Waste Product Handling
The food industry produces waste materials as a by-product of food processing and preparation. Many food processors consume large quantities of water, but food manufacturing sectors vary in their major purposes for using water.
For example, 60% of the water used by meat processors is for cooling, 62% of the water consumed by sauce manufactures is for cleaning; while starch millers use 55% of their water for granule separation.Water serves several functions in food processing including clean-ing, conveying, steam generation, heat exchange, and as an ingredient. Thus, the industry should accept the challenge of handling residues and wastes as part of the production process and apply techniques to improve productivity, quality, and efficiency.
Waste materials generated from food processing and foodservice facilities can present difficulties because they contain large amounts of carbohydrates, proteins, fats, and mineral salts. For example, the wastes from dairy plants; food freezing, and dehydration plants; and processing plants for red meats, poultry, and seafood can produce distinct odors and heavy pollution of water if the discharge is not properly treated. Organic matter from waste materials should be treated through biological stabilization processes before it is discharged into a body of water. Improper waste disposal is a hazard to humans and to aquatic forms of life. This treatment incorporates biological processes to treat the effluent to meet EPA discharge limits and is critical to the treatment plant’s operation.
Accumulation of wastes, even for short periods of time, can attract insects and rodents, pro-duce odors, and become a public nuisance or an unsightly condition inside or outside the plant. The integration among controlled production processes, with low level of losses, and the treatment system and handling of residues (solids, liquids, andg gases is fundamental to administration of waste product handling with an acceptable cost.
Determination of Extent of Pollution
A large percentage of the waste discharged in fruit and vegetable waters, wash water from animal slaughter, and cleanup water discharge are, product pieces (larger pieces can be removed by screening). Finer solids, which pass through a screen, and organic matter in colloidal and true solution usually have an oxygen demand in excess of the dis-solved oxygen content of the water.
Biochemical Oxygen Demand. A frequently used method of measuring pollution strength is the 5-day BOD test. The BOD of sewage, sewage effluents, and waters of industrial wastes is the oxygen (in ppm) required during stabilization of the decomposable organic matter by action of aerobic microorganisms. The sample is stored in an airtight container for a specified period of time and temperature. Complete stabilization can require more than 100 days at 20ºC.
Although BOD is a common measurement of pollution of water and the test is relatively easy to conduct, it is time-consuming and lacks reproducibility. Tests such as chemical oxygen demand (COD) and total organic carbon (TOC) are quicker, more reli-able, and more reproducible.Chemical Oxygen Demand. The COD test for measuring pollution strength oxidizes compounds chemically rather than biologically by a dichromate (K2Cr2O7) acid reflux method.
Dissolved Oxygen. Dissolved oxygen (DO) concentration is of major concern for both wastewater and receiving water because it affects aquatic life and is important in treatment systems such as aerated lagoons. Determination of dissolved oxygen can be accomplished by an iodometric titration procedure using the azide and permanganate procedures to remove interfering nitrite and ferrous ions, even though this method isnot considered to be very reliable. Alternatively, electrode probes can be used for this measurement..
Total Organic Carbon. Total organic car-bon determines all materials that are organic. It measures the amount of CO2 pro-duced from the catalytic oxidation at 900ºC of solid matter in wastewater. This method of pollution measurement is rapid and reproducible, and correlates highly with standard BOD5 and COD tests, but it is difficult to conduct, and requires sophisticated laboratory equipment. Residue in Wastewater. Residue can be considered pollution because it affects the measurements that have been previously dis-cussed. Residues of evaporation (total solids) and the volatile (organic) and fixed (ash) fractions are routinely recognized.Settleable solids (SS) settle to the bottom in 1 hour. They are usually measured in a graduated Imhoff cone and reported as mL/L SS. Settleable solids are an indication of the amount of waste solids that will settle out in clarifiers and settling ponds. This examination technique is easy to perform and can be conducted at field sites.
Total suspended solids, sometimes referred to as nonfilterable residue, are determined by filtration of a measured volume of waste-water through a tared membrane filter (or glass fiber mat) in a Gouch crucible. The dry weight of the total suspended solids (TSS) is obtained after 1 hour at 103 to 105ºC.Total dissolved solids (TDS), or filterable residue, is determined by the weight of the evaporated filtered sample or as the differ-ence between the weight of the residue on evaporation and the weight of TSS. These pollutants are difficult to remove from wastewater, so knowledge of them is essen-tial. Treatment requires microorganisms, which are normally present, for conversion to particulate matter, i.e., microbial cells.
