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Lesson 17. TREATMENT AND DISPOSAL OF DAIRY WASTE WATER
Module 3 Plant hygiene and sanitation
Lesson 17
TREATMENT AND DISPOSAL OF DAIRY WASTE WATER
TREATMENT AND DISPOSAL OF DAIRY WASTE WATER
17.1 Introduction
With increase in demand for milk and milk products, many dairies of different capacities have come up in different places. These dairies collect milk from the producers and then either simply bottle it for marketing, or produce different milk products according to their capacities. Large quantities of waste water originate due to their different operations. The organic substances in the wastes comes either in the form in which they were present in milk or in a degraded form due to their processing. As such, the dairy waste, though biodegradable, are very strong in nature.
Dairy plants process a wide variety of products including milk, cheese, butter, ice cream, yogurt, non-fat dry milk, whey and lactose. The volume and composition of dairy wastes from each plant depends on the types of products produced, waste minimization practices, types of cleaners used, and water management in the plant. Because most dairy plants process several milk products, waste streams may vary widely from day to day.
17.2 Sources of Dairy Wastes
The liquid waste from a large dairy originates from the following sections or plants: receiving stations, bottling plant, cheese plant, casein plant, condensed milk plant, dried milk plant, and ice cream plant. The main sources of dairy effluents are those arising from the following:
Dairy plant operators may choose from a wide variety of methods for treating dairy wastes from their plants. This may range from land application for small plants to operation of biological waste-water treatment systems for larger plants. Some dairy plants may pre-treat the effluents and discharge them to a municipal waste-water treatment plant.
In addition to the wastes from all the above milk processing units, some amount of uncontaminated cooling water comes as waste; these are very often re-circulated.
17.3 Objectives of Treating Dairy Wastes
The objectives of treating dairy wastes are to
With increase in demand for milk and milk products, many dairies of different capacities have come up in different places. These dairies collect milk from the producers and then either simply bottle it for marketing, or produce different milk products according to their capacities. Large quantities of waste water originate due to their different operations. The organic substances in the wastes comes either in the form in which they were present in milk or in a degraded form due to their processing. As such, the dairy waste, though biodegradable, are very strong in nature.
Dairy plants process a wide variety of products including milk, cheese, butter, ice cream, yogurt, non-fat dry milk, whey and lactose. The volume and composition of dairy wastes from each plant depends on the types of products produced, waste minimization practices, types of cleaners used, and water management in the plant. Because most dairy plants process several milk products, waste streams may vary widely from day to day.
17.2 Sources of Dairy Wastes
The liquid waste from a large dairy originates from the following sections or plants: receiving stations, bottling plant, cheese plant, casein plant, condensed milk plant, dried milk plant, and ice cream plant. The main sources of dairy effluents are those arising from the following:
- Spills and leaks of products or by-products
- Residual milk or milk products in piping and equipment before cleaning
- Wash solutions from equipment and floors
- Condensate from evaporation processes
- Pressings and brines from cheese manufacture
Dairy plant operators may choose from a wide variety of methods for treating dairy wastes from their plants. This may range from land application for small plants to operation of biological waste-water treatment systems for larger plants. Some dairy plants may pre-treat the effluents and discharge them to a municipal waste-water treatment plant.
In addition to the wastes from all the above milk processing units, some amount of uncontaminated cooling water comes as waste; these are very often re-circulated.
17.3 Objectives of Treating Dairy Wastes
The objectives of treating dairy wastes are to
1. Reduce the organic content of the waste-water,
2. Remove or reduce nutrients that could cause pollution of receiving surface waters or groundwater, and
3. Remove or inactivate potential pathogenic microorganisms or parasites.
2. Remove or reduce nutrients that could cause pollution of receiving surface waters or groundwater, and
3. Remove or inactivate potential pathogenic microorganisms or parasites.
The level of treatment needed for dairy waste-water for each plant is dictated by the environmental regulations applicable to the location of the dairy plant. The Environmental Protection Agency (EPA) establishes general regulations concerning discharges to surface waters and groundwater. Each state environmental regulatory agency is responsible for ensuring compliance with those regulations. Each plant must have a discharge permit for each outfall discharging to surface waters. The limits within that permit depend on the flow and type of surface water into which the treated waste-water is discharged. If a plant discharges waste-water to municipal sewers for treatment, the municipal treatment system may require pre-treatment of high-strength wastes to bring the waste load down to domestic sewage strength. This allows for proper treatment of waste-water before it is discharged to surface water. For land applications, state regulatory agencies dictate hydraulic loadings and maximum levels of toxic substances that can be land spread on each unit of land.
