1 the chemistry of solid waste waste always a byproduct of human societies –food scraps, paper,...
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The chemistry of solid waste
Waste always a byproduct of human societies– food scraps, paper, packaging; agricultural waste; sewage;
scrap metal and rubber; metals, plastics, ceramics in consumable products
– wealthy, modern societies produce large amounts of waste
Distinguish between bulk waste (typical waste produced in large volumes) and other, specific waste types (medical waste, various types of chemical wastes, etc.)
Desirable to decrease the amount of waste produced– Reduce– Reuse– Recycle
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Disposal of bulk solid waste
Solid wastes are usually disposed of on landE.g., municipal garbage, animal manures, sewage, mine tailings
Waste interacts physically and chemically with its surroundings– Time scale varies depending on material and degree of exposure– Water and oxygen play an essential role in many chemical
reactions in the waste– Products are gases, liquids, and residual solids– 2 approaches to disposal on land:
1. Maximise interaction of soil and waste (e.g., manure)2. Contain and compact waste as much as possible (landfill)
Examine direct disposal of animal waste on land, sewage sludge, biogas synthesis, landfilling, and incineration
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Direct disposal of animal wastes on land
Daily animal faeces production: ~60kg per 1000 kg mass of animalManure consists of organic matter and nutrients, especially N, P, and
K, which help to improve the condition of soilManure composition is variable, depending on type of animal and
feed. Typically, about 10% solids, 0.6% N, 0.1% P, 0.3% KAesthetic and health issues raised by this disposal route
Environmental issues:- not all nutrients may be taken up- incorporation into surface water - nitrogen release is a problem: released as NH4
+, undergoes nitrification to form NO3
-, and migrates into surface and ground water- leaching of K and P less common → eutrophication
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Direct disposal of animal wastes on land
Proper management is essential. Take into account:- soil type, drainage, proximity to water courses and aquifers- how manure applied (wet or dry)
These factors limit amount of manure that can be applied to land, usually to about 30 t ha-1 per annum
Crops should be planted soon after manure application
Other problems with disposal on land include increased salinity- total salt content (chlorides of alkali and alkali earth metals) can range from 1 – 10% dry mass- leachate concentrations can be high enough to impede plant growth or adversely affect groundwater quality
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Sewage sludge
Waste water from domestic & industrial sources
Waste water treatment produces:(1) treated water – released into lakes & rivers(2) sewage sludge – disposed of in various ways
Sewage sludge composition: slurry of 1% solid material- organic carbon = 30% of dried mass- inorganic components from original sewage & added metal-based coagulants during treatment- nutrients include N, P, K: may be a suitable soil amendment
However, relatively high levels of toxic elements, esp. Zn, Cu, Ni, Hg, Pb
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Sewage sludge
Sludge may be applied either as a slurry with 1% solids or as dewatered, partially dried form
Obvious precautions must be observed in its application:- not too close to residences and soils- far from surface water- the depth of the groundwater table (not on shallow soils)- observe a waiting period before planting to avoid pathogens
Must not be used on soils with low pH or on land with an existing high concentration of toxic metals
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Sewage sludge
Suitability of sludge for application to land determined by either:
1. Ratio of N (as NH3 and NO3-) to metal
2. Assigning relative toxicity factors to different metals (esp. Cu, Ni, Zn) – total application of sludge to soils must be less than 560 kg ha-1 over 30 years
Metals in sludge originally present in organic forms – this is released when the organic material decomposes on the land:- associates with other solid phases in soil:- largely immobile & confined to topsoil- taken up by plants or accumulates over time
Soluble salts removed mainly in treated water- salinity generally not a problem
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Small scale biogas synthesis
Biological waste material may be used to produce two products: biogas and a residual organic slurry
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Small scale biogas synthesis
Biogas formed by anaerobic processes:
- mixture of 60-70% CH4 and 30-40% CO2
- minor components include H2, NH3, H2S- useful for cooking and heating- clean burning with low sulfur and particulate emissions
Reaction conditions must be controlled to optimise gas heat content:- pH = 6.5 – 8.5- temperature = 20 – 60ºC- C:N ratio of feedstock is important and should be about 30 or less
Residual sludge retains N, K, P, and other nutrients; organic content is about 30% of original value & suitable for soil amendment
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Urban Waste in Ireland
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Municipal Solid Waste Generation
Ireland – 1995, 1.8 million tonnes
– 2002, 2.7 million tonnes
– Household waste = 375 kg per person per year
– waste includes household, commercial and street cleaning waste.
