Utilization of dairy industry wastes

The production technology of dairy industry products, main sources of wastes and ways of its utilization. Description of milk processing. Waste generating processes. Handling of by-products and treatment of waste. Waste reduction. Economic considerations.

Рубрика Экология и охрана природы
Вид курсовая работа
Язык английский
Дата добавления 23.10.2012

NATIONAL AVIATION UNIVERSITY

Institute of Ecological Safety

Department of Biotechnology

TASK

on the execution of term paper

The theme of course work: Utilization of dairy industry wastes

student Anna Pokusaieva

Abstract

Explanatory notes to the term paper from discipline “Utilization and recuperation of biotechnological industries wastes”: 39 pages, 2 fig., 6 tables, 45 references.

Object of investigation - technology of wastes utilization on dairy industry plants.

Aim of work - to get acquainted with production technology of dairy industry products, main sources of wastes and ways of its utilization.

Method of investigation - analytic review of literature.

Key words: DAIRY INDASTRY, ORGANIC WASTES, WASTEWATERS, UTILIZATION.

Content

1. Dairy industry (farming)

1.1 Liquid milk products

1.2 Cheese/Whey/Curd

1.3 Butter/Ghee

1.4 Milk powder

1.5 Condensate/Cream/Khoa

2. Description of milk processing

3. Dairy industry products. Emissions

3.1 Solid waste

3.2 Wastewater. Whole milk products

3.3 Cheese/Whey/Curd

3.4 Butter/Ghee

3.5 Milk powder

3.6 Condensed milk/Cream/Khoa

4. Waste generating processes

5. Prevention of waste production

6. Handling of by-products and treatment of waste

6.1 Several ways may be followed to reduce the occurrence of waste.

6.2 Treatment of Dairy Waste Water

7. Main wastewater problems

7.1 Most treatment plants follow the following steps

8. Waste production and its consequences

8.1 Data availability and reliability

9. Waste reduction

10. Economic considerations

Conclusions

References

1. Dairy industry (farming)

Dairy farming is a class of agricultural, or an animal husbandry, enterprise, for long-term production of milk, usually from dairy cows but also from goats and sheep, which may be either processed on-site or transported to a dairy factory for processing and eventual retail sale.

Most dairy farms sell the male calves born by their cows, usually for veal production, or breeding depending on quality of the bull calf, rather than raising non-milk-producing stock. Many dairy farms also grow their own feed, typically including corn, and hay. This is fed directly to the cows, or is stored as silage for use during the winter season [3].

1.1 Liquid milk products

Dairy products are generally defined as food produced from the milk of mammals (the Food Standards Agency of the United Kingdom defines dairy as "foodstuffs made from mammalian milk"). They are usually high energy-yielding food products. A production plant for the processing of milk is called a dairy or a dairy factory. Apart from breastfed infants, the human consumption of dairy products is sourced primarily from the milk of cows, yet goats, sheep, yaks, camels, and other mammals are other sources of dairy products consumed by humans. Dairy products are commonly found in European, Middle Eastern and Indian cuisine, whereas they are almost unknown in East Asian cuisine [8].

1.2 Cheese/Whey/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[32]. For the purpose of discussing the environmental impact, the production of cheese will be 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 [11].

1.3 Butter/Ghee

In developed countries, butter is made from cream that has been churned (separation of sweet butter and sweet buttermilk). In developing regions the technology in use for the making of butter and ghee is closely related to the technology to make fermented milk. Traditional butter is made from fully soured whole milk that is churned.

1.4 Milk powder

Milk powder is made from raw milk, skim milk or sweet buttermilk. After pasteurization, decreaming etc. the water from the milk is removed through evaporation [28].

1.5 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 neighboring 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.

2. Description of milk processing

Dairy plants are found all over the world, but because their sizes and the types of manufactured products vary tremendously, it is hard to give general characteristics. The dairy industry can be divided into several production sectors. Each division produces wastewater of a characteristic composition, depending on the kind of product that is produced (milk, cheese, butter, milkpowder, condensate). Figure 1 presents a schematic flow sheet of the main dairy products[33].

Fig.1 A schematic flow sheet of the main dairy products

3. Dairy industry products. Emissions

3.1 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 wastage and in anaerobic systems about 0.1 kg per kg of removed wastage [41].

