Water containing 5 ppb of microcystin (a toxin produced by algae) is treated to

Question

Water containing 5 ppb of microcystin (a toxin produced by algae) is treated to reduce the concentration to 0.2 ppb. Note that 1 ppb = 1 part per billion = 1E-9 mass fraction.

To treat the water, a solid adsorbent is mixed continuously with the water which adsorbs 0.1 g of microcystin per kg of adsorbent. The solid adsorbent settles by gravity to the bottom of the mixer where it is removed continuously. The treated water is withdrawn continuously from the top. You may assume no water is withdrawn with the adsorbent from the bottom of the tank. Additionally, no adsorbent is withdrawn with the treated water from the top. The process is designed to treat 1 m3/s of inlet water (density = 1000 kg/m3) and operates at steady state.

1.Draw a process flow diagram.

2.Prove the number of unknowns is equal to the number of equations.

3.Prove 0.048 kg/s of adsorbent is required by the process.

Explanation / Answer

1 ppb = 10-6 g/L of the solution

Thus, 5 ppb = 5*10-6 g /L of the solution

0.2 ppb = 2*10-7 g/L of the solution

Thus, required concentration to be reduced = (5-0.2)*10-6 g/L = 4.8*10-6 g/L

Now, flow of water = 1m3/s = 103 L/s

Thus, total mass of microcystin to be removed per second = (4.8*10-6)*103 = 4.8 mg = 0.0048 g

Now, 1 kg of absorbent is required to remove 0.1 g of microcystin

Thus, mass of absorbent required per second = (0.0048/0.1)*1 = 0.048 Kg/s

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