Comparison Of Different Waste Management Techniques Environmental Sciences
Like incineration, gasification is a thermal process that uses high temperatures to break down waste. It is still classed as incineration in the European Unions Waste Incineration Directive and has to meet the mandatory emission limits that it sets.
Gasification is a process in which materials are exposed to some oxygen, but not enough to allow combustion to occur. The resulting gas mixture of carbon monoxide, hydrogen and methane (with smaller quantities of carbon dioxide and nitrogen) is called syngas and is itself a fuel. It has a calorific value so can be used as a fuel to generate electricity or steam or as a basic chemical feedstock in the petrochemical and refining industries. The calorific value of the syngas will depend on the composition of the input waste waste to the gasifier.
Both gasification and incineration are capable of converting hydrocarbon-based hazardous materials to simple, nonhazardous byproducts(A Comparison of Gasification and Incineration of Hazardous Wastes Final Report Prepared for: U.S. Department of Energy March 30, 2000.
There is not much unbiased data available on gasification but the companies developing gasification claim the technology has significant advantages over traditional incineration of waste. These are as follows
The process uses less oxygen meaning that fewer air emissions may be produced containing potential pollutants.
Less carbon dioxide is produced meaning less impact on global climate change. Any carbon dioxide produced during gasification is present at much higher concentrations and at higher pressures than in streams produced from conventional combustion , making them easier to capture(http://www.netl.doe.gov/technologies/coalpower/gasification/basics/2.html )
The plants are are made up of small units (modular) which can be added to or taken away from as waste streams or volumes change (e.g. with increased recycling in country) and are therefore more flexible and can operate at a smaller scale than mass burn incinerators
They are quicker to build than conventional incinerators
The processes are claimed to produce a more useful product than standard incineration that can be used as a fuel(syngas)
The syngas may be used to generate energy more efficiently, if a gas engine (and potentially a fuel cell) is used, while incineration can only generate energy less efficiently via steam turbines.( Source - Eunomia Research and Consulting (2008). Greenhouse gas balances of waste management scenarios - report for the Greater London Authority).The syngas produced by gasification can be converted into many valuable products, ranging from electricity and steam to liquid fuels, basic chemicals, and hydrogen. Integration of multiple products of gasification into industrial applications increases opportunities for added revenues
The energy produced from gasification may be eligible for more Renewals Obligations Certificates (ROC's) than conventional incineration thus increasing the potential income
The treated flue gas from an incinerator goes directly out into the atmosphere. The treated syngas from the gasification plant is used as a fuel in itself.
When solid waste is incinerated one of the by products is bottom ash which then has to be disposed of or treated and then disposed of depending on the content. When solid is processed in a gasifier, slag is produced which can be can be sold, used as feedstock in chemical production processes, or recycled in other in-plant process operations.
Sulfur compounds (H2S and COS) in the particulate-free syngas, normally a byproduct of liquid gasification are typically removed and recovered using conventional gas treatment technologies from the refinery and natural gas industries. The resulting byproduct is high-purity liquid sulfur which can then be sold and reused. (A comparasion of gasification and incineration of hazardous wastes. Final Report. Prepared for the U.S. Department of Energy. March 2000)
Emission levels of SOx, NOx, and particulate from gasification systems are reduced significantly compared to incineration systems. In an oxidative incineration environment, sulfur and nitrogen compounds in the feed are converted to SOx and NOx. In contrast, syngas cleanup systems for modern gasification systems are designed to
recover 95 to 99% of the sulfur in the fuel as a high-purity sulfur byproduct A comparasion of gasification and incineration of hazardous wastes. Final Report. Prepared for the U.S. Department of Energy. March 2000)The factors affecting the choice of bag filters or ESP in waste incinerators.
Factor one : The type of waste being incinerated
It all depends on what is being incinerated. .
The characteristics of the dust produced by the incineration plays a role in the choice as the combustibility of some fine materials rules out the use of electrostatic precipitators.
