The water tank is the one place nobody looks, and every glass of water in the flat comes through it. Within six months there is a layer of silt on the floor of it, algae on the walls and biofilm under the lid.
We drain the tank, lift out the sludge, high-pressure jet the walls and ceiling, apply a food-grade disinfectant, rinse and refill. Photos are taken before and after — useful when the building committee wants proof the job was done.
For building committees: every tank in the building in one day, on a twice-yearly schedule at a contract rate, with a separate invoice each time.