Innovative on-site sewage system development and management: a pilot program in Craven County, North Carolina.
The Coastal Plain physiographic province of the southeastern United States contains extensive areas of flat landscapes with wet soils that have significant limitations for the use of on-site wastewater treatment and disposal systems. Land development in these rural areas, in many instances, is not feasible without extensive use of alternative and innovative on-site technologies. One important limitation of widespread adoption of these technologies is a lack of information from field scale systems concerning appropriate sitting constraints, design characteristics, construction and inspection standards, and long term monitoring and maintenance requirements. Craven County, North Carolina has undertaken a unique program to develop innovative and alternative on-site sewage treatment systems and a management plan for those systems that will protect public health and the environment while allowing the use of traditionally-considered problem soil and site conditions.Craven County is located in the central coastal area of North Carolina, with a population of 83,000 people. Approximately 50% of the county's population is served by individual on-site sewage treatment systems. Conventional septic tank systems do not function properly in many of the soil conditions located in Craven County. Problems occur under three general scenarios: Residences with failing on-site systems, homes lacking indoor or complete plumbing systems and properties that do not meet current North Carolina sewage rules (1) and are classified unsuitable. If located in poor soil conditions, these systems can cause public health and environmental problems through contamination of ground water and exposure of sewage to the soil surface or open water where disease agents can come in contact with humans and animals (2, 3, 4).Due to the fact that Craven County is one of the fastest growing counties in North Carolina and much of the soil suited for the installation of conventional septic tank systems is already utilized for development or is in agricultural production, problems have been experienced in acquiring the necessary permits to install septic tank systems in many areas of the county.The Craven County commissioners, in conjunction with the Craven County Health Department, the North Carolina Department of Environmental, Health and Natural Resources, North Carolina State University, the North Carolina Cooperative Extension Service, concerned citizens and private consultants began the first steps in mid-year 1988 to formulate a program that would allow the use of innovative on-site wastewater treatment systems and a county controlled operation and maintenance plan to ensure the proper functioning of those systems. Before a county-wide program could be initiated those involved with this effort recognized that a pilot program would be necessary to develop the innovative on-site systems and a workable system of operation and management.In February 1989, the Craven County Innovative and Alternative Demonstration Program (IADP) began with the first meeting of the program's 12-member Technical Advisory Committee (TAC). The committee is made up of local citizens, local and state wastewater officials and university representatives who are technical leaders in environmental assessment and on-site wastewater management. Program direction and technical guidance to the county and local staff are the primary duties of the TAC. One of the first duties of the TAC was the formulation of the program objectives, which are as follows:* Develop site criteria and maintenance requirements for a wide range of alternatives to be utilized in the program.* Conduct appropriate scientific evaluations to determine system performance.* Develop local technical and management skills necessary to institute a county-wide operation and management program.* Utilize data collected to suggest appropriate changes to applicable rules.Craven County intends to accomplish these objectives by selecting 18 sites through criteria established by the TAC on which innovative systems will be installed and monitored for two years from the date of installation to determine performance. Following the two-year monitoring and evaluation period, the TAC and responsible state agencies will decide, upon review of the data collected, the types of systems to be further utilized in the county-wide operation and management phase of the program.Soil resource groups and system designOne of the major concepts of this program is that soils with similar properties can be grouped together for system design purposes into what are called Soil Resource Groups (5). All of the soils in Craven County are divided into six resource groups. Although this concept does allow similar design criteria to be utilized for the soils within any given resource groups, individual site characteristics ultimately determine the type