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Introduction

Zero-liquid discharge (ZLD) is a robust wastewater management solution that utilises a sequence of advanced treatment processes to minimise wastewater volumes, increase water recycling, and concentrate waste solids to the greatest extent possible. Numerous different treatment processes and combinations of technologies  can be utilised as part of a ZLD system, but a common denominator among the vast majority of available ZLD options includes a large thermal evaporation stage that is extremely energy-intensive, meaning high and often prohibitive operating costs. This challenge is driving the need for innovative ZLD solutions that offer effective and reliable treatment, and also overcome cost barriers to implementation, energy use, and operation.

Environmental regulations & water scarcity driving ZLD use

In global regions with high to extreme water stress, and in countries where intensifying growth and industrialisation present mounting risks to water quality, industries that use large volumes of process water and generate highly contaminated waste streams are increasingly required to adopt measures that reduce water consumption and thereby mitigate the impact on receiving waters. These trends, which are evident in China, India, the USA and elsewhere, are driving the introduction of stricter environmental regulations that mandate the use of ZLD.

By enabling a “closed-loop” solution that virtually eliminates discharge and facilitates the maximum reuse of water in process operations, ZLD is receiving ever greater recognition as a “best available” solution for managing the most difficult-to-treat industrial wastewaters.

In China, ZLD systems are now mandatory in new and existing coal-fired power plants and coal-to-chemical refining facilities for treating complex industrial waste streams. This requirement is a result of China’s 12th Five-Year Plan, which includes objectives to enhance environmental protection. In India, ZLD is mandatory across many industrial sectors including thermal power, steel, pulp & paper, pharmaceutical, chemical, textile, and food and beverage, as the country looks to mitigate surface water pollution and enhance industrial water efficiency in drought-prone, water-stressed regions.

ZLD requirements were also recently adopted in the USA for the steam electric power generating category. Last year, the U.S. Environmental Protection Agency finalized new Effluent Limit Guidelines (ELGs) that require ZLD for all pollutants associated with fly ash transport water, bottom ash transport water and flue gas mercury control wastewater in power plants that are 50 megawatts or higher.

As governments continue to place more emphasis on the need to protect water resources, address rising concerns related to water scarcity, and target heavy metals and other toxic contaminants in wastewater discharges, the use of ZLD systems is anticipated to increase.

ZLD cost barrier

ZLD systems typically integrate pre-treatment and pre-concentration steps that can include reverse osmosis (RO), biological treatment, ion exchange, ultrafiltration, and other advanced treatment processes followed by a thermal evaporation and crystallization stage, which works to concentrate and thereby minimize waste volumes. But, due to the high energy demand associated with thermal evaporative technologies, operating costs for ZLD systems can be very high and most often represent the single largest expenditure in ZLD adoption. For this reason, reliable and cost-effective solutions are increasingly needed to overcome this prevailing cost barrier and enable for more affordable and sustainable long-term ZLD operation.

Since thermal evaporative processes can account for more than 90% of the total operating cost for a ZLD plant, innovations that aim to improve the operational cost-efficiency of ZLD systems should target the evaporator stage in order to realise the most optimum cost reduction benefits.

ZLD cost solution

Hager + Elsässer, an Aquarion Group company, has addressed this cost challenge by developing an innovative ZLD hybrid system that optimizes the energy efficiency of ZLD operation by minimizing the thermal requirements of the evaporation stage. The ZLD.eco2 system employs a series of innovative filtration and separation technologies to reduce the residual waste volume to a minimum, thereby significantly decreasing the subsequent evaporation requirement.

As a robust solution for managing heavy-loaded, contaminated wastewater from industrial processes, ZLD.eco2 includes pre-treatment, ultrafiltration, low-pressure RO stages, and a new high-pressure RO stage utilising Circular Disc (CD) Module technology from Membran-Filtrations-Technik (another Aquarion Group company) to concentrate non-organic waste material far more than is normally achievable. With waste volumes minimised, the size of the evaporator is also minimised, thereby reducing capital investment. In addition, since the volume needing evaporation is much reduced, the energy requirement is also far less, resulting in much reduced operating costs.

When compared to conventional systems without pre-concentration of the wastewater, ZLD.eco2 uses approximately 20 percent less energy and, since the evaporation system is smaller, less cooling energy is needed for the condensation process. In total, this can result in as much as 80% saving in energy requirement. Even more savings result from the use of heat exchangers that recover thermal energy which is subsequently re-used, again boosting overall energy efficiency.

About Aquarion Group

A total, enhanced EPC solution provider, the Aquarion Group offers advanced water treatment technologies and systems for the global process industries, thereby providing solutions to today’s critical water and wastewater needs. The group specialises in addressing the most challenging water quality issues and in designing advanced and sustainable water treatment solutions, all provided by its group companies in many different regions throughout the world.

Offering engineered recycling and reuse solution, including ZLD, for difficult-to-treat industrial wastewaters, Aquarion Group has a successful track record across numerous industrial sectors including power generation, oil & gas, pulp & paper, chemicals, semiconductors, pharmaceuticals, food & beverage, aerospace & transport, and mining where wastewater solutions include acid mine drainage treatment, precious metals recovery, tailings volume reductions, and removing hazardous and toxic elements such as arsenic and cyanide.

A key advantage of Aquarion Group’s solution platform includes its capacity to design, build and operate customized, field-deployable and packaged treatment units that are engineered for durability and treating the most problematic waste streams in extreme climates across the globe.