Svetlana B. Zueva Voronezh State Technology ...

Svetlana B. Zueva Voronezh State Technology Academy 19 Revolution Ave., Voronezh, 394000 Russian Federation [email protected]

VARIREI 2009 – L’Aquila, Italy Paper Purification of Waste Water from Food Industry

Preservation and protection of water resources is one of the most serious socio-economical problems of contemporary society, all over the world. Potable water consumption by industries has grown steadily throughout the 20th century. Presently, consumption by industry is equal to about 20% of total water consumption. This paper gives an overview of the methods currently adopted in treating waste-water from food industries, lending recommendations in order to select the best solution for a given case. The paper also highlights the interesting perspectives opened by a research conducted in Russia on the utilization of by-products of aluminum industry which are currently wasted. Compared to other industries, the foods industry is one of the least voracious of water. It does however consume large amounts of water for heating systems, cooling and freezing of foodstuff and technological equipment, cleaning and disposal of waste foodstuff, disinfection, etc. The most peculiar feature of waste-water from food industry is its extremely high content of organic materials. This is best characterized by OCI (Oxygen Consumption Index by natural oxidizing process) and OBCUI (Bio-Chemical Oxygen Consumption Index). Furthermore, these waters contain plenty of organic materials resistant to microbiological decomposition (e.g. fats and some suspended matters, synthetic surface-active reagents etc.). Finally, waste water from food industry has a very high content of bacteria of all kinds. Suspended matter in waste-water from a milk processing plant consists of solid wastes and by-products of the milk processing steps (small and micro particles of scum, curds, curd grain etc.) as well as technological dirt (sand, soil particles) washed-off or cleaned-off from the floors, pickings’, equipment, from personnel garments and footwear etc. These suspended matters are mainly in the form of highly concentrated disperse systems of ‘emulated’ organic matter.



The main source of waste water contamination at meat packing plants are animal carcasses, fat, blood, manure, body tissues, hair and fragmented bones. Other contaminants are also present, the most frequent ones being salt, nitrates, detergents and sand. Overall water consumption and contaminants in waste-water depend largely on the water supply system, which can be either ‘straight flow’ or a combination of ‘straight flow’ and ‘return’ water lines. Return water is used mainly by freezers and refrigerators, vacuum equipment and other equipment where there is no direct contact of water with foodstuff. Cooling water collected after cooling of pasteurized foods and condensed steam from vacuum chambers can be used for cleaning, watering etc. The correct selection of a food-plant-waste-water processing and cleaning system depends also (and considerably) on the chemical composition of water sources. Typically, the purpose of a food-industry waste-water treatment plant is twofold: 1) lowering concentration of fats and suspended matter (primarily to avoid clogging of sewage systems) and 2) extraction proteins and fats. In the case of plants located outside of inhabited areas, biological filtration can and must be provided. Current recommendable waste-water treatment systems for food industry are shown in Fig. 1. System I consists in extracting fats and suspended matter from waste-water by means of a ‘grease catcher’. The settling process takes 30-60 minutes, with the extraction of 30-60% of fat and other organic matter. The resulting organic fatty slug can be processed by means of existing utilization technologies and the non-organic slug is usually sent to community landfills.



Pumping station Grease catcher

I

Grease

II

Electrical coagulation station

Grease catcher Sewer system

Skimmer

Grease

Separator

III

Electrical floto-coagulation station Skimmer

Separator

IV а Sand trap

Grease

IV б Clarifier

Aerotank

Secondary setting basin

Sludge Sediment Sludge lagoon

Chlorination

Contact tank Sewage farms Water body Dehydrated sludge

Exhibit 1 Food Industry Waste-Water Processing Systems



Experience has shown that sorption treatment gives good results only at the final cleaning stages, after mechanical filtering and other low-cost extraction methods of rough dispersed matters, colloids, and dissolved contaminants. Summarizing, a typical System I process usually consists of slug collection (first step), water filtering and coagulation (second step) and finally sorption (last step). A variant of System I allows for the use of chemical reagents ( the process is in fact called ‘physical-chemical cleaning’). The process requires a more complex waste water treatment station, which comprises slug tanks for mechanical-chemical cleaning. As a result, better precipitation of the slugs (Fig. 2) is achieved. Interesting new perspectives are being opened by a research conducted in Russia on the utilization of wasted by-products from aluminum industry. In fact, the findings of a Research Team of the Voronezh State Academy of Technology (Team Leader: Doct. Svetlana B. Zueva) show that the introduction of a particular chemical composition, which is actually a by-product (currently considered as a waste product) of aluminum alloys etching technology, enhances both extraction and coagulation of fats and other suspended matter in food-industry wastewater treatment. The Structural X-ray analysis of this material dried up at 100°C reveales the presence of modified aluminum oxides, viz., – Al2O3∙3Н2О; Al2O3∙2Н2О (96–98 %) as well as some minor quotients of hydroxides of the metals which were the components of the aluminum alloy (2–4 %). Presently, this material is not used anywhere and is treated as industrial waste. Whereas instead it can significantly enhance the waste-water treatment. In fact, research has shown that its particles become the condensation nuclei of hydrolysis products, thus enforcing coagulation and aggregation of large flocks of suspended matter in



slightly turbid waste-water. As a result, a much better sorption of a number of dissolved contaminants is achieved, improving both their extraction process and the conditions for coagulation. When a higher standard of waste-water cleaning is required, Systems II and III should be used. They allow the extraction of 9097% of fats and suspended matter and a total lowering of the biochemical reactivity coefficient equal to 65 – 70 %. In System II waste water is pumped to the electro coagulation station and then to the settling tank. It is de-foamed with a skimmer and the resulting waste, together with the fatty slug from precipitation tank, is sent to further processing. The return water is pumped to the header of the water treatment station for further cleaning. In System III electro floto-coagulation in combination with flotation is used. This water treatment technology requires less space. by-product

waste water

Setting basin

Grease catcher

Secondary setting basin

Grease Firing range hard home departure Sediment

Sewage farms

Exhibit 2 Food Industry Waste-Water Physical-Chemical Treatment



All described systems do not provide the necessary quality of return water. Consequently, waste-water from plants applying any of them cannot be disposed by direct introduction in natural bodies of water. Further treatment in town treatment plants is required. Drainage of waste-water from small foods plants and farms implies the provision of a biological cleaning station. System IV provides biological treatment of waste water from dairy products plants at the city sewer together with the wastes of other industries and home wastes. Biological treatment can be performed in the form of filtering fields, pools, biological filtering stations, circulation oxidation trenches and aeration tanks. Biological treatment structures must be preceded by mechanical treatment – sand traps, water clarification tanks and digestion tanks (IVа). For local waste water treatment System IVb, the biological treatment area is reduced by 1.2 – 2 times by excluding the mechanical waste water treatment, reducing the volume of aeration tanks (or digestion tanks and biological filters) and slug compacting fields because the principal decontamination of waste water made by the electro-coagulator station. However, electric coagulation before the biological waste water treatment can be recommended only for small dairy foods production where there is not enough area for full scale water treatment. The reason is that electro-coagulation requires special electric power equipment and abundant use of non-ferrous metals (aluminum) and steel. It also requires periodical electrodes replacement.