of treatment system to be designed on a site.Soil Resource Group A, which encompasses 9.3% of Craven County, has only slight to moderate limitations for the installation of sewage treatment systems. These soils are well drained to excessively well drained on nearly level to undulating ridges along stream terraces and lower coastal plain surfaces. The seasonal water table is generally greater than 48 inches below the ground surface. This resource group is generally suitable for the installation of conventional sewage treatment systems. Two systems are to be evaluated in this resource group.Soil Resource Group B makes up 12.8% of Craven County. These soils generally are unsuitable for conventional sewage treatment systems, which means alternative or modified systems must be utilized. The shallow depth to a seasonal water table which is generally close to 24 inches, and in some instances soil permeability, are limitations. The sewage treatment systems of choice in this resource group are shallow low pressure pipe, modified conventional and shallow alternating dual field systems. Four systems are installed and are being evaluated in this resource group.Soil Resource Group C makes up 14.3% of Craven County. The soils within this group have seasonal high water tables that range from 12 to 24 inches, which requires alternative and modified sewage treatment systems to be utilized. Site modification by the utilization of sub-surface drainage is necessary on many of these sites. Fill and/or drainage with pressure distribution; pretreatment, disinfection and subsurface application; and pretreatment disinfection and surface application will be utilized. Five systems are to be installed and monitored in this resource group.Soil Resource Group D makes up 25.7% of Craven County. These soils generally have seasonal water tables less than 12 inches below the ground surface, which like the previous two resource groups requires the utilization of the site modification and alternative sewage treatment system design. Drainage modifications are necessary on most sites within this resource group. The sewage treatment systems utilized are fill, drainage and pressure distribution; pretreatment, disinfection and subsurface application; pretreatment, disinfection and surface application. Three systems are to be installed and monitored in this resource group.Soil Resource Group E makes up 15% of Craven County. With the seasonal water table located near the ground surface and very slow sub-soil permeability, sites located within this group have severe limitations for treating and disposing of sewage. Pretreatment, disinfection and surface application is the only type of treatment which can reasonably be expected to function on these soils. Sites utilized for surface application in this resource group will be graded or filled sufficiently so as to create a slight turtle-back slope to divert any rainwater. Application rates in the range of .025 to .035 gallons per square foot per day (g/|ft.sup.2~/day) will be used for calculating required spray area. Four systems are to be installed and monitored in this resource group.The last resource group, F, makes up 21.5% of the land area in Craven County. These soils generally are organic and are found in depressional areas of the county. The organic content and severe wetness of these soils prevent their utilization for sewage treatment and disposal during this program.Sand filters, both dosed and recirculation, National Sanitation Foundation (NSF) class I aerobic treatment units and artificial wetlands are pretreatment systems to be utilized in the program. Spray irrigation is the surface application method of choice due to its ability to apply effluent at the ground surface where the soil conditions are generally more suited to the treatment of effluent. The North Carolina Division of Environmental Management requires an area of approximately five acres, depending in site conditions and configurations, to install a single family spray irrigation treatment system, due to buffer requirements (6). Most sites proposed to be utilized for residences are, in many cases, less than five acres. Many of the spray irrigation systems to be monitored during this program are proposed on sites less than five acres, which dictates reduced buffers from adjacent property lines and residences. To compensate for buffer reduction, effluent quality will, in many instances, be improved before being sprayed by combining pretreatment systems or by using recirculating sand filters. We hope that through the development and monitoring of highly efficient and dependable pretreatment and disinfection systems, that effluent may in the future be applied on the lawn or in flower and plant beds around the home.Many soils in Craven County have seasonal high water tables located close to the ground surface. In order to provide proper effluent treatment on these sites the seasonal water tables have to be lowered. On many sites this is