Water Quality and Food Safety – VII

Safety and Quality of Water

From the processor’s perspective, water quality and safety begins at the source of the raw materials and the key to good water safety and quality management is for all manufacturing facilities to have effective programs in place to control water microbiological, chemical and physical quality and to verify that the water meets specified requirements for both direct and indirect product uses. Because, water may be adulterated by a number of chemical, heavy metal, microbial and physical hazards that pose potential public health risks if they are present at high levels. The water risk management plans need to consider above risks in risk analysis process, where microbial hazards include waterborne pathogens such as E. coli, Salmonella and Listeria monocytogenes, Vibrio cholera, viruses and parasites, such as Hepatitis A virus, Giardia and Crytosporidium parvum. Chemical and heavy metal hazards in water range from the presence of lead, copper, methyl-tertiary-butyl-ether (MTBE), total trihalomethanes (TTHM), arsenic and benzene, to name a few. Physical hazards may include natural particulates, and glass and metal fragments. The potential for any one of these hazards to be present in a facility’s water supply requires, where food manufacturers to develop a solid water quality and safety management strategy.

Developing a risk-based water monitoring program, or water safety/quality assurance procedure, as part of ongoing food safety management system while considering Hazard Analysis and Critical Control Points (HACCP) requirements to comply with any national or international quality/ safety requirements is an excellent way to ensure that effective controls for water are achieved to prevent product exposure to spoilage or pathogenic microorganisms, to potential chemical contaminants and to possible physical hazards. Assessing and monitoring water quality and safety includes considering incoming water requirements, water utilities controls and treatments, and microbiological and chemical sampling and testing protocols that best meet your operation’s needs.

Water Quality Analysis

All food processors should test water in the plant from different outlets at least once each year—and preferably more often. Operators should collect water samples from the farthest faucet from the line in the facility and preferably from the cold side. This should be done even if water is obtained from a city water system. The water quality as it leaves a treatment plant and its condition when it gets to your plant may vary, which is especially true in cities where pipelines are old. If the water pipes are iron, it is quite easy to pick up that metal from the lines. High iron water, whether from old pipes or a natural source, is quite easy to detect, what necessary is to look for iron stains wherever there are leaks or drips.

Along these lines, processors should always request that the city provide them with water test results, but provided results are those obtained at the water treatment facilities. Having city water records does not preclude the processor from testing water from their own operations, however, it is your duty to test your supply. If water from multiple sources is being used (wells, city or wherever), be sure that samples from each source are tested separately. Considering the testing schedule, both microbiological and chemical parameters should be tested, because these analyses may be used to do more than just assure safety of your food and ingredients. Knowing the chemistry of the water coming into the plant will help in other areas, where microbial analyses should include total counts and coliforms. If there are concerns that the water may have been contaminated with runoff from fields or elsewhere, you may want to look for pathogens or parasites. Chemical tests should include pH, water hardness, heavy metals, pesticides, iron and nitrates. Water samples for complete chemical analyses should collected at least once a year and submitted to a recognized and accredited water testing laboratory.

Testing the microbiological quality of the water should be done more frequently, and be sure to establish documented programs for water sampling. The instructions should include how to sample, how often to sample and where to sample. The procedure should also instruct what tests should be done and the methods for doing the work as well as reference standard test and identification codes for verification purposes. If third parties are to be used for sampling and/or testing, be sure that the follow your procedures. Maintain all your records and testing procedures in a separate file or binder so that test results may be quickly and easily accessed. 

Installing sample ports on water lines is a good idea, provided they are installed properly. Don’t leave a large deadleg. It is also a good idea to allow the sample port to “run” for a short period to flush the port. If water samples are being collected for microbiological testing and the water is chlorinated, be sure that the sampling program includes a step to neutralize any residual chlorine (your sampling schedule should guide to include a thiosulfate tablet in the field sampling kit, that will meet this need). 

There are processors who have built additional safety into their systems by treating all waters entering the plant with chemicals or by UV light systems. Whether the added costs are worthwhile, only time will tell, but no effort to assure safety should be criticized.

Standard Operation Procedure for Water Safety and Quality – An Example

The following example is a generic water quality and safety procedure which might not be a very comprehensive document, but it provides the way you need to consider about your water treatment program based on your own requirements.   

1.0       SCOPE AND APPLICATION

1.1       This SOP covers the initial processing and preservation of water quality samples collected for analysis of pH, alkalinity, turbidity, apparent color, specific conductivity, total and volatile suspended solids, soluble reactive phosphorus, dissolved and total P, ammoniatot, nitrate + nitrite, dissolved and total N, dissolved and total organic carbon, major cations (Ca, Mg, Na, K), metals (e.g. Fe, Mn), and major anions (SO₄, Cl, F).

