Waste Treatment Technologies

  Types of Waste Treatment Technologies


The waste can undergo various changes in its properties. Based on the type of process being followed for its treatment; it is divided into 3 main categories

  • Physical technologies which process organic waste to yield fuel. Example RDF
  • Thermal technologies which can yield heat, fuel oil, or gas from organic wastes. Example gasification
  • Biological technologies bacterial fermentation is used to digest organic wastes to yield fuel.  Example Composting

We will study following techniques in this unit:

  • Refuse Derived Fuel
  • Composting
    • Windrow Composting
    • Vermi – Composting
  • Anaerobic Digestion/ Biomethanation
  • Mass burning/ Incineration:
  • Pyrolysis
  • Gasification
  • Combustion of RDF
  • Engineered Landfill

  Refuse Derived Fuel (RDF)


Refuse Derived Fuel (RDF) is also known as Solid Recovered Fuel (SRF) or Specified Recovered Fuel (SRF).

It is a process by which municipal, industrial and bulky waste, construction refuse and surplus products with high-calorific value are processed into homogenous RDF for cement kilns or power stations

It is considered as Alternative Fuel Resource (AFR). Indian Government mandates all industries to substitute their 3-5% fuel usage by alternate fuels and thus they are compelled to use one or the other renewable energy. RDF is one of the common fuel substitutes for cement kilns and power stations.

RDF is a physical process, as it does not alters the chemical and biological property of waste. Fuel produced by shredding and dehydrating segregated high calorific fraction of processed MSW. Non-combustible materials such as glass and metals are removed during the post-treatment processing cycle by mechanical/magnetic separation etc. The residual material can be sold in its processed form or it may be compressed into pellets, bricks or logs.

Using RDF prevents GHG emissions from landfills, displaces fossil fuels, and reduces the volume of waste that needs to be landfilled. RDF can also be fed into plasma arc gasification or pyrolysis where the RDF is capable of being combusted cleanly.

There are a number of different processes for preparing RDF from MSW

  • Acceptance of segregated (separation at source) or unsegregated waste at the processing plant
  • Sorting by different rotating drum or plain sieves into main fractions (biodegradable, combustible, mineral)
  • mechanical separation of recyclables like metals
  • Size reduction (shredding, chipping and milling)
  • Separation and screening of main fractions into products
  • Drying and pelletising
  • Packaging and Storage

Input material for RDF

  • Plastic and paper waste
  • Wood
  • Mixed and bulk waste from household, industries etc.

 

RDF characteristics

RDF characteristics vary with the composition of input being used. The calorific value generally ranges from 3000 – 4000 KCal/kg for household waste.

Moisture is only 3-8% as it is dried during RDF processing. Carbon and nitrogen also varies based on the input. 

RDF from household

Marketability of the RDF in India        

The RDF in India is usually sold by the MSWM operators to industrial units as alternate fuels to coal and fire wood. Textile units, dyeing units, industrial boilers, hot air generators are some of the places which need RDF. The consumers of RDF use them to produce thermal energy and are situated in rather close proximity to the production of RDF.

According to information from the MSW plant operators the current market prices of RDF are in the range of Rs 2700 to Rs 3000 per ton. The RDF sold in current retail market has a gross calorific value (GCV) from 3200 - 3800 kcal/kg.

Difficulties in marketing RDF originate from bad or not stable qualities of the product (limitations are due to odour/foul smell and variation in GCV of the RDF). The low grade coal price ranges between Rs. 1500 – 1800 per ton which though have less GCV than RDF but is more attractive option for the users.

Facts on RDF

The total waste quantity in India is about 600,000 tons per day (6.0 lakhs MT/d). Assuming that 20% can be utilised as RDF, this translates to 120,000 tons per day (1.2 Lakh MTD) of RDF fuel. This can theoretically effect a substitution of 240,000 tons (2.4 lakh MT) of coal every day, assuming a 50% lower GCV of coal compared to RDF. A power potential of 15,000 MW exists if all the RDF can be utilised in India but this will account for approximately 2-4 % of the total energy demand of the country. The numbers are small but can make a remarkable difference in present energy scenario.

