By Michael Hines, M.S., P.E.
It is estimated that 40% of all new homes constructed will come equipped with a septic system to process human waste. Conventional septic systems consist of two major components: the septic tank that collects the sewage from the home and provides primary solids separation and digestion, and the disposal field that disperses the settled septic tank effluent into the soil column. While this article will focus on the septic tank, it should be stressed that for a septic system to function properly, the tank and disposal field must be designed as a unit.
As a wastewater system designer, you need to know how to properly treat human waste for one very important reason: Improperly treated human feces are the prime breeding ground of potentially deadly diseases.1 This means that septic tanks are critical components of the local environment – they can be buried but should never be forgotten.
Well-built and well-maintained septic tank systems are remarkable engineering designs. In fact, efficient, properly managed on-site wastewater treatment systems that include a septic tank system built to industry standards are a healthy, natural and environmentally responsible way for homeowners to process wastewater from toilets, showers, washing machines and sinks.
When septic systems fail, it isn’t pretty
When periodic checks of the water level in the septic tank reveal high levels during wet weather and/or low levels during dry weather, the tank is not watertight and could be failing. When a tank leaks, ground water passes through the tank during wet weather and untreated sewage leeches into the ground water during dry weather. Perhaps unnoticeable at first, soggy, unsanitary soil above the system’s failed drain field may be lying in wait for unsuspecting children or the family pet. No matter where the failure occurs, this situation poses potential health risks, professional liability and costly major repairs.2
Seven typical causes of septic system failure:
1. The septic tank is not designed or constructed according to standards to be watertight and structurally sound.
2. The septic tank is too small for the number of people and the wastewater flow served by the tank.
3. The septic tank is not properly maintained; tank solids have built up and discharged to the drain field, causing clogging failure of the field.
4. Excess flows are discharged into the tank, often from nondomestic sources.
5. Toxic chemicals, large volume of solids or other materials besides sewage are discharged into the septic tank.
6. The drain field is too small for the intended use, or drain field soils are inadequate to treat and move flows away from the site.
7. The drain field is poorly designed or installed and does not allow equal distribution of the applied flow across the field.
ASTM C1227 and local standards:
With the above factors in mind, how can you make sure that your tank design meets the highest industry standards? A designer needs to know one fact: Quality septic tanks that comply with ASTM C1227, “Standard Specification for Precast Concrete Septic Tanks,” are:
• structurally sound;
• absolutely watertight; and
• comply with the industry standard for design requirements, manufacturing practices and performance requirements.
An oversimplification? Actually, no. Just as you would expect a roof to leak if covered with poor-quality shingles installed by an unqualified roofer, a poorly made septic tank can fail because of a poor design, nonstandard manufacturing or shoddy installation. Septic tank failures can result in dangerous situations, from a cave-in to contact with untreated human waste.
Obviously, ASTM C1227 is not the only standard for septic tanks. In some states and localities, this standard may be adopted by reference into the state and local codes. When there is adequate enforcement and oversight, regulatory agencies act on the citizens’ behalf to make sure all producers of septic tanks meet the standard. Some state and local ordinances define their own standards and require septic tank producers to comply with regional specifications to be certified.
So the problem − and it is, unfortunately, a widespread problem − is that ASTM C1227 is not referenced consistently by all septic tank regulatory agencies, whether state, province or local. To make matters worse, even where ASTM C1227 or comparable regional standards exist, septic tank inspectors and regulators do not always enforce the standard. In fact, many regulatory jurisdictions pay lip service to standards that clearly describe how septic tanks are to be designed and constructed to be structurally sound.
Enforcement and Economics: the Dark Side
While some inspectors may specify (on paper) that tanks within their jurisdiction must be structurally sound and watertight, these regulators often do not enforce (in practice) these requirements. As a result, thin-walled tanks with little or no steel reinforcing may be installed.
Installers of poor-quality tanks know that they are relying on the subsoil for structural support. Without the soils that surround it and hold it together, a poor-quality tank will collapse when filled with water. On the other hand, a quality precast concrete septic tank built in compliance with industry standards is structurally sound before and after installation. So what incentive drives some producers to market tanks that are not watertight or structurally sound?
