Musings Of A Chemical Engineer
Posted On : 26th February 2015

In this column I now address the treatment and recycle of wastewater in chemical manufacturing facilities and also in textile processing. Conventionally, the treatment of wastewater has been performed in four sequential stages i.e. equalization, primary treatment, secondary treatment and tertiary treatment. In a typical chemical factory, the wastewater is collected in an equalization tank, where the various effluent streams are mixed and then subjected to the three treatment stages in the conventional sequence. It seems that some pollution control boards have been insisting on zero discharge from chemical manufacturing facilities. Whereas this can be a desirable goal, it is often impractical to achieve using these sequence. There are, however, ways to improve the discharge quality to US standards. I will describe my experience in developing wastewater treatment schemes for high polluting chemicals manufacture and textile processing.

In late 1990's we were providing complete design and engineering services for a large dyestuff intermediates project. Design of the effluent treatment plant was also included in our scope. The plant was located in an industrial estate near the coast with access to the ocean for treated effluent. The unit processes used in the plant were:

  • Sulphonation
  • Reduction
  • Nitration
  • Halogenation
  • Amination
  • Hydrolysis
  • Alkylation reactions.

We found that it would be impossible to treat the combined effluent and meet the stipulated discharge standards. We therefore decided to study the possibility of segregating all effluent streams and treating them individually or combining only some selected streams before finally treating them. Some of these treated streams could be reused in the process, as make-up cooling water and for washing. We were able to find cost effective techniques to separate objectionable compounds from the various streams such that the individually treated stream were combined and subjected to biological treatment. Some of the unit operations that were used to treat the individual streams were:

  • Ion exchange
  • Membrane separation
  • Liquid-liquid-extraction, Crystallization
  • Thermal concentration
  • Adsorption
  • Biological treatment

As is clearly evident we were only able to succeed in meeting discharge standards because we modified the conventional treatment sequence and because we used unit processes that were not normally used in wastewater treatment.

WE WERE ABLE TO FIND COST EFFECTIVE TECHNIQUES TO SEPARATE OBJECTIONABLE COMPOUNDS FROM THE VARIOUS STREAMS SUCH THAT THE INDIVIDUALLY TREATED STREAM WERE COMBINED AND SUBJECTED TO BIOLOGICAL TREATMENT.

In the days when the large German companies were producing dyes and intermediates, they used to generate effluent streams that contained organics refractory to biological treatment. Naphthalene compounds tend to be resistant to biological treatment resulting in high COD values. Hoechst used to subject these streams to Wet Air Oxidation, whereby the organics get converted to acetic acid, which can then be subjected to biological oxidation. This is another example where segregation of effluent streams was practiced and the treatment sequence was changed to meet the discharge standards. Wet Air Oxidation can be a very useful technique to convert effluent streams containing refractory compounds to acetic acid that can be treated in conventional activated sludge plants.

Decades ago, DuPont developed the PACT process. PACT stands for powdered activated carbon treatment. This process combines biological oxidation via activated sludge with powdered activated carbon and has been reported to be very effective in treating effluent streams containing toxic organics. Wet Air Oxidation can be used to regenerate the activated carbon thereby reducing overall treatment costs. This process can be tested in a laboratory and is relatively easy to scale-up.

It is known that effluent streams containing a relatively high concentration of dissolved salts are not amenable to biological treatment. Such streams need to be segregated and if possible treated separately and can only be mixed with other streams prior to tertiary treatment.

In 1989, I was also involved in designing a wastewater recycling plant for a textile processing plant in Oman. In Oman, water is considered a national resource and water conservation is actively encouraged. The plant used a range of textile auxiliaries, dyes and pigments. Here also we segregated the various streams generated and the unit processes to treat the effluent streams were a combination of:

  • Coagulation,
  • Ion Exchange,
  • Membrane separation,
  • Oxidation,
  • Carbon adsorption
  • Electro-deionization.

The treated wastewater was recycled to the textile processing sections.

Traditionally effluent treatment has been the domain of civil engineers and recently environmental engineers are being employed to design and engineer wastewater treatment facilities. To meet present and future effluent discharge standards, it has become necessary to develop treatment processes specifically for each manufacturing site. This requires the same level of knowledge and skills that are used for chemical process development.

I believe, therefore, it is unlikely that civil or environmental engineers possess the necessary knowledge or training to be able to undertake such involved tasks. Chemical engineers together with Chemists are best qualified to undertake such assignments.

ABOUT THE AUTHOR:

RAJAN SHAH received his BS in 1966 and MS in 1967 in Chemical Engineering from the Massachusetts Institute of Technology (MIT), USA. He then worked as a development engineer with MIT Prof. Dr. Ralph Landau's company Halcon International Inc. (part of the Halcon/Scientific Design Group) in New York. He returned to India in 1968 and joined Aniline Dyestuffs & Pharmaceuticals Pvt. Ltd. (ADP). The company had been started by his father, late Shri. Shivanand J. Shah, who was a past president of ICMA (now ICC) and past Chairman of CHEMEXCIL. In 1990 Rajan started Chemet Design & Engineering (I) Pvt. Ltd. to provide design, engineering and construction services for the chemical process industries. He moved to Australia in 1998 and has continued to provide design and process consulting services in Australasia, India and USA. Since 2005, Rajan has also been associated with Biospecialties International Pty. Ltd. as CEO, where he was engaged in developing the manufacturing process for a novel peptide for use as an ingredient in health supplements.

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