Abstract
Industrial wastewater management is an increasingly important environmental and economic challenge due to tightening discharge regulations, coupled with increasing water scarcity, climate change and rising costs, especially energy. Regulators are looking to Zero Liquid Discharge (ZLD) solutions as a sustainable strategy for water management. The aim is to eliminate liquid effluent discharges and to convert the contamination into easier-to-manage solid waste.
Existing ZLD systems typically rely on highly energy-intensive evaporation and crystallisation technologies, with associated high capital and operating costs.
The innovative Low temperature Ambient pressure Technology (LAT) process, which has been proven in full-scale operation at a variety of industrial users, offers an alternative approach, using low-grade waste heat to recover high levels of clean water compared with legacy systems. The low cost and high water recovery rates mean that LAT Technology has a high potential to form the key part of ZLD systems in the future, across a wide range of industrial uses.
Introduction
Industrial activities generate large volumes of wastewater containing salts, organic compounds, ammonia, heavy metals and other contaminants. Regulators are increasingly requiring industry to clean up its water discharges, reduce or eliminate liquid discharges, and reuse the water used in the industrial process.
Zero Liquid Discharge (ZLD) is a strategic wastewater management system and a key part of the circular economy, ensuring that there is no discharge of industrial wastewater into the environment. It is achieved by processing the wastewater through recycling, then recovery and reuse for industrial purposes. Hence ZLD is a closed loop with no discharge. ZLD is currently a complex and costly process, but it paves the way for economic benefits by recovering salts and other chemical compounds from the solid waste produced. A diverse group of countries are progressively mandating ZLD for a variety of reasons, driven principally by increasing water scarcity.
Different methods are currently used to achieve ZLD. These involve maximising water recovery through processes such as reverse osmosis (RO), followed by evaporation and crystallisation. RO is relatively simple and cheap to operate, but struggles to deal with water containing different classes of compounds and can only recover around 50% at best of the clean water in the effluent. Further processes, usually evaporation, crystallisation and centrifuging, are then carried out to drive off more water. This is especially challenging given the high energy demands of these processes.
Principle of LAT Technology
LAT is an integrated evaporation treatment solution for highly contaminated waste streams, using low-grade waste heat as an energy source. The LAT system partially evaporates an incoming waste stream by heating it with the available waste heat and fragmenting the effluent into a cascade of small droplets in a sealed evaporation column. The water vapour is transferred into a second condensation column, where it condenses into hot distilled water. The heat is collected from this pure water and used to preheat the incoming stream and residual concentrate.
The design of the evaporation–condensation towers, together with their interconnections and the control algorithm that maximises water recovery, is the subject of a series of international patents; the control algorithm and mathematical model are kept confidential. The waste heat can be supplied by a wide range of sources, such as exhaust gases, cooling liquids and renewable energy sources.
The system is economic for facilities requiring treatment of up to 25,000 m³/day. Based on operating experience, where waste heat is available LAT can generate total cost savings for clean water recovery of up to 40% compared with legacy processes.
The system has been proven at a variety of sites in the UK and China, achieving clean water recovery of over 95%.
To prevent volatile discharges, suitable carbon filters are installed at bleeder valves.
Application of LAT Technology to ZLD
Given LAT Technology's experience in achieving high levels of water recovery, there are obvious advantages in using it in ZLD applications. The constraint on water recovery for LAT is when the concentrate becomes too viscous or reaches the saturation level for the key contaminants. For example, in the simple case of a salt (sodium chloride) solution, the saturation point is approximately 36%, with little variation by temperature. In this case the LAT system can be run to achieve a concentration just below 36%, and the concentrate extracted for final drying.
If a simple reverse osmosis system were used, the maximum achievable concentration would be around 8%; that is, the LAT design would extract 30% more water, with a consequent fall in the energy and cost required for final crystallisation. Crystallisers require of the order of 35 kWh of electrical power to dry one cubic metre of water, so the savings are substantial.
"Crystallisers require of the order of 35 kWh of electrical power to dry one cubic metre of water, so the savings are substantial."
Initially, a standard drier or crystalliser could be used to achieve ZLD. However, LAT Technologies is researching a number of innovations and developments to the core LAT unit to enable further efficiency gains.
Conclusion
LAT Technology offers a simpler and cheaper method of achieving ZLD from industrial water compared with legacy systems. LAT Technology is undertaking research into the overall economics of a ZLD system, using the LAT facility as its core, over the next 24 months, with the objective of rolling out a commercial, scalable solution.
References
- LAT Water Limited. Low Temperature Ambient Pressure Unit: product information.
- LAT Water case study: waste heat powered treatment of leachate. Industry Europe, "LAT Launches Pioneering Landfill Wastewater Technology".
- Tong, T. and Elimelech, M. (2016). The Global Rise of Zero Liquid Discharge for Wastewater Management. Environmental Science & Technology, 50(13), 6846–6855.
- Ahmed, M., Shayya, W., Hoey, D., Al-Handaly, J., Bruggeman, A. and Nayar, K. (2000). Use of evaporation systems in zero liquid discharge applications. Desalination, 130, 155–168.