For too long, septic tanks have been treated as a commodity rather than as an engineered product that requires skill and care in manufacturing. As a commodity, septic tanks are typically purchased based on cost alone − often the cheapest price available. As a designer or specifier, you should advise the homeowner or your home-builder client to consider the relative economics of paying $30,000 to $50,000 for a top-of-the-line kitchen with all the latest appliances while settling for a cheap, poorly constructed septic tank that could endanger a family’s health and lead to expensive remediation down the road.
With a strong ASTM standard, why are there so many septic tank problems?
In the real world, poor-quality septic tanks are installed every day and these are the tanks that are likely to fail structurally and create health and environmental risks. Here is the inside track to help you understand the current dilemma:
• By their nature, septic tank regulatory programs are very politically sensitive. Tank manufacturers resist any strengthening of requirements/standards that will increase cost of production.
• Homeowners generally choose the lowest-cost system available.
• When a system fails (often the property owner has no lot space left for system repair), the regulator’s only options are to:
– enforce the regulations that may include possible eviction; or
– ignore the failure (regulators seldom return to malfunctioning system unless neighbors’ complaints force subsequent inspections).
• Many regulators are poorly trained in the science of wastewater treatment, septic tank design or tank maintenance requirements. Typically, regulators are handed a “cookbook” set of rules to apply to every situation regardless of differences in sites, soils, tanks and drain field installations.
• Many jurisdictions in the United States operate on standards and regulations that have been essentially unchanged for 20 to 40 years. Modern advancements in treatment or disposal technologies go unrecognized until mandated by changes in political perceptions.
• Not all state and local regulators have adopted industry standards like ASTM C1227, and not all septic tanks are manufactured by NPCA-certified precast plants.
• Not all septic tanks are made of steel-reinforced precast concrete.
How Septic Systems Should be Maintained (and what will happen if they are not)
Now that your client’s septic tank is installed and in use, what must your client do to protect the tank and care for it? If the client simply forgets the tank and pays no attention to it, one day the homeowner will be standing in a mucky, stinky yard while a guy with a backhoe digs large holes in the lawn. If this happens, the dollar signs will flash in front of your client’s eyes. Septic tanks require inspection and maintenance if they are to function properly over their expected service life of 50 years.4
Conclusion and Responsibilities
Many players are involved in providing a quality septic tank system:
• The engineer who designs the tank and its companion disposal field must be competent in concrete and structural design as well as the biology and hydrology processes involved in soil dispersal of septic tank effluent.
• The tank manufacturer must adhere to the engineered design, use quality materials and exercise quality control throughout the manufacturing process.
• The installer must be competent and install the tank and disposal field in such a manner to ensure the integrity of the tank and watertightness of the connected system.
• The regulatory inspector must enforce diligently to ensure that all tanks and disposal fields within his or her jurisdiction are constructed, installed and maintained in accordance with competent regulatory requirements.
• The homeowner must provide attentive (not passive) care and maintenance of the septic tank system for as long as he or she uses the residence.
Providing a quality septic tank system involves choices. Choosing quality septic tank components and materials from certified manufacturers ensures the value of your homeowners’ investments, the safety of their families and the protection of our environment.
Checklist for Quality
A quality septic tank should meet this eight-point checklist:
1. The tank manufacturer should be willing to provide certification by a professional engineer that the tank was designed in accordance with appropriate recognized standards and engineering principles.
2. The tank should be one piece or a monolithically poured body with a separate lid that is permanently affixed to the body with a watertight seal.
3. The tank should be made of high-strength concrete (minimum of 5,000 psi) and contain sufficient steel reinforcement to ensure the structural integrity of the tank at full design operating conditions.
4. To ensure watertightness, the inlets and outlets must be equipped with rubber boots integrated into the tank wall (inlet and outlet pipes pass through these boots, and the boots must be sealed against the pipes with stainless steel clamps).
5. The inlet inspection plug in the tank lid should be sealed to the tank with high-quality mastic or other sealant that will prevent water leaks into the tank.
6. The inlet inspection riser must be cast as part of the tank lid, extended to the ground surface and be equipped with a watertight cap.
7. If the tank is not equipped with an effluent pump, an effluent filter should be installed on the outlet.
8. A 24-in. diameter riser ring should be poured in the tank lid over the tank outlet to allow access to the pump or filter. A watertight riser must be permanently sealed to the riser ring, extended to grade and be equipped with a locked or bolted watertight lid.
