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 Anaerobic digestion of solid waste

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تاريخ التسجيل : 16/01/2012
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مُساهمةموضوع: Anaerobic digestion of solid waste   الخميس فبراير 23, 2012 12:29 am

From a naturally occurring process to a high-tech industry – anaerobic digestion has come a long way and should now be seen as a vital weapon in the fight against the world’s stockpiling of waste and its finite fossil-fuel resources.
by Luc De Baere and Bruno Mattheeuws
Anaerobic digestion for the treatment of the organic fraction from municipal solid waste was developed more than 20 years ago and has been developing rapidly since the mid-nineties. The implementation of anaerobic digestion (AD) for the treatment of biowaste, or the organic fraction in municipal solid waste, could even be considered as the major development in the field of waste treatment in Europe during the past two decades. Nowadays, anaerobic digestion has captured a significant part of the European market for the biological treatment of the organic fraction in municipal solid waste (MSW). But how well established is this technology? And what trends can be observed?
To allow us to give an overview of the development of anaerobic digestion, we created a set of criteria that European treatment plants should meet in order to be included.
• At least 10% of organic solid waste from household origin needs to be treated in the plant, with a minimum capacity of 3000 tonnes per year.
• The capacity taken into consideration is the designed capacity for the plant, unless specified differently by the supplier/operator. For biowaste, the total capacity of the biowaste plant was used, while for mixed and residual waste plants, the actual capacity going into the digesters was used.
• Plants were not eliminated if operation ceased.
• The plants taken into consideration have to be at least under construction or contracted, and situated in Europe. An estimate was made for 2009 and 2010 based on tenders decided in 2008.
Digestion capacity development in Europe
The initial capacity of plants that met the criteria was about 87,000 tonnes per year in 1990, provided by three plants. Since then, capacity has greatly increased. The increase in additional digestion capacity was initially rapid but has levelled off during the past five years. The current situation is that there will be 171 plants with a total installed capacity of 5,204,000 tonnes per year by the end of 2010. This capacity is spread over 17 European countries.
During the period of 1991–1995, 38,800 tonnes per year was added in capacity. During 1996–2000, this increased to 223,500 tonnes per year while this increased again dramatically in the period 2001–2005 with an annual increase in capacity of 415,590 tonnes per year. In the last period (2006–2010) an annual increase of 345,540 tonnes per year is noted with about 11 plants being constructed every year with an average size of 31,413 tonnes per year (see Figure 1 and Table 1).


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Figure 1. Evolution of installed capacity



