation rates within the tanks of Glide and Idleyld Park. After 8 years, data taken from 336 residential, single-compartment, 1000-gallon tanks showed rates of sludge and scum accumulation matching average rates published by the Douglas County Public Health Service. After 8 years, only 55% of the maximum sludge and 43% of the maximum scum volumes had been filled. In other words, the tanks were only half of the way to needing to be pumped out. This implies that the average pump out rate for a 1,000-gallon residential tank is once every 15 years. Pump-out costs were, at maximum, $150 dollars per tank in the Glide area. Other areas of the country (those that combine the strictest waste disposal guidelines with the greatest distance from the landfill) have pump-out costs as high as $250 per tank. Even at that exorbitant rate, the cost to the homeowner for pump-out will be, on average, $17/year, or $1,700 per year per 100 homes. It should be noted that the cost of pump-outs are the responsibility of the operating utility.
Cost of Repair/Replacement and Electricity
Turbine pumps need to be replaced or repaired every 20 to 30 years. They currently cost about $400 to $550 each. The cost of a pump is approximately $22 per year per home, or $2,200 per year per 100 homes. Pump replacement costs are also typically the responsibility of the homeowner. The following calculation shows the estimation of electricity costs: 150 gpd/home / 10 gpm = 15 minutes of pumping per day. 15 minutes * ½ HP / 50% efficiency (conservative) = 0.19 kWh/d. 0.19 kWh/d * $0.10/kWh * 365 days/year = $6.94 per year per home. Add extra for energy the pumps use when powering on, and $8 to $10 per year per home is a reasonable estimate.
Summary: Total Operations and Maintenance Costs
Tables 3 through 5, list the annual budget for the Glide, Oregon system. The total cost of labor is $94,000/year, a total of $135 per year per equivalent dwelling unit (EDU). Materials, including power for the treatment system (not shown in the following charts), cost $76 per EDU. Service calls cost less than $1,600 per year at 15 minutes per service call and $40/hour, a total of $3 per home per year. Thus, the total cost of operations and maintenance of the collection and treatment system is $214 per home per year ($18 per month). That’s $27 per month in 2006 dollars. (click to enlarge)
1) Bounds, Terry. “Glide Audit 1986-1987 Summary of Sludge and Scum Accumulation Rates”, Department of Public Works, Douglas County, Oregon; 1988. 2) Bounds, T.R. “Design and Performance of Septic Tanks”, published 1997 Conference of the American Society of Testing and Materials. 3) Bounds, Terry R., P.E. “STEP Systems: Glide’s Contribution to the State of the Art”, originally presented for Rural Community Assistance Corporation Conference, San Francisco, CA.