Water is a limited resource, yet dairy processing characteristically requires very large quantities of fresh water. Water is used primarily for cleaning process equipment and work areas to maintain hygiene standards. The dominant environmental problem caused by dairy processing is the discharge of large quantities of liquid effluent. For plants located near urban areas, effluent is often discharged to municipal sewage treatment systems. In extreme cases, the organic load of waste milk solids entering a sewage system may well exceed that of the township's domestic waste, overloading the system. In rural areas, dairy processing effluent may also be irrigated on to land. If not managed correctly, dissolved salts contained in the effluent can adversely affect soil structure and cause salinity. Contaminants in the effluent can also leach into underlying groundwater and affect its quality (UNEP 2000).
Dairy processing effluents generally exhibit the following properties:
• high organic load due to the presence of milk components;
• fluctuations in pH due to the presence of caustic and acidic cleaning agents, plus other chemicals;
• high levels of nitrogen and phosphorus;
• fluctuations in temperature.
Meanwhile, large volumes of water are available for reuse within the typical dairy factory. Milk contains 87% water, whey contains 94% water, yet when converted into dried products the bulk of this water is lost in the waste as condensate, steam or permeate. Dairy processing also generates large quantities of waste water. At start-up, during interruptions and rinsing, the waste is diluted milk, whey or cream without chemicals and could potentially be recovered. The development of processes for the recovery and reuse of water from dairy processing, requires consideration of the following: assessment and maintenance of quality of the recycled water; reuse options for recycled water within the dairy plant; treatment options for recycling water such as membrane filtration, ion exchange, etc.
Water recovered from condensate, steam or permeate often contains low levels of organics from the medium being concentrated. The type and amount is dependent on the process equipment, the product and the degree of concentration achieved. Monitoring quality parameters such as: pH, conductivity, solids, BOD, total plate count and coliforms, is necessary for safety and quality, with the degree of testing determined by the intended reuse of the water. Water containing even small amounts of organic matter poses a biological hazard if stored without treatment for any length of time. Recycled-water storage tanks need to be monitored with regular flushing and cleaning cycles.
Guidelines for the hygienic reuse of process water have been published by the Codex Alimentarius Commission (2001) and by the New Zealand Food Safety Authority (NZFSA, 2003) (see Fig. 14.8). These guidelines can be used to set the framework of water reuse within the factory. Matching water quality requirements with the type of water available requires analysis of the critical control points and an evaluation of the potential contamination of the food products. For example, water used to wash floors does not need to be treated to the same level as water used to wash equipment that is in contact with the food. The water reuse plan should be integrated with existing Hazard Analysis and Critical Control Points (HACCP) programmes, with programmes for monitoring and control of recycled water (Kirby 2004).
Pursuant to the publication of the water reuse guidelines, the Codex Committee for Food Hygiene decided during the 36th session in 2004 to discontinue the consideration of the draft guidelines for the time being, with the understanding that this decision would be reviewed at a later time (Codex Alimentarius Commission 2004). As yet, the Codex committee have not yet revisited the water reuse guidelines.
In the dairy factory there are two main scenarios for water reuse, either reused water will come into contact with food (raw, intermediate or final product) or it will not come into contact with food (Kirby 2004). The intended use of the water will therefore determine the degree of hazard and type of water treatment required.
The CODEX guidelines for the hygienic reuse of processing water in plants specify the following:
■ Reuse water shall be safe for its intended use and shall not jeopardise the safety of the product through the introduction of chemical, microbiological or physical contaminants in amounts that represent a health risk to the consumer.
■ Reuse water should not adversely affect the quality (flavour, colour, texture) of the product.
■ Reuse water intended for incorporation into a food product shall at least meet the microbiological and, as deemed necessary, chemical specification for potable water. In certain cases, physical specifications may be appropriate.
■ Reuse water shall be subjected to ongoing monitoring and testing to ensure its safety and quality. The frequency of monitoring and testing are dictated by the source of the water or its prior condition and the intended reuse of the water; more critical applications normally require greater levels of reconditioning than less critical uses.
■ The water treatment system(s) chosen should be such that it will provide the level of reconditioning appropriate for the intended water reuse;
■ Proper maintenance of water reconditioning systems is critical;
■ Treatment of water must be undertaken with knowledge of the types of contaminants the water may have acquired from its previous use; and
■ Container cooling water should be sanitised (e.g. chlorine) because there is always the possibility that leakage could contaminate the product.
Fig. 14.8 Proposed draft guidelines for the hygienic reuse of processing water in plants. Based on: Codex Alimentarius Commission (2001).
Food-contact applications include: cleaning/rinsing of equipment; diafiltration water for ultrafiltration or nanofiltration processes; ion exchange resin rinsing or elution; decanter washing of lactose crystals or calcium phosphate precipitates; and dissolution of ingredients such as lactose for secondary crystallisation. Non-food-contact applications include: counter current cooling processes; boiler water and steam generation; floor washing and general sanitation; and fire fighting. It should be noted that these applications may lead to incidental contact with food.
The choice of treatment is determined by the intended reuse of the recycled water and the quality of the available water. It is possible to treat water to such an extent that it reaches potable quality. Wastewater treatments can be classified as primary (physical), secondary (biological) or tertiary (chemical). Waste water that has undergone all three levels of treatment and tested to meet the standards is classified as potable (Palumbo et al. 1997).
Early treatment options for water reuse include the addition of disinfectants to stop bacteria and mould growth such as silver ions, chlorine and chlorine compounds, or mixtures of peracetic acid and H2O2. Such chemical addition may have enabled water to be used for cleaning, but not for food contact. Other treatment options include carbon filtration to remove organic contaminants and ion exchange to remove minerals, particularly for boiler feed water (IDF 1988).
Improvements in membrane materials, cost and process efficiency have enhanced the options for water reuse. Gungerich (1996) examined the use of ceramic ultrafiltration to clean up evaporator condensate from milk concentration. The ultrafilter removed all the micro-organisms and residual protein material thereby improving downstream processing by reverse osmosis. Without pretreatment, the reverse osmosis rapidly lost flux and needed cleaning after only 15 h, with ultrafiltration pretreatment the reverse osmosis maintained flux at 50 L/m2/h and could be run over 24 h. Additionally, the ceramic membrane can be backflushed and sanitised with chlorine or steam without damaging the membrane. Recent studies by Vourch et al. (2005) examined the treatment of dairy rinse waste water and found that single-stage reverse osmosis or two-stage nanofiltration/reverse osmosis was sufficient for water reuse as heating, cooling, cleaning or boiler feed water. Higher quality water, which complied with the drinking water limit, was produced using a two-stage reverse osmosis/reverse osmosis process.
The final water quality relies on continued integrity of the membrane during repeated cycles of processing and cleaning. Membrane failure can occur, either as a slow leak or as sudden failure. Water quality monitoring is integral to the membrane process to assure water quality and safety, and identify leaks as they occur. It is recommended that the recycled water be pasteurised prior to reuse, to reduce microbial contamination and delay microbial growth during storage. Finally storage time of water before use should be minimised (e.g. less than 1 day) at cool to ambient temperatures to minimise microbial growth. The storage tank should also be flushed with a regular cleaning programme to prevent the build up of contamination over time.
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