  Vermi – Composting


Composting using the earthworms is known as vermi-composting. Earthworms can consume practically all kinds of organic matter and they can eat their own body weight per day, e.g. 1 kg of worms can consume 1 kg of residues every day. The excreta (castings) of the worms are rich in nitrate, available forms of P, K, Ca and Mg.

The passage of soil through earthworms promotes the growth of bacteria and actinomycetes. Actinomycetes thrive in the presence of worms and their content in worm casts is more than six times that in the original soil. A moist compost heap of 2.4 m by 1.2 m and 0.6 m high can support a population of more than 50,000 worms.

Turning the heaps is not necessary where earthworms are present to do the mixing and aeration. The ideal environment for the worms is a shallow pit and the right sort of worm is necessary.

Common varity of earthworms are:

  • Lumbricus rubellus (red worm) and Eisenia fetida are thermo-tolerant and so particularly useful. Field worms (Allolobophora caliginosa) and night crawlers (Lumbricus terrestris) attack organic matter from below but the latter do not thrive during active composting, being killed more easily than the others at high temperature.

Vermi – Composting Procedure

A series of pits (the number depending on the space available) are dug approximately 3 m × 4 m × 1 m deep, with sloping sides. Necessary drainage is provided as the worms could not have survived in a waterlogged environment. The pits are lined with old feedstuff sacks to prevent the worms from escaping into the surrounding soil and yet permit drainage of excess water. The pits are then filled with organic residues such as straw and other crop residues, animal manure, green weeds, and leaves. Covered loosely with soil and kept moist for a week or so. One or two spots on the heap are then well watered and worms from the breeding boxes were place on top. The worms burrowed down immediately into the damp soil. The compost pits are left for a period of two months; ideally such pits should be shaded from hot sunshine and kept moist.

Within two months, about 10 kg of castings can be produced per kilogram of worms. The pits are then excavated to an extent of about two-thirds to three-quarters and the bulk of the worms removed by hand or by sieving. This left sufficient worms in the pit for further composting, and the pit can be refilled with fresh organic residues. The compost is sun-dried and sieved to produce good quality material. A typical analysis of vermin-compost are: organic matter - 9.3 percent; N - 8.3 percent; P - 4.5 percent; K - 1.0 percent (water soluble); Ca - 0.4 percent; and Mg - 0.1 percent

The excess worms harvested from the pits, can be sold to other farmers for the same purpose, used or sold as animal feed supplement, used or sold as fish food, or used in certain human food preparations.

Factors that enhance vermi-composting

Sieving and shredding - decomposition can be accelerated by shredding raw materials into small pieces

Blending - carbonaceous substances such as sawdust, paper and straw can be mixed with N-rich materials such as sewage sludge, biogas slurry and fish scraps to obtain a near optimum C:N ratio.

Half digestion - the raw materials should be kept in piles and the temperature allowed to reach 50-55 °C. The piles should remain at this temperature for seven to ten days.

Maintaining moisture, temperature and pH - the optimum moisture level for maintaining aerobic conditions is 40-45 percent. Proper moisture and aeration can be maintained by mixing fibrous with N-rich materials. The temperature of the piles should be 28-30 °C. Higher or lower temperatures reduce the activity of microflora and earthworms. The height of the bed can help control the rise in temperature. The pH of the raw material should not exceed 6.5-7.

Vermi-composting by Eisenia fetida

The species used commonly in India is Eisenia fetida species of earthworm. The species need a temperature of 15–25 °C, moisture 40-50%, pH (slightly alkaline) etc for proper composting. Bed should be protected from predators like red ants, white ants, centipedes and others like toads,rats, cats , poultry birds and even dogs. Minimum space required is 2 square meter per 2000 worms with 30-45 cm thick bed. Cattle ,sheep, and horse dung with vegetable wastes forms ideal feed for worms.

Addition of neem cake in small quantity enhances growth of worms, biogas slurry aged aerobically for 15 days enhances vermi composting process.

 

Vermi-composting give many bye-products like vermi-casting and vermi-wash which are very rich in nutrients.

