New method cuts overall cost for ZLD wastewater treatment

In this review, water recovery using MLD is maximised using spiral wound ultra-high pressure reverse osmosis elements (UHP RO) (i.e., elements designed to operate at feed pressures up to 120 bar) to efficiently desalinate hypersaline brines.

Industrial water users in Tamil Nadu, India are finding it more challenging to manage their water utility because of reduced access to reliable, low-cost water sources, as well as the higher costs associated with water discharge. As a result, investing in wastewater reuse systems to recover and recycle 70 per cent or more of the water in textile plants has become a common practice. The region’s textile industry has been employing minimal liquid discharge (MLD) using reverse osmosis (RO), followed by evaporation for nearly 10 years to meet local zero liquid discharge (ZLD) requirements. In this review, water recovery using MLD is maximised using spiral wound ultra-high pressure reverse osmosis elements (UHP RO) (i.e., elements designed to operate at feed pressures up to 120 bar ) to efficiently desalinate hypersaline brines.

The challenge

An aggressive wastewater management approach is critical to minimalise freshwater withdrawals and eliminate wastewater discharge, both of which can be achieved via ZLD. Unfortunately, in some of India’s textile plants, brine management practices in place are either expensive or environmentally unsustainable. Typically, when ZLD is mandated, inefficient and costly thermal evaporation processes are used. Advanced RO elements designed to treat challenging brine waters are needed to achieve more reliable, higher-recovery MLD processes to lower the cost of ZLD systems.

The system

Currently, the process for the tertiary treatment of wastewater using MLD and ZLD applications for Common Effluent Treatment Plants (CETP) starts with a three-stage RO system, followed by mechanical vapour recompression (MVR), and then multi-effect evaporation. The total dissolved solids (TDS) concentration after RO is approximately 50 g/L, and would, theoretically, be around 100 g/L at the MVR stage. After the MVR stage, wastewater passes to the multi effect evaporator, which raises TDS up to 250-300 g/L. There are disc-type RO systems competing with MVR systems to increase the salinity to 100-120 g/L, but both alternatives have technical and commercial challenges in terms of installation and operation.

Key benefits

• UHP RO membranes are a cost-effective alternative in textile MLD/ZLD wastewater applications compared to costly all-thermal evaporation processes
• UHP RO membranes can potentially eliminate the MVR phase
• UHP RO membranes can achieve reject concentrations of 105 g/L

The solution

A CETP textile wastewater treatment plant in Tiruppur, a city in Tamil Nadu, carried out an 80-day field trial in cooperation with the end user, a system integrator and a consultant, to better understand the flow, recovery and salt rejection performance of UHP RO membranes. These advanced RO elements are designed to treat high concentration brine waters that have previously been concentrated by RO elements operated at conventional pressures. The elements help operators squeeze out even more water for reuse and further reduce the amount of water sent to the ZLD system. The UHP RO element used has a spiral wound design with a pressure rating of up to 120 bar.

The results

The field trial was conducted with a single 4-inch diameter UHP RO element in a single-element RO system. The membrane was subjected to four phases of operation. The first phase was simply treating the water from the RO reject of a textile wastewater treatment plant, which had high TDS (58.6 g/L, comprising a 0.68:1 mixture of sodium chloride and sodium sulphate) along with the severe fouling conditions – including high chemical oxygen demand (COD = 1024 mg/L) and high colour (1,400 Pt/Co) – that are common in textile wastewaters. Because only a single element was used in this pilot, in order to evaluate the element performance at higher TDS levels additional sodium chloride and sodium sulphate were added to the feed in increments. This achieved a phase II TDS level of 68-80 g/L, a phase III TDS level of 83-85 g/L, and a phase IV TDS level of 94-105 g/L while maintaining the 0.68:1 sodium chloride to sodium sulphate composition. During each phase the rejection and the required feed pressure to achieve an operating flux of 10 lmh litres per meter squared of membrane was monitored.

• The field trial demonstrated the efficacy of UHP RO elements in challenging, high pressure MLD and ZLD applications at operating pressures up to 100 bar and feed TDS levels as high as 105 g/L.
• Colour removal and >98 per cent TDS rejection was achieved when treating water TDS levels as high as 105 g/L.
• Despite high fouling feed conditions of high salinity, COD and colour, the normalised differential pressure observed throughout each phase of the test was maintained at about 0.2 bar.

The benefits

The potential of Spiral Wound UHP RO membranes lowers the overall system cost by more than 50 percent in comparison with the MVC and drastically reduces the economic burden in MLD/ZLD treatment applications. This trial also highlighted the high energy efficiency of UHP RO compared with thermal processes, such as MVC, in an aggressive waste management approach of ZLD. In general, MVC would need two to three times more energy than UHP RO.

The water-intense textile industry has long been a contributor to the growing water crisis in India. With a competition for resources and costs being driven by local government regulations – including strict Zero Liquid Discharge (ZLD) standards – textile plants face challenges to manage their water utility. Looking to maximise recovery, efficiency and profitability, a CETP textile wastewater treatment plant in Tirupur implemented a field trial using DuPont Water Solutions’ Spiral Wound Ultra-High Pressure Reverse Osmosis (UHP RO) elements to desalinate hypersaline brines. The trial found that UHP RO membranes reduce overall ZLD system cost by 50 percent, while increasing operational efficiency in challenging, high-pressure MLD and ZLD applications. Read the full success story here.