Waste Processing Techniques
In this section of Solid waste management we will discuss the technologies used for treatment of solid waste to recover some valuable products. But before treatment of waste; it should be processed as per the desired technology.
Waste processing means the physical alteration in waste to make it best suited for technology adopted for its treatment. The processing of wastes helps in achieving the best possible benefit from every functional element of the solid waste management (SWM) system. It requires proper selection of techniques and equipment for every element to derive maximum economical value. Various methods used in waste processing are baling, shredding, compaction, drying, metal segregation etc.
Objectives of processing techniques:
Three major objectives of any processing done on waste are:-
Improving efficiency of SWM system: the technology should always help in making the waste transport and storage effective.
Example – waste paper is baled to reduce transporting and storage volume requirements, compaction and shredding of waste is done to improve the efficiency of the transportation and disposal.
Recovery of resources: materials with high market value and in sufficient quantity are recovered from mixed waste by various segregation techniques
Example - paper, cardboard, plastic, glass, ferrous metal, aluminium and other residual metals
Recovering conversion products and energy: Combustible organic materials can be converted to intermediate products and ultimately to usable energy. Incineration, pyrolysis, composting or bio-digestion.
Shredding and drying is necessary before the waste material can be used for power generation to reduce moisture content.
At the stage of storage and transportation, it is very important to store maximum waste in one truck and small storage area. This reduces the fright of trucks and storage space thus reduces the investment. Various methods are used to reduce the volume of waste. The most widely used techniques are mechanical, chemical and thermal techniques of volume reduction.
Types of Volume Reduction for SWM
- Mechanical Volume reduction
- Thermal Volume reduction
- Chemical Volume reduction
Mechanical Volume Reduction
Mechanical Volume reduction
- Size reduction and shredding
Mechanical Volume Reduction: Compaction
The main purpose is to reduce the volume (amount) and size of waste, as compared to its original form, and produce waste of uniform size. The process increases the density of waste thus more waste can be accommodated in small space.
Volume reduction or compaction refers to densifying wastes in order to reduce their volume.
Compaction is done by stationary as well as movable equipments.
This represents the equipment in which wastes are brought to, and loaded into, either manually or mechanically. In fact, the compaction mechanism used to compress waste in a collection vehicle, is a stationary compactor. It uses steel containers that can be subsequently moved manually or mechanically; chambers where the compressed blocks are banded or tied by some means before being removed; chambers where they are compressed into a block and then released and hauled away untied; transport vehicles directly.
This represents the wheeled and tracked equipment used to place and compact solid wastes, as in a sanitary landfill.
Some of the advantages and problems associated with waste compaction are given it the columns.
One of the loopholes of compaction is; after compressing waste segregation for recyclables becomes very difficult. The wet waste in mixed waste destroys the calorific value of combustible waste on compression.
Compactors are based on compaction pressure applied to the waste mechanically, for example
(i) Low-pressure (less than 7kg/cm2) compaction: In low-pressure compaction, wastes are compacted in large containers.
(ii) High-pressure (more than 7kg/cm2) compaction: pulverised wastes are extruded after compaction in the form of logs/blocks/bale. Typically, the reduction ranges from about 3 to 1 through 8 to 1.
The formula used to study waste compaction is given below. Vi and Vf are initial and final volume in m3
Size reduction and shredding
This is required to convert large sized wastes (as they are collected) into smaller pieces. Size reduction helps in obtaining the final product in a reasonably uniform and considerably reduced size in comparison to the original form
This process not necessarily imply to volume reduction. The waste once shredded will increase in volume but if compacted then can further be reduced in volume.
Equipments Used :
- Hammer Mill
- Chipper/Grinder/Shredder/ jaw crusher
Solid wastes and recycled water are added to the hydropulper. The high-speed cutting blades, mounted on a rotor in the bottom of the unit, convert pulpable materials into slurry with a solid content varying from 2.5 to 3.5%. Metal, tins, cans and other non-pulpable materials are removed from it.
These are used most often in large commercial operations for reducing the size of wastes. Hammer mill is an impact device consisting of a number of hammers, fastened flexibly to an inner disk.
The shredder is also known as Chipper/Grinder/Shredder/ jaw crusher as per its requirement. The waste is shredded as per the required size for treatment technology.
Thermal Volume Reduction
Thermal processing of solid waste can be defined as the conversion of wastes into gaseous, liquid and solid production, with or without energy valorisation.
