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concern for regulators because high concentrations of BOD(S) increase the risk of <br /> infiltration-restrictive biomat formation on the dispersal component infiltration surface. <br /> Excessive biomat formation can result in eventual discharge of sewage to the ground <br /> surface from the soil component. BOD(5) concentration by itself, however, is not the <br /> only risk factor. The ultimate consideration is the organic loading rate (unit mass/unit <br /> time) relative to the total surface area (unit area) of the soil dispersal component <br /> (Ibs/day/ft2 in English units). The implied organic loading rate limit for sandy soils in the <br /> Wisconsin administrative code is 0.0013 Ibs/day/ft2 given a specified upper limit <br /> hydraulic loading rate of 0.7 gal/day/ft2 for septic tank effluent and a specified upper limit <br /> BOD(S) concentration of 220 mg/L. <br /> Applying this principle to your septic system using its installed "equivalent' surface area <br /> (EISA) of 3,640 ft2 and its approved plan parameters of 2,379 gallons per day design <br /> flow and 220 mg/L BOD(S), the organic loading rate would be 0.0012 Ibs/day/ft2, a value <br /> within the implicit limits of the code. We know, however, that the highest measured <br /> BOD(S) concentration from sampling in 2010 was 550 mg/L. In that case, the organic <br /> loading rate was 0.0030 Ibs/day/ft2, a value exceeding the implicit limits of the code. <br /> But if the hydraulic flow is no more than 1,000 gallons per day with a BOD(S) <br /> concentration of 550 mg/L, then the organic loading rate becomes 0.0013 Ibs/day/ft2, <br /> the code-implied organic loading rate limit for sandy soils. <br /> So as you may be able to discern from the foregoing examples, it is possible to maintain <br /> an organic loading rate at or below the code-implied rate by either individually or in <br /> combination adjusting the design flow, the BOD(S) concentration, or the soil dispersal <br /> area. The fact that the dispersal area component has been operating without problems <br /> since its installation in 2004 suggests that the system has been functioning since that <br /> time with an average organic loading rate of less than 0.0013 Ibs/day/ft2; otherwise, <br /> surface discharge by the dispersal component or pump re-cycling would have occurred <br /> by now. It is most likely that actual water use in the restaurant is significantly less than <br /> the plan-approved design flow. The shallow installation and the wide range of <br /> weekly/seasonal fluctuations in business would also explain its failure-free operation. <br /> In order to choose a future plan of action for the system, it would be prudent to meter <br /> the daily water use in the facility during the busy season (Memorial Day through Labor <br /> Day). I would also recommend that an additional series of three samples be analyzed <br /> for BOD(S),TSS, and FOG concentrations. Details on when and how sampling should <br /> be accomplished will be specified at a later time. With accurate water use and organic <br /> load information, it will be possible to make any necessary modifications reliably without <br /> adding an aerobic pretreatment device whose only function would be to reduce the <br /> BOD(S) concentration. (Since 2004, when they were first being installed in the state, <br /> experience with such devices for restaurant applications has proved to be less than <br /> satisfactory for various reasons.) Furthermore, the state administrative code does not <br />