PS1_ESM202_2008_fin

# PS1_ESM202_2008_fin - Assignment 1 ESM 202 Winter 2008 Due...

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1 Assignment # 1 ESM 202, Winter 2008 Due Tuesday 1/22/08 I. OXYGEN DEMAND IN POLLUTED RIVERS NOTE: THIS PROBLEM IS IN A “WORKBOOK” FORMAT. WORK THE PROBLEM SECTIONS IN ORDER. THE QUESTIONS ARE IN “BOLD”. SHOW YOUR CALCULATIONS. SHOW UNITS. Background problems When the dissolved oxygen (DO) drops below approx. 5 mg/L in rivers, aerobic aquatic organisms including fish are stressed and some cannot survive. Organic waste (sewage and certain industrial wastes) discharged into rivers “demands” oxygen. Wastes are biodegraded by oxygen-utilizing (aerobic) microorganisms, and this depletes the DO that aerobic organisms need. Rivers are modeled as systems that mix across their width, not along their axis of flow. As river water flows from upstream to downstream, wastes that have entered upstream are decomposed by microbes. This consumes DO. Simultaneously, due to river turbulence, water is reaerated (i.e., DO is added back to the water). The “balance” of DO depletion by waste-eating microbes and reaeration determines how far downstream the river will be oxygen-depleted and just how “bad” it will be for the fish and other organisms. The removal rate of DO by microbes is expressed as follows: Deoxygenation rate = d(DO)/dt = -k d L t ( 1 ) where DO = dissolved oxygen, mg/L k d = deoxygenation rate (1/day) L t = BOD remaining (the BOD in the river), mg/L a. Explain Eq. 1., i.e. describe in writing what controls how fast DO changes. Is DO decreasing or increasing? Explain why a “minus sign’ is needed on either the left or right side of the equation. The organics in the waste that microbes metabolize are measured as biochemical oxygen demand (BOD), and BOD changes over time as microbes consume it: L t = L 0 e -k d t ( 2 ) where L 0 = ultimate BOD (total BOD of waste) k d = decay constant (1/day)

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## This note was uploaded on 08/06/2008 for the course ESM 202 taught by Professor Keller during the Winter '08 term at UCSB.

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PS1_ESM202_2008_fin - Assignment 1 ESM 202 Winter 2008 Due...

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