A wastewater sample contains 120 mg L⁻¹ phenol and a COD of 300 mg L⁻¹. It is treated in two successive stages. The first stage is an adsorption process that removes 40 % of the phenol and reduces the COD by 25 % of its initial value. The second stage is a biological treatment that removes 70 % of the phenol remaining after the first stage and 60 % of the COD remaining after the first stage. The effluent standards are phenol ≤ 10 mg L⁻¹ and COD ≤ 50 mg L⁻¹.

(a) Determine whether the combined treatment meets both standards.
(b) If the phenol standard is not met, calculate the minimum overall percentage removal of phenol required in the second stage (keeping the COD removal unchanged) so that the phenol concentration just meets the standard.

Question

A wastewater sample contains 120 mg L⁻¹ phenol and a COD of 300 mg L⁻¹. It is treated in two successive stages. The first stage is an adsorption process that removes 40 % of the phenol and reduces the COD by 25 % of its initial value. The second stage is a biological treatment that removes 70 % of the phenol remaining after the first stage and 60 % of the COD remaining after the first stage. The effluent standards are phenol ≤ 10 mg L⁻¹ and COD ≤ 50 mg L⁻¹.

(a) Determine whether the combined treatment meets both standards.
(b) If the phenol standard is not met, calculate the minimum overall percentage removal of phenol required in the second stage (keeping the COD removal unchanged) so that the phenol concentration just meets the standard.

Accepted Answer

The treatment does not meet either standard. To meet the phenol limit the second stage must remove at least 86 % of the phenol (≈16 % more than the 70 % originally assumed).

Neco Model Questions Vol1 2024 Chemistry Question 16

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