Anaerobic digestion (AD) has, for some time, been considered an important technology in the treatment of waste and in the development of energy recovery solutions. Historically, many anaerobic digestion plants have tended to specialize in the treatment of manure or sludge. In today’s market, the latest AD plants have to handle more complex substrates and varying volume streams. As a result, the demands placed on this technology in terms of reliability, stability and robustness are significant.
Also significant is the potential contribution AD could make to solving our most pressing environmental concern namely a reduction in the anthropogenic emission of greenhouse gases (GHG). AD technology can reduce unwanted and uncontrolled emissions of methane by tapping the energy potential of this gas while reducing the volume of waste going to landfill. How can this potential be realized?The role of AD technology in today’s market
Anaerobic digestion is a biochemical process where, in the absence of oxygen, bacteria break down organic matter to produce biogas plus a liquor and a fibre.
The biogas consists of 55%70% methane (CH4) and 30%45% carbon dioxide (CO2) and can be used to generate energy through a generator the energy content of biogas is 2025 MJ/standard m3. Alternatively, the gas can be cleaned and then either compressed for use in vehicle transport (compressed natural gas) or injected into the gas distribution network. An average CH4 yield per metric tonne of treated waste (sludge, manure) ranges from 5090 Nm3 per tonne and for municipal solid waste (MSW) the yield increases to 75120 Nm3 per tonne. The liquid fraction, with a high nutrient content, and the fibre fraction can be used as a soil improver.
As mentioned earlier, older applications of this technology tended to focus on the treatment of sewage sludge and agricultural manures. More modern plants have been developed to process MSW, industrial solid wastes and industrial wastewaters, but impurities and the varying content of lipids, proteins and carbohydrates can cause problems. As a result, a pre-treatment step, involving removal of impurities and particle size reduction, is needed to allow stable digestion performance.The substrate is key
Wastes of the type mentioned above can be characterized according to their COD concentration. COD refers to the total quantity of oxygen required for oxidation to carbon dioxide and water and is a measure of the organic content of the waste.
Systems have been invented to process substrates with a minor COD concentration (<25gO2/litre raw material), for example:upflow anaerobic sludge blanket (UASB) expanded granular sludge blanket (EGSB) internal circulation (IC).
A completely stirred tank reactor (CSTR) or plug-flow-tank reactor (PFTR) is used to treat substrate with a high COD concentration, and high content of fat, lipids and particles. Such fermenters tend to operate with low loading rates when compared with the systems developed for minor COD concentration in order to guarantee complete anaerobic digestion. COD loading rate is the daily quantity of organic matter, expressed in COD, feed per m³ digester volume per day i.e. kg COD/m3/d.
One of the important goals for industry has been to develop a technology to treat both large volume streams and complex high concentrated substrates. And one company believes that it has solved this puzzle.The Enbasys approach
Enbasys, based in Grambach, Austria, has developed a technology called a High Load Hybrid Reactor that is designed to handle large volume treatment of different organic waste and biofuel residue streams, which include complex substrates. The technology unifies a classical digester system (high total solids and COD content of the substrate) with high performance digesters (UASB). The combination of process, fermenter and mixing technology makes it possible to process various kinds of organic residues. With a loading rate of ≥15 kg COD/m³ fermenter/d, Enbasys believes that it possesses a sharp differentiation to traditional biogas plants.
