Introduction
Food plants basically produce three types of waste: liquid, solid and sanitary. At one time liquid or suspended wastes from food plants were simply flushed into the local sanitary system or they were pumped into a nearby river without any treatment. Solid waste was sent to a convenient landfill for burial or burned in an incinerator located ‘out back’ on the plant ground.
These expedients were not without problems, however, because:
- Rivers lakes and streams receiving liquid waste often became fouled with resulting death of wildlife/or unpleasant smell.
- Municipal disposal system often became inoperative or worked poorly when large amounts of food plant wastes were suddenly flushed into them. This also resulted in odors and pollution from improperly treated outfall.
- The burial of solid waste also began to encounter resistance because materials and by-products from these landfills began to percolate into the ground water in many areas.
- In addition many landfills were regarded as nuisance because of the odors they created and legislation began to appear that severely restricted the location and operation of these facilities.
- Burning of solid waste was not considered to be satisfactory because of the certainty of air pollution that accompanies this activity.
Liquid waste
Depending on the product being processed and the type of process being used, from 50-90% of food plants waste will be liquid(Air & Waste Management Association, 1995, p. 78). Because these wastes normally contain high levels of nutrients such as carbohydrates or proteins, their treatment as waste can be difficult and costly.
On the other hand, many food plant wastes, because they contain nutrients, can be processed to obtain some recovery of these nutrients as feed for animals or as fertilizers for valuable crops. Liquid waste also can be used to provide moisture for the growth of such crops.
Characteristics of food plant waste
Biochemical oxygen Demand (BOD).
The capacity to pollute bodies of water is, understandably, one of the most important criteria used to evaluate waste water.The BOD test is a valuable determination of this capacity. What is actually measured in this analysis is the amount of oxygen utilized by an inoculums obtained preferably from the effluent of the treatment facility that will treat the wastewater (Nathanson, 2003, p. 132).Samples usually are diluted to obtain optimum reaction levels during the usual 5-day incubation period. Following this interval, the amount of oxygen present is determined by means of a sodium thiosulphate titration to a starch –iodine end point.A dissolved oxygen level obtained at the beginning of the test is then subtracted from the final dissolved oxygen value to obtain the BOD.
A commonly used maximum BOD limit from receiving wastes to a municipal system from a food plant is 300mg/l. Generally waste from dairy and meat processing will produce high BOD levels. While vegetable and fruit processing operations contribute somewhat lower levels.
Chemical Oxygen Demand
The COD analysis also determines the amount of organic material that is easily oxidized; however, this determination includes such materials as nitrites and sulfites in addition to ‘normal ‘organic matter. Although generally comparable with the BOD, the COD is much faster and more reliable than the BOD.
The BOD/COD ratio can be used to estimate the oxidizability of the waste.eg a low BOD/COD ratio indicates that there is a small biodegradable fraction present and large non biodegradable fraction that must be treated by the waste treatment facility. However, high BOD/COD ratios, such as those from animal waste, indicate a large biodegradable fraction. Generally a ratio above 0.5 refers to degradable waste and ratios below this refer to progressively less biodegradable wastes.
Phosphates
Phosphates, important bacterial nutrients in small amounts, are significant contributors to pollution if permitted to enter natural bodies of water in large quantities.
The principal concern is the unrestricted growth of the algae that is stimulated by phosphates with the result that oxygen levels are depleted and fish and other aquatic flora and fauna die.
In addition the proliferation of algae speeds the natural eutrophication of lakes, resulting in the destruction of their aesthetic and recreational value.
Total solids
This parameter is a measure of the total amount of matter of all types(except organic solvents)) in wastewater.These are sum of dissolved, suspended and settle-able solids.
Grease and oils
Because of their tendency to adhere and clog as well as their tendency to be malodorous when decomposing, fats and oils can be extremely difficult forms of waste to treat effectively. In addition these materials float on the surface of the water where they prevent the dissolution of atmospheric oxygen. This contributes to the difficulty encountered in treating this class of waste.
Treatment of waste
Most conventional waste treatment systems consist of stages or series of treatment steps to which the waste is sequentially exposed. During these stages, the BOD/COD levels are gradually reduced until finally, in the case of tertiary treatment systems, an effluent is produced that contains little organic materials and may be added to streams and other bodies of water without harm to wild life or other users.
Depending on the type of process and waste produced by it, the series of treatment may be interrupted with a specific terminal step such as land disposal.
Pretreatment
Pretreatment is primarily used when it is necessary to reduce the amount of solids and BOD relatively quickly to avoid excessively high disposal costs.
Methods of pretreatment
Screens
A major problem with the removal of solids by use of screens is that these devices tend to become clogged and thereby interrupt the waste flow. To prevent this from occurring, various designs have been developed that permit flow of the waste stream while removing the necessary quantities of solids e.g.
Static screens- They tend to clog and their use is often labor intensive because they require frequent cleaning.
Rotary screens- usually consist of large mesh drums through which the waste stream is conducted. As the screen turns, solids are retained within it and conducted out of one end of the drum by gravity.
Tangential screen- waste streams are directed onto a screen located diagonally to the flow. The effluent flows through this tangential screen and is recovered for further treatment. Because the waste stream strikes the screen at an angle (tangentially), the screen operates generally without clogging.