17.4 Composition of Dairy Wastes
Because more than 95% of the waste load from dairy plants comes from milk or milk products, it is of value to know the average composition of these products. Milk solids are primarily composed of fats, proteins, and carbohydrates. Other constituents in dairy waste water may include sweeteners, gums, flavouring, salt, cleaners, and sanitizers. Biochemical oxygen demand (BOD) is the amount of dissolved oxygen (DO) consumed by microorganisms for biochemical oxidation of organic solids in waste water. The analytical procedure for determining BOD measures dissolved oxygen consumed by a seeded, diluted waste water sample incubated at 20°C for 5 days. One gram of milk fat has a BOD of 0.89 g, whereas milk protein, lactose, and lactic acid have BOD value of 1.03, 0.65, and 0.63 g, respectively. Roughly, one kg of BOD in dairy waste water represents 9 kg of whole milk. Chemical oxygen demand (COD) is the amount of oxygen necessary to oxidize the organic carbon completely to CO2, H2O and ammonia. The COD is measured calorimetrically after refluxing a sample of waste water in a mixture of chromic and sulphuric acid. If the BOD/ COD ratio of waste water is less than 0.5, then the organic solids in the waste are not easily biodegraded. The BOD/ COD ratio for dairy wastes has been reported to range from 0.50 to 0.78.
Some minor constituents, such as phosphorus and chloride, are also very important in the treatment of dairy wastes. Phosphorus is the element that limits plant and algal growth in surface waters. Discharge of any significant levels of phosphorus in waste effluents to surface waters can lead to decreased water quality in lakes and streams. Milk and milk by-products can contribute significant quantities of phosphorus to dairy wastes. The phosphorus content of milk is approximately 1000 mg/ l, whereas whey contains 450 to 575 mg/l. Salty whey and brines can contribute significant levels of chloride to dairy waste water. Chloride concentrations in excess of 400 mg/ l in effluents discharged to streams can result in chronic toxicity to sensitive water insects such as Daphnia magna. Because chloride cannot be removed with biological or chemical treatments, waste minimization is the only method for reducing chloride in dairy wastes. BOD5 values and percentage contribution of various milk components to such values has been reported in Table 17.1.
Table 17.1 BOD5 values and percentage contribution of milk components
The dairy wastes are very often discharged intermittently the nature and composition of wastes also depend on the types of products produced, and the size of the plant. Characteristics of the wastes of a typical Indian dairy, handling about 3,00,000 to 4,00,000 litres of milk in a day have been compiled in Table 17.2.
Table 17.2 Composition of waste water of a typical dairy
17.5 Aerobic Treatment of Dairy Waste Water
Wastes from processing milk products are almost entirely composed of organic material in solution or colloidal suspension, although some larger suspended solids may be present in waste water from cheese or casein manufacturing plants. Sand and other foreign material is present in limited amounts as a result of floor or truck washes. Because milk waste contains very little suspended matter, preliminary settling of solids does not result in any appreciable reduction of BOD.
However, a screen and grit chamber with 0.95-cm mesh wire screen is recommended to remove large particles to prevent clogging of pipes and pumps in the treatment system. This is especially important if the waste is to be pumped with high-pressure pumps, as in spray irrigation. After preliminary treatment in the screen and grit chamber, the waste should be pumped to an equalization tank. With wide variations in waste water flow, strength, temperature, and pH, some reaction time is required to allow neutralization of acid and alkaline cleaning compounds and to allow for complete reaction of residual oxidants from cleaning solutions with organic solids of dairy waste. Ideally, a minimum of 6–12 hours of equalization should be provided to allow for waste stabilization. The equilibrated waste can then be treated with one of the following systems or a combination of treatment systems as explained below:
17.5.1 Treatment ponds or lagoons
Dairy plants in rural areas with insufficient farmland available for land application may be able to use ponds or lagoons for economical treatment of dairy wastes. A pond or lagoon normally consists of a shallow basin designed for treatment of dairy waste water without extensive equipment and controls. The three types of ponds used are aerobic, facultative, and anaerobic.
