ZLD compliance for a Gujarat dyes-intermediates plant

In Q2 2024, GPCB issued a ZLD directive to a mid-scale dyes-intermediates manufacturer in Gujarat's chemical belt. The plant processing reactive and disperse dye intermediates was generating approximately 180 KLD of effluent with TDS 28,000-34,000 mg/L, colour ADMI > 2,000, COD 4,500-6,000 mg/L, and variable pH swings tied to batch synthesis schedules. The directive gave the plant 12 months to demonstrate zero liquid discharge to the drain boundary.

Baseline conditions and treatability assessment

Terraquaer conducted a bench-scale treatability study on composite samples collected over five production days. The effluent showed two distinct streams: a high-colour, high-COD synthesis stream (COD 5,800 mg/L, colour ADMI 2,400) and a high-TDS washing stream (TDS 32,000 mg/L, COD 800 mg/L). Commingling ahead of the existing single-stage biological plant had driven MLSS to collapse during high-load batches. Treatability confirmed two-stage biological treatment - anaerobic followed by aerobic - was required before the RO feed point. Colour removal ahead of RO was the critical constraint: reactive dye fragments are poorly rejected by standard RO membranes unless COD and colour are first brought below threshold.

Process train: ABR-ASP pretreatment, two-pass RO, and MEE

The pretreatment block selected was ABR-ASP-PSF-ACF, targeting COD below 250 mg/L and colour below ADMI 100 at the RO feed. The RO section used a two-pass configuration: first pass at 75% recovery giving permeate TDS below 500 mg/L (returned as process wash water), first-pass concentrate feeding a second-pass RO at 60% recovery. Combined RO recovery reached 85% of the biological train output. The RO reject, approximately 27 KLD at TDS 180,000-200,000 mg/L, fed a three-effect falling-film MEE sized for 22 KLD evaporation duty, concentrating to 35-40% dissolved solids. MEE condensate recovered at conductivity below 200 uS/cm was returned as process wash water.

MEE concentrate fed a forced-circulation crystalliser producing a mixed-salt cake - predominantly sodium sulphate - at approximately 92% dryness by centrifuge. After characterisation, the salt qualified for co-processing rather than hazardous waste disposal, reducing the plant's Category H waste quantum under GPCB's hazardous-waste authorisation framework.

Commissioning results and process lessons

At commissioning: RO permeate TDS 380-420 mg/L returned as process water, MEE condensate conductivity 140-180 uS/cm, zero liquid discharge to boundary drain, salt generation approximately 1.2 MT/day. Net freshwater drawal fell by 78% within the first quarter of operation. GPCB CTO was renewed on schedule without conditions. Three lessons for similar projects: stream segregation before the biological stage reduced ABR volume by 35% versus a fully commingled design; first-pass RO required a specific antiscalant formulated for high-sulphate, high-colour feeds (standard carbonate-scale antiscalants caused 40% flux decline in commissioning trials); crystalliser steady-state required MEE concentrate TDS held above 220,000 mg/L consistently, below which crystal nucleation was erratic and centrifuge cake dryness fell below 80%.