Food and Industrial Microbiology

Module 10. Dairy waste management

Lesson 32
TREATMENT AND DISPOSAL OF WASTE WATER AND EFFLUENTS

32.1 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, nonfat 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.

32.2 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:

1. Spills and leaks of products or by-products

2. Residual milk or milk products in piping and equipment before cleaning

3. Wash solutions from equipment and floors

4. Condensate from evaporation processes

5. 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 pretreat 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.

32.3 Objectives of Treating Dairy Wastes

The objectives of treating dairy wastes are to

a. Reduce the organic content of the waste water,

b. Remove or reduce nutrients that could cause pollution of receiving surface waters or ground water, and

c. 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 ground water. 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 pretreatment 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 spread on each unit of land.

32.4 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, flavoring, 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, 1 kg of BOD in dairy wastewater 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 wastewater in a mixture of chromic and sulfuric 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. Reported BOD values and percentage contribution of milk 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. The BOD values of various dairy products are shown in Table 32.1.

Table 32.1 BOD values and percentage contribution of milk components to product BOD

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 plants. The Table 32.2 gives the characteristics of the wastes of a typical Indian dairy, handling about 300000 to 400000 lts of milk in a day.

Table 32.2 Composition of waste water of a typical dairy

32.5 Treatment of Milk Wastes

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 h 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: (a) land application, (b) treatment ponds or lagoons, (c) activated sludge, (d) biological filtration, or (e) anaerobic digestion.

32.6 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 wastewater without extensive equipment and controls. The three types of ponds used are aerobic, facultative, and anaerobic.

32.7 Aerobic Ponds

Aerobic ponds are generally 0.5–2.0 m 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.

32.8 Anaerobic Ponds

Anaerobic ponds are generally used to pretreat 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.

32.9 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 (Figure.32.1.).

Fig. 32.1 Activated sludge process

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 32.3 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 favor 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 32.3 Bacterial genera found in activated sludge

32.10 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 h. However, dairy waste waters may require longer detention times, 15–40 h, to reduce BODs to an acceptable level. This type of process is called an extended aeration system.