Zero liquid discharge (ZLD) is a liquid waste stream treatment. It involves transforming liquid waste streams from industrial plants into clean water, which can be reused in the process, and the minimum amount of solid residues, which often include valuable by-products that can be sold or reused.
It is believed that the tightening of environmental regulations in the United States in the 1970s led to the development of the first ZLD units, which used various combinations of evaporation and crystallisation. With increasingly high environmental standards being implemented by companies, coupled with sometimes onerous regulation, the technique is now becoming more and more popular around the world. It can also help businesses deal with challenges such as raw material costs and water scarcity.
ZLD is suitable for a wide range of industries, including power production, chemical and fuel refining, mining, distillation, food production, and waste treatment, and a variety of equipment is available to treat different waste streams and processes. However, this diversity is also one of the drawbacks of ZLD, as every system must be designed on its own merits, considering factors such as the contamination or chemicals present in the water, flow rate, how pure the returned water needs to be, etc. This makes it impossible to design a ‘general purpose’ ZLD system, and makes the necessary custom-made solutions more expensive.
The effective design of any ZLD system is dependent on the correct analysis of the water/waste stream. It is essential to have accurate estimates of composition, flow rates, chemistry, etc. Without this, any designed solution will fail to deliver the required results, if it works at all. For example, calcium, ammonium and some heavy metal salts are difficult to crystallise by evaporation, and so other treatment techniques will need to be incorporated into the overall system design.
Although every ZLD system will be different, many will comprise a pre-treatment phase, an evaporation phase to remove most of the water, and a further concentration or crystallisation phase to produce the final solid residue. Pre-treatment often focuses on removing organic elements and any chemicals which could damage evaporation or other equipment later in the process. As ZLD has become more widely used, more and more techniques have been employed, with varying levels of success. Most common water treatments, such as pH adjustment, flocculation, membrane processing, degasification, oxidation, separation and even aerobic and anaerobic digestion, have all been used as pre-treatments for ZLD systems.
Traditionally, vapour compression evaporation has been the main method employed for ZLD processing, 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.
Working with a reduced boiling point means that multi effect evaporation can be made possible. In multi-effect evaporation, 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 and as a result energy savings are obtained. For many components, crystal precipitation is favoured at lower temperatures, therefore lowering evaporation temperatures helps to increase the solids yield.
Whatever kind of evaporator is employed, 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.
ZLD in action
HRS is currently installing a ZLD system for an industrial client in Europe. This contains the following three process steps:
- Evaporation/concentration: A concentrated solution is taken from an existing multi effect evaporator to a second (new) evaporator, resulting in a solution containing elevated salt levels very close to the saturation point.
- Cooling: The solution is then cooled to provoke the formation of salt crystals.
- Crystallisation: Further crystallisation occurs in specially designed crystallisation tanks, with separation of the crystals that are formed. A supernatant layer of concentrated solution remains after this stage, and is returned to the second evaporator for reprocessing.
Both the evaporation and cooling steps result in a high degree of material fouling on the inside of the equipment, so HRS Unicus Series scraped-surface evaporators are used to maintain thermal efficiency and remove fouling as it occurs in the evaporation process. HRS R series scraped surface coolers are also used for cooling the crystal loaded slurry that is obtained in the crystallisation tanks. The result is an efficient process which can work continuously without requiring scheduled downtime.
Removing waste, reducing costs
“The level of concentration required for Zero Liquid Discharge systems of this type means that the product concentration will have to be above the saturation point, and so solid crystals will appear in the solution,” says Arnold Kleijn, HRS Heat Exchangers’ Product Development Director. “Solid-liquid mixtures are difficult to handle in a process installation; therefore, a research study was done to evaluate the nature of the saturated product. An operation point was found where solid’s precipitation in the evaporation step could be avoided (evaporating just under saturation point) and crystal formation was handled by cooling in crystallisation tanks. The evaporator concentrates and removes pure water from the solution, which can be used elsewhere. The coolers and crystallisers produce solid crystals and the remaining solution returns to the evaporation process. No liquid waste stream remains after the process. This way, any waste management costs are reduced to an absolute minimum.”