5 Waste water characteristics of dairy and sugar industry

J.S. Laura

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6.1 Introduction

 

In the previous module the characteristics of wastewater from various sources were discussed in general along with the wastewater discharge standards. In the followind modules the wastewater characteristics of different industries will be discussed in details. Presented in the present chapter are the wastewater characteristics of dairy and sugar industries.

 

 

6.2 Dairy Industry

 

A dairy is a business enterprise established for the harvesting or processing (or both) of animal milk – mostly from cows, goats, sheep, horses, buffaloes and camels for human consumption. In last two decades the Indian dairy industry has acquired substantial growth. The dairy sector is now the largest contributor from the agricultural sector to the nation’s GDP. The huge increase in milk supply has led to a White Revolution. India is positioned at second rank milk producer in the world accounting for 10% of the total milk produced in the world. India has been producing 61 million kilograms of milk whereas United States at first position with 92 billion kilogram of milk production on annual basis.

 

A dairy plant has a chain of operations involving receiving and storing of raw materials, processing of raw materials into finished products, packaging and storing of finished products, and a group of other ancillary operations (e.g., heat transfer and cleaning) will produce wastewater. The dairy industry is one of the most polluting of industries, not only in terms of the volume of effluent generated, but also in terms of its characteristics as well. In the associated milk processing factories, most of the waste is washing water that is treated, usually by composting, and spread on farm fields in either liquid or solid form.

 

Dairy Industry milk processing chain

 

The various steps involved in a dairy plant from collection to processing of the milk are shown in Fig.1. The main steps are 1. Collection of milk from various sources, 2. Transportation to the plant, 3. Receiving at the plant, 4. Storage, 5. Clarification, 6. Pasteurization 7. Processing 8.Packing. Water is used for cleaning of milk cans, storage tanks, for steam during pasteurization and during various steps in the milk processing steps leading to the production of effluent. The wastewater generation standard for dairy plant is 3000 liter of waste water/1000 liter of milk processed.

 

Fig.1  Schematic flow sheet of the main dairy products

Milk is first received in a dump tank from the road milk tankers. It is then chilled in a chilling unit to 4oC. This chilled milk is then pasteurized in a milk pasteurizer and cream is separated from milk in by the cream separator to get skimmed milk. Milk is standardized depending on the requirement of toned milk, double toned milk, skimmed milk or full cream milk. The separated cream is further processed to manufacture ghee and butter. The pasteurized skimmed milk, toned milk or double toned milk is sent for packaging in pouched in the packing machine for various capacities like 0.5 Kg, 1 Kg pouches.

 

Products

 

 

Milk Powder- Milk powder is a processed dairy product made by evaporating milk to dryness. Drying extends the shelf life of milk apart from reducing the weight and volume. The initial solid in the milk varies from 7.5% to 12% depending upon type of milk and final moisture in the powder, ranges between 2.5% and 5%. Cheese/way/curd- There are about 500 varieties of cheese produced throughout the world. These are classified in nine major cheese families. These varieties come about as a result of different types of production processes. The composition of the wastewater of each specific production

 

process varies from variety to variety. For the purpose of discussing the environmental impact, the production of cheese is related to the production of whey. For hard cheeses (Cheddar cheese, Dutch cheese, etc.), the quantity of whey produced is high and equals more or less the amount of milk used. During the production of other types of cheeses, such as soft types, the whey production is much lower or there is no production of whey at all.

 

Condensate/Cream/Khoa- For condensed milk and cream, a portion of the water is removed by evaporation. Khoa is a product typically found in India and neighbouring countries. It is produced by thermal evaporation of milk to 65-70% solid state and serves as base material for a variety of Indian sweets.

 

Emissions

 

 

Solid waste – Hardly any solid waste is produced by the dairy industry. The main solid waste produced by the dairy industry is the sludge resulting from wastewater purification. There are figures available about the amount of sludge production: in aerobic systems the sludge production is about 0.5 kg per kg of removed COD and in anaerobic systems about 0.1 kg per kg of removed COD.

