Waste
Once you have finished using materials or equipment you should take steps to see if a) the materials or equipment can be used elsewhere (i.e. prolong the useful life) or b) safely dispose of it in an environmentally responsible manner.
The waste issue is also linked to the purchasing or acquisition phase as the cost of disposal and the environmental impact of disposal can be minimised by acquiring materials and equipment with good design features (e.g. purchase materials and equipment that has been designed for ease of dismantling for recycling and that uses components manufactured to minimise the use of hazardous waste).