All most every by-product of slaughter house can be utilized. However, various circumstances do not always permit by-product recovery. The reasons may be inadequate quantity of materials, lack of markets, cost of processing etc. In such instances, they simply form part of waste lot for which different methods of processing and disposal have to be considered. For the slaughter house wastes composting, biomethanation and rendering systems are suggested. Selection of appropriate method, however, depends mainly on type of wastes and its quantity. Incineration is also an option for treatment of slaughter house waste.
Practically, all slaughter house waste i.e. type I and type II waste, can be used for compost making. The agriculture residue and dung from the lairage, ruminal and intestinal contents, blood, meat cuttings, floor sweepings, hair, feathers, hide trimmings can be stabilized by composting.
For preparation of compost stack, it is suggested that alternate layers of type I waste and type II waste should be built up to a height of 4 to 5 feet as shown in Fig 1. The heap should preferably be laid direct on the ground. It is advisable to put a layer of about 6-inch of course material, such as maize or millet stalks, banana stumps, straw, grass, small twigs etc. underneath in order to achieve proper ventilation. In case type II waste contains large organs such as kidneys and lungs or other similar wastes, then they are not put in whole but need to be minced or chopped into 2 to 3 inch pieces. For better results it is advised to mix these pieces with earth and evenly spread out in the centre of the heap where the temperature is high. Higher temperature in compost keeps rats, dogs or other vermin away. The ruminal and intestinal contents provide sufficient moisture for a start of bacterial activities. As such no water is required initially.
To achieve optimum conditions for the bacteria, moisture and proper aeration must be maintained from start to finish. A gradual reduction in height will follow, because of the shrinkage of decomposed matter. At least two turnings are required to obtain a uniform compost material. The first turning is normally advised after 2 to 3 weeks and the second turning after 3 to 4 weeks. The compost can be removed after 4 to 5 weeks. The total time required is about 90 days. This is reasonably enough time for composting, although it depends on many factors, such as type of material, size of heap, ambient temperature etc.
The quality of compost can be improved gradually with experience by proper combination of different wastes, providing appropriate time intervals for mixing, and moisture control.
When a clean, neat and tidy heap is required, compost bunkers can be constructed. As shown in Fig. 2(b), bricks or cement blocks may be used to build a wall, leaving open spaces between bricks. Brick walls are preferred over wood, because the wood tends to rot very quickly unless it is properly preserved.
The size of the compost bunkers depends on the quantity of raw material to be converted. Fig. 2(a) shows the recommended layout, which facilitates easy turning of material and removal of finished product (compost). This consists of four raw material bunkers namely A, A1 and C, C1. Each of these bunkers has four walls. Bunkers B and D share two walls with A and C and have one outer wall each. The fourth side of B and D is formed by inserting wooden planks. Bunker E is used for the finished material. It has one outer wall, and one side closed by wooden planks or door. The wooden partitions are provided to facilitate turning or loading. The floor of the whole area should be preferably earth.
Raw material is stacked into bunkers A and A1 first. When these bunkers are full, start using C and C1. The first layer forming the base of A, A1 and C, C1 is suggested be coarse vegetable matter of about 6 inch thick. The bunker is now ready to receive the first lot of material from the slaughter house. The method for preparing heap and time of turnings remains the same as in case of compost stack.
A biomethanation plant can be constructed in two ways. The gas is produced in one or more digesters and then it can be stored in a separate gas holder from where it is drawn as and when required. The other alternative is that the digester and gas holder are built so as to form one single unit. The gas is produced in the lower part of the structure, while the upper tank serves as a gas holder. While the second option is extremely simple and cheap in construction, but it has the disadvantage that gas production is affected during recharge. On the other hand, with a separate gas holder, continuous supply of gas can be assured even when one or more digesters are being charged. It is, therefore, more practicable for larger units to have separate gas holders.
Conventional Biogas Plant: Fig. 3 shows a conventional floating drum type biomethanation (biogas) plant. An inverted drum with a diameter slightly less than that of cylindrical digester serves as gas holder. The plant delivers gas at uniform pressure and provide good seal against gas leakage. It is reliable and has proven performance for cattle dung processing. However, the plant feed on slaughter house wastes such as rumen and paunch contents, dung etc. will also exhibit same performance when loading rate is maintained about 0.5 - 0.6 kg volatile solids / m3/day. The waste should be suitably diluted before feed. The plant can handle feed with solid content up to 8 percent.
