Water Quality and Food Safety – VI

This article is a continuation from the last article Water Quality and Food Safety - V

Ultraviolet Radiation (UV Light)

Ultraviolet (UV) radiation is a proven technology against microbes in water which has been used in many industries for inactivation or destruction of microorganisms for disinfection purposes. Ultraviolet light is the name given to electromagnetic radiation lying in the wavelength band immediately beyond the violet end of the visible spectrum but preceding the X-ray radiation band. The spectral band is, by definition, between 100 and 400 nm. The UV spectrum is arbitrarily subdivided into three bands: Wavelength (nm) UV-A (long-wave) 320–400 UV-B (medium-wave) 280–320 UV-C (short-wave) 100–280 Microorganisms like bacteria, moulds, yeasts, and protozoa can be inactivated by short-wave UV radiation. UV treatment should ensure a 4 log reduction of test organisms proven by biodosimetry. The UV radiation used should be comparable to 400 J/m2 where proper maintenance is of utmost importance to ensure reliable operation. UV treatment will never produce sterile water, but it is capable of significantly reducing the number of microorganisms. Thus water sources with a high initial contamination level, UV treatment may not be sufficiently effective to obtain potable quality water. Some bacteria, particularly micrococci, protozoa, algae and moulds, exhibit varying levels of resistance and may need higher doses for inactivation. A decision to install a UV system should be based on a thorough investigation of the water source, since the effectiveness of UV is strongly dependent on the composition of water (turbidity, adsorption, concentration of organic material).

The main advantages include:

No chemicals used;

A clean process;

It can be used synergistically with ozone and can be used to remove ozone residues in water;

It has a broad spectrum of activity.

The main disadvantages include:

It is only effective in non-turbid water;

Particulates can protect organisms in shadows;

Contact times required may limit flow rates;

Lamps need regular maintenance and quite frequent replacement;

No residual activity requiring a high level of hygiene after UV treatment in order to maintain water quality.

Particle Filtration and System Pre-filtration

The removal of sediment and other suspended particles is critical to the clean operation of any system. Even the water used in flumes to carry raw fruits and vegetables for processing should be filtered to remove sediment and avoid fouling piping or depositing visible particles on the products. Of course, removing sediment and small particulates is key to the efficient, cost effective operation of any purified water treatment system. Performed using depth filtration media, the particle filtration and pre-filtration processes preserve system performance and can reduce operating costs.

Trap Filtration

Many water treatment systems involve water passing through either a water softener or deionization. These systems are based on specially made plastic resin beads. Resin can break down over time and particulate can escape resin vessels, thus filters are used to trap the particles and prevent their contamination of downstream processes. Again, depth filtration is most often used for this function.

Bacteria Control

Controlling the level of organisms in a system is not the same as removing all of them. In fact, sterilizing filtration is defined by a different set of performance measures (see below). Controlling the number of organisms, sometimes called the "bioburden", is done using filters similar to those used for sterilizing filtration, but the performance requirements are not as stringent. That does not mean that bioburden control is a secondary consideration. Reducing the bioburden in a system keeps the system cleaner, making cleaning and disinfection easier and, most important in most systems, protects the expensive sterilizing filters from excessive loading which will shorten their life.

The choice of filter to use for bioburden reduction is based on the type and number of bacteria likely to be found in the system. In general, organisms that may be carried by the raw ingredients (Alicyclobacillus species, Bacillus species, Cryptosporidium, coliforms and others) are the primary targets for removal. However, microorganisms’ endemic to the location of the facility (molds, airborne bacteria, etc) also need to be considered. Depending on the size of the organisms and the nature of the product, membrane filters with anywhere from 1.2µm to 0.45µm pore size ratings may be used.

These filters may be used for bioburden reduction, for sterilizing filtration or for ultrafine particle removal. Membrane filters with pore sizes of 0.65µm or 0.45µm are usually chosen for these applications.

