Conventional Onsite Wastewater Treatment System

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R. B. Reneau Jr., Professor, Waste Management and Water Quality
Charles Hagedorn, Biotechnology Specialist

A conventional onsite wastewater treatment and disposal system (onsite system) consists of three major components: a septic tank, a distribution box, and a subsurface soil absorption field (consisting of individual trenches). This system relies on gravity to: 1) carry household waste to the septic tank, 2) move effluent from the septic tank to the distribution box, and 3) distribute effluent from the distribution box throughout the subsurface soil absorption field. All of these components are essential for a conventional onsite system to function in an acceptable manner. In the remainder of this article we will describe the function of each of these components and some problems that may occur if not installed and maintained properly. The information in this article is compatible with the Virginia Department of Health Sewage Handling and Disposal Regulations. These regulations can be viewed at Link [1]. An overview of a conventional onsite system is shown at Link [2].

We have linked to the web sites of several companies in the onsite industry. We included these links with the idea that the information available at each link gives the reader the opportunity to better understand the components and design of a conventional onsite system. Reference to these links does not constitute our endorsement of these products and in many instances there are other similar products on the market.

The Septic Tank
The septic tank is a buried, watertight receptacle designed and constructed to receive wastewater from a home, to separate solids from the liquid, to provide limited digestion of organic matter, to store solids, and to allow the clarified liquid to discharge for further treatment and disposal. Solids and partially decomposed sludge settle to the bottom of the tank and accumulate. A scum of lightweight material including fats and greases rises to the top. The partially clarified liquid, present between the sludge and scum layers flows through the outlet Tee and into a distribution box or equivalent. Anaerobic digestion in the septic tank provides some pretreatment of waste before application to the soil. This pretreatment results in a reduction in sludge volume (40%), BOD (60%), suspended solids (70%), and conversion of much of the organic N to NH4+ -N. Biological contaminants are present in relatively high numbers in septic tank effluent. A typical septic tank showing the separation of sludge, scum, and liquid layers can be viewed at Link [3].

In Virginia the minimum hydraulic detention time in the tank is 48 hours, based on daily design flow, and the tank must have a minimum capacity of 750 gallons. The inlet and outlet Tees in the tank are designed to function as a baffle. The inlet Tee extends six to eight inches below and eight to 10 inches above the normal liquid level. The outlet Tee extends below the normal liquid surface to a distance 35 to 40% of the liquid depth and eight to 10 inches above the normal liquid level. Filters can be placed in the septic tank to reduce the quantity of solids leaving the tank and subsequently being deposited into the subsurface soil absorption field. Most onsite systems that employ filters will use a filter that can be placed in the outlet Tee. However, larger filters are available that replace the outlet Tee and are occasionally used for individual residences. Link [4] gives an example of a filter that fits directly into the outlet Tee.

Reports on the effectiveness of these filters indicate that total suspended solids (TSS) in septic tank effluent, at two sites, were reduced from 190.5 and 131.6 to 68.0 and 56.6 mg per liter, respectively, with an average reduction of 61%. These data can be reviewed in more detail at Link [5]. For data most applicable to individual residences refer to information supplied for the model 1800 filter. Based on existing information placing filters in the septic tank is an option that should be for both new installation and existing systems.

Maintenance of the septic tank is essential for obtaining best performance. It is currently recommended that septage (a combination of sludge, effluent, and scum) be removed from the tank at least once ever five years. However, since families use varying amounts of water and employ appliances that may result in faster accumulation of sludge (e.g., garbage disposal), we recommend the installation of an observation port that allows the accumulation of solids in the tank to be measured. Septage should be removed when the total depth of sludge exceeds one-third of the liquid depth in the tank. The consequence of allowing the sludge layer to accumulate beyond the recommended depth is the carry over of solids from the septic tank into the subsurface soil absorption field with a subsequent reduction in the life of the system. In many instances where the solids are allowed to carry over into the absorption field, the field undergoes hydraulically failure and requires installation of a new field. In this case a small investment in preventative maintenance could have resulted in a large savings in construction and associated costs.

Placing a filter in the septic tank can play an integral role in maintenance of the onsite system. The need for an observation port to measure solids depth in the septic tank is eliminated since the access port for the filter can serve this purpose. The filter can also act as a trigger to initiate routine maintenance of the system. How is this possible? With increased use, a larger percentage of the filter will clog with solids. After a large percentage of the filter clogs, the plumbing fixtures in the house are slower to drain. This is a signal to the homeowner to have the tank serviced (this includes cleaning the filter and measuring the depth of solids in the tank). If the filter is not cleaned, eventually the toilets, when flushed, will overflow into the house. This is a highly undesirable situation for any homeowner. Filters are now available with a float switch that activates an alarm, continuously, when the filter is approximately 94% clogged. Thus warning the homeowner that it is time to have the tank serviced. A link is not currently available for this product, but, in the future, you will be able to obtain information at Link [5].

