1. Unit IV Off Site Processing
Syllabus Processing techniques & Equipments – Resource Recovery from solid wastes – Composting – Incineration – Pyrolysis – Options under Indian conditions.

2. Processing Techniques & Equipments
Purpose of Processing
The purposes of processing are:
(a) Improving efficiency of SWM system (e.g.) Shredding
(b) Recovering material for reuse
( c ) Recovering conversion products & energy

3. Various Techniques & Equipments
(a) Mechanical Volume & Size reduction
(b) Component Separation
- Air Separation
- Magnetic Separation
- Screening
( c ) Drying & Dewatering

4. (A) Mechanical Volume & Size Reduction
Mechanical volume and size reduction is an important factor in the development and operation of any SWM system.
The main purpose is to reduce the volume and size of waste, as compared to the original form, and produce waste of uniform size.

5. Volume Reduction (or) Compaction
Volume reduction (or) compaction refers to densifying the wastes in order to reduce their volume.
The benefits of compaction are:
(a) Reduction in the quantity of materials to be handled at the disposal site
(b) Improved efficiency of collection and disposal of wastes
( c ) Increased life of landfills
(d) Economically viable waste management system

6. Equipments for Volume reduction (or) Compaction
Stationary Equipments:
This represents the equipment in which wastes are brought to, and loaded into, either manually (or) mechanically.
Movable Equipment:
This represents the wheeled and tracked equipment used to place and compact solid wastes.

7. Low Pressure ( < 7 kg / cm2 ) Compactors
These compactors are used at apartments, commercial establishments.
Example:
Baling equipment – For waste papers & card boards
Stationary compactors – For transfer station
In low pressure compaction, wastes are compacted in large containers.

8. High Pressure ( > 7Kg / cm2 )Compactors
Compact systems with a capacity up to Kg / cm2 came under this category.
Here, specialized compaction equipment are used to compress solid wastes in to blocks (or) bales of various sizes.
Typically, the reduction ranges from about 3 to 1 through 8 to 1
Compaction Ratio = V1 / V2
V1 = Volume of waste before compaction
V2 = Volume of waste after compaction

9. Size Reduction (or) Shredding
This is required to convert the large sized wastes into smaller pieces.
Size reduction helps in obtaining the final product in a reasonable size in comparison to the original form.

10. Equipments for Size Reduction (or) Shredding
Hammer Mill:
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, which rotates at a very high speed.
Solid wastes as they enter the mill, are hit by sufficient force, which crush (or) tear them with a velocity so that they do not adhere to the hammers.
Wastes are further reduced in size by being struck between breaker plates & cutting bars fixed around the periphery of the inner chamber.

12. Hydro-pulper
An alternate method of size reduction involves the use of a hydropulper as shown below:
Solid wastes and recycled water are added to the hydro-pulper.
The high speed cutting blades, mounted on a rotor in the bottom of the unit, convert palpable and friable materials in to slurry with a solid content varying from 25 to 35%
The rejected material passes down a chute that is connected to a bucket elevator, while the solid slurry passes out through the bottom of the pulper tank and is pumped to the next processing operation.

14. (B) Component Separation
Component separation is a necessary operation in which the waste components are identified and either manually (or) mechanically to aid further processing.

15. Air Separation
This technique has been in use for a number of years in industrial operations for segregating various components from dry mixtures.
Air separation is primarily used to separate lighter materials from the heavier ones.
The lighter materials may include plastics, paper products and other organic materials.

17. Equipments used for Air Separation
Conventional Chute Type
Its one of the simplest type of air classifier.
In this type, when the processed solid wastes are dropped into the vertical chute, the lighter materials are carried by the air flow to the top while the heavier materials fall to the bottom of the chute.
A rotary air lock feed mechanism is required to introduce the shredded wastes into the classifier.

18. Zig-Zag Classifier
It consists of a continuous vertical column with internal zig-zag deflectors through which air is drawn at high rate.
Shredded wastes are introduced at the top of the column at a controlled rate, and air is introduced at the bottom of the column.
As the wastes drop into the air stream, the lighter friction is fluidized and moves upward and out of column, while the heavy fraction falls to the bottom.

20. Magnetic Separation
The most common method of recovering ferrous scrap from shredded solid wastes involves the use of magnetic recovery system.
Ferrous materials are usually recovered either after shredding (or) before air separation.

