A city of 500,000 people is living in Baltimore city. Each person discharges 0.5

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A city of 500,000 people is living in Baltimore city. Each person discharges 0.5 m^3 of sewage (wastewater) per day, Wastewater sample was collected and bring to the laboratory. The waste water sample volume of 100 ml was used in performing all analysis. Tare mass of evaporating dish is 24.3520g. Mass of evaporating dish plus residue after evaporation @ 105 degree C is 24.3970 g. Mass of evaporating dish plus residue after ignition @ 550 degree C is 24.3850 g. Mass of Whatman filter and tare is 1.5103 g. Mass of Whatman filter and tare after drying @ 105 degree C is 1.5439 g. Residue on Whatman filter and tare after ignition @ 550 degree C is 1.5199 g. The sewage having a 7-day BOD test performed on a sample was 10 mg/L. The base e BOD rate constant determined from previous studies was estimated to be 0.10 days-1 and DO was 1.8 mg/L. A multiple-tube fermentation test was also performed on the sewage. A set of 15 test tubes with sample sizes of 10, 1.0, and 0.1 ml were used in the multiple-tube fermentation test, resulting in the following number of positive tubes: 5-2-1. The sewage contains dichloroacetic acid (CHCl_3COOH) formed during disinfection of water. The half-life of dichloroacetic acid is approximately 3 days. The Back River wastewater treatment plant has ten rectangular settling basins, 40.0 m long 5.0m wide and 3.0m deep of basins are operated in parallel of coagulated water. The effluent weir length in each basin is equal to 3 tank widths. After treatment, treated water released to the Back River which has a BOD of 3.6 mg/L and a DO of 8.6 mg/L. The saturation value of DO is 9.5 mg/L. The deoxygenation coefficient k_d is 0.61/day and the reaeration coefficient k_d is 0.76/day. U.S census bureau of Maryland expected that Baltimore city population will be keep growing and reach 600,000 after 10years. Therefore Deportment of Public Work, decided to expand Back River wastewater treatment plant facility to handle future wastewater volume. Determine 5-day BOD of the sewage. Calculate current overflow rate (m/h) and the detention time of each basin in hours

Explanation / Answer

The test for biochemical oxygen demand (BOD) is a bioassay procedure that measures the oxygen consumed by bacteria from the decomposition of organic matter (Sawyer and McCarty, 1978). The change in DO concentration is measured over a given period of time in water samples at a specified temperature. Procedures used to determine DO concentration are described in NFM 6.2. It is important to be familiar with the correct procedures for determining DO concentrations before making BOD measurements. BOD is measured in a laboratory environment, generally at a local or USGS laboratory. w organic matter affects the concentration of dissolved oxygen (DO) in a stream or lake is integral to waterquality management. The decay of organic matter in water is measured as biochemical or chemical oxygen demand. Oxygen demand is a measure of the amount of oxidizable substances in a water sample that can lower DO concentrations (Nemerow, 1974; Tchobanoglous and Schroeder, 1985).There are two stages of decomposition in the BOD test: a carbonaceous stage and a nitrogenous stage . X The carbonaceous stage, or first stage, represents that portion of oxygen demand involved in the conversion of organic carbon to carbon dioxide. X The nitrogenous stage, or second stage, represents a combined carbonaceous plus nitrogeneous demand, when organic nitrogen, ammonia, and nitrite are converted to nitrate. Nitrogenous oxygen demand generally begins after about 6 days. For some sewage, especially discharge from wastewater treatment plants utilizing biological treatment processes, nitrification can occur in less than 5 days if ammonia, nitrite, and nitrifying bacteria are present. In this case, a chemical compound that prevents nitrification should be added to the sample if the intent is to measure only the carbonaceous demand. The results are reported as carbonaceous BOD (CBOD), or as CBOD5 when a nitrification inhibitor is used. The standard oxidation (or incubation) test period for BOD is 5 days at 20 degrees Celsius (°C) (BOD5). The BOD5 value has been used and reported for many applications, most commonly to indicate the effects of sewage and other organic wastes on dissolved oxygen in surface waters (see TECHNICAL NOTE). The 5-day value, however, represents only a portion of the total biochemical oxygen demand. Twenty days is considered, by convention, adequate time for a complete biochemical oxidation of organic matter in a water sample, but a 20-day test often is impractical when data are needed to address an immediate concern. X The BOD5 and CBOD5 tests have limited value by themselves in the assessment of stream pollution and do not provide all of the relevant information to satisfy every study objective (Nemerow, 1974; Stamer and others, 1983; Veltz, 1984). Additional analyses of water samples for chemical oxygen demand, fecal bacteria, and nutrients can aid in the interpretation of BOD5. X An ultimate carbonaceous BOD (CBODu) test is needed to obtain additional BOD information, and can be used for modeling DO regimes in rivers and estuaries (Hines and others, 1978; Stamer and others, 1983). Guidelines for the CBODu determination are described in Stamer and others (1979, 1983). X Note that BOD results represent approximate stream oxygen demands because the laboratory environment does not reproduce ambient stream conditions such as temperature, sunlight, biological populations, and water movement.

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