Recycling the benefits

Much has been written recently regarding the attractions of water recycling as one way in which the laundry and textile rental industry can counter the swingeing increases in water supply charges and effluent disposal costs. In some parts of the country annual increases in excess of 30% are crippling even the most prudent budgets and making hitherto profitable contracts uneconomic. Recently there have been advances in water recycling technology which might alleviate the problem.

General principles

Many laundries have practised water recycling for years. This has generally involved the collection of last rinse liquor from washer-extractors and using it for wetting out, first wash and main wash. The volume of the last rinse generally represents a third of the water consumption for the process, so savings of up to and over 30% are possible by this relatively inexpensive technique.

The drawback is that it generally requires two sets of drains and a separate supply mains. Service to each washer-extractor then requires an additional air-operated inlet valve and of course the extra drain requires a separate dump valve. Even so, the payback period is generally less than twelve months at present prices.

Those rental operations which have moved from washer-extractors to continuous batch washers have already seen their water consumption drop from 30-40 litres per kilogram of work processed down to 8 litres per kilogram of work by virtue of the internal recycling within the continuous washer design.

The only supply of fresh water to a conventionally operated continuous washer should be that which goes into the rinse, so of necessity this must be pure because it is the final cleansing stage for sheets, towels and pillowslips which will eventually come into direct contact with the hotel guest.

How pure?

It is difficult to produce a sound economic case for water recycling until the bulk of the laundry’s work is being processed through a continuous batch washer because the financial benefits of changing from washer-extractors to a continuous washer are irresistible. Such a change would certainly take precedence over any scheme to purify dirty effluent from washer-extractors alone.

It follows that the water to be recycled must be purified to the extent that it can be used as rinse water on a continuous washer, so its quality must essentially match that of the town’s mains. This includes freedom from bacteria and other micro organisms as well as from heavy metals and dissolved salts.

It is for this reason that as far as the laundry is concerned the only filtration or purification technique which is likely to be acceptable in the long term is the process referred to as reverse osmosis. This can be considered as filtration which is so fine as to be almost on the molecular scale, removing far more than ultra-filtration or micro- filtration or systems with similarly impressive names. We are talking about filtration down below 0.001 micrometres which is sufficient to filter out most bacteria and many dissolved salts.

The most space efficient reverse osmosis membranes tend to be those which are spirally wound and contained in horizontal tubes which can be wall mounted alongside a continuous washer. The optimum performance per tube is generally achieved by operating at the design pressure and temperature with contamination concentrations which are generally fairly stable. Typical pH ranges are pH 4-11 which covers most laundry systems, but it is also important to control colloidal material in suspension and the oxidation (bleaching) potential of the effluent.

There are significant benefits to be obtained by designing the water purification unit in conjunction with the soap system and applying this first to hotel flatwork which is generally lightly soiled. In this way it is possible not only to maximise the production of purified water per square metre of membrane, but also to optimise membrane life. This will produce the lowest initial cost and minimum maintenance cost.

To date, two companies have worked in conjunction with detergent suppliers to produce reverse osmosis units specifically designed for continuous washers in the textile rental sector serving the hotel flatwork market. Aqua Technology Hanson has a system designed to work with any compatible detergent system. Henkel-Ecolab has developed its own unit designed to operate with Henkel detergent systems. The diagrams shown refer to the Henkel unit but the basic principles are essentially the same.

Deciding strategy

There is no point in contemplating water recycling involving reverse osmosis until the bulk of the laundry production is passing through a continuous washer. Even then, it is only likely to be worth considering reverse osmosis for the continuous washers themselves.

If there are good reasons for retaining significant numbers of washer-extractors, then the best strategy is probably to look at last rinse recovery for these and to try to devise a simple but cost effective means for the drain collection and tankage required. For example, on most installations it is possible to lay large diameter plastic piping on the floor behind a bank of washer-extractors rather than attempting to dig out extra drainage channels in the floor.

When evaluating a reverse osmosis system, it is always worth calculating the economics at this year’s cost to determine the payback, then repeating the calculation using cost inflation of approximately 10% per annum to find out at what point a payback of less than two or three years will be achieved. Now is the time to start planning for a 2001 installation.

The old washing machines of the 1950s consumed around 45 litres per kilogram of work. Washer-extractors, with the benefit of interspins, reduced this to 30 litres and the continuous washers cut the figure to 8 litres. Now we are looking at water recycling systems using reverse osmosis that will take the market leaders down to a water consumption around 2 litres per kilogram, at which point, even quite steep rises in effluent charges become much less critical.

The concentrated dirty water from a reverse osmosis unit will still contain all of the suspended solids and the chemical oxygen demanded of the original effluent, so this part of the effluent charge equation will not be affected by recycling. However, the laundry should save at least three-quarters of the incoming water supply charge plus three-quarters of the reception and volume components of the effluent charge equation. So while the savings are far from total they are usually sufficient to make this whole exercise well worthwhile to the astute laundry engineer.