Increasing interest in ZLD driven by costs and environment

Membrane can only work to bring the product up to a certain concentration. To achieve complete separation, evaporation / crystallization processes are needed for completing the process. As explained before, evaporation (due to the latent heat) is highly energy consuming. Therefore, it is wise to choose an evaporation process that involves ways of energy optimization, the most popular being:

• Multistage evaporation: using the latent heat of the evaporated water as energy source in a next evaporation stage reduces the overall consumption of the boiler to the evaporation plant.

• Thermal Vapor Recompression (TVR): evaporated steam is mixed with boiler steam. The reuse of the evaporate steam reduces the energy demand.

• Mechanical Vapor Recompression (MVR): An MVR compressor (driven by an electrical motor) can be used to compress the evaporated steam, thus increasing its pressure, and use this steam as the energy input for the process. MVR compression is very efficient in terms of energy consumption.

Due to the factors outlined above, (multistage) vapour compression plants remain the main method employed for ZLD processing globally, with evaporation typically recovering around 95 per cent of wastewater as distillate. Any remaining concentrate is then further treated physically or chemically to produce solid residues (such as crystals) and water. Evaporators used in ZLD systems are often run at lower pressures in order to reduce the boiling point of the liquid being treated.

The HRS ZLD solution

Depending on the product to be concentrated, HRS can select from a series of technologies for designing the most optimal ZLD process. Energy optimization methods (multistage, TVR, MVR) can be combined with several types of heat transfer technologies (plate evaporators, corrugated tube evaporators, scraped surface evaporators). Whatever the technology applied, the overall process can be separated into three steps:

1. Evaporation / concentration: The product is concentrated to just below its maximum concentration (saturation). The evaporation plant is usually a multistage evaporator setup.

2. Cooling: if the maximum solubility curve is steep (large concentration at high temperature, low concentration at low temperature), the product obtained in step 1 is cooled, provoking immediate precipitation of dissolved solids.

3. Crystallisation: Crystallisation / sedimentation of the solids produced in step 2 occurs in specially designed crystallisation tanks. A supernatant layer of concentrated solution remains after this stage and is returned to step 1 for reprocessing. For products without a steep solubility curve, it is necessary to concentrate inside the evaporator to above the maximum solubility. This means that the step 1 process is equipped with a final evaporator stage (finisher) that is specially designed to work with suspended solids. The fluid with suspended solids is then transferred directly to the crystallisation tanks in step 3.

Despite the economic climate we have seen steady growth in the demand for e-waste recycling and the export of processed materials. We have also been able to maintain strong relationships with our suppliers overseas. It’s important to note that e-waste is a global issue, and the need for responsible disposal and recovery of materials remains a priority regardless of economic conditions.

The brine cooler and evaporator finisher work with solids in suspension and often this means dealing with fouling products. A typical HRS evaporator / finisher will use Unicus scraped surface evaporators that are self-cleaning and maintain optimal evaporation rates.

Typically, our R series scraped surface coolers are used for cooling the saturated brines that are send to the crystallization tanks. The result is an efficient process which can work continuously without requiring scheduled downtime.

Whatever kind of evaporator is employed, heat exchangers have a crucial role to play in ZLD systems in reducing running costs 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.