Minimal Liquid Discharge: Adopting A 'Less Is More' Mindset
Fei Wu
Regional Marketing Manager- RO/NF
Dow Water & Process Solutions

Minimal Liquid Discharge (MLD) is a cost effective water management approach that helps increase recovery and also mitigates discharge. If zero liquid discharge (ZLD) is too costly and the associated infrastructure is a latent deterrent, water reuse and sustainability can still be prioritized and realized through MLD. It can assist municipal and industrial plants to make substantial gains in minimizing liquid discharge and help plants considerably abate their capital and operating costs, since achieving the final 3-5 percent of liquid elimination to accomplish ZLD can prove contemptibly expensive. The article deliberates in-depth on benefits of adopting MLD approach over ZLD approach. It also explains its role in reducing CAPEX, OPEX and other environmental impacts.

Industry challenge
Due to urbanisation and industrialisation, India faces major challenges in the water sector with the per capita availability going down and demand going up. Inefficiency of water use in agriculture, over-exploitation of underground water and contamination are other issues associated with water management in India. Since the amount of water available is more or less constant, rising demands due to increasing population and economic growth are straining the demand-supply balance. Also with environmental norms becoming increasingly stringent and water becoming a "scarce commodity", treating water to make it reusable has become an imperative for water intensive industries across the globe. As we are heading towards developing a circular economy, where raw materials are recycled and reused, many industries realise that the wastewater they generate could well be their cheapest & most reliable source of water and MLD technology can lead the way in that direction.

Why MLD?
Maximizing the efficient use of water by industry is critical for both the environment and the bottom line. Depending on individual regulatory and environmental needs and requirements, as well as the capital and operating budget, MLD can be a good fit for many industrial and municipal sites. By conducting a water audit to match waste streams and appropriate water requirements, companies can better identify how much wastewater requires processing and the approach that will most economically and sustainably match their plant's needs. MLD can help municipal and industrial plants make substantial gains in minimizing liquid discharge and help plants significantly minimize their capital and operating costs, since achieving the final 3 to 5 percent of liquid elimination to achieve ZLD can prove outrageously costly. (See graphic below.)
Cost Comparison Of Minimal Liquid Discharge (MLD) VS Zero Liquid Discharge (ZLD)

Benefits of MLD over ZLD
While zero liquid discharge (ZLD) - a water treatment process in which all wastewater produced is purified and recycled, leaving zero liquid discharge at the end of the treatment cycle - is a viable solution for some companies; it's not the most realistic solution for all. Due to the additional material , energy and disposal resources they typically require, it is technically daunting, highly expensive and not necessarily environmentally friendly. Hence when facing significant discharge mitigation costs, businesses often wonder, "What are my other options besides ZLD?" One solution is minimal liquid discharge(MLD), a more cost-effective and sustainable way for companies to improve their water footprint, enabling up to 95 percent liquid discharge recovery at a fraction of ZLD's costs. Coined by Dow, the processes on which it is based rely on proven water filtration technologies such as ultrafiltration, reverse osmosis (RO), and nanofiltration.

Minimal Liquid Discharge

An excellent example of an MLD mind-set in action can be found at the General Motors(GM) vehicle assembly plant in San Luis Potosi, Mexico. The plant, which employs up to 1,800 and has an annual capacity of 160,000 cars, is located in an arid, remote area with no receiving stream or municipal sewer available to discharge wastewater. Yet by using a combination of RO technology, a proprietary high-rate chemical softening process, the plant can convert up to 90 percent of its tertiary wastewater into reusable water, leaving less than 10 percent of liquid waste for discharge into adjacent solar ponds for evaporation. MLD provides users with a spectrum of options to achieve the results most optimal for a particular plant, based on a variety of factors. For some, it could be 90 percent, for others, it could be 80 or even 70 percent.

