Across the world, we use more than 9 billion cubic metres of water daily – of which 56% becomes wastewater. Usually locally contained, nearly 80% of the wastewater goes untreated and is left to be managed by natural biogeophysical processes. Unfortunately, nature can’t keep up with the pace of human influences, and as result, our limited global freshwater resources are becoming more challenged by pollution and scarcity.
UN Sustainable Development Goal 6.3 aims to improve water quality by halving the proportion of untreated wastewater and substantially increasing recycling and safe reuse globally by 2030. How can advances in wastewater treatment technology help achieve this?
Let’s take the case of zero-liquid discharge (ZLD) wastewater treatment. It is an extreme wastewater treatment process that maximizes water recovery for reuse and avoids discharging contaminants. However, ZLD has traditionally relied on thermal treatment methods, making it cost-prohibitive and unsustainable with high energy demand.
Despite this, in water-stressed regions like China and India, many industries have been driven to adopt ZLD to comply with stringent regulations. As such, advanced approaches were needed to improve sustainability (and affordability) for achieving ZLD. Evaluating how these industries, such as the textile industry in Southern India, have been able to implement ZLD without going bankrupt provides valuable learnings.
With more than 10 years of commercial practice, textile wastewater treatment to achieve ZLD has been optimized on two fronts. Firstly, the energy demand and cost of the total water recovery process were reduced by approximately 52% by using an energy-efficient reverse osmosis membrane process where 85-90% of the wastewater is recovered for reuse. The process, referred to as Minimal Liquid Discharge (MLD), reduces the volume of water needing the more energy-intensive and expensive thermal ZLD treatment to only 10-15%. Coupling MLD with ZLD lowers the cost for total water recovery from approximately $3.48 per cubic metre to $1.76.
By adopting a value-driven refinery mindset, industries will increase the adoption of technologies that will help us achieve SDG 6.3
Secondly, they make the entire wastewater treatment process more attractive and affordable by capturing value for not only the water, but also the salts. Both are recovered for reuse in the textile process for a combined reuse value of $1.06 per cubic metre of wastewater treated. Together, using MLD and circular economic benefits, the cost of achieving ZLD could be reduced from $3.48 per cubic metre to $0.70.
The textile industry has pioneered an MLD-ZLD wastewater treatment process with salt and water recovery which can be managed and optimized much like a petrochemical refinery that refines petroleum. Impaired water can be refined into fractions that have reuse value that offsets the cost of treatment.
Other industries can learn from this and should consider shifting from treatment to a refinery mindset when managing industrial wastewater. Unlike treatment processes, where the objective is defined by local regulations, refinery processes are value driven and aim to generate an income from the process stream that outweighs the expenses. For wastewater, this income can be water value, salt value, recovered organic raw materials, or even biogases.
Proven technologies exist for practicing water refining – including membrane bioreactors, ion exchange resins, ultrafiltration membranes, and reverse osmosis and nanofiltration membranes. These technologies offer the complementary benefits of both resource and water recovery. Advances in the energy and separation efficiencies of these technologies coupled with adaptable processes guided by digital monitoring will serve to further optimize the refinery process.
By adopting this sustainable and value-driven refinery mindset, industries will increase the adoption of processes and technologies that will help us achieve SDG 6.3 to maximize the benefits and reduce the risks related to such a large percentage of wastewater going untreated.