Neco 2023 Chemistry Question 20

A wastewater treatment plant receives \(5000\) L of effluent containing \(120\) mg L^{-1} of ammonia‑nitrogen (\(\mathrm{NH_3\text{-}N}\)) and \(80\) mg L^{-1} of phosphate (\(\mathrm{PO_4^{3-}}\)). The plant operates as follows:\n1. A nitrification step converts \(85\)% of the ammonia‑nitrogen to nitrate‑nitrogen (\(\mathrm{NO_3\text{-}N}\)).\n2. A denitrification step removes \(90\)% of the nitrate‑nitrogen formed.\n3. A chemical precipitation step removes \(75\)% of the phosphate as calcium phosphate, using calcium hydroxide \(\mathrm{Ca(OH)_2}\) (M = 74.09 g mol^{-1}).\n\n(a) Determine the final concentrations (mg L^{-1}) of total nitrogen (as \(\mathrm{N}\)) and phosphate (as \(\mathrm{PO_4^{3-}}\)) in the treated water.\n(b) Calculate the mass of \(\mathrm{Ca(OH)_2}\) required per litre of effluent to achieve the phosphate removal.