Ultrasonic Pretreatment in Anaerobic Digestion

Anaerobic digestion process comprises of four major steps – hydrolysis, acidogenesis, acetogenesis and methanogenesis. The biological hydrolysis is the rate limiting step and pretreatment of sludge by chemical, mechanical or thermal disintegration can improve the anaerobic digestion process. Ultrasonic disintegration is a method for breakup of microbial cells to extract intracellular material.

Ultrasound activated sludge disintegration could positively affect sludge anaerobic digestion. Due to sludge disintegration, organic compounds are transferred from the sludge solids into the aqueous phase resulting in an enhanced biodegradability. Therefore disintegration of sewage sludge is a promising method to enhance anaerobic digestion rates and lead to reduce the volume of sludge digesters.

The addition of disintegrated surplus activated sludge and/or foam to the process of sludge anaerobic digestion can lead to markedly better effects of sludge handling at wastewater treatment plants. In the case of disintegrated activated sludge and/or foam addition to the process of anaerobic digestion it is possible to achieve an even twice a higher production of biogas. Here are few examples:

STP Bad Bramstedt, Germany (4.49 MGD)

  • First fundamental study on pilot scale by Technical University of Hamburg-Harburg, 3 years, 1997 – 1999
  • reduction in digestion time from 20 to 4 days without losses in degradation efficiency
  • increase in biogas production by a factor of 4
  • reduction of digested sludge mass of 25%

STP Ahrensburg, Germany (2.64 MGD)

  • Preliminary test on pilot-scale by Technical University of Hamburg-Harburg, 6 months, 1999
  • increase in VS destruction of 20%
  • increase in biogas production of 20%

STP Bamberg, Germany (12.15 MGD)

  • Preliminary full-scale test, 4 months, 2002 2) Full-scale installation since June 2004
  • increase in VS destruction of 30%
  • increase in biogas production of 30%
  • avoided the construction of a new anaerobic digester

STP Freising, Germany (6.87 MGD)

  • Fundamental full-scale study by University of Armed Forces, Munich, 4 months, 2003
  • increase in biogas production of 15%
  • improved sludge dewatering of 10%

STP Meldorf, Germany (1.06 MGD)

  • Preliminary full-scale test, 3 months, 2004 2) Full-scale installation since December 2004
  • increase in VS destruction of 25%
  • increase in biogas production of 25%
  • no foam or filamentous organisms present in the anaerobic sludge digester

STP Ergolz 2, Switzerland (3.43 MGD)

  • Full-scale test, 3 months, 2004
  • increase in VS destruction of 15%
  • increase in biogas production of 25%

STP Beverungen, Germany (2.64 MGD)

  • Full-scale test, 3 months, 2004/2005
  • increase in VS destruction of 25%
  • increase in biogas production of 25%

To sum up, ultrasonication has a positive effect on sludge solubilisation, sludge volume, biogas production, flock size reduction and cells lyses. Ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better degradation of volatile solids and an increased production of biogas.

The use of low power ultrasound in bioreactors may present a significant improvement in cost reduction. Therefore, ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better sludge digestion and efficient recovery of valuables.

Anaerobic Digestion of Tannery Wastes

The conventional leather tanning technology is highly polluting as it produces large amounts of organic and chemical pollutants. Wastes generated by tanneries pose a major challenge to the environment. Anaerobic digestion of tannery wastes is an attractive method to recover energy from tannery wastes.

According to conservative estimates, more than 600,000 tons per year of solid waste are generated worldwide by leather industry and approximately 40–50% of the hides are lost to shavings and trimmings. Everyday a huge quantity of solid waste, including trimmings of finished leather, shaving dusts, hair, fleshing, trimming of raw hides and skins, are being produced from the industries. Chromium, sulphur, oils and noxious gas (methane, ammonia, and hydrogen sulphide) are the elements of liquid, gas and solid waste of tannery industries.

Biogas from Tannery Wastes

Anaerobic digestion (or biomethanation) systems are mature and proven processes that have the potential to convert tannery wastes into energy efficiently, and achieve the goals of pollution prevention/reduction, elimination of uncontrolled methane emissions and odour, recovery of biomass energy potential as biogas, production of stabilized residue for use as low grade fertilizer.

Anaerobic digestion of tannery wastes is an attractive method to recover energy from tannery wastes. This method degrades a substantial part of the organic matter contained in the sludge and tannery solid wastes, generating valuable biogas, contributing to alleviate the environmental problem, giving time to set-up more sustainable treatment and disposal routes. Digested solid waste is biologically stabilized and can be reused in agriculture.

Until now, biogas generation from tannery wastewater was considered that the complexity of the waste water stream originating from tanneries in combination with the presence of chroming would result in the poisoning of the process in a high loaded anaerobic reactor.

