Analysis of Alternative Methods of Rendering Waste Harmless

V.G. Sister (Â.Ã. Ñèñòåð), A.N. Mirniy (À.Í. Ìèðíûé).

Along with the most widely used methods of power-driven rendering solid waste harmless and recycling it in world practice – burning, aerobic composting and a combination of these two methods – all over the world, alternative technologies of rendering solid waste harmless and recycling it are being developed, aimed at obtaining new materials and extracting valuable utility fractions from waste.  Work on integrated sorting of solid waste with extraction of valuable secondary materials, anaerobic fermentation with obtaining combustion gas and organic fertilizer was begun.  A technology for extracting combustion fractions from solid waste and producing fuel blocks or granulated fuel has been developed and implemented, pressing of solid waste is used for production of building blocks, etc.

INTEGRATED SORTING AND RECYCLING OF WASTE

The main goal of integrated sorting is maximum power-driven extraction of utility components from the entire mass of solid waste.  Each separate procedure uses its own set of technological equipment, which allows more or less to sort out utility fractions.  Table 1 represents the various ways of extracting utility fractions from solid waste.

Table 1 – Various ways of extracting coal fractions from solid waste

 

Separation of solid waste is generally looked upon as a way of improving “traditional” methods of its processing (the quality of compost improves due to the elimination of ballast fractions, clogging of fire grates decreases during waste burning) and not only as a method that allows utilizing some valuable waste components.

Thus, the effectiveness of waste recycling method with an emphasis on utility components extraction is defined by the purchase cost of extracted components considering their quality.  Some examples of this are some fundamentally different approved technological schemes of integrated separation of solid waste.

Companies “Rutiere”, “Sorain Cecchini” (Italy) developed a technology for power-driven separation of SW that considers linear dimensions, density, wind resistance, magnetic abilities, etc.  At the first stage of the process SW is freed from plastic bags, into which garbage is packed in Rome.  Then, at the fire grate, waste is separated into three fractions.  After electromagnetic separation, large fraction is sent to be burned, small fraction is sent for composting.  The company thinks that extracting utility components from the middle fraction is most rational.  Paper is sucked out during reloading of the material with special equipment.

Black scrap metal is extracted by a magnetic separator, textile is extracted by a drum fork mechanism.  Middle fraction material left over after extracting paper, textile and black metal is sent to the grate, where it, in turn, is separated into three fractions.  Just like after the first grating, the small fraction is sent to the compost segment, middle and large fractions go to the machines for paper extraction.  Then the material proceeds to the machines for extracting organic parts suitable for feeding substance production.  On belt conveyors paper is moved onto a paper cleaning sieve for final blowing and cleaning and then onto a press, where it is packed into stacks ready for shipment.

Food waste is separated into two parts.  One, containing valuable organic substances, is sent to a shop for feeding substance production, the other, containing mainly glass, bones, goes through magnetic separators and ballast separators and is sent to the machine for separating glass and bones.  Black scrap metal processed by magnetic separators is sent to the furnace for cleaning.  Cleaned metal is sent to the press for packing.  Food waste, after washing down with water with vigorous shaking, is sent to a grinder with cutting knives.  Grinded material is sent to the sterilizer, where steam is forced.  Inside the sterilizer is a blender mixing the material in the process of sterilization.  The material is in the sterilizer for 40-50 minutes and is heated to 100-110 °Ñ.  Such temperature is sufficient for inactivation of all pathogenic microorganisms. The sterilizer works in cycles. Unloading is done automatically by switching the blades that are mixing and moving the materials.  Harmless material is fed into a spinning dryer chamber.  At the moment of entering the chamber it is aerated with air heated to 80 °Ñ and is sent upwards, where it is exposed to air heated to 100 °Ñ and more for 20-25 minutes.  In the process of drying the humidity of the material lowers to 10-12 %.

Sterilized and dried material is mixed with corn, vitamins and mineral substances.  The finished mixture is sent to a machine for granulating.  The resulting feeding product satisfies hygienic requirements and has the chemical characteristics necessary for deeding all animals, although due to its specifics it is recommended for feeding ruminants.

The production zone for feeding products (sterilization) is securely separated from waste sorting and processing zones.  Movement of employees between the zones is not allowed.

Organic substance with low feeding value is partially sent to biothermal drums to make compost.  In recent years the company has been making a new product instead of compost: a protein-organic fertilizer in the form of dry granules, which have been experimentally used as fuel.  The new process involves stoppage of fermentation, heating with drying, additional ballast cleaning.

