Production Expansion, Effluent Consent and Water Reuse in a Cotton Dyehouse
A vertically integrated cotton fabric mill processes 30,000 kg/day. Its dyehouse uses 120 L of water per kg of fabric. The effluent treatment plant (ETP) has a design capacity of 3,000 m³/day, but current discharge is 3,600 m³/day. Influent COD is 1,800 mg/L; the consent limit is 150 mg/L COD and 200 mg/L suspended solids. The existing activated-sludge ETP has a 12-hour hydraulic retention time and no tertiary treatment; outlet COD averages 220 mg/L. The mill plans a 20% production increase and a water-reuse project. The regulator demands compliance within six months and has signalled a water tariff of $0.60/m³ and an effluent penalty of $1.50/kg COD above the limit. Management wants to know whether the production increase can be approved, what water use target is needed, and which treatment upgrade will actually meet consent.
1. Calculate current and post-expansion flows and COD loads. Determine whether the ETP capacity and consent limits are met.
Read the worked answer
Current flow = 30,000 kg/day × 120 L/kg = 3,600,000 L/day = 3,600 m³/day. The ETP design capacity is 3,000 m³/day, so it is already 600 m³/day, or 20%, over capacity. Influent COD load = 3,600 m³/day × 1.8 kg/m³ = 6,480 kg COD/day. Outlet load at 220 mg/L = 3,600 × 0.22 = 792 kg COD/day. The consent load at 150 mg/L is 3,600 × 0.15 = 540 kg COD/day. The plant is therefore 252 kg COD/day over the consent load, which would attract a penalty of 252 × $1.50 = $378/day if enforced. Removal efficiency is (6,480 − 792)/6,480 = 87.8%.
After a 20% increase, production is 36,000 kg/day. At unchanged water use, flow = 36,000 × 120 = 4,320 m³/day, which is 1,320 m³/day above ETP capacity. Influent load = 4,320 × 1.8 = 7,776 kg COD/day. At 220 mg/L the outlet load would be 950.4 kg/day; the consent load at 150 mg/L would be 648 kg/day, so the excess would be 302.4 kg/day, or $453.60/day. To meet 150 mg/L at 4,320 m³/day, required removal = (7,776 − 648)/7,776 = 91.7%. The existing plant at 87.8% cannot achieve that. The production increase cannot be approved on the current water and treatment basis.
2. Set a water-use target that allows the 20% expansion within the existing 3,000 m³/day capacity, and list measures with estimated savings.
Read the worked answer
Required specific water use = 3,000,000 L/day ÷ 36,000 kg/day = 83.3 L/kg. The current use is 120 L/kg, so the mill must reduce by 36.7 L/kg, or 30.6%. That is ambitious but feasible if the dyehouse is the main consumer. Measures: convert to low-liquor dyeing, for example from 1:8 to 1:6, which can cut dyehouse water by roughly 20–25%; use counter-current washing and combined rinse baths, which can save another 20–30% of washing water; recover cooling water and condensate; reuse final rinse water for the next first rinse; repair leaks and install submetering by department. If these deliver 30–35%, the 83.3 L/kg target is achievable.
The economics are significant: at 36 t/day, 120 L/kg gives 4,320 m³/day; at 83.3 L/kg it gives 3,000 m³/day, a saving of 1,320 m³/day. At $0.60/m³ that is $792/day, or about $237,600/year at 300 days. That saving can part-fund the treatment upgrade. The target must be written into production planning as a specific water budget per kg of fabric, not left as a general instruction.
3. Recommend the treatment upgrade and discuss the reuse trade-off. Will the existing activated sludge meet 150 mg/L COD?
Read the worked answer
The existing activated sludge at 12 h HRT and 87.8% COD removal is unlikely to reach 91.7% removal reliably, especially with reactive dye colour, salt and fluctuating dyehouse loads. Recommend source segregation first: separate desizing wastewater (very high COD), mercerizing wash (high pH), and dyehouse wastewater. Then upgrade with equalization, extended aeration to 18–24 h HRT, nutrient dosing and adequate secondary clarification; add tertiary coagulation–flocculation plus filtration or ozonation for colour and residual COD. If the water is to be reused in dyeing, add ultrafiltration and reverse osmosis; reuse of treated effluent without demineralization will accumulate salt and hardness and cause shade and levelness problems.
The reuse trade-off is real: water recovery of 50% concentrates salts, COD and colour in the remaining stream, increasing treatment load and risk of scaling. Use RO permeate for sensitive dyeing and use the concentrate for washing or discharge after treatment. A pilot should run for at least four weeks and monitor outlet COD, colour, conductivity, hardness and dyehouse shade reproducibility. Approval of the production increase should be conditional on the water target, the ETP upgrade and a successful reuse pilot; otherwise the mill will meet neither the consent nor the expansion plan.