Laboratory Water Purification Questions

Tap water can contain particulate matter, chlorine, inorganic salts, organic compounds which can affect various types of analysis if not reduced or removed. Take for example the inorganic salts responsible for the electrical conductivity of water. Although drinking water does not taste salty, it contains about 300 parts per million (ppm) of dissolved salts, which give it a conductivity of around 600 μS/cm. Compare this against the ASTM standard specifications for three types of reagent water: 

 

Basically, conductivity is the measurement of electrical conductance of the water and is measured in Siemens (or microSiemens, μS/cm). The less dissolved inorganic salts, the lower the conductance and the purer the water. The purest water is Type 1 which has a conductivity of 0.056 μS/cm.

Resistivity is the measurement of electrical resistance of the water and is measured in Ohms (or microhms, MΩxcm.) . The less dissolved inorganic salts, the higher resistance and the purer the water. Type 1 water has a resistivity of 18.2 MΩxcm.

Conductivity is the inverse of Resistivity i.e. 1 / Resistivity = Conductivity (1 / 18.2 [MΩxcm.] ~ 0.055 [μS/cm]. So as conductivity increases so resistivity decreases proportionally. 

 

Pre-treatment may be necessary to treat tap water before any upstream purification is reached. The most common methods are to have a cartridge with resin to stabilise any hardness in the tap water and another cartridge with a depth filter and activated carbon to remove particulate matter and chlorine.

Alternatively, a pre-treatment system containing high quality mixed bed resins removes practically 100% of the salts from water and this product water can be used in many situations. A conductivity meter with adjustable limiting value enables the product water quality to be monitored and the ion exchange cartridge can be replaced when necessary.

Distillation is the oldest method and used to be the most commonly used method but is expensive both economically and environmentally as it uses a lot of power with electric boiler elements and a lot of water to supply the still and also for the condensers. Basically, tap water is boiled and the ‘pure’ water vapour is condensed and collected. The impurities are left behind in the boiler compartment. However, with the requirements of more stringent levels of pure water in areas such as tissue culture, molecular biology, microbiology, etc., other means of water purification have taken over in many areas from distillation.

Reverse osmosis (RO) has become an indispensible part of modern water purification. Although it is not quite as effective as ion exchange for salt removal (TKA ROmembranes reduce the salt content by 98% on average), it is very cost effective and is self-cleaning. The system works by feedwater being forced through a membrane leaving the salts on the feedwater side of the membrane and purified ‘permeate’ on the other which goes to a storage tank. Some of the feedwater is pressurised so that it does not pass through the membrane but moves across the membrane to continually wash away rejected salts. This ‘tangential flow’ is so effective in TKA membranes that they provide excellent service for at least 3 years (using specified feedwater quality).

Optional UV (Ultra-violet photo-oxidation) light of 254nm. wavelength can be incorporated to destroy bacteria and can also improve the quality of the water recirculated between a storage tank and the RO system.

RO water can be stored in a storage tank and used for rinsing glassware, making up general solutions, feeding an autoclave or glassware washer, etc. RO water can also be used as a feedwater source for deionisation cartridges to produce ultrapure water.

Suitable feedwater such as distilled or RO water can be further purified to give ultrapure water using a series of cartridges such as activated carbon + adsorber resin (to effectively remove traces of organic substances and the TOC value to a value of 2 – 10 ppb) and special ion exchange resin (effectively removes remaining traces of inorganic substances to bring the conductivity of the treated water to 18.2 MΩxcm. (0.055 μS/cm).

This ASTM Type 1 water can be used:

An Ultrafilter can be incorporated in a system for bacterial endotoxin removal and also to retain particles, macromolecules, viruses, colloidal silica, bacteria and high molecular weight organic compounds.

This product water can be used for electrophoresis buffers, sensitive tissue culturesand cell culture media preparation.

Using a UV lamp with 185/254nm. wavelengths, UV photo-oxidation destroys traces of small organic molecules and bacteria and further improves TOC values to 1-5 ppb.

A combination of UF and UV

This ‘final filter’ removes bacteria and particles < 0.2μm from 18.2 MΩcm water directly before the point of delivery.

This is dependent on (a) the quality of the feedwater (tap water, etc.) (b) the quality and use(s) required for the water and (c) the quantity of water required per day.

TKA have the Smart2Pure system which can be directly attached to tap water supply to produce RO water (ASTM Type 2) into a 6 litre incorporated storage tank (at 3 litre/hour or 6 litre/hour) which then passes through DI cartridges (and optional UV, UF or UV/UF) to give ASTM Type 1 water (1 litre/minute or 0.6 litre/minute for UF or UV/UF models).

For larger water requirements TKA have modular systems consisting of the following components:

a)      Pre-treatment cartridge(s) to treat tap water

b)      Pacific RO system (with optional UV lamp)
c)      30 or 60 litre storage tank (with optional recirculation pump and/or UV lamp)d)     GenPure ultrapure water system (standard, UV, UF or UV/UF models)

NOTE: It is a lot more cost effective to have some sort of pre-treatment before your ultrapure water deionisation cartridges so that these more expensive cartridges are not used up quickly removing the main contaminants in tap water but rather for final ‘polishing’ of treated water.