VERMI WASH is the liquid fertilizer collected after the passage of water through a column of worm culture. It is very useful as a foliar spray. It is a collection of excretory products and excess secretions of earthworms along with micronutrients from soil organic molecules.

Point to remember:-

  • Composting provide a good soil conditioner but no nutrients to soil whereas, vermi-composting provide a good soil conditioner with nitrogen, phosphorus and potassium rich casting.
  • Vermi-composting does not need mixing of waste; as it is done by earthworms.
  • Vermi-composting gives verm-cast and vermin wash together with compost.

  Bio-methanation / Bio-gasification


The production of Bio-gas is called Bio-gasification and as the gas is primarily methane thus called Bio-methanation. Biogas is a mixture of gases composed of:

  1. Methane 40 – 70 vol.%,
  2. Carbon dioxide  30 – 60 vol.%,
  3. Other gases 1 – 5 vol.% (Hydrogen 0 – 1 vol.% and H2S) 0 – 3 vol.%.

 It originates from bacteria in the process of bio-degradation of organic material under anaerobic (without air) conditions. Due to absence of oxygen rather than production of CO2; CH4 is produced.

Benefits of Bio-gasification

  • production of energy (heat, light, electricity);
  • transformation of organic waste into high quality fertiliser;
  • improvement of hygienic conditions through reduction of pathogens, worm eggs and flies generally generated from waste dumps;
  • reduction of workload in firewood collection and cooking;
  • micro-economical benefits through energy and fertiliser substitution,;
  • macro-economic benefits through decentralised energy generation, import substitution and environmental protection.

Biogasification or Biomethanation is the process of conversion of organic matter in the waste (liquid or solid) to Bio-Methane (sometimes referred to as "Biogas” with high energy density) and manure (bio compost) by microbial action in the absence of air, known as "anaerobic processing or digestion."

Bio-gasification is particularly suitable for wet substrates, such as sludges or food waste, which present difficulties in composting, as the lack of structural material restricts air circulation. The anaerobic process is used sometimes to digest sewage sludge, and this has been extended to fractions of household solid waste.

Process Bio-methanation : Exothermic Process

In contrast to aerobic processes (i.e., composting), biogasification is mildly exothermic. Heat needs to be supplied to maintain the process temperature, especially for thermophilic processes. The advantage of high temperature is that the reaction will occur at a faster rate, and so a shorter residence time is required in the reactor vessel.

Types of Digestion

Dry anaerobic digestion

In dry anaerobic digestion, semi-solid wastes are digested to produce biogas in a single stage, either as a batch process or a continuous process. It takes place at a total solid concentration of over 25%, and below this level of solid, the process is described as wet digestion.

The process is initially mesophilic (operating between 30 and 40 0C) and later thermophilic (operating between 50 and 65 0C). This anaerobic process usually takes between 12 and 18 days, followed by several days in post-digestion for residue stabilisation and maturation

Wet anaerobic digestion

This process consists of a single stage in a completely mixed mesophilic digester, operating at a total solid content of around 3 – 8%. To produce this level of dilution, a considerable amount of water has to be added and heated, and then removed after the digestion process.

The single-stage wet process can suffer from several practical problems such as the formation of a hard scum layer in the digester and difficulty in keeping the content completely mixed.

The development of the two-stage digestion process solves this problem as hydrolysis and acidification occur in the first reactor vessel, kept at a pH of around 6.0 and methanogenesis occurs in the second vessel, operated at a pH of 7.5 – 8.2.

Maturing or refining

The process facilitate the release of entrapped methane, elimination of phytotoxins (i.e., substances that are harmful for plant growth, such as volatile organic acids) and reduce the moisture content to an acceptable level. These residues contain a high level of water – even the dry process residue contains around 65% water.

Factors affecting biogasification

Temperature: A temperature range of about 25 – 40 0C (mesophilic) is generally optimal. It can be achieved without additional heating, thus very economical. In some cases, additional energy input is provided to increase temperature to 50 – 600C (thermophilic range) for greater gas production. Often digesters are constructed below ground to conserve heat.

pH and alkalinity: pH close to neutral, i.e ., 7, is optimum. At lower pH values (below 5.5), some bacteria carrying out the process are inhibited. Excess loading and presence of toxic materials can lower pH levels to below 6.5 and can cause difficulties. The presence of alkalinity between 2500 and 5000 mg/L can provide good buffering against excessive pH changes.

