Aerobic versus anaerobic treatment
When compared to other alternatives for treating industrial effluents, suchas aerobic effluent treatment, anaerobic processes have significant environmental and financial advantages for achieving industrial compliance.
In aerobic treatment processes, such as the Activated Sludge process, Sequencing Batch Reactors (SBRs) or Biological Aerated Filters, aerobic microoganisms metabolise the polluting organic compounds (measured as COD) to generate more microorganisms which must be then disposed of as a troublesome sludge.
At a time when disposal routes for sludges (popularly called biosolids) are under increasing competition from other sources, such as sewage sludge treatment and industrial processes, the number of options are also diminishing. For example, sludges can no longer be sent to landfill or be disposed of at sea, and there are tight restrictions on land application in order to protect the food chain. In addition, aerobic processes are very expensive in terms of the electricity needed for aeration with prices increasing as fuel prices rise.
While equivalent anaerobic processes can exhibit a larger footprint, they generate little, if any excess sludge and do not require expensive aeration. Where the COD density is sufficient to generate excess biogas (>5,000-7,000mg/l) after heating the effluent to process temperature (35-37ºC), there are opportunities to generate and export electricity in CHP sets, or to use the excess biogas as a boiler fuel.
As a result anaerobic processes, their characteristics and instrumentation needs are now part of a well established portfolio of effluent treatment alternatives for industry as well as for the water industry worldwide.
The anaerobic digestion process has also been the focus of attention for dedicated renewable energy applications. Not simply as a staple for the treatment of industrial effluents or sewage, but as a stand alone energy generating alternative. Early examples included tractors powered by biogas during the second world war, to generating a substitute natural gas for cooking and lighting at rural scale as in India, Nepal and China.
In the early 1980s there was significant research into generating biogas from crops grown specifically for the purpose of producing energy. There is now a substantial database of biogas potential from so called ‘energy crops’, or ‘catch crops’, grown in between the harvest and planting of food crops. Generally, the activity remained in the research stage in Europe and the US for many years until the economic drivers shifted for commercial implementation, although there was one notable installation in Disneyworld, where water hyacinth was harvested to generate biogas.
While the environmental argument has always been clear, that biogas is a viable and sustainable way to generate renewable energy with a wide range of secondary benefits, such as pollution reduction, odour reduction and the generation of a fertiliser or peat replacement, the economic drivers have taken much longer to materialise.
Sceptics still argue that renewable energy technologies are still not economically viable, and that they can only contribute to the global energy solution through significant economic subsidy. However, the opposite is also true. Most of our existing fossil fuel and atomic based electricity generating technologies do not compare well with renewable sources if you take into consideration the economic implication of using them, e.g. nuclear waste disposal; Green House Gas (GHG) emissions; climate change and pollution.
It was perhaps one of the most ambitious of economic instruments in this sector that led to the most recent trend for increased implementation of biogas systems across Europe, and that was the Renewable Energy Sources Act (EEG) implemented in Germany on April 1, 2000. Very attractive incentives were offered including substantial grants, low interest loans for the remaining investment, subsidised long term electricity purchase rates, that led to an almost exponential increase in biogas plants installed to generate energy from animal wastes, crop wastes, crops and other biomass sources. Some farmers even gave up farming to generate energy from biogas plants, because the financial rewards were so attractive.
Some years later, the German biogas sector is still very active, the rest of Europe is fast catching up with renewable energy plans published across Europe, and even the UK is finally incorporating biogas solutions.
Although the UK has boasted several centres of excellence for the research into biogas technologies for many years, for a long time it has remained one of the European countries with the least number of biogas plants per head of population. The UK is only now beginning to see the potential benefits from biogas systems and the incentives needed to secure them2. Interestingly, many of the major companies offering biogas services at full scale in the UK have developed from the German market, as a result of the ground breaking incentives there. In spite of this, the largest biogas plants in Europe can only generate 4 MWe of power3. In other parts of the world the biogas revolution has also been in full sway.
Warm climate digestion
One of the major issues with the anaerobic digestion process is that it functions best at two temperature optima, mesophilic temperatures (35-37°C) and thermophilic temperatures (55-57°C). The process can operate at lower temperatures (psychrophilic temperatures), but the size of the bioreactors needed increases as the temperature drops (the rate of reaction doubles for each 10°C rise in temperature).
The most common compromise by far is to operate digesters at body temperature (mesophilic), but as a typical rule of thumb for, say, pig waste digesters in Europe and North America, about one third of all the biogas produced is required to heat the incoming waste to the digester. Of course some of this heat can be recovered from the effluent stream and waste heat from CHP sets can also be used, but this only increases the capital costs of the plant, lengthening payback times and reducing the interest of investors and further opportunities for implementation, unless the incentives are right.
As a result, digester designs are often limited to shapes which reduce heat loss (low surface area to volume ratio, with a height to width ratio of 1:1) and must be insulated. These are most often steel or concrete cylindrical tanks with fixed or flexible roofs.