Nine Steps to Properly Maintain a Septic Tank System
Quality precast concrete septic tanks that are structurally strong and watertight, and serviced for long-term, trouble-free operation must be maintained by the following:
1. Confirm that the sizing of the septic tank and drain field match the features of the home. If the permit calls for a three-bedroom system, don’t remodel and make an office or den into another bedroom. Septic tanks are sized to allow collection of all the wastewater and allow settling of the solids. Exceeding the designed limits reduces the ability of the septic tank and drain field to function.
2. Ensure that the amount of wastewater entering the system does not exceed the permit design for the operation of the dispersal field. In North Carolina, for example, a three-bedroom design means the dispersal field is sized for 360 gal./day). This is 120 gal. per bedroom assuming two people per bedroom (60 gal./person/day). Unfortunately, every locality uses different design rates. Some design rates provide very large safety factors; others are designed based on personal preference and may have little basis in science or engineering. Regardless of the validity of locally established design rates, they must be followed.
3. Understand the first rule of septic tanks: Other than laundry and bathing wastewater, nothing goes into the septic system that has not first gone through the human digestive system (or was intended to). Do not discharge paint, motor oil, cigarette butts, chewing gum, rug cleaners, bleach, drain cleaners, large volumes of cooking oils or grease, or anything else toxic to a septic system’s working bacteria. Cleaning and laundry using normal amounts of disinfectant, detergent, softener and bleach should not pose a problem, but any of these products used in excess can effectively “kill” a septic tank’s biology. A septic system relies on viable bacteria to breakdown human waste. Killing the bacteria means removing the ability of the septic system to function – never a pretty sight.
4. Ensure that commercial additives are NOT used in the septic tank. TV and radio airwaves are full of ads for miracle additives, enzymes and special bacteria to add to the septic tank to make it work better, smell like flowers and look good. These products are, at best, a waste of money and can be harmful to the biological processes in the tank or drain field. Historically, EPA and other professional organizations have determined that such additives have not been shown to serve any useful purpose. Business development seminars held for the septic tank installers and pumpers encourage the sale of such additives to homeowners as a high-profit revenue source for the industry. Ignore such sales pitches.
5. Routinely inspect toilets to ensure they are not leaking. Leaking toilets are the No. 1 cause of hydraulic failure of septic tank systems. A single toilet leaking just ¼ gal./min. adds 360 gal. in 24 hours, the full design flow for a three-bedroom home.
6. Ensure that the septic tank is routinely pumped. The usually recommended clean-out schedule is every three to five years (or longer for bigger tanks). Specify that the pumping will be done by a professional septage firm that services and reinstalls the effluent filter (if installed) and make sure all access devices are secured back into place.
7. Specify that no down spouts, AC condenser drains or other clear-water discharges ever be connected to the tank. Water softener regeneration backwash should be diverted around the tank to a separate seepage pit.
8. Limit garbage disposal use. Some regulatory jurisdictions try to prevent installation of garbage disposal units in homes with septic tanks. Most homeowners want an in-sink garbage disposal whether it comes with the house or is added later. The solution to this homeowner preference is to initially size the septic system to handle the extra solids produced by garbage disposals. All residential septic tanks should have minimum capacity of 1,000 gallons. Larger tanks are required if garbage disposal units are planned or as the number of occupants in the house exceeds eight.
9. Including the following items in the annual inspection checklist:
• Depth of sludge and scum layer
• Condition of scum layer
• Condition of inlet tee (should be clear)
• Proper operation of any installed pump
• Cleanliness of any effluent filter present
In addition, walk the drain field or drip dispersal area to check for any wet or soggy spots that would indicate pending or actual system failure.
Michael Hines, M.S., P.E., has 44 years experience in the field of wastewater engineering as a statewide regulator, a Tennessee Valley Authority environmental manager and a consulting engineer. He is founding principal of Southeast Environmental Engineering LLC and president of Utility Capacity Corporation Inc., a construction company.
Hines serves on the Board of Directors of the National Onsite Wastewater Recycling Association (NOWRA) and its National Model Performance Code Committee.