impossible by conventional means because no outlet for drainage exists or the outlet is not deep enough to provide the required water table draw-down distance. Sites which participate in this program that require drainage will utilize pumped drainage outlets when natural outlets are unavailable.TABULAR DATA OMITTEDIn addition to the types of treatment systems described above, low pressure pipe (LPP), drip irrigation (DI) and pressure manifold (PM) ground absorption systems are being utilized in various configurations and designs. Several of these systems have been designed with pretreatment and disinfection prior to final subsurface application.Disinfection of the effluent from systems which require pretreatment is being accomplished by using small ozone generators, ultraviolet light and chlorination. The performance of all methods utilized will be evaluated. Small ozone generators may be a promising alternative to chlorination. Ozone can reduce organic and inorganic contaminant levels in effluent after pretreatment. Currently, Craven County is working with manufacturers of small ozone generators in an attempt to produce a unit for small on-site systems which can be installed with the necessary alarm systems to alert the user of unit failure.Due to the fact that the design flow of many of the systems being utilized in the program is greater than the actual flow, valves are being added to pretreatment units and nitrification fields to allow system capacity to be shut down to allow actual soil application rates to approach design rates.System monitoring, maintenance and evaluationSewage treatment systems installed under this program are monitored for groundwater contamination and performance for two years from the date of first operation. Groundwater samples are collected monthly from five shallow wells located around the nitrification or spray field area. Samples are analyzed for chloride, pH, total organic carbon, nitrate, phosphorous, fecal coliform, ammonia, and total dissolved solids. Volatile organic compounds are analyzed bi-annually from each groundwater well on several sites. In addition, all pretreatment components are sampled monthly and analyzed for |BOD.sub.5~, TSS, nitrate, ammonia, chloride, phosphorous, pH and fecal coliform to determine performance.Once every week the operation of each system is also evaluated. All tanks, pretreatment systems, disinfection equipment, pump and alarm operation, groundwater level on the site and nitrification or spray field condition, among other areas, are evaluated. Water flow into the home is recorded and compared with effluent flow in the system, which is determined by pump run times. Should any minor problems arise with any system components or landscaping, local staff will make corrections during the system evaluation. Major problems, such as pump failure, will be corrected by a qualified company or individual through contracted services. All routine maintenance of these systems is performed by local staff.As mentioned earlier, following the two-year monitoring and evaluation period, the TAC and responsible state agencies will decide, upon review of the data collected, the types of systems and components to be further utilized in the county-wide operation and management phase of the program.The sitesCurrently, all 18 sites to be utilized in the program have been selected. Twelve of the 18 sites have been installed, six of which are under operation and being monitored as described earlier. Five sites have completed the two year evaluation period while two installed systems have yet to begin operation. The remaining five sites have completed the design/permitting process and should be installed soon. The following information is provided on representative sites currently installed, by order of installation:Site 1Type of structure: 2 bedroom house Number of people: Three Soil Resource Group: C Design Wastewater Flow: 240 gallons per day (gpd) Actual Wastewater Flow: Approx. 240 gpd System Cost: $9,998 Construction: Began October 1989, completed February 1990 Type: Septic tank, constructed wetlands pretreatment; ozone disinfection; subsurface application (LPP)Primary treatment for this system is provided by one 1,000-gallon septic tank. The effluent from the tank flows by gravity into three 4-foot by 23-foot constructed wetland cells, which provide secondary treatment. The cells are each planted with different types of vegetation, including Scirpus (Bulrushes), Phragmite (Common Reeds), and Typha (Cattails). Appurtenances have been installed on each of the three cells, which allows sampling to determine treatment capability of each vegetation type. Cells can also be operated in various combinations to determine optimum treatment capacities. Once through the cells, effluent flows into a 1,000 gallon pump chamber where ozone which is produced by a small ozonator is pumped in. Effluent is then pumped into an LPP distribution network installed in 18 inches of loamy