Undoubtedly, operating in a climate that is already at ambient mesophilic temperatures is not only ideal for biogas applications, but allows a much wider and more cost effective range of bioreactor designs to be implemented. While digester designs can still be optimised for use as advanced bioreactors, less expensive designs can be used.
Of course much of the world has ambient temperatures in this range for much of the year including Thailand, Vietnam, South China, South and Central America, Burma, India, Malaysia or Indonesia. Most of these countries are considered to be ‘developing’ and exhibit a unique range of characteristics which are attractive for industrial development and biogas applications.
Indeed, large sections of global manufacturing capacity have been relocated to developing countries where labour and development costs are low, and environmental and other regulations are much less stringent than in Europe and the United States. In fact it could be argued that by elevating our environmental and social standards in the west, we have become unattractive for siting new factories. In addition, there are unique agricultural products which can only be grown in these countries, such as tapioca starch, rubber, ethanol, palm oil and sugar cane.
Local regulations often favour employment rather than environmental considerations and control on water use is often limited. As a result, effluent streams from many industrial sectors are often between 1,000-15,000m3/d. Similarly, existing practice often involves direct discharge of effluent to open lagoons, so effluent concentration is poorly controlled and COD levels are often as high as 10,000 – 250,000 mg/l. This has resulted in the installation of some of the largest biogas systems in the world.
In one typical example, at Sanguan Wongse Industries (SWI), a 750t/d tapioca starch plant in Thailand, the first in a generation of new CDM compliant biogas plants was constructed in 2004. Due to the high ambient temperatures, it was possible to abandon the traditional tank design and install an uninsulated covered lagoon digester based on a 100m x 100m x 10m deep (100,000m3 volume) earthen supported lagoon. The biogas plant is covered with a high density polyethylene (HDPE) flexible liner and designed to treat up to 8,000m3/d effluent at an average COD of 27,000mg/l.
The system can generate as much as 80,000Nm3/d biogas (at approx 60% methane). This is equivalent to 7MWe of electrical generation capacity, but in fact most of the biogas is used in six industrial boilers with the remainder used to generate 3MWe power, which is exported to the grid. This is a significant increase in scale in comparison to European and North American systems.
Flexible liner reactors
One of the reasons for the success of this application is the ability to deploy appropriate biogas technologies such as flexible liner reactors (FLR). These were pioneered in Taiwan in the 1970s where it was found that a cheap rubber material could be manufactured from a by-product of the local alumina industry. ‘Red Mud Plastic’, as it was called, could be used to cover simple concrete trenches resulting in simple but efficient and cost effective pig waste digesters used to reduce odour, pollution and to generate biogas.
In the late 1970s researchers at Cornell University, USA, took this concept a stage further in an effort to design a northern climate flexible liner reactor using an insulated earthen lagoon, or submerged concrete structure, with a range of flexible materials such as Hypalon, XR5 and Butyl rubber. This spawned a range of similar developments over the years in the US, Canada and Asia, such that these systems are now commonplace in warmer climates and almost always use HDPE as the flexible material of choice. This is relatively inexpensive, tried and tested as a geotextile for lining landfills, and also one of the least methane permeable membranes available.
In addition to the obvious financial benefits of renewable energy generation at this scale, the SWI project also reduced annual emissions of methane equivalent to 314,000 tonnes of CO2e which routinely occurred from 72 lagoons which were used to treat the effluent before the digester was installed. At a typical price of €10/tonne carbon dioxide equivalent this is an additional revenue stream of more than €3m per year. Like the German Renewable Energy Act in Europe, the generation of carbon credits is one of the major reasons for the revolution of biogas plants in Asia.
The key driver in Asia was the 1997 Kyoto Protocol, which reflected general global concern about the need to reduce the emissions of GHGs into the environment, setting binding targets for all 41 industrialised countries which signed, and later ratified, the agreement (so called Annex 1 countries). Perhaps most notably the US refused to sign the agreement for as long as India and China did not make formal commitments to reduce emissions, and many have seen this as an effort to extend trade protectionism which has wrankled with the global environmental community ever since. The main feature within the Protocol which was of relevance to biogas development in Asia was the Clean Development Mechanism (CDM)4.
While Annex 1 countries would make every effort to reduce carbon emissions to the levels agreed for each country by the end of 2012, it was accepted that there would be diminishing returns and the final savings would be more difficult and more expensive to accomplish. The CDM mechanism addressed this by allowing Annex 1 countries to purchase carbon savings (in the form of Carbon Credits or CERs – Certificates of Emissions Reduction, each equivalent to 1 tonne of CO2) made in developing, non Annex 1 countries, to offset against their national targets. This was to have multiple benefits in terms of technology transfer to developing countries, which would then be in a better position to make their own savings in the next round of the Kyoto Protocol, as well as financing development projects.