Nutrient concentration: An ideal C: N ratio of 25:1 is to be maintained in any digester. It is an important parameter, as anaerobic bacteria need nitrogen compound to grow and multiply. Too much nitrogen, however, can inhibit methanogenic activity. If the C: N ratio is high, then gas production can be enhanced by adding nitrogen, and if the C: N ratio is low, it can be increased by adding carbon, i.e., adding chicken manure, etc., which reduces

Loading: Feed rate (Q, measured in m3/day)

Hydraulic loading or retention time (R, measured in days)

The feed rate (Q) is given by the mass of total solid (m, kg) fed daily, divided by the proportion of total solid (TS)

The retention time (R) of any digester is given by the volume of the digester pit (V, m3), divided by the volume of the daily feed (Q, m3/day).

R  = V /Q days

Effect of toxins: The main cause of biogas plants receiving flak is the presence of toxic substance. Chlorinated hydrocarbons, such as chloroforms and other organic solvents, are particularly toxic to biogas digestion. The chlorine mainly comes from water. If the digester has been badly poisoned, it may be difficult to remove the toxins. In that case, the digester must be emptied, cleaned with plenty of water and refilled with fresh slurry

Digester are modified technically to give high treatment is less time. Some of such modifications are:

Standard rate single-stage digester

Untreated waste sludge is directly added to the zone, where the sludge is actively digested and the gas is being released.The sludge is heated by means of an external heat exchanger. As the gas rises to the surface, it lifts sludge particles and other minerals such as grease oil and fats, ultimately giving rise to the formation of scum layer.

Stratify by forming a supernatant layer above the digesting sludge and become more mineralised, i.e., the percentage of fixed solid increases. Due to stratification and lack of mixing, the standard rate process is used principally for small installations. Detention time for standard rate processes vary from 30 to 60 days.

High rate single-stage digester

The single-stage high rate digester differs from the single-stage standard rate digester in that the solid-loading rate is much greater. The sludge is mixed intimately by gas recirculation, mechanical mixing, pumping and heated to achieve optimum digestion rates. Digestion tank may have fixed roof or floating covers along with gasholder facility, which provide extra gas storage capacity. The required detention time for a high rate digestion is typically 15 days or less.

Two-stage digester

The combination of the two digesters is known as a two-stage digester. The first stage digester is a high rate complete mix digester used for digestion, mixing and heating of waste sludge, second stage is to separate the digested solid from the supernatant liquor, and in the process, additional digestion and gas production may occur. The tanks are made identical, in which case either one may be the primary digester. They may have fixed roofs or floating covers along with gasholder facility.

Components of biogas plant

  • a digester in which the slurry (e.g., dung mixed with water) is fermented;
  • an inlet tank used to mix the feed and let it into the digester;
  • a gas holder/dome in which the generated gas is collected;
  • an outlet tank to remove the spent slurry;
  • distribution pipeline(s) to carry the gas into the kitchen;
  • a manure pit, where the spent slurry is stored.

In India at many places biomethanation plants are installed for commercial and domestic uses; i.e Magarpatta, Maharashtra, Wipro, Karanataka.

           

  Factors that affect composting process


 

Chemical Factors

Carbon orEnergy source:

All living organism need food and energy to work; since, composting is a biological process therefore; the organism involved in the process do need continuous source of energy. Microorganisms in the composting process rely on carbon in the organic material. Since most municipal and agricultural organics and yard trimmings contain an adequate amount of biodegradable forms of carbon, it is not a limiting factor in the composting process

Nutrients or C/N ratio

Apart from carbon another important nutrient required by organisms is nitrogen. Perhaps; it’s the carbon nitrogen ration which plays very important role in composting. To aid the decomposition process, the bulk of the organic matter should be carbon with just enough nitrogen. In general, an initial ratio of 30:1 (C: N or Carbon: Nitrogen) is considered ideal for composting.