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A lot of digestion capacity has been constructed in Germany, where a capacity of 1,531,555 tonnes per year will be available by the end of 2010 in 69 plants. About 1,319,000 tonnes per year of capacity will be available in Spain in only 24 plants. This means that plants in Germany have an average annual capacity of 22,196 tonnes while the more recent installations being added in Spain have a rather large average capacity of 67,900 tonnes. The trend towards larger plants (mainly because of the ability to capture the benefits of economies of scale) has been stopped. However, several large-scale facilities have been implemented or are planned in Spain, France and the UK (where source separation of household waste is less actively encouraged), resulting in large MSW-digestion plants.
On a per person basis, the highest digestion capacity will be available in Malta with a capacity of 112,500 tonnes per million inhabitants (45,000 tonnes installed capacity for 0.4 million inhabitants), followed by Luxembourg with 76,000 tonnes per million and Spain with 29,181 tonnes per million people by the end of 2010. Switzerland will have a digestion capacity of 26,200 tonnes per million while Germany only follows in 5th place with 18,632 tonnes per million people.
Assuming a potential digestible solid waste fraction of 300kg/person/year in all countries, a potential capacity of 300,000 tonnes per year and per million inhabitants is available for digestion. This would mean that 37.5% of all potential tonnage is being treated in Malta by anaerobic digestion, 25.33% for Luxembourg, followed by Spain with 9.73%. Switzerland treats 8.73% of all biodegradable solid waste by means of anaerobic digestion, while in the other European countries, 0.2% to 6.9% of treatment capacity is provided by digestion. Europe has about 300 million tonnes of MSW every year of which 180 million tonnes is said to be organic. In 2010, a capacity of more than 5 million tonnes of AD will be installed, which means that the average in the EU is almost 3% of total organics treated by anaerobic digestion. About 15% of the organic fraction of MSW (OFMSW) is treated biologically in Europe; anaerobic digestion represents about 20% of all biological treatment capacity.
Underlying drivers of growth
Some general factors can be identified for this growth. First of all the alternative techniques suffered from bad market penetration due to poor public acceptance (such as incineration) or too high development costs and time needed to prove a technology (such as pyrolysis and gasification).
Secondly, both recycling and source separation became more prevalent, resulting in the active encouragement to collect the putrescible organic fraction of MSW separately and to treat this clean material in dedicated plants in order to produce high quality compost. In countries where source separation is less common, the combination of centralized separation and biological treatment of residual household waste as well as for mixed waste was preferred by many solid waste authorities because of the need for an alternative treatment.
This combination is called Mechanical Biological Treatment (MBT). The high prices for oil, the public opinion towards renewable energy and the resulting stimulating regulatory and financial framework also has had a big influence in the rising interest in this technology in many countries.
Analysis of the installed capacity
Anaerobic digestion is a biochemical process where, in the absence of oxygen, bacteria break down organic matter to produce biogas and digestate. This is what all projects and concepts have in common. But the biogas and the digestate can be produced out of many different kinds of biomass such as energy crops, putrescible organic waste, manure or a combination of these input materials. Furthermore, these materials can be treated under different types of conditions such as temperature, solids concentrations in the digester, one or two phases (even three phases have been investigated), method of mixing and agitation, and so on. As a result, there are many different kinds of digestion plants with a lot of different parameters. In the assessment, a distinction was made between the most important types of process conditions.
Mesophilic versus thermophilic digestion
Anaerobic digestion can be performed at mesophilic (35° to 40°C) or thermophilic (50° to 55°C) operating temperatures. Until the beginning of the nineties, all plants were operating at mesophilic conditions, mainly because this was considered as a more stable process and because of the lesser need for heating. The first thermophilic plants were dry fermentation and came on line in 1992 and 1993. The capacity of both mesophilic and thermophilic operation increased very much during the past two decades with mesophilic digestion being predominant (see Figure 2 and Table 2).


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Figure 2. Mesophilic (35º–40ºC) versus thermophilic (50º–55ºC) digestion