It highlights the advantages of its system as follows:
Its technology was first installed in north Italy in 2005 and has been operating continuously for three years. The plant in Italy consists of two 2900 m3 fermenters which process 120,000 tonnes of pre-treated organic waste per year. This results in a loading rate of 12 to 15 kg COD/m3/d from which 1250 Nm3/h biogas is produced. The biogas is used to run a combined heat and power (CHP) plant with 3.0 MWel. Studies conducted by Enbasys for this article of 33 MSW anaerobic digestion plants worldwide, built since 2004 or still under construction, have shown that the average fermenter volume is about 6220m3 converting 54,900 tonnes MSW into biogas. This equates to 9 tpa (tonnes per annum) per m3 fermenter volume. The Enbasys plant in Italy processes 21 tpa pre-treated organic waste per m3 fermenter volume, giving a higher system efficiency (specific waste throughput) than conventional plants.Click here to enlarge image
The High Load Hybrid Reactor technology represents the culmination of more than 10 years of experience in design, engineering, building, operation and research in the field of anaerobic digestion technology. To spread this new generation technology internationally the inventors needed a strong partner, which they found in VTU Holding. Enbasys is a subsidiary of VTU Holding. Contracts for the foundation of Enbasys GmbH were signed in August 2007.
Two views of the fermenter plant Click here to enlarge image
In January 2008 BDI BioDiesel International AG acquired an interest of more than 25% in VTU Holding GmbH. BDI BioDiesel is one of the world’s leading suppliers of complete biodiesel production plants, with expertise in a range of raw materials, such as vegetable oils, waste edible oils and animal fats. Enbasys GmbH therefore benefits from the long established engineering expertise of VTU and the know-how of BDI in international plant supply and construction.The development of AD in Europe
It is perhaps not surprising that new companies specializing in AD are emerging in the European market, where there has been active political interest in promoting industries that link waste treatment with renewable energy. In countries such as Italy (Certificati Verde, i.e. green certificates) or Germany (EEG renewable energy law) there is now scope for technology suppliers to operate within a regulatory framework that makes AD cost effective, with high gate fees for wastes and special premium prices paid for the production of renewable energy. Developments in the UK market exemplify current interest and are summarized in Box 1. In the face of Europe’s increasing dependency on fossil fuels, using biomass is one of the key ways of ensuring the security of supply and sustainable energy in Europe. More detail on the legislative developments can be found in Box 2.
In April the UK Environment Agency (EA) and WRAP (Waste & Resources Action Plan) published a draft protocol on anaerobic digestion. The document is out for consultation until 27 June. ‘The quality protocol for the production and use of quality outputs from anaerobic digestion of source-segregated biodegradable waste’ aims to inspire confidence in the products made from waste in order to develop a market for these products, encourage increased recovery and recycling and thereby divert material from landfill. Among the main obstacles to increased development of AD facilities are uncertainty over what constitutes waste and the fact that the products of anaerobic digestion whole digestate, the separated fibre fraction and the separated liquor are still classed in the UK as waste. This has hindered the development of a market for materials produced from waste, and has also adversely affected recycling rates and diversion from landfill. The introduction and acceptance of a quality protocol are seen by many in the industry as key steps in its development.
The protocol is part of the Waste Protocols Project, a joint initiative between the EA and WRAP in collaboration with industry, and funded by the UK government. The aim of the Waste Protocols Project is to identify the point at which materials cease to be waste and are, therefore, not subject to waste regulation controls. In March 2007 the EA published the UK’s first quality protocol for compost, which sets out the criteria for the production of a quality compost from source-segregated biodegradable waste. The new protocol will do the same for anaerobic digestion.
The thirty-page quality protocol sets out criteria for the production of useable by-products of the anaerobic digestion process. If all the criteria are met, the ‘quality outputs’, as they are called, will be considered to have been fully recovered and therefore no longer waste when it is despatched to a customer. The quality protocol also sets out strict guidelines on the input materials that can be used in the digestion process and the uses to which the resulting products can be put. These ‘designated market sectors’ are agriculture and forestry (excluding horticulture), and land restoration. The quality digestate can be used as a soil improver or fertilizer, for soil manufacture/blending and for land reclamation processes such as contouring, reconstruction, re-vegetation and so on. The protocol qualifies this further by stating that if it cannot be demonstrated that the digestate will be used in one of the designated market sectors, then the AD products will be classed as waste.
