Treatment of Effluents of Dairy Industry | Environmental Engineering

In this article we will discuss about the treatment and disposal of effluents of dairy industry.

Products and Processes Involved in Dairies:

The number of products that are made in each dairy depends upon the demand and supply of milk in the region. Many dairies in India restrict themselves to bottling pasteurized and homogenized milk and making ghee from soured milk.

A few dairies, where supply is larger than local demand, produce milk products such as butter, condensed milk, powered milk, baby food, ice-cream, cheese and CHHANA. A few daries are also preparing casein.

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Processes:

a. Milk Receiving Section:

The milk cans are unloaded at the receiving station and emptied into a receiving tank after testing for fitness and freshness. Cans which are turned sour are segregated. The milk from each lot of cans is weighed and conveyed to the pasteurization and other units.

b. Pasteurization:

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Pasteurization is accomplished by heating either to 62°C for 30 minutes or at 71°C for 15 minutes followed by chilling to 4°C. The milk is then bottled for distribution.

c. Cheese Making:

Cheese is made from milk in which fat to protein ratio has been suitably adjusted. For this, milk is run into large cheese vats and the temperature is adjusted. A culture of bacteria, called the ‘starter’ is added to induce the formation of lactic acid. Rennet is then added to induce coagulation of curd which causes its separation from the whey.

When the proper degree of setting of the curd has been reached, it is cut into pieces and after further adjustment of temperature, is allowed to settle; whey is then run off. The curd is then subjected to various processes depending on the type of cheese to be made.

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d. Butter:

Butter Milk and Skim Milk-Milk is first passed through centrifugal machines to separate cream and skim milk. Cream is then churned either as such or after ripening with or culture until butter separates from the liquid and butter milk is left over.

e. Ghee Making:

Ghee is a very commonly used dairy product in India and is made from cream/butter obtained from fresh or sour milk.

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f. Casein Making:

The casein content of whole milk is 2.5 to 3.0 percent. When milk becomes sour during hot weather it is used for the preparation of casein. The sour milk is separated to remove fat and the skim milk thus obtained is fed into large vats and treated with acids to precipitate casein which is washed and dehydrated.

g. Baby Foods:

The baby food is prepared either by spray drying or roller of the milk of cow or buffalo or a mixture there of.

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h. Powdered Milk:

Milk is dried in spray drier or roller drier to manufacture powdered milk.

i. Other Products:

In recent years an increasing range of products have been made from milk. These products include lactic acid, calcium lactate, albumin, milk sugar and proprietary mixed foods for human consumption or for farm animals.

Sources, Quantity and Characteristics of Effluents:

Sources:

Effluents arising out of dairy plants include wash water from milk cans, equipment, bottles and floors. They also include portions of spilled milk, spoiled or sour milk, skimmed milk, whey and buttermilk.

These effluents arise out of:

(a) Rinsing and washing of cans and bottles, tanks or drums, equipment, pipe line and doors,

(b) Overflow, spillages and leakages from pumps and other equipment, and

Sources of waste waters are given process-wise in the following clauses:

a. Effluents from Milk Receiving Section:

The empty cans are drained so as to allow residual milk in the cans to drip into a receptacle and the cans are washed mechanically in an automatic milk can washer. The wash water consists of milk drippings, rinses and washings.

b. Effluents from Pasteurization Plant:

Effluents from this section consist of washings of equipment containing acids, alkalies and detergents and floor washing, spills and leaks. It is an accepted practice to wash the entire unit after each shift or batch.

c. Effluents from Manufacture of Butter and GHEE:

Butter-milk, washing of churns, separators and other equipment contain small portions of whole milk, skim milk or cream.

d. Effluents from Manufacture of Cheese:

Effluents from cheese manufacture mainly comprise whey, washings from vats, drains, floor, pressed and other equipment. The main bulk of the effluent from this unit consists of a mixture of discarded whey and wash water used for cleaning and rinsing the vats.

e. Effluents from Casein Plant:

Effluents from this plant primarily contain whey of milk, mineral acids when used for precipitation, and washings. This unit generally functions on a batch basis and hence discharge of effluent is intermittent.

f. Effluent from Bottle-Washing Plant:

Bottle and crates are thoroughly washed in this plant using detergents and/or caustic or washing soda in solution. The process is a batch one and effluents are discharge intermittently (generally twice a day). The effluents are generally alkaline due to the use of caustic or washing soda.

g. Effluent from Water:

Softening Plants and Boiler-House-Effluents from water softening plants and boiler blow-down are usually small ill volume. The boiler blow-down contains suspended solids of minerals origin. Effluents from softening or demineralization plant depend upon exchange resins are used or highly acidic or saline if only cation exchange is used.

h. Uncontaminated Cooling Water:

Cooling water is utilized in the pasteurization of milk. The spent water though hot, when it leaves the plant, is not usually contaminated and can be reused after cooling. Some cooling water has to be bled off to prevent build-up of inorganics and consequent scaling effects.