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A decrease in the amount of thermophilic capacity occurred in the years 2002 to 2004, as a number of large-scale, mostly wet, mesophilic plants were constructed, so that at the end of 2004, 77% of the capacity was provided by mesophilic plants. A large number of thermophilic plants were constructed in 2005 and 2006 and this trend seems to be continuing. In the period 2006–2010, about 41% of the installed capacity will be thermophilic. At the end of 2010, mesophilic operation will provide a capacity of 3,606,000 tonnes per year or 69% of the existing plant capacity.
Wet versus dry digestion
Wet digestion can be defined as digestion with less than 15% dry solids in the digester. All digesters with a higher percentage of dry solids inside are considered dry systems in this assessment. Although both applications have continued to increase in total capacity, dry digestion has been dominant since the beginning of the nineties. An increase of wet systems was observed between 2000 and 2005 as a number of large-scale wet plants were put into operation, while more dry fermentation plants are being installed since 2005. Dry anaerobic fermentation currently provides almost 54% of the capacity while wet fermentation is used in 46% of the total installed capacity.
One-phase versus two-phase digestion
In the digesters there are several processes running continuously and consecutively. These processes require different optimal conditions, especially regarding acidity. Hence, it was claimed that the separation of the methanization phase and the hydrolysis/acidification phase would have resulted in important process and economic advantages.
This separation is carried out by installing two different reactors with different bacteria and operating under different conditions. In this way, both processes can be optimized. But market penetration of the wet and dry two-step digesters is very moderate. The advantage of having a faster degradation during the digestion step is usually not enough to compensate for the higher capital cost of the hydrolysis-step. Therefore, two-phase digestion has been decreasing and is currently limited to around 7% of the installed capacity. In the period 2006–2010, it is expected that only 2% of capacity will be added by using two-phase systems. Most of this capacity will be derived from batch-type systems.
Single feedstock versus codigestion
In Europe, a lot of digesters are codigesting. This means these digesters operate with a mixture of different types of input materials. In many cases the term ‘codigestion’ is used for manure treating plants that bleed in an amount of energy crops, agro-industrial or organic household waste as a co-product.
However, the tonnage of organic household waste is usually far less than the 10% threshold for codigestion and/or the required 3000 tonnes as defined here, therefore agricultural digesters as well as sludge digesters are left out of this analysis. In any case, codigestion with solid waste has been rather the exception, as barely 9.7% or 440,000 tonnes of capacity is provided by plants using codigestion in 2008. The percentage of installed codigestion plants has dropped from 23% in the period 1990–1995 to 5% in the period 2006–2010 (see Table 2). However, due to the high prices for agricultural crops, many energy crop digestion plants are looking for organic waste feedstocks.
Generally, the wet concepts are not ideal for digesting the OFMSW without co-products and thus these systems lend themselves somewhat more for co-digesting OFMSW with more dilute feedstocks.
Residual/mixed waste versus biowaste
Initially most of the digestion technologies were developed using mixed household waste, so that most of the treatment capacity available in the beginning of the nineties was designed to treat mixed waste. But in the mid-nineties, both source separation and recycling became more prevalent and thus source separation of biowaste was introduced. Therefore plants dealing with mixed waste were not constructed any longer and all new digestion plants treated source separated organics. For digesting, this source separation has some clear advantages.
On the one hand, this stream usually has a lower degree of contamination compared with mixed waste streams where the digestate or compost can have a higher content of heavy metals and plastics. On the other hand, the waste has a more homogenous and consistent composition, which makes it easier to ‘manage’ the process and to achieve a steady level of biogas production.
In 1997, the first digestion plant on residual waste was put into operation (residual waste being the remaining waste after source separate collection of the biowaste). From then on, many plants were added, operating not only on residual waste in the countries where biowaste collection was established, but also again on mixed waste in countries such as Spain and France, where source separate collection is much less common. Almost 90% of the capacity was derived from plants treating biowaste in the mid-nineties, but by the end of 2010, barely 52% of the capacity will be handling biowaste.


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Mechanical biological treatment plant Hille, Germany, for the treatment of 100,000 tonnes per year of residual household waste including digestion of 38,000 tonnes per year of organic fraction
In 2010 more than 2.5 million tonnes per year of residual or mixed waste will be digested in 48 different plants with an average capacity of 52,094 tonnes going into the digester, while 123 plants are treating biowaste with a capacity of only 21,982 tonnes per year. In the period 2006–2010, the percentage of biowaste/residual-plants is about fifty-fifty. The figures in Table 2 clearly mark that the percentage of biowaste-plants is not decreasing any more as it did during the period 1998–2004. Since 2004 there is a steady state in the cumulative percentage of biowaste plants installed. This is mainly due to the revamping or extension of existing biowaste composting plants. Many of these aerobic plants are 10 to 15 years old and are implementing anaerobic digestion in their system as a way of improvement (for example for improving odours, compost quality and so on).
Anaerobic digestion is also used for extending the capacity of existing plants. The insertion of a partial stream anaerobic digestion can increase existing capacity by up to 50% with minimal surface requirement. In most cases this is an economically very attractive option.
OWS and the Dranco concept in today’s market
A total of about 40 different system providers were identified in this assessment, that each have constructed (or shall construct before the end of 2010) between one and 33 plants for digestion of organic municipal waste. The eight main technology providers represent a market share of more than 80% by volume and 77% by number of plants. Taking Organic Waste Systems (OWS) as an example, this company is behind more than 11% of the total installed capacity in Europe.
The dry technologies are probably among the best adapted systems and are most commonly used for digestion of MSW and biowaste. The Dranco concept developed by OWS is a dry system and consists of a single-step vertical plug flow reactor. Phase separation is prevented due to the very high dry matter concentrations of 30%–40% in the digestate. This is made possible by a vertical digestion whereby the material moves vertically from top to bottom by gravitational force only.
Because there are no moving parts in the digester, this system relies on an external recycle of a large proportion of the outgoing digestate to inoculate the incoming raw feedstock and to mix the ingoing material. The requirement for only minimal water additions makes the overall heat balance favourable for operating at thermophilic digestion temperature. This results in a high biogas production rate and yield per tonnage fed (see Figure 3).


