Principles of non-organic waste water treatment.

Physico-chemical or non-organic waste water treatment usually converts unwanted dissolved components into insoluble matter which can be separated from the water rather easily, be it by sedimentation or by filtration. There’s one common exception: when cyanide is present, this will be converted into nitrogen and carbon dioxide and thus in non-toxic gases, leaving no solid residue.

Most non-organic pollution can be made insoluble quite readily. Often, it’s just a matter of pH adjustment. For instance, many heavy metals are only soluble at extreme pH values. In etching or pickling baths for example, the metal remains in solution because of the low pH (high acid content) of the bath. Usually the bath itself will not be treated in a WWTP, because of the high acid and metal content it would consume a lot of chemicals and produce lots of sludge after neutralization. But almost always, metallic parts that have been immersed in such a bath, are rinsed afterward in order to remove the concentrated etching liquid from the metal surface. This rinse water contains lots of harmful substances and therefor needs to be treated in a WWTP.

Often a simple pH correction is enough to make the toxic components insoluble. The easiest way of neutralizing an acid waste water is done by adding caustic soda. Though in waste water treatment, in general hydrated lime is used instead, because:

On the other hand, hydrated lime has also some disadvantages:

In some cases, a simple pH correction doesn’t do for removing toxic components. For example if the waste water contains hexavalent chrome (Cr6+) or monovalent copper (Cu1+) or cyanide, those substances must be subjected to a redox (oxidation-reduction) conversion, in order to neutralize the toxicity and to make these matters insoluble. One can compare a redox correction with a pH neutralization, only we use oxidizing/reducing substances instead of acid/caustic products. Thus we can for example convert hexavalent chrome into trivalent chrome (Cr3+), which is far less toxic and also much easier to convert in insoluble components.

In general, after detoxification and neutralization, a coagulant is added to the treated waste water in order to improve the settling and filter- ability of the sludge. Sometimes (this depends on the local effluent standards and on the pollutant itself), a straightforward neutralization doesn’t reach the discharge standards. For example in Europe, the limit value for heavy metals (Ni, Zn, Cr, Pb, Cu) is currently 0,5mg/l. This is quite difficult to meet, in particularly for zinc and nickel. Therefor a supplementary treatment might be necessary. The most reliable way of removing the traces of heavy metals is by filtration over a selective ion exchange resin.

So in most non-organic waste water plants, we’ll find the following items:

Though the principle of this waste water treatment may seem quite simple, it takes a lot of engineering skills and a solid experience to build a reliable and cost-effective plant.

The most recent development in non-organic waste water treatment consists of a closed loop system: this is a process-integrated waste water treatment, in which after treatment, the water is reused and thus no waste water is discharged in the environment anymore. Only dewatered sludge (filter cake) and –in some cases- concentrated chemical baths are disposed off for treatment in a specialized facility. It goes without saying that this kind of treatment is quite costly, as well with regard to the initial investment as with regard to the exploitation.

View of a physical-chemical water treatment plant