3.2 Wastewater. Whole milk products

Wastewater from dairy industry may originate from the following sources:

Wastewater is mainly produced during cleaning operations. Especially when different types of product are produced in a specific production unit, clean-up operations between product changes are necessary. In developing countries, the main problem is pollution through spoilage of milk [1].

3.3 Cheese/Whey/Curd

Waste results mainly from the production of whey, wash water, curd particles etc. Cottage cheese curd for example is more fragile than rennet curd which is used for other types of cheese. Thus the whey and wash water from cottage cheese may contain appreciably more fine curd particles than that from other cheeses. The amount of fine particles in the wash water increases if mechanical washing processes are used [9].

3.4 Butter/Ghee

Butter washing steps produce wash water containing buttermilk.

Skim milk and buttermilk can be used to produce skim milk powder in the factory itself or these materials may be shipped to another dairy food plant by tank truck.

The continuous butter production process materially reduces the potential waste load by eliminating the buttermilk production and the washing steps[14].

3.5 Milk powder

Environmental problems are caused by high energy consumption (= emission of CO2, CO etc.), by cleaning and by emission of fine dust during the drying process.

3.6 Condensed milk/Cream/Khoa

Environmental problems related to the production of condensate and khoa are mainly caused by the high energy consumption during the evaporation process.

The main suspended solids mentioned in the literature are coagulated milk and fine particles of cheese curd[15].

The effect of the type of product produced is illustrated in Table 1.

Table.1.

The ranges in Table 1 also indicate that the production of wastewater is highly influenced by management practices. It is not possible to identify particular waste producing practices. The way in which the water consuming and operation processes are carried out is indicative of the management quality. The major contribution to the waste load comes from cleaning operations, which take place throughout the production process. Only in the production process of (hard) cheese, is whey sewering one of the main contributors to the waste load[32].

4. Waste generating processes

Waste generating processes of major significance include:

- Washing, cleaning and sanitizing of pipelines (metals), pumps, processing equipment, tanks, trucks and filling machines (high N load);

- Start-up, product change over and shut down of HTST and UHT pasteurizers;

- Breaking down of equipment and breaking of packages resulting in spilling during filling operations;

- Lubrication of casers, stackers and conveyors [32]

Air pollution

In dairy plants air pollution is mainly caused because of the need for energy. In the process gasses may be discharged such as CO2, CO, NOx and SO2.

Table 2 gives the emissions into the air as a result of gas- and oil-combustion. No figures are available about the emissions into the air resulting from the use of electricity.

Emissions of CFC's and NH3 into the air may come about as a result of leakage and stripping of chilling machines when out of use [22].

Table.2.

5. Prevention of waste production

The waste load, expressed as BOD depends to a large degree on the style of management. Table 3 gives an example of the relationship between management practices and waste production in terms of BOD and the amount of wastewater produced. The table shows that a large quantity of processed milk does not necessarily lead to higher waste loads or to higher levels of wastewater production [7].

Management practices cover a wide range of water consumption and process operation activities. Well controlled processes reflect good management qualifications, while bad practices are a reflection of poor management. Table 3 shows the relationships. The qualification “fair” signifies that good as well as bad practises occur. With good management practices, values of BOD 1 kg/ton and produced wastewater below 1 kg/kg may be reached. Poor management will result in values greater than 3 kg/ton resp. 3 kg/kg [45].

For the evaluation of management practices, the following indicators are useful: 1. Housekeeping practices;

2. Water control practices; frequency with which hoses and other sources of water are left running when not in actual use;

3. Degree of supervision of operations contributing to either the volume of wastewater or to BOD coefficients;

4. Extent of spillage, pipe-line leaks, valve leaks and pump seals;

5. Extent of carton breakage and product damage in casing, stacking and cooler operations;

6. Practices followed during the handling of whey;

7. Practices followed in handling spilled curd particles during cottage cheese transfer and/or filling operations

8. The following of practices that reduce the amount of wash water from cottage cheese or butter operations;

9. Extent to which the plant uses procedures to segregate and recover milk solids in the form of rinses and/or products from pasteurization start-up and product change-over;

10. The procedures used to handle returned products;

11. Management attitude towards waste control [18].

Table.3. Description of management level

Poor = 1. no steps taken to reduce waste, 2. whey included, 3. many drips, leaks, 4. returns included, 5. sloppy operations, 6. spillage leaks, 7. hoses running,