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Figure 3. DRANCO-plant Brecht II: Six year overview of input in reactor and biogas production
The average biogas production in the Dranco plant in Brecht kept increasing for six years in a row up to 126.3 m³ per tonne fed. The reactor started in 2000 with a gas production rate of 5.8 m³ biogas per m³ of active digester volume to reach up to 7.4 as an average annual rate in 2006. Peak weekly production rates of up to 10–11 were obtained.
Future perspective
The impressive growth-curve up to 2005 seems to stabilize. In the period 2006–2010, about the same number of plants per year (11) will be constructed as during the period of 2001–2005, with a slightly smaller average size however. Further progress in capacity extension is expected mainly by new MSW-plants and the revamping of existing composting plants for biowaste and residual waste digestion. A new trend is the use of technologies that were developed for solid waste digestion, for the recovery of biogas from energy crops. Both horizontal and vertical dry digestion techniques for solid waste have been applied on a large scale for the digestion of energy crops, as these technologies are less sensitive to the presence of heavies and light materials in the feedstock.


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Extraction DRANCO-digester at the bottom of the conical outlet
The high prices for oil and for agricultural crops together with a decreasing capacity for landfilling will keep stimulating the growth of anaerobic digestion. On the other hand, the main barrier will remain the higher investment cost of AD in comparison to aerobic in-vessel-composting units. Other factors hampering the expansion of AD, but also biological treatment in general, are the stringent hygiene and air emission requirements.
Batch systems are being developed to reduce the initial investment and operating costs. Several different suppliers have developed new ways to reduce the risks and uncertainties associated with batch operation, and are constructing and implementing new plants. The application of batch digestion has so far been limited to biowaste digestion.
Conclusion
The number of plants treating the organic fraction of household waste in Europe has grown from 3 in 1990, to 62 in 2000, to up to more than 170 plants that will be installed by the end of 2010. The digestion capacity of more than 5,000,000 tonne per year can handle almost 3% of the OFMSW produced in Europe by 2010. This may seem small, but represents 20%–30% of the biological treatment capacity for organics derived from household waste.


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Horizontal digestion plant with recovery of waste heat for the drying of wood chips
The basic technology is proven and well implemented, but still improvements are under way in terms of optimization and pre- and post-treatment of the end-products. Companies are making substantial efforts to find markets for the AD-technology and new applications for biogas. This results in new countries implementing this technology and an increase in capacity in the countries that already have some digestion experience. Some serious breakthroughs in the field of upgrading could mean another stepping stone for further implementation of biogas in Europe.
The most important advantage remains the fact that anaerobic digestion allows the recovery of renewable energy from organic waste. The current dramatic rise in energy prices and the need for renewable energy from a greenhouse gas perspective will only stimulate further implementation of digestion technology for the treatment of organic solid waste, and replace conventional aerobic treatment of organics more and more.
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Anaerobic digestion of solid waste
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