 

Wastewater– The volume, concentration, and composition of the effluent arising in a dairy plant are dependent on the type of product being processed, the production program, operation methods, design of the processing plant, the degree of water management being applied, and subsequently the amount of water being conserved. Dairy wastewater may be divided into three major Categories:

 

  1. Processing water, which include water used in the cooling and heating processes. These effluents are normally free of pollutants and can with minimum treatment be reused or just discharged into the water system.
  2. Cleaning wastewater emanate mainly from the cleaning of experiment that has been in contact with milk or milk product, whey, pressing and brines, and water resulting from equipment malfunctions and even operational errors.
  3. Sanitary wastewater, which is normally piped directly to sewage works.

 

Wastewater loading for a typical dairy industry is summarized in Table 1

Milk has BOD content 250 times greater than that of sewage. It can therefore be expected that dairy wastewaters will have relatively high organic loads, with the main contributors being lactose, fats, and proteins (mainly casein), as well as high levels of nitrogen and phosphorus that are largely associated with milk proteins. The COD and BOD for whey have, for instance, been established to be between 35,000-68,000 mg/l, with lactose being responsible for 90% of the COD and BOD contribution. Characteristics of wastewater are given below- It contains milk solids, detergents, sanitizers, milk wastes, and cleaning water.

 

It is characterized by high concentrations of nutrients, and organic and inorganic contents. Salting activities during cheese production may result in high salinity levels.

 

Wastewater  may  also  contain  acids,  alkali  with  a  number  of  active  ingredients,  and disinfectants, as well as a significant microbiological load, pathogenic viruses, and bacteria.

 

Standards of wastewater discharge for dairy industries

 

 

Wastewater from dairies and cheese industries contain mainly organic and biodegradable materials that can disrupt aquatic and terrestrial ecosystems. It generates about 0.2–10 litres of effluent per litre of processed milk with an average generation of about 2.5 litres of wastewater per litre of the milk processed. Due to the high pollution load of dairy wastewater, the milk-processing industries discharging untreated/partially treated wastewater cause serious environmental problems. Hence, the importance of carrying out a whey treatment as a starting point in order to optimize a simple and economic method to treat the whole dairy effluent.

 

Moreover, the Indian government has imposed very strict rules and regulations for the effluent discharge to protect the environment (Table 2).

 

 

6.3 Sugar industry

 

It encompasses the production, processing and marketing of sugars (fructose and saccharose).Sugar is manufacture from two raw materials, sugarcane and sugar beet, 80 % of th sugar is extracted from sugarcane (In tropical and subtropical climate) and 20% from sugar beet (In temperate climate). In terms of quantity India is at second position with 341,200 thousand metric ton(TMT) annual production of sugarcane whereas Brazil ranked first position with 739,300 TMT per annum. Sugar industry can be broadly classified into two sub sectors, the organized sector i.e., sugar factories and the unorganized sector i.e., manufacturers of traditional sweeteners like gur and khandsari. The latter is considered to be a rural industry and enjoys much greater freedom than sugar mills. Sugar companies have been established in large cane growing states likes Uttar Pradesh (U.P), Maharashtra, Tamil Nadu, Karnataka, Punjab, Gujarat. U.P leads the tally by contributing 24% of the country’s total sugar production and Maharashtra stands next with 20% contribution.

 

Sugar industry –manufacturing process

 

  1. Sugarcane harvesting– Harvesting is done when the crop is fully matured. Maximum portion of the stem is separated from leaves and transported to industry where cutting and shredding of sugarcane have is done for juice extraction.
  2. Juice extraction– there are two processes for extracting juice from cane- milling and diffusion. In milling process squeezing of cane under a set mills using high pressure between heavy iron rollers.In diffusion extracting the sucrose from the cane is done without the squeezing by mills. Shredded cane is introduced into the diffuser at the feed end, Hot water is poured over the shredded cane just before the discharge end of the diffuser. The hot water percolates through the bed of cane and removes sucrose from the cane.
  3. Clarification- The cane juice received from mill is turbid liquid with varies colors from light gray to green. It contains many soluble and insoluble impurities. The impurities in cane juice are coming from cane origin itself. The main aim of clarification is to remove these non-sugar impurities of raw juices and to neutralize the juice. There are four processes for clarification are- Liming- Calcium hydroxide (Milk of lime or lime water) is added to adjust pH to 7. Lime prevent decay of sucrose into glucose and fructose. After lime addition pH changes from 5.2 to 7.2-7.6 (alkaline) and the number of substances originally present in raw juice in colloidal form precipitates.

