Introduction to Dairy Industries:
The rapid expansion of dairy industry is one of the significant developments in the food processing field during this period. The subject dairy and food engineering occupies a major importance in the food science curriculum and this emphasis is likely to continue.
In dairy industry, water has a multipurpose use. Water used for the purpose of processing, cleaning and other general uses should be of potable standard and absolutely free from microbial contaminants. Use of contaminated water in the plant, results in milk and milk products to be unsafe for human consumption.
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Several outbreaks of epidemics have been traced to contaminated milk and milk products. Contamination of water invariably arises from the activities of man. It is always desirable to use water of good quality and avoid chances of its contamination either at source or transit.
The dairy industry has to meet the rapidly intensifying quality standards with regard to microchemical/microbiological parameters. These quality standards can be achieved by having an adequate system which covers quality of raw milk, water supplies, cleaning materials and procedures and solid/liquid waste management system, without any compromise on aesthetic, public health and stipulated technical standards.
An adequate ‘safety screen’ approach requires that:
(a) Potential contamination of milk and milk products with any of the group factors like bacteria, heavy metal, viruses etc., should be minimised through deliberate ‘point source’ control with stringent inspection.
(b) A secondary protective screen system be installed at critical unit process where water loops may be in contact with products.
They should adequately remove all potentially questionable factors, usually by subtractive filtration, absorption methods, appropriate for each factor in type, size and frequency.
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These can usually be designed with ample safety factor to meet present and future needs. It is necessary to carefully design and build proper supply and operating systems of water for various uses in the plant based on sound engineering principles.
The milk industry is one of the most widely spread of all industries. Fig. 28.1 is a composite flow diagram showing the major operations for the processing of the more common milk products. Few plants are equipped for all of the operations shown in Fig. 28.1, but may employ any one or a combination of several.
Sources of Waste from Dairy Industries:
Wastes from milk products manufacture contain milk solids in a more or less dilute condition, but in varying concentration. These solids enter the wastes from almost all of the operations.
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In general the wastes and their sources may be classified as follows:
(i) Spoiled raw or manufactured products.
(ii) By-products (buttermilk, skim milk and whey).
(iii) Spillage or overflow due to inefficient equipment and careless operations.
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(iv) Rinsings and washings from cans, equipment and floors.
(v) Condenser water and condensate from vacuum pans.
(vi) Water from coolers, ice machine, boilers and roof drains.
BOD of Whole Milk and By-Products:
Milk solids are composed of the three general classifications of organic material, namely, fats, proteins and carbohydrates. Roughly, the BOD of one kg of milk fat is 0.90 kg. The milk solids content of whole milk and hence that of skim milk, buttermilk and whey varies to some extent.
The average compositions of these products are given in Table 28.1 Since milk wastes are made up of dilutions of whole milk and the by-products it is of value to know the range of BOD concentrations of these products.
The following data have been obtained from the analysis of a large number of samples:
Waste Prevention:
Waste disposal in the milk industry may be divided into two programmes, first, waste prevention or saving, and second, waste treatment. The utilization of by-products and a waste- saving programme will materially reduce the loss of milk solids and simplify the requirements for treatment. Such a programme should always precede the design of treatment facilities.
The first step in the programme is to segregate all possible clean water from the water containing milk solids. Segregation necessitates changes in the drain system of the plant in order to provide a separate line for cooling water, ice machine water, boiler blow-down, roof drains and vacuum pan water. The condenser water from the vacuum pan will contain entrained solids, but because of its large volume it must be segregated from the plant wastes.
After as much of the clean water has been segregated as can be economically accomplished, a weir box containing a device for measuring the rate of flow and an automatic sampler is installed on the waste line. The laboratory is then provided with facilities and instructed in the procedure for the BOD test. Fig. 28.2 shows a weir box with a 90° V-notch weir, hook gage, and Trebler automatic sampler.
A regular programme of sampling and analysis is initiated before any waste prevention activities are started and is continued until losses are reduced to a minimum. After this point has been reached occasional measurements and analyses are necessary to prevent a return to careless operations.
The prevention programme consists of a study of the various sources of waste, and the initiation of good housekeeping methods, careful operations, by-product utilisation, employee education and adequate and efficient facilities.
Disposal of Spoiled Products:
Spoiling usually occurs during periods of hot weather when cooling facilities may prove to be inadequate. More attention has been given in recent years to the handling of milk by the producer and spoilage is not as great as it was at one time. Spoiling may occur in the plant due to prolonged power failure, breakdown of equipment, or lack of adequate storage.
