The biological effluent treatment plant is one of the most preferable technology in the world for its low operating cost and durability. Though capital expenditure at the beginning is quite higher than other Physico-chemical treatment plant. Still, most of the textile industry owners in Bangladesh are going for replacement of their conventional effluent treatment plant with 100% biological effluent treatment system.
But the biggest challenges that factories are facing regarding biological effluent treatment plant is the removal of color. The outlet wastewater after treatment does not seem very transparent as like Physico-chemical treatment plant. Though the legal requirements for color in Bangladesh or in most of the countries in the world are not so stringent but still it’s a matter of mental satisfaction and aesthetical issue.
So, a common question is arising from factories- then what is the way of removing color in biological effluent treatment system?
First of all, we have to understand the reason for color in wastewater. There could be two reasons- undecomposed/ un-oxidized suspended solid and the presence of soluble solids which show optical color properties. For textile wastewater- I can say mostly the first one, i.e. suspended solid is responsible. Normally biological effluent treatment system is not able to decompose/ oxidize (biologically) all of the suspended solids present in the wastewater, especially those which contain complex chemical structure, high molecular weight and have longer half-lives.
One can imagine easily that some of the new generations of organic dyes contain all of these behaviors. In some special case-biological effluent treatment plant with modern technology is allowing very high number microbial colony (very high MLSS), the induced oxidation process can breakdown and decompose most of those, however, still, there is something which is not suitable for a micro-organism to decompose-this is the reason for color. But obviously, there are some acceptance limit in regulations for color, such as ECR’97 for BD allow 15 co-pt. And I believe the after treatment wastewater of advance biological process is able to deliver wastewater within this limit.
If you want the complete removal of color in conventional biological treatment plant- you have to go for the chemical color removal process after biological treatment. Chlorine, H202, O3 (or any other commercial color remover). The use of Chlorine is very effective but it itself is very hazardous-proper caring is required to handle this-and of course after chlorination, the wastewater needs to go through the dechlorinating process to remove residual amount.
H202 is also very active regent for color removal but as hazardous as Chlorine, but the removal of residue may not be required. O3 is the safest decoloring agent which does not remain any residue but the generation of Ozone is a costly process. I prefer Chlorine gas to use in this regard, because it also kills micro-organism in the outgoing wastewater. And Chlorine gas can be found as a byproduct from industrial synthesis (e.g. HCl plant.). But a big remark for that-“Proper Caring is required to Handle Chlorine gas and Dechlorination is Mandatory.”