Aox and Ozone

AOX levels in waste water of power plants and paper Industry Ozone solution

Halogenated organic compounds are by products formed when chlorine is used as disinfectant in water and waste water and in industrial processes.

What is it?
Adsorbable organically bound halogens (AOX) are a group of chemicals which can be adsorbed from water onto activated carbon. Most AOXs are compounds containing chlorine, but some also contain the similar elements like bromine or iodine.

The properties of AOXs vary: some evaporate easily (such as trichloromethane or chloroform); some are complex molecules such as dioxins and furans. THMs are part of AOX. AOX once formed cannot be destroyed by any chemical means or process. They have to be removed from water or waste water.

Industry Water Guzzlers.
Both the power plants as well as paper and pulp plant use more than 60% of the total water used in the country. Therefore, they also discharge large quantities of water as waste water. Both the power plants as well as the paper and pulp plants use lots of chlorine in their water treatment/process etc.

Where does it come from?
The main sources of AOX are from the chlorine chemicals used for treating cooling water in power plants and chlorine used to bleach pulp /fibers in the paper and pulp industry. AOX are also formed during the routine chlorination (disinfection) of drinking water, swimming pools and industrial effluents.

How will it affect the environment?
Some AOXs are toxic to fish and aquatic organisms – Even at low concentrations. Many are persistent and have a tendency to accumulate in the environment which cause concerns at global level. AOX once formed remains, either in the ground, polluting water source or in humans when they are consumed with water.

How exposure to AOX affect human health.
Exposure of AOXs to humans can occur through eating contaminated food, drinking contaminated water – and the fact that AOXs can concentrate up the food chain means that humans could be exposed to dangerous levels. The type and severity of adverse effects depend on the AOX in question. Some (such as dioxins) are known to be very toxic. Many AOX/THMs are cancerous to humans and known to be precursors to rectum and colon cancer

In the UK (including Scotland) releases of AOX are controlled through the Pollution Prevention and Control (PPC) regulations. Chlorinated AOXs have been on the European Commission’s “black list” of toxic substances since 1976. In India pollution control board provide levels of AOX that industries must control in their waste water discharge.

For example, according the German classification system regarding AOX levels in river water, a concentration below 5mgCl/l is slightly polluted, 5–20mgCl/l is moderately polluted, 20–40mgCl/l is strongly polluted, and over 40mg Cl/l is very strongly polluted.17 This classification is relatively severe. It is normal to choose the maximum AOX data in river water as the nonpolluting level.

In India, except one paper and pulp plant, no plants have taken conscious efforts to reduce AOX levels. AOX levels can be reduced by replacing part of chlorine in their process with Ozone. (ITC is the first plant) The main by product of ozone chemistry is oxygen. It is an ideal choice to limit the formation of chlorinated byproducts (AOX) formed during conventional bleaching processes. Similarly, power plants have already taken conscious efforts to replace chlorine with ozone in cooling water treatment and in waste water treatment. We have not seen industrial cooling towers adopting ozone technology. It may be noted that not all benefits of ozone can be quantified as many benefits are health saviors

However, the incorporation of ozone as a replacement of part of the bleaching sequence, would depend on many factors including availability of suitable raw materials. The first plant emerged in India in the year 2008 and many plants that went for expansion opted to continue with chlorine as an add on to the chlorine plant, would be more economical than installing a new ozone plant. However, a stage has come that installing a new ozone plant would be more economical than installing a new chlorine plant for the expansion. R & D has been continuing in the use of ozone in pulp bleaching and various concerns of the paper mills such as scaling in the contact tanks, reuse of oxygen, lower power requirement for ozone generation, higher temperature chilled water requirement, use of ozone in recycled paper etc. have been addressed to increase the confidence of paper plants to consider ozone.