It is a general rule that spoiled products are not to be dumped into the drain system, except perhaps in very large cities where the quantity of sewage is so large that the spoiled material will have no apparent significance.
Provision should be made to prevent spoiling by installing adequate equipment and emergency power. When products do spoil, they should be returned to the producer for feeding purposes.
Utilisation of By-Products:
It should be a general rule in all plants where milk is processed that by-products should not be allowed to enter the drain systems. The quantity of these by-products is not always amenable to processing for use as food products or animal feed. In these cases adequate provision must be made to return the entire amount of by-product which cannot be sold as such to the farms for feeding purposes.
Where the volume is sufficiently large to warrant processing, adequate provision must be made to take care of the by-product at peak season. In some cases, it may be feasible for several plants to combine for processing by-products, if the length of haul is not excessive. There are numerous plants designed especially for by-product processing which take the material from a fairly wide area.
In general, these processes consist of removing the water and recovering the solids in a semi-solid or dry condition.
Skim Milk and Buttermilk:
Considerable quantities of skim milk are processed for the manufacture of both plain and sweetened condensed skim and for dry powder. Single or double-effect evaporators are used for condensing.
Dry powder is produced by either spray or roll drying. A good share of these products goes into the manufacture of food. Buttermilk is also condensed or powdered. The concentrate is used extensively in feed for animals.
Since buttermilk has a much higher solids content than whey, economical processing is possible with smaller volumes, although it is usually an advantage to haul to a central condensing plant if the creameries are suitably located.
Dry powder is usually produced by roll drying the buttermilk directly or after pre-condensing. It may be spray dried in certain types of driers. As much of the butter washings as possible should be included with the buttermilk in processing.
The first rinse should therefore be of as small a volume as possible and should be collected. Where condensing or drying facilities are not available, the buttermilk and first washing should be collected and used for feeding in the fluid form. There is a limited market for fluid buttermilk for human consumption.
Cheese and Casein Whey:
Whey is the most dilute of the by-products and for this reason larger volumes are necessary to make processing economical. It has been investigated that 100,000 kg of whey per day should be available during the flush season to economically operate a double-effect evaporator, and 250,000 kg of fluid whey for the operation of a spray drier.
Most of the whey powder produced is made by pre-condensing in single or double-effect evaporators and then spraying to form the powder. This means that the condensing plant must serve several cheese factories and that the drying plant must take the condensate from several condensing plants. It is estimated that the economical haul for fluid whey is up to 40 miles and up to 150 miles for concentrate.
Roll drying can be used for fluid whey and has the advantage of lower equipment cost. The principal use for powdered whey is in the manufacture of poultry and animal feeds. A relatively smaller amount is mixed with other materials for food products.
In addition to whey powder, a number of other products are produced from whey, such as milk sugar, lactalbumin, lactic acid, alcohol, vinegar and sweetened condensed whey. The manufacture of all of these, except whey concentrate, is accompanied by a waste disposal problem which, mainly because of the comparatively high concentration of solids is of greater intensity and sometimes more difficult than most other milk waste problems.
The manufacture of whey concentrate and powder adds to the waste problem because of losses during operation and clean up, but the production of products such as milk sugar, etc., results in a residue which is very concentrated and for which the market is limited. These residues can be dried in the same manner as whey concentrate.
Both sweet and sour whey concentrate are manufactured. Because of the high acid content the sour concentrate is not nearly as perishable as the sweet, however, it is very corrosive to pans and equipment. The investment in equipment for the manufacture of the concentrate is smaller than for whey powder, but the difficulties and cost of transportation are greater.
Spillage and Overflow of Milk:
Spillage and overflow of milk and milk products are caused by inadequate equipment and careless operations.
Following are some of the more common methods and facilities used to prevent these losses:
(i) Electronic liquid level controls and alarms are used on storage tanks, vats, upper and lower cooler troughs and other receptacles where there is danger of an overflow of milk or other products.
(ii) Temperature controls are installed on coolers to prevent the milk from freezing on the plates.
(iii) Temperature regulators are used to prevent hot-wells from boiling over.
(iv) Where possible, stand-by electric power is provided to avoid the necessity of dumping milk and by-products when the usual source of power fails.
(v) Adequate storage tanks and processing equipment are necessary for handling the peak volume of milk to avoid waste and spoilage.
(vi) Preventive maintenance of equipment and piping and replacement of worn-out or old style parts prevents or corrects leakage.