sand fill material. The bottom of the lateral trenches are installed at about the original soil surface. The LPP network consists of 11 60-foot laterals, based on an application of .075 g/|ft.sup.2~/d. The side slope of the fill material provided for the nitrification field is 15 feet from the lateral lines, which provides a basal absorption area of 7,200 |ft.sup.2~ or an application rate of .033 g/|ft.sup.2~/d. The lateral network is divided into four subfields to allow sections to be completely shut off from effluent. This design allows absorption area and actual sewage flow to be properly balanced according to the soil application rate.As indicated earlier, the installation of this system was completed in February 1990, although construction of the residence, which the system services, was not completed until February 1991. Since the wetland cells were planted in April 1990, in anticipation of the house being completed much earlier, local staff visited the site twice weekly to fill the cells with water, until the house was completed. During the period from April 1990 to February 1991 when the cells received no effluent, the Phragmites grew very well, while the Scirpus grew little and the Typha appeared to suffer badly. This same trend has continued since sampling began in March 1991. Observations and measurements suggest that Scirpus and Typha require effluent to be located closer to the rock surface of the cells to maintain healthy growth. Maintaining higher cell effluent levels was accomplished in February 1992 by the addition of control structures at the end of each cell. The average values of the analyzed parameters in the pretreatment train is provided in Table 1.Site 2Type of structure: Three two-bedroom mobile homes Number of people: Five (variable) Soil Resource Group: E Design wastewater flow: 720 gpd Actual flow: Approx. 320 gpdTABULAR DATA OMITTEDSystem cost: $13,065 Construction: Began February 1990, completed May 1990 Type: Septic tank, sandfilter pretreatment; ozone disinfection, spray irrigationThis system currently is being utilized for three two-bedroom mobile home units. Each unit utilizes one 1,200-gallon septic tank for primary treatment. The effluent flows by gravity into a 1,200-gallon pumping chamber where it is dosed into an 18-foot by 36-foot single pass sand filter, via an LPP lateral network. The LPP laterals are installed in 12 inches of 1/2-inch limestone which rests on the filter sand. The design application rate of the filter surface is 1.11 g/|ft.sup.2~/d. Once dosed, effluent passes through 6 inches of rock and 2 feet of graded filter sand before being collected by three 1,200 gallon pump tanks connected to form one 3,600-gallon pump chamber. Ozone is applied to the effluent in the pump chamber and dosed to a 21,000 |ft.sup.2~ spray field with an application rate of .035 g/|ft.sup.2~/d for final treatment and disposal.Groundwater and effluent sampling for this system began in June of 1990. No adverse effects from system operation have been found in the groundwater samples taken close to the spray field. Average values of the data collected on the pretreatment system is provided in Table 2.Site 18Type of structure: Two-bedroom house Number of people: Three Soil resource group: D Design wastewater flow: 240 gpd Actual flow: Approx. 280 gpd System cost: $12,359.36 Construction: Began December 1990, completed January 1991 Type: Septic tank, sand filter pretreatment; subsurface application LPPA 1,000-gallon septic tank is utilized to provide primary treatment for this system. Effluent flows from the septic tank into a 1,000-gallon dosing chamber where it is dosed onto the surface of three sand filters built above-ground in 1,000-gallon one piece pump tanks. This site is located in a 10-year flood frequency zone on the Neuse River. The filters were constructed above-ground to prevent storm overwash. Each filter has a surface area of 32 |ft.sup.2~. The application rate for the filters is 2.5 g/|ft.sup.2~/d. Each filter is dosed by two LPP lateral lines 7 feet in length, encased in 4-inch perforated ABS tubing. The holes in the 4-inch tubing are directed toward the surface of the filter, while the lateral holes are pointed up. The purpose for this design is to promote even distribution across the filter surface and to prevent scouring of the filter surface by the discharge of effluent under pressure. The graded filter sand is 2 feet deep and rests on 12 inches of 1/2-inch stone with a collection tube extending the filter length. All filters are connected at the bottom. The effluent from the filter is conveyed to another 1,000-gallon pump chamber which doses a 1,200 |ft.sup.2~ LPP lateral network (four 60-foot laterals) installed in 24 inches of sandy loam fill material. The design soil application rate is .2 g/|ft.sup.2~/d, although considering the fill area side slopes (11 feet on all sides), the actual basal area application rate is .08 g/|ft.sup.2~/d.Groundwater and pretreatment