MECHANICAL COMPOSTING
Though manual methods are preferable in countries where labour is comparatively cheap, mechanical processes are preferred (Gotaas 1956) where higher labour costs and limitations of space exist. In 1922, Becari in Italy patented a process using a combination of aerobic and anaerobic decomposition in enclosed containers. The first full scale plant was established in 1932 in the Netherlands by a non profit utility company-VAM using Van Maanen Process in which raw refuse is composted in large windrows, which are turned at intervals by mobile cranes moving on rails. The Dano Process was developed in Denmark in 1930. Several other processes were subsequently developed using different methods of processing of solid waste using different designs of digester.
14.6.1 Unit Processes
A mechanical composting plant is a combination of various units which perform specific functions. Fig.14.3 gives a general flowchart of a mechanical compost plant.
Solid waste collected from various areas reaches the plant site at a variable rate depending upon the distance of collection point. As the compost plant operates at a constant rate, a balancing storage has to be provided to absorb the fluctuations in the waste input to the plant. This is provided in a storage hopper of 8 to 24 hours storage capacity, the exact value depending upon the schedule of incoming trucks, the number of shifts and the number of days the plant and solid waste collection system works.
The waste is then fed to a slowly moving (5metres/minute) conveyor belt and the non-decomposable material such as plastics, glass, metals are manually removed by labourers standing on either side of the conveyor belt. The labourers are provided with hand gloves and manually remove the material from the moving belt (the thickness over the belt is kept less than 15cms) and the removed material is stored separately.
The metals are then removed from the waste by either a suspended magnet system(Fig.14.4a) or a magnetic pulley system (Fig.14.4b). Majority of the metals are recycled at the source itself and hence are not contained in the waste.
Magnetic removal of metals hence is not very efficient and therefore not used is India.
In developed countries glass and metals are present in larger concentration and are removed by using ballistic separators. In these units, the waste is thrown with a large force when different constituents take different tragectories and get separated (Fig.14.5). This unit is energy intensive and due to smaller content of glass and metals in Indian municipal solid waste, it is not used in India.
The waste is thus subjected to size reduction when the surface area per unit weight is increased for faster biological decomposition. Size reduction also helps in reducing fly breeding in the decomposing mass. This is commonly carried out either in Hammermills or Rasp mills. Hammermills are high speed (600-1200 revolutions per minute) compact machines but consume large energy (Fig.14.6). Rasp mills are slow moving large units that require lesser energy (Fig.14.7). The capital cost of a hammer mill is less but its operating cost is more than that of a rasp mill mainly due to the larger energy requirement as well as more frequent replacement / retipping of hammers.
The stabilisation is carried out in open windrows provided over flagstone paved or cement concrete paved ground. These windrows are turned every 5 days to ensure aerobic decomposition. Various types of equipment such as front end loaders/windrows reshifters are used for turning of windrows.
At the end of the 3 to 4 weeks period, the material is known as green or fresh compost wherein the cellulose has not been fully stabilised. It is hence stored in large sized windrows for 1-2 months either at the plant or the farms. At the end of the storage period, it is known as ripe compost. It may be sometimes subjected to size reduction to suit kitchen garden and horticulture requirements.