Diversity of Dischargers - MLD Solutions
The diversity of types of discharge streams across industries, markets and municipalities calls for MLD solutions tailored to individual needs. Potential discharge sources and opportunities for MLD-driven improvement include

Cooling tower blowdown - in heavy industry and power plants
Ion exchange regenerative streams - particularly in food and beverage processing Flue gas desulfurization - wet wastewater stream
Municipal potable water systems - wastewater streams
Process water reuse - from agricultural, industrial and municipal streams
In general, all of these discharge sources can be broken into two main themes: volume streams and complexity streams. A tailored approach for MLD will depend on whether it's tackling a volume stream, a complex stream, or some kind of combination, as well as the geography.

New MLD-related technologies are enhancing these efforts. For e.g., for municipal desalination plants, it's possible to get to 93 percent recovery using a traditional dual-membrane treatment system, chemical adjustments and precipitation. It's typically a combination of brackish water reverse osmosis, nanofiltration and seawater reverse osmosis. To dispose the remaining concentrate, for many desalination plants, the preferred method of concentrate disposal is discharge into the ocean and if that is not an option, alternatives vary by geography.

In the United States, which primarily has small-scale desalination plants, the primary locales for concentrate disposal are evaporation ponds and injection wells. But in much of Asia, given its population densities and environmental factors, surface- or injection-based concentrate disposal are not viable options. In such instances, a reverse osmosis system is typically used to pre-concentrate the brine. Newer technology enhancements for this process include disk tube reverse osmosis, in which an open-channel structure RO system is fed by a turbulent feed water flow system. Through this process, the system can avoid settling of suspended solids carried in the feed water inside the membrane module.

Another newer technology variant is the use of forward osmosis, which uses a water permeable, salt-rejecting membrane to separate water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient, so that a "draw" solution of high concentration (relative to that of the process feed solution) is used to induce a net flow of water through the membrane and into the draw solution. This process helps eliminate the need and use of high-pressure pumps and can thus potentially operate using a lower energy footprint than evaporative technologies.

Reduced CAPEX, OPEX and Environmental Impact
The cost of membrane-based pretreatment and filtration technologies applied in MLD is relatively minimal compared with the exponentially greater capital and operating costs of ZLD-related evaporators and crystallizers. That’s why the advent of new technologies, such as disc-tube reverse osmosis and forward osmosis, is so important to users. They not only extend recovery but can also minimize or even eliminate the size or need for evaporators and crystallizers. Waste heat can be used from some of these operations to provide additional benefit to an MLD approach.

Evaluating MLD Needs
So what is the best way to determine if your plant is a candidate for MLD? Often, the first step is to conduct a water audit to identify the sources and types of wastewater generated in a facility, especially in terms of flow and total dissolved solids content. By matching waste streams and appropriate water requirements, you can better identify how much wastewater (if any) requires processing and the approach that will most economically and sustainably match your plant's needs. For e.g., some wastewater sources, such as condensate and storm water require minimal cleaning while those with high concentrations of organic compounds, salts, metals and suspended solids , will likely require more rigorous treatment. Similarly, cooling water and process water do not need to be treated to the same standards as potable water. The most important factor is that an MLD approach enables your plant to address the substantial 'limbo' that exists between minimizing discharge, and go-for-broke ZLD.

Real-Life MLD Approach Benefits: India Due to considerable effluent discharge by the textile mills of Tiruppur, Tamil Nadu, the Madras High Court, in February 2011, decreed that all dyeing units in Tiruppur would have to shut down for violating pollution norms and would only be reopened when they implemented zerodischarge protocols in order to protect the surrounding farmlands and rivers. In their effort to comply with regulation, minimize environmental impact while reducing operating costs, the mill owners partnered with Dow Water & Process Solutions (DW & PS). As a result of this engagement, the research team in DW & PS developed a new product line called FORTILIFE™, which is designed to handle the most challenging water applications such as waste water reuse and MLD and give these applications the reliability and efficiency they badly need. With success of the existing pilots, the FORTILIFE™ products are now well on their way to be rapidly adopted by the industry.