Higher C/N ratio

Higher ratios (means more carbon or less nitrogen) retard decomposition; ratios below 25:1 may result in odour problems. This ratio decreases as composting proceed due to CO2 losses.

 Finished compost should have ratios of 15:1 to 20:1

Adding 3 – 4 kg of nitrogen material for every 100 kg of carbon should be satisfactory for efficient and rapid composting. To make condition ideal for composting generally co-composting is done. Co-composting is done with two or more feedstock with different characteristics.

Reducing C/N ratio

Nitrogen-rich materials such as yard trimmings, animal manures, or bio-solids are often added to compost to increase nitrogen content. Adding partially decomposed or composted materials (with a lower carbon: nitrogen ratio) as inoculums may also lower the ratio. As the temperature in the compost pile rises and carbon: nitrogen ratio falls below 25:1, the nitrogen in the fertiliser is lost as gas (ammonia) to the atmosphere.

The composting process slows, if there is not enough nitrogen, and too much nitrogen may cause the generation of ammonia gas, which can create unpleasant odours.

Moisture

Moisture percentage of 40-60% is the ideal requirement for composting. Excessive moisture and flowing water form leachate, which creates potential liquid management problems including water and air pollution (e.g., odour). Excess moisture impedes oxygen transfer to the microbial cells, can increase the possibility of developing anaerobic conditions and may lead to rotting and obnoxious odours.

Moisture added to composting mixture is often lost through evaporation. Controlling the size of piles can minimise evaporation from compost piles, as piles with larger volumes have less evaporating surface/unit volume than smaller piles. Properly wetted compost having the consistency of a wet sponge is preferred.  

Oxygen

Composting requires ideally 10 to 15% oxygen concentration. Composting is considered an aerobic process. Decomposition can occur under both aerobic (requiring oxygen) and anaerobic (lacking oxygen) conditions. The compost pile should have enough void space to allow free air movement so that oxygen from the atmosphere and carbon dioxide and other gases emitted can be exhausted.

The compost piles are either mechanically aerated or turned frequently for fast composting, it expose the microbes to the atmosphere and create more air spaces by fluffing up the pile. Excess air can remove heat, which cools the pile and also promotes excess evaporation. In other words, excess air slows down the rate of composting. Excess aeration is also an added expense that increases production costs.

pH

The pH factor affects the amount of nutrients available for the microorganisms, the solubility of heavy metals and the overall metabolic activity of the microorganisms. A pH between 6 and 8 is considered optimum. pH can be adjusted upward by the addition of lime, or downward with sulphur. The composting process itself produces carbon dioxide, which, when combined with water, produces carbonic acid, which could lower the pH of the compost. Wide swings in pH are unusual, since organic materials are naturally well buffered with respect to pH changes.

Some of the physical factors also play very important role in composting like:

 

Particle size:

Particle size refers to the average size of solid waste particles. Composting is a natural process of size reduction. Smaller particles have more surface area per unit weight, thus facilitate more microbial activity on their surfaces. Optimum particle size provides high surface area and void space for rapid microbial activity and respiration.

Temperature:

Optimum temperature range is between 32° and 60° C. Temperatures above 65° C are not ideal, thermal destruction of cell proteins kill the organisms beneficial for composting. Low temperatures reduce the metabolic activity of the cells thus it should be also avoided. Temperatures can be lowered, by either increasing the frequency of mechanical agitation or using airflow throttling, temperature feedback control or blowers controlled with timers.

At 55° C or higher temperature for at least three days, pathogen destruction occurs. It is important that all portions of the compost material are exposed to such temperatures to ensure pathogen destruction throughout the compost.

Mixing:

Mixing of feedstock, water and inoculants is important and is done by turning or mixing the piles. Mixing and agitation distribute moisture and air evenly, and promote the breakdown of compost clumps. Excessive agitation of open vessels or piles, however, can cool them and retard microbial activity.

 

  Composting


Composting is the biochemical degradation of the organic fraction of solid waste material having a humus-like final product that could be used primarily for soil conditioning. Any organic material that can be biologically decomposed is compostable.

