An 18–month field evaluation of four micro–irrigation 'drip' treatment and disposal systems was conducted to determine if reducing the separation distance between the bottom of the drip–line and the seasonal high water table (limiting condition) could be reduced from 18 inches to 12 inches. In this study three systems did not have advanced treatment while the remaining system was used in conjunction with an aerobic advanced treatment unit (ATU).
The seasonal high water table (SHWT) was estimated during the site evaluation and monitor wells were installed to collect grab samples quarterly. The monitor wells were placed up–gradient of the system, inside the drainfield and down-gradient of the system. The grab samples were analyzed for Nitrate/Nitrite–N, Ammonia–N, Kjeldahl–N, Total N, pH, specific conductivity and fecal coliform bacteria. Two of the three systems, without the ATU, had SHWT¹s at 20 inches while the third was estimated at 12 inches. The system with the ATU had a SHWT at or near the soil surface.
All of the systems appear to be functioning properly with no adverse environmental impacts. Nitrate/Nitrite–N did not increase significantly from the background samples taken before system operation. The ATU system had a significant increase in the Nitrate/Nitrite–N levels within the wells that were located within the drainfield. This is expected due to the aerobic properties of the ATU which nitrifies the wastewater creating Nitrate/Nitrite-N. This form of nitrogen is readily absorbed by plant roots. Fecal coliform bacteria levels were all generally less than 20/100mL with 79% of the total being less than 10/100mL.
The reduction of the isolation distance from 18 inches to 12 inches appears to have no adverse impact on the ground water quality within the soils of Delaware. Further water sampling, plus soil sampling at four inch intervals, during the next year will provide more complete results and allow to see longer term exposure of the sites to wastewater treatment and disposal. Keywords: on–site wastewater treatment and disposal system, advanced treatment unit, seasonal high water table.
It is a generally accepted principle that soils with seasonally or permanently high water tables are not suitable for siting on–site wastewater treatment and disposal systems due to the inadequate renovation of the effluent prior to its merging with ground water (Cogger, 1984). In Delaware, as well as the entire eastern coastline, many soils fit this description and, with the constant downward development from New York, New Jersey and southeastern Pennsylvania, are being utilized more and more. Central sewerage systems are being brought on-line to handle some of the load but on–site wastewater treatment and disposal systems (OWTDS) will continue to be the workhorse for future expansion.
Throughout the past ten years numerous alternative technologies have been developed to address the need for cleaner wastewater and better attenuation of the wastewater once it has been introduced into the soil environment. Several advanced treatment units are now on the market to satisfy the cleaner wastewater concern such as biological contactor units, fixed film units, recirculating sand filters and sequential batch reactors to name a few. The low pressure pipe (LPP) treatment and disposal system has been the latest innovation that has worked well in Delaware since the trench width was increased to twelve inches and an eighteen inch separation distance is maintained from a limiting condition (DNREC, 1985). The principal behind the LPP is to incorporate small diameter piping with equal distribution over a larger area with less volume per dose to allow the natural cleansing processes of the soil to attenuate the wastewater (Cogger, 1984).
This is where the micro-irrigation 'drip' system fits in perfectly. 'Drip', as it is commonly referred to within the industry, uses this concept but takes it a step further. The drip–line is usually installed 6 – 8 inches below the soil surface which allows for vegetation to readily absorb and breakdown the disposed wastewater. The drip-line consists of plastic tubing with pressure compensating emitters spaced along its entire length. The exact spacing varies by state but, in Delaware, we allow 2 foot spacing between emitters and two foot on center but are flexible with this distance. Another benefit with the ³drip² system is that each is comprised of two zones which are alternately dosed so it allows for aerobic treatment for longer periods between doses and this compliments the small volume dose being emitted (Am. Man. Co., Inc., 1997).
The purpose of this project was to determine if siting the 'drip' system in soils with a limiting condition less than 24 inches (18 inch separation + 6 inch installation depth) and without the requirement for advanced treatment (secondary) would show adverse ground water impacts. The movement of N and fecal coliform was evaluated. Nitrogen was the determining factor because of its mobility and its usefulness as an indicator of aerobic and anaerobic conditions.
There have been numerous studies conducted on the movement of chemical and bacterial contaminants from on–site wastewater treatment and disposal systems in soils with seasonal or permanent high water tables so this is not an attempt to recreate those results. Also, work has been done with fluctuating water tables (Reneau, 1977), artificial drainage (De Vos, 2000, Gambrell et al 1975 and Reneau, 1978) and three-dimensional flow of contaminants through ground water (Hagedorn, 1978). This study will incorporate the results from these studies to make conclusions from the data collected at each site. After a thorough literature review, very few studies have looked into micro–irrigation 'drip' treatment and disposal system impacts. This may be the first attempt to determine a comfortable separation distance to a limiting condition without the need for advanced treatment and an eighteen inch standoff.