20% household waste, 51% commercial waste recovered in 2004 –
remainder went to landfill!
Recycling in Ireland has increased significantly recently but 75%
of material is recycled elsewhere
Consider waste disposal in landfills and using incineration
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Landfill
Landfilling may follow removal of components for composting
or recycling, or it may be used for disposal of ash from
incineration
For health and aesthetic reasons landfills should not be located in
the immediate vicinity of population centres
To maximise space, rubbish is often compacted with heavy
machinery
What then happens to this highly concentrated mixture of solid
material?
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Landfill
Most rubbish contains amounts of degradable organic matter
(OM) – food, wood, paper, and more inert materials
OM – degradation is microbial and as long as oxygen is present
the eventual products will be CO2 and H2O with nitrogen being
converted to nitrate
However, the compact nature of the rubbish means that the
environment becomes anaerobic
Eventually methane gas will be produced – a greenhouse gas but
also a useful energy source
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Landfill
Engineered collection systems of methane:
– seal the landfill to prevent the escape of the gas
– install pipes to collect and transport the gas
Unfortunately, degradation does not produce only gases
A liquid leachate of variable composition is also produced
pH of the leachate is low so it can dissolve some metals,
including those that are toxic
Leachate must be contained!
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Landfill
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Alternative to landfill
Advantages include:
- Efficient energy recovery from waste
- Can be set up in an area near to population centres
- Reduces the waste volume → less land is required for
disposal
- Eliminates landfill problems associated with methane
generation and leachate
Incineration
Proposed Meath incinerator
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Incineration
Consider the energy from 1 ton waste:
Landfill:
– would produce about 100 m3 methane (recovered)
– When methane burned it would produce about 4 × 109 J energy
Incineration:
– With efficient energy collection → 1.2 × 1010 J energy
Incineration gives a 3-fold gain in energy recovery compared with
methane recovery from a landfill and use as fuel
What are the incineration products?
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Incineration
Most organic matter is converted to CO2 and H2O
Depending on the waste composition and on the combustion
conditions, other gases may be produced:
– SO2, NOx, PAHs and chlorinated organic substances
Incineration reduces the waste but residual solids remain– ash
– Some solids (< 1%) are emitted through the stack – fly-ash
– > 99% of the solid component is present as bottom ash that
remains as a residue after combustion is complete
Bottom ash must be disposed – creating its own problems
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Human health and Incineration
Research studies of possible health outcomes in populations living
close to incinerators have not given clear indications of the presence
or absence of an effect
Many studies have produced evidence of association between a health
outcome and an environmental pollutant, but cannot demonstrate a
cause and effect relationship
Health Research Board’s 2003 review of the international literature
finds there is some evidence that incinerator emissions may be
associated with health effects – respiratory morbidity, respiratory
symptoms, reproductive effects, cancer – but concluded that the
results are inconclusive
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Human health and Incineration
A recent UK review:
– “looked in detail at studies of incineration facilities, and found no
consistent or convincing evidence of a link between cancer and
incineration” (Review of Environmental and Health Effects of Waste
Management: Municipal Solid Waste and Similar Wastes, Department
of the Environment, Food and Rural Affairs, London, 2004)
Most studies refer to incineration facilities from 1960s to 1990s whose
emission profile is significantly different from today’s modern
incinerators.
Most old incinerators now closed or have been upgraded to meet new
EU requirements
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Economics of Incineration
Landfill used to be cheap but costs have increased a lot
– waste licensing and more stringent environmental controls on
landfill construction and operation
– increased costs have indirectly supported other waste
management options – recycling
Incineration in Ireland is now becoming a commercially viable
waste management option
– no recurring land acquisition costs and greater energy recovery
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Economics of Incineration
Incineration requires a steady stream of waste to be financially
viable
– Would the need to feed waste incinerators eventually overturn
increases in recycling and reuse
– Incineration is not a cheap alternative:
• high capital and operating costs
In the event of recyclables being diverted to incineration a
financial penalty could be levied
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The EPA’s Licensing Role
The EPA operates a licensing system in line with all relevant
national and EU legislation
It must ensure that all standards are complied with and that any
decision to grant a license is based on the merits of a license
application
The EPA attaches conditions to licenses it grants to ensure both
the facilities are properly managed and that risk of pollution is
minimised
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