- Destruction of the organic components of wastes, specially the dangerous ones; Reducing their volume;
- Obtain solid and/or gaseous inert products;
- Achieve a significant energetic valorisation
Unlike chemical and biological processes, thermal process don’t depend on waste specificity and operating conditions. Its sufficient guarantee that certain temperatures are achieved in a minimum gap time. For the heavy metals, the situation is more complex, as some of them can be volatilize during process and causing severe environmental impacts
Combustion is an exothermic process, which means, it release spontaneously, significant energy to the process become autonomous and, also, to export energy in heat, or, most important, in electric energy
On the contrary, the gasification and pyrolysis processes are endothermic, which means, it’s necessary to supply thermal energy to perform the pyrolysis reactions.
These methods will be discussed in details in thermal waste treatment technologies.
Chemical Volume Reduction
Chemical volume reduction is a method, wherein volume reduction occurs through chemical changes brought within the waste either through an addition of chemicals or changes in temperature.
These other chemical methods used to reduce volume of waste chemically include pyrolysis, hydrolysis and chemical conversions.
Component separation is a necessary operation in which the waste components are identified and sorted either manually or mechanically to aid further processing.
- recovery of valuable materials for recycling;
- preparation of solid wastes by removing certain components prior to incineration, energy recovery, composting and biogas production
Types of component separation
- Air Separation
- Magnetic Separation
This technique has been in use for a number of years in industrial operations for segregating various components from dry mixture. Air separation is primarily used to separate lighter materials (usually organic) from heavier (usually inorganic) ones.
The lighter material may include plastics, paper and paper products and other organic materials.
It is done by conveying air streams, which is usually done in a cyclone separator. As you can see in picture above, the shredded waste is introduced to the unit, because of air the lighter component come up and heavy stays at bottom. The two components are than collected separately. The light fraction may be used, with or without further size reduction, as fuel for incinerators or as compost material.
Ferrous materials are usually recovered either after shredding or before air classification. When wastes are mass-fired in incinerators, the magnetic separator is used to remove the ferrous material from the incinerator residue. Magnetic recovery systems have also been used at landfill disposal sites.
Suspended magnet: a permanent magnet is used to attract the ferrous metal from the waste stream. When the attracted metal reaches the area, where there is no magnetism, it falls away freely. This ferrous metal is then collected in a container.
Magnetic pulley: This consists of a drum type device containing permanent magnets or electromagnets over which a conveyor or a similar transfer mechanism carries the waste stream. The conveyor belt conforms to the rounded shape of the magnetic drum and the magnetic force pulls the ferrous material away from the falling stream of solid waste
Screening of waste
Screening is the most common form of separating solid wastes, depending on their size by the use of one or more screening surfaces. Screening has a number of applications in solid waste resource and energy recovery systems. Screens can be used before or after shredding and after air separation of wastes in various applications dealing with both light and heavy fraction materials. The size of the sieve is decided based on the waste size required for treatment.
The most commonly used screens are rotary drum screens and vibrating screens.
Rotating wire screens with relatively large openings are used for separation of cardboard and paper products, Vibrating screens and rotating drum screens are typically used for the removal of glass and related materials from the shredded solid wastes. The vibration of screens further fastens the process of screening.
Hand-sorting or previewing: Previewing of the waste stream and manual removal of large sized materials is necessary, prior to most types of separation or size reduction techniques. This is done to prevent damage or stoppage of equipment such as shredders or screens, due to items such as rugs, pillows, mattresses, large metallic or plastic objects, wood or other construction materials, paint cans, etc.
Inertial separation: Inertial methods rely on ballistic or gravity separation principles to separate shredded solid wastes into light (i.e., organic) and heavy (i.e., inorganic) particles.
The motor or the rotor throw waste with pressure, based on the inertia and gravity forces the waste is separated in different fractions.
Separation by Flotation:
In the flotation process, glass-rich feedstock, which is produced by screening the heavy fraction of the air-classified wastes after ferrous metal separation, is immersed in water in a soluble tank. Heavy material settles and lighter are removed from floatables
Optical sorting is used mostly to separate glass from the waste stream, and this can be accomplished by identification of the transparent properties of glass to sort it from opaque materials (e.g., stones, ceramics, bottle caps, corks, etc.) in the waste stream. Optical sorting involves a compressed air blast that removes or separates the glasses – plain or coloured
Drying and Dewatering
Drying and dewatering operations are used primarily for incineration systems, with or without energy recovery systems. These are also used for drying of sludges in wastewater treatment plants, prior to their incineration or transport to land disposal. The purpose of drying and dewatering operation is to remove moisture from wastes and thereby make it a better fuel. Sometimes, the light fraction is pelletised after drying to make the fuel easier to transport and store, prior to use in an incinerator or energy recovery facility.
- Convection drying: In this method, hot air is in direct contact with the wet solid waste stream
- Conduction drying: In this method, the wet solid waste stream is in contact with a heated surface
- Radiation drying: In this method, heat is transmitted directly to the wet solid waste stream by radiation from the heated body