23. Equipments used for Magnetic Separation
Suspended Magnet
In this type of separator, 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 separate container.

24. Magnetic Pulley
This consists of a drum type device containing 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.

25. Screening
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 & energy recovery systems.
Screens can be used before (or) after shredding and after air separation of wastes in various applications dealing with both light & heavy fraction materials.

27. (C) Drying & 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.

28. Drying
The following methods are used to apply the heat required for drying the wastes.
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.

30. Dewatering
When drying beds, lagoons (or) spreading on land are not feasible,
Other mechanical means of dewatering are used.
The objective is to reduce the liquid volume in the solid waste stream.
Once dewatered, the sludge can be mixed with other solid waste, and the resulting mixture can be:
(i) incinerated to reduce volume
(ii) used for the production of recoverable by-products
(iii) used for production of compost
(iv) buried in a landfill

31. Resource Recovery from solid wastes – MRF
A Materials Recovery Facility (or) Materials Reclamation Facility (or) Materials Recycling Facility (MRF) is a specialized plant that receives, separates and prepares recyclable materials for marketing to end-user manufacturers.
Generally, there are two different types of MRF are there:
Clean MRF & Dirty MRF

32. Clean MRF
A clean MRF accepts recyclable commingled materials that have already been separated at the source from municipal solid waste generated either by residential (or) commercial sources.
There are varieties of clean MRFs.
The most common are “Single Stream Type” where all recyclable material is mixed (or) “Dual Stream Type”

33. Dirty MRF
A dirty MRF accepts a mixed solid waste stream and then proceeds to separate out recyclable materials through a combination of manual and mechanical sorting.
The sorted recyclable materials undergo further processing to meet technical specifications established by end-markets

35. Composting
Composting is one of the important technologies for solid waste management.
Any organic material that can be biologically decomposed is compostable.
Composting is depending upon type of organic materials being composted and the designed properties of final product.
The overall composting process can be explained as follows:
Organic matter + O2 + Aerobic bacteria → Co2 + NH3 + H2O + other end products + Energy

36. Principles of Composting
Decomposition and stabilization of organic waste matter is a natural phenomenon.
Composting is an organized method of producing compost manure by adopting this natural phenomenon.
Composting can be carried out in two ways, aerobically & anaerobically.

37. Aerobic Composting
During composting, aerobic micro-organisms oxidize organic compounds to CO2, Nitrite and Nitrate.
Carbon from organic compounds is used as a source of energy.
Due to exothermic reaction, temperature of the mass rises.

38. Anaerobic Composting
The anaerobic micro-organisms, while metabolizing the nutrients, breakdown the organic compounds through a process of reduction.
A very small amount of energy is released during the process and the temperature of composting mass does not rise much.
The gases evolved are mainly Methane & Carbon dioxide.
An anaerobic process is a reduction process and the final product is subjected to some minor oxidation when applied to land.

41. Methods of Composting
Manual composting was systemized by Howard and his associates.
It was further developed by Acharya & Subramanyam and the methods are conventionally referred as Indore and Bangalore methods of Composting.

42. Bangalore Method
This is an anaerobic method conventionally carried out in pits.
Formerly the waste was anaerobically stabilizes in pits where alternate layers of MSW and night soil were laid.
The pit is completely filled and a final soil layer is laid to prevent fly breeding, entry of rain water into the pit and for conservation of the released energy.
The material is allowed to decompose for 4 to 6 months after which the stabilized material is taken out and used as compost.

43. Indore Method
This method of composting in pits involves filling of alternate layers of similar thickness as in Bangalore method.
However, to ensure aerobic condition, the material is turned at specific intervals for which a 60 cm strip on the longitudinal side of the pit is kept vacant.

44. For starting the turning operation, the first turn is manually given using long handled rakes 4 to 7 days after filling.
The second turn is given after 5 to 10 days. Further turning is normally not required and the compost is ready in 2 to 4 weeks.

45. Comparison of the methods:
The Bangalore method requires longer time for stabilization of the material & hence needs larger load space.
The gases generated in this anaerobic process also pose smell & odour problems.
The Indore method on the other hand stabilizes the material in shorter time & needs lesser land space.
As no odourous gases are generated in this process, it is environment friendly & hence commonly preferred.

