PROCESSING & RECOVERY

1. PROCESSING & RECOVERY

2. Second fundamental function of solid waste management. • Improves the efficiency of solid waste disposal and prepares solid waste for subsequent recovery of materials and energy. • Some material in solid waste can be recovered and recycled for manufacture of the same or new products.

3. RECYCLING • The recovery of solid waste components for possible use as raw materials. • Industries used recycled materials in manufacture as processing of raw materials is more economical than processing recycled materials • Support from the government and enforcing laws for wastes to be recycled • Future for recycling is bright

4. PROCESSING FOR RECOVERY OFMATERIALS FOR RECYCLING • To segregate solid waste components may be done at point of generation or at a central processing facility. • On site processing needs the cooperation of the waste producer. • Different containers for different wastes: paper, cans, bottles. • At central processing facility, segregation into components using screening, air classifying and magnetic separations. • Size reduction using shredders (hammer mill, flail mill and shear shredder), glass crusher and wood grinders. • Magnetic separation involved the use of electromagnet to separate the ferrous materials from the rest of the solid

5. Hammer mill • Mechanical impacts device in which the raw solid waste material is hit with force sufficient to crush or tear individual pieces of waste. • Size reduction between 25mm to 50mm. • A typical hammer mill is a 150 hp unit capable of processing about 12 tons of solid waste per hour. • Disadvantage – high cost and maintenance, noisy and cause dust generation

7. Screening • Is a unit operation of separating feed into • Oversized and undersized products • Separation into light combustibles and heavy non-combustibleties • Recovery of paper, plastics and other light materials from glass and metals • Separation of glass grit, sand from combustibles materials • Separation of rocks and other oversized debris from soil excavated at construction site • Removal of oversized materials from combustion ash • Oversized products are those that do not pass the opening of the screen • Undersized products are those that do pass the opening at the screen • Screen can be classified as primary, secondary or tertiary screens depending on where in the flow sheet the unit is located • The screen with the largest size opening is called the primary screen.

8. Trommels or rotary screens • Is a primary screen • Placed ahead of all separations unit in a separation facility • Can be equipped with metal blades, use as bag breakers • Separate materials into several size fractions • Materials to be separated are introduced at front end of the inclined rotating trommel • Small particles will fall through the holes in the screen while oversized material will pass through the screen

10. An operating trommel processing mix waste

11. Vibrating screens • Used to remove undersized materials from commingled waste • Process demolition and construction waste • Vibrate from side to side, vertically or lengthwise • To separate MSW are inclined and vertical motion • Vertical motion allows the materials to be separated to contact the screen at different locations each time • Disc screens • Consist of parallel set of horizontal shaft. The undersized material fall between the spaces of in the discs and oversized materials will ride over the top of the discs • Different sized materials can be separated using the same screen by adjusting the spacing between the rotating discs. • Advantages – self cleaning and adjustability with the respect to the spacing between the discs on the driving shaft

12. Density separation (air classification) • To separate light materials such as paper and plastics from heavier materials such as ferrous metal, based on the weight of materials in the air stream • If materials introduced in the air stream moving with sufficient velocity, the light material will be carried away with the air while the heavier material will fall in the countercurrent direction • Magnetic separation • Recover ferrous material from commingled and shredded waste • Ferrous material are recovered either after shredding and before air classification or after shredding and air classification

13. Densification (compaction) • Unit operation that increases the density of waste materials so that they can be stored and transported more efficiently – reduce handling and shipping cost • Several technologies are available – baling, cubing and pelleting • Bales are typically 1.5m3 in size weigh roughly 1 ton • Solid waste can be compacted under high pressures (700 kPa or 100 psi) in either vertical or horizontal process • Volume reduction may be expressed in terms of compaction ratio or in percent • An understanding of the relationship between percent volume reduction and compaction ration is important

14. % volume reduction = initial volume – final volume x 100 initial volume Compaction ratio = initial volume final volume

15. MRFS (MATERIAL RECOVERYFACILITIES) • To separate out commingled MSW. • Commingled MSW enters the MRF at the tipping point. • Then, accumulated wastes push into hoppers or loaded onto conveyor belts using front end loaders. • Conveyors carry the waste to the next stage which is the sorting system. • Manual sorting platform or a multi staged waste processing system designed to automatically remove different recyclables. • Shredders and pulverizers may be used prior to separation to sort material by size. • The extraction products sent through densifier, can flatteners and baling systems before stockpiling.

16. Layout and design should consider: • Waste deliveries • Material delivery rates • Loading rates including storage for peak times • Material flow and handling patterns • Performance criteria for equipment selection • Equipment • Environmental controls • Aesthetics • Issues for MRFs • Siting – remote locations as much buffer as possible • Environmental issues – traffic, noise, dust, odour, vectorcontrol, airborne, litter, aesthetics problems • Public health and safety – general public and employees • Economics – sensitive to market prices

17. COMBUSTION • Reduction in the volume (85 – 95% reduction) and weight of wastes that required disposal • Recovery of energy in the form of heat • Major concerns – air pollution and siting • Air pollution is controlled including furans, dioxins, NOx, SO2 and particulates • Clean gases are discharged to the stack • Ash and unburned material are quenched (cool with water) • The water and residue must be properly disposed of.

18. Types of combustors • Mass fired. • Use unseparated, commingled MSW. The energy content is extremely variable • RDF (Refuse Derived Fuel) fired. • Produced from the organic fraction and can be made with consistency to meet energy, moisture, ash content specifications • Forms include: shredded, fluff, pellets or cubes. • Air emissions are cleaner because metals and plastics not burned • Issues • Siting – remote location with plenty of buffer zone • Air emission – important consideration in the decision between mass fired and RDF systems • Disposal of residue, bottom ash, fly ash, scrubber product. • Typically disposed in landfill • Liquid emissions – sources: ash removal, wet scrubbers, washings and housekeeping, cooling tower • Economics