Bag filters are very efficient at collecting fine particulates but not so good at large particulates so it depends on the product of the incineration.(Source Should I replace my Electrostatic Precipitator with a fabric filter, I. Fanthom, C. Cottingham.)
Most common ESP filtration is best used for ambient capture of light atmospheric dust. Unless a elf cleaning electrostatic precipitator is used, source capture or direct ducting from a heavy dust producing incineration will quickly fill up the collection plates. Heavy dust collection requires storage for a large volume of dust. The surface area of bag filters is much greater than surface area of electrostatic collection plates and work better for dust capture of heavy dust producing incineration than ESP would.(http://www.dustcollectorexperts.com/electrostatic/)
Factor two : Characteristics of the airstream
The characteristics of the airstream can have significant impacts on the collector system. For example cotton fabric bag filters cannot be used where air temperatures exceed 82 degrees centigrade. Also condensation of steam or water vapour can blind bags making them ineffective. Various chemicals created in the airstream can react with the water in the airstream and form corrosive liquids such as sulphuric acid which can corrode any metal in the bag i.e. if it is reverse jet bag filter with a metal cage. ESP's can withstand corrosive material making collection possible.
The single most important factor influencing the Elecrostatic precipitator is the resistivity of the gas being caught. Fabric filters remove dust from a gas stream by passing gas through a fabric and leaving dust on the surface of the fabric. It is therefore not sensitive to dust resistivity.
A fabric filter can work on emission levels of 10-20 mg/NM3 whereas an ESP needs to be sized to suit requirements.
Factor three - Cost
With most designs of ESP's they have to shut the line down in order to maintain them which incurs a cost. With most bag filters they can be changed online, not incurring a cost of shut down. The power consumption using a bag filter is higher than using an ESP, obviously incurring more costs for more power.
Bag filters need to be changed more frequently than an Electrostatic Precipitator. Typically bags need changing every 4 years. An ESP needs a full service every 20-30 years.
Bag filters are highly efficient and cost effective due to the dust cake which is formed on the surface.( (Source Should I replace my Electrostatic Precipitator with a fabric filter, I. Fanthom, C. Cottingham.) They can achieve a collection efficiency of more than 99% for very fine particulates.
The ESP's are more expensive to install than the bag filters
Dust loads may be needed to be reduced before the Electrostatic Precipitation process (precleaner may be needed) hence adding to the cost.
Factor four - Characteristics of the dust. Hygroscopic (i.e. a material which attracts moisture from the atmosphere. If not protected from contact with the atmosphere (by being stored under vacuum or under a dry gas) some hygroscopic materials will eventually attract so much water that they will form solutions) and these can blind bag filters making them ineffectual.
Factor Five - Compliance with Environmental regulations and law.
In 1990the Environmental protection Act (EPA) introduced Integrated Pollution Control (IPC) requiring higher control of emissions in most industries. More recently the waste Incineration Directive was introduced and has imposed significant changes on any process burning waste materials. For example the total emission value for cadmium is 0.05 mg/Nm3 .(Source the Waste Incineration Directive). Hence the type of treatment needs to be chosen in order to satisfy these regulations.
It will also depend how near the waste incinerator is to buildings and the type of building i.e. is it near a residential area. Hence more regulations need to be considered regarding public health.
Factor Six- Space
How much space is there for the installations. ESP's are larger than bag filters and hence take up more space.Methods for reducing heavy metals in landfill leachate.
There are various methods for reducing heavy metals in landfill leachate -biological, biodegredation using anaerobic and aerobic processes and chemical and physical methods.
One such biological treatment that has been investigated is using vertiver grass (N. Roongtanakiat,T.Nirunrach, S.Chanyotha, D. Hengchaovanich. Uptake of heavy metals in landfill leachate by vertiver grass' Natural Science 37: 168-175. 2003). They investigated the plants ability to uptake heavy metals from the leachate. The Surat ecotype vertiver plants were planted in pots and treated with landfill leachate. The vertiver grass took up more heavy metals as the strength of the leachate increased and the heavy metals were evenly distributed in the shoot and the root. The results of the field trial at the landfill site also indicated that vertiver could be used in rehabilitating landfills and nearby areas. The vertiver plants were shown to die after 80-85 days if 100% leachate was used so they could not e directly used on young landfills, but could be used on young landfills if limited leachate were used. The shoot of the plant should be harvested periodically in order to remove the heavy metals from the contaminated soil and to stimulate new growth for more uptake.
