PROCESSING & RECOVERY

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.

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

PROCESSING FOR RECOVERY OF MATERIALS 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

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

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.

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

An operating trommel processing mix waste

Vibrating screens Disc 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

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

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

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

MRFS (MATERIAL RECOVERY FACILITIES) 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.

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, vector control, airborne, litter, aesthetics problems Public health and safety – general public and employees Economics – sensitive to market prices

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.

Types of combustors Issues 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