Sterilizing Filtration

Sterilizing filters are used as insurance against adding waterborne bacteria to processes. This last process water filtration step assures that microorganisms that may have entered the treatment system are removed and protects the quality and safety of your processes and products. Sterilizing filtration is defined as removing all bacteria from the fluid stream. This process is critical for the final product quality and shelf life. For beverage production, the filters must remove the microorganisms that might adversely affect shelf life and product safety while preserving the flavour and character of the wine, beer, juice, soft drink or bottled water. Filtration products making the sterilizing claim must be supplied with proof that they can perform as claimed. That proof is usually in the form of a certificate of compliance stating that the filter has been tested during production to assure that it will remove organisms the size of those targeted in the user’s system.

Sterilizing filters are the last process step as a beverage is packaged.

Tank Vent Filtration

After purification, water may be stored in a tank system to assure an adequate supply during peak demand periods. Filtration acts as a critical control step for protecting your water from particle and/or bacterial contamination from the environment around the tank. Hydrophobic filters are used to block particles ranging in size from visible dust to micro-particles as small as 0.03 microns.

Validating Performance

The pharmaceutical industry has accepted successful removal of surrogate organisms by specific filter membranes as proof that a filter can remove organisms of similar size. The organisms are defined by an ASTM standard procedure for each pore size rating. The ASTM standard (ASTM F838-05 rev 2013) requires COMPLETE removal of all test bacteria when a filter is challenged with at least 107 organisms per cm² of membrane surface area. This level of challenge is extremely unlikely in actual applications.

Critical process filtration provides filters tested using the ASTM standard with 0.10µm filters challenged with Acholeplasma laidlawii; 0.22µm challenged with Brevundimonas diminuta; 0.45µm challenged with Serratia marcescens; 0.65µm with Saccharomyces cerevisiae.

Neutralisation

After water treatment, particularly using one of the above (previous article and this) processes, it may be necessary to bring pH back into the desired range using acid or alkali dosing as appropriate. Care must be taken to ensure that this introduction of ions will not cause damage, for example, sulphates in concrete and chlorides on stainless steel are a particular problem. The plant should provide proper protection against overdose of chemicals.

Activated Carbon

Adsorption refers to the ability of certain materials to retain molecules on their surface in a more or less reversible way. The main applications of adsorption are the removal of micro pollutants from water in a concentration area of less than one milligram to some tens of milligram per litre. Thus most applied adsorbent is the activated carbon, in which the adsorption capacity of the material depends on the specific surface area of carbon, the particle size, the contact time, the type of carbon and the nature of adsorbance–adsorbent bond. A good pre-treatment by sand filtration is necessary to prevent the pollution of carbon bed with suspended solids, where granular activated carbon (GAC) is used with an internal surface area of 500–1500 m2 /g for water treatment. The GAC can be reactivated with steam or at high temperatures (800–900 8C).

Growth of microorganisms also is a serious risk. In carbon filters, a strong biological activity is possible. This can result not only in microbiological risks, but also in the production of hazardous compounds such as toxins and lipopolysaccharides. 

Chemical Coagulation

Coagulation is suitable for the removal of certain heavy metals and low solubility organic chemicals such as certain organochlorine pesticides, which is generally ineffective for other organic chemicals. Chemical coagulants are dosed to the raw water under controlled conditions to form a solid flocculent metal hydroxide. The flocculent is removed from the treated water by subsequent solid–liquid separation processes such as sedimentation or flotation, and/or rapid gravity filtration. Typical coagulant doses are 2–5 mg/l as Al or 4–10 mg/l as Fe. The compounds used are various salts of aluminium (e.g. alum) or iron (ferric sulphate). In some countries local law defines maximal values than can be used. Nevertheless, consideration must be taken of residual chemicals that are in the water after it has been processed.

Electrolytic Treatments

There are a few companies on the market that offer electrolytic treatment for water. Generally, a high electric tension is used to produce radicals in a separate small water circuit. These compounds exhibit bactericidal actions due to their unpaired electrons. However, the value of the electrolytic process depends on a number of factors. No general statement can be made in support of the technology as only a careful analysis of the reactions that occur can determine if the technology is of an advantage without creating harmful by-products.