Evaluating the Impact of Closed-Loop RO Systems on ZLD in Textile Industry: Case Study from Tirupur, India

The textile industry is known for high-water consumption and substantial wastewater generation, with dyeing contributing the largest share. Dyeing wastewater is a complex mix of salts, dyes, cleaning agents, and fixing agents, posing severe environmental risks if discharged untreated. In Tirupur, Tamil Nadu, a hub for textile dyeing, untreated wastewater caused significant groundwater and surface water contamination in the early 2000s. In response, the Madras High Court mandated zero liquid discharge (ZLD) systems for dyeing units. While this regulation curbed pollution, it led to increased operational costs, forcing many facilities to shut down or relocate, while others adopted ZLD at significant expense. Currently, central effluent treatment plants (CEPTs) in Tirupur treat 100 000 m³/day, ensuring ZLD compliance. These systems recover up to 98 % of water and 75 % of salts, depending on the salts used and treatment technologies. Field visits to dyeing units revealed wastewater with total dissolved solids (TDS) ranging from 5 to 10 g/L, primarily sodium sulphate, sodium chloride, and sodium hydroxide. After biological treatment, effluent undergoes RO treatment, which is key to water recovery. Most units employ 3-4 RO stages: brackish water membranes in the first stages, followed by seawater membranes. RO recovery rates typically range from 85-90 %, with specific energy consumption (SEC) of 4-7 kWh/m³. However, many units suffer from suboptimal designs, which cause energy loss, higher operational costs, increased maintenance, and elevated capital costs. This study evaluates the benefits of a semi-batch closed-loop RO (CLRO) system through theoretical analysis of three dyeing units, using a modeling tool developed at Fraunhofer ISE. Results show that CLRO achieves recovery rates of up to 95%, reducing brine volume by 50% compared to the 90% recovery of existing systems. Despite the higher recovery rate, there is a reduction in SEC up to 75 % by CLRO system. Additionally, CLRO decreases system size, requiring 30-35% fewer membrane elements. The system also adapts to variable feed volumes by adjusting RO membrane recovery rates and recirculation flow in CL mode. This optimization significantly lowers ZLD costs, reduces dyeing expenses, and enhances the competitiveness of dyeing units equipped with ZLD systems.