EVALUATION OF GRAVELLESS PIPE SYSTEMS FOR ONSITE WASTEWATER TREATMENT ACROSS MINNESOTA

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1 EVALUATION OF GRAVELLESS PIPE SYSTEMS FOR ONSITE WASTEWATER TREATMENT ACROSS MINNESOTA INTRODUCTION S. H. Christopherson, D. M. Gustafson, J. L. Anderson Approximately 30% of Minnesota's residents rely on onsite technologies for their wastewater treatment. Gravelless pipe systems became a standard choice for trench systems in 1989 in Minnesota Rules Chapter This system is called gravelless because no rock or gravel is placed in the trenches. The gravelless pipe evaluated in this study was 10 inner diameter corrugated polyethylene tubing covered with a permeable nylon fabric. Two rows of holes, approximately 1/2 inch in diameter are located at the four o clock and eight o clock position. Sizing requirements for these systems are based on research performed at the University of Minnesota in the early 1980s (Anderson, 1985). Figure 1. Installation of gravelless pipe The Minnesota Onsite Sewage Treatment Contractors Association (MOSTCA) received concerns from its members about gravelless pipe systems in MOSTCA contacted several suppliers of gravelless pipe who agreed to fund a research study to see if problems do, in fact, exist. The purpose of this study was to determine the reason(s) for success and failure of Individual Sewage Treatment Systems that use gravelless technology in Minnesota.

2 PROCEDURES Of the fourteen systems evaluated, eleven systems had experienced problems (surfacing to the ground and backups into home) while the remaining three did not exhibit any obvious signs of failure. Typically, if the gravelless system exhibited functional problems, either the homeowner or a septic system professional contacted the University for assistance to determine why there was a problem. A system usage survey was given to each homeowner. Information requested included: flow data (measured or estimated), number of people living in home, design (including size and location of septic tank and soil treatment system), age and performance of system, maintenance history and usage of garbage disposal, dishwasher, laundry, water softener, antibacterial soap and cleaning supplies. Individual locations had a detailed site analysis performed to evaluate the soil, system, location and condition. The trenches were evaluated to determine the depth of ponding in each trench and amount of biomat development. Soil borings were conducted to determine depth to water table or other limiting soil conditions. Soil samples were taken to determine soil texture classification from particle size analysis distribution analysis, (US Department of Agriculture, 1993). Effluent samples were collected from the septic tank and from inside the gravelless pipe. These were taken as grab samples. Samples were taken at the 40% liquid depth at the outlet of the septic tank and in the gravelless pipe with a Masterflex pump. Samples were analyzed for dissolved oxygen (DO), biological oxygen demand (BOD), and total suspended solids (TSS). All samples were collected and analyzed in accordance with Standard Methods for Examination of Water and Wastewater (American Public Health Association, 1992).

3 RESULTS AND DISCUSSION System locations are shown in Figure 2. The number in the county represents the number of systems evaluated. The map illustrates that the study was concentrated in and around the northern metropolitan area and central Minnesota. These locations were selected based on homeowner and/or contractor requests. As a result, this provided a focus on problem areas and, in particular, sites with fine sand soil textures. Figure 2. Location of Gravelless Pipe Systems Evaluated. Soil texture was identified by the feel method at each site and followed up with a sieve analysis to confirm those field results. Table 1 summarizes the soil textures and number of systems evaluated with and without problems for each soil texture. Sieve results are included in the individual reports.

