A guide to water recycling

Rises in water and effluent charges have been well above inflation and now account for a significant part of laundering costs. The industry has found ways to tackle the problem but some have been more successful than others, so water consumption now varies from 2litre/kg textiles to over 30litre/kg.

Dip levels, flow-rates

Before investing in any water recycling system, laundries need to systematically reduce their washer-extractor dips to the levels recommended by the British Launderers’ Research Association’s work in the 1960s and 70s.

Similarly, tunnel washer users should first reduce the rinse flow down to the design rate for the unit.

The first step for the washer-extractor section is to make sure that each machine is fully loaded on every cycle. This may mean mixing work and sorting it by customer later but it is essential. Even if these washers only process 10% of the workload they still use over 30% of the process water.

Laundries that have suffered a series of tunnel washer blockages often underload first one classification, then another in an effort to find the cause. The blockages may well be unrelated to the load factor (for example a double drop is more likely) but frequently laundries forget that they need to return to their original load weights when their investigations are over.

Correcting both these points will usually lead to a small reduction in litres/kg, without any capital investment.

Last rinse recycling

Washer-extractor based laundries will find that it is cost-effective to re-use the last rinse liquor in the pre-wash and main wash.

The quantities usually match well, with an overflow of about 10% from the recycle collection tank, which keeps it clear of scum and floating lint. The last rinse is usually a few degrees warmer than the incoming cold water so this method also re-uses a small amount of energy.

This method saves around 30% of the supply charge and 15% of the effluent charge on a two wash, two rinse system.

The effluent saving is always about half the supply saving, because the effluent carries the removed soiling and the concentration is greater when the water volume is reduced.

The big advantage of last rinse recycling for a washer-extractor based plant is that it can be linked with flash steam recovery.

On a steam-based plant, the energy in the flash steam usually represents about 10% of the fuel bill, which can be extracted cheaply from the condensate main and injected via the sparge pipe into the recycled water tank. The total system involves piping, pumps and relatively inexpensive fittings, so the investment is usually modest – well-under £20,000 and will pay for itself in less than a year, especially if the work is carried out in-house.

Recycling for tunnels

Laundries usually reduce their water use and the effluent volume by around 75% when they switch from washer-extractors to a tunnel washer.

However, if they want to stay competitive, even using 6litre/kg is still too high. Lavatec’s split rinse concept was probably the first attempt to solve the problem.

This modification involves taking a pipe from the press tank and pumping about 2litre/kg back into the centre of the rinse zone. It greatly improves efficiency in the first rinse compartment and does not affect the water balance, because all of the recycled water eventually emerges from the machine, either into the press or over the rinse weir.

Split rinsing can be applied to any machine with a double skin in the centre of the rinse. It allows a reduction of 1 – 2 litre/kg in the main fresh water supply to the rinse and saves at least 20% in water supply and effluent volume. Split rinsing is particularly beneficial on very dirty work.

Chemicals suppliers have been much involved in water and energy saving. Ecolab’s PerFormance system has three main variants including an advanced system which combines water and heat recovery, but its Basis system incorporates split rinsing – see diagram.

Taking a different approach, Christeyns has combined recycling around the pre-wash with neutralisation (as opposed to dilution) in its Sanoxy Plus system.

Contamination

The last rinse from coloured and starched work is unlikely to be suitable for recovery and heavily-soiled liquors with scum should also be avoided but it is essential to monitor all recycling streams for micro-organic contamination.

The excrement from bacteria is usually highly malodorous and objectionable so bacterial growth must be halted. The cheapest way of doing this is to maintain a low level of sodium hypochlorite bleach in the recycle water, – the level used in swimming pools is a guide. Launderers that use a level of around 1 – 2ppm report few if any odour problems. The more refined alternative is ultraviolet radiation.

Straining the recycle stream to remove lint and other solid contaminants is essential.

Many systems use mesh screens but these are difficult to clean and quickly fall into disuse for this reason. A far better alternative is the wedge wire screen which does not get blocked by laundry lint and which can be sloped so it is self-cleaning.

This type of screen is widely used on tunnel washers to improve rinse water that is recycled into the wash zones. It can be specified to remove particles either down to 25mm (recommended) or even down to 0.125mm if the laundry is handling very sensitive work.

Ceramic micro-filters will remove even finer particles and these have been successfully installed in many laundries to achieve total water recycling.

However, they do not remove dissolved contaminants and they certainly cannot reduce bacteria, so careful design is essential. The secret to avoiding contamination is to forget about recycling 100% of the water selected for recycling and allow a purge of 20 – 30% to drain to prevent a build-up of these dissolved species.

Ultra-filtration is possible using reverse osmosis, which is effectively filtration at the molecular level, sometimes called nano-filtration. The finest semi-permeable membranes will take out large dissolved molecules and micro-organisms such as viruses and bacteria but it is unwise to rely on this and the best systems back-up the filtration with ultraviolet radiation to ensure an adequate bug kill.

Integrated systems

Most plants pay less for water and effluent than they do for energy but the gap is closing. Investment that integrates recycling of both water and energy generally gives much better payback and it also avoids later problems when a laundry team invests in one type of water recovery and then discovers they cannot add heat recovery later.

Recycling heat in a tunnel washer system requires special skills. If the recovered heat is used to warm up the previously cold fresh rinse flow, then the temperature in the membrane press (or

hydro-extractor on a garment line) will rise, often to 40 – 50C.

This will improve moisture extraction, pre-heat the textiles and so speed ironing times for pillowcases, duvet covers and sheets.

It will also reduce tumble drying times for towels (which is very valuable if the productivity is limited by the tumblers.)

However, it will increase creasing on all classifications and so reveal any weaknesses in the ironer tuning.

Hot pressure creases in polyester cotton workwear could reduce quality standards markedly, because tunnel finishers are not normally designed to remove this type of creasing.

The second problem with putting heat into the rinse is that this will raise the temperature of the front two compartments when press liquor is recycled to the pre-wash and it becomes difficult to keep these compartments below 40C. As a result protein stains will set onto the cloth and become very difficult to remove.

They will gradually build up in the circulating stock and may lead to significant downgrading of the stock a few weeks later.

One way of controlling the latter problem is to combine heat recovery with water recycling and to sacrifice 0.5 – 1.0litre/kg of the water saving to cool the front end with cold soft water.