Let's agree on definitions!

The use of the term "septic-system malfunction rate," or sometimes "failure rate," has been inconsistent in the industry and has caused confusion. The following discussion proposes definitions for the terms "septic-system malfunction rate" and "percentage of septic systems in malfunctioning status."

Two definitions are proposed:

* Annual malfunction rate is the number of new malfunctioning septic systems divided by the total number of septic systems in the study area during a 12-month period.

* The percentage of septic systems in malfunctioning status is the number of septic systems that are in malfunction condition at any time divided by the number of systems in the study area.

Onsite systems are susceptible to malfunctions of two main types-functional malfunctions and performance-related malfunctions. A functional malfunction prevents the onsite system from operating as planned or designed; the term covers malfunctions that impair the proper functioning of the system. Symptoms that might indicate such malfunctions include

* excessive septage pumping;

* surface breakout;

* plumbing backup--possibly due to plumbing problems, but sometimes to the onsite-system drainfield system; and

* direct discharge.

Functional malfunctions have public health implications. The first three symptoms generally occur when the drainfield has insufficient capacity to treat and transmit wastewater, usually as a result of biomat issues.

Performance malfunctions exist when the septic system inadequately treats wastewater as compared to performance standards. These malfunctions have implications for water quality and possibly public health. Symptoms of such malfunctions include

* excessive nutrient (nitrogen and phosphorous) release to groundwater and surface waters; and

* inadequate bacterial treatment, which may result in bacterial degradation of surface-water and groundwater quality.

There is a significant difference between annual malfunction rate and the percentage of septic systems in malfunction status. Annual malfunction rate, again, is the number of newly failing systems in a 12-month period divided by the total number of systems in the study area. The percentage of systems in malfunction status includes failed systems that have not been repaired as well as newly malfunctioning systems. Systems in malfunction status may not have been repaired for a number of reasons: No onsite solution may be possible (i.e., waiting for an offsite sewer solution), economic hardship may be involved, a solution may be in the process of being developed and implemented, and so forth.

Annual septic-system malfunction rates are a critical component of cost-effectiveness analysis because the annual malfunction rate is also a way of stating economic useful life. Table 1 presents the economic useful lives of septic systems with associated malfunction rates.

Septic-system annual malfunction rates of 1 to 2 percent are not uncommon in communities with good onsite-wastewater-management programs. So what are reasonable and cost-effective annual malfunction rates? In sewerage systems, treatment plants generally have an economic life of 20 years and sewers of 60 years. Since sewers can represent 60 percent of the cost of the sewerage system, a weighted average for the useful economic life of sewerage systems is approximately 44 years. When the capital cost for onsite systems is similar to that of sewers, the onsite systems need to have an economic life of at least 40 years to be cost-effective. When on-site systems are less expensive than sewers (which is likely in many areas where they are technically feasible), they can have an economic life of less than 40 years and be cost-effective. Thus, communities with annual failure rates in the range of I to 3 percent per year have economically attractive and sustainable wastewater management systems.

The percentage of septic systems in malfunction status is an indication of how severe the onsite wastewater situation is in a community. If, at a given time, this value exceeds a certain level, say 15 to 20 percent, the community is likely at serious risk from the standpoints of public health, water quality, and economics. Our professional judgment, based on work in numerous communities, is that the percentage of septic systems in malfunction status should not generally exceed 5 to 10 percent, plus or minus. High rates indicate that either the annual malfunction rate is high and its causes should be addressed, or that malfunctioning systems are not being repaired, with the associated public health and environmental consequences.

Water Quality Considerations

We need to recognize that the annual malfunction rate and, especially, the percentage of systems in malfunction status may significantly increase as water quality studies determine that higher levels of treatment are necessary for installed onsite systems or that sewers are needed. Total-maximum-daily-load (TMDL) studies are now used by state and federal regulations to determine watershed wastewater treatment requirements and achieve water quality standards. Many areas of the United States are recognizing that advanced-treatment onsite systems with nutrient removal are now necessary to protect water quality. A brief description of performance malfunctions follows.

Bacteria Malfunctions

Insufficient bacterial purification of onsite septic systems is usually associated with insufficient depth of unsaturated adequate soils. Improved groundwater information may reveal that some septic systems are in bacterial-performance malfunction status.

Nitrogen

Many coastal areas are considering and proposing sewering to address the nitrogen contributions from conventional septic systems; advanced onsite systems can, however, perform as well as centralized treatment. In areas where the receiving water body is a drinking-water aquifer, onsite-system effluent requirements of <10 mg/L of total nitrogen (TN) are now becoming commonplace. In coastal areas, water quality conditions may require standards at the accepted limits of current technology for nitrogen removal, TN < 3 mg/L. Although conventional onsite systems may be in malfunction status in terms of meeting effluent requirements, advanced onsite systems can achieve those standards, often through retrofit, in a more cost-effective manner than sewering.

Phosphorus 

Although many onsite systems have relied upon the ability of soils to remove phosphorus, recent research has shown that soils have limited and possibly reversible sorption capacity and that some soils have little to no phosphorous removal capability. Also, a number of bio-geochemical reactions control phosphorous removal in drainfield soils. Using advanced treatment systems that remove biochemical oxygen demand, total suspended solids, and nitrogen before effluent reaches the drainfield may prevent the phosphorus-removing bio-geochemical reactions from occurring and thereby possibly create a performance failure in phosphorus-sensitive areas. Knowledge of the soil mechanisms in phosphorous removal is imperative to ensure that phosphorous performance failures do not occur with newly installed systems.

Emerging Contaminants

Consideration of emerging containments, such as endocrine disrupters and personal-care products, may increase treatment performance requirements in the future and thereby create a new malfunction definition. In the interim, however, communities whose drinking-water aquifers receive effluent from onsite systems may wish to utilize indirect-water-reuse guidelines to determine if malfunction is occurring. These guidelines specify minimum treatment requirements depending upon the percentage of the water supply that is wastewater derived.

Conclusion

The definitions this paper lays out for annual malfunction rate and percentage of septic systems in malfunctioning status can be good matrices for communities attempting to benchmark their onsite-wastewater-management programs. Also, because the use of these definitions allows problems to be clearly identified by type and location, it makes possible a rational analysis of the most appropriate solutions for a community.

Corresponding Author: Pio Lombardo, President, Lombardo Associates, Inc., 49 Edge Hill Road, Newton, MA 02467. E-mail: [e-mail].

Pio Lombardo, P.E., D.E.E.

TABLE 1 Failure Rates Versus Economic Useful Lives of Septic Systems

Annual Failure Rate  Economic Useful Life (Years)*

1.0%                 100
1.5%                  67
2.0%                  50
2.5%                  40
3.0%                  33
4.0%                  25
5.0%                  20

*Inverse of malfunction rate.