Alternative On-site Wastewater Treatment and Disposal Options
Author: The Water Quality Program Committee, Virginia Tech*
*Funding for this project was provided, in part, by Extension Service, USDA, under grant number 91-EWQI-1-9034, "Residential Watershed Management," and by the Virginia Department of Conservation and Recreation, under grant number 94-0612-10, "Residential Water Quality Management."
Publication Number 448-403, July 1996
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Introduction
The most common methods of on-site wastewater treatment currently being used in Virginia are the conventional septic system, low pressure distribution system, and the mound system. However, there are a number of other alternative systems available if none of these systems are appropriate for the site. The Virginia Department of Health is the agency that oversees the design and inspection of new on-site wastewater systems. The department maintains local offices in all jurisdictions within the state. This office as well as the local government in each jurisdiction should be contacted to obtain information on any regulatory requirements for a particular alternative system that may be of interest.
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Alternative Treatment Systems
Aerobic Treatment Units
Aerobic treatment units are designed to treat wastewater in an oxygen rich environment and, therefore, tend to produce a cleaner effluent than septic tanks alone. Typically, the first compartment is a settling chamber where solids and liquids separate and some anaerobic digestion occurs. This chamber performs the same function as a septic tank, and if the unit is preceded by a septic tank, this first compartment may not be necessary. From the settling chamber, wastewater then passes into the aeration chamber where a pump supplying a constant flow of air and a stirring mechanism are used to oxygenate the water, creating optimum conditions for aerobic organisms to decompose organic compounds. This allows more of the organic matter in the water to be digested, reducing the amount of pathogens and other pollutants. The third compartment is another settling chamber which allows for further removal of particulate matter before the effluent leaves the unit. All aeration treatment units are required to be equipped with an alarm system that detects both failure of the aeration pump and high water levels. The main disadvantages associated with aerobic treatment units are the need for an outside power source and the higher amount of maintenance required to ensure proper system operation.
Sand Filters
Sand filters are an alternative that is commonly used to provide additional treatment for effluent before it is discharged. These filters can be located either above or below ground, depending on the site conditions, and can be used in conjunction with a septic tank or an aerobic treatment unit. As with a normal system, wastewater first enters a septic tank for the primary treatment stage. From there it passes to the sand filter. The filter acts to reduce the amount of suspended solids and dissolved organic material present in the water. Microorganisms attached to the sand particles are able to aerobically digest the organic material within the wastewater. Underneath the sand bed is a layer of gravel that prevents the sand from being washed out of the system and also acts to further treat the effluent. At the bottom of the sand filter there is underdrain piping which either carries the effluent away from the filter, or in the case of a recirculating sand filter, rechannels a portion of the effluent back to the sand filter to be treated again. Owners need to periodically rake and replace clogged surface sand.
Constructed Wetlands
The purpose of constructed wetlands is to artificially recreate the filtering capacity of natural wetlands. A constructed wetland provides an additional treatment stage, necessary when a conventional system alone is not adequate to overcome the limitations of a site. These artificial wetlands typically consist of one or more trenches referred to as cells. Each cell has a lining which can either be impermeable or permeable, depending on the system, and contains vegetation anchored in a medium such as gravel. When planning to use such a system, proper care needs to be taken to ensure that it is designed to accommodate the specific characteristics of the site. Although they can be constructed to conform to many different situations, there are just two basic designs for constructed wetlands: Surface Flow systems, where the wastewater passes over the medium; and Subsurface Flow systems, where the wastewater passes directly through the medium.
The medium used in the cell is typically gravel, although sand or a gravel/sand mixture can be used as well if dictated by the design. Bulrushes, cattails, reeds, rushes, and sedges are types of vegetation used in constructed wetlands. Irises are very attractive wetland plants that perform well in constructed wetlands. The medium and vegetation present in the cells provide several functions for the system. The principal function of the plant roots is to transpire oxygen and thus aerate the water. Aerobic conditions allow for a larger variety of microorganisms to attach themselves to the surface of the roots and medium. These microbes are the primary source of treatment, feeding on the waste products in the water.The medium and plant roots help polish the water by trapping tiny particles. Another positive function of the plant roots is to take up some of the water, reducing the amount that will need to be discharged.