Fair = 8. whey excluded, 9. good water volume control, 10. wash water excluded, 11. no entertainment losses, 12. all powder handled as solid waste, 13. no leaks/drips, 14. continuous churn,

Good = 15. fines screened out, 16. wash water to drain, 17. spilled curd handled as solid waste, 18. rinses segregated, 19. rinses saved, 20. returns to feed use, 21. returns excluded, 22. good water control, 23. buttermilk excluded, 24. few leaks,

Excellent = 25. hoses off, 26. filler drip pans, 27. cooling tower, 28. dry floor conditions

In the following a summary is given of suggestions for the prevention of dairy waste. At the same time they are indicative of what is to be understood when speaking about good management of waste control:

1. Instruction of plant personnel concerning the proper operation and handling of dairy processing equipment. Major losses are due to poorly maintained equipment and to negligence by inadequately trained and insufficiently supervised personnel.

2. The carrying out of a study of the plant and the development of a material balance to determine where losses occur. Modification and replacement of ill-functioning equipment. Where improper maintenance is the cause of losses, a specific maintenance programs should be set up.

3. The use of adequate equipment for receiving, cooling, storing and processing of milk, so as to take care of the maximum volume of flush production and of special products. All piping, around storage tanks and other areas, should be checked on mis-assembly and damage that may lead to leakage.

4. Accurate temperature control on plate, tubular and surface coolers to prevent freeze-on, which may result in loss of products.

5. Elimination of valves on the outlet sides of internal tubular or plate heaters and coolers and maintenance of plates and gaskets in good repair so as to eliminate waste due to blown or broken gaskets

6. Installation of suitable liquid level controls with automatic pump stops, alarms, and other devices at all points where overflows could occur (storage tanks, processing tanks, filler bowls etc).

7. Keeping in good order of vats, tanks and pipelines so as to eliminate and reduce to a minimum the number of leaky joints, gaskets, packing glands and rotary seals.

8. Proper design and installation of vats and tanks at a level high enough above the floor for easy drainage and rinsing if hand cleaned. Tanks should be pitched to insure draining.

9. Correct connections on plate type heat exchangers so as to avoid milk being pumped into the water side of the exchanger or water being pumped into the milk side.

10. Provision and use of proper drip shields on surface coolers and fillers so as to avoid that products reach the floor. Avoidance of cheese vats, vat processors or cooling tanks being overfilled so that no spillage occurs during product agitation. The liquid level in cheese vats should be at least three inches below the top-edge of the vat.

11. Avoidance of foaming of fluid dairy products, since foam readily runs over processing vats and other supply bowls and contains large amounts of solids and BOD. The use of air tight separators, proper seals on pumps and proper line connections to prevent inflow of air when lines are under partial vacuum, will avoid foam production.

12. Turning off of water hoses when not in use. Use should be made of hoses equipped with automatic shut-off valves so as to avoid excessive water usage[8], [5], [37].

6. Handling of by-products and treatment of waste

As mentioned before, generally three different types of waste are distinguished:

(1): solid waste, which may cause problems with respect to dumping grounds;

(2): wastewater, which may decrease the quality of surface waters; and

(3): volatile compounds, which cause air pollution [5].

6.1 Several ways may be followed to reduce the occurrence of waste

(1): waste prevention;

(2): development of clean processing methods; and

(3): end of pipe treatment.

The most important one: the prevention of waste production. Through a careful examination of the production processes, one may identify the source(s) of pollution. The development of new and clean processing methods is the next step. In the present report this approach will not receive attention, because of the specific technical and economic know-how involved [14].

The third step is the treatment of the produced waste before it will be discharged into the environment. This is generally referred to as `end-of-pipe' treatment which will be discussed below. Because this treatment is usually very expensive, the amount of waste to be treated “at the end-of-pipe” should be as small as possible [2].

Practically all by-products may be used in one way or another. The amounts of solid waste that need to be dumped can be kept small. With the exception of cooling water which in most cases is not polluted, wastewater can usually not be re-used. There are several methods of treating wastewater that can be applied before it is discharged into the sewer system or surface water. Polluted air can be filtered before discharge.