Interested parties have until 27 June to respond to 25 questions relating to the technology, an approved composting standard (PAS 110), and the financial impact of the protocol, which is expected to be implemented on 1 September. A pdf of the quality protocol can be downloaded at http://qpyr1.dialoguebydesign.net/docs/QP_AnaerobicDigestion.pdf
The UK government’s Waste Strategy 2007 identified anaerobic digestion as an effective way of recycling food waste and proposed further support for the process via the New Technologies Demonstrator Programme (NTDP). And, more recently, in February this year the government announced an increase in funding for clean energy technologies to over £400 million ($788 million) over the next three years including £10 million ($20 million) for a new anaerobic digestion demonstration programme. Furthermore, funding for AD facilities is also offered via the Capital Grant Programme managed by WRAP and the Welsh Assembly’s Materials Action Programme (MAP), which can fund up to 30% of capital set-up costs.
Sarah Wisson, Waste Management WorldBox 2. An evolving regulatory framework for AD technology suppliers in Europe
The EU policy on sustainable energy dates back to a Council resolution in 1986 to develop new and renewable energy sources.
In 1997 a ‘White Paper on renewable energy sources’ was published by the European Commission. This set a target of 12% of gross inland energy consumption from renewables for the EU-15 by 2010, of which electricity would represent 22.1%. With the 2004 enlargement, the EU’s overall objective increased to 21%.
The EU has committed to reducing its overall GHG emissions to at least 20% below 1990 levels by 2020. And the climate action and renewable energy package published earlier this year sets out sustainability criteria that biofuels must meet to ensure they deliver real environmental benefits.
In the waste sector the EU landfill directive (1999/31/EC) requires the reduction of biodegradable municipal waste going to landfills to 75% by 2006, 50% by 2009 and 35% by 2016 calculated on the basis of the total amount of biodegradable municipal waste produced in 1995.
In February 2007, the European parliament adopted a resolution on a revised Waste Directive. In April 2008, the Environment Committee of the European Parliament voted on this Directive (see comment on page 8).
Further national regulations include strategies and limiting values intended to promote successful and sustainable operation of biowaste treatment facilities.
Biomass currently meets 4% of the European Union’s energy needs that is 69 million metric tonnes of oil equivalents (toe). According to the European Commission’s Biomass Action Plan this should be increased by 2010 to 150 million tonnes of oil equivalents.
The plant is designed to handle large volumes of organic waste and biofuel residues Click here to enlarge image
An increase of this magnitude could bring the following benefits: diversifying Europe’s energy supply; significantly reducing greenhouse gas emissions (209 million tonnes); direct employment for 250 to 300,000 people; and potentially lowering the price of oil as a result of lower demand.
The digester at the Enbasys plant Click here to enlarge image
The majority of municipal waste in the EU is currently disposed of through landfill (49%), followed by incineration (18%) and recycling and composting (33%). In the new Member States the situation is evolving rapidly but landfill is still the main destination for MSW. According to the European Environment Agency, 30%40% of the MSW produced across Europe could be used for AD. In 2005 Europe (EU-25) produced 120 million tonnes of waste that could have been digested anaerobically, which would have produced about 9000 million m3 of biogas or 4.6 million tonnes of oil equivalents.What lies ahead?
There are a number of factors that will give rise to greater interest in technologies such as AD. These include:
The most practical environmental solution will be deriving energy from waste, not only municipal solid waste but also the residues industry. Anaerobic digestion has significant potential for industries with organic waste streams, such as food processing, the paper and textile industry, pharmaceutical industry and biofuel production. Anaerobic digestion combines several advantages. As a technology it can be regarded as being ‘CO2 neutral’ because there is no net addition of CO2 to the atmosphere. The Enbasys High Load Hybrid Reactor could be the key to developing the potential of AD as a waste management strategy. It degrades waste while producing biogas and a fertilizer product that contains a high nutrient content (nitrogen, phosphorus and potassium), but in order for the full potential of the waste/organic substrate/input to be realized, it is vital that the waste management industry is able to develop markets for all the by-products.
Gertrud Aichberger is Project Manager for Enbasys