Lec40_Sediment_Oxygen_Demand

Lec40_Sediment_Oxygen_Demand - Lecture 40 Sediment Oxygen...

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ecture 0 Lecture 40 Sediment Oxygen Demand
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Reading Assignment • Read the SOD chapter, in particular, CSOD, NSOD and Total SOD models and numerical SOD models.
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SOD Sediment Oxygen Demand (SOD) is the rate of oxygen consumption exerted by the bottom sediment on the overlying water.
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The Problem ettleable waste in the vicinity of an outfall may result in the Settleable waste in the vicinity of an outfall may result in the formation of "sludge beds" • If the velocities are high, scouring of the river bottom will prevent ild d t d iti build up due to deposition • At low velocities, however, deposits may build up over time • As depth of deposited solids increases, anaerobic decomposition begins • Carbon dioxide, methane (CH 4 ) and H 2 S are formed as a result of is decomposition this decomposition • These gases escape to the overlying water column • At high rates of gas production, the buoyancy may result in the oating of the bottom sl dge creating aesthetic problems floating of the bottom sludge creating aesthetic problems • SOD is not always created due to sewage. Certain benthic organisms such as Sphaerotilus and zerbra mussel also withdraw substantial amounts of oxygen.
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Introduction A zero-order source term was used earlier to model sediment demand: B S dD D P R =- - ¢ + •S B ' has units of g/m 2 .day This approach is inadequate to address how SOD f t t t t i t h dh a kD P R dt H ++ changes after treatment since the zero-order approach treats SOD as an input rather than as a calculated variable. We need to link the sediment organic matter to the di t sediment oxygen.
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Measuring SOD Method 1 (Model based). Develop a DO model and determine all parameters except the SOD. Then adjust the SOD until the model and observations match. Disadvantage: It is difficult to estimate all other parameters with confidence. This introduces higehuncertainties in SOD. Method 2 (Direct "Chamber" Measurement). Can be used in the lab or in situ . The sediment and some overlying water are enclosed in a chamber and the DO concentration in water is measured as a function of time. Disadvantage: The chamber environment can be different from the natural environment.
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Typical SOD values 20 ,20 T BB SS  B S is the areal SOD rate at 20 C. is the temperature coefficient. A value of 1.065 is usually used. B S Typical ranges of values: Outfall vicinity: 2-10 g/m 2 .day Downstream of outfall: 1-2 g/m 2 .day g y Sandy bottom: 0.1-1 g/m 2 .day Areal Hypolimnetic Oxygen Demand (AHOD) - lakes 0.06-2 g/m 2 .day In addition to bacteria, benthic organisms (e.g., zebra mussel) can also exert SOD.
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Lec40_Sediment_Oxygen_Demand - Lecture 40 Sediment Oxygen...

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