The KTPCP-project (Kasur Tannery Pollution Control Project) may serve as an example of why it is worth paying attention to environmental problems, which in this case had been caused mainly by the 160 tanneries of Kasur, Pakistan. Owing to insufficient attention to waste disposal from these tanneries, artificial and stagnant lakes had received environmental pollutants. This in turn led to major health problems, decreased crop yields (up to 50%) and to contaminated groundwater and fish [18].

Except for packing material etc. and sludge (in case of wastewater purification), dairies do not produce solid waste. The potential use as fertilizer of stabilized sludge from wastewater treatment plants of dairies has no environmental limitations, provided the sludge contains no toxic compounds [11].

6.2 Treatment of Dairy Waste Water

The average volume of waste water in dairies is currently 1.3 l/kg milk. This results in considerable waste water disposal costs. The centrifugal separation technology from GEA Westfalia Separator paves the way for major potential savings by minimizing the use of fresh water and reducing the volume of residual sludge [17].

Hygiene is the most important factor in milk processing and the production of dairy products. This necessarily results in the use of considerable volumes of water for cleaning purposes. In addition, considerable quantities of waste water with volatile milk constituents, fats and proteins occur when milk is being processed, particularly during evaporation and spray-drying [33].

Fig.2 Waste water treatment in dairies

After aerobic or anaerobic biological treatment of dairy waste waters, the residual sludge is sent through a clarifying decanter which efficiently dewaters the sludge before the clean water is recycled back into the process. A calculation by the Verband der deutschen Milchwirtschaft (Association of German Dairying) shows how in-plant waste water treatment pays: direct dischargers, i. e. operations with their own waste water processing facility, operate with costs that are up to two thirds lower than users of municipal waste water treatment plants [16].

7. Main wastewater problems

The problems of the wastewater from the slaughterhouses, tanneries and dairies result from the discharge of:

a: large amounts of BOD (slaughterhouses, tanneries and dairies).

BOD-problems can be handled, as already mentioned, by biological wastewater treatment.

b: high values of NKj (slaughterhouses).

NKj can be lowered by oxidation of organic compounds (proteins) followed by nitrification: conversion of ammonium (NH4+) into nitrate (NO3-). To reduce the eutrophication potential of the wastewater, nitrate must be removed. This can be achieved by denitrification: conversion of nitrate (NO3-) into nitrogen (N2).

c: chromium (tanneries).

Chromium can be handled by precipitation reactions, these are simple processes[37].

There are basically two types of biological wastewater treatment systems: aerobic and anaerobic systems. In Tables 4 and 5 represents the characteristics and the (dis)advantages of these systems are mentioned.

Table 4 *: depends on BOD-load.

In view of the high BOD-load in the wastewater of tanneries, dairies and slaughterhouses, anaerobic systems seem to be appropriate wastewater purification systems. Simple anaerobic systems may achieve 50% of BOD-purification (Table 4), while high-rate anaerobic systems may result in 90% of BOD-purification (Table 5). Anaerobic systems do not remove such nutrients as ammonium-nitrogen. If liquid and slurry are used as fertilizer this does not need to pose specific problems. Nutrient removal systems should be applied only if water authorities set limits for the discharge of nutrients. As in most countries this is not the case, there are no reasons for industry to make high investment costs for tertiary treatment[20].

Table 5. General:

The process that may be used for the treatment of wastewater produced by the industries mentioned in this report do not differ very much from each other. In general, these systems are applied to a large extent in developed countries. In developing countries adoption rates are much lower. Especially for these latter countries, treatment methodologies and technologies should be cheap, efficient and easy to operate. Important differences of wastewater treatment in the different industries will be mentioned [10].

For large dairies in many developing countries, treatment of wastewater is not even considered as an option. Because in developing countries the amount of milk processed industrially is minor, wastewater problems will mainly occur at the plant site and the surrounding surface waters. This implies that dairy wastewater problems in these countries are very local in contrast to those in developed countries. The dairy wastewater problem is larger in developed countries because all milk is processed industrially. For dairies in these countries it is very important that proper wastewater treatment system are installed [25].

As mentioned before, the primary action to reduce pollution by wastewater discharge, is efficient water management. After a thorough search for ways to reduce water use and wastewater production, the inevitable produced wastewater can be treated in different ways as discussed below[19].

Usually wastewater produced during the day has a variable composition. For the optimal performance of most treatment system it is necessary that the load is rather constant and that the plant is fed with a rather constant wastewater flow. Wastewater is therefore collected in equalization or balance tanks[32].