(vii) Foaming is prevented by avoiding air leaks in pump suction lines, pump packing or rotary seals.
(viii) Modern style separators are installed to avoid the loss of solids which occurs by foam from older types.
(ix) Cheese vats are not filled above 3 inches from the top to prevent spillage.
(x) Storage tanks with a capacity of one and one-half the maximum daily volume are necessary in order to make adequate disposition of whey.
(xi) Entrainment separators on vacuum pans are strongly recommended to minimise the carryover of milk solids in the condenser water.
(xii) In cases where the pollution problem is extremely acute, surface condensers are installed on vacuum pans to collect the condensate for treatment.
(xiii) Electronic water level controls are used to operate an alarm and to break the vacuum on pans to prevent them from boiling over.
(xiv) A stand-by pump installed to pump whey from the cheese vats to storage will avoid the discharge of whey to the drains.
Rinsings and Washings:
The only unavoidable wastes from a milk products plant are those resulting from rinsing and washing tanks, cans, equipment, and floors. Considerable waste can be prevented here by careful operations and the installation of adequate waste-saving facilities.
The following list includes some of the more common methods used to control these wastes:
(i) Much of the milk which remains in the can after the milk is dumped to the weigh vat can be collected by installing a drip saver and pre-rinse on the can washer. Very little water is required for the pre-rinse (about 3 ounces per can).
However, it should be injected into the cans in the form of a fine spray. The drip and rinse are collected in a special can or tank and used in the product, if possible, or for animal feeding.
(ii) The use of a constantly discharging water hose in the receiving room or other rooms increases the volume of waste to be treated and should be avoided.
(iii) Pipelines used to transport milk or milk products are installed in such a manner that they can be easily drained into standard buckets before they are disassembled for washing.
(iv) Tanks and vats are drained thoroughly and the milk or product collected.
(v) Cheese vats are directly connected to the whey pump suction line to avoid spillage of whey.
Cheese washings, especially the first and more concentrated, are added to the whey for processing or feeding purposes.
(vi) The first butter washings are collected and processed with the buttermilk.
Standard Maximum Load:
The “standard maximum load” is the average BOD load for “the 10 consecutive days during the year which give the highest average daily milk volume processed, assuming that the plant has good management and personnel and adequate modern equipment to handle its maximum volume and that every effort is made to follow good housekeeping and normal waste saving practices.”
This calculated value gives the plant management something to strive for in a waste prevention program, although it cannot be expected that every plant will meet the value. Some may arrive at a value considerably less than the calculated one, while others will find it impossible to reach at all.
Table 28.2 can be used for calculating the standard maximum load for any given plant. This table gives the lbs of BOD for each 5,000 kg of milk handled in the various process steps.
The following example is used to calculate of the standard load from the values in Table 28.2. An evaporated whole milk plant receives an average of 80,000 kg of milk during 10 top days. Of this, 60,000 kg are received direct from the producer in cans, and 20,000 kg comes in tank trucks.
The standard load is calculated as follows:
In addition the normal entrainment loss in the pan water will be 16 kg per day.
Treatment of Milk Waste:
The wastes from the processing of milk products are almost entirely composed of organic material in solution, or colloidal suspension, although some larger suspended organic solids may be present in such wastes as those from cheese and casein operations. Sand and other foreign particles will be present in limited amounts as a result of cleaning up the floors.
If these wastes are held for a short time, fermentation of the milk sugar will produce lactic acid and cause precipitation of the casein and other protein material. If stored in a condition which allows anaerobic action the waste rapidly becomes septic and odourous. Milk solids, especially the carbohydrate material, are readily oxidized biologically under aerobic conditions.
Treatment Processes:
Since milk waste contains very little suspended matter, preliminary settling for solids removal does not result in an appreciable reduction of the BOD. In most cases the amount of sand and other floating material in the waste is sufficient to warrant a small screen and grit removal chamber.
Aerobic processes are best adapted to the treatment of milk waste. The selection of the process for any particular plant will depend upon the size of the problem, location of the plant, and the necessary degree of treatment.
In some cases it has been considered desirable to treat the concentrated by-products, such as whey or “mother-liquor” from milk sugar manufacture, rather than to utilize them in the manufacture of feeds or other products. This is not an economical practice, but if it is considered desirable for any reason, these valve by-products can be successfully treated by the anaerobic digestion process.
