Neco 2019 Chemistry Question 18

A wastewater stream contains 0.025 mol\,L\(^{-1}\) nitrate‑N (as NO\(_3^{-}\)) and 0.015 mol\,L\(^{-1}\) phosphate‑P (as PO\(_4^{3-}\)). A biological denitrification step reduces nitrate to N\(_2\) according to\n\\(2\,\text{NO}_{3}^{-}+5\,\text{C}_{6}\text{H}_{12}\text{O}_{6}+6\,\text{H}^{+}\rightarrow\text{N}_{2}+5\,\text{C}_{6}\text{H}_{12}\text{O}_{7}\\).\nThe same stream also contains calcium ions at 0.030 mol\,L\(^{-1}\), which precipitate phosphate as calcium phosphate according to\n\\(3\,\text{Ca}^{2+}+2\,\text{PO}_{4}^{3-}\rightarrow\text{Ca}_{3}(\text{PO}_{4})_{2}\\).\nAssuming complete nitrate removal and complete phosphate precipitation limited only by calcium, calculate (a) the minimum moles of glucose required per litre, (b) the concentration of calcium remaining after precipitation, and (c) the reduction in chemical oxygen demand (COD) in mg\,L\(^{-1}\), given that oxidation of one mole of glucose consumes six moles of O\(_2\) (M\(_{O2}=32\) g\,mol\(^{-1}\)).