37 construction, repair, maintenance, and installation of waste water and sewage treatment plants; solid waste treatment and disposal units and air pollution control units; water purification units, water purifying units for potable water for domestic and commercial use; water distillation units; steam/electric radiators for heating buildings; furnace boilers, gas boilers; air conditioners, air purifiers for commercial and industrial use, air ventilators, refrigerators, steam generators for commercial use; waste disposal services for others; collection and disposal of domestic and industrial refuse, residues and sludge