Optimizing the performance of onsite wastewater systems
The septic system, once thought of as a temporary solution for the treatment of domestic wastewater, is still the best choice for individual residences and small communities where it would be cost-prohibitive to access public sewer systems. In the U.S., these onsite systems collect, treat and release about 4 billion gal of wastewater per day from an estimated 26 million homes.
Concerns about the impact of these systems on groundwater and surface water quality have increased interest in optimizing system performance. It is now accepted that these onsite systems are not just temporary installations that eventually will be replaced by centralized sewers but are a permanent part of the wastewater infrastructure.
Septic System Design
Septic systems are typically simple in design, which makes them generally less expensive to install and maintain. By using natural processes to treat the wastewater on site, usually in a homeowner’s backyard, septic systems do not require the installation of miles of sewer lines, so they are less disruptive to the environment. In addition, there are many innovative designs for septic systems that allow them to be placed in areas with shallow soils or other site related conditions previously considered to be unsuitable for on-lot treatment and dispersal.
Although the septic tank settles out most of the heavier solids and breaks down almost half of the suspended solids from household wastewater, the effluent still has a high amount of biodegradable organic materials along with a high bacterial content that may include pathogens. Therefore, septic tank effluent is not suitable for direct discharge into surface waters or onto land surfaces. Further treatment is needed to remove these harmful pathogens. The most common way to do this and dispose of the partially treated wastewater is through subsurface soil absorption through the drainfield.
When properly designed, installed and maintained, septic systems have a minimum life expectancy of 20 to 30 years. Neglecting maintenance of system components only leads to failure.
Causes for system failure are many and varied—ranging from improper siting, design or construction—to the simple overuse of water-generating appliances. When a septic system fails, it can pollute nearby water resources and endanger public health. Children are most susceptible to these health problems because they often come into contact with the contaminated areas.
The soil absorption system, or drainfield, is an arrangement of perforated pipes or chambers buried underground that channel the pretreated wastewater—the liquid discharge (effluent) from the septic tank—out over a large area of the soil. The effluent then moves slowly down through the soil o become naturally purified before returning to the aquifer. The drainfield acts as a natural filter for effluent by absorbing the organic materials, reducing or removing bacteria and viruses, and removing some nutrients.
The most obvious sign of drainfield failure is surfacing effluent. If the soils can no longer accept the effluent being delivered, the effluent will either rise to the ground surface or “blow out” at the end of the last trench.
The reason the soil can no longer accept the pretreated effluent is most often the biomat. As the effluent enters the drainfield, bacteria in the soil begin to thrive on the new food source. As these bacteria grow, they form a thick slimy colony called a biomat, which restricts the flow of effluent to the surrounding soil.
Causes of Drainfield Failure
Drainfield failure can be caused by many things, including excessive rainfall, tree roots interfering with the drainlines, the disposal of decay-resistant materials or vehicles driving over the system and cracking pipes. The two most common causes are hydraulic and biological overloading.
Hydraulic overloading occurs when too much water is sent to an under-designed system. The initial design of a system is based on soil and site characteristics, including depth to groundwater or bedrock. Part of the design includes the system’s capacity, which takes into account the number of people living in the home. Capacity is usually based on the number of bedrooms in the home, but this may not be an accurate way to determine flow generation. Extra people or the addition of a hot tub, for instance, can create more wastewater than the system and drainfield can handle. It is important to avoid putting too much water into the system at one time.
Biological overloading is the result of too much organic matter in the effluent. The addition of appliances, such as garbage disposals and dishwashers, can greatly change the quality of the wastewater sent to the system. These appliances send increased amounts of solids to the system, possibly causing biological overloading. Many local and state regulatory authorities require onsite systems to be sized larger to handle the additional load from appliances.
When an onsite system fails, it is important to gather specific information about the system in order to diagnose the problem and determine the appropriate corrective action. The following steps are helpful when gathering specific data about the system:
- Visual observation of the failure should be made to confirm the problem. All system components should be inspected, and any mechanical components, such as float switches and flow diverters, should be tested.
- A complete history of operation and maintenance of the system should be reviewed. Frequently, a study of the past three to five years of operation and maintenance will reveal a possible problem. The correction may be as simple as pumping the tank or cleaning a tank filter. The age of the system should also be determined.
- Obtain a copy of the original permit and any updates. This permit will contain a layout of the system from a site survey or drawings of the original design.
- Determine approximate loading rates from the original design and permit.
- Review soil test results. If they are not included in the permit, soil samples should be taken to determine the soil profile and to locate any soil boundaries that may be present.
- Obtain a complete report of the symptoms of failure. For example, surfacing effluent above the drain field suggests that the soil may be overloaded, either with too much total water or water with inappropriate amounts of organic matter that has clogged the soil pores. Additionally, if the failure is seasonal, wet weather conditions are likely to be the cause.
- Determine the amount of waste-water entering the system. Using data from the dwelling’s water meters, actual flow (even if estimated) should be compared to the design loadings. This will yield a good approximation of how much wastewater is entering the wastewater system. Leaking plumbing fixtures will skew this number, causing more water to enter the system. Thus, all leaking fixtures must be repaired.