Like the German Renewable Energy Laws in Europe, the CDM mechanism has proved a major influence on the development of the biogas sector in non Annex 1 countries and can typically add 30% or more economic benefit in practice to any given biogas project. In fact, according to complex rules established by the governing body (the UNFCCC), each project is supposed to be marginal, prior to the receipt of CDM support. In practice, these rules (known as the test for ‘additionality’) have been widely flouted with many highly profitable projects receiving additional CDM funding. In many countries the largest renewable energy sector receiving carbon credits by far is the biogas sector.
One of the main reasons for this is that biogas projects not only displace fossil fuels when the renewable energy is used (as with wind, solar or thermal energy), but they also mitigate methane emissions that would have otherwise been discharged directly into the atmosphere. Since methane has a global warming potential (GWP) of 21, e.g. one tonne of methane is equivalent to 21 tonnes CO2e, 21 CERs are generated for each tonne of methane mitigated.
That’s about 111Nm3 biogas for each CER. With projects typically generating between 30,000-80,000Nm3/d biogas that’s a major incentive.
In the SWI example, the industrial effluent was being discharged into 72 lagoons where anaerobic processes were naturally taking place and emitting methane to the atmosphere. By using one covered lagoon with a rigorous bioreactor design (feed distribution, recycle, biogas collection) a reduction of 90% or more of the COD, and therefore biogas potential, can be achieved in one reactor. This results in a very attractive system and it is no surprise that many such biogas systems are now installed in Thailand, Indonesia, India, Malaysia and China, with new markets opening in Vietnam, Cambodia and Laos.
The CDM process is increasingly rigorous, but with ever more conservative features being introduced each year, and fewer projects passing the strict rules laid out. If the SWI project was registered today it would qualify for less than half the number of carbon credits awarded in 2006. However, the financial benefit depends on the value of a CER, which has varied widely from around €5-20 depending on the stage of the project with a current value of around €12.70.
The formal market is set to finish at the end of 2012 due to continued lack of agreement on the establishment of future binding emissions targets after the failure of the Copenhagen summit in 2010. However, a limited number of CERs can be traded into the European Trading Scheme (ETS), and there is an increasing ‘voluntary’ market where lower priced VERs (voluntary emissions reductions, about 20% of the price), which are subject to less stringent regulations, can still be generated. These are often purchased for CSR (corporate social responsibility) purposes by boards wishing to offset the carbon footprint of the corporate travel budget.
One interesting spin-off from the CDM process has been the rigorous nature of the annual auditing or ‘verification’ process. Plans to monitor the plant must be put in place with approved equipment and standards. In many cases this has led to the implementation and installation of some of the highest quality instrumentation and laboratory equipment for the first time in this sector.
The results of COD analyses are rigorously checked, as well as meter calibration certificates and liquid and gas flow meter readings. Methane, CO2 and Oxygen analysers are now routinely installed depending on the configuration, and with installation costs to generate and export electricity at almost US $1m per MWe the stakes are high. This has presented a significant opportunity for European and US instrument manufacturers where accurate instrumentation is a precursor to securing CERs. Markets that would normally compromise on quality over price now have no choice but to install the most accurate instrumentation.
While 2012 is said to be the death knell for the CDM market, short of any last minute agreement within the global community there are new opportunities. Ironically, the mad rush to generate Ethanol from maize (in the US), cassava (SE Asia) and molasses (similar) to use as a renewable vehicle fuel, has presented further opportunities for biogas applications. Most of the new generation of Ethanol plants in China, Vietnam and Thailand, for example, incorporate biogas plants with at least one of these capable of generating more than 10 MWe of electrical energy.
At the same time, the new generation of high solids digesters from Europe are also beginning to find favour in SE Asia with applications for pineapple waste, cassava wet cake or palm press cake.
Biogas developments will no doubt continue with new reactors, new substrates and new applications. The relentless increase in fossil fuel prices is already breaching cost thresholds and opening opportunities for purifying and bottling methane derived from biogas for direct use as vehicle fuel or for grid exported gas. Biogas systems have become a staple of the renewable energy sector and are here to stay.
1. http://www.aebiom.org/IMG/pdf/Brochure_BiogasRoadmap_WEB.pdf 2. http://www.adbiogas.co.uk/ta_content.asp?id=43 3. http://cleantech.com/news/1470/schmack-to-build-biggest-biogas-plant- 4. http://cdm.unfccc.int/faq/index.html
Dr Stephen P Etheridge, Co-founder and CEO of Biotrix
Dr Stephen Etheridge has a PhD in Anerobic Digestion Design and has worked in the sector for more than 30 years in Europe, US, China and SE Asia. He co-founded the biogas company Biotrix in 2010 (www.biogas.asia) and has designed and constructed some of the largest industrial and agricultural biogas plants. He was a biogas advisor for the DTIs Biowise programme (1992-2004) and advisor for China’s National Biogas Action Plan developed under a UN GEF programme. He was one of the original authors of the first Biogas CDM methodology AM022 for the SWI project, and he also acts as a verifier for biogas projects in SE Asia.
Published: 01st Mar 2012 in AWE International