Wastes from processing milk products are almost entirely composed of organic material in solution or colloidal suspension, although some larger suspended solids may be present in waste water from cheese or casein manufacturing plants. Sand and other foreign material is present in limited amounts as a result of floor or truck washes. Because milk waste contains very little suspended matter, preliminary settling of solids does not result in any appreciable reduction of BOD.
However, a screen and grit chamber with 0.95-cm mesh wire screen is recommended to remove large particles to prevent clogging of pipes and pumps in the treatment system. This is especially important if the waste is to be pumped with high-pressure pumps, as in spray irrigation. After preliminary treatment in the screen and grit chamber, the waste should be pumped to an equalization tank. With wide variations in waste water flow, strength, temperature, and pH, some reaction time is required to allow neutralization of acid and alkaline cleaning compounds and to allow for complete reaction of residual oxidants from cleaning solutions with organic solids of dairy waste. Ideally, a minimum of 6–12 hours of equalization should be provided to allow for waste stabilization. The equilibrated waste can then be treated with one of the following systems or a combination of treatment systems as explained below:
17.5.1 Treatment ponds or lagoons
Dairy plants in rural areas with insufficient farmland available for land application may be able to use ponds or lagoons for economical treatment of dairy wastes. A pond or lagoon normally consists of a shallow basin designed for treatment of dairy waste water without extensive equipment and controls. The three types of ponds used are aerobic, facultative, and anaerobic.
Aerobic ponds are generally 0.5–2.0 meters deep, and contents are mechanically mixed and aerated to allow penetration of sunlight necessary for growth of algae. The algae produce oxygen through photosynthesis and use waste products from the bacteria involved in the biological breakdown of milk wastes. At 20°C, a BOD removal of 85% can be experienced with an aeration period of 5 days.
17.5.3 Anaerobic ponds
Anaerobic ponds are generally used to pre-treat dairy wastes with high protein and fat levels or for stabilizing settled solids. Organic matter is biodegraded and gases such as CH4, CO2, and H2S are produced. To reduce effectively the BOD in anaerobic effluent, an aerobic process must follow to allow aerobic microorganisms to use up the residual breakdown products. The typical retention time for anaerobic treatment ponds ranges from 20 to 50 days.
17.5.4 Activated sludge
Activated sludge is one of the most popular methods for treating dairy wastes. The process consists of aerobic oxidation of organic matter to CO2, H2O, NH3 and cell biomass followed by sedimentation of activated sludge. A portion of the activated sludge is returned to the aeration tank to continue the treatment cycle.
Activated sludge contains a large mass of various microorganisms plus organic and inorganic particles. The concentration of biomass in the aeration or contact tank is normally called the mixed liquor suspended solids (MLSS). Bacteria make up the largest portion of activated sludge in the aeration process. Bacteria are primarily responsible for oxidation of organic matter and formation of polysaccharides and other polymeric materials that aid in flocculation of the microbial biomass. Table 17.3 lists the bacterial genera found in activated sludge. Estimates of aerobic bacterial counts in activated sludge are approximately 1010/g of MLSS or 107–108/ ml. The active fraction of bacteria in activated sludge flocs represents only 1%–3% of total bacteria present. This indicates that the major portion of activated sludge is actually dead cells and extracellular material. Activated sludge does not normally favour growth of yeast, algae, or fungi. Protozoa may represent up to 5% of the MLSS. Protozoa are predators of bacteria in activated sludge; they help reduce effluent suspended solids and soluble BOD.
Activated sludge is one of the most popular methods for treating dairy wastes. The process consists of aerobic oxidation of organic matter to CO2, H2O, NH3 and cell biomass followed by sedimentation of activated sludge. A portion of the activated sludge is returned to the aeration tank to continue the treatment cycle.