 

Sulphitation- The sulphitation process can be considered to take place in two steps, first step is neutralization of juice acidity this being a juice defecation. The other step is neutralization of the excess variables on juice clarification lime with sulphur dioxide. After the addition of this, the pH value becomes 7.3 to 7.4. The sugar juice is heated to a temperature of about 70 degree Celsius. The juice in this stage contains impurities and hence it has to be filtered. Finally the purified juice is obtained which is sent to the chemical department and the crystallization process takes place.

 

Phosphatation/floatation- the process consist of precipitating insoluble calcium phosphates of variable composition in hot melt liquor by first adding phosphoric acid, or an acid phosphate, followed by calcium hydroxide either as slurry with water or dissolved in sugar syrup as lime sucrate, to give a final pH of 7.2-7.4. The two following reaction take place-

 

Ca+2 + HPO4-3    CaHPO4  (Calcium biphosphate)

 

3Ca+2 + 2PO4-3       Ca3(PO4)2 (Tricalcium phosphate)

 

The first reaction is explained as being more rapid because it is a second order reaction involving two ions while other reaction is a higher order reaction involving five ions. The floc and the precipitated calcium phosphate, absorb other particles such as clay, cane wax, bagacillo, bacteria, poly saccharides including starch etc. Phosphatation generally achieves a decoloration in the range of 25-30%.

 

Carbonation- In this process calcium hydroxide slurry added to the raw melt solution. Then carbon dioxide gas is bubbled into the sugar solution in saturators, under controlled conditions of pH and temperature. This process removes color (55%), turbidity (90%), starch (93%), sulphates (86%), phosphates (100%), magnesium (67%), and Gums (29%).

 

  1. Juice concentration- The clarified juice is concentrated to about 65% solids from about 15% solids before entering the first multiple effect evaporator sending steam in the first evaporator. Vapours from the first evaporation are fed to the second evaporator and so on. Spent steam from the first evaporator is returned to the boiler for reuse as feed water for steam generation. Spent steam from the second and third evaporator is used for process, and vapours from the last evaporator are condensed through condensers.
  2. Crystallization: crystallization is the process just after evaporation. During evaporation the clarified sugar cane juice is boiled in evaporators which remove most of the water leaving a thick syrup. Then in the crystallization process the syrup is boiled at low temperatures under partial vacuum and some seedings are added which causes the development and growth of sugar crystals and the outcome is called massecuite (raw sugar crystals mixed with molasses). The various seeding techniques include:

1) Traditional (secondary, or shock) seeding- The old traditional method of seeding practiced by the “artisan” pan-men is characterized by the addition of some amount of fine icing sugar, or nowadays slurry to the supersaturated solution. If supersaturation is large enough new crystals will be formed (nucleation). Slurry is a suspension of finely ground sugar mixed for example in isopropyl alcohol.

 