7.1 Most treatment plants follow the following steps

Preliminary treatment. This type of treatment includes screening, skimming and settling which can lead to the recovery of by-products, grease and fat and removal of coarse solids. For an optimal performance and to avoid overload of the screening devices, it is important that large amounts of produced solids such as (hog)hair, feathers etc. are collected during the processing itself. For developing countries, the salt laden tannery effluent from the soaking process can be collected in solar evaporations pans, possibly pretreated with coagulant, after which salt can be recovered. In case of chrome tanning effluents, the wastewater that contains chromium should not be allowed to become mixed with other types of wastewater: it must be collected separately. Depending on the quality of the composite effluents, neutralising chemicals like lime alum, ferric chloride etc. should be added for an effective precipitation of chromium and removal of suspended solids in the sedimentation process. From this material chrome can be recovered, or dumped separately [43].

Primary treatment. This involves separation of solids in a settling tank (primary clarifier), or by flotation. The settleable solids and up to 60% of the suspended solids corresponding to approximately 35% of the BOD, can be eliminated during the primary treatment. Subsequently the solids may be treated by anaerobic sludge digestion. This produces biogas and solids that are suitable for soil conditioning and fertilization. Primary treatment is a essential activity that needs to be undertaken for a proper application of various secondary treatment systems. In case of aerobic secondary treatment, a further function of this step is the reduction of electric energy required for aeration [11].

Secondary treatment. This usually consists of biological treatments by means of high rate anaerobic treatment systems, anaerobic (lagoons) suitable for high organic loads, or aerobic (lagoons) suitable for low organic loads, activated sludge, oxidation ditch or a combination. Present research is mainly focused on low energy demand and low volume treatment systems and optimum process control. Usually, a combination of high rate anaerobic treatment and aerobic activated sludge is required to meet effluent quality demands. Removal efficiencies reached with these kinds of combination are up to 98-99%. Depending on the operational conditions, removal efficiencies for slaughterhouses range from 70 to more than 99% for BOD and grease and from 80 to more than 97% for Suspended Solids (SS). The process performance depends strongly on the amounts of SS that can be removed in the primary treatment phase [38].

Tertiary treatment. This includes chemical-physical methods such as adsorption, stripping, coagulation, sedimentation, chlorination as well as biological methods like slow sand filtration and maturation ponds. This post-treatment serve to remove nutrients such as phosphorus, sulphide, suspended solids, remaining BOD as well as pathogens [1].

Another method of wastewater treatment is that of irrigation on land. Before wastewater is applied on land, toxic compounds such as chromium, salt sulphide, etc. have to be removed. Small amounts of nitrate and phosphate however may serve as fertilizers. The BOD value is usually not allowed to be higher than 300 mg/l.

At present, this kind of wastewater treatment is carried out mainly in developing countries. The method is cheap, rather easy to perform, does not require highly sophisticated techniques and can be apllied because of the usually low pollutional strength of the produced wastewater [33].

8. Waste production and its consequences

In the three types of animal-product-processing industries (slaughtering, tanning and milk processing), wastewater problems appear to be the most severe ones. Processing activities inevitably produce wastewater, frequently in large quantities. This wastewater is polluted with biodegradable organic compounds, suspended solids, nutrients and toxic compounds (particularly chromium and tannins from tanneries). Via the reduction of dissolved oxygen this pollution directly or indirectly leads to a decreasing suitability of (surface) water for aquatic life, and drinking, swimming or other purposes [32].

If the density of animal product processing is so low that the concentration of pollutants in the receiving water bodies remains low, the production of wastewater does not necessarily lead to environmental problems. However, when from the comparison of the values of Table 26 with the European target values for urban wastewater discharge (e.g. 25 mg BOD, 10-15 mg N and 1-2 mg P per litre), it becomes clear that, from a wastewater production point of view, that there is a trend towards increasing densities of product processing even at relatively small amounts of processed animal products [15].

The heavy metal Chromium, occurring in the waste of tanneries, has caused and will in all likelihood continue to cause, serious environmental problems. It is common practice that most of the chromium is released in wastewater. There are no indications of other heavy metals in the waste of the animal processing industry causing environmental problems.

Problems caused by air pollution and solid waste disposal are minor in comparison to those related to wastewater production. The main cause of air pollution is the use of fossil energy, with as major exception the volatile organic compounds in the leather industry [18].