1.2       It assumes that one 1-liter sample has been collected for measurement of turbidity, apparent color, and suspended solids, 1-2 syringes have been collected for analysis of pH and alkalinity, and one 1-liter sample has been collected for the analysis of the remaining water quality parameters.

2.0       DESCRIPTION OF METHOD

A.        Definitions - N/A

B.         Health and Safety Warnings - Lab coats, gloves, and safety glasses should be worn when working with acids. Addition of strong acids to samples for preservation should be done under a hood.

C.   Interferences - Samples can be contaminated through hand contact (e.g. Na, Cl, PO₄), through contact with unclean surfaces, through aerial deposition (dust), or through dissolution of acid aerosols released from strong acids. Disposable gloves should be worn when processing samples to avoid hand contact. Samples and filters should not be touch directly; filters may be handled with forceps. Samples should not be left standing uncovered. Samples should be processed in the order described below to avoid cross-contamination from acids used in preservation. Ideally, acidification of samples with HNO₃ or H₃PO₄ should take place in a room other than that used for processing of nutrient or anion samples. Samples should be divided and filtered within 24 hours of collection. Samples must be shaken well before splitting; otherwise sample characteristics will be biased and subsequent analyses invalid. Nutrient and organic carbon samples may deteriorate (mineralize) if left at room temperature for too long; these should be kept at 4ºC until processing, and then analyzed or frozen or preserved with acid until analysis.

3.0       PERSONNEL QUALIFICATIONS  

3.1       Personnel should be familiar with general lab safety practices and steps necessary to avoid contaminating samples. Personnel preparing samples for the first time should be supervised by a staff member familiar with these procedures, and blanks samples be prepared and processed to ensure good techniques have been followed.

4.0       MATERIALS AND PROCEDURES

4.1       Materials

4.1.1         Nalgene-type filtration units (250 mL capacity), Vacuum pump, or hand pump, or lab vacuum (least desirable), Vacuum tubing or thick-walled Tygon tubing

4.1.2          Ultrex HNO₃ (or equivalent)

4.1.3          Phosphoric acid (H₃PO₄) 50%

4.1.4          Calibrated Eppendorf (or equivalent) pipettes (Repeating Eppendorf or 10-100 μL pipette)

4.1.5          Deionized water in wash bottle

4.1.6          10% HCl in wash bottle

4.1.7          10% HNO₃ in wash bottle

4.1.8          Membrane filters (Millipore) (< 0.45 μ pore diameter; 47 mm diameter) and forceps

4.1.9          Glass fiber filters as prefilters for turbid samples (47 mm diameter, e.g. GF A/E)

4.1.10        Pre-cleaned sample bottles (see pages 10-11 for volumes needed)

4.1.11        Pre-printed sample labels (corresponding to numbers on field-collected samples)

4.1.11.1  Sample numbers on the 1-L composite samples (C-nnnn) should match those on the subsamples poured off and/or filtered from the main sample

4.1.12        Sample tracking sheet

4.2       Sample filtering (for filtered sample processing)

4.2.1          Clean and soak filtration apparatus for at least one hour in deionized (DIW) water or 10% HCl or 10% HNO₃ prior to filtering samples.

4.2.2          Rinse each filtration apparatus at least 3x with DI water prior to filtering samples.

4.2.2.1    Use forceps when handling o-rings to avoid contamination.

4.2.2.2    When putting filtration apparatus back together, check to make sure that all o-rings are in place and in good condition to prevent leakage.

4.2.2.3    Using clean forceps, place filter(s) on the base of the filtration apparatus.

4.2.2.3.1    Do not touch filter or inside of filtration unit with your hands; disposable gloves should be worn.

4.2.2.3.2    Make sure filter is centered and does not overlap around edges of unit.

4.2.2.4    Replace top of filtering apparatus on bottom and tighten using the white ring around bottom of top unit, while holding down top of unit.

4.2.2.5    Wiggle the top of the unit to make sure it is screwed down snugly; if it is not, it will leak around the edges.

4.2.3          Filter cation, anion, dissolved nutrient, and dissolved organic carbon subsamples from 1-L composite sample (C-nnnn) bottle as follows:

4.2.3.1    Anion and samples will be filtered with the DI-water washed filtration apparatus.

4.2.3.2    Nutrient samples will be filtered with the HCl-washed filtration apparatus.

4.2.3.3    Cation and organic carbon samples will be filtered with the HNO₃ -washed filtration apparatus.

4.2.4          The composite sample (C-nnnn) should be shaken thoroughly before dividing into samples as to eliminate analysis bias.