In many cases it will be necessary to provide aerobic treatment for the effluent from the digestion process in order to reduce the BOD to the desired level.
Each of the above aerobic processes is discussed below:
The following processes are recommended:
1. Land irrigation
2. Aeration
3. Biological filtration
4. Activated sludge
1. Land Irrigation:
Surface irrigation may be used as a means of waste disposal by small plants located on farms or in sparsely populated areas. This method can be used only during the summer months in temperate zones, but has a much wider application in warm and arid regions.
The land to which the waste is applied must be kept under constant cultivation in order to prevent pooling and the production of odours.
The land may be furrowed and crops planted between the furrows. The soil must be light and sandy and may be under-drained with parallel rows of farm tile located about 15 inches below the surface. Drainage is not necessary, but aids in removing the excess water from the soil and increases the capacity for handling the waste.
The waste may flow by gravity or be pumped to the field depending upon the topography of the area, but in either case provision should be made to apply the waste to alternate portions of the field in order that each area can be allowed to dry and can be cultivated between applications.
If odours become a factor in the use of this method of disposal they may be controlled by the application of liquid chlorine or sodium hypochlorite to the waste before it is applied to the field. It has been found that a dose of 20 ppm chlorine is sufficient to control odours.
Screen and Grit Chambers:
Screening to remove large particles such as matches and milk bottle caps prevents much of the trouble with clogging of pumps and other equipment. A screen of 1/2 inch mesh wire is installed in a chamber in the drain line, if the line is not too deep. It is built on a frame which can be removed for cleaning. The screen chamber can also be used for the removal of grit by building it of a size which will reduce the average velocity of flow to about 1 foot per second.
This velocity is sufficient to keep most of the organic solids in suspension while allowing the sand particles to settle. The chamber requires frequent cleaning and a closed container should be available to hold the grit and screening until they can be disposed of by burying. This screen and grit chamber should precede all types of treatment except perhaps those for very small plants.
2. Aeration and Pre-Aeration:
The operations of a milk plant are not uniform and the flow and concentration of the waste will vary over a wide range. There are periods of great activity in the plant when the waste may be very strong. This condition may also prevail during the final wash-up period.
At other times, especially at night, the waste may consist entirely of clean water. Because of this wide variation of the waste, and since it is desirable to operate treatment facilities at a more uniform rate over as much of the entire day as possible, provision is made for storage and equalisation.
Air is applied to the waste in the equalisation tank to avoid the production of odours and septic conditions. It has been found that this application of air, if extended over a period of 18 to 20 hours, results in a reduction of the BOD varying from 30 to 60 per cent and averaging about 45 per cent. Also, during this aeration and storage period much of the milk sugar (lactose) is converted to lactic acid by the action of fermenting organisms.
In this condition it is more readily oxidised on the biological filter or by activated sludge.
A period of aeration is, therefore, desirable either as a means of treatment in itself or preliminary to other aerobic processes and for the equalisation of flow and concentration.
In some cases the reduction obtained from simple aeration of this type may meet the requirements for treatment of the waste. This is especially true if the waste is to be discharged to the sewerage system of a municipality.
The process will gradually develop a sludge which materially aids in the reduction of BOD and approaches activated sludge. No attempt is made, however, to remove this sludge by settling and much of it will leave by way of the aeration tank effluent.
The aeration tank is usually constructed of reinforced concrete and at such an elevation as to receive the waste by gravity. If the tank is used for pre-aeration ahead of filtration or activated sludge, a capacity for about 12-hours flow of waste is provided. If it is used as the only means of treatment, the capacity is for the average 24 hours flow.
The depth is usually from 6 to 8 feet and some provision is made for the collection and removal of sludge, since some of the solids are sufficiently heavy to settle in spite of the agitation.
In large installations a sludge collecting mechanism may be used for this purpose. For smaller operation the bottom may consist of a series of hoppers in which sludge lines are located.
Air is provided by means of a positive pressure-type blower. The quantity of air necessary is about one-half cubic foot per minute (cfm) per kg of BOD per day for the maximum day. Air is applied to the waste by a series of pipelines to which self-cleaning jet nozzles are attached (Fig. 28.3) or any other type of diffuser except those employing porous plates. If jet nozzles are used (3/16 inch bore), about 3 jets are required for each cfm of air.
3. Biological Filtration:
Biological filtration consists essentially of applying the waste to a bed of stone on which a film of active organisms has developed. The standard or one-pass filter is no longer used to any great extent for the treatment of milk waste.
