Water Quality and Food Safety - V

Common Treatment Techniques and Their Main Hazards

Water is a critical part of many food and beverage processes where it acts as a raw material, ingredient or rinsing/washing and cleaning medium. Thus water needs to be treated before it is been used for any processing operations to guarantee the safety of the food products manufactured. Water may be treated using anything from sediment filters that remove only visible particles to reverse osmosis systems with sterilizing filters. Below is a more detailed discussion of treatment technologies use in process water systems, but not all of the treatment steps given are necessary for all food and beverage processes. Nevertheless, each treatment or filtration step produces water suitable for some food and beverage processing steps. In many cases, only water used as an ingredient/raw material will require all of the components of water treatment steps given below.

The choice of water treatment depends on the water source and the intended application of water, which often requires a combination of techniques. There are three main sources of hazards in relation to water treatment techniques:

Hazards in relation to design and building;

Hazards during the normal operation (including stops);

Hazards introduced by external factors;

Usually, general rules of hygienic design for installation in the food industry are critically important to prevent design and building hazards, i.e., there are specific design considerations for some techniques. Hazards during normal operation and due to malfunctioning can be minimized by good hygienic practice and adequate preventative maintenance. Examples of external hazards are: incoming water, used chemicals and materials, lack of water supply or inferior utilities (energy, cooling water, air). A good monitoring and guarding system will decrease these hazards.

Filtration

Filtration is a separation process that consists of passing a solid and liquid mixture through a porous material (filter) that retains the solids and allows the liquid (filtrate) to pass through. Filtration is used for the removal of suspended solids from the water.

There are three types of filtration:

1. Filtration on a support (micro straining, filtration with cartridges and candles);

2. Filtration with a filter cake;

3. Filtration through a granular solid material filter bed (e.g. sand and anthracite). 

For the treatment of process water, filter cartridges and granular filtration are generally used.

Filtration hazards

Damaged or badly designed filters can lead to fine sand passing into the downstream pipe work. Pressure drop over a filter is a good indication that a filter is saturated, where replacing or back-washing of the filter is necessary. Sufficient aeration and right pH are required to remove iron and manganese, which takes some time after the start-up of a new installation before filtration works effectively. Nonetheless, growth of microorganisms is a problem when the installation is out of operation, where recirculation of water over the filter during these periods might be a solution. Another risk is the breakthrough of material to be filtered due to:

Too long running time;

Insufficient back-wash program;

Wrong installation of cartridges in pressure vessel.

Ion exchange processes

Ion exchange processes modify or reduce the ionic content of waters and include de-alkalising, softening and de-mineralising.

Hygienic aspects of ion exchange processes

During normal operation of ion exchange processes for product water, the following precautions should be taken:

Recirculate water over the beds to avoid stagnant water.

Pre-rinse the unit when the operation starts.

If the unit has been out of operation for longer than 6 h, drain two bed volumes before starting to use the water for process purposes.

Ion exchange plants must be disinfected if they become infected with microorganisms where disinfection agents such as sodium hypochlorite must be approved for food application. Free chlorine or any other oxidising agent may substantially harm the resin bed, where disinfectant should be compatible with the particular resin being used. Disinfection frequency should not exceed once a month. Ion exchange plants can also become fouled with iron and suspended solids, where any chemicals used to remove such materials must be approved for food factory use as a general guide. The ion exchange unit must be thoroughly rinsed with at least four bed volumes of water, after disinfection and/or cleaning.

Resin replacement

Ion exchange resins will last on an average for about 5 years, where annual resin analysis should be used to indicate when a replacement needs to be installed.

Membrane filtration

Membrane filtration is a pressure-driven technology using a broad range of pore sizes, which can be applied with both cross-flow and dead-end filtration. The inlet feed is pumped over the membranes with a turbulent high flow velocity with cross-flow filtration. The pressure differential across the membrane forces part of the fluid through the filter membrane, while the remainder flows over the membrane and removes residues, whereas dead-end filtration forces all of the fluid through the filter membrane by pressure. This technology is generally used for fluids with a low solid content.

Hygienic aspects of membrane filtration

Cleanability depends on both the type of membrane and material it is made of, where tubular membranes are easier to clean than other types in general. Ceramic is ideal for sanitary applications as well as for products with extremes in pH, temperature or solvents. On the other hand, stainless steel is effective for applications with aggressive process conditions.