4 Soil Classification Table 1. Soil Classification by System # of systems with problems # of systems with no problems Sand 3 1 Fine sand 5 2 Loam 1 >NA Sandy clay 1 NA loam Loamy sand 1 NA Results of Effluent Analyses The results from the septic tanks and gravelless pipe for BOD, TSS and DO were all within normal levels to be expected in a septic tank effluent. The BOD values were all below 220 mg/1 and the TSS values were below 65 mg/1. Many of these were skewed because the first thing many homeowners do when there is a problem is to pump the septic tank. This pumping affects these results and the typical operation of these tanks may have been different. Visual inspection indicates that there is reason for concern that some of these tanks may not be watertight. This was not apparent during the site evaluations but these were not done during the spring when such high water table conditions may exist, in the vicinity of the tanks. Flow Results Five of the eleven homeowners installed flow meters to monitor the flow. Flows were monitored for periods of seven to fourteen days. Three of the homes had flows well below the design levels. Two were close to or above their design flow rates. It is not possible to make a definitive conclusion based on such limited time periods. However, consistent flows at or above design values will shorten the life and reduce performance in any onsite system. The other homes refused or could not install a flow meter. In these instances we do not know if the system is being hydraulically overloaded. Based on the number of people living in these homes and the number of water using devices, none of them appear to be exceeding the daily estimated sewage flow rates.

5 Homeowner Usage Results The table below summarizes some of the homeowner water use patterns. House Age Garage Disposal Table 2. Homeowner Data Summary Water Softener Dish washer Anti-bacterial Soap Long Term Prescription Drug(s) Date of Last Pumping 1 10 x x x x Summer * 2 Spring na na na na na na 4 9 x x Summer 5 4 x x x x Winter x x Summer 7 7 x x x Fall na na na na na 9 5 x x Summer 10 5 x x Summer na na na na na na 12 5 x x Summer 13* 3 x x x Spring 14* 7 x x x Fall 1999 * Systems with no apparent problems Reasons for Failure The average age of the gravelless system evaluated was six years with a minimum age of two and the maximum of ten years. Age is important because Chapter 7080 became mandatory in Previously using 7080 was voluntary, so a person could choose to downsize gravelless systems by 20-50%, which was a manufacturers recommendation. This evaluation looked at systems using various brands of gravelless pipe. There are four main

6 suppliers of gravelless pipe in Minnesota. Problems were seen among all four brands. The products, particularly the fabric, vary by brand and year of installation. Hydraulic failure of an onsite system is often caused by either daily sewage flows exceeding the design rate, or an incorrect evaluation of the long-term acceptance rate. The long-term acceptance rate can be negatively affected by improper site ands soils evaluation, improper system location, improper construction practices and lack of maintenance (Anderson, 1982). One or more of these factors can result in system failure. All the systems evaluated with problems were either designed or installed improperly. Not one site with problems was designed according to Chapter At each site, all trenches were ponded with effluent. Four primary factors were identified that negatively affected the systems. 1. Nine of the eleven problem systems evaluated were undersized according to design criteria. Of these nine, two were designed 50% too small, four - 40% too small, one - 30% too small and two - 20% too small. The table below from Chapter indicates the required sizing factors. Two of these systems did not have three feet of separation as well. A common error was to size a fine sand as a coarse or medium sand or as a sandy loam. This general reduced size, combined with near design flows will limit the long-term performance of these systems.for example, a system for a 3 bedroom is designed to treat 450 gallons per day. If a system is designed based on a medium sand texture it may, in fact, be large enough for 225 gallons per day flow if the soil texture is fine sand. Figure 3. Soil Characteristics and Sizing Factors According to Chapter Three of the eleven were designed and installed with less than the required three-foot separation to the water table. Chapter 7080 requires three feet of unsaturated soil be present beneath the trench to adequately treat sewage. (See Chapter ). At one site the standing water level was visible at the same depth as the gravelless pipe. At the other two, redoximorphic features (mottling) were used to determine the seasonally high water table. Two of these systems were undersized as well. One was 30% too small and the other was 40% too small. 3. One of the systems was installed improperly. Trench systems must be installed level following land contours. (See Chapter ). There was a four-inch drop from one end of the pipe to the other over the fifty-foot trench length. Therefore as the sewage seeks its own level it will surface at the point of lowest elevation. This occurred at the far end, where the soil cover was lower than the elevation of the outlet from the distribution box.