Wastewater from the house goes directly to a septic tank, where solids and liquids are allowed to separate. Bacteria that thrive under anaerobic conditions begin the process of breaking down the waste products within the effluent. The water then passes to the first cell of the system. Here the microorganisms attached to the substrate further break down the waste in the water. There are then two options for the disposal of the effluent. In the case of a very small system, the water can flow to an unlined cell where it evaporates, is taken up by the roots of the vegetation, or is allowed to percolate into the soil below. For larger volumes of water flow, the addition of more cells or a drainfield may be necessary.
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Alternative Disposal Methods
Leaching Field Chambers
Leaching field chambers are an alternative to the conventional network of distribution pipes used in a drainfield. Depending on the drainfield size requirements, one or more chambers can be used, each being connected to form a large underground cavity. The chambers are usually made of a sturdy plastic and do not require gravel fill around them. The sides and bottom of each chamber have a network of openings to allow for the seepage of wastewater into the soil. These systems allow more of the soil profile to be used, since the effluent is distributed not only to the ground below, but also to the soil surrounding the chamber. This allows leaching chambers to work more effectively than traditional drainfields, especially when the drainfield must be located on a steep slope. Leaching chambers do not require any additional maintenance than a conventional distribution system, and their sturdy construction makes them less vulnerable to damage from the weight of heavy objects placed on the surface of the ground above them.
Drip and Spray Irrigation
Irrigation systems are similar to conventional drainfields in that they use the natural assimilative capacity of the land to dispose of wastewater. The principal difference is that irrigation systems are designed to allow the water and nutrients to be used by plants. A vegetative ground cover extracts nutrients such as nitrogen from the effluent and also serves to reduce soil erosion and maintain soil permeability. The uptake of water through the roots of the plants reduces the amount of water that percolates through the soil, lessening the possibility of oversaturation problems. The roots also introduce oxygen, allowing aerobic microorganisms at the top of the soil profile to digest some of the organic matter in the effluent. An additional benefit from the use of irrigation systems is the possibility that non-food crops can be grown on the land.
Spray irrigation systems use sprinklers to distribute the effluent evenly over the surface of the ground. This allows for greater water loss through evapotranspiration, using the assimilative capacity of the entire soil profile. Since the effluent is distributed through the air, pathogens can be carried by the wind and cause a possible health hazard. Because of this the wastewater must first be treated by either an aerobic treatment unit or a septic tank-sand filter combination, and then it must be disinfected before it can be sprayed.
Drip irrigation systems utilize pressure compensated drip tubing to slowly and evenly dispense the wastewater just below the soil surface, but still within the root zone of the vegetation. After first being treated by a septic tank, the wastewater goes to a dosing chamber. The dosing chamber then periodically sends effluent through a series of disk filters before delivering it to the network of tubing. The system regularly back flushes effluent through the filters to prevent them from becoming clogged. The pressure compensated tubing is designed to distribute wastewater uniformly over the entire drainfield. Since the effluent is never exposed to the air, as it is with spray irrigation, aerobic treatment and disinfection are not always necessary with drip irrigation. This also enables this system to avoid the potential problem of odors. Both irrigation systems require an outside power source for the operation of the dosing chamber.
Direct Discharges from On-site System
It may be necessary to discharge the treated effluent from an on-site system directly into an all weather stream, intermittent stream, or dry ditch. If so, the system must receive a discharge permit from the Department of Environmental Quality. The permit sets the discharge limitations for various pollutants that are commonly found in wastewater. The Virginia Department of Health is then responsible for ensuring that the proposed on-site system is capable of meeting the discharge limits set by the permit.
Most permits will likely require additional treatment to remove more nitrogen, phosphorus, and organic matter from the effluent before it can be discharged. Virginia state regulations do require that all discharging on-site systems have a final treatment stage where the effluent goes through a disinfection process, which removes any possibly harmful pathogens before the effluent is discharged. Typically, this is accomplished by first chlorinating the effluent and then removing the chlorine before discharging. Ozone and ultraviolet light are two other common methods of disinfecting wastewater.
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