The anaerobically digested sludge has higher nitrogen content than compost manure. The sludge should be dewatered by sand filtration or filter press. The dried sludge can be utilized as manure in field. The filtrate is recycled for preparation of feed slurry, which contains microorganism. The biogas can be used for boiler or power generation.
The economics of a typical biogas plant processing 1250 kg/day of waste is presented in Table 6. It can be seen that the plant can save up to Rs. 83,800/- on account biogas and manure.
Table 6: Economics of a biogas plant
The success of a biomethanation plant depends on several factors, such as the quality of the raw materials, temperature, ratio of water to solids, and also on the type of bacteria present.
Fig. 3 Schematic Diagram of Conventional Biogas Plant
High Rate Biomethanation: The essential elements of a high rate biomethanation are complete mixing and uniform temperature with more or less uniform feeding of the substrate. Pre-thickening or dilution of the digester contents are optional features of high rate digestion system. The benefits of high rate biomethanation are reduced digester volume requirement and increased process stability. In high rate biomethanation system, there is proper arrangement for operation control and safety measures.
Complete mixing of substrate in the high rate digester creates a homogeneous environment throughout the digester. It also quickly brings the feed into contact with microorganisms and evenly distributes toxic substances, if any, present in the wastes. The entire digester is available for active decomposition, thereby, increasing the effective solids retention time. Temperature is one of the important environmental factor. In the cold climate areas, digester heating is beneficial because the rate of microbial growth and therefore, the rate of digestion increases with temperature.
A schematic diagram of a typical high rate biomethanation plant is given in Fig. 4. Wastes consisting of rumen and paunch contents, dung, agriculture residue, fat and blood is processed in the high rate plant. The solid wastes from different sections are collected in dissolution tank. The dissolution tank is used to adjust moisture and solid ratio and mixing the waste thoroughly. The waste containing up to 12 per cent solids is passed through shredder, which reduces solid waste size to required level. Waste is now pumped to digester. Hydraulic retention time of waste in the digester is about 25 days. At organic loading rate up to 2.5 kg/m3/day, the digester can give up to 55 per cent efficiency in terms of volatile solid destruction. In digester optimum temperature of about 36 °C is maintained with the help of heat exchanger. The digester has all accessories such as temperature and pressure indicators, overflow, safety valve etc. Specific biogas production in high rate plant is about 0.8 m3/kg of volatile solids destroyed, having electrical equivalent of 2.11 kwh/m3 of gas.
Economics of a typical high rate biomethanation plant catering to 60 tonnes/day of waste is worked out in Table 7. The plant generates about 2600 m3 biogas and 7 tonnes manure in a day which can give additional income of Rs. 40 lakhs per annum.
Table 7: Economics of a high rate biomethanation plant
All the animal matter i.e. type II wastes such as inedible offal, tissues, meat trimmings, waste and condemned meat, bones etc. can be processed in rendering system. The main constituents of animal matter are fat, water and solids. The objective of rendering process is to physically separate the fat, the water and the solids. This is effected by heating and rupturing connective tissue of individual fat and muscle cells so that raw fat and other material bound within is freed. In rendering, fat recovered is used for industrial purposes, such as soap and greases. Fat recovered from flesh of healthy parts can also be used for edible purposes. Solid portion, which is known as meat meal or bone meal, is utilsed for the manufacture of stock feed and fertilizers.
Rendering is carried out in dry rendering or wet rendering plants. In both the processes, large pieces such as heads, bones etc are reduced in size by shredders or other machinery. Large soft offals are also cut to size before processing. Intestines, stomach and similar soft materials contain manure and, therefore, they are opened and cleaned before feeding to rendering plant.
Wet Rendering: The name wet-rendering is applied where the raw material is processed with added water or condensate derived from steam. The wet-rendering tank is usually a vertical, cylindrical boiler, having a cone-shaped bottom, with a gate valve outlet. Fig. 5 shows a typical batch type wet rendering plant.
Fig. 5 Diagram of Batch Rendering Plant
At the top of the tank there is a manhole through which raw material is loaded, and also a valve through which obnoxious gases escape without reducing the pressure. Steam is injected from the bottom of the tank. Several draw-off cocks on the side of the tank, at different levels, enable the fat and water to be removed.