4.2.5          Put approximately 20 mL of sample from composite (C-nnnn) bottle into the top of the unit, swirl, and apply suction to the filter.

4.2.5.1    The filtering apparatus is attached to the vacuum tubing and uses the vacuum pump to suction.

4.2.5.2    Release vacuum from base of unit by releasing cap on side port of filtration unit.

4.2.6          Swirl the water around the bottom of the apparatus and discard, or use to rinse out subsample bottles if only a small amount of parent sample is available.

4.2.6.1    Rinse pre-cleaned cation (M), anion (AN), nutrient (FN), or organic carbon (FC) bottle with a little filtered sample and discard before filling bottle completely

4.2.7          Fill FN bottle to just below neck to allow for sample expansion during freezing. Place FN bottle in freezer, AN bottle in refrigerator, and FC and M bottles in hood for acid additions.

4.2.8          Rinse filtering apparatus with appropriate 10% acid solution and 3X DIW between samples.

4.2.8.1    If an organic matter film builds up on the HCl-apparatus, it may need to be wiped out or scrubbed and re-soaked before additional use.

4.2.9          If sample is very turbid, a pre-filter may be needed on top of the membrane filter.

4.3       Sample splitting (for unfiltered sample processing)

4.3.1          The composite sample (C-nnnn) should be shaken thoroughly before dividing into samples as to eliminate analysis bias.

4.3.2          The composite sample for unfiltered samples will be poured into 2 sample bottles:

1) unfiltered nutrients (labeled UN-nnnn) and

2) total organic carbon (labeled UC-nnnn).

4.3.3          Fill UN bottle to just below neck to allow for sample expansion during freezing. Place UN bottle in freezer, and UC bottle in hood for acid additions.

4.4       Sample preservation

4.4.1          Freeze nutrient samples (FN, UN).

4.4.2          Refrigerate anion samples (AN).

4.4.3          Acidify cation (M) samples to pH 1-2 with Ultrex HNO₃ and refrigerate. (Start with 100 uL, check pH, add in 50 uL increments until desired pH).

4.4.3.1    Check pH on a subset of samples by pouring a small amount of sample into a beaker and testing with pH paper.

4.4.4          Acidify organic carbon samples (FC, UC) to pH 1-2 with 50% phosphoric acid (use 4 drops ~50uL then check pH) and refrigerate.

4.4.4.1    Check pH on a subset of samples by pouring a small amount of sample into a beaker and testing with pH paper.

4.5       Sample storage and tracking

4.5.1          Samples will be stored in CT rooms (refrigerated samples: UC, FC, AN, M) or in the core freezer (FN, UN) in labeled boxes by sample type.

4.5.2          Check the labels on the shelves for the proper location of different sample types for the WSDT project.

4.5.2.1    The box should be labeled with the date, batch number (corresponds to date of collection, e.g. 6/10/16 = batch 910), team, project, sample type, and holding time. Use preprinted labels provided for this purpose.

4.5.3          Fill out sample tracking sheet with each sample set processed.

5.0 DATA ACQUISITION, CALCULATIONS, AND DATA REDUCTION

A.        Computer Hardware and Software - Sample tracking sheets are generated in Paradox after sample numbers are entered from water quality sample field sheets into the R:\WATERSHD\WQ97\SAMPLES.DB file and WQTRACK.SAS is run.

B.         Data Management and Records Management - See Water Quality Information Management Plan for details. Samples are tracked by a unique sample code and batch number (corresponding to collection date).

6.0 QUALITY CONTROL AND QUALITY ASSURANCE SECTION

6.1       Field sample blanks, field bottle blanks, and field duplicate samples are all processed the same as blind samples at this point. Laboratory filter blanks are done with every filtering event. These blanks are used in analyses for background determinations.

6.2       QA sample types are recorded on field sheet sample forms and tracked through the information management system.

7.0 REFERENCES

7.1       APHA. 1992. Standard methods for the examination of water and wastewater, 18th edition. American Public Health Association, Washington, D.C.

7.2     U.S. EPA. 1983. Methods for chemical analysis of water and wastes. U.S. Environmental Protection Agency, Cincinnati, Ohio. EPA_600/4-79-020.

7.3     U.S. EPA. 1988. Chemical characteristics of streams in the Mid-Atlantic and Southeastern United States (National Stream Survey - Phase I): Volume I: Population descriptions and physico-chemical relationships. Office of Acid Deposition, Environmental Monitoring and Quality Assurance, Washington, D.C. EPA/600/3-88/021a.

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.

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.