Summary of Real Science Project 2011
Setting the Scene
Milliken Speciality Woollen Products are manufacturers of felt which is used primarily for tennis balls and snooker tables. Their markets include Dunlop, Slazenger and Wilson.
At Milliken’s Lodgemore Mill the washing, scouring and dyeing of the cloth produces around 150 000m3 of waste water each year.
The waste water contains fibres, dye liquors and salts. The water is also depleted in oxygen due to the action of bacteria and chemicals. The waste water is sent down a “private” pipeline straight to Severn Trent Water Treatment Works near Stonehouse.
Severn Trent charge Milliken around £130 000 per annum for processing Milliken’s effluent before it is released into the river. The price depends on the volume of waste water, the volume of suspended solids (mainly wool fibres) and the biological/chemical oxygen demand (BOD/COD) of the waste water
Real Science’s Challenge
Find a way to reduce the processing costs of Lodgemore Mill’s effluent
The Real Science group established that the largest contribution (around 44%) to the cost of water treatment was biological/chemical oxygen demand (BOD/COD) – this relates to how much dissolved oxygen has been removed from the water - Fish cannot survive in waters with high BOD/COD.
The cost of removing suspended solids from the effluent is around £5500 per annum (5% of the total waste-water processing charge).
- How much suspended solids are in the effluent?
- How can we measure this?
- Is there a way for Milliken to reduce the suspended solids?
Although the waste dye liquors are not harmful, they do make the effluent unsightly. There are no direct charges made by Severn Trent to Milliken due to the colour of the effluent, but Milliken wanted the Real Science Group to look into ways of removing the unsightly colour.
Key questions explored:
- How can ‘colour’ be measured?
- Which chemicals give rise to colour?
- Do coloured chemicals give rise to a chemical oxygen demand?
- How can the colour be removed?
The most effective method of colour removal was found to be the addition of small quantities of activated charcoal. However, the mass of activated charcoal required to decolourise 150 000 tonnes of water would run into hundreds of tonnes which is not feasible.
There was a correlation between dissolved oxygen content and bacterial activity. This relationship was believed to be causal. That means, oxygen depletion was primarily caused by bacteria. Hence de-oxygenation of the effluent was thought to be mainly through its BOD.
The source of bacteria was confirmed to be in the wool fibres and not the softened river water used for dyeing, scouring and rinsing.
BOD can be limited by adding trace amounts of enzyme inhibiting chemicals such as EDTA, or small quantities of disinfectant chemicals such as sodium metabisulphite, bleach or hydrogen peroxide. However, the optimum stage in which to introduce these chemicals still needs to be investigated.
The main sources of oxygen depleted water can be traced back to just a few processes taking place in the mill. Milliken can, therefore, target where to apply disinfecting chemicals.