sampling began in January 1991. Table 3 provides average levels for the effluent before and after filtration. It is of interest to note that during the first three months of operation (January, February, March), in cooler weather, the filters did a poor job of converting ammonia to nitrate, along with reducing BO|D.sub.5~ and TSS. As the temperature began to rise in the spring and early summer the filters began to provide a higher level of treatment for those parameters. It appears this is due to the more favorable conditions (warmer temperatures) for the development of the filter biology.Site 15Type of structure: Small business Number of employees: Ten Soil resource group: A Design wastewater flow: 400 gpd Actual flow: Not occupied System cost: $11,190 Construction: Began December 1990, completed January 1991 Type: Septic tank, constructed wetland pretreatment, subsurface application (LPP)Primary treatment is provided by two 1,000-gallon septic tanks for this system. Effluent is conveyed from one septic tank located in the rear of the structure via a pressure supply line to the front tank where it flows into two 15-foot by 18-foot constructed wetland cells. Each cell contains 12 inches of limestone aggregate and will be planted with different vegetation. The cells will be monitored to determine the treatment efficiency of shallower aggregate depths and greater surface areas with varying types of vegetation. Each cell can be utilized independently of the other or both cells can be by-passed completely. Both cells are constructed so the effluent depth can be regulated. Once through the cells, the effluent flows into a 1,000-gallon dosing chamber where it is conveyed under pressure to an 1,875 |ft.sup.2~ lateral network (five 75-foot laterals), designed at an application rate of .21 g/|ft.sup.2~/d (based on the C soil horizon). Due to the low saturated conductivity (Ksat) of the Bt soil horizon, the nitrification trenches for this system were installed with the bottoms located 5 feet below the soil surface, into the more permeable C soil horizon. The trenches (2 feet wide) were then back-filled with 3-1/2 feet of construction sand. The 1-1/4-inch lateral lines were then installed above the sand surface in 12 inches of stone, encased in 4-inch perforated ABS pipe. This system is not operational, so no data has been collected from the site to date.Site 20Type of structure: Three bedroom house Soil resource groups: E Design wastewater flow: 360 gpd Actual flow: 60 gpd System cost: $10,400 Construction: Began November 1991, completed December 1991 Type: Septic tank, constructed wetlands, ozone disinfection, spray irrigationPrimary treatment for this system is provided by a 1,000 gallon septic tank, the effluent from which flows into two 15 x 20 foot constructed wetland cells where it receives secondary treatment. The depth of aggregate in one cell is 2 feet while the other is 1 foot. This design configuration was utilized to compare treatment ability of the cells with various aggregate depths utilizing the same vegetation in each cell. Both cells contain a variety of plant materials including Waxed Myrtles, Virginia Sweet Briar, Soft Stem and Hard Stem Needle Rush.Effluent receives treatment as it passes through the cells and into control structures which regulates the liquid level in the cells. Once effluent passes through the control structures, it is held in a 1,500 gallon spray field dosing tank prior to application to a 22,500 |ft.sup.2~ field. The soil application rate is .016 g/|ft.sup.2~/day. Effluent is dosed once per day prior to dawn to minimize human contact. The spray field contains 12 sprinklers which are located in a wooded area. Disinfection is provided by utilizing ozone, which is pumped into the spray field dosing tank prior to effluent application. The average valves of the analyzed parameters in the pretreatment train are provided in Table 4. No high levels of analyzed parameters have been detected in the groundwater samples to date.Site 19Type of structure: Construction company offices Number of employees: 18 Soil resource group: CTABULAR DATA OMITTEDDesign wastewater flow: 450 gpd Actual flow: Approx. 150 gpd System cost: $15,380 Construction: Began June 1992, completed June 1992 Type: Septic tank, recirculating sandfilter, ultraviolet disinfection, spray irrigationPrimary treatment is provided by a 1,200 gallon septic tank. The effluent from the septic tank flows into a 1,000 gallon recirculation tank, which holds effluent until dosed onto the surface of a 180 |ft.sup.2~ (9 x 20) recirculating sandfilter, where it receives additional treatment. Three 20-foot (1-1/4 inch) LPP laterals are utilized to distribute effluent over the filter surface. the filter is constructed of 1 foot of 1/2-inch washed rock on the surface, 2 feet of filter sand (.35 to .5mn, UC|is less than~3, dust|is less than~.5%) and 1 foot of 1/2-inch washed rock underlying the sand as an underdrain. The filter is constructed so