The processing of solid waste usually consists of various mechanical processes, which serve to change the physical and fuel properties of the waste input and its composition in such a way that subsequent processes and possibilities of the waste utilization can be improved.
The minimum technical equipment for efficient treatment includes the following installations:
- Storage and loading facilities
- Removal of impurities and foreign matter
Solid Waste Treatment Process
Storage and Loading
Shallow bunkers or deep bunkers receive the delivered waste. Bulky impurities can be roughly removed in the flat bunker with the help of wheel loaders or special grabs. Irrespective of this, the delivered waste can be checked here without any major problems. Problematic deliveries can be excluded from the treatment. Furthermore, the separate storage of different fractions (e.g. dry commercial waste, bulky waste, wet household waste) is easier to realize in flat bunkers. They are cheaper than underground bunkers, but require a larger area.
The delivered waste can easily be mixed in the underground bunker. On the other hand, however, sorting out impurities is relatively difficult. Underground bunkers are particularly suitable for receiving wet household waste. For dry waste, the flat bunker is the better option. In general, flat bunkers are therefore the preferred solution for mechanical-biological waste treatment.
Separation of Foreign and Disruptive Substances
In the case of the operation of flat bunkers, bulky materials that are in the way can be easily removed with special grabs (grab excavators) or wheel loaders. Other impurities (such as batteries, large plastic foils) are usually removed at the conveyor or assembly line. In the case of dry, bulky and commercial waste, manual selection is also practiced in ventilated cabins. This practice is not suitable for wet household waste due to the potential health risks. Mechanical separation (e.g. with a grab excavator) is required here.
In addition to generating a homogeneous waste mixture, the reaction surface is increased in this step and materials that are still packaged are made available for the subsequent process steps. Because pre-shredding is the most energy-intensive step in mechanical treatment, it is only used in selected cases. However, bulky and commercial waste must always be pre-shredded. Cutting devices, shredders and crushers are used for pre-shredding. The main crushing is carried out with pre-shredder and double-shaft shredder. Fine crushing is carried out by single-shaft shredder or hammer mill.
Ferrous Metal Separation
Large metal parts are already separated in the storage area, while small parts can remain in the waste. The small iron parts can be separated from the sufficiently distributed material flow passing underneath by overbelt magnets. Because ferrous metals can be separated and recycled very easily, their separation is one of the frequent components of an MBT.
Non-ferrous Metal Separation
The separation of non-ferrous metals is also possible, especially in the material flow < 80 mm. High revenues can be achieved for recyclable non-ferrous metals.
If the waste contains significant amounts of plastic and wood, it is often separated together with paper/cardboard in a screening drum. Sieving with a grain size of 100 to 150 mm produces a high-calorific fraction in the overflow (paper/cardboard, plastics, wood). The biologically degradable materials are concentrated in the sieve passage. The screening drum is not used for bulky waste. If secondary fuel is produced from the screen overflow, it is then shredded and possibly conditioned and compacted.
Light and Heavy fraction Separation
During classification, for example using an air classifier, glass and stones are removed from the high-calorie fraction. However, this procedure is less common than screening.
Manual sorting can take place where the dry waste (particularly commercial, bulky and construction waste) contains high proportions of recyclable materials. The sorting steps usually take place after pre-screening. Wind sifting is a valuable support in separating the waste mixture and generating alternative fuels.
If the high-calorific fraction is used as a fuel, e.g. in co-incineration (see also "Industrial co-incineration" data sheet), further shredding is often required. High-speed machines are preferred. A grain size of 60-80 mm can be achieved with this shredder If further shredding is necessary, the waste must first be pelleted, which involves a great deal of technical effort.
For optimized storage and transport, the separated material (mostly plastics and paper) is often pressed into bales.
In the case of the MBS process approach, usually only the metal and coarse mineral matter are mechanically separated after biological stabilization. Before the biological process, it may be necessary to separate the impurities and carry out pre-shredding.