Report

Ohio Northern University CE 3231 - Introduction to Environmental Engineering and Science Introduction Chemistry, Microbiology & Material Balance Water & Air Pollution Dissolved Oxygen Sag Curve When organic carbon is placed into a stream environment, the dissolved oxygen drops following a characteristic pattern. The resulting “sag curve” is able to be modeled to predict the minimum oxygen concentration expected and the point along the river stretch where the deficit will occur. Env Risk Management Readings for This Class: 5.5-5.6 Lecture 25 DO Sag Curve – Streeter Phelps Model (Water Quality V) BOD and DO Definition of BOD Calculating BODx from equation DO Sag Curves Simple Plug Flow River Model Objective: derive a mathematical model to predict DO downstream: DO(x,t) Wastewater discharge (High BOD, Low DO) Assumptions: 1. completely and uniformly mixed in the cross flow and vertical direction 2. neglect pollutant dispersion Formulation: Mass balance DO replenish from air A control volume in the river A A: cross section area (m2) u: flow velocity u DOin Δx DOout DO consumed due to oxidation Formulation: Mass balance Formulation: Mass balance Formulation: Mass balance k d L0 kd t krt krt D e e D0e kr kd Critical Point • Time/Distance @ DO=Domin – Minimal occurs when dDO/dt=0 – Taking the sag equation with respect to t and setting it equal to zero, then solve for t kr kr kd tc ln 1 D0 kr kd k d L0 kd 1 Distance downstream where DOmin occurs = tc x u u: flow speed of the river Streeter Phelps solved for Dmax Streeter Phelps solved for critical time ö æ k L -kd t -kr t -kr t d 0 + D0e ÷÷ e -e D max = çç ø è kr - kd ( ) kr kr kd tc ln 1 D0 kr kd k d L0 kd 1 In-class Problem The initial BOD of a river just below a sewage outfall is 25 mg/L. The oxygen deficit just upstream from the outfall is 2 mg/L. The deoxygenation rate coefficient kd is 0.4/day, and the reacation rate coefficient kr is 0.7/day. The river is flowing at a speed of 20 miles/day. You have been asked to determine the impact of this single source on the river: a) Find the critical distance downstream at which DO is a minimum a) Find the minimum DO kr kr kd tc ln 1 D0 kr kd k d L0 kd 1 Solution (a) æ 0.7 æ 1 0.7 - 0.4 öö tc = ln ç ç1- 2× ÷÷ = 1.65days 0.7 - 0.4 è 0.4 è 0.4 × 25 øø dc = tc × u =1.65days× 20miles / day = 33miles In-class Problem The initial BOD of a river just below a sewage outfall is 25 mg/L. The oxygen deficit just upstream from the outfall is 2 mg/L. The deoxygenation rate coefficient kd is 0.4/day, and the reacation rate coefficient kr is 0.7/day. The river is flowing at a speed of 20 miles/day. You have been asked to determine the impact of this single source on the river: a) Find the critical distance downstream at which DO is a minimum a) Find the minimum DO ö æ k L -kd t -kr t -kr t d 0 + D0e ÷÷ e -e D max = çç ø è kr - kd ) ( Solution (b) æ 0.4 × 25 -0.4×1.65 -0.7×1.65 ö -0.7×1.65 D max = ç e -e + 2× e ÷ = 7.36 è 0.7 - 0.4 ø ( ) DO min = DOsat - Dmax =10 - 7.36 = 2.65mg / L In-class Problem The initial BOD of a river just below a sewage outfall is 25 mg/L. The oxygen deficit just upstream from the outfall is 2 mg/L. The deoxygenation rate coefficient kd is 0.4/day, and the reacation rate coefficient kr is 0.7/day. The river is flowing at a speed of 20 miles/day. You have been asked to determine the impact of this single source on the river: a) Find the critical distance downstream at which DO is a minimum a) Find the minimum DO Streeter Phelps solved for Dmax Streeter Phelps solved for critical time ö æ k L -kd t -kr t -kr t d 0 + D0e ÷÷ e -e D max = çç ø è kr - kd ( ) kr kr kd tc ln 1 D0 kr kd k d L0 kd 1 What happens when there is no initial deficit? What happens where the organic loading increases? What happens if the river increases velocity?