Sorting during solid waste processing is also widely used in Sweden.  At the waste processing plant in Stremstad SW is chopped, fed under a magnetic separator and sorted in a cylindrical grate.  Small fraction is sent to the blender, where the sediment from sewage waters is sent, and then to the maturation field, where it is being piled into stacks.

As a result of simplified sorting the plant produces 2,4 % of black scrap metal, 26,3 % of fuel and 71,3 % of fractions for composting.  Before composting, 24 % (from initial SW mass) of sediment from sewage waters is added to the last fraction.

The sorting plant in Wijster (Netherlands) by the Swedish company “Flect” processes 20% of the country’s solid waste, being part of the largest composting plant with productivity of 3 mln m³ of SW per year. Productivity of the sorting plant is 600 thousand m³ of SW per year (125 thousand tons/year). The plant produces annually:

• 9,1 thousand tons of light paper mass;

• 7,5 thousand tons of heavy paper mass;

• 3,5 thousand tons of black metals;

• 4,5 thousand tons of plastic;

• 39 thousand tons of organic substances for composting;

• 47 thousand tons of ballast fractions sent to the dump.

The German company IMPO Maschinenbau GMBH developed a complex for sorting waste that includes, along with a system of conveyors, a vibrator, a suspended separator for black scrap metal and a separator for non-ferrous metals.

The principle underlying the work of non-ferrous metal separator is based on using flurry currents (Foucault currents) created in a conductor placed in an alternating magnetic field.  These currents create a secondary magnetic field that interacts with the primary magnetic field.  As a result, the conductor is thrown out of the effective zone of the primary field.  Thanks to the vibrator, the material on the separator belt is loosens.  A magnetic drum is built into the pulley of the belt, rotating with a speed that is significantly higher than the speed of the pulley.  Because of created flurry currents non-ferrous scrap metal is thrown farther than non-metal fractions.  The full scheme of the complex includes other kinds of sorting equipment as well.

Certainly, an interesting solution to waste sorting and processing problem was developed by the alliance SYS-ÒÅÑ - Gesellschaft fur Systemtechnologie mbH from Cologne.  The so-called “Technology of the future” presented by this alliance involves a significant amount of technological equipment for various operations that include successive dry power-driven processing of solid waste, hydro-processing of the large sifting extracted at the first stage of the process and separated paper and, as well as special processing (ennoblement) of extracted utility fractions.

The goal of this technology is maximal extraction of utility fractions from solid waste, which increases the economic effectiveness of the entire complex, decreases areas for SW grounds, decreases gas emissions of garbage trucks.

Taking into consideration that most SW is collected by Germans into plastic bags, all waste that comes to the plant first goes through the roller milling machine to tear the polymer bags.  It is then sent to the drum grate for preliminary separation into large and small fractions.

MODERN TECHNOLOGIES FOR RENDERING SOLID WASTE HARMLESS AND UTILISING THEM

Large fractions then go through a pneumatic separator, where light paper fractions are separated by wind resistance (the speed of moving in an air stream).

Using a magnetic separator black scrap metal is extracted from heavy fractions.  The remaining material goes through an optical separator, where plastic bottles and cardboard boxes (from food products) are extracted.  The remaining large siftings and separated paper fractions are sent separately into two hydro-pulpators, where they are turned into pulp, and undergo further processing, including:

• secondary grinding;

• separation of aluminum by flurry currents in an alternating magnetic field on a special non-ferrous metal separator;

• turning paper into grain;

• dehydration of produced materials in centrifuges;

• agglomeration (producing pellets) in a drum machine;

• granulation in a screw machine.

HYDRO-SEPARATION OF WASTE

A particularity of this method is the use of equipment for sorting and processing SW manufactured for the paper industry.  The system of hydro-separation of SW was developed by the company “Black Clawson” (USA).  Waste was supplied by a tabular feeder from the receiving bunker to a water-filled mixing reservoir “hydro-pulpator”, where it was intensively mixed by stirrers and partially grinded.

The pulp (sludge) went to the separator, where large scrap metal and ballast was extracted from it, and then to the cyclone, where glass, sand and small metal fractions were extracted.  In the next cyclone, textile, paper and other fiber fractions were extracted.  Then they were dehydrated and piled into stacks.  After these operations, a sediment useful for composting was extracted from the pulp.  Purified water was returned to the hydro-pulpator.  The full cycle of processing lasted 90 minutes.