Particularly in slaughterhouses solid waste disposal may lead to hygienic problems, but in principle these are relatively easy to solve. An exception is the leather waste that contains chromium. This waste must be dumped on special grounds.

For a proper discussion of the environmental impact of slaughtering, tanning, and dairy industry, the effects of related activities such as transportation, spoilage by the consumer, durability of the product etc. also have to be taken into account. These activities are especially important for the discussion concerning the advantages and disadvantages of the various production processes and the scale at which processing is undertaken [10].

8.1 Data availability and reliability

In Table 26 typical values of wastewater production for several processes are given. They are given for most common parameters to characterize wastewater production. The data originate mainly, though not exclusively, from OECD-countries. Data from developing countries on waste production and its environmental impact are difficult to find. Those data that have been reported can often not be interpreted adequately owing to major shortcomings in the description of the relevant processes or the data collection methods. In some cases waste parameters have been recorded without indications of relationships with other parameters. Examples of these are:

- data on suspended solids without any reference to solid waste;

- solid waste data of 5.5 kg manure per ton carcass weight, obviously referring to minor components of the manure, probably the scrapings, but with no reference to other solid waste production (e.g. rumen content).

Most of the reported values originate from EPA-studies published in the period 1970-1975. Even studies published at the end of the eighties refer mainly to these studies. Moreover, huge variations in waste production per unit of product processed have been found. This variation can be partly explained by looking at the types of products made or processes used, but some variation remains unexplained. But even worse, also in OECD-countries exceptions have been recorded which exceed emission values by manifold, without mention of a possible clue of explanation [1].

The conclusion that needs to be drawn is clear. There is an urgent demand for proper, well described, reference values on waste production. Monitoring programmes need to be set up to allow for a more reliable environmental impact assessment of animal product processing than is presently the case. These monitoring programmes should result in emission factors per unit of product processed. Because of the diversity in processes and waste production, proper data collection on waste processing will be an expensive and time consuming undertaking [15].

The obtained reference values on waste production will always need to be translated to locally relevant processing methods and production situations. Thus, monitoring programs must be arranged so as to make it possible to give a correct interpretation of the reference values.

To this end monitoring programmes should:

- cover all important pollutant parameters (particularly solid waste, water consumption, SS, BOD, N, P, heavy metals and energy consumption);

- give a clear description of the production processes to which the data relate, including the quantity and type of product processed;

In addition steps must be made that measurements are taken prior to wastewater treatment and before the water is diluted with other (waste-)water. If a wastewater treatment plant is available, the reactor performance should be determined for the evaluation of the effectiveness of the water treatment. In such cases related parameters should also be measured: precipitation not only results in a reduction of BOD, SS, etc., but also increases the amount of solid waste [44].

9. Waste reduction

There are several ways to reduce the waste load:

- prevention of the production of waste;

- development of new clean processing methods;

- treatment of waste (“end-of-pipe treatment”).

In this study possibilities for waste prevention and end of pipe treatment have been treated. No attention has been given to the development of clean processing methods as these entail specific fundamental technical and economical knowledge.

For a reduction of environmental problems that occur because of discharge of waste, improved housekeeping practises and management practices are of more importance than end-of-pipe waste treatment. Good house-keeping practices are not easy to describe, but it is clear that, as the amount of water used is major factor in all industries (if more water is used, total wastewater production per unit of product processed may increase manifold) proper water management is one of the first aspects deserving attention. A reduction of water consumption without decreasing hygienic standards, is often possible. This reduction may be reached by good-house keeping practices, but also by the introduction of new technics such as dry cleaning prior to washing[27].

Furthermore, environmental problems may also be reduced by converting as much waste as possible into a solid product instead of washing the waste away into the wastewater. In general solid waste is fairly easy to control, requires less energy and is cheaper than wastewater treatment.

For tanneries, it is of prime importance to prevent chromium from polluting wastewater. Precipitation of chromium is an easy process. Solid waste containing chromium should be dumped in special dumping grounds where facilities should be available to minimize the amount of percolation water. Precipitation also results in large reductions of SS and BOD emissions[6].

In slaughterhouses, blood and paunch contribute enormously to the wastewater load. These and other solid by-products should be prevented being washed away. By-products can be used for several purposes and unusable solid waste can be easily handled properly, e.g. via composting. This process and more sophisticated processes for by-product handling may even result in valuable products.