 

Basic Equation for composting

Organic matter + O2 + aerobic bacteria = CO2 + NH3 + H2O + other end products + energy

Compost is the end product of the composting process. The by-products of this process are carbon dioxide and water. Compost is peaty humus, dark in colour and has a crumbly texture, an earthy odour, and resembles rich topsoil.

Benefits of composting

Composting does offer many benefits from environment and economic point of view. It is a form of source reduction with least management (70% by weight of organic materials and food processing, agricultural and paper industries). It reduces the waste volume to landfills, tipping fees and land destruction by waste dumps in landfill.

It is economically viable option to waste management; attractive economic advantage for communities where the costs of using other options are high. Compost, because of its high organic matter content, makes a valuable soil amendment and is used to provide nutrients for plants. When mixed with soil, compost promotes a proper balance between air and water in the resulting mixture, helps reduce soil erosion and serves as a slow-release fertiliser.

 

Process of Composting

Composting is considered to be physical, chemical and biological processes. The physical and chemical aspects are because many physic-chemical parameters change during the whole composting process. However; the major role is played by microorganism; thus it is a biological process.

Biological process - Microorganisms such as bacteria, fungi and actinomycetes as well as larger organisms such as insects and earthworms play an active role in decomposing the organic material. They break down of organic matter produce carbon dioxide, water, heat and humus (the relatively stable organic end product). Microorganisms consume some of the carbon to form new microbial cells, as they increase their population. Preferences based succession takes place during composting. Final compost is made up of microbial cells, microbial skeletons and by-products of microbial decomposition and un-decomposed particles of organic and inorganic origin. Decomposition is slow initially but speeds up with high microbial population.

 

Phases of Composting

Mesophilic Phase

The first stage of composting is called mesophillic phase; it is a moderate-temperature phase. In this phase, bacteria in waste combine carbon with oxygen to produce carbon dioxide and energy, some of the energy is used by them for growth and rest of the heat is emitted out. Mesophilic bacteria (include E. coli and other bacteria from the human intestinal tract) proliferate during this phase this raises the temperature of the composting mass up to 44°C. As the temperature increase; mesophillic bacteria are not able to resist that high temperature thus their growth is inhibited. At this phase; thermophilic bacteria take over in the transition range of 44°C – 52°C.

Thermophilic Phase

As the name indicate, temperature loving; it is a high-temperature phase, temperature is generally above 550C. High temperatures accelerate the breakdown of proteins, fats, and complex carbohydrates like cellulose and hemicellulose, the major structural molecules in plants. Thermophilic microorganisms take over this phase, they are very active and produce heat. The temperature thus sometime reaches up to about 70°C (such high temperatures are neither common nor desirable in compost). At around 65°C most of the pathogens in composting mixture die.

This phase last only a few days or week (localised at upper portion of a compost pile where the fresh material is being added). After this phase, manure will appear but coarser organic material is yet to be digested.

Cooling Phase

The temperature again goes down in this phase, which makes it difficult for thermophillic bacteria to survive. Thus; during this phase, the microorganisms that were replaced by the thermophiles migrate back into the compost and digest the more resistant organic materials. Fungi and macro-organisms such as earthworms and sow bugs that break the coarser elements again appear in compost during this phase. As the supply of these high-energy compounds become exhausted, the compost temperature gradually decreases and mesophilic microorganisms once again take over the final phase of curing or maturation of the remaining organic matter.

Maturation Phase

The phase is said to be Curing, ageing, or maturing stage. The phase is important as all pathogen die in this phase, the longer the phase better will be the quality of compost. Immature compost can be harmful to plants. Uncured compost can, for example, produce phytotoxins (i.e., substances toxic to plants), robbing the soil of oxygen and nitrogen and contain high levels of organic acids

 

  Types/Techniques of Composting


  • Windrow composting
  • Aerated static pile composting
  • Vertical Composting Reactor
  • Horizontal composting reactors
  • Rotating drum

Windrow Composting

Windrows are defined as regularly turned elongated piles, shaped like a haystack in cross section and up to a hundred meters or more in length. The cross-sectional dimensions vary with feedstock and turning equipment, but most municipal solid waste (MSW) windrows are 1.5 to 3 meters high and 3 to 6 meters wide.

