1 Chapter 8 Water Quality 8 1 A stream at 25 C has a dissolved oxygen concentration of 4 mg L What is the dissolvedoxygen deficit mg L Solution Determine DOsat from the appropriate temperature dependent Henry s Law constant and the oxygen partial pressure DOsat moles O2 1 29 10 3 moles 0 21 atm 2 71 10 4 L atm L and converting to mg O2 L DOsat 2 71 10 4 moles O2 32 g O2 1 000 mg O2 mg O2 8 7 L mole O2 g O2 L Now calculate the oxygen deficit D DOsat DOact D 8 7 mg mg mg 4 4 7 L L L 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 8 2 The oxygen concentration of a stream is 4 mg L and DO saturation is 10 mg L What is the oxygen deficit Solution D DOsat DOact D 10 mg mg mg 4 6 L L L 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 8 3 Calculate the dissolved oxygen deficit for a river at 30 C and a measured dissolvedoxygen concentration of 3 mg L The Henry s law constant at that temperature is 1 125 10 3 mole L atm and the partial pressure of oxygen is 0 21 atm Solution Determine DOsat from the appropriate temperature dependent Henry s Law constant and the oxygen partial pressure DOsat 2 36 10 4 moles O2 1 125 10 3 moles 0 21 atm L atm L and converting to mg O2 L DOsat 2 36 10 4 moles O2 32 g O2 1 000 mg O2 7 6 mg O2 L mole O2 g O2 L Now calculate the oxygen deficit D DOsat DOact D 7 6 mg mg mg 3 4 6 L L L 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 8 4 A wastewater treatment plant discharges an effluent containing 2 mg L of dissolved oxygen to a river that has a dissolved oxygen concentration of 8 mg L upstream of the discharge Calculate the dissolved oxygen deficit at the mixing basin if the saturation dissolved oxygen for the river is 9 mg L Assume that the river and plant discharge have the same flow rate Solution Determine the actual oxygen concentration at the mixing basin Qriver Qdisch arg e Q mg mg Qriver 8 Qdisch arg e 2 mg L L 5 DO 2Q L Now determine the dissolved oxygen deficit D DOsat DOact D 9 mg mg mg 5 4 L L L 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 8 5 A river traveling at a velocity of 10 km day has a dissolved oxygen content of 5 mg L and an ultimate CBOD of 25 mg L at distance x 0 km that is immediately downstream of a waste discharge The waste has a CBOD decay coefficient k1 of 0 2 day The stream has a reaeration rate coefficient k2 of 0 4 day and a saturation dissolved oxygen concentration of 9 mg L a What is the initial dissolved oxygen deficit b What is the location of the critical point in time and distance c What is the dissolved oxygen deficit at the critical point d What is the dissolved oxygen concentration at the critical point Solution a D0 DOsat DOact 9 5 4 mg L b Use Equation 8 7 to determine the critical time and knowledge of the river s velocity to determine the critical distance k D k2 k1 1 tcrit ln 2 1 0 k1 k2 k1 k L 1 0 mg 4 0 4 day 0 2 day 1 0 4 day tcrit ln 1 L mg 0 4 day 0 2 day 0 2 day 0 2 day 25 L tcrit 2 6 day xcrit 2 6 day 10 km day 26 km c Use Equation 8 5 to determine the oxygen deficit and Dt Dt k1 L0 e k1 t e k2 t D0 e k2 t k2 k1 0 2 day 25 mg L 0 4 day 0 2 day Dt 7 4 e 0 2 day 2 6 day e 0 4 day 2 6 day 4 mg e 0 4 day 2 6 day L mg L 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 d Use Equation 8 2 to determine the actual dissolved oxygen concentration for the critical time as just calculated mg DO 9 7 4 1 6 L 8 6 The wastewater treatment plant for Pine City discharges 1 105 m3 day of treated waste to the Pine River Immediately upstream of the treatment plant the Pine River has an ultimate CBOD of 2 mg L and a flow of 9 105 m3 day At a distance of 20 km downstream of the treatment plant the Pine River has an ultimate CBOD of 10 mg L The state s Department of Environmental Quality DEQ has set an ultimate CBOD discharge limit for the treatment plant of 2 000 kg day The river has a velocity of 20 km day The CBOD decay coefficient is 0 1 day Is the plant in violation of the DEQ discharge limit Solution Determine the initial concentration at the mixing basin from Lt L0 e kL t t 10 x 20km 1 day U 20 km day mg Lo e 0 1 1 L Lo 11 mg L Apply mass balance to determine the ultimate CBOD of the discharged wastewater CBOD5 after mixing Qriver CBOD5 river Qwaste CBOD5 waste Qriver Qwaste 3 3 mg mg 5 m 5 m 2 x 1 10 9 10 day L day L mg 11 3 3 L 5 m 5 m 1 10 9 10 day day Solve for x 92 mg L ultimate CBOD concentration of the wastewater discharge 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 92 mg kg 1000 L 1 105 m3 9200 kg day L 1 106 mg m3 day 9200 kg day 2000 kg day so the plant is in violation 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 8 8 7 An industry discharges 0 5 m3 s of a waste with a 5 day CBOD of 500 mg L to a river with a flow of 2 m3 s …