The consumption and discharge of water by food industries, in particular the meat processing industry, could be considerably minimized by means of water re-use (Casani et al., 2005). However, water re-use has not been widely practised due to a number of constraints and barriers. These constraints range from economic and legislative barriers, to the perceived complexity of testing and documentation required, and the negative perception and poor acceptance of water re-use by the consumer.
Previously, meat processors have combined wastewater from all operations and directed them to a single treatment system. More recently, meat processors have seen the benefits of segregating various process water streams so that direct re-use is possible or water treatment can be optimized. An example of direct re-use is slaughter-floor knife and equipment sterilizer water being collected for stock and stockyard washing. Most sterilizer water is only lightly contaminated, and due to the high temperatures, is free from pathogenic bacteria. Other examples of optimized water treatment are re-use of chiller shower water from meat processing (Mavrov and Belieres, 2000), carcass scalding water from swine processing (Miller et al., 1994), and carcass washing water from poultry processing (Chang and Sheldon, 1989).
A significant number of practical suggestions and recommendations on how water utilization can be minimized and waste discharge reduced can be found in McNeil and Husband (1995). In particular, a significant body of knowledge addressing the waste issues facing the meat processing industry has been assembled by the European Commission (BREF SA, 2003). At the time of publication, November 2003, this BREF document on the slaughterhouse and animal by-product industries, represented a compendium of the BAT for meat processing in Europe.
The treatment of wastewater involves the removal of foreign suspended and/or dissolved solids, organic compounds and inorganic salts. A hierarchy of treatments is commonly used to effect this removal. These treatments are categorized as primary, secondary and tertiary treatments. Primary treatment might involve mechanical screening, fat separation, dissolved air flotation or coagulation/flocculation (BREF SA, 2003). Secondary treatment generally involves a form of biological treatment, which converts soluble and colloidal materials into biosolids. Both aerobic digestion (activated sludge) and anaerobic digestion are used either individually or in combination. Tertiary treatment such as filtration - e.g. using sand filters, reed beds (constructed wetlands), coagulation/flocculation or precipitation - are sometimes used to make the treated effluent suitable for discharge to a watercourse.
Constructed wetlands have found favour in the Czech Republic where over 100 operations have been established (Vymazal, 2002). Construction costs are equivalent to those for conventional treatment systems, but the operational and maintenance costs are significantly lower.
A recent publication on coagulation/flocculation of slaughterhouse wastewater reported improved performance through the use of anionic polyacrylamide as a coagulant aid, in combination with a range of commercially available coagulants (Aguilar et al., 2005). The use of anionic polyacrylamide increased the efficiency of flocculation and the speed of settling, while reducing the overall cost of the process.
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