Windrows must be placed on a firm impermeable surface to turn the piles with ease and should be turned as frequently as once per week. Any leachate or runoff created is collected and treated or added to a batch of incoming feedstock to increase its moisture content. To avoid leachate or runoff, piles can be placed under a roof, but it increases the cost

Aerated static pile composting

Requires the composting mixture (i.e., a mixture of pre-processed materials and liquids) to be placed in piles that are mechanically aerated. The piles are placed over a network of pipes connected to a blower; air supply blower either forces air into the pile or draws air out of it. When the composting process is nearly complete, the piles are broken up for the first time since their construction. A controlled air supply enables construction of large piles, which decreases the need for land.

Odours from the exhaust air could be substantial, but traps or filters can be used to control them. The temperatures in the inner portion of a pile are usually adequate to destroy a significant number of the pathogens and weed seeds present. But in upper layer, need to place a layer of finished compost of 15 to 30 cms thick over the compost pile to enhance killing of pathogens. It takes 6 to 12 weeks of time in complete process.

The mechanically aerated composting can also be done in large vessels. These vessels can be single- or multi-compartment units. In some cases, the vessel rotates, and in others, it is stationary and a mixing/agitating mechanism moves the material around. The material to be composted is frequently turned and mixed to homogenise the compost and promote rapid oxygen transfer. Retention times range from less than one week to as long as four weeks. These systems, if properly operated, produce minimal odours and little or no leachate. In-vessel systems enable exhaust gases from the vessel to be captured and are subjected to odour control and treatment.

Vertical Composting Reactor

As the name indicates; the waste is fed to a vertical container. Organic material, typically fed into the reactor at the top through a distribution mechanism, moves by gravity to an unloading mechanism at the bottom. Process control is usually by pressure-induced aeration, where the airflow is opposite to the downward materials flow. The height of these reactors makes process control difficult due to the high rates of airflow required per unit of distribution surface area.


Horizontal composting reactors

Horizontal composting reactors can overcome the problems faced in vertical reactor. It avoids high temperature, oxygen and moisture gradients of vertical reactors by maintaining a short airflow pathway. The system can be agitated mechanically for air management inside the waste. Systems with agitation and bed depths less than two to three meters appear effective in dealing with the heterogeneity of MSW.


Rotating drum

Rotating drum reactors is another easy and innovative design for composting. It reduce the reactor cost and compost residence time to an even further extreme than the horizontal or vertical in-vessel systems. These reactors, also known as digesters, retain the material for only a few hours or days. The tumbling action can help homogenise and shred materials; the short residence time usually means the processing is more physical than biological. While rotating drums can play an important role in MSW composting, they are normally followed by other biological processing.

 

Various design and parameter changes can help in making composting efficient and economical with minimum land and energy requirement. The composting can be enhanced and catalysed with the use of earthworms; the process is called Vermi-composting.

  Thermal Techniques


Thermal Techniques

  • Incineration
  • Gasification
  • Pyrolysis

Incineration

Incineration is a chemical and thermal reaction in which carbon, hydrogen and other elements in the waste mix with oxygen in the combustion zone and generates heat. The air requirements for combustion of solid wastes are considerable. For example, approximately 5000 kg of air is required for each tonne of solid wastes burned. The principal gas products of combustion are carbon dioxide, carbon monoxide, water, oxygen and oxides of nitrogen. Combustion zone of 900°C–1100°C is required. Incinerator systems are designed to maximise waste burn out and heat output, while minimising emissions by balancing the oxygen (air) and the three “Ts”, i.e., time, temperature and turbulence. 90% of volume reduction takes place.

Objectives : Volume reduction,  Stabilisation of waste, Recovery of energy from waste (EFW) and Sterilisation of waste (mainly for clinical or biomedical waste). Incineration of solid wastes will also ensure destruction of pathogens prior to final disposal in a landfill.