Quantity:

The volume of effluents from different dairies seems to vary from 6 to 10 litres per litre of milk processed depending upon the processes used, products made, care taken in the use of water and the quantity of water available.

Within a particular dairy, the rate of discharge of the 24 hours at a large product-oriented dairy is shown in Fig. 17.1. Maximum flow of 5 times the average occurred in the afternoon (1400 to 1600 h) and a minor peak of about twice the average occurred in the early morning hours (0300 to 0500 h).

Characteristics:

Individual Effluents:

The nature and composition of effluent from a dairy depend upon the quantity of milk processed and the type of products made.

The characteristics of individual effluents from a medium sized dairy processing 360000 litres of milk per day (mostly bottling, preparation of butter and casein) are given in Table 17.2.

Characteristics of individual effluents from large dairy processing 360000 litres of milk per day and producing 30 tonnes of spray dried milk, 10 tonnes of drum-dried milk (baby food), 16.5 tonnes of butter and GHEE, 1.5 tonnes of cheese and 3 tonnes of casien per day given in Table 17.3.

Note 1:

Flow was 6 to 8 litres of effluents per litre of milk processed.

Note 2:

Temperature ranged between 29.5 and 35.5°C, the average temperature being 32.8°C

Composition of Milk and By-products:

It is well known that milk contains carbohydrates such as lactose, protein such as casein, and fats such as butter fat, in addition to minor organic and inorganic materials.

There is no set of figures that can be used as average composition of milk under Indian conditions. To incorporate various degrees of variations in percentage Figures reported for protein, fat, lactose, etc. would be impracticable. An approximate average composition of milk and its by-products is shown in Table 17.4.

Combined Dairy Effluent:

The composition of the combined effluent from a dairy depends upon the quantity of milk processed and the types of products made. The characteristics of effluents from three different dairies are shown in Table 17.5.

The biochemical oxygen demand in the pasteurization and bottling plants is primarily due to loss of milk into the waste streams. A BOD value of 1000 mg/l can be considered as a usual average under Indian conditions.

Suspended solids form a good portion of the total solids in the combined effluents; oil and grease are relatively high. Volatile solids constitute about 70 to 90 percent of the total solids. The COD to BOD ratio of dairy effluents is in the range of 1.4 to 1.6 indicating their easy biodegradability. The BOD load is 2 to 8 kg per cubic metre of milk processed.

Pollutional Effects:

Discharged into Inland Surface Waters:

a. Depletion of Dissolved Oxygen:

The most important problem of water pollution associated with dairy effluents is the depletion of dissolved oxygen. As dairy effluents contain all the nutrients essential for bacterial life and, as the water temperatures encountered in most parts of India are ideal for Bacterial growth, the rate of decomposition is high resulting in anaerobic conditions followed by bad odours, conditions suitable for fly and mosquito breeding.

When discharged into open NALLAH and ponds the following adverse conditions are likely to arise:

(a) Putrefaction due to rapid degradation of lactose and production of lactic acid and butyric leading to foul odours.

(b) Ply breeding on the sludge rafts floated by anaerobic decomposition, and

b. Effect on Fish:

It has been reported that at a dilution of 1: 15, dairy effluent (excluding whey) had distressing effect on fish. At a dilution of 1: 35, whey had been found to be toxic to fish in a few hours. Lactic acid can be toxic to fish at a concentration of 654 mg/l of hard water or even less in soft waters and when the dissolved oxygen content is low. Dairy effluent are reported to contains soaps that are toxic to fish at concentrations of 600 mg/l.

Bytric acid, a product of anaerobic metabolism, has a very unpleasant rancid odour Beyond 100 mg/l concentration, it is known to adversely effect and above 400 mg/l to kill trout fish. It is also know to effect Daphnia (a crustacean) and Scenedesmus (an algae).

c. Pathogens:

Dairy effluents can occasionally carry pathogenic bacteria.

d. Radioactivity:

Milk as well as its effluents can concentrates tronstium 90, a beta-emitting radio-isotope that seeks the bone. This property is of extreme significance when pasture land is contaminated with this isotope due to radioactive spills or fall out of fission products.

e. Discharge on Land:

While disposal of dairy effluents on land is a common method, not all soils are suitable for this purpose. Heavy soils like clay can lead to ponding, anaerobic conditions and foul odours if the irrigation system is not properly designed.

f. Discharge into Public Sewers:

As dairy effluents are known to be readily biodegradable, it is often presumed that there is no problem in treating them along with sewage. As they contain significant amount of carbohydrate and the temperature of discharge is ideal, the possibility of sewage water waste mixture becoming septic in the sewer itself is very great and this may need additional pre-aeration in the sewage treatment plant.