This type has been replaced by the recirculating filter because of its efficiency and greater load capacity. The recirculating filter may be single or two-stage depending upon the degree of removal required.
It is preceded by an aeration tank which is also used to receive the recirculated effluent, and is followed by a settling tank for the removal of suspended material from the effluent. Some provision is made for the disposal of the sludge from the aeration and settling tanks, depending upon the size of the operations.
A heated digestion tank and sludge beds are most satisfactory for this purpose, but in some cases the cost of such an installation is not warranted due to the relatively small amount of sludge obtained. In these cases it may be ponded or hauled and applied to land.
The design of large units to accommodate the recirculating principle is a matter which should be referred to an experienced engineer. The design is governed mainly by the size of the operation, local conditions of topography, and the desired effluent.
i. Fill-and-Draw Recirculating Filter:
Milk plants vary considerably in size and the treatment process must be adapted to this factory. Fig. 28.4 shows a recirculating filter using the fill-and-draw principle and adapted for use at very small plants where the daily waste volume does not exceed 3,000 to 4,000 gallons. The tanks may be of wood stave or steel plate construction and are both set above ground.
The waste is collected in a small sump and pumped to the equalising tank which has a capacity to hold the total daily flow but may or may not be equipped for aeration. The waste is pumped over and over the filter at a rate which will provide from 8 to 10 re-circulations. The tank is emptied each morning before a new batch of waste is applied.
The filter is composed to 3 to 4 inch diameter hard granite stone or slag and is 6 feet in depth. It is supported on a grid located about 6 inches above the bottom of the filter tank. Holes bored just below the grid provide ventilation to the filter.
The size of the filter is such as to provide one cubic yard of filter media per kg of BOD per day. Up to 95 per cent reduction in BOD is possible with this batch filtration process, depending upon the number of times the waste is recirculated and its initial concentration.
ii. Recirculating Filter, Single-Stage:
The suggested arrangement of units shown in the flow diagram, Fig. 28.5 is adapted to plants where an effluent having a BOD concentration of 70 to 100 ppm is satisfactory. The raw waste enters the aeration tank through the screen and grit chamber where it mixes with the settled filter effluent.
A vertical type centrifugal pump is used to pump the waste to a rotary or other type of distributor which applies it to the filter media. The capacity of this pump is about 8 to 10 times the average rate of flow of raw waste.
The filter bed is 6 feet deep and under-drained in the usual manner. The filter media is composed of hard granite stone or slag, entirely free of sand and having no stone smaller than 2½ inches or larger than 4½ inches. The average size is 3½ inches.
The filtered waste flows into a weir box in which the weir is set at the desired level of water in the aeration tank. The rate of flow over the weir is therefore the same as that of the incoming raw sewage. The balance of the filter effluent is returned to the aeration tank.
The waste which flows over the weir is passed through a settling tank having a capacity for the maximum one-hour rate of flow of the raw waste. Provision is made in both the aeration and settling tank for the collection of sludge.
In the smaller installations this may be accomplished by constructing hoppers in floors of the tanks and installing sludge lines in these hoppers. In larger installations it may be desirable to install mechanical sludge collection equipment.
The volume capacity of the filter is based on the maximum raw waste BOD load. The recommended loading is one kg of BOD per cubic yard of filter media. A reduction of from 75 to 80 per cent of the BOD is possible by this process.
iii. Recirculating Filter, Two-Stage:
In case the reduction provided by single-stage filtration is not sufficient to meet the requirements, two filters operating in a series may be used. Fig. 28.6 shows a flow diagram of a two-stage filter.
The size or arrangement of the screen, grit chamber, aeration tank or settling tank need not be altered. The waste from the aeration tank is pumped to the first filter and returned to a weir box in which the weir is set at the desired water level in the aeration tank. The capacity of the pump provides for 6 to 8 re-circulations.
The overflow of the weir enters a pump sump in which a second pump of like capacity is installed. The waste is pumped from this sump to the second filter and returned to a second weir box.
The weir in this box is set at the desired elevation of water in the pump sump and at least 6 inches below the weir in the first box. The overflow of this second weir passes through the settling tank to the outfall line. The remaining portion of the second filter effluent enters the pump sump for recirculation.
The tendency in recent years has been toward shallow filters, especially those used for two-stage filtration. Filters 4 feet deep are usually recommended. The stone of the first filter has a diameter between 2½ and 4 inches.
