Semester /2014 ERT 319 Industrial Waste Treatment

Landfill & Incineration Waste Treatment Systems

LANDFILL Introduction -A landfill site (also known as tip, dump, rubbish dump or dumping ground) is a site for the disposal of waste materials by burial and is the oldest form of waste treatment. -Historically, landfills have been the most common methods of organized waste disposal and remain so in many places around the world. -In Europe, follows EC Waste Landfill Directive (1999).

-Major advantages: Waste landfill is low cost compared with other disposal options and the fact that a wide variety of wastes are suitable for landfill. Collection & utilization of landfill gas as a fuel for energy generation. However, landfill achieves a lower conversion of the wastes into energy with about one-third less energy recovery per tonne from landfill gas than incineration. This is due to the conversion of the organic materials in the waste into non-combustible gases and leachate and general losses from the system. -Leachate represents the water which passes through the waste from precipitation, and water generated from the waste within the landfill site, resulting in a liquid containing suspended solids, soluble components of the wastes and products from the degradation of the waste by various microorganisms.

- Disadvantages:  Older sites of landfill – still under current use or have long been disused, were constructed before the environmental impacts of leachate and landfill gas were realized. - now, sources of pollution with uncontrolled leakage. - landfill gas can be hazardous, ex. largest component, methane can reach explosive concentrations, and also is a ‘greenhouse gas’, which has about 30 times the effect of CO 2. - constructed close to housing areas, and some have been built on disused landfill sites.

Site Selection and Assessment -Depends on a wide range of criteria, including the proximity (closeness) of the sites to the source of waste generation, the suitability of access roads, the impact on the local environment of site operations and the geological and hydrogeological stability of the site. -  the chosen sites will have no significant impact on environment. -Location requirements: -The distance from the boundary of the site to residential areas, waterways, and other agricultural or urban sites. -The operator should report on the geological and hydrogeological conditions of the area and the risks of hazards such as flooding, landslides and avalanches (large mass slides, flood). -levels of water and air quality.

Considerations for Landfills 1)Final landform profile – dictates after-use of the site, waste capacity, and settlement of the site after completion and landscaping. 2)Site capacity 3)Waste density – typical range: 0.4 – 1.00 tonnes/m 3 4)Settlement: – due to physical rearrangement of wastes soon after emplacement. - typical long term settlement for municipal solid wastes: % reduction. - can take place over 50 years, but the major settlement (up to 90%) occurs within first 5 years of the final emplacement of wastes. - involves 3 stages:

a) Initial compression: virtually instantaneous and due to compaction of the void space and particles caused by compression of overlying waste and the compaction vehicles. b) Primary compression: due to dissipation of pore water and gases from the void space and usually takes around 30 days. c) Secondary compression: may take many years, due to waste creep and biodegradation processes of wastes which produce leachate and landfill gas thus, reduce mass and volume. 5) Material requirements – clay, sand, gravel and soil. 6) Drainage – drainage of rainwater on the site is required to ensure that excessive water does not infiltrate the waste directly or from run-off from surrounding areas 7) Operational practice

Types of Waste Landfilled 1)Hazardous waste: waste which is dangerous or difficult to keep, treat or dispose of and may contain substances which are corrosive, toxic, reactive, carcinogenic, infectious, irritant, toxic to human and environment. 2)Non-hazardous waste: includes municipal solid wastes, and wide range of industrial wastes, such as organic and inorganic materials provided that they are non-hazardous  and biodegradable. 3)Inert waste: waste that does not undergo any significant physical, chemical or biological transformations  do not pose a significant environemental risk, not endanger surface / ground water, no reactivity. Ex: bricks, glass, tiles and ceramic, concrete, stones, etc.

Landfill Design and Engineering -Key to design: barrier system at the base, sides and cap of the waste landfill. -Barrier liner system – from natural or synthetic materials – for protection of surrounding soil and groundwater.  To provide sufficient permeability and thickness requirements at least equivalent to the following: 1)For hazardous waste landfill sites, hydraulic conductivity (permeability) of ≤ 1.0 x m/s, and liner material thickness of ≥ 5m. 2)For non-hazardous waste landfill sites, hydraulic conductivity of ≤ 1.0 x m/s, and liner material thickness of ≥ 1m. 3)For inert waste landfill sites, hydraulic conductivity of ≤ 1.0 x m/s, and liner materialthickness of ≥ 1m. **hydraulic conductivity (permeability)-is the ease with which a fluid such as leachate will flow through the liner material

Landfill liner materials 1)Natural clay – un-joined rocks composed of clay minerals formed as breakdown products from weathering of pre- existing rocks. 2) Bentonite-enhanced soils A mixture of clay minerals 3) Geosynthetic clay liners- A mixture of bentonite clay, adhered to a geotextile fabric 4) Flexible membrane liners – synthetic, polymeric plastic material e.g. PVC, polyamide etc.

