Zero/Minimum Liquid Discharge
Zero liquid discharge (ZLD) is a treatment approach where all water is recovered and waste is reduced to a solid. This can be a difficult and expensive target to achieve, with recovery becoming more difficult as the waste stream becomes more concentrated. Governments sometimes mandate ZLD in environmentally sensitive areas, or sometimes for entire industries (such as mining, power plants or pharmaceuticals), requiring the plant to recycle water and produce only solid waste. Plants may also be located in areas where there is nowhere to discharge water to, and when building storage areas or evaporation ponds either isn’t allowed or isn’t economical. Producing a smaller volume of solid waste decreases the cost involved with waste management, and in some cases the solid waste can be valuable resources that can be reused or sold.
Minimum liquid discharge (MLD) are near-zero liquid discharge processes which produce highly concentrated waste streams. Typically MLD is defined as a process that recovers at least 95% of the water. In some cases it can be more economical to use a MLD solution and store or dispose of this waste, compared to a ZLD solution which can have lower waste costs.
Many treatment processes exist that can achieve ZLD, with the suitability and economic efficiency dependent on the composition of the feed water. Evaporation is one method of achieving ZLD, where the feed water is evaporated until solids precipitate as crystals. This process typically has a high operational cost to heat the feed water, especially when waste heat isn’t available. Other solutions are processes such as electrodialysis, forward osmosis and membrane distillation.
Our EVAPX™ evaporation and crystallisation technology reduces higly concentrated saline (>5%) waste steams in a low temperate, ambient pressure humidification chamber, which evaporates water using a dry air stream. The evaporated water and heat are both recovered in a dehumidification chamber. The low energy process distills the water and leaves behind salts, and the low temperature aspect provides excellent scaling resistance, even for highly polluted waste streams.
Our DESALX® and HIROX® technologies can both achieve MLD (>95% recovery) depending on the quality of the feed water. These processes are both are fouling resistant and have great tolerance to solids and varying feed qualities.
In the mining industry our DESALX® technology has been combined with high density sludge (HDS) technology to achieve ZLD. The DESALX® portion of the system first removes multivalent cations such as heavy metals and hardness, then removes multivalent anions such as sulphate and nitrate, creating a gypsum based brine. The treated water can then be reused or treated further, and the brine is recycled to the HDS system resulting in ZLD.
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