The sources and uses of water will become of ever increasing concern as the fragile balance between human and environmental needs continue to be stressed. Most utilities and agencies charged with meeting community and commercial demands while protecting the environment are looking at more integrated approaches to conservation and reuse of water. However, in some cases, water needs are outside a utility service area. In those cases, care must be taken to carefully manage wastewater so as to conserve water, minimize environmental damage, and control costs. In these cases, one of the treatment alternatives is the Onsite Disposal System.
In their older form, the standard septic tank and drainfield have received justified criticism over the years. These treatment systems can be significant contributors of non-point source pollution that can endanger our fresh water resources. According to some sources, about 30 percent of households in the U.S. are using Onsite Disposal Systems. Conventional systems collect wastewater in a septic tank where it goes through primary settling and biological reaction during its retention time. The supernatant or effluent is discharged from the tank and disposed to a drainfield for percolation through an unsaturated soil zone which provides advanced aerobic treatment of the effluent. The challenge faced by this system is its reliance on the soil properties to function properly. Either nutrient or hydraulic overload of the drainfield is, in many cases, the cause of system failures which result in discharge that endangers the environment. Management of the effluent quantity and quality is very important when developing the parameters of each system while maintaining protection of our natural resources.
To address these issues much research, field experimentation, and innovative design is underway. The objective is to establish viable, cost effective alternatives to manage quantity and quality of septic tank effluent at the disposal point. While most of these alternatives are not cost effective for the individual residence, they are viable options for small community treatment systems or corporate/public centers.
Often, soils with restrictive percolation rates are encountered on a site. These soils require dosing at a lower rate over a larger area and can be unsuitable altogether for disposal. In 2006, Auburn University conducted a field evaluation in a high clay content soil of a subsurface drip irrigation system drainfield delivering septic tank effluent to an integrated seasonal crop. The seasonal crops were employed to provide additional moisture and nutrient uptake from the drainfield soil while moisture sensors located in the drainfield controlled the dosing rate. The purpose of this field evaluation was to test effects of select plant uptake integrated with controlled low dosing rates to overcome the shortcomings of a conventional system in high clay soils. The results of this evaluation was published in a technical paper titled Innovative Subsurface Drip Irrigation (SDI) Alternatives For On-Site Wastewater Disposal In The Alabama Black Belt which is available for reference on the internet.
The benefits from introducing plants either at the point of treatment or at the point of disposal can be significant in improving the final effluent discharge quality. One patented system employs algae biomass (one celled aquatic plants) to enhance bacterial reduction of organic matter in the wastewater. The algae flourish in the presence of the nutrients in the wastewater and carbon dioxide that is produced by the bacteria. The algae also consume additional nitrogen and phosphorus. As an added bonus, the algae takes in carbon dioxide and produces oxygen which is, in turn, needed by the bacteria to complete a symbiotic relationship between the two. This relationship ultimately lowers the emission of carbon dioxide which is a greenhouse gas. This patented system is known as Algaewheel Technology and has been licensed to OldCastle Precast. More information about this system can be obtained at their website www.oldcastleprecastonsite.com.
More recently, compact fill and drain wetland technology has been developed to produce high levels of treatment for wastewater generated in larger office and public/institutional buildings. Two such buildings were showcased in an article published in the June 2012 issue of CE News, titled Wastewater Wetlands On Display. Both of the buildings were designed to use innovative onsite wastewater treatment with effluent reuse for flushing of toilets as an aid in their seeking LEED certification. The treatment system they employed was The Living Machine by Worrell Water Technologies. The Living Machine is comprised of a series of watertight cells with under drains. The cells are filled with engineered media that support microbial film growth and planted with select wetland plant species that root in the media. After solids are removed in a septic tank, filtered wastewater is pumped into the wetland cells and allowed to migrate through the plant roots and microbial biomass which provide treatment. The wastewater is then recovered in the under drain network and cycled through additional cells for further treatment. Once the designed dosing cycles are completed, the final effluent is filtered and disinfected before storing for reuse or routed to final disposal. A more detailed description of the process and system components can be found at www.livingmachines.com for those interested.
The field evaluation and innovative technologies that are referred to above are a small sampling of the ongoing research and development that are taking place in the wastewater industry. Although there are specific characteristics and limitations with each system type, it is encouraging to have these additional alternatives available to improve the operational capabilities of onsite systems. As pressure increases to retain and reuse wastewater, these onsite systems can provide a necessary solution in more remote settings that don’t have access to central systems. When that is the case, our industry engineers and scientists are developing solutions to provide quality wastewater treatment and protect one of our most valuable natural resources – our fresh water supply. Chastain-Skillman, Inc. is continually monitoring new developments in our industry and is proud to be an active member of this engineering community.
By H. Robin Losh
Robin Losh is a Senior Project Manager in the Environmental Engineering Department of Chastain-Skillman’s Tallahassee office and has been with the firm for 23 years. He can be reached at  942-9883 or firstname.lastname@example.org.