UF-membranes can be cleaned by either enzymatic or chemical processes, because membrane fouling is one of the main problems during operation, which is most frequently caused by build-up of colloidal material, but metal oxides can also cause fouling. In addition, appropriate cleaning schedules and monitoring (e.g. turbidity, conductivity, pressure differential measurement) of the treated water should be established to prevent bacterial growth through the filter. Working conditions (e.g. velocity) of the filtration should be chosen in a way to minimize the risk of fouling. Water produced by reverse osmosis may have corrosive properties due to the removal of minerals where a degree of re-mineralisation might be required for some applications.

Chlorination and Ozonation

Chlorination and ozonisation are oxidation techniques that used to degrade organic chemicals and for disinfection. General hazards of the application of oxidation techniques are:

The reaction with other compounds and formation of toxic agents;

The occurrence of residues of chemicals;

Damage to personnel by inhalation or skin contact. 

Chlorine

Chlorination has been the most commonly used procedure for the control of microbiological contamination in both potable and utility water for many years, where chlorination can either be done with NaOCl or with chlorine gas. Depending on the reliability of purchased water supply, it may be required to chlorinate the incoming water, where NaOCl is the major oxidizing agent in food industry. In many cases, chlorinated water is unsuitable as ingredient water because of the taste, where a combined chlorination/dechlorination process is required, in which dechlorination usually taking place via activated carbon treatment. Chlorine exhibits a broad spectrum of anti-microbial activity, which is extremely cost effective in use and has a rapid killing action. However, for effective disinfection there should be a residual concentration of free chlorine of 0.5 mg/l or greater for at least 30 min contact time at pH 8.0.

However, disadvantages include:

Increasingly reduced effectiveness at pH O8.

Reacts with nitrogenous compounds to give chloramines, which are poor biocides and can give rise to unpleasant odours.

Reactive with other organic materials and may yield environmentally unacceptable compounds, e.g. trihalomethanes.

Reacts with naturally occurring phenolic compounds to form chlorophenol taint materials.

Easily quenched by organic matter and turbidity in the water.

Highly corrosive.

Chlorine Dioxide

The first reported use of chlorine dioxide treatment of drinking water was in the 1940 s in the USA. Chlorine dioxide can be prepared by acidification of sodium chlorite or by its reaction with chlorine gas which was made possible when sodium chlorite became commercially available. Chlorine dioxide is extremely reactive and cannot be stored in its active state, which must therefore be generated on site, close to the point of use. The process is usually designed to ensure that the chlorine dioxide produced is delivered as a dilute solution. As to the recently introduced method, ClO₂ can also be generated directly converting sodium chlorite to chlorine dioxide via a patented electrochemical process, which is safer to use than the traditional generators.

Chlorine dioxide has a number of advantages over chlorine and bromine, as follows:

Broad spectrum anti-microbial activity at lower concentrations.

Maintains its effectiveness up to pH 10.

Does not react with nitrogenous compounds.

Does not react readily with organics so does not produce environmentally unfriendly compounds.

More effective in the presence of organic matter.

Approved for use in potable water.

Considerably less likely to produce taint compounds.

Chlorine dioxide also have some disadvantages:

Much more expensive than chlorine.

It has to be generated at the point of use.

With some generation processes, significant levels of chlorine may be produced which cancels out some of the advantages.

As it is a gas dissolved in water, some will be lost to atmosphere in processes, which involve spraying under pressure or high agitation of heated water.

Generators have a high capital cost, are complex to install, and may require frequent servicing.

Due to higher volatility than chlorine or bromine, it may demonstrate a greater corrosion potential, e.g. in vapour spaces of hydrostatic sterilizers.

Ozone

Ozone has many potential applications for oxidation and disinfection, which has to be generated on site by means of a silent electrical discharge. The concentration required for disinfection of water potable supply is 0.4 mg/l which should be maintained for 5 min and for sporicidal activity, 2 mg/l is required. Apart from water disinfection, ozone is increasingly being applied in cooling tower systems and before sand or active carbon filtration for the removal (oxidation) of certain dissolved organic compounds, e.g. phenols, turbidity, iron, manganese and colour.

The main advantages of ozone include:

O₃ can be generated easily as and when it is required, thereby not requiring any storage facilities.

O₃ does not form carcinogenic organic residues.

O₃ has a broad spectrum of activity and is an excellent viricide.

Microorganisms do not develop resistance towards ozone.

O₃ breaks down easily to oxygen so is unlikely to pose any risk of taint.

The main disadvantages of ozone include:

Discoloration, bleaching.

Easily quenched by organic matter in the water.

Possible formation of by-product, for instance bromate in the presence of bromide.

Generating has a high energy demand.

Capital and maintenance costs of generators can be high.

Because of its high volatility, ozone is not recommended for evaporative cooling systems.

Please continue reading in the next article for rest of the treatment methods and their impacts to get a complete understanding.