maintenance, repair, unblocking, cleaning, clearing, descaling, sterilizing, disinfecting, fumigating and deodorising of drainage and of piping systems, sewers, manholes, gratings, culverts, gullies, flues and channels for the passage of liquids, solids or gases; cleaning and clearing of pipework systems used in the manufacturing and refining industries; clearing of tree roots; cleaning and degreasing of exterior and interior floor, ceiling and wall surfaces; cleaning by means of sanding, scouring or blasting; hygienic cleaning of buildings, tanks, and pools