When the locally available industrial wastewater treatment plant is not provided by anaerobic digester, a large scale digestion can be planned in regions accommodating a big cluster of tanneries, if there is enough waste to make the facility economically attractive.

In this circumstance, an anaerobic co-digestion plant based on sludge and tanneries may be a recommendable option, which reduces the quantity of landfilled waste and recovers its energy potential. It can also incorporate any other domestic, industrial or agricultural wastes. Chrome-free digested tannery sludge also has a definite value as a fertilizer based on its nutrient content.

Potential Applications of Biogas

Biogas produced in anaerobic digesters consists of methane (50%–80%), carbon dioxide (20%–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide.  Biogas can be used for producing electricity and heat, as a natural gas substitute and also a transportation fuel. A combined heat and power plant (CHP) not only generates power but also produces heat for in-house requirements to maintain desired temperature level in the digester during cold season.

CHP systems cover a range of technologies but indicative energy outputs per m3 of biogas are approximately 1.7 kWh electricity and 2.5kWh heat. The combined production of electricity and heat is highly desirable because it displaces non-renewable energy demand elsewhere and therefore reduces the amount of carbon dioxide released into the atmosphere.

AD Plant at ECCO’s Tannery (Netherlands)

A highly advanced wastewater treatment plant and biogas system became fully operational in 2012 at ECCO’s tannery in the Netherlands. A large percentage of the waste is piped directly into the wastewater plant to be converted into biogas. This biogas digester provides a source of renewable fuel and also helps to dispose of waste materials by converting waste from both the leather-making processes, and the wastewater treatment plant, into biogas. All excess organic material from the hides is also converted into biogas.

This project enables ECCO Tannery to reduce waste and to substitute virtually all of its consumption of non-renewable natural gas with renewable biogas. The aim is to use more than 40% of the total tannery waste and replace up to 60% of the total natural gas consumption with biogas.

Biogas Prospects in Rural Areas: Perspectives

Biogas, sometimes called renewable natural gas, could be part of the solution for providing people in rural areas with reliable, clean and cheap energy. In fact, it could provide various benefits beyond clean fuel as well, including improved sanitation, health and environmental sustainability.

What Is Biogas?

Biogas is the high calorific value gas produced by anaerobic decomposition of organic wastes. Biogas can come from a variety of sources including organic fraction of MSW, animal wastes, poultry litter, crop residues, food waste, sewage and organic industrial effluents. Biogas can be used to produce electricity, for heating, for lighting and to power vehicles.

Using manure for energy might seem unappealing, but you don’t burn the organic matter directly. Instead, you burn the methane gas it produces, which is odorless and clean burning.

Biogas Prospects in Rural Areas

Biogas finds wide application in all parts of the world, but it could be especially useful to developing countries, especially in rural areas. People that live in these places likely already use a form of biomass energy — burning wood. Using wood fires for heat, light and cooking releases large amounts of greenhouse gases into the atmosphere.

The smoke they release also has harmful health impacts, particularly when used indoors. You also need a lot to burn a lot of wood when it’s your primary energy source. Collecting this wood is a time-consuming and sometimes difficult as well as dangerous task.

Many of these same communities that rely on wood fires, however, also have an abundant supply of another fuel source. They just need the tools to capture and use it. Many of these have a lot of dung from livestock and lack sanitation equipment. This lack of sanitation creates health hazards.

Turning that waste into biogas could solve both the energy problem and the sanitation problem. Creating a biogas system for a rural home is much simpler than building other types of systems. It requires an airtight pit lined and covered with concrete and a way to feed waste from animals and latrines into the pit. Because the pit is sealed, the waste will decompose quickly, releasing methane.

This methane flows through a PCV pipe to the home where you can turn it on and light on when you need to use it. This system also produces manure that is free of pathogens, which farmers can use as fertilizer.

A similar but larger setup can provide similar benefits for urban areas in developing countries and elsewhere.

Benefits of Biogas

Anaerobic digestion systems are beneficial to developing countries because they are low-cost compared to other technologies, low-tech, low-maintenance and safe. They provide reliable fuel as well as improved public health and sanitation. Also, they save people the labor of collecting large amounts of firewood, freeing them up to do other activities. Thus, biomass-based energy systems can help in rural development.

Biogas also has environmental benefits. It reduces the need to burn wood fires, which helps to slow deforestation and eliminates the emissions those fires would have produced. On average, a single home biogas system can replace approximately 4.5 tons of firewood annually and eliminate the associated four tons of annual greenhouse gas emissions, according to the World Wildlife Fund.