It is common practice to use somewhat smaller diameter stone in the second filter, but this often leads to trouble with clogging, especially during winter operation when the efficiency of the entire system is low. Use of the same size stone in both filters is preferred.
A somewhat higher filter loading is used in the design of the first filter than is recommended for the single-stage. Loadings of from 1.5 to 2.0 pound per cubic yard of media based on the raw waste BOD are generally considered adequate.
In calculating the load for the second filter a reduction of 60 to 70 per cent is assumed for the first unit and a loading figure of 0.75 pound of BOD per cubic yard applied. Using these loadings, the two-stage filtration unit is capable of efficiencies of 90 to 95 per cent and effluents with BOD values below 50 ppm.
4. Activated Sludge:
Experience has demonstrated that the activated sludge process can be successfully used for the complete treatment of milk waste. It consists of developing naturally by aeration of the waste a sludge containing large numbers of organisms which are active in the oxidation of organic solids.
These organisms are supplied with sufficient air to maintain a dissolved oxygen content in the liquid media. Air is supplied either by blowers under pressure or by mechanical aerators. Aeration is followed by a period of sedimentation during which the sludge is removed and returned to the aeration unit.
Excess sludge beyond that required for adequate oxidation is removed from the process and subjected to anaerobic digestion in a heated digestion tank. There is no need to provide facilities for the disposal of digested sludge since all of the solids are liquefied or gasified in the digestion tank. The supernatant liquor from the anaerobic process is returned to the aerator.
There is evidence, but not definite proof at the present time, that if a sufficient aeration period is provided, there will be no excess activated sludge from the process. Whether or not it is more economical to provide the needed aeration period than to construct a digester for the excess sludge has not been determined. For simplicity of design and operation, however, the elimination of the digester, if possible, is desirable.
Perhaps the largest plant in which the process is demonstrated is that of the Mead Johnson Company at Zeeland, Michigan. This plant uses the Mallory oxidised sludge method of control, but is essentially what is conventionally known as the activated sludge process.
There seems to be a consensus of opinion that this process requires more technical control than is usually available at milk products plants and that it is much more easily upset than is the biological filtration process.
Actual experience has proved that this is not the case. This viewpoint has resulted from observations at plants which have not been properly or adequately designed or operated. Properly designed plants which have adequate facilities for providing sufficient air and for the control of the return sludge are not easily upset nor is the control procedure difficult.
The process has certain advantages over filtration such as lower construction costs, less area required for the plant, and much lower effluent BOD values. However, power costs are somewhat higher. It can be adapted to small as well as large milk plants.
There are many types of construction and many possible arrangements of the aeration and settling units which are used to accommodate the activated sludge process and if they provide the essential facilities they should be equal in performance.
Steel construction is considered superior to concrete for small installations in that the tanks may be prefabricated. The aeration unit is in the upper portion of the taller tank and the settling tank in the smaller. There is a room under the aeration tank for housing the pumps and blowers.
The unusual design of the Mead Johnson plant in which the rectangular settling tanks are contained inside of a large steel circular tank. The aeration compartment is that portion of the circular tank outside of the settling units.
The top rim of the circular tank is hollow and is used as a main line air supply. The individual diffuser units are connected to this rim. Aeration is accomplished by means of the self- cleaning nozzles shown in Fig. 28.3. Sludge is returned from hoppers in the settling tank to the aeration compartment by air lifts.
A plant of steel tank consists of separate aeration and settling tanks. This plant was designed for 300 pound of BOD per day and an effluent BOD not to exceed 50 ppm. Actually the BOD of the effluent has averaged about 20 ppm with an average load of 250 kg per day.
The basic capacity of the aeration unit to accommodate the activated sludge process for the treatment of milk waste is about 80 gallons per pound of BOD. Air requirements depend upon the nature of the air diffuser units. If self-cleaning nozzles are used, these requirements are approximately one cfm per pound of BOD per day.
The rate of return sludge, and the concentration of solids in the return sludge and mixed liquor, is governed by the BOD concentration. For waste having between 800 and 1,200 ppm BOD the return sludge flow for optimum treatment is between 600 and 650 per cent of the waste flow. In this range the concentration of solids in the mixed liquor is from 5,000 to 6,000 ppm. Settling rates and the sludge index are used as a means of control of the process.
Sufficient air capacity must be available in order to maintain dissolved oxygen in both the effluent from the aeration tank and the clarifier. Dual blowers and ample return sludge facilities are important features for the design of units for this process. BOD values below 10 ppm are possible under adequate design and operating conditions.