Artificial wetlands combined with aerobic treatments have also been studied as a removal method for heavy metals in leachate. The study was undertaken at Alback landfill site in Sweden In 2003. (Source - http://warrr.org/168/) . The leachate treatment system consists firstly of an aeration step, followed by several wetlands with different depths and vegetation, intermediate mixing and aeration in a ditch, and finally sedimentation in a pond. An approximate total of 120,000 m3 of leachate passes through the treatment steps annually. Leachate samples were collected at different stages along the treatment path during a period of two months and the concentrations of cadmium ,copper, zinc, nickel, lead and chromium were studied. The leachate samples were filtered with three different membranes with different pore sizes . Lead and chromium could not be detected at all in the leachate. The total rates of reduction in the whole wetland system for cadmium, copper, and zinc concentrations were on average - 83% 74% and 68 % respectively. Nickel passed unaltered through the wetlands. The largest amount of metals in the leachate was already reduced during the first few meters in the wetland system, provided by sedimentation and aeration. Results of fractionation indicate that nickel and copperoccurred mainly complex-bound to humic substances. Which are hard to access mechanically or biologically. Zinc occurs mostly in different ionic forms or bonded to particles in the water. According to environmental quality criteria for natural waters in Sweden controlled by the Swedish Environmental Protection Agency, metal concentrations in the treated leachate are low and give rise to no or little risk of biological effects. Further improvement to the wetland system's heavy metal removal rates is probably limited since a large amount of the metals appear as complexes, which are hard to access mechanically or biologically. (Source - Persson, K. M., Van Praahg,M and Olsberg.G, E. (2007) Removal of Heavy Metals From Landfill Leachate by an Artificial Wetland During a Nordic Autumn. In: Eleventh International Waste Management and Landfill Symposium, 1-5 October 2007, S.Margherita di Pula - Cagliari, Sardinia, Italy. http://warrr.org/168/)
Aerobic treatment can be used where leachate is recirculated through the waste mass and air is injected into the waste mass. An investigation by M.Sartaj, M. Ahmardifar, A.Karmi Jastini ' (Assessment of in-situ aerobic treatment of municipal landfill leachate at laboratory level - Iranian Journal of Science and Technology, Transaction B, Engineering. Vol 34 No.Bl. Pp107-116 2010) found that the removal efficiency for Magnesium, Iron, Lead and Zinc was 93%, 90%, 43% and 76% respectively. Leachate was collected in a container at the bottom and pumped into another container at the top, from which leachate was recirculated back into the waste mass into which air was injected.
Bacteria can be used to treat leachate for heavy metals. Bacterial floc on the on the leachate surviving in an aerated system with oxygen levels maintained above 5mg/l. The heavy metals are taken in by the bacteria and incorporated into their cell biomass. (Source - Arden Quarry Landfill - www.drydenaqua.com/leachate/leachate/leachpapers/123pdf)
Chemical treatment is also used - Three tanks are used in which pH is adjusted, metal precipate particles coagulate and are flocculated and nutrients are added to encourage microbial growth The use of ferrous and ferric oxides as coagulates separate and coagulate the heavy metals allowing removal. The use of oxidants such as hydrogen peroxide or potassium permanganate react with the heavy metals and draw them out of the leachate allowing removal. Simple pH adjustment of the leachate causes the heavy metals to precipitate from the leachate and hence be removed. (Source - www.epa.gov/nrmrl)
Other methods include rotating biological contractors, trickle filters, aerated lagoons, up flow anaerobic sludge blanket reactors, chemical oxidation, adsorption, sedimentation, flotation, reverse osmosis and air stripping.Techniques for the separation of plastic types arising from municipal wastes
Plastics can be separated by their resin identification code, a method of categorization developed by the Society of the Plastics Industry in 168. See below
Polyethylene phenolphthalein - Fizzy drink bottles and oven-ready meal trays.
