| Under ideal conditions, septic tank effluent is discharged into a leach field where it is allowed to infiltrate into the soil. There, natural physical, chemical, and biological mechanisms remove organic material, suspended solids, and microorganisms from the effluent as it percolates downward.
The successful placement of a leach field is dependent on several site-specific factors, including soil permeability, depth to groundwater, the presence of subsurface rocks or low-permeability soil layers, and site terrain. When site conditions disallow the use of a leach field, other treatment methods must be considered. Septic tanks are not highly effective at removing bacteria and viruses, and the effluent can be high in biological oxygen demand (BOD), total suspended solids (TSS), and nitrogen content. Therefore, some type of effluent treatment must be conducted before the effluent is released to the environment.
There are numerous effluent treatment options available. Many of these treatment methods, while highly effective, depend on mechanical systems that are expensive to install and maintain, difficult for the average homeowner to operate and maintain, and prone to malfunction. Fortunately, many onsite treatment locations are amenable to natural treatment processes using land application technologies.
Natural wastewater treatment methods attempt to mimic the natural degradation of pollutants by using systems that rely primarily on the biological processes of plants and bacteria. Manufactured mechanical systems are kept to a minimum and are employed primarily for delivery of the effluent to the treatment unit.
Most natural treatment systems fall into one of three general land application categories: spray irrigation, rapid infiltration, and overland flow. These systems require minimal effort for operation and maintenance. Direct benefits to the operator are simplicity, cost-effectiveness, efficiency, and reliability, while environmental benefits include minimal use of chemicals..
Spray Irrigation
Spray irrigation is a well-proven treatment technology that has seen over 100 years of use in the US. Used primarily for secondary or tertiary treatment, spray irrigation is a viable option where soil conditions are not conducive to the use of conventional drainfields. Sites with low soil permeability, shallow bedrock, near-surface groundwater aquifers, or other limiting characteristics may be candidates for the use of spray irrigation.
Spray irrigation systems are a relatively simple concept, using sprinklers to evenly distribute effluent over a large area of vegetation. However, despite the simple mechanical aspects of this treatment method, several natural mechanisms are responsible for effluent treatment.
Evapotranspiration—the combined effect of evaporation and transpiration from plants—plays a significantly larger role in spray irrigation than it does in other effluent disposal methods due to the aerial delivery of effluent. Nitrogen and other nutrients will be taken up by the plants and removed from the effluent. Also, oxygen from the plant roots provides an aerobic environment that is conducive to the growth of aerobic microorganisms, which remove organic constituents from the effluent as it passes through the upper portion of the soil profile. These mechanisms are in addition to those provided by a conventional drainfield, where filtering and anaerobic treatment occurs in deeper portions of the soil profile.
As stated earlier, spray irrigation is usually the final wastewater treatment step before effluent is released to the environment. At a minimum, pretreatment in a lagoon or settling pond is usually necessary to reduce suspended solids that can clog the spray heads. However, the presence of pathogens in raw sewage is a serious concern because the effluent is sprayed through the air and onto vegetative surfaces. Wind can blow pathogen-laden effluent off-site and contaminate surrounding areas, and pathogens can be transported off-site by runoff. Although these transport mechanisms must be addressed through good design practices, pathogen loading must also be reduced prior to spraying. Conventional onsite sewage treatment is a common treatment approach, using aerobic treatment or a septic tank followed by sand filter treatment. Some states also require disinfection prior to spraying, which can be achieved by chlorination or UV treatment.
The vegetated areas to be sprayed must be relatively flat to reduce runoff. To further reduce the potential risks posed by pathogens in the effluent, several states have established buffer-zone requirements around the treatment area. Buffer zones vary by state, but range from 25 feet for roadways to 100 feet for streams and lakes, wells and other water supplies, and residential dwellings. The buffer zone surrounding the treatment area, along with the relatively large area required for spraying, can place severe limitations on the use of spray irrigation unless a large land area is readily available.
In addition to accounting for possible pathogen threats, a number of other factors must be considered in the evaluation and design of a spray irrigation system. Site-specific factors to be evaluated will include the site topography, soil characteristics, and water-table elevations. The soil permeability, texture, and depth must allow for proper infiltration of effluent, although the soil property requirements are less stringent for spray irrigation than they are for conventional leach fields. Groundwater must be deep enough to allow for thorough effluent treatment in the soil profile before impacting potable water sources.