Given the high BOD-load in the wastewater of tanneries, dairies and slaughterhouses, anaerobic systems would seem to be the most suitable wastewater purification systems. Simple anaerobic systems reach 50% BOD-purification, while high-rate anaerobic systems may achieve a 90% BOD-purification rate [11].

In a few developed countries, environmental problems have led to the formulation of high quality standards for discharged water. To meet these standards, a combination of anaerobic and aerobic is required, often coupled to nutrient removal systems.

As most of the air pollution is related to fossil energy consumption, prevention as a method to reduce environmental pollution is even more important than it is for wastewater. For some components (e.g. VOC, dust) methods exist for the treatment of polluted air, however frequently at high costs [32].

10. Economic considerations

The costs of wastewater treatment are a factor of major importance for the selection the appropriate treatment system. Estimates should be made of the investment costs and the expected annual costs. The investment costs are largely determined by construction costs, the costs of land and the required degree of removal of pollutants. The annual costs will depend on the price of the energy and chemicals required for the operation of the plant, the discharge fees and the capital costs on investment. A problem for the estimation of the costs of treatment plants is that prices are rapidly changing. Cost estimates should therefore be referenced to an index [34].

From a comparison of the costs of 6 treatment systems (stabilisation ponds; aerated ponds; high rate anaerobic treatment + ponds; high rate anaerobic treatment + trickling filters; activated sludge process; and oxidation ditch; DHV, 1993) it can be concluded that high rate anaerobic treatment + post treatment of the effluent offers a very economic and effective solution [32].

The relatively high initial costs are compensated for by the low costs of energy and maintenance, which results in low running costs and a limited need for land. Costs of a stabilisation pond, high-rate anaerobic treatment plant + post-treatment in a pond and an activated sludge process for the sewage treatment plant for a town of 50,000 inhabitants (producing ca 550 ton BOD and ca 135 ton N) given as a reference, are: resp. around 3,5.106, 2.106 and 2.106 USD for investment costs and running cost resp. around 400,000, 300,000 and 430,000 USD on an annual basis. In this calculation it is assumed that electricity costs are 0.10 USD per kWh, sludge disposal costs 10 USD per 1000 kg and that the price of land is 25 USD per m2. Lagoons will become more economical if land costs are below 10 to 20 USD/m2. Wastewater from slaughterhouses, tanneries, and the dairy industry are more heavily loaded with pollutants than sewage. This will have the effect that anaerobic processes are more competitive than aerobic processes owing to the much lower energy costs of anaerobic treatment [23].

Taiganides (1987) gives an overview of relative cost indices and ranking for various treatment systems. According to him, the selection of the treatment system has to be undertaken on the basis of economic costs, environmental considerations, and the technical complexity of the system. Both the initial investment and the operating costs of the system must be taken into consideration. However, environmental and technical aspects cannot be quantified. Therefore subjective rankings must be used. Table 6 indicates that aerobic ponds is the least desirable method of concentrated wastewater treatment in places were productive land is to be used for construction of the ponds. Anaerobic lagoons are the least expensive and are used more often than any other treatment in the management of wastewater from feedlots. However, they are not recommended as a permanent solution [17].

Table 6.

a Exclusive of land acquisition costs. It is assumed that land used in the construction of the treatment plant is owned by the feedlot.

b Index is the ratio of the treatment cost to that of the least cost treatment. Thus, the least cost treatment would have an index of 1. An index of 6 means 6 times more expensive than the least cost treatment in that category.

c Ranking is a judgement ranking of the six potential systems ranked in order of preference from 1 to 6. The ranking is not on the basis of cost, nor does a ranking of 6 means it is 6 times less diserable than that ranked 1 in the same category.

d Index/rank is a combination of cost rations and judgement rankings reflecting the author's preference based on technical, economic, and ecological feasibility of the system[33].

Conclusions

The conclusion that needs to be drawn is clear. There is an urgent demand for proper, well described, reference values on waste production. Monitoring programs need to be set up to allow for a more reliable environmental impact assessment of animal product processing than is presently the case.

These monitoring programs should result in emission factors per unit of product processed. Because of the diversity in processes and waste production, proper data collection on waste processing will be an expensive and time consuming undertaking.

References

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