After the raw material is loaded, the manhole is tightly closed and steam is let into the mass. The steam pressure used will very with the material. The higher the pressure, the quicker the disintegration. For this reason, large plants often render the offal at a pressure of 4 kg/cm2. However, high pressure may reduce the quality of the material, especially of the fat. For this reason, a pressure of 3 kg/cm2 is usually maintained in the tank.
The time required to disintegrate the tissue and free all the fat varies from four to six hours, depending on the character of the offal. After cooking is completed, the contents of the tank are allowed to settle for about two hours. After settling, clean divisions is formed between the digested material, water and fat. The fat, having the lowest specific gravity, will be on the top, the sludge and solids having the high specific gravity will be at the bottom. The center will be occupied by water. Gradually the pressure is reduced to that of the atmosphere, and then the water and fat are ready to be drawn off through the side cocks. If the fat level is below a cock, the level can be raised as required by addition of water.
After the fat and tank water have been removed, the gate valve is opened and the digested mass of meat and bones is taken out. At this stage the mass may contain up to 55 percent moisture and about 15 percent fat which can be dried in the dryer to obtain meat meal or bone and meat meal.
For large operation, integrated continuous rendering plants are used. An integrated rendering plant consists of pre-breaker, metal detector, fat separator, dryer and hammer mill. Total yield of bone and meat meal by wet rendering system is about 30 per cent of raw material weight and tallow about 10 per cent.
For large operation, integrated continuos rendering plants are used. An integrated rendering plant consists of pre-breaker, metal detector, fat separator, dryer and hammer mill. A schematic diagram of the integrated wet rendering plant is given in Fig. 6.
Dry Rendering: In this process, all the unwanted moisture is eliminated from type II wastes without the loss of any nutrient by using specially designed cooker. The dry rendering cooker, is a horizontal steam jacket equipped with a set of agitators, which keep the material in continuous motion. The steam is applied to the jacket only and not to the material to be processed, as in wet-rendering.
The material remains in the cookers for about 4 to 5 hours in most plants. Steam pressure in the cooker jackets usually ranges from 3 to 4 kg/cm2. The dry heat transmitted from the steam jacket to the raw material converts the moisture present in material into steam, which gradually builds up the internal pressure of cooker. This pressure, combined with agitation, disintegrates the material and breaks down the fat cell. Dry rendering therefore works on steam pressure develop from the moisture contained in the raw material itself, and not as in wet-rendering, from the pressure created by injected steam.
In the wet-rendering process, the fat floats on top of the liquid and is separated out. In dry-rendering, the fat is released from the fat cells but is still dispersed throughout the material. The fat in the solids may be removed by either a hydraulic press or by using a centrifugal turbine fat extractor.
As seen from the above, the whole process, i.e., sterilization, digestion, and drying, take place in cooker only. Therefore, there is no loss of nutrient. The dry rendering process allows approximately 20 percent higher yield than the wet-rendering, as the water containing water-soluble extractives and proteinous suspended matter is not discarded.
The dry rendering plants have units such as metal detector, pre-breaker, cooker, fat extractor and hammer mill. A schematic diagram of a typical dry rendering plant is shown in Fig. 7.
Economics of dry rendering plant installed in a mechanised slaughter house has been worked out and presented in Table 8. On an average the plant renders 65 tonne/day of animal matters to produce 20.25 tonnes bone and meat meal and 8 tonnes tallow in a day.
Incineration can be used for treatment of many wastes. Unlike previous methods, incineration provides no by-products but recovery of heat is possible. Incineration is a controlled combustion process for destruction of combustible wastes.
Fig. 7 Schematic Diagram of Dry Rendering Plant
Table 8: Economics of a dry rendering plant
The wastes after combustion are converted to gaseous constituents and a non-combustible residue. The gases are released to atmosphere and the residue is usually disposed to landfill.
In incineration, waste is burnt at temperatures between 850 0C and 1100 0C in specially designed combustion chambers. An auxiliary fuel is required to start ignition and sustenance of combustion of wastes. Incineration is immediate, it does not require long residence as in case of other methods. Proper temperature control, mixing and turbulence are necessary for effective combustion. It requires skilled manpower for operations. Capital cost and recurring expenses of incinerator are high. By using heat recovery system, the cost of operation can be reduced through use or sale of energy. Incineration technique is yet to be practiced for treatment of slaughter house wastes in the country.