Chromium in the Cuttack Plating Cluster
A cluster of 14 small electroplating units near Cuttack discharges about 12,000 L of rinse water per day into a common drain. Analysis shows Cr(VI) at 45 mg/L, pH 3.8, sulphate 2,800 mg/L, and occasional oil and grease. The Odisha State Pollution Control Board limit for Cr(VI) is 0.1 mg/L. The units can jointly spend about ₹8 lakh for capital and ₹40,000 per month for operation. A vendor offers a chemical reduction system using sodium metabisulfite and lime; it reaches 0.05 mg/L within 45 minutes but produces 60 kg of sludge per day and costs ₹55,000 per month. A university team proposes a biological option: a native Bacillus strain reduces Cr(VI) to Cr(III) in a 72-hour batch with molasses as electron donor, followed by vetiver wetland polishing. Their pilot, run in September at 28–32°C, achieved 8 mg/L after the bioreactor and 0.4 mg/L after the wetland. Winter temperatures in the shed fall to 16°C. No full-scale biological plant exists in Odisha.
1. Which treatment route should the cluster adopt for immediate compliance, and how should the biological option be positioned? Justify using the pilot numbers and site constraints.
Read the worked answer
Immediate compliance must drive the first decision. The biological pilot ends at 0.4 mg/L Cr(VI), which is four times the 0.1 mg/L consent limit, and it has not been demonstrated at the winter shed temperature of 16°C. The chemical route reaches 0.05 mg/L in 45 minutes and is a known, controllable process. Therefore install chemical reduction now, with pH adjustment, ORP-controlled metabisulfite dosing, sludge dewatering and secured disposal of Cr-bearing sludge. The 60 kg/day sludge is hazardous waste, so its disposal cost must be added to the ₹55,000/month figure before comparing operating costs. Position the biological route as a side-stream pilot for at least 12 months, covering winter and monsoon, with a clear trigger: it must meet 0.1 mg/L consistently for 90 days before it can replace or polish the chemical plant. Keep chemical reduction as a standby. Also install an oil–grease trap and equalisation tank upstream, because both options will be upset by oily shocks. This is a staged decision: compliance first, lower-cost innovation second.
2. Identify three plausible failure modes of the Bacillus–vetiver system at this site and propose a control for each.
Read the worked answer
Low winter temperature. The pilot worked at 28–32°C; at 16°C the enzymatic reduction rate will fall and the 72-hour batch will not finish. Control: insulate and heat the bioreactor, or isolate a psychrotolerant culture, and keep a chemical backup for winter.
High sulphate and competing electron acceptors. At 2,800 mg/L sulphate, sulphate-reducing bacteria can compete for molasses and produce sulphide, reducing Cr(VI) removal and creating odour. Control: monitor sulphate and ORP, dose molasses on an ORP set point, and consider a pre-treatment step.
Oil and grease fouling. Oily rinse water can coat biomass and block the vetiver root zone. Control: grease trap and equalisation before the bioreactor.
A fourth risk is Cr(VI) shock toxicity to the culture; an equalisation tank and gradual acclimatisation would manage it. For each failure mode, define a measurable trigger and a fallback, rather than assuming the pilot will scale linearly.
3. Design a 90-day monitoring programme that would convince the Pollution Control Board to accept a biological system later.
Read the worked answer
Use daily flow-proportional composite samples at inlet and outlet, with pH, temperature, Cr(VI) by diphenylcarbazide colorimetry, total Cr by AAS or ICP, COD, sulphate, and oil and grease. Analyse Cr(VI) at least twice daily during the pilot because it is the consent parameter. Monitor bioreactor ORP, molasses dose and biomass density continuously. Analyse sludge for total Cr and leachability before disposal. Use an independent NABL-accredited laboratory for fortnightly check samples. Set pre-agreed trigger values: if outlet Cr(VI) exceeds 0.1 mg/L on two consecutive samples, stop the biological route and use the chemical standby. Report monthly to the Board, with a final 90-day performance report stating the minimum, maximum and 95th percentile. Include vetiver growth and metal accumulation data, but do not count wetland polishing as reliable until it has operated through a monsoon. This plan gives the Board auditable evidence, not a one-off pilot result.