With this technology there was no need for further purification of the compost from ballast fractions.  At the experimental plant 13 % of paper mass, 4 % of glass, 9 % of black and 0,3 % of non-ferrous metals was extracted from SW.

Considering the difficulties in the sale of paper mass, the company later decided to use fiber fractions after dehydration for granulated fuel production.

GRANULATED FUEL PRODUCTION

The calorific value of specially selected and dried light combustion components from solid waste is 2 times higher than the calorific value of initial SW.  The fuel made from waste, unlike initial SW, can be stored for a long time and transported, has a more homogenous fraction composition, lower humidity and ash content, contains less metal infractions, has high calorific capacity, because it is composed of such fractions as paper and cardboard.  Because of this, a series of foreign companies are conducting large scale experiments in power-driven extraction of light combustion components from SW for further use, after proper preparation, as energy fuel.

Usually in fuel production people don’t stop at grinding SE and using magnetic separation, but use pneumatic separators, grates and other equipment, and fuel production is combined with extracting utility components or organic substances for composting.

In England, three sorting plants use the technology developed by the research laboratory “Warren Spring”, in accordance with which solid waste undergoes rough grinding (particle size – up to 200 mm), and is then sent to the grate for separation into two fractions.  Large fraction is used for producing paper mass and fuel, black metal and glass is extracted from the small fraction.  The plant design includes a pneumatic drum separator separating paper from heavier fractions.

In the city of Soria (Spain) an experimental machine was built, working using the “Ferrospack” method for producing fuel blocks from a mixture of SW with industrial waste of vegetable origin.  Pre-composted waste is mixed at the ratio of 1:5 with “fresh” SW and loaded for one day into a biothermal chamber.  Then the material is sent to the grate, magnetic separation and grinder for rough (preliminary) grinding, after which it is sent to a biothermal tower, grinder for fine grinding (to fractions of 1-5 mm) and into a second biothermal tower.  Due to partial fermentation, the mechanical durability of the compost decreases, which contributes to lower wear and tear of grinder hammers and lower energy consumption for grinding.

From biothermal towers the material is sent to the drier, where gas is supplied with initial temperature 300-350 °Ñ (the temperature of material is raised to 120-150 °Ñ).  Compost dried to 3-8 % humidity is sent to a block-forming press that makes blocks 80 mm in diameter.  Block density is 1.2 tons/m³. Calorific value of the blocks (at least 4 000 Cal/kg) is ensured by the addition of a significant amount of sawdust and other such materials to SW.

RECYCLING WASTE IN ANAEROBIC CONDITIONS

In recent years work on methane fermentation of SW has been active.  Companies “Valorga” and “Sofregas” (France) tried the technology of waste processing in anaerobic conditions in production settings with obtaining combustion gas and organic fertilizers.  The first experimental plant working with this technology was built and is operating near Grenoble.

The specifics of this technology is this: SW is dumped into a receiving bunker, from which a claw crane transfers it to a feeder and then into a grinder with vertical axle.  Grinded waste is reloaded from the grinder onto a belt conveyor located under the black scrap metal separator.  Material cleared from black metal is sent to the methane tank (500 m³), where it remains for 10-16 days under temperature of 25 °Ñ. Under these conditions fermentation of organic mass happens.  From each ton of SW 120- 140 m³ of gas is produced, which is sent to the gas tank.

Part of the produced gas is pumped out by a compressor and using a balancing chamber sent under pressure under a layer of material, which is being processed, which is necessary for mixing the mass. The hard fraction from methane tank is sent to the screw press for partial dehydration and then to the loosener.  Then the material goes to a cylindrical grate, where it is separated into mass used as an organic fertilizer and large siftings.

From 1 ton of SW 170 kg (140 m³) of biogas is produced, consisting 65 % of methane, 410 kg of organic fertilizer with 30 % humidity, 50 kg of scrap metal and ballast fractions (which are extracted by a magnetic separator and thrown out by a grinder), 250 kg of large siftings from the cylindrical grate. 120 kg goes for gas losses and filtered material.  5% of the biogas obtained is consumed by the plant for its needs. Biogas can be used in its initial state, getting 23 400 kJ/m³ of heat, or after clearing it from carbon dioxide and hydrogen sulfide, getting 35 600 kJ/m³ of heat.

