The application of water treatment technologies to achieve “minimal liquid discharge” (MLD) or “zero liquid discharge” (ZLD) is increasingly under consideration in today’s water stressed world. RO and IonExchange have been successfully implemented in textile and chemical industries.
MLD and ZLD are processes that minimize or eliminate the discharge of water or liquids from a plant process. The reasons to implement these processes can be
• Cost saving from water recycling, and reduction of water discharge costs
• Environmental – Increase sustainability of industrial sites in water stressed regions
There are two treatment options for process water: internal recycling, and the end-of-pipe treatment.
The internal recycling is conducted inside the plant processes, and implemented by analyzing the water composition and the water quantity to make a so-called “pinch point” analysis. In this analysis, determined for a specific production step, and a decision is made as to whether it is possible to use the wastewater from a certain production step as the water supply for a separate production step. If usage is not possible, an interception technology may be used to get the water quality to the level suitable so that it can be reused.
A typical technology for this is membrane technology. Depending on the type of contaminants, it can be microfiltration (MF) to remove bacterial substances, ultrafiltration (UF) to remove particulate and high molecular weight organics, nanofiltration (NF) to remove divalent salts and organics, or reverse osmosis (RO) to remove salts and small organic compounds. Another technology that can be used is ion exchange treatment (IX). This technology should be applied if a selective separation is preferred, e.g., to remove specifically some ionic contaminants from the process water or to recover valuable ionic or non-ionic substances selectively. This separation can be achieved using a special IX product called scavenger, selective resin or adsorber resin.
When interception technology is not possible, the client should consider an end-of-pipe treatment solution. In this case, the plant wastewater is collected and treated depending on the type of contaminants by either mechanical, biological, or filtration steps to produce a water quality that can be reused in an upstream plant process.
In MLD/ZLD processes, it is common that both interception and end-of-pipe treatment options can be applied for overall water management. Selecting the treatment process depends on the size of the plant, and whether the treatment process is added to an existing plant. If an MLD/ZLD process implemented to an existing plant, in most cases, an end-of-pipe treatment is preferred since less new piping, and less interference with the existing process is required.
In the following industrial examples, two MLD (minimal liquid discharge) processes, which were implemented in existing plants in India, are presented. These examples represent two different industries: the textile and the chemicals industry.
This zero liquid discharge wastewater treatment plant is located inside the water treatment facility section of a major specialty chemicals producer. The wastewater plant processes treated sewage water via UF (ultrafiltration) pretreatment, cartridge filtration, and RO membrane desalination. The desalinated water is further treated by degasification, and ion exchange mixed bed columns installed with Lewatit®MonoPlusS108H and MonoPlusM800 to prepare boiler feed water. The RO installation is a two-stage system with a capacity of 38 m3/h. The feed water to the RO has a TDS in the range of 600–1,300 mg/l. The permeate has a TDS below 20 mg/l, and after mix bed application, a final boiler feed water quality of < 0.1mg/l TDS and 0.02 mg/l silica.
The concentrate is further treated by high pressure RO. The permeate of this concentrate RO is reused in the process, and, finally the brine concentrate is treated via an evaporation step to achieve the zero liquid discharge operation.