SOLID WASTE DISPOSAL
Disposal of solid wastes is a major challenge for the food industry. In food industries such as canneries, up to 65% of the raw materials received must be disposed of as solid waste. The most common method for disposal has been to truck the wastes to municipal garbage dumps. If composting is used, the organic matter in waste material must be stabilized through microbial action. Humus, which results from stabilization of waste material, improves fertility and tillage properties. The basic com-posting procedures has four steps:
Solid waste material should be comminuted (pulverized) to expose the organic matter to microbial attack.The comminuted waste should be stacked in windrows approximately 2 m high and 3 m wide.Aeration should be provided.
After extensive aeration, the compost should be comminuted again.Addition of an inoculum will accelerate the composting process. This process is pro-duced through those aerobic thermophilic microorganisms present in the waste mate-rial in 10 to 20 days, depending on temperature and waste composition.
LIQUID WASTE DISPOSAL
Whenever food is handled, processed, packaged, and stored, wastewater is generated. Quantity, pollutant strength, and nature of constituents of processing wastewater have both economic and environmental consequences concerning treatability and disposal.
Economics of treatment are affected by the amount of product loss from the processing operations and by the treatment costs of this waste material. Significant characteristics that determine the cost for wastewater treatment are, the relative strength of the wastewater and the daily volume of discharge.
Pretreatment
The pretreatment of food processing wastewater is frequently required prior to discharge into a municipal waste treatment system. Most common pretreatment processes include flow equalization and the separation of floatable matter and SS. Separation is frequently increased by the addition of lime and alum, ferric chloride (FeCl3), or a selected polymer.
Screening
The most frequently used process for pre-treatment is screening, which normally employs vibrating screens, static screens, or a rotary screen. Vibrating and rotary screens are more frequently used because they can permit pretreatment of a larger quantity of wastewater that contains more organic mat-ter.
Skimming
This process is frequently incorporated if large, floatable solids are present. These solids are collected and transferred into some disposal unit or preceding equipment. Lime and FeCl3, or a selected polymer may be added to enhance separation of solids, and paddle flocculation may follow to assist with the coagulation of these solids.
PRIMARY TREATMENT
The principal purpose of primary treatment is to remove particles from the wastewater. Sedimentation and flotation techniques are used.
Sedimentation
Sedimentation is the most common primary treatment technique used to remove solids from wastewater influent because most sewage contains a substantial amount of readily settleable solid material. As much as 40 to 60% of the solids, or approximately25 to 35% of the BOD5 load, can be removed by pretreatment screening and primary sedimentation.
A rectangular settling tank or a circular tank clarifier is most frequently used in pri-mary treatment. Many settling tanks incorporate slowly rotating collectors with attached flights (paddles) that scrape settled sludge from the bottom of the tank and skim floating scum from the surface.
Flotation
In this treatment process, oil, grease, and other suspended matter are removed from wastewater. A primary reason that flotation is used in the food industry is that it is effective in removing oil from wastewater.
Dissolved air flotation (DAF) removes suspended matter from wastewater by using small air bubbles. Flocculants and polymers are added to the wastewater to separate grease, oils, and fats from the water. Flocculating agents are commonly used to pretreat wastewater prior to treatment by a DAF unit. Treatment by DAF is widespread because of the relatively fast passage and because solids of nearly the same as, or lower density than water, can be removed. This treatment technique requires high invest-ment and operating costs, especially for chemical additives and sludge handling.
Secondary Treatment
Treatment through biological (or bacterial) degradation of dissolved organic matter through biological oxidation is the most common technique for secondary treatment.
However, secondary treatment can range from the use of lagoons to sophisticated acti-vated sludge processes and may also include chemical treatment to remove phosphorous and nitrogen or to aid in the flocculation of solids.Most lagoons are earthen basins that contain a mixture of water and waste. A dike or berm usually surrounds a lagoon as a lip of the basin that prevents spills and overflows. The depth of an impoundment (lagoon) depends on the volume of waste to be handled,
with increased depth necessary to contain unforeseeable events, such as weather.
Anaerobic Lagoons
Anaerobic lagoons can be designed with either a single stage or multiple stages. The
disadvantages of multiple stage lagoons are increased construction and land costs.