High-density polyethylene - Bottles for milk and washing-up liquids.
Polyvinyl chloride - Food trays, cling film, bottles for squash, mineral water and shampoo.
Low density polyethylene - Carrier bags and bin liners.
Polypropylene - Margarine tubs, microwaveable meal trays.
Polystyrene - Yoghurt pots, foam meat or fish trays, hamburger boxes and egg cartons, vending cups, plastic cutlery, protective packaging for electronic goods and toys.
Any other plastics that do not fall into any of the above categories. - An example is melamine, which is often used in plastic plates and cups.
( Source http://www.wasteonline.org.uk/resources/informationsheets/plastics.htm)
The first point of separation can be at the time of collection. The resin code is identified, as seen above, by a triangle formed by three chasing arrows with a number inside. This system allows segregation by what is desirable for a municipal recycling segregation system and what should not be included.
Other types of separation include:
DRY SEPARATION , using the following techniques:
Air classifiers -Air separation is used to separate different plastics, or even the same plastic, by the difference of the ratio between the surface of the flake and its mass.
This is done by an air counter-flow, - an air flow lifts up plastics of light density stuff and the high density plastic stays down using gravity.
Mechanical classifiers -These are used on flakes of plastic and are mostly used to separate flakes by size.
Classifiers can be circular, flat, inclined, with slow or high frequency vibrations etc.
NIR (Near Infrared Rays) - These give a certain quantity of energy to every single piece of plastic and measure the response; this happens in terms of milliseconds.
Its limit is the fact it can be used only on transparent items (mainly to sort PVC from PET bottles and flakes)
Laser spectral analysis -This penetrates the surface and measures emission spectra which depends upon heat capacity and thermal conductivity so colour doesn't matter.
The response time is long on this method so it is not largely used.
Polarized light -This used to check differences of crystallinity and applies mainly to the sorting of PVC from PET bottles but it can be used to sort any mixture of two plastics.
UV light - This is used to separate polymers that exhibit different UV induced fluorescence.
To human eyes, PET will stay clear while PVC turns black therefore this is a very common way to manually sort bottles.
Electrostatic separation is a system to attract or repulse different plastics according to their charge.-
WET SEPARATION, using the following techniques
Hydro cyclones enhance the difference of specific weight by centrifugal force, so seperating the plastics. It can be used on plastics of very similar weights
Sink-float by preferred solvent absorption is used when two polymers with same specific weight need to be separated; a solvent (alcohol, ketone, etc) makes one of the two lighter hence they can be separated.
Hydrophobicity is the dislike of water; some polymers react in a different way when going into water under certain conditions therefore separation becomes possible.
Froth flotation means air bubbles attach to one solid promoting floating in a liquid and leave the other(s) to sink.
Chemical Separation can also be used including Hydrolysis ,Glycolysis, hydroglycolysis
A new separation technique for mixed plastics utilizing selective wetting characteristics has been developed. The surface of specific plastics can be selectively changed from hydrophobic to hydrophilic by using a wetting agent. Then, when small air bubbles are introduced into a separation cell, they adhere to the surface of the hydrophobic plastics and float them to the water surface. The new separation technique is entirely different from conventional methods based on only differences in density. Plastics with the same density can be separated by this process. The plastic separator can be used for many purposes: for separating plastics from mixtures, elimination of foreign matter such as paper, fibers, aluminium foil, copper wire chips, sand, and glass from plastics; clarification of waste water containing fine resin powder; etc. (Source K.Saitoh, I. Naguna, S.Izuni. ' New Separation technique for waste plastics. Central Research Laboratory, Mitsui Mining and Smelting Company. 1976)
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