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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