7 4. All systems evaluated had biomat development, varying from one to four inches surrounding the pipe. When septic tank effluent is applied to soil, a biomat forms at the interface of the soil and the gravelless pipe. Development of the biomat occurs more quickly under anaerobic conditions. Therefore, separation distance to the water table is critical to provide an aerated soil below the biomat. Table 3. System Summary House Age Amount of Biomat Problem(s) (inches) Does not have 3 feet of separation 40% too small 2* 2 <1 na % too small % too small % too small % too small Does not have 3 feet of separation 30% too small Installed incorrectly % too small % too small Does not have 3 feet of separation % too small High water use 13* 3 <1 na 14* 7 1 na

8 The biomat extends into the soil; therefore the fabric is not inhibiting biomat development in the soil (Anderson, 1982). When the biomat was removed from around the pipe the effluent would run out of the pipe. This is a normal function, it is why effluent ponds in any sewage treatment trench. This indicates that the effluent was being prevented from leaving the gravelless pipe by the biomat. There is the possibility that the pipe could be plugged from the inside as well, but this was not apparent at any of the sites evaluated in this study. There was no apparent accumulation of sludge or other material in the pipe. All three systems that did not have three feet of separation (two of which were downsized as well) had 3 inches of biomat surrounding the pipe. All of systems that were sized 30-50% too small had 2-4 inches of biomat surround the pipe. These values indicate the systems are probably hydraulically overloaded. Reasons for Success Two of three systems visited had two septic tanks. This is above what Chapter 7080 requires. It provides additional settling time and storage space. All three systems were sized appropriately based on estimated daily sewage flow and soil. All three systems had three feet or more of vertical separation to the water table. On average, only half of the trenches had effluent ponded in the pipe. CONCLUSIONS 1. The sample size was too small to provide definitive answers about whether gravelless pipe has problems. 2. All of the systems in this study evaluated had design or construction errors. These errors strongly contributed to the failure of the systems evaluated. These errors may be due to lack of knowledge by designer/installer and local unit of government staff, pricing/bidding issues or simply a mistake. 3. The sites we evaluated appeared to be plugging from the outside of the pipe due to biomat development that would indicate that the problem is not due to the product itself, but how it is sized and installed.

9 RECOMMENDATIONS More systems should be evaluated to determine if there is a problem with gravelless pipe. If there is doubt about how a soil should be sized the larger soil-sizing factor should be chosen. Sieve analysis should be done with sandy soils that appear to have large amounts of fines. A technique to accurately judge the amount of buildup on the fabric material should be developed. Tests that have been done are unable to differentiate between plugging on the inside or outside and soil particles that remain attached to the media. ACKNOWLEDGEMENTS The Minnesota Onsite Sewage Treatment Contractors Association (MOSTCA) funded this project. Thanks to the homeowners, contractors and local units of government that assisted. For more information about the individual sites evaluated please call (612) or heger001@umn.edu REFERENCES American Public Health Association Standard Methods for Examination of Water and Wastewater (18 th edition). American Public Health Association, Washington D.C. Anderson, J. L., R. E. Machmeier and M. P. Gaffron Evaluation and Performance of Nylon Wrapped Corrugated Tubing in Minnesota. Proceedings of the Forth National Symposium on Individual and Small Community Sewage Systems. American Society of Agricultural Engineers, St. Joseph, MI. Anderson, J. L., R. E. Machmeier and M. J. Hansel Long-Term Acceptance Rates of Soils For Wastewater. Proceedings of the Third National Symposium on Individual and Small Community Sewage Systems. American Society of Agricultural Engineers, St. Joseph, MI. Minnesota Pollution Control Agency. 1999a. Minnesota rules chapter 7080 individual sewage treatment systems program. Office of Reviser of Statutes, St. Paul, MN. 5. United States Department of Agriculture Soil Survey Manual. US Government Printing Office, Washington, DC.

Onsite Sewage Treatment Program. Presented by: Ed Melzark SR Environmental Technician / Zoning Administrator Pine County

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