Thanks 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 that 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 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 compared with 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 environmentally friendly discharge, and a solid residue suitable for further processing (often to recover valuable components for use elsewhere) or for safe disposal.
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, preprocessing 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 desulfurisation, 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.
Vapour compression evaporation is a common method used in ZLD as evaporation can recover up to 95% wastewater as distillate. Any remaining concentrate is further treated physically or chemically to produce solid residues (such as crystals) and water.
By running the evaporators at lower pressures, the boiling point of the liquid being treated is reduced. This means that multi-effect evaporation can be made possible; that is, steam from a previous evaporation stage is used as thermal energy in the next stage which works at a lower boiling point.
This way, multiple evaporation stages are combined, generating significant energy savings. For many components, crystal precipitation is favoured at lower temperatures, therefore lowering evaporation temperatures helps to increase the solids yield.
ZLD is certainly not the only waste treatment solution available: it will not be suitable for all situations and it is not a panacea for the treatment of hazardous wastes. However, in the right situation it can play a key role in reducing waste volumes or preparing waste streams for final, safer, disposal.
Heat exchangers play a crucial role in reducing the running costs of a ZLD system by utilising heat from process water and other existing sources, and also recapturing heat at the end of the process and reusing it to boost the energy efficiency of the overall ZLD system.
Where there is a hazardous liquid waste stream to deal with, then the potential to utilise ZLD techniques as part of the overall treatment solution should definitely be investigated.