Activated sludge contains a large mass of various microorganisms plus organic and inorganic particles. The concentration of biomass in the aeration or contact tank is normally called the mixed liquor suspended solids (MLSS). Bacteria make up the largest portion of activated sludge in the aeration process. Bacteria are primarily responsible for oxidation of organic matter and formation of polysaccharides and other polymeric materials that aid in flocculation of the microbial biomass. Table 17.3 lists the bacterial genera found in activated sludge. Estimates of aerobic bacterial counts in activated sludge are approximately 1010/g of MLSS or 107–108/ ml. The active fraction of bacteria in activated sludge flocs represents only 1%–3% of total bacteria present. This indicates that the major portion of activated sludge is actually dead cells and extracellular material. Activated sludge does not normally favour growth of yeast, algae, or fungi. Protozoa may represent up to 5% of the MLSS. Protozoa are predators of bacteria in activated sludge; they help reduce effluent suspended solids and soluble BOD.
Table 17.3 Bacterial genera found in activated sludge
17.5.4.1 Conventional process
In the conventional activated sludge process, dairy waste-water is introduced into the aeration tank along with a portion of activated sludge from the clarifier. Air is incorporated into the waste mixture with diffusers or mechanical aerators. The air serves two purposes in the aeration tank: first, to supply oxygen to aerobic microorganisms and, second, to keep the activated sludge floc thoroughly mixed with incoming waste-water to allow maximal efficiency in oxidation of organic matter. Key parameters controlling operation of the activated sludge process are rate of (a) aeration in the tank, (b) return of activated sludge to the aeration tank, and (c) waste or excess sludge discharged from the treatment system. Normal detention time for conventional activated sludge treatment of municipal or low strength waste-water is 4–8 hours. However, dairy waste-waters may require longer detention times, 15–40 hours, to reduce BOD5 to an acceptable level. This type of process is called an extended aeration system.
17.5.4.2 Contact stabilization process
Another modification of the activated sludge treatment is a three-step process known as the contact stabilization process (Figure 17.1). This process allows for a 30 minutes detention time in the contact tank in which microorganisms obtain their food. Sludge containing the organisms and their food is separated in the clarifier. Sludge that is to be returned to the contact tank is first sent to an aerated stabilization tank for 4–8 hours during which time microorganisms finish digesting their food. By aerating only sludge that is being returned to the initial contact tank, less tank space and less air are required. This system produces less sludge and is better suited for shock loading. The BOD of dairy waste-water could be reduced by 99% and total Kjeldahl nitrogen by 91% after a total detention time of 19.8 hours in this type of system.
In the conventional activated sludge process, dairy waste-water is introduced into the aeration tank along with a portion of activated sludge from the clarifier. Air is incorporated into the waste mixture with diffusers or mechanical aerators. The air serves two purposes in the aeration tank: first, to supply oxygen to aerobic microorganisms and, second, to keep the activated sludge floc thoroughly mixed with incoming waste-water to allow maximal efficiency in oxidation of organic matter. Key parameters controlling operation of the activated sludge process are rate of (a) aeration in the tank, (b) return of activated sludge to the aeration tank, and (c) waste or excess sludge discharged from the treatment system. Normal detention time for conventional activated sludge treatment of municipal or low strength waste-water is 4–8 hours. However, dairy waste-waters may require longer detention times, 15–40 hours, to reduce BOD5 to an acceptable level. This type of process is called an extended aeration system.
17.5.4.2 Contact stabilization process
Another modification of the activated sludge treatment is a three-step process known as the contact stabilization process (Figure 17.1). This process allows for a 30 minutes detention time in the contact tank in which microorganisms obtain their food. Sludge containing the organisms and their food is separated in the clarifier. Sludge that is to be returned to the contact tank is first sent to an aerated stabilization tank for 4–8 hours during which time microorganisms finish digesting their food. By aerating only sludge that is being returned to the initial contact tank, less tank space and less air are required. This system produces less sludge and is better suited for shock loading. The BOD of dairy waste-water could be reduced by 99% and total Kjeldahl nitrogen by 91% after a total detention time of 19.8 hours in this type of system.
