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7 C. WASTEWATER FLOW CALCULATIONS AND CHARACTERISTICS Domestic wastewater flows managed by on-site septic systems have been studied for decades with variable data published in the scientific literature. Wastewater flows for this project will be assessed on a"future"basis whereby it will be assumed the existing household and a potential second unit dwelling will have 4 persons per house (husband, wife and two children). The total daily water use is a combination of household and personal uses, as itemized below: HOUSEHOLD PERSONAL USE 25 gal for laundry 2 gal for drinking and cooking 10 gal for dishwashing 3 gal for oral hygiene 5 gal for misc. use 14 gal for bathing 16 gal for toilet flushing/sink washing FLAW RATE in gals/home/day= 40 gals/household/day+(35 gal/person/day x number of persons/home). The daily wastewater flow rate can be estimated at: 40 gals+(35 gals x 4 people/home) = 180 gals/day for an average flow per resident of 45 gals. As a safety factor,the total daily wastewater flow per house and average daily flow per resident will be increased 1.5x to 270 gals/day and 68 als/resident,respectively. The peaking factor for individual residences on septic tanks will vary with the day of the week and usage. Peak hourly rates are normally associated with the discharge of clothes wash machines, dishwashers and shower/bath water. The ratio of the peak day to the average day can vary, thus a 1.5 safety factor. The characteristics or composition of septic tank effluent is dependent upon several parameters because of idiosyncrasies within each household. Certain appurtenances change the characteristics of the wastewater effluent such as filter vaults within septic tanks and kitchen food-waste grinders. Since this study is concerned with nitrogen, this constituent is quantified in the effluent, as illustrated below: In determining nitrogen concentrations of septic influent and effluent,per capita intake of nitrogen per day must be estimated. The average American consumes between 70 and 140 g of N/day. Since nitrogen comprises 16% of the weight of the average protein, 16% of 70 g N= 11 g N/capita/day and 16% of 140 g N= 22 g N/capita/day. The average of these two values is 17 g N/capitalday of theoretical influent. The concentration of nitrogen in septic system effluent is normally specified in the scientific literature as units of mg N/L. Unfortunately, these units superimpose wastewater flow variability onto nitrogen influent variability, thus creating even greater variability. One researcher, (Bauman), has averaged the means from 20 other studies to arrive at a concentration of 62-L 21 mg NIL in septic tank effluent. If the above figure for wastewater discharge is estimated at 68 gals/resident/day(257 liters) and using the figure of 17 g N/cap/day excreted= 17 g+ 257 L= 66 Lug N/L/dav estimated for tie pfo_ op sed pra_„iect This concentration will be the estimated nitrogen loading in the effluent discharged from a second unit dwelling. Page -8- Chesney Consulting