energies Review A Key Review of Non-Industrial Greywater Heat Harnessing Abdur Rehman Mazhar 1, Shuli Liu 1,2,* and Ashish Shukla 1 1 School of Energy, Construction and Environment, Coventry University, Coventry CV1 2HF, UK; mazhara4@uni.coventry.ac.uk (A.R.M.); ab4322@coventry.ac....