Climate is also an important design consideration, as spray irrigation is effective only during the growing season. Subfreezing temperatures will also cause aboveground piping and spray heads to freeze. Spray irrigation can be successfully employed in colder climates, but sufficient onsite storage must be available to hold effluent that is generated during the winter. Climate also impacts the effluent delivery rate, as evapotranspiration and plant uptake will be higher in warmer and dryer climates and seasons.
Water quality may also need to be evaluated to ensure that it is appropriate for irrigation, especially when the effluent originates from non-residential sources. Total salt concentrations, cations, and toxic constituents should be evaluated if the effluent comes from an industrial source, or if the original water source is non-potable. The level of nitrogen in the effluent is also important to avoid overfertilization of the vegetation.
The type of vegetation grown must take into consideration local conditions. Crops cannot be used for human consumption, and any vegetation should be based on the climate and soil type at the treatment area.
As mentioned above, spray irrigation has a long history of use as an effluent treatment technology, and performance has been well proven. BOD has been reported to be reduced by an average of 90% to 99%, while TSS are reduced by a similar percentage. Total nitrogen reductions range from 80% to 99%. A system designed under consideration of the limitations discussed above should be expected to provide similar improvements in effluent quality.
Operation and maintenance requirements are greater than for conventional drainfields, but still within the abilities of the average homeowner or site operator. Routine inspection and maintenance of the spray system is needed to ensure that pumps are operating properly, spray heads aren't clogged, and pipes are not leaking. Chlorination systems will need periodic replenishment, and vegetation must be periodically mowed or cut. Some jurisdictions require that a maintenance contract be in place to assure that all O&M activities are properly conducted.
In summary, spray irrigation offers an excellent alternative to conventional drainfields where site conditions do not allow for drainfield placement. However, lot size is a major constraint, as a large area of land is needed for the spray area and buffer zones. Climate is also a limiting factor, and large effluent holding ponds may be needed in colder climates.
Overland Flow
The overland flow process differs from other land treatment processes in that treatment of the effluent occurs on or above the land surface, rather than in the subsurface. In this process, effluent is spread evenly along the top of a vegetated slope. The slope is gentle enough so that sheet flow occurs over the vegetated land, where chemical, physical, and biological processes improve the quality of the effluent.
Sedimentation, filtration, and biochemical activity are the primary treatment processes at work in overland flow. Besides controlling erosion, the vegetation removes nitrogen and other nutrients from the effluent, and also filters out suspended solids. Microorganisms such as bacteria and algae, attach to the vegetation and break down dissolved organics. Additional biochemical activity occurs in the top layer of saturated soil. Overall, effluent treated by overland flow is of relatively high quality. Suitability for discharge to streams, however, will be dependent on local discharge requirements, as well as the characteristics of the receiving water body.
Evapotranspiration and percolation into the subsurface provide some reduction in the amount of water that is eventually discharged. Percolation, however, is necessarily limited by the low-permeability soils that are required at the treatment site. Overland flow depends on sheet flow across the land and vegetation surfaces, so this option can be implemented only where underlying soils will restrict infiltration. For this reason, overland flow can provide an alternative to other land treatment options where site conditions limit the use of drainage fields, spray irrigation, rapid infiltration, and other methods that require percolation into the subsurface.
As with other land treatment options, design of an overland flow system requires careful consideration of site-specific characteristics. As discussed earlier, low soil permeability is a requirement, since significant infiltration of effluent is undesirable. However, this also means that depth to groundwater is usually not a limiting factor and overland flow can be used in locations with shallow groundwater tables.
The growing season is another important consideration, but need not be a disqualifying factor. Overland flow systems can operate even in cold climates, although nitrogen removal is significantly reduced when vegetation growth is not active. Sufficient onsite storage volume will allow system operations to be suspended during extended periods of extreme cold.
Topography can be a severe limiting factor, as finished slopes are typically between 2% and 8% grade. On steeper slopes, the effluent flow rate increases to the point where treatment is not sufficient to provide adequate final water quality. It may be possible to construct terraces to provide a series of lower-sloped steps, but this also depends on site-specific characteristics.
Aside from the large land area needed for this option, another factor that often limits the use of overland flow for onsite wastewater treatment is the availability of a suitable receiving stream. Effluent at the bottom of the treatment slope is collected and channeled to the receiving stream, which must be near the collection area. Stream flow volume, existing water quality, other dischargers, and downstream users are some of the factors that determine the suitability of the stream for receiving treated effluent. A discharge permit may be required by local regulatory authorities, and this should be considered before selecting overland flow as a treatment solution.