Centrifugal screens-they rely on centrifugal force to separate solid matter from suspending flows of water. To achieve this, the waste is conducted with some pressure into the center of a rotating drum or screen. Centrifugal force moves water through the screen while solids are retained on the surface. While more waste moves through the screen in this situation, waste tends to be broken into smaller particles and little removal of solids is obtained.
Sedimentation
Sedimentation tanks or clarifiers may be constructed on site or prefabricated, usually of steel of other materials and shipped to the treatment area. The velocity of the incoming waste is decreased and particles aggregate and slowly settle to the bottom of the tank.
The clarified waste water is then removed. Additives such as ferrous sulfate promote sedimentation of certain waste and may result in as much as 40% increase in BOD removal.
The sediment or sludge can be returned to the fields or taken for further treatment depending on the composition.
Flotation
Flotation units treat the incoming waste stream with air, which attaches to waste particles and floats to the surface of the treatment vessel. This material is then removed from the surface by means of a continuous skimmer. The aggregation and floatation process can be augmented by pretreatment with a flocculating agent such as Chitosan or compounds known as polyelectrolyte. Operating costs of these are high but it is an effective process.
Secondary treatment of food wastes
Land disposal
This type of disposal is usually carried out in one of three ways:
Spraying, flooding or ridge and furrow application. Flooding is mostly used for food processing wastes. Soil disposal is efficient at reducing BOD levels and phosphorus content, provided that the absorbing crops are carefully selected. Usually grasses and legumes are particularly effective in achieving high nutrient uptake rates.
Lagoons and holding ponds
Lagoons ordinarily obtain their high oxygen levels either by mechanical agitation by the addition of compressed air from pipes in the bottom of the lagoon or by means of floating aerators.
Holding ponds on the other hand, rely on the growth of algae or the dissolution of atmospheric oxygen. Holding ponds tend to be slow systems and are designed to retain wastewater for 60-90 days.
Activated sludge systems
These systems normally consist of two stages; a strongly aerobic or reactor stage involving the introduction of large quantities of oxygen to the treated waste; and a subsequent digestion or clarifier stage in which microorganisms settle out and are returned as inoculums to the clarifier.
The aerobic reactor can take many forms. A common type is the trickling filter, which contains a bed of gravel, plastic balls or other media that provide a support for bacterial mats creating maximal exposure to the nutrients in the wastewater and to ambient oxygen. Other types use pumped air, in which compressors force air through orifices beneath the reactor surface or static diffusers may be used to optimize exposure to oxygen. Following aeration, the wastewater flows to a clarifier where a sludge, composed primarily of bacteria, settles to the bottom of the tank, at this point the effluent, if sufficiently free of organic matter, may be pumped into steams or used to irrigate tillable fields. The sludge is pumped from the bottom of the clarifier and part of the returned solids are moved into open containment’ pens’ where further settling and evaporation occur.
The final dried sludge solids may then be used as a soil conditioner or fertilizer, burned or sent to a land fill .Apart of the pumped sludge stream is added to the aerobic reactor to provide seed or inoculums for the continuing process
Tertiary treatment
The most common and least expensive tertiary treatment is the use of coagulants which are to remove suspended solids. Coagulants are chemicals such as lime or ferric chloride that form a dense precipitate and carry or sweep solids from the waste stream. Activated carbon is occasionally used in the treatment of waste from chemical or pharmaceutical plants to adsorb specific chemical from a waste stream, but seldom used for the treatment of food plant wastes
SOLID WASTE
Solid waste collection and treatment areas should be located in remote and segregated area within the plant or preferably at a specific outside the plant building, but on the premises. Under no circumstances should they be located near stored raw materials or exposed product. Most sanitarian recommend that remotely located solid waste processing area have roofs and be constructed of concrete and placed on a concrete pad.
This building should be clean and well maintained and should a wet bay area to clean trash receptacles and transporting devices. The building should be designed with proper attention to the exclusion of rodents and insects.The waste should then be collected on regular basis for disposal.Solid waste from fruit pits, animal bones, bird feathers and so forth.In almost every case these materials can be processed and recycled into useful and even profitable by products.
Fruit pits can be converted into charcoal and bones are ground and used as fertilizers. In some instances, the disposal of these by – products is a profitable operation whereas in others the food processor eliminates a waste problem without cost, or little cost relative to the expenses of burying, incinerating or otherwise disposing of these materials.
Whatever means selected to dispose of solid waste, it is important that the plants management team be aware of specific environmental regulations related to, for example, burning materials and types of waste permissible in landfills.
ECONOMIC UTILIZATION OF WASTE FROM THE FOOD INDUSTRY
FPR recovery for human uses
Food processing residuals (FPR) contain principally biodegradable organic matter and improper disposal of the materials can have detrimental effect on the environment. In recent years attempts have been made to convert FPR into a variety of valuable products. FPR characteristics of commercial interest include antioxidants, carbohydrates (sugar, oligo- and polysaccharides), fats and oils, pigments,
- starch. Fruit and vegetable residuals are rich in antioxidants.
- Grape seeds, for example, are a good source of natural antioxidants.
- Graviton is a commercial grape seed extract capable of inhibiting oxidative deterioration of lipids, vitamins, and other valuable components.
- Beet and grape residuals are a good source of natural pigments.
- Concord grape pomace, for example, is extremely rich in anthocyanins that have been commercially extracted for food industrial uses.
