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Aerobic Septic System Failure Rates & Costs InspectAPedia®  -     This article discusses Aerobic Septic System Failure Rates & Costs. This website provides designs and products for aerobic septic treatment units (ATUs) for onsite waste disposal, also called fine bubble aeration systems. We address aerobic septic system design, features, inspection, repair, and maintenance. Product sources are also listed.

Aerobic Treatment Unit Failure Rates and ATU Installation & Operating Costs

Failure Rates for Aerobic Septic Systems

Some readers have written expressing concern that they've heard that ATU's have a high failure rate, "as high as 95%". That number doesn't mean much to me without more specific data, in particular, one would need to know what was meant by "failure" and what was the cause of specific failures of ATUs. Important septic system failures are the failure to adequately treat wastewater or the failure to successfully dispose of it.

ATU Failures due to lack of maintenance: In the study discussed below, while a high failure rate was reported for ATUs (where failure here means the aerobic treatment system was not treating the effluent satisfactorily), the failures were specifically linked to a lack of proper maintenance. Every septic treatment system requires periodic maintenance and repair, and I expect that every septic system eventually fails and needs extensive repair or replacement. So we could put the septic system failure rate at 100%. What would be more useful would be a statement of the typical operating cost for various alternative septic treatment systems, the typical component life, and the amount and cost of ongoing maintenance that are required.

NESC, in an issue of their Pipelines magazine (Note 1), , cited a 1998 survey conducted in West Virginia which looked at Aerobic Treatment Units to evaluate the quality of their effluent discharge. 419 ATUs were tested for TSS (total suspended solids) and BOD5 (5-day biochemical oxygen demand), and 92 percent of the systems were determined to be producing unacceptable effluent discharge.

The main cause of these aerobic septic system treatment unit failures was attributed to poor or improper ATU maintenance. I suspect that homeowners, accustomed to the tradition of inattention to septic systems until they are visibly failed, simply were performing no maintenance at all. However with proper inspection and maintenance, ATU's can continue to provide effluent which has been treated better than by a conventional septic tank and drainfield.

For example, of the 419 units, only 85 were selected for chemical tests. Of the 419 units inspected, field inspectors found (by visual inspection) deficiencies in 272 units (65%). 71% of the 85 units selected for testing were found to have deficiencies.

What was particularly remarkable, while 93% of the ATU's with a visually detectable deficiency were producing unacceptable effluent output, more stunning, another 80% of the ATU's with no apparent deficiencies by visual inspection, were producing unacceptable effluent discharge! (Op. Cit., p. 43). Here is a description of the study: "Of the 419 ATUs examined, 85 were sampled for additional laboratory analyses, which included BOD, TSS, and fecal coliforms (FC).

Approximately 150 units were tested for chlorine residual and turbidity. The survey was performed by the Environmental Services and Training Division and the Environmental Microbiology Laboratory, both at West Virginia University (WVU), and six county health departments, and with assistance from the West Virginia Bureau of Public Health. The project objectives were to survey existing ATUs for proper operation and maintenance and to determine the chemical and microbiological quality of their effluents."

Installation & Operating Costs for Aerobic Treatment Units ATUs

The NESC cites several obvious components in ATU installation and operating costs. Installation costs for an ATU include the cost of the ATU installation, including excavation and electrical wiring, the cost of construction of the post processing facility such as raised bed or sand bed.

Operating costs for aerobic septic systems include electricity for the ATU compressor or air pump, annual inspection and maintenance, usually by contract with the installer, and cost of effluent disinfection if that step is used. One website, www.toolbase.org which has some nice simple summaries of types of septic systems, cites a cost of $3200. to $5000. to install an aerobic treatment unit.

This number cannot possibly be correct for a complete system installation. More likely it is referring to the aerobic treatment unit itself, the tank and aeration system. Add to these the cost of excavation and piping for the sewer line from the building, from the ATU to the absorption system, and the installation of the wastewater absorption system, and you're probably looking at two to three times that number.

It is a bit tricky trying to compare alternative septic system costs. After all, we choose a particular system because it best fits the limitations of the site. A very limited site that requires installation of an ATU and a raised bed or septic mound system to handle the effluent, such as in a rocky building site in the Northeastern U.S., may involve a total installed cost of $25,000. This troubling number cannot be compared with the probably lower cost of a conventional septic tank and drain field ($10,000) because the latter simply won't work on the site in question.

What we can say about ATUs is that if regular inspection and maintenance are not performed, the system has a good chance of not working properly.

The cost of electricity to operate the septic aeration system is likely to be trivial, perhaps $4./month. We can also say that the trash tank or primary treatment tank will require more frequent pumping than a conventional septic tank. I expect that pumping cost, often $125. to $250. per pumpout depending on location and tank size and effort to access the tank, probably is more significant than the operating cost to supply electrical power to the equipment.

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• Note 1: NESC Pipelines, Summer 2005, Vol. 16., No. 3, citing an excellent, juried article: "A Survey of Home Aerobic Treatment Systems Operating in Six West Virginia Counties", Sextone, Alan et. al., Small Flows Quarterly, National Environmental Service Center, Fall, 2000, www.nesc.wvu.edu/nsfc/sfqarchiveframe.html - unfortunately the NESC online copy is illegible. Fortunately NESC permits public download of the entire SFQ issue as a PDF, which will permit you to read this excellent article. A more careful reading than the citation above by the NESC is in order.

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