Fig. 17.1 Activated sludge system with contact stabilization
17.5.4.3 Flocculation
Settling of sludge in the clarifier usually proceeds best when the microbial growth rate is slow and nutrient concentrations are very low. Extracellular polysaccharides and slimes produced by Zooglea ramigera and other activated sludge organisms play a leading role in bacterial flocculation and floc formation. Good sludge settling and BOD removal occurs at high MLSS concentrations. Microbial flocculation can be enhanced with addition of poly-electrolytes, alum, or iron salts as coagulants. Poor settling of sludges may be observed if excess production of exo-polysaccharides by bacteria occurs in activated sludge. This non-filamentous bulking may be corrected with chlorination. Filamentous bulking may be caused by excessive growth of filamentous bacteria such as Sphaerotilus spp. or Nostocoida limicola. A low level of dissolved oxygen in the aeration tank is the primary factor contributing to growth of this filamentous bacterium in activated sludge.
17.5.5 Biological filtration
17.5.5.1 Trickling filters
Biological filters, such as trickling or percolating filters, are one of the earliest types of biological waste treatment. In a biological filter, the biofilm is attached to a support substance such as gravel, stones, or plastic materials. As wastewater is pumped over the biofilm, it oxidizes organic matter and removes nutrients such as nitrogen and phosphorus. A basic trickling filter is composed of a tank containing a filter medium to a depth of 1.0–2.5 metres, a wastewater distributor that applies the waste solution evenly over the medium bed, and a final clarifying tank to remove sludge and solids sloughing off the filter medium (Figure 17.2).
Settling of sludge in the clarifier usually proceeds best when the microbial growth rate is slow and nutrient concentrations are very low. Extracellular polysaccharides and slimes produced by Zooglea ramigera and other activated sludge organisms play a leading role in bacterial flocculation and floc formation. Good sludge settling and BOD removal occurs at high MLSS concentrations. Microbial flocculation can be enhanced with addition of poly-electrolytes, alum, or iron salts as coagulants. Poor settling of sludges may be observed if excess production of exo-polysaccharides by bacteria occurs in activated sludge. This non-filamentous bulking may be corrected with chlorination. Filamentous bulking may be caused by excessive growth of filamentous bacteria such as Sphaerotilus spp. or Nostocoida limicola. A low level of dissolved oxygen in the aeration tank is the primary factor contributing to growth of this filamentous bacterium in activated sludge.
17.5.5 Biological filtration
17.5.5.1 Trickling filters
Biological filters, such as trickling or percolating filters, are one of the earliest types of biological waste treatment. In a biological filter, the biofilm is attached to a support substance such as gravel, stones, or plastic materials. As wastewater is pumped over the biofilm, it oxidizes organic matter and removes nutrients such as nitrogen and phosphorus. A basic trickling filter is composed of a tank containing a filter medium to a depth of 1.0–2.5 metres, a wastewater distributor that applies the waste solution evenly over the medium bed, and a final clarifying tank to remove sludge and solids sloughing off the filter medium (Figure 17.2).
Fig. 17.2 Trickling filtration process
In some instances, wastewater is re-circulated through the system to provide for added dissolved oxygen to primary influent and greater removal of BOD. The two most important factors affecting microbial growth on the support medium are flow rate of wastewater and size and geometrical configuration of support material. In the initial start-up of the filter, the medium surface is colonized by gram-negative bacteria followed by filamentous bacteria. The biofilm formed on support material is called a zoogleal film and is composed of bacteria, fungi, algae, protozoa, and other life forms such as rotifers, nematodes, snails, and insect larvae. Some of the bacterial genera active in trickling filters are Achromobacter, Flavobacterium, Pseudomonas and filamentous bacteria such as Sphaerotilus. Growth conditions on the outer surface of the biofilm are aerobic but the inner portion of the biofilm next to support material tends to be anaerobic. Trickling filters are categorized by the loading rate to the filter medium. Low-rate trickling filters (40 kg BOD/ 100 m3/ day) allow for nitrification and more complete removal of nutrients from wastewater. High-rate filters (60–160 kg BOD/ 100 m3/ day) rarely have nitrification take place and have lower treatment efficiencies. BOD removal by trickling filters is approximately 85% for low-rate filters and 65–75% for high rate filters.