Waste Reduction and Byproduct Recovery in Dairies:

Waste Reduction:

Waste reduction from dairies can be achieved by adopting the following steps:

(a) Providing more time for draining the cans either manually or mechanically;

(b) Using a small quantity of water in a fine spray after draining the cans and providing further time for draining;

(d) Preventing boil over in vacuum pans;

(e) Checking joints, packing glands and seals dairy to prevent leakage;

(f) Prevention of foaming in the pipelines by avoiding air leaks in suction lines and splashing in tanks;

(g) Prevention of entrainment of milk solids in condenser water in vacuum pans as entrainment can cause losses as high as 1 percent of the total milk processed; and

Process Change for Reducing Pollutional Load:

It has been long known that the traditional depilation method using lime and sulphide, in addition to being wasteful in regard to chemical usage is without doubt the largest contributor to high levels of pollution.

Tanneries in advanced countries have for years attempted to utilize new processes, namely, enzyme depilation, oxidation processes, dimethyl amine, caustic soda as a substitute for lime. Water can be saved from the deliming, bating and pickling stages.

(h) Reduction of the water volume by segregation of all the clean water streams like condensates and cooling water, from the strong waste.

Reclamation of Waste Products:

The waste waters from dairy industry have been used abroad for the following purpose:

(a) Milk drippings and primary rising animals,

(b) Combined effluents for breeding craps,

(d) Yeasts produced on whey to be used as animal feed.

Byproduct Recovery:

The recovery processes for usable byproducts are outlined below:

a. Butter Milk:

Butter milk resulting from butter manufacture has higher total proportion of total solids than whey and these can be recovered by roller drying of butter milk after preliminary condensation. The dried powder can be sold as such or incorporated as source of protein.

b. Whey:

A considerable amount of whey result from the manufacture of cheese and because of its high dissolved organic content; it could be economically recovered as a dry powder by pre-condensing in single or two-stage evaporators and spray drying. Spary dried whey can also be used as a source of protein in animal feeds.

c. Lactose:

Whey and milk are the main sources of lactose. Recovery of lactose from milk a such may be uneconomical, but lactose present in the effluent, especially whey, can be utilized for culturing of yeast which may be harvested and used as animal feed.

d. Casein:

Casein can be obtained from sour milk by treating with acid, which can be washed and dehydrated. In terms of the total milk handled, daily spoilage of milk in India varies from 5.0 to 7.0 percent and this can be used for recovery of casein which is used in the manufacture of resins, plastics, paints and varnishes.

e. Butter:

Butter that has turned very sour or rancid or cream which has deteriorated in the quality may find as raw materials in vegetable oil factories.

Methods of Treatment and Disposal:

Agricultural utilization system-Land disposal of dairy effluents is one of the most commonly practised methods in India. It can give good results provided the irrigation system is designed properly and the effluent is diluted sufficiently to bring BOD below 500 mg/l.

Different irrigation systems tried included:

(a) Ridge and furrow,

(b) Flood irrigation with under-drains, and

Treatment Methods Recommended in India. Reviewing the methods mentioned above, treatment of diary effluents in India should consist of the following:

(a) Preliminary treatment of effluents,

(b) Primary treatment of effluent, and

(a) Preliminary Treatment:

This involves removal of oil and grease and coarse solids. Grease taps and screens are efficient and economical and are invariable used as an essential part of treatment by the dairies. This step is essential for any rational method of treatment or disposal. Provision for equalization of flows, and aeration to avoid production of odours and septic conditions is necessary.

(b) Primary Treatment:

Primary treatment as generally described in the waste treatment practise, that is, setting and digesting the sludge is not very useful for dairy effluents. However, dilution with water and anaerobic lagooning or aerated lagooning for a short period can reduce the BOD and suspended solids enough to facilitate disposal into sewers or on land for irrigation.

Dilution water required will be approximately 1: 1 as the combined effluent from a common dairy is likely to have below 1200 mg/l suspended solids and about 1000 mg/l BOD (A in Fig. 2). The BOD and suspended solids can also be reduced to 50 percent to suit the limits of the sewers and land irrigation by treating in anaerobic lagoons (6 days) (B in Fig. 2) or aerated lagoons (1.5 days) (C in Fig. 2).

The dilution requirement and the detention period required in the lagoons may have to be modified depending upon the effluent characteristics.

(c) Secondary Treatment:

Secondary treatment is required for further reduction of BOD to suit the inland surface water standard.

The secondary treatment would consist of one of the following system:

i. Oxidation ditch,

ii. Aerated lagoon, and

iii. Anaerobic lagoon followed by stabilisation pond.

Oxidation ditch is preferred by several diaries because of its compact nature and simplicity in construction and operation. The oxidation ditch (A in Fig. 3) can be loaded at 0.2 kg BOD/ kg MLSS/day. The BOD reduction that can be expected would be 05 to 98 percent.

The aerated lagoon (B in Fig. 17.3) is designed to have a detention time of 3.7 days to get a 90 percent BOD reduction.

Anaerobic lagoon treatment for 10 days would help reduce the BOD by 90 percent. It is desirable to residual BOD by aerated lagoons or oxidation pond or by dilution with water. One of the above methods would help to ‘Sweeten’ the effluent, that is, to reduce the septicity or increase the oxidation reduction potential.