impact of water recycling and clean techniquesReading time:
Two successive initiatives from industry have resulted in reduced volumes of discharge and of pollution flows.
Recycling was first imposed in cooling applications in order to reduce water consumption. Its next target was discharge control. Recycling has been very thorough in steel-making (instantaneous consumption of 200 m³.t–1 of steel for make-up volumes of 5 m³ or even 3 m³ per tonne) and in paper manufacturing (reductions from more than 50-100 m³.t-1 to less than 5). This was made easier by the fact that the main pollution is insoluble and can be removed using one single physical-chemical process located on the circuit or, more frequently on a by-pass.
This recycling and/or re-use has produced smaller but more concentrated effluent outputs even when the total pollution flow is almost always reduced.
Becoming part of a "sustainable development" approach, producing less pollution while improving production output is a challenge that has been set to industry by those in charge of the environment.
Many actions have been undertaken and have been successful and here, obviously, we shall quote the most pertinent examples dealing exclusively with liquid effluent.
When a gas emission is replaced by a liquid discharge produced from a washing process, this replacement must not result in transferred pollution and must be justified on the grounds that this aqueous discharge is easier to purify or because the wash water can be recycled direct to the process (case of ammonium nitrate plants).
These actions can be identified by a number of features:
- eliminating effluent by creating new "dry" processes; this method can be contemplated, for instance in surface treatment applications:
- replacing chromeplating with hot ionic nitriding;
- replacing cadmium-plating by an ionised vapor aluminium application;
- replacing galvanisation with rilsanisation (plastic);
- using supercritical CO2 instead of organic solvents, or even water;
- separation and, if necessary, recovery of toxic or costly dissolved raw materials, for instance:
- solvents separated by distillation in paint (HC) and in sulfonated resins (dichloro-ethane) manufacturing, in pharmaceuticals (ethanol) and in tawery (oil industry);
- protein, latex (agri-food industry – paper coatings) recovery via ultrafiltration membranes;
- trivalent chromium in chromeplating baths, fixed onto resins and then recovered as chromate, chromic acid from passivation baths with the eluates being recycled in the baths;
- separation of compounds in suspensions created by the production process with possible re-incorporation into the process:
- sedimentation sludge, even biological sludge to the paperboard industry;
- oils from food refineries and margarine producers to soap makers...;
- grease and proteins from abbatoirs to the manufactuer of animal foods...;
- separation of dissolved compounds that are synthesised in the processes:
- ammonium from coking plant formation water to amino acid production, in digestion returns …, separated by air or vapour entrainment and then recovered through condensation or sulphatation.