Some additional steps might be necessary to test ideas before any corrective actions are taken. Wastewater metering or testing, equipment testing and monitoring, or additional soil testing might help more clearly define the cause of the system failure.
Repair permits may be required before any corrective action begins. Contact the local health department or permitting agency to find out what is required to obtain such a permit.
There are various repair or remediation techniques that may be considered, depending on the investigation into the causes of failure as described above. It is also important to take into account economic considerations and the flexibility of the local permitting entities. State and local statutes vary as to what technologies are permitted.
If the neighborhood is scheduled to receive public sewerage within a short time, it might be practical to use a short-term technique such as water conservation. Conservation and other management techniques, however, are only part of most solutions. Drainfield failure must be considered a serious health hazard and should be taken care of with long-term goals in mind.
Sometimes the overloaded drainfield can recover if a strict policy of water conservation is observed by the homeowner. If the soil around the piping is allowed to dry out, it may be able to function properly. This method obviously requires a good deal of homeowner commitment. It usually takes a 30% reduction in water use to allow the drainfield to recover.
In cases of physical damage, system restoration may only require leveling of the distribution box or repairing crushed or broken pipe. If tree roots are interfering with the operation of the soil absorption field, they can be removed. Broken or deteriorated baffles in the septic tank can allow solids to go to the drainfield; these should be replaced or repaired.
There are some new technologies that may provide temporary relief to drainfield failure. The first is “jetting,” a procedure that utilizes special pumps to inject high-pressure water into the drainlines to break up silt deposits and other solids, coupled with powerful vacuum lines that suck the broken-up solids out of the lines before they can settle again.
If the problem stems from poor or compacted soil, hope may come in the form of another new technology known as soil fracturing. Highly specialized equipment uses a pneumatic hammer to drive narrow probes down into the soil of the drainfield, typically to a depth of between 3 and 6 ft.
Air is then forced into the soil at a controlled rate, which fractures the hard soil and creates tiny open channels through it. Next, polystyrene pellets are injected into the newly aerated soil, which keeps the passages open so that the soil will not simply compact again. This technology has produced mixed results and is only approved by certain states. It is important to check with local health officials to find out if this technology or a similar process is approved for the situation. Some of these more extreme procedures may provide some temporary relief for a failing system that is soon to be replaced or connected to a municipal system. In many states, the process falls between the regulatory cracks, whether or not it is a repair and requires a repair permit.
In some cases, corrective measures are not enough; a new soil absorption system must be constructed. New soil absorption systems can be placed either in an isolated area so that the old system is not disturbed, or between the existing trenches if there is adequate room. These additional lines are considered part of an alternating drainfield system.
A diversion valve is installed so that in the future, it will be possible to direct the flow from the septic tank to either of the soil absorption systems. After the new drainfield is in place, the flow is diverted from the old field, which will slowly rejuvenate itself and be available for use in the future.
The rejuvenation process takes about two years. During this time, naturally occurring organisms decompose the clogging mat that has formed and return the absorptive system to near original capacity. (The old drainfield can recover faster if a septic tank pumper can open the field and remove as much of the ponded wastewater as possible.)
After a replacement system has been installed, a homeowner should switch back to the old drainfield after two years, and then switch back and forth between the two systems annually. This will result in a continuous use and rejuvenation cycle for both drainfields and should prevent future failures. An observation tube in each drainfield may be used to monitor the condition of the drainfields and can help the homeowner determine the frequency of alternating between the two fields.
If an adequate area for a new system does not exist and the old system is a trench system with at least 6 ft of undisturbed soil between the trenches, it is possible to install new replacement trenches interlaced between the old ones. The plumbing for the new and old systems, however, must be entirely separate so that when one is in operation, the other has the opportunity to completely dry out.
Another option to reduce the organic load on the drainfield is the addition of an advanced treatment system such as an aerobic treatment unit or a sand filter. Sand filters and aerobic treatment units (ATUs) are systems that use natural processes to treat wastewater and are frequently used to renovate biologically clogged, failing septic tank soil absorption units.
Typically, sand filters are used as the second step in wastewater treatment after the septic tank where solids in raw wastewater have been separated out. Constructed of a bed of sand about 2- or 3-ft deep and often contained in a liner, sand filters receive the partially treated effluent in intermittent doses. The effluent slowly trickles through the media, is collected in an underdrain and flows to further treatment and/or disposal.
Sand filters are very effective at removing high levels of suspended solids and are capable of handling heavy hydraulic loads. These two qualities make them particularly useful in cases of drainfields that have been overloaded either hydraulically or biologically.
Aerobic treatment units are similar to septic tanks in that they use natural processes to treat wastewater, but unlike septic treatment, the ATU process requires oxygen. ATUs use a mechanism to inject and circulate air inside the treatment tank. Bacteria that thrive in oxygen-rich environments work to break down and digest the wastewater inside the aerobic treatment unit.
Aerobically treated effluent is defined as effluent exiting a properly operating ATU or sand filter. This additional step reduces the amount of total suspended solids to less than 10 to 15 mg/L, compared to typical septic tank effluent with suspended solids in the range of 100 to 250 mg/L.
In situations where the soil absorption units have failed due to excessive biomat formation, aerobic effluent reduces the symptoms.