| On Topic
The role of Zero Liquid Discharge in reducing hazardous wastes By Matt Hale, International Sales & Marketing Director, HRS Heat Exchangers
hanks to tighter environmental regulations and greater public awareness, companies are increasingly looking to reduce or eliminate the waste that they produce. In recent years, Zero Liquid Discharge (ZLD) has become an important waste reduction technique, but its potential in dealing with hazardous waste streams has not been fully appreciated until now. Hazardous waste is waste which is dangerous or potentially harmful to the environment or human health. It can come in any form: solid, gaseous, sludge or liquid. Many vital materials such as cleaning products, pesticides and industrial chemicals are, by their nature, hazardous and therefore present disposal challenges. Traditionally, hazardous wastes have been treated by a number of different physical, thermal, chemical and biological methods, including precipitation, high temperature incineration and even burial in specialist secure sites. However, in parts of the world, particular
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10 | Farming Monthly | February 2019
industries have become associated with environmental pollution through the inappropriate disposal of hazardous wastes, with the textiles industry in India and South East Asia being such an example; one which has attracted NGO interest and the development of new cleaner waste disposal schemes. One of the advantages of ZLD over other treatment techniques is its theoretical ability to separate unwanted materials from water, whether they are benign, hazardous or toxic. The resulting solid residue is often more stable, making it suitable for recycling or landfill. A well-designed ZLD system should minimise or even eliminate liquid waste streams, resulting in clean water for reuse or environmentallyfriendly discharge, and a solid residue suitable for further processing (often to recover valuable components for use elsewhere) or for safe disposal. Correct analysis is crucial The composition of wastewater streams varies greatly, even where the same basic processes are involved. Certain wastewater sources, such as power plants and boilers with
wet gas scrubbing, often contain salts which may be hazardous or valuable, or even both. Environmental regulation usually means that treatment is required to reduce or remove such toxic compounds before wastewater can be discharged. Where the initial wastewater is relatively dilute, pre-processing can also be required prior to the evaporation phase, often using common water treatment techniques such as reverse osmosis. Other sources, such as wet flue gas desulfurization, may contain highly soluble calcium and aluminium salts, as well as heavy metals, which are not easily crystallised by evaporation. Such sources therefore need significant pre-treatment, often using lime or soda ash to introduce sodium ions so that a crystalline solid can be produced by the evaporation stage. The effective design of any ZLD system is therefore dependent on the correct analysis of the water/waste stream, making it essential to have an accurate analysis of composition, flow rates, chemistry, etc. Without this, any designed solution will fail to deliver the required results, if it works at all.
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