Landfill Liner Systems 1)Single Liner System

- Suitable for low-risk wastes in sites where escape of leachate poses negligible risk of contamination. - Comprises of primary barrier consisting of a layer of clay, bentonite-enhanced soil or hydraulic asphalt - Above and below primary liner or barrier would be a separation/protection layer of geotextile material, (ex: non-woven, needlepunch fabrics-polyester / polypropylene fiber  acts as a protective layer and filter for fine suspended solids. - Between the waste and separation/protection layer would be a leachate collection system consisting of a series of drainpipes or drainage layer. - Beneath the liner, may be a groundwater collection system.

2) Double liner system

3) Multiple liner system

Typical liner materials used in landfill liner systems

Process operating in waste landfills

Comparison of early and late stage leachate Stage II / IIIStage IV High content of fatty acidsLow content of fatty acids Acidic (low pH)Neutral or alkaline (pH 7-8) High BOD/COD ratioLow BOD/COD ratio High ammoniacal nitrogenLower ammoniacal nitrogen High organic nitrogen Heavy metals in solution e.g Cr, Fe, Mn

Hydrogeological requirements for sustainable landfill design

Landfill Leachate -Leachate represents the water which passes through the waste from precipitation, and water generated from the waste within the landfill site, resulting in a liquid containing suspended solids, soluble components of the wastes and products from the degradation of the waste by various microorganisms. -Associated with malodorous smell, due to presence of organic acids. -Composts of highly variable concentrations of wide range of components such as salts, halogenated organic compounds, trace metals, organic compounds (high ammonia, etc).

Characteristics / categories of landfill leachate: 1) Dissolved organic material (quantifies as COD and TOC), volatile fatty acids and fluvic and humic-like material. 2) Inorganic macro-components including Ca, Mg, Na, K, Ammonium, Fe, Mn, Cl, Sulphate, and Bicarbonate compounds. 3) Heavy metals such as Cd, Cr, Cu, Pb, Ni, Zn compounds. 4) XOC which are compounds not degraded by organisms in the environment and include aromatic hydrocarbons, pesticides, plasticizers, chlorinated aliphatic compounds, etc. (originated from household and industrial chemicals, present in low concentrations)

Treatment process for leachate

Energy Recovery from Landfill Gas

INCINERATION

Incineration -Alternative to landfill. -Incineration is the oxidation of the combustible material in the waste to produce heat, water vapor, nitrogen, CO 2, and O 2. -Others may be formed, ex: CO, HCl, HF, NO, SO 2, heavy metals, etc. -Incineration now  for waste disposal, energy recovery and economic necessity.

-Advantages over landfill: 1 ) can be carried out near the point of waste collection. 2) the waste is reduced into a biologically sterile ash product  ~ 10% of its pre-burnt volume & 33% of its pre-burnt weight. 3) does not produce methane, unlike landfill ( greenhouse gas) 4) as low-cost source of energy to produce steam, for electric power generation, industrial process heating or hot water  conserving valuable primary fuel resources. 5) the bottom ash residues can be used for materials recovery or as secondary aggregates in construction. 6) the best practicable environmental option for many hazardous wastes such as highly flammable, volatile, toxic and infectious waste.

Disadvantages: 1)Higher cost, longer pay-back periods, due to high capital investment. 2)Lack of flexibility in the choice of waste disposal once chosen, because high capital cost tied to long-term waste disposal contracts. 3)Removal of materials such as paper and plastics for recycling may reduce the overall calorific value of waste  may affect incinerator performance. 4)Some public concern that the emitted levels may still have an adverse effect on health. 5)Incineration still produce a solid waste residue which require further management.

Types of incinerator: 1)Mass-burn incineration 2)Other types – fluidized bed, cyclonic, starved air or pyrolytic, rotary kiln, etc.

Mass burn incineration Sections: 1)Waste delivery, bunker and feeding system 2)Furnace 3)Heat recovery 4)Emissions control 5)Energy recovery via district heating and electricity generation.

Gas clean-up

Cyclones- remove particles larger than 15 µm, but incinerator particulates < 15 µm size. Thus, its only used as a preliminary collector, prior to electrostatic precipitator of fabric filter. Electrostatic Precipitators- the dust-laden gas stream enters EP, the particles are charged by –ve ions by corona discharge in an intense electrostatic field, then attracted to oppositely charged collector electrode plates, then the particles accumulate. Fabric Filters- consist of a series of elongated, permeable fabric bags through which the particulate-laden gas flow and filter out the particles from gas stream. Can remove particles with submicron size. Can be coated with additives where adsorption take place.

Cyclones

Wet Scrubbers (Dry or semi-dry scrubbers)- remove soluble acid gases such as hydrogen chloride, hydrogen fluoride and sulfur dioxide. The acid gases passes through spray of liquid (alkaline solution such as CaOH or NaOH). Then, hydrochloric or hydrofluoric acids produced, thus remove heavy metals which are soluble in acid solutions. De- NO X systems- Nitrogen oxides, NO X (NO & NO2) are formed from nitrogen in the air used for combustion and in fuel itself at high temperature. Contributes to acid rain and act as photochemical oxidant in the atm. To reduce NO X  control the combustion conditions (lower T, lower O 2 levels)  use Non-catalytic reduction using ammonia (high T ⁰C)  controlled by selective catalytic reduction, SCR (ammonia added, low T, wide T range, Pt, Pd, etc)

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