The Distribution Box
The distribution box is designed to distribute effluent (from the septic tank) equally among multiple effluent ports. All effluent ports are at the same height and diameter and are located at a lower elevation than the influent port. Each of these ports distributes effluent to an individual subsurface soil absorption trench. Under ideal condition, the distribution box should ensure that effluent from the septic tank is uniformly distributed to all trenches. However, after installation, the distribution box often settles and results in uneven flow of effluent to the trenches. In many instances failing onsite systems can be attributed to the uneven flow of effluent to the subsurface absorption trenches. In these cases effluent may be ponded on the surface as a result of overloading of one or two trenches while the other trenches may be receiving little or no effluent. The settling of the distribution box can be reduced by placing the distribution box on an undisturbed block of soil that is covered with a bed of sand or a concrete slab. Link [6] shows a distribution box in relation to the septic tank and the subsurface soil absorption field.

Even if the settling of the distribution box is not a problem, flow to the individual subsurface soil absorption trenches may not be uniform. Thus overloading part of the subsurface soil absorption field may still be a problem. Cement dams are normally constructed in the effluent ports to try and distribute effluent uniformly to all trenches. Plastic inserts have been developed that fit directly into the effluent ports and allow flow to each trench to be adjusted rapidly and probably more accurately than can achieved with cement dams. Also, these inserts allow the flow to be adjusted at a later date, when the distribution box may have undergone settling, as part of ongoing maintenance program. Link [7] shows an example of a plastic insert.

A flow divider constructed from PVC can be used in place of the traditional distribution box. Link [8] shows an example of a commercially available flow divider.

The Subsurface Soil Absorption Field
The subsurface soil absorption field consists of a series of trenches that allows wastewater to be discharged below the ground surface where it is absorbed and is treated as it percolates through the soil before eventually reaching ground and surface waters. Trenches are placed on contour, can have a maximum length of 100 feet, can vary in width from 18 to 36 inches, and are separated by a center to center distance of at least three times the width of the trench (for slopes up to 10%). Trenches consist of a layer of clean gravel or crushed stone (stone) with a perforated, percolation line placed in the stone layer. Stone is placed into the trench until six inches is below and two inches is above the percolation pipe. A layer of untreated building paper or geotextile is placed on top of the stone to prevent migration of soil into the stone while the trench is being backfilled. There may be other aggregates that can be substituted for stone, but it will be necessary to check with your local Health Department for information. Link [9] shows a sectional view of a conventional subsurface soil absorption trench.

Chambers can be used in place of stone in the absorption field trenches. Link [10] shows the installation of a plastic chamber in a trench. One advantage of the plastic chambers is their lightweight, which allows them to be easily carried to and placed in the trench. This reduces the traffic around the subsurface absorption field, thus reducing the potential for compaction, and also reduces in the number of trees that need to be removed from the field.

Link [11] shows a cross sectional view of both the conventional stone filled trench and a trench where plastic chambers have been used in place of gravel.

Wastewater introduced into the trench infilters into the soil from the bottom and sidewall of the trench as shown in Link [12]. However, uneven distribution of effluent along the trenches, in conventional onsite systems, results in high loading rates to a relatively small portion of the trench. These high loading rates result in saturated flow which increases the distance biological and chemical agents are transported and subsequently the potential for ground and surface water contamination. Saturated soil conditions encourages flow through macropores and result in 'shortcircuiting' of the soil purification process. This is of particular concern in soils overlying creviced bedrock or high water tables.

The saturated soil also produces anaerobic conditions that accelerates the formation of a biological clogging mat that decreases effluent infiltration into the soil. The clogging mat that develops due to poor effluent distribution normally begins where effluent enters the trench and progresses along the trench with time. This phenomenon is well documented and is normally referred to as 'creeping failure'. A relatively simple way of reducing, but not eliminating, a problem associated with effluent distribution is to divide the absorption field into multiple fields. Effluent application is then altered between fields with a diversion valve. Alternating operation of the fields permits part of the system to "rest" periodically so that the infiltrative surface can be rejuvenated naturally through biodegradation of the clogging mat. A common practice is to switch fields on a semiannual or annual basis. Link [13] show an alternating trench system with a diversion valve.

Link [14] shows a product that can be purchased commercially to adjust flow between alternating subsurface soil absorption fields.

The most reliable way to reduce high, localized effluent loading rates is to uniformly apply effluent over the entire soil area available for treatment to more fully utilize the soil's renovation capacity. This also reduces the rate of clogging and the potential for biological and chemical contamination of ground and surface waters. Additional treatment (treatment prior to soil application) of effluent beyond that provided by the septic tank also reduces the potential for contamination of waters as well as the potential for hydraulic failure because of excessive clogging. In future articles we will examine alternative systems that rely either on better effluent distribution or production of a higher quality wastewater or both to solve some of the problems associated with conventional onsite systems. We will also discuss site and soil problems associated with onsite systems and how alternative systems can overcome these constraints.

Trade names are used in this publication for information purposes only. Virginia Cooperative Extension, Virginia Polytechnic Institute and State University, and Virginia State University do not warrant those mentioned nor do they intend to imply discrimination against those not mentioned.