EXPLORING THE "ZERO LIQUID EFFLUENT DISCHARGE" WATER MANAGEMENT PHILOSOPHY

Industrial sites use water for various purposes, but in most instances an effluent stream is generated. The effluent is typically a composite of various individual effluent streams, and can vary with respect to volume, chemical characteristics and physical characteristics, such as temperature. There are a number of industrial organisations I have worked with who seek to eliminate this effluent discharge. This is known as a “zero liquid effluent discharge” approach. 

While that definition seems simple enough, implementing ZLED can be a costly and challenging exercise. In this post I will examine why organisations pursue the approach and what the implications for water management are.

It should be noted that while point effluent discharges from an industrial site can be eliminated, this does not necessarily mean that no liquid wastes are discharged to the environment. Contaminated aqueous waste streams can still arise due to seepage from impoundments, uncollected leachate from solid waste storage facilities or uncontrolled releases and spills. Water and effluent transfer infrastructure can also fail, leading to leakage of both contaminated effluents and water of good quality. In some cases, these spills could mean the loss of water of superior quality to that supplied (since water quality may have been upgraded using specialised treatment facilities), which can be very costly. The implementation of ZLED therefore requires consideration of a far broader range of issues than the elimination of known effluent streams produced on an industrial site, and must consider operations and maintenance factors in an integrated way.

Organisations seeking to implement ZLED generally have the following motivations:

• They wish to recycle as much water as possible, thereby reducing the amount of fresh water consumed. This reduces the strain on local water resources, and for large water users can also have a significant impact on the extent of the infrastructure needed to supply water and the impacts associated with its operation.
• They wish to completely avoid waterborne pollution arising from effluent streams. This reduces negative impacts on the environment and on downstream water users.

The pursuit of ZLED is therefore driven by considerations associated with water use efficiency and water quality impacts. One or more of these considerations may be in turn driven by a need for regulatory compliance. Alternatively, water management may simply be high on the organisation’s sustainability agenda. Economics will also be a factor as water prices and the penalties for pollutant discharges increase. Whatever the motivation, it has to be appreciated that ZLED comes at a cost. The question to be answered by organisations considering its implementation is whether the costs are less than the benefits. These costs and benefits have to be considered from a long term perspective, and are not only financial but also social and environmental.

The economic costs associated with ZLED typically concern the treatment costs associated with improving the quality of effluent streams to a level that is sufficient to allow their reuse. This could entail the use of a range of technologies as well as the ongoing operational costs associated with their use. The benefits to the implementing organisation are a reduction in water usage (and hence the costs of supply) and the avoidance of effluent charges. In a broader sense, there is clearly a significant environmental benefit in the form of reduced pressure on local water resources as well as reduced levels of water pollution, making more water available to other water users. In developing countries where many people may lack access to piped potable water supplies, pollution may have very serious social consequences through impacts on human health as vulnerable communities access raw water resources.  It should be clear therefore that the benefits of ZLED are many, and would have particular appeal for large water users and those that produce harmful effluent streams.

Be wary however of falling into the trap of thinking that ZLED is a panacea for industrial water management issues. An industrial site could implement ZLED, but still employ wasteful water management practices, such as excessive evaporation. Such a site could be placing more stress on local water resources than an equivalent site which discharges effluent of a quality level that makes the effluent available to downstream water users. Bear in mind also that water treatment processes all generate some form of waste, which could be in the form of concentrated contaminants, organic sludge, additional discharges (e.g. when regenerating treatment media) and/or local emissions of GHG’s. Contaminants removed or generated during treatment would need to be disposed of safely, failing which they could still enter local watercourses. Treatment processes also require energy and chemicals, with consequent life-cycle impacts. The point is that in evaluating whether ZLED is the right option to pursue, the risks need to acknowledged and mitigated.  You then need to reconcile the expected end-state of the ZLED implementation with the reasons you wish to implement ZLED in the first place.

A final thought is that not all industries can implement ZLED at acceptable financial cost, since recycling is ultimately about sources and sinks, and these have to be considered against the backdrop of the costs of treatment. It may however be possible to generate a partially treated effluent stream that is of acceptable quality for use as a supply to the processes of a nearby site, and in this regard, collaboration with local industries can assist with individual sites attaining ZLED, or at least significantly reducing discharges. Of course, you will then need to take a keen interest in the fate of the water supplied in order to ensure that the strategic objectives of your ZLED implementation are indeed met.