LARGE BLOCK PRODUCTION

Pressing SW under high pressure is one of the ways to improve operational conditions on the grounds.  Packed SW release less filtered materials and gas emissions, and the risk of fires decreases, ground area is used more effectively.

Packers for SW pressing on the grounds are produced by the company “American Hoisted Derrick”.  Packer productivity is 450 tons per shift, block mass is 1.2 – 1.4 tons, dimensions – 0.9 õ 0.9 õ 1.2 m. A block is pressed for 1.5 minutes with maximum pressure of 19 MPa.

Japanese company “Tezuka-Kosan” has developed and is implementing production of building blocks for sinking refuse in the sea on the basis of its own equipment. SW is pressed by several plungers of various sections that are sequentially used on the material.

Pressure in the zone of contact with small plungers reaches 36 MPa with general pressure of 5-6 MPa.  The level of compression under this method amounts to 1:10 despite the high humidity of incoming SW (up to 56-65 %). Dimensions of the produced block are 1.1 õ 1.1 X 1.2 m, density -1.2-1.7 t/m³. The productivity of this plant is 100 tons per shift.  In the pressing process filtered material is produced, which comprises 2-5 % from the mass of pressed materials.  Finished blocks are wrapped into a wire net or metal sheets and are used as large building elements.  If blocks are to be used for building dams in the sea, then they are covered with hot asphalt or plastic film.

Tests conducted by the company showed only a little surface corrosion of blocks covered with metal sheets over two years. No aerobic or anaerobic processes accompanied with rising temperature or secreting unpleasant smells were found.

WASTE HYDROLYSIS AND FERMENTATION

The main fractions of SW are paper and food waste, which contain a significant amount of cellulose.  Experiments on obtaining industrial ethyl alcohol (ethanol) from cellulose contained in SW were conducted in USA and Great Britain.  Ethanol is obtained in the following manner:  first, cellulose undergoes hydrolysis, during which it reacts with water under the presence of hydrochloric acid as a catalyst:

To speed up the process and increase ethanol output, the reaction is held under high temperature.  Sugars are obtained as a result.  Sugar solution is fermented, resulting in ethyl alcohol solution.

Then comes fast cooling with water, neutralizing with calcium carbonate and filtration.  After that, fermentation is done for about 20 hours under temperature of 30-38 °Ñ.  Resulting water solution of ethyl alcohol is purified and distilled, yielding 95% alcohol.

During hydrolysis two reactions take place:  cellulose is restored to sugar, which dissociates under the action of hot diluted acid, and the speed of restoration and dissociation depends on the concentration of acid, temperature and time.  The reaction’s energy is independent of acid concentration and compounds to 42 900 cal/ mole during cellulose restoration to sugar and 32 800 cal/ mole during sugar dissociation.

Increasing acid concentration or temperature (or both factors simultaneously) leads to raising the effectiveness of sugar restoration, and in the range 170-190 °Ñ raising the temperature by 10 °Ñ leads to increasing the speed of sugar restoration reaction by 186 % and the speed of sugar dissociation by 125 %. Calculations allow choosing acid concentration and temperature that correspond to optimal sugar output.

Below are the results of calculating 95 % ethanol output with the intake of 250 tons of SW per day.

 

Experiments in production of industrial ethanol are of definite interest.  But like for any other technology, the most important indicators are economic.  Creating  plant for ethanol production does not yet allow getting rid of dumps, because sludge from the plant that processes 250 tons of SW per day is 200 tons per day.

Analysis of discussed technologies for rendering SW harmless shows that most of them in one way or another combine with composting lightly decomposable fractions and burning high calorific fractions cleared from ballast and plastic.

Wide use has been gained by technologies, where SW is cleared from ballast, dried and turned into fuel granules.  It is worth noting that this method can only be used in places, where consumers for such fuel exist (for example, cement plants), who can ensure its combustion at temperatures higher than 1200 °Ñ.

Extracting utility and ballast fractions from SW, especially power-driven, using specialized equipment, will lead to a significant improvement in economic effectiveness of a waste processing company.

The diversity of new methods of processing and utilizing waste shows the urgency of this problem.  Using new local and foreign materials for SW grounds will allow decreasing laboriousness of the works both during construction and during operation and revegetation of the grounds.

However, it should be noted that traditional methods of rendering SW harmless and burying it on the grounds, composting and burning, combination of sorting, composting and thermal neutralization, being constantly modernized, also remain reliable, effective methods for utilizing solid waste.