Biogas is also a clean, renewable energy source and reduces the need for fossil fuels. Chemically, biogas is the same as natural gas. Biogas, however, is a renewable fuel source, while natural gas is a fossil fuel. The methane in organic wastes would release into the atmosphere through natural processes if left alone, while the greenhouse gases in natural gas would stay trapped underground. Using biogas as a fuel source reduces the amount of methane released by matter decomposing out in the open.

What Can We Do?

Although biogas systems cost less than some other technologies, affording them is often still a challenge for low-income families in developing countries, especially in villages. Many of these families need financial and technical assistance to build them. Both governments and non-governmental organizations can step in to help in this area.

Once people do have biogas systems in place though, with minimal maintenance of the system, they can live healthier, more comfortable lives, while also reducing their impacts on the environment.

Food Waste Management in UK

Food-Waste-UKFood waste in the United Kingdom is a matter of serious environmental, economic and social concern that has been attracting widespread attention in recent years. According to ‘Feeding the 5K’ organisation, 13,000 slices of crusts are thrown away every day by a single sandwich factory which is featured in the figure above. More recently, Tesco, one of the largest UK food retailers, has published its sustainability report admitting that the company generated 28,500 tonnes of food waste in the first six months of 2013. TESCO’s report also state that 47% of the bakery produced is wasted. In terms of GHG emissions, DEFRA estimated that food waste is associated with 20 Mt of CO2 equivalent/year, which is equivalent to 3% of the total annual GHG emissions.

Globally, 1.2 to 2 billion tonnes (30%-50%) of food produced is thrown away before it reaches a human stomach. Food waste, if conceived as a state, is responsible for 3.3 Bt-CO2 equivalent/year, which would make it the third biggest carbon emitter after China and USA. What makes food waste an even more significant issue is the substantially high demand for food which is estimated to grow 70% by 2050 due to the dramatic increase of population which is expected to reach 9.5 billion by 2075. Therefore, there is an urgent need to address food waste as a globally challenging issue which should be considered and tackled by sustainable initiatives.

A War on Food Waste

The overarching consensus to tackle the food waste issue has led to the implementation of various policies. For instance, the European Landfill Directive (1999/31/EC) set targets to reduce organic waste disposed to landfill in 2020 to 35% of that disposed in 1995 (EC 1999). More recently, the European Parliament discussed a proposal to “apply radical measures” to halve food waste by 2025 and to designate the 2014 year as “the European Year Against Food Waste”. In the light of IMechE’s report (2013), the United Nations Environment Programme (UNEP) in cooperation with FAO has launched the Save Food Initiative in an attempt to reduce food waste generated in the global scale.

In the UK, WRAP declared a war on food waste by expanding its organic waste programme in 2008 which was primarily designed to “establish the most cost-effective and environmentally sustainable ways of diverting household food waste from landfill that leads to the production of a saleable product”. DEFRA has also identified food waste as a “priority waste stream” in order to achieve better waste management performance. In addition to governmental policies, various voluntary schemes have been introduced by local authorities such as Nottingham Declaration which aims to cut local CO2 emissions 60% by 2050.

Sustainable Food Waste Management

Engineering has introduced numerous technologies to deal with food waste. Many studies have been carried out to examine the environmental and socio-economic impacts of food waste management options. This article covers the two most preferable options; anaerobic digestion and composting.

In-vessel composting (IVC) is a well-established technology which is widely used to treat food waste aerobically and convert it into a valuable fertilizer. IVC is considered a sustainable option because it helps by reducing the amount of food waste landfilled. Hence, complying with the EU regulations, and producing a saleable product avoiding the use of natural resources. IVC is considered an environmentally favourable technology compared with other conventional options (i.e. landfill and incineration). It contributes less than 0.06% to the national greenhouse gas inventories. However, considering its high energy-intensive collection activities, the overall environmental performance is “relatively poor”.

Anaerobic Digestion (AD) is a leading technology which has had a rapidly growing market over the last few years. AD is a biologically natural process in which micro-organisms anaerobically break down food waste and producing biogas which can be used for both Combined Heat & Power (CHP) and digestate that can be used as soil fertilizers or conditioners. AD has been considered as the “best option” for food waste treatment. Therefore, governmental and financial support has been given to expand AD in the UK.

AD is not only a food waste treatment technology, but also a renewable source of energy. For instance, It is expected that AD would help the UK to meet the target of supplying 15% of its energy from renewable sources by 2020. Furthermore, AD technology has the potential to boost the UK economy by providing 35,000 new jobs if the technology is adopted nationally to process food waste. This economic growth will significantly improve the quality of life among potential beneficiaries and thus all sustainability elements are considered.