Overland flow is not suitable for primary treatment of raw sewage, so a primary treatment system, such as a septic system, must be constructed. As with spray irrigation, effluent disinfection may be required to reduce potential threats posed by pathogens in the effluent. This risk is smaller in overland flow, however, because the effluent is not sprayed into the air and is, therefore, more controlled and able to be contained onsite. If disinfection is not required prior to the overland flow system, it may still be required prior to discharge to the receiving stream.
The volume of wastewater to be produced is another consideration in the design of an overland flow system. Onsite storage must be adequate to allow for periodic system shutdown during cold weather, and during routine maintenance that includes mowing the vegetation or harvesting of non-food crops. Storage is also required because effluent is not continuously applied to the treatment area. The timing of application cycles and the volume of effluent applied during these cycles are selected so that active microorganism growth is maintained, while minimizing severe anaerobic conditions and preventing surface ponding.
Rapid Infiltration
In regard to site conditions, rapid infiltration is less restrictive than other types of land treatment. A basic rapid infiltration system is little more than a shallow basin constructed in soils of relatively high permeability. The basin is flooded with effluent, as produced by a conventional septic system, and allowed to percolate through the soil at a high rate. In the upper several feet of the soil profile, biological and chemical processes act to break down organic constituents and other pollutants. High rates of removal BOD, suspended solids, and fecal coliform have all been reported using this system.
Water is delivered to the rapid infiltration basin using a cycling pump. In a typical system, the basin is flooded and then allowed to dry out before being flooded again. This aids in the breakdown of organic matter that remains on the basin surface after the effluent has percolated into the subsurface, and can also produce a reduction in nitrogen levels. The basin does not need to be vegetated, but doing so may provide further nitrogen reductions.
Rapid infiltration basins offer the advantage of being less dependent on climate than other natural treatments systems. Vegetation is not a required design element, so the length of the growing season does not impact effectiveness unless a high degree of nitrogen removal is necessary. However, the life spans of rapid infiltration systems have been reported to be potentially reduced due to saturation of the soil with phosphorus and heavy metals.
For onsite wastewater treatment, sand mounds are more common than basins when rapid infiltration is implemented. Sand mounds are used when only the upper portion of the soil profile is conducive to rapid infiltration, and may be appropriate where soil permeability is too slow or too fast, or where shallow bedrock or a shallow water table exists.
A sand mound creates a system where effluent is treated above the natural ground surface. The mound is constructed of fill material with suitable permeability. Effluent is discharged to the top of the sand mound, often through a series of pipes similar to a conventional below-ground leach field. Effluent is treated by natural mechanical, biological, and chemical processes as it percolates through the sand mound and upper portion of the soil profile. These treatment processes are the same as would occur in other land application systems where treatment occurs within the soil profile.
The effluent distribution system in a sand-mound system is usually placed within highly permeable fill, and capped with vegetated topsoil to reduce infiltration from precipitation. Because the effluent is delivered below the mound surface, effluent disinfection is not required, as it is in some other land treatment systems. However, improper design or poor construction of the sand mound could lead to leaking from the slopes, which may result in exposure to pathogens from partially treated effluent.
The most important factor to be considered in the design of a sand mound is the volume of effluent that is anticipated, which directly affects the size of the sand mound footprint. Treatment occurs below ground, so climate is not a primary consideration. The mound can be constructed from off-site fill material, so a wide range of material specifications can be accommodated.
An advantage of the sand mound for onsite treatment is the ability to design the shape of the mound to blend into the site more naturally than other land treatment systems. The slopes of the mound can be vegetated with shallow-rooted grasses and perennials, so that the mound can be constructed and vegetated to serve as a windbreak or a privacy barrier.
Maintenance requirements pertain mostly to inspecting and servicing the dosing chamber and pumps that deliver the effluent from the septic system to the sand mound. If the system is operating properly, routine maintenance of the mound itself should require only periodic maintenance of the vegetation on the cap and slopes. Trees and other deep-rooted plants must not be allowed to establish themselves, and foot traffic should be minimized to prevent compaction of the fill material, especially when the mound is wet.
Summary
The selection of a natural wastewater treatment system requires the consideration of a number of factors, including wastewater volume and pollutant characteristics, site soils and geology, and climate. Land application systems also require a large land area and are not appropriate for small residential lots. Not all sites will be candidates for land application, but for those sites that do qualify, natural treatment will offer the owner and operator many benefits over systems that employ mechanical and chemical treatment.
THOMAS M. ROTH, P.E., is a senior project manager for
CH2M HILL in Atlanta.
OW
- September/October 2005
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