Hard Water, Grey Towels and Harsh Woollens
Your laundry serves a 90-room hostel and uses borewell water that tests at 640 mg/L hardness as CaCO3 and pH 8.3. Over three weeks, customers return white cotton towels that look grey and feel stiff, and two woollen shawls come back harsh and slightly shrunken. Staff have been using a soap-based washing powder at roughly double the recommended dose and adding bleaching powder to 'brighten' the whites. The washer is a 10 kg semi-automatic drum, and rinse water is reused for the next load. Weekly throughput is about 180 kg, of which 15 kg is wool and silk. The owner can either install a 1,000 L softener with a capital cost or change chemicals. Water is not metered and the borewell yield drops in summer.
1. Explain what is causing each symptom - grey towels, stiff handle, and shrunken harsh shawls - and link each cause to a specific practice in the workshop.
Read the worked answer
The symptoms are different consequences of the same two decisions: soap in hard water, and no fibre-based sorting.
- Grey towels. The calcium and magnesium in 640 mg/L water react with soap to form insoluble lime soap. That scum deposits on the cotton, traps soil, and is then partly redeposited because rinse water is reused. Doubling the soap dose makes it worse: more scum, more residue, and more greying. Bleaching powder adds calcium hypochlorite, which leaves its own insoluble calcium salts and can yellow whites rather than brighten them.
- Stiff handle. Lime soap and detergent residue dry into the fibre, and without a souring step the fabric stays alkaline. Reused rinse water keeps the hardness and residue in contact with the cloth.
- Shrunken, harsh shawls. Wool is a protein fibre. Alkaline soap, bleaching powder, heat and drum agitation together cause felting: the scales lock and the fabric shrinks irreversibly. The harsh handle is the felted surface plus chemical damage. The shawls were almost certainly run in the same load and programme as the cotton.
- The key judgement. The greyness is not a 'dirty wash' problem that can be solved by more soap; it is a water-chemistry problem. Adding more soap or bleach will increase deposits and damage.
2. The owner cannot decide between installing a softener and simply switching chemicals. Recommend a route and set out separate washing programmes for the white cotton towels and the wool/silk items.
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Recommendation: install the softener if the capital is available, because it treats the cause and reduces chemical cost across all 180 kg per week. If it is not available now, switch to built synthetic detergents and keep wool away from the cotton programme.
Why the softener wins: at 640 mg/L, every load carries hardness into the drum. A softener removes it before it meets the detergent, so soap scum, redeposition and overdose all fall. It also protects the 15 kg of wool and silk that cannot tolerate alkaline builders.
Cotton towel programme:
- Synthetic detergent with builders (zeolite/STPP) at the recommended dose, plus 1–2 g/L soda ash for soil removal.
- Main wash at 60 °C; do not reuse rinse water for whites.
- Three rinses, then a souring rinse with acetic acid to pH 5.5–6.5.
- Oxygen bleach only if needed; never bleaching powder.
Wool/silk programme:
- Neutral non-ionic wool detergent, no soda ash, no bleach, no enzyme.
- 30 °C, short gentle cycle, low spin, mesh bags.
- Flat dry in shade; never tumble.
- Separate from cotton and from coloured items.
Chemical fallback: use a sequestrant (EDTA or a proprietary hardness controller) in the wool loads and a built detergent for cotton. This costs more per load than softened water, which is why it is a fallback rather than the first choice.
3. Staff resist giving up bleaching powder because 'it makes whites white'. How will you control or replace it, and what simple checks will you institute?
Read the worked answer
Replace it for wool, silk and coloured work immediately, and restrict any chlorine source to white cotton under dose control.
- Explain the chemistry in workshop terms. Bleaching powder is calcium hypochlorite: it is alkaline, leaves calcium deposits, and is unpredictable in hard water. It attacks wool and silk and can yellow cotton if overdosed or inadequately rinsed. 'Whiter' immediately after bleaching often becomes 'greyer' after two washes because of deposits.
- Use the right product for the job. Sodium hypochlorite solution for white cotton only, dosed to about 100–150 ppm available chlorine, added after the detergent wash, with a dedicated bleach cycle and thorough rinsing. Oxygen bleach for coloured cotton and blends. No chlorine at all for wool, silk, elastane or metallic trims.
- Control the dose, not the habit. Keep a measured jug, a dosing chart by load size, and test papers for available chlorine. Never mix bleach with acid, ammonia or a souring bath; store it separately and label containers clearly.
- Simple checks. Test water hardness weekly with a kit and record it. Check wash-bath pH and final rinse pH; the rinse should be close to neutral or slightly acidic. Do a whiteness comparison of a retained towel under daylight. Check for residue by feel and by a simple shake test after drying.
- Use data to win the argument. Show the complaint log before and after the change. Staff accept a new routine faster when the grey-towel returns stop.