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Nw = 103 mg N/L: 20%reduction from the septic tank =(1-0.2)103 =82 mg N/L d = Assume 15%due to the fine-grained soil at the potential soil-effluent interface soil chemistry results. R = 12 in/yr. Nb= Assumed to he 0.1 ppm NO3-N 0.036 in/yr(82 ma N/L) (1 -0.15)+(12 in/yr x 0.1 mg/L) Nr = (0.036 in/yr+ 12 in/yr) Nr 1.4 ppm itrate as nitrate(NO3) The c ated nitrate loading concentration in the recharge effluent from the new modular building is calculated to be under the Maximum Contaminant Level limit for nitratg water. The variables used in the calculations above are conservative. For example,the ADF 84 g s/day is assumed 365 days per year. Secondly,the use of 103 mg N/L,is a much higher nitrogen c ncen tion than those concentrations published from household or domestic sources. Various researchers place the nitrogen reduction occurring in septic tanks at 10%to 20%. From the May 8,2003 Symposium"Onsite Wastewater Treatment -Nitrogen Removal,"Dr. Stuart Oakley of Chico State University states that"approximately 20%of the total nitrogen generated is removed in the septic tank." IV. WASTEWATER DISPOSAL SYSTEM OPERATIONAL PARAMETERS Septic Tank Sizing Since the new modular building will be elevated,influent can gravity flow into a 1,200 gallon septic tank that is installed at grade. The effluent can then gravity flow from the tank into the disposal area structure. The EHD will require that two-to-three inches of soil is placed over the tank to mitigate odors. The tank may have to be pumped on a prescribed interval basis depending on direct observations of paper and scum accumulation;e.g.,every six months to every three years,depending on sludge/scum accumulation to be determined by a septic system contractor. Peaking factors for septic tanks vary with usage during the hour of the day,and day of the week,with peak hourly rats associated with normal working hours. The ratio of the peak day to the average day can vary from 1.5 to>5, depending upon the usage. A peaking factor of 2.5 (average between 0 and 5)will be used in the calculation below: Septic Tank Hyd lic Detention Time Detention Time 12,00 septic tank ca ac' 'x 0.70 30% volume lost to sludge/scum accumulation / Factoryz 84 gals/day ADF Detention Time d 4.00 d4. The minimum acceptable detention time is 1.0 days. Therefore, a 1,2W-gailbh septic tank is adequate for the new modular building. Primary and 100% Replacement Filter Bed Areas for the new Modular Building The filter bed area is calculated in the procedure described in Section 8.7 of the San Joaquin County Sewage Standards and EHD Document 42-01-004 Calculation Requirements for Commercial Septic Systems: Base Tank Capacity(BTC)= 1,200 gals 8