17.6 Anaerobic Digestion
Anaerobic digestion has been used to stabilize waste treatment sludges for many years. However, in recent years, it has also been designed to treat high-strength dairy wastes. In anaerobic breakdown of dairy wastewater, lactose is first fermented to lactic acid and fats and proteins are hydrolyzed to organic acids, amino acids, aldehydes, and alcohols. Second, the intermediate organic compounds are converted to methane and CO2. Because anaerobic digestion does not require oxygen for decomposition of organic material, operating costs for treatment are greatly reduced from that of aerobic treatments. However, it is a much slower treatment process that is more susceptible to toxic upsets.
17.6.1 Conventional process
The anaerobic digester is a large fermentation tank in which fermentation, sludge settling, sludge digestion, and gas collection take place simultaneously. Many dairy plants use a two-stage system in which the first stage is complete mixing of the contents of a fermentation tank and the second stage is a digester in which the contents are allowed to stratify. The two-stage anaerobic process allows for higher loading rates and shorter hydraulic retention times. In anaerobic treatment of wastewater, fermentation of sugars, amino acids, and fatty acids is primarily carried out by strict and facultative anaerobic bacteria such as Bacteroides, Bifidobacterium, Clostridium, Lactobacillus and Streptococcus. Production of methane from fermentation intermediate compounds is accomplished by methanogenic bacteria, which are strict anaerobes. Approximately two-thirds of the methane is derived from acetate conversion by acetotrophic methanogens and the other one-third is the result of carbon dioxide reduction by hydrogen. Methanogens are difficult to grow in pure culture. The milk fat was inhibitory to methanogenic bacteria, and dairy effluents should be treated by anaerobic digestion only after the milk fat concentration was less than 100 mg/ l. They also indicated that anaerobic cultures at the start-up of anaerobic digestion should be acclimatized to casein to ensure proper degradation of casein in the process. Methanogenic bacteria are also sensitive to acidic conditions with complete inhibition below pH 6. With efficient operation of one- or two-stage anaerobic digesters, dairy plants should experience BOD reductions of 78–95%. Biogas from the digester contains up to 67– 75% methane.
17.7 BOD Measurement
Biological (or biochemical) oxygen demand (BOD) is an important parameter in water resource management. BOD is a parameter used to measure the quality of water and treatment results in wastewater. In addition, BOD analysis potential is used in the planning and design wastewater treatment facilities. In routine use BOD determination is used to check the wastewater in the inflow and discharge of wastewater treatment plants. The BOD determination is an empirical test in which standardized laboratory procedures are used to determine the relative oxygen requirements of Waste waters, effluents, and polluted waters. The test has its widest application in measuring waste loadings to treatment plants and in evaluating the BOD-removal efficiency of such treatment systems. The test measures the molecular oxygen utilized during a specified incubation period for the biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidize inorganic material such as sulfides and ferrous iron. It also may measure the amount of oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand) unless their oxidation is prevented by an inhibitor. The seeding and dilution procedures provide an estimate of the BOD at pH 6.5 to 7.5.
The method (5-day BOD) consists of filling with sample, to overflowing, an airtight bottle of the specified size and incubating it at the specified temperature for 5 days. Dissolved oxygen is measured initially and after incubation, and the BOD is computed from the difference between initial and final DO. Because the initial DO is determined shortly after the dilution is made, all oxygen uptake occurring after this measurement is included in the BOD measurement.
17.8 Treated Dairy Effluents and Its Disposal
Dairy plants discharging waste waters directly to streams, bays, rivers, creeks and /or estuaries must have a permit for this discharge. Dairy plants that use non-discharge systems such as land disposal will also need a permit. Permits for discharge are usually obtained from the state government control agency. Effluents from waste treatment systems must be sufficiently reduced in BOD and biological nutrients (e.g., P, NH3) that discharge to surface waters does not significantly affect aquatic life. Environmental regulatory agencies specify limits for composition of effluents discharged to each type of stream or watershed. To reduce the volume of dairy wastewater to be treated and reduce treatment costs, careful attention must be given to minimizing losses of milk and milk products in the dairy plant. With good product conservation and selection of an effective waste treatment process, dairy plant operators should be able to operate profitably and meet environmental requirements.
Last modified: Wednesday, 7 November 2012, 4:13 AM