the lower 2 feet of filter depth is below ground while the upper 2 feet is above. This was done to help prevent groundwater infiltration.A 20 mil PVC liner encases the filter to prevent groundwater contamination. Two 4-inch drainage tubes are installed in the bottom layer of rock to collect effluent which is conveyed through two flow splitters. The flow splitters allow 75% of the sandfilter flow to be diverted to the recirculation tank while 25% flows into the TABULAR DATA OMITTED spray field holding tank for final disposal. The effluent dosing pump is controlled by a timer allowing effluent to be sprayed once per day to a 19,610 |ft.sup.2~ spray irrigation field.The soil application rate is .023 g/|ft.sup.2~/ day. The effluent is disinfected prior to application utilizing a closed vessel ultraviolet (UV) radiation reaction chamber. The UV bulb which produces germicidal radiation operates 24 hours per day. Effluent is disinfected as it passes through the vessel and around the bulb during a dosing event to the spray field. The average valves of the analyzed parameters in the pretreatment train are provided in Table 5. Due to the location of the site near a mining operation, which artificially lowers the natural water table, no groundwater data have been collected.Site 24Type of structure: Three bedroom mobile home Number of people: One Soil resource group: D Design wastewater flow: 360 gpd Actual flow: Not operational System cost: $7,825.00 Construction: Began May 1992, completed June 1992 Type: Septic tank, low pressure pipe (LPP) nitrification field, pumped drainage systemPrimary treatment for this site is provided by a 1,000 gallon septic tank, the effluent from which flows into a 1,000 gallon dosing chamber. No further treatment is provided. Effluent from the dosing chamber is pumped to a 1,200 |ft.sup.2~ (four 60 foot, 1-1/14-inch laterals) LPP nitrification field. The 12 x 12 inch LPP nitrification trenches are installed with the trench bottom located 12 inches below the original soil surface. The entire field is covered with a 6 inch cap of soil and shaped to promote surface water runoff. Due to the location of the seasonal water table (less than 12 inches below the soil surface) artificial drainage had to be incorporated as an integral component of the system design. No existing outlets adjacent to the property were deep enough to allow sufficient drainage tube depth so pump drainage had to be utilized. Drainage tubes were installed 3-1/2 feet deep at 1% grade extending to an 800 gallon fiberglass drainage basin. A 25 gpm pump is activated on demand via float controls to convey groundwater from around the system area to a shallow ditch where it exits the site. The system is not currently operational so no water and effluent quality data have been collected.ConclusionCraven County, with the help of many individuals and state agencies, has developed an innovative approach to address the problems of on-site sewage disposal. While it is clear that more complex on-site sewage treatment systems can improve treatment and disposal problems on many sites, it is also clear without continued maintenance, malfunction of those systems is certain. It is the goal of this program to develop innovative sewage treatment systems and a management scheme for those systems that will protect public health and our environment, while allowing the use of traditionally problem soil not only in Craven County, but in other areas as well.References1. Laws and Rules for Sanitary Sewage Collection, Treatment and Disposal, Title 15A NCAC 18A. 1900, (1990), North Carolina Department of Environment, Health and Natural Resources, Division of Environmental Health, Environmental Health Services.2. Beneson, A.S. (1985), Control of Communicable Diseases in Man, 14th Edition, American Public Health Association, Washington, D.C., pp. 1-455.3. Salvato, J.A., P.E. (1982), Environmental Engineering and Sanitation, 3rd Edition, Wiley-Interscience, John Wiley & Sons, New York, NY, pp. 376-382.4. Hagedorn, C., E.L. McCoy and T.M. Rahe (1990), "The Potential for Groundwater Contamination from Septic Effluents," J. Env. Quality 10(1):1-8.5. Carlile, B.L. (1988), Guide to Soil Suitability for On-Site Wastewater Systems for Craven County, North Carolina, pp. 5-31.6. Waste Not Discharged to Surface Water, Title 15A NCAC 2H.0219(h)(5), (1990), North Carolina Department of Environment, Health and Natural Resources, Division of Environmental Health.7. Amoozegar, A. and A.W. Warrick (1986), "Hydraulic Conductivity of Saturated Soil: Field Methods," in Methods of Soil Analyses Part 1, Physical and Mineralogical Methods, 2nd Edition, American Society of Agronomy, pp. 735-770.John Myers, R.S., Craven County Health Dept., Division of Environmental Health, P.O. Box 1390, New Bern, NC 28563.
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|Publication:||Journal of Environmental Health|
|Date:||Mar 1, 1993|
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