There are basically three processes by which it can be done:
(i) Physical Processes (ii) Chemical Processes (iii) Biological Processes
(i) Physical processes:
In physical processes, the solid and liquid components can be separated through sedimentation of the solid in a sedimentation tank, followed by filtration for solids which do not settle and float.
Liquid wastes which do not settle or float can be separated through adsorption in granular activated carbon having surface area as large as 1000 m2/g.
Removal of some specific components can be achieved by reverse osmosis, ion exchange process, dialysis, electro-dialysis and simple distillation process.
(ii) Chemical processes:
In chemical treatment, various techniques adopted are:
Acidic waste water (pH2) is neutralized by adding lime slurries, and alkaline waste water (pH12.5) is neutralized by adding either acid directly or by passing C02 through the solution, the H2C03 thus formed catalyses the reaction. When the pH of the solution approaches 7 (near neutrality), it becomes less hazardous.
(b) Oxidation – reduction:
Materials with some elements of variable oxidation states differ in toxicity. In other words, the oxidation states of some specific elements in a material determine its toxicity.
Under such conditions, if the lower oxidation state of that element shows greater toxicity, it needs to be converted to higher oxidation states by oxidation. On the other hand, if the higher oxidation states of that element show greater toxicity, it needs to be converted to lower oxidation states by reduction.
For example, cyanide containing waste needs to be oxidized to cyan ate to reduce toxicity and finally to nitrogen and C02 for complete reduction of toxicity.
In NaCN, the oxidation states of C and N are respectively +2 and -3 and when converted to NaCNO (cyanate) by alkaline chlorination, oxidation states of C changes to +4 and finally with excess chlorination oxidation states of N become zero and there is a complete reduction of toxicity.
The wastes containing benzene, toluene, phenol, arsenic as toxic materials can be detoxified through oxidation.
In case of material with hexavalent chromium, Cr6+ is found to be more toxic compared to trivalent chromium Cr3+. Thus, hexavalent chromium is reduced to trivalent chromium by passing S02 in the medium.
More toxic less toxic:
The oxidation states of chromium changes from +6 to +3, reducing toxicity.
The wastes containing chromium (IV), lead, silver, mercury can be detoxified through reduction.
(c) Chemical precipitation:
In this process, the soluble toxic heavy metals like Pb, Hg are precipitated out as sulphides due to their low solubility. Although this is an efficient process, there remains the possibility of formation of toxic H2S gas.
The other alternative process could be precipitation as hydroxide at a specific pH. Thus, the metal ion (M2+) reacts with Ca(OH)2 and removes M(OH)2,leaving Ca2‘ ion in the solution which is not toxic.
The precipitate so formed can be removed by settling and filtration.
In this technique the hazardous liquid waste is forced through a column of solid resin beads of zeolites or any synthetic resin. In the ion-exchange unit, the toxic heavy metals get exchanged with sodium ion of resin.
(e) Ultraviolet radiation oxidation:
This is an excellent technique of destruction of dissolved organic compounds. In this case, a potent oxidizer, hydroxyl free radical (OH) is generated. Ozonized solution is exposed to UV rays and OH forms readily which simultaneously oxidizes Land Pollution and Control organic molecule to C02 and water.
(iii) Biological processes:
In the biological process, the microorganism (bacteria, fungi) mainly bacteria, decomposes the organic substances like oils, phenols and refinery waste to C02. Although the living organisms are generally susceptible to toxic substances in hazardous wastes, for some microorganisms under specific temperature, pH of solution and concentration of toxic chemicals is an ideal condition for survival.
They consume the heavy metals and as well as decompose organic wastes into C02 and H20, thereby reducing overall toxicity.