46. Windrow Composting
The organic material present in Municipal Waste can be converted into a stable mass by aerobic decomposition.
Aerobic micro-organisms oxidize organic compounds to carbon di oxide and oxides of Nitrogen and carbon from organic compounds is used as a source of energy, while Nitrogen is recycled.

47. In areas / regions were higher ambient temperatures are available, composting in open windrows is to be preferred.
In this method, refuse is delivered on a paved / unpaved open space but leveled and well drained land in about 20 windrows with each windrow 3m long x 2m width x 1.5m high with a total volume not exceeding 9 m3
Each windrow would be turned on 6th & 11th days outside to the centre to destroy insects larvae and to provide aeration.

48. On 16th day, windrow would be broken down and passed through manually operated rotary screens of about 25mm square mesh to remove the oversize contrary material.
The screened compost is stored about 30 days in helps about 2m x 1.5m high and up to 20m long to ensure stabilization before sales

56. Mechanical Composting
Mechanical composting preferred where higher labour costs and limitations of space exist.
Mechanical composting plant is a combination of various units which perform specific functions.

61. Factors affecting Composting (i) Organisms (ii) Use of cultures (iii) Moisture (iv) Temperature (v) C/N Ratio (vi) Aeration (vii) Addition of sewage & sewage sludge

62. Incineration Definition
Incineration process can be defined as an engineered process using controlled flame combustion to thermally degrade waste materials in presence of oxygen.

63. Necessity of Incineration
Waste volume reduced to less than 5%
At sufficiently high temperature, and residence time, any hydrocarbon vapour can be oxidized to carbon di oxide and water.
Relatively simple devices capable of achieving very high removal efficiencies.
Heat can be recovered
Most gases are burnt – well designed systems
Easy to maintain
Only solution for certain waste types.

64. Incineration system components
Waste pre-treatment ( pre-heating (or) shredding )
Waste loading systems ( conveyors, hoppers, sprayers )
Burner management system
Combustion chambers
Heat recovery
Air pollution control device
Ash disposal
Emission monitoring systems

65. Process Description
The System concept consists of the following three (3) essential main components:
1. Rotary kiln with subsequent afterburner chamber
2. Heat exchanger or steam boiler for energy utilization
3. Multi-layer flue gas purification, including catalyst

67. Pyrolysis
Definition
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen.
It involves the simultaneous change of chemical composition and physical phase, and is irreversible.
Pyro = fire; lysis = separating

68. Advantages of Pyrolysis
Conversion up to 100%
No Green house and other harmful gas emission into the atmosphere
No toxic waste to dispose of
Reduced public health risk
Simultaneous treatment of various types of waste materials
Reliable energy source, high energy output
Low capital and operation cost
Low energy consumption

69. System Components Pre-processing & feeding system Pyrolysis reactor
Thermal oxidizer
Energy generator
Off gas depuration system

70. Process Description Step 1: Waste pre-processing and feeding system
Pre-processing- filtering materials that are not suitable for pyrolysis (metals, glass and inerts)
The remaining waste stream is chipped into small pieces up to 50mm in diameter.
In the dryer its moisture content is reduced down to 20% in order to provide higher conversion results.

71. Step 2: Pyrolysis of waste
The high-temperature pyrolysis process takes place in an indirectly heated pyrolytic chamber at C in an oxygen-free environment
Allowing conversion of the waste into synthesis gas (90-98%) and a solid cocking residue, or carbon char (2-10%), without any liquid tar fractions being formed
The specific temperature maintained eliminates any dioxins or furans in the carbon char residue thereby allowing further use of this byproduct.
Dioxins are contained in syngas only.

72. Step 3: Thermal Oxidizer
Syngas is mixed with air in the main burner and directed to the oxidizer for combustion at 1200ο C.
The oxidized gasses have high thermal capacity and are partly utilized by the system to maintain the temperature in the pyrolytic chamber

76. Step 4: Energy Generation
As oxidized gas leaves the thermal oxidizer, its high temperature is heat exchanged to a waste heat boiler.
Thermal energy captured from the oxidized gas is converted into high temperature steam supplying energy to turbine generators which in turn produce clean electric power

77. Step 5: Pollution Control System
The cooled (flue gasses) are passed through a multi-stage pollution control system to be cleaned of harmful impurities and safely released into the environment.
Waste off gasses utilized for pyrolysis reactor heating are also directed to the waste heat boiler, and, after being cooled, pass through a similar depuration system.