Incineration technologies

  1. Mass burning system
  2. Refuse derived fuel system
  3. Modular incineration
  4. Fluidised bed incineration

The two most widely used and technically proven incineration technologies are mass-burning incineration and modular incineration. Fluidised-bed incineration has been employed to a lesser extent, although its use has been expanding and experience with this relatively new technology has increased. Refuse-derived fuel production and incineration has also been used, with limited success. Some facilities have been used in conjunction with pyrolysis, gasification and other related processes that convert solid waste to gaseous, liquid, or solid fuel through thermal processing

Mass-burning system

Mass-burn systems generally consist of either two or three incineration units ranging in capacity from 50 to 1,000 tonnes per day. That is to say, the facility capacity ranges from about 100 – 150 to 2,000 – 3,000 tonnes per day.

The steam generator generally consists of refractory-coated water wall systems, i.e., walls comprised of tubes through which water circulates to absorb the heat of combustion

Modular incineration  - Modular incinerator units are usually prefabricated units with relatively small capacities between 5 and 120 tonnes of solid waste per day. Typical facilities have between 1 and 4 units with a total plant capacity of about 15 to 400 tonnes per day. Use for small communities or for commercial and industrial operations.

Involving two combustion chambers, and combustion is typically achieved in two stages. The first stage is less than the theoretical amount of air necessary for complete combustion. The controlled air condition creates volatile gases, which are fed into the secondary chamber, mixed with additional combustion air, and under controlled conditions, completely burned. Combustion temperatures in the secondary chamber are regulated by controlling the air supply, and when necessary, through the use of an auxiliary fuel. The hot combustion gases then pass through a waste heat boiler to produce steam for electrical generation or for heating purposes.

Fluidised-bed incineration : Fluidised-bed incineration of MSW is typically medium scale, with processing capacity from 50 to 150 tonnes per day. In this system, a bed of limestone or sand that can withstand high temperatures, fed by an air distribution system, replaces the grate. The heating of the bed and an increase in the air velocities cause the bed to bubble, which gives rise to the term fluidised. There are two types of fluidised-bed technologies, viz., bubbling bed and circulating bed.

Fluidised-bed systems are more consistent in their operation than mass burn and can be controlled more effectively to achieve higher energy conversion efficiency, less residual ash and lower air emissions.

Incineration/Waste-to-Energy

  • Thermal Technology : Waste-to-Energy combustion (WTE) is defined as a process of combustion in excess of oxygen/air, using an enclosed device to thermally breakdown combustible solid waste to an ash residue that contains little or no combustible material and that produces, electricity, steam or other energy as a result
  • Must meet the prescribed emission standards
  • Temperature 900 – 1200 0C

Pyrolysis

  • Pyrolysis can be defined as the thermal decomposition of carbon-based materials in an oxygen-deficient atmosphere using heat to produce synthesis gas (syngas CO+H2). No air or oxygen is present and no direct burning takes place. The process is endothermic (400 -9000 C)
  • A mixture of un-reacted carbon char (bio-char) (the non-volatile components) and ash remains as a residual
  • Used in producing transportation fuels and other chemical products

Gasification

  • It occurs in a higher temperature range of 700 - 1600°C with very little air or oxygen (controlled oxygen). In addition to the thermal decomposition of the volatile components of the substance, the non-volatile carbon char that would remain from pyrolysis is converted to additional syngas.
  • Steam may also be added to the gasifier to convert the carbon to syngas. Gasification uses only a fraction of the oxygen that would be needed to burn the material. Heat is supplied directly by partial oxidation of the carbon in the feedstock. Ash remains as a residual.

Plasma arc Gasification

  • Plasma arc gasification is a high-temperature pyrolysis without burning, process whereby the organics of waste solids (carbon-based materials) are converted into syngas and inorganic materials and minerals of the waste solids produce a rocklike glassy by-product called vitrified slag
  • The heart of the PAG “plasma converter” is its “electrical arc gasifier,” which passes very high voltage electrical current through two electrodes, creating an arc in the space between them
  • The plasma arc gasification reactor is typically operated between 4000 – 7000 0C
  • The vetrified slag can be processed into bricks, synthetic gravel or asphalt, and other materials

 

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