Air oxygen (A) dissolves in a shallow stagnant pond and is consumed by microorganisms. The rate of the consumption can be approximated by a first order reaction, i.e. rA = −kCA, where k is the reaction rate constant in 1/time and CA is the oxygen concentration in mol/volume. The pond can be considered dilute in oxygen content due to the low solubility of oxygen in water (B). The diffusion coefficient of oxygen in water is DAB. Oxygen concentration at the pond surface, CAo, is known. The depth and surface area of the pond are L and S, respectively.
a. Derive a relation for the steady state oxygen concentration distribution in the pond.
b. Obtain steady state oxygen consumption rate in the pond.
(This is transport type problem. Please answer it completely and correctly)

The value of L will be equal to the square root of the diffusion coefficient of oxygen in water times the reaction rate constant. The steady-state oxygen consumption rate in the pond is given by: Q = S*rA = −S*kCAo*2πL2.

a. Steady-state oxygen concentration distribution in the pond: Air oxygen (A) dissolves in a shallow stagnant pond and is consumed by microorganisms. The rate of the consumption can be approximated by a first order reaction, i.e. rA = −kCA, where k is the reaction rate constant in 1/time and CA is the oxygen concentration in mol/volume. The pond can be considered dilute in oxygen content due to the low solubility of oxygen in water (B). The diffusion coefficient of oxygen in water is DAB. Oxygen concentration at the pond surface, CAo, is known. The depth and surface area of the pond are L and S, respectively.

The equation for steady-state oxygen concentration distribution in the pond is expressed as:r''(r) + (1/r)(r'(r)) = 0where r is the distance from the centre of the pond and r'(r) is the concentration gradient. The equation can be integrated as:ln(r'(r)) = ln(A) − ln(r),where A is a constant of integration which can be determined using boundary conditions.At the surface of the pond, oxygen concentration is CAo and at the bottom of the pond, oxygen concentration is zero, therefore:r'(R) = 0 and r'(0) = CAo.The above equation becomes:ln(r'(r)) = ln(CAo) − (ln(R)/L)*r.Substituting for A and integrating we have:CA(r) = CAo*exp(-r/L),where L is the characteristic length of oxygen concentration decay in the pond. The value of L will be equal to the square root of the diffusion coefficient of oxygen in water times the reaction rate constant, i.e. L = √DAB/k.

b. Steady-state oxygen consumption rate in the pond: Oxygen consumption rate in the pond can be calculated by integrating the rate of oxygen consumption across the pond surface and taking into account the steady-state oxygen concentration distribution obtained above. The rate of oxygen consumption at any point in the pond is given by:rA = −kCA.

The rate of oxygen consumption at the pond surface is given by: rA = −kCAo.

Integrating the rate of oxygen consumption across the pond surface we have: rA = −k∫∫CA(r)dS = −k∫∫CAo*exp(-r/L)dS.

Integrating over the surface area of the pond and substituting for the steady-state oxygen concentration distribution obtained above we have: rA = −kCAo*∫∫exp(-r/L)dS.

The integral over the surface area of the pond is equal to S and the integral of exp(-r/L) over the radial direction is equal to 2πL2.Therefore,rA = −kCAo*S*2πL2. The steady-state oxygen consumption rate in the pond is given by:Q = S*rA = −S*kCAo*2πL2.

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