Introduction:

All Industries use fresh water for cooling tower make up. The utilized water finds its way into drain and collected for treatment and is called as cooling tower blow down. The major contaminants in any cooling tower blow down are suspended solids, some organic matter (BOD and COD) and dissolved solids. Up to certain limit they are not harmful to natural resources hence could be discharged without removal but while reusing they creates problems such as scaling of pipelines, corrosion, biological contamination and fouling, reaction with other chemicals and changing the quality of product from processes etc. Hence, they need to be removed before reusing and this is where UF/RO plays a vital role.

Cooling tower blow down is recycled and reused as make up water by treating through chlorination, clarification, filtration and by membrane processes like ultra filtration, reverse osmosis etc.

Disinfection is done for the removal of bacterial growth if any. Clarification and filtration is done for removal of suspended solids. Membrane technologies are pressure driven processes used for removal of un-dissolved as well as dissolved impurities present in water. The wastewater recycle treatment plants mainly involves treatment with Ultra filtration (UF) followed by Reverse Osmosis (RO).

A Typical cooling tower blow recycle plant

Chlorination:

Chlorination is done for the removal of bacterial growth and organic matter. It is achieved by the use of oxidizing agents such as chlorine, hypochlorite, chlorine dioxide, etc.

Clarification:

Clarification is a physico-chemical process where the suspended solids are reduced by the use of coagulant and flocculants. It is carried out by the use of Clarifier, lamella clarifier, High rate solid contact clarifier, etc depending upon the inlet and outlet water quality expected.

Filtration:

Filtration is done for reduction of suspended solids to an extent such that the water can be further passed through membrane processes. Thus it acts as a pretreatment for downstream membrane processes. It is done by use of filters like multigrade filter, dual media filter etc.

Dechlorination:

In dechlorination the chlorine carry forward from the upstream disinfection process is removed as well as also reduces BOD to some extent. It is achieved by the use of activated carbon filter, dosing of sodium bisulphite, etc.

>Ultra Filtration (UF):

Ultra filtration is a tangential flow, low pressure driven membrane process that separates particles on the basis of their molecular size. It removes of virtually all-particulate matter, suspended solids, bacteria, viruses and pyrogenic species, colloidal material (non-reactive silica, iron, aluminum etc) and high molecular weight organics. UF also reduces TOC, Colour components, SDI etc. Hence, it is used to improve downstream RO performance with respect to flux rates, recoveries and reduced maintenance, cleaning frequencies and associated costs.

Reverse Osmosis:

Reverse Osmosis is a high pressure membrane process that rejects practically all undissolved and dissolved impurities present in water, wastewater and process liquids. The process employs a semi permeable membrane that allows selective removal of impurities.