Methods for Hydrogen Sulphide Removal from Biogas

The major contaminant in biogas is H2S which is both poisonous and corrosive, and causes significant damage to piping, equipment and instrumentation. The concentration of various components of biogas has an impact on its ultimate end use. While boilers can withstand concentrations of H2S up to 1000 ppm, and relatively low pressures, internal combustion engines operate best when H2S is maintained below 100 ppm. The commonly used methods for hydrogen sulphide removal from biogas are internal to the anaerobic digestion process – air/oxygen dosing to digester biogas and iron chloride dosing to digester slurry.

Biological Desulphurization

Biological desulphurization of biogas can be performed by using micro-organisms. Most of the sulphide oxidising micro-organisms belong to the family of Thiobacillus. For the microbiological oxidation of sulphide it is essential to add stoichiometric amounts of oxygen to the biogas. Depending on the concentration of hydrogen sulphide this corresponds to 2 to 6 % air in biogas.

The simplest method of desulphurization is the addition of oxygen or air directly into the digester or in a storage tank serving at the same time as gas holder. Thiobacilli are ubiquitous and thus systems do not require inoculation. They grow on the surface of the digestate, which offers the necessary micro-aerophilic surface and at the same time the necessary nutrients. They form yellow clusters of sulphur. Depending on the temperature, the reaction time, the amount and place of the air added the hydrogen sulphide concentration can be reduced by 95 % to less than 50 ppm.

Measures of safety have to be taken to avoid overdosing of air in case of pump failures. Biogas in air is explosive in the range of 6 to 12 %, depending on the methane content). In steel digesters without rust protection there is a small risk of corrosion at the gas/liquid interface.

Iron Chloride Dosing

Iron chloride can be fed directly to the digester slurry or to the feed substrate in a pre-storage tank. Iron chloride then reacts with produced hydrogen sulphide and form iron sulphide salt (particles). This method is extremely effective in reducing high hydrogen sulphide levels but less effective in attaining a low and stable level of hydrogen sulphide in the range of vehicle fuel demands.

In this respect the method with iron chloride dosing to digester slurry can only be regarded as a partial removal process in order to avoid corrosion in the rest of the upgrading process equipment. The method need to be complemented with a final removal down to about 10 ppm.

The investment cost for such a removal process is limited since the only investment needed is a storage tank for iron chloride solution and a dosing pump. On the other hand the operational cost will be high due to the prime cost for iron chloride.

Biogas-to-Biomethane Conversion Technologies

biogas-biomethaneRaw biogas contains approximately 30-45% of CO2, and some H2S and other compounds that have to be removed prior to utilization as natural gas, CNG or LNG replacement. Removing these components can be performed by several biogas upgrading techniques. Each process has its own advantages and disadvantages, depending on the biogas origin, composition and geographical orientation of the plant. The biogas-to-biomethane conversion technologies taken into account are pressurized water scrubbing (PWS), catalytic absorption/amine wash (CA), pressure swing absorption (PSA), highly selective membrane separation (MS) and cryogenic liquefaction (CL) which are the most common used biogas cleanup techniques.

The Table below shows a comparison of performance for these techniques at 8 bar (grid) injection.

Table:  Comparison of performance for various upgrading techniques (result at 8 bar) (Robert Lems, 2010) , (Lems R., 2012)

  PWS CA PSA MS CL Unit
Produced gas quality*2 98 99 97-99 99 99.5 CH4%
Methane slip 1 0.1-0.2 1-3 0.3-0.5 0.5 %
Electrical use 0.23-0.25 0.15-0.18 0.25 0.21-0.24 0.35 kWh/Nm3 feed
Thermal energy use 0,82-1.3 kWth/Nm3 prod.
Reliability / up time 96 94 94 98 94 %
Turn down ratio 50-100 50-100 85-100 0-100 75-100 %
CAPEX Medium Medium Medium Low High  
Operation cost Low Medium Medium Low High  
Foot print Large Large Medium Small Large  
Maintenance needed Medium Medium+ Medium+ Low High  
Ease of operation Medium Medium+ Medium Easy Complex  
Consumables &

waste streams

AC*3/Water AC*3/amines AC*3/ absorbents AC*3/None AC*3/None  
References Many Many Medium Medium Very few  

*2 If no oxygen of nitrogen is present in the raw biogas

*3 Activated carbon (AC) consumption is depending on the presence of certain pollutants (trace components) within the raw biogas.

From the above Table, it can be concluded that the differences between technologies with respect to performance seem to be relatively small. However, some “soft factors” can have a significant impact on technology selection. For example, water scrubber technology is a broadly applied technology. The requirement for clean process water, to make up for discharge and condensation, could be a challenging constraint for remote locations.

Moreover, PWS systems are prone to biological contamination (resulting in