1 Chapter 11 Wastewater Treatment 11 1 Research an emerging chemical of concern that might be discharged to a wastewater treatment plant or household septic systems Examples would include pharmaceuticals caffeine surfactants found in detergents fragrances and perfumes Write an essay of up to three pages on the concentration of these chemicals found in wastewater influent Determine whether the chemical you are interested in researching is treated in the plant passes through untreated or accumulates on the sludge Identify any adverse ecosystem or human health impacts have been found for this chemical Solution Students responses will vary Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 2 11 2 Research whether there are state or regional pollution prevention programs to keep mercury out of your local municipal wastewater treatment plant This mercury might come from your university laboratories or local dental offices and hospitals What are some of the specifics of these programs How much mercury have they kept out of the environment since the program s inception Solution Students responses will vary Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 3 11 3 A wastewater treatment plant receives a flow of 35 000 m3day Calculate the required volume m3 for a 3 m deep horizontal flow grit chamber that will remove particles with a specific gravity of more than 1 9 and a size greater than 0 2 mm diameter Solution The basic design equation is V hQ vs Calculate the particle settling velocity vs using Stokes Law vs vs g p w d 2 18 9 81 1 9 1 0 998 0 2 10 3 18 1 002 10 3 2 v s 0 02 m s or 1 728 m day The design particle and all larger denser particles will be removed m3 3 m 35 000 day hQ V 60 8 m 3 m vs 1 728 day Consider the same plant utilizing an upward flow clarifier Calculate the required surface area of the tank The basic design equation is m3 Q day 20 3 m 2 m vs 1 728 day 35 000 Atop Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 4 11 4 A wastewater treatment plant will receive a flow of 35 000 m3 day Calculate the surface area m2 diameter m volume m3 and hydraulic retention time of a 3 m deep circular primary clarifier that would remove 50 percent of suspended solids Assume the surface overflow rate used for design is 60 m3 m2 day Solution The basic design equation is SOR Q Atop Calculate the required surface area Atop Q 35 000 m3 day 3 2 583 m2 SOR 60 m m day Calculate the tank diameter Atop r 2 r Atop 583 13 6 m D 2r 27 2 m Calculate the tank volume V Atop h 583 m 2 3 m 1 749 m 3 Calculate the retention time V 1 749 m3 0 05 day 1 2 h Q m3 35 000 day From the design plot a BOD removal efficiency of 25 could be expected Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 5 11 5 Assume a plant flow of 12 000 m3 day Determine the actual detention time observed in the field of two circular settling tanks with depth 3 5 m that was designed to have an overflow rate not exceeding 60 m3 m2 day and a detention time of at least 2 hour Solution Size settling tanks based on both overflow rate and hydraulic detention time Select largest reactor to ensure both design criteria are met Solving by overflow rate OR Q m3 60 2 A m day 12 000 m3 day A A 200 m 2 However t 200 m 2 3 5 m 24 h V 1 4 h Q 12 000m3 day day Solving by detention time V 2 hours Q day 3 5 m 2 h A m3 24 h 12 000 day 2 A 286 m 12 000 m3 day m3 OR 42 m 2 day 286 m 2 Therefore the larger area should be used Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 6 11 6 A wastewater treatment plant has a flow of 35 000 m3 day Calculate the mass of sludge wasted each day QwXw expressed in kg day for an activated sludge system operated at a solids retention time SRT of 5 days Assume an aeration tank volume of 1 640 m3 and an MLSS concentration of 2 000 mg L Solution 2 000 mg L 2 kg m3 Set up the SRT design equation c XV XV X w Qw X w Qw c 1 640 m 2 kg m 3 X w Qw 5 days 3 656 kg day Solutions Manual prepared by Ziad Katirji and Heather E Wright Wendel Environmental Engineering Fundamentals Sustainability Design James R Mihelcic and Julie Beth Zimmerman John Wiley Sons New York 2009 7 11 7 You are provided the following information about a municipal wastewater treatment plant This plant uses the traditional activated sludge process Assume the microorganisms are 55 percent efficient at converting food to biomass the organisms have a first order death rate constant of 0 05 day and the microbes reach half of their maximum growth rate when the BOD5 concentration is 10 mg L There are 150 000 people in the community their wastewater production is 225 L day capita 0 1 kg BOD5 capita day The effluent standard is BOD5 20 mg L and TSS 20 mg L Suspended solids were measured as 4 300 mg L in a wastewater sample obtained from the biological reactor 15 000 mg L in the secondary sludge 200 mg L in the plant influent and 100 mg L in the primary clarifier effluent SRT is equal to 4 days a What is the design volume of the aeration basin m3 b What is the plant s aeration period days c How many kg of secondary dry solids need to be processed daily from the treatment plants d If the sludge wastage rate Qw is increased in the plant will the solids retention time go up go down or remain the same e Determine the F M ratio in units of kg BOD5 kg MLVSS day f What is the mean cell residence time Solution a First determine Qo and BOD influent L 6 L 150 000 225 33 75 10 capita day day mg 106 kg BOD5 mg kg 444 BOD influent 150 000 0 1 L person day 33 75 106 L day Qw X w …