Laboratory Water Purification Systems

Laboratory Water Purification Systems

Decide on what water purification systems you really need

Four things to decide when choosing water purification systems:

• Quality of water required - ultrapure for critical analysis use or lesser quality for downstream equipment such as autoclave, glassware washer rinse cycle, etc.
• Quantity per day of water required (reverse osmosis water can be stored, deionised is drawn off as required)
• Feedwater required (reverse osmosis, deionised, distilled) or pre-treatment of tap water as feedwater
• Do you require complete systems to obtain ultrapure water from tap water (i.e. pre-treatment of tap water, reverse osmosis, deionisation with optional UV, UF or UV/UF) or a 'polishing' system (deionisation with optional UV, UF or UV/UF) to add on to your current supply of treated water (reverse osmosis, deionised, distilled) to produce ultrapure water?

Water purification systems for cost-conscious customers

Many companies offer laboratory water purification equipment. All make good products and clearly point out technical differences that they believe make their products best. But the mass of various information can make objective comparisons difficult and can cause confusion rather than simplify the choice of systems. It helps here first to decide in what you really need in terms of the purified and/or ultrapure water quality, probable daily volumes, etc., and to consider the anticipated outlay for maintenance and after sales service, and so to make a good start in cost management.

Keeping it simple

We have therefore limited this guide to explanations of the individual steps in our water purification systems, starting with the feedwater pre-treatment that is so important for the economy and effectiveness of the purified and/or ultrapure water production.

Choosing water purification systems by comparing investment and running costs

When deciding to buy a system, the purchase price is often taken as the decisive factor. As indicated above, however, the running costs for maintenance, service contracts, replacement parts, e.g. over a 3 year service life or longer, should also be considered. The water systems can differ greatly and exert a large influence on the actual cost of your water purification process, calculated as cost per litre!

Determine the water quality you need:

ASTM CAP/NCCLS Standard Specifications for Reagent Water

How clean must 'purified' water be?

Let us take the inorganic salts responsible for the electrical conductivity of water as an example. Drinking water does not taste salty, but contains about 300 parts per million (ppm) of dissolved salts, which give it a conductivity of around 600 µS/cm.

As a comparison:

ASTM Type 2 water = 1.0 µS/cm

ASTM Type 1 water = 0.056 µS/cm

The ion-exchange resins and reverse osmosis membranes used to reach these values must be highly efficient and inherently pure. And, as purification costs increase with increasing water quality, Type 1 water should never be used for Type water jobs.

The ideal combination for salts

Pretreatment Systems containing high quality mixed bed resins removes practically 100% of the salts from water. A conductivity meter with adjustable limiting value enables the product water quality to be monitored, so that replacement of the ion exchange cartridge can be made at the right time.

Reverse Osmosis (RO) , an even finer purification step than ultrafiltration, has become an indispensable part of modern water purification. Although it is not quite as effective as ion-exchange for salt removal (TKA's RO-membranes reduce the salt content by 98% on average), it is very cost effective as it is self-cleaning (a small portion of the feedwater is not forced through the membrane as permeate, but is pressurised across the membrane surface to continually wash away rejected salts). This so-called tangential flow is so effective that TKA's RO-membranes provide excellent service for at least 3 years (with feedwater of quality specified by TKA). A further benefit is that they also similarly retain particles, bacteria and pyrogens.

Because of this 3 year service life, TKA RO is used as the first purification step in TKA Systems. It supplies down-stream ion exchanger cartridges with such low salt water that they reach service lives of at least 8,000 - 10,000 litres. An ideal combination for an optimally low cost per litre.

Tap water pre-treatment

Specifications for the feedwater for TKA Reverse Osmosis units:

• Hardness stabilised water
• Manganese and iron each < 0.05 mg/l
• Salt content max. 2000 mg/l
• Colloid index max. 3
• Temperature 1 - 30oC
• Pressure 2 - 6 bar
• Chlorine concentration < 0.01 mg/l

The TKA-Pretreatment Systems

Pretreatment is required to reduce the concentration of contaminants that would otherwise negatively effect the economy of the subsequent purification steps.

The necessary pre-treatment steps are:

1.  Hardness stabilisation

If not treated, the calcium and magnesium ions responsible for hardness in water would form scale on water carrying surfaces in the water purification system. A continual deposition of calcium and magnesium ions on the surface os the reverse osmosis membrane would cause the flow of permeate to steadily decrease until it eventually ceased.

A hardness stabilising cartridge reduces the content of hardness formers and so protects the membrane from blockage and premature replacement.

2.  Removal of chlorine and coarse particles from water

Residues of any chlorine used to control micro-organisms in tap water could damage reverse osmosis membranes. Activated carbon absorbs chlorine. Coarse particles resulting from the decomposition or agglomeration of matter in drinking water, or given up to the water from aged piping, could exert the same effect as water hardness. A low cost, high capacity 5 ¥ìm. depth-type filter retains such particles.

Contaminants and purification methods that remove / reduce them

The TKA pre-treatment systems

This consists of two 10" plastic housings in series, one containing a cartridge for hardness stabilisation and the other a cartridge for chlorine and coarse particle removal.

Summary for Water Purification Systems

The pre-treatment systems described above is optimal for a subsequent purification system consisting of a serial combination of multiple purification methods, and will suffice when clean and relatively soft drinking water is used as feedwater. Should the feedwater be of a less than favourable quality, however, it must be supplemented by appropriate additional pre-treatment.

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