  • 2) Full seeding- the basic idea of full seeding is very simple; enter the number of crystals when seeding equaling the one in the end product. This means that no new crystals should be generated during the complete strike, and no crystals should be lost from those entered during seeding. Full seeding can be implemented in two ways: with footing magma. The only difference between the two methods is the difference in the crystal sizes and the difference between the required quantity of slurry, or footing magma.
  1. Seeding with slurry- the mean crystal size with slurry seeding is in 5- 10 micron range. The crystals are prepared locally by ball mills or are provided by companies producing them. When using ball mills it is typical to give the time of grinding instead (4-6 hours). The first step is to calculate the number of crystals needed in the slurry by taking into account the product crystal size, the useful volume of the pan and the target crystal content when dropping the change. It is then followed by calculating the (dry) weight of the seeding sugar needed for a batch of slurry.
  2. Seeding with footing magma- procedure for footing magma is as following- transfer the concentrated syrup into a cooling crystallizer and seed it with slurry. Drop the magma crystals 0.07-0.1 mm mean size into a magma receiver with stirrer. Use the magma to seed a vacuum pan containing concentrated syrup and then continue crystallization up to 0.25-0.3 mm mean crystal size (second magma). Use the second magma to seed the product pans.
  3. Sugar crystal separation, drying, packing and molasses handling- The Mixture of crystals and liquor, called ‘massecuite’, is sent to high-speed centrifuges. The liquor is re-concentrated and cooled successively to obtain more than one crops of crystals. The final mother liquor, called ‘Molasses’, which is still very rich in Sugar content is sent to steel storage tanks. Molasses is sold to various distilleries and other users against permit issued by excise department. The separated crystals are passed on to hopper conveyors where hot and cold air is passed through the crystals. The appropriate size is dried and sent to elevators. The elevators feed the grading system bins. The fine crystals are reused for seeding. Finally the finished product is bagged and stored in godowns.
  4. Bagasse utilization– The pulp expelled after extraction of juice is called ‘Bagasse’. As it comes out of the mill house, it contains about 50% moisture. A number of drying processes have been tried in the industry but unfortunately none of these were found industrially viable. Therefore, the wet bagasse with 50% moisture is carried to boiler house by bagasse carrier. It isable to generate about 2 kg of steam per kg on wet basis itself. With efficient boilers coming in the market, the factories are able to save about 10-20% bagasse.

 

The excess bagasse is carried to bagasse yard from where it is sold to paper mills and other users. During the general cleaning or shut down, bagasse return carrier brings the bagasse back to the boiler. A small fraction is sent to bailing plant where bagasse is compressed and tied by G.I. wire to form small bails. The size of these bails is about 40 to 50 centimetre cube. The bagasse can be stacked in the form of bails. This is done very carefully as bagasse is known to be susceptible to auto ignition.

 

Water Requirement of a Sugar Industry: Major units that consume water in a Sugar plant are:

 

  • (i) Boiler feed water.
  • (ii) Cooling water for condenser.
  • (iii) Process water for maceration, lime preparation, dilution for control of brix, dilution in evaporators and massecuite dilution, filter mud, fly ash handling, and cane wastewater.The Sugar cane received from the field contains about 70% moisture on an average. Majority of this water has to be discharged as factory wastewater. Material balance of a typical plant with respect to water requirement and wastewater generation is show in Fig 2.

Sources of Wastewater in a Sugar Industry: The wastewater discharge standard for sugar industry is 400 liter of waste water per ton of sugarcan crushed. However the generation changes with quantitative capacity of the mill.

 

                        Table 1: Quantity of effluent generation based on factory size

 

Since the sugar industry operates on a seasonal basis, wastewater production is also obviously seasonal. A large variation is observed in the quality and quantity of wastewater generated in various sugar mills. The wastewater has a high COD, BOD, Suspended Solids and also most acidic pH. Equalization and lime treatment is needed prior to further treatment. The wastewater has very low nutrient content such as nitrogen and phosphorus.

 

Discharge standards

 

Wastewater discharge standards have been made stricter, by limiting the same to ‘200 litre per tonne of cane crushed’, as against the earlier limit of ‘400 litre per tonne cane crushed’

The number of effluent quality parameters to be monitored for ascertaining compliance have now been increased to six (6) – i.e. pH, Bio-chemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Total Dissolved Solids (TDS) and Oil & Grease (O&G). Earlier, the notified parameters were only BOD & TSS.

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References

 

  1. Quazi, H. A study of the Dairy Industry in India. http://gsw.shikshamandal.org/wp-content/uploads/2015/10/A_STUDY_OF_DAIRY_INDUSTRY_IN_INDIA.pdf
  2. Spring Conference, 1966. Research and Development in the dairy industry: Milk Production. . http://onlinelibrary.wiley.com/doi/10.1111/j.1471-0307.1966.tb02801.x/abstract
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  6. LS. 8682-1983 Indian Standards for Dairy Wastewater.
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