Advantages are:
There is less floating debris on the sec-ond and third stages, with a reduction in
clogging of the flushing system or irri-gation pump.
The first lagoon, containing a higher concentration of waste, will not over-flow.
An adequate amount of bacteria will be available for waste treatment.
The resulting effluent will be treated more thoroughly.
Aerobic Lagoons
Aerobic lagoons use mechanical aerators to supply atmospheric oxygen for aiding
bio-logical oxidation. Mechanical agitators, designed to pull air under water and
circulate it horizontally, can maintain a dissolved oxy-gen concentration of 1 to 3 mg/L
at a BOD loading rate of up to 450 kg/ha/day. BOD influent is reduced by 70% to 90%.
The solids produced will partially decompose in anaerobic sludge banks in facultative
lagoons, but the completely mixed effluents usually require additional treatment, such
as clarifica-tion or polishing pond treatment.
Trickling Filters
Trickling filters reduce BOD and SS by bacterial action and biological oxidation as
wastewater passes in a thin layer over stationary media (usually rocks) arranged above
an overdrain. Biological degradation occurs almost exactly as in the activated sludge
process, except that the filter is a three-phase system in which the biofilm is fixed on
the solid medium (stones or plastic). Aeration is accomplished by exposing large surface areas of wastewater to the atmos-phere. Layers of zoogloea (filter sludge) grow on and attach to the medium surface. Primary treatment should precede this process if the wastewater suspended solids concentration exceeds 100 mg/L.
Activated Sludge
The activated sludge process is widely used for wastewater treatment. It requires a reactor that is an aeration tank or basin, a clarifier, and a pumping arrangement for returning a portion of the settled sludge to the reactor and discharging the balance to waste disposal. Primary treatment is optional. A portion of the clarifier-settled sludge is returned and mixed with wastewater enter-ing the reactor. The resulting biological solids concentration is much higher than what could be maintained without the recy-cle. The term “activated sludge” applies because this returned sludge has microorgan-isms that actively decompose the waste being treated. This mixture of influent wastewater and returned biological suspended solids is termed the mixed liquor. The activated sludge process is frequently called the fluid-bed biological oxidation system, whereas the trickling filter is referred to as a fixed-bed system.
Land Application
The two types of land application techniques that are the most efficient are infiltra-tion and overland flow. With land application techniques, the pollutants can harm vegetation, soil, and surface and ground waters if not properly operated. However, both of these treatment techniques can effectively remove organic carbon from high-strength wastewater. Although land application has been a standby in the past for discharge of some food processing wastes, this approach is now limited. Hydraulic loads that are high may necessitate an unreasonably large amount of land. Runoff and proper utilization of nutri-ents can restrict the vegetation. Buildup of minerals and other materials in the soil has the potential for long-term liability for residues possibly as yet undiscovered
Tertiary Lagoons
These maturation lagoons, which are known as polishing ponds, are used for terti-ary treatment of secondary effluents from activated sludge or trickling filter systems. This type of lagoon is usually from 0.3 to 1.5 m deep. Natural aeration, mechanical aeration, or photosynthesis provides the oxygen source. BOD5 loading rates normally range from 17 to 34 kg/ha/day, with a reduction range for BOD and SS of 80 to 90%.
The waste removal efficiency of this system is influenced by temperature. This simple method of treatment requires practically no equipment or power, and minimal atten-tion is required for the day-to-day opera-tion. However, the land requirement of this process is the highest of the treatment methods.
Disinfection
The major purpose of disinfection is to reduce the total bacterial concentration andeliminate the pathogenic bacteria in water. A potable water supply requires zero or very low bacterial concentration to avoid disease transmission. The total number of coliforms, instead of the presence of specific pathogens, is often used as an indicator for sanitary quality and the efficiency of disinfection. There are many chemical disinfectants and physical methods that can be incorporated for disinfection. For public health reasons, treated waste-waters should be disinfected before final dis-charge. Addition of a chemical disinfectant to water provides a maximal time of contact between the chemical and organisms, assuring efficient bactericidal action
Deodorization
Treated water may be safe to drink, yet have an unpleasant taste and odor because of the activity of some microscopic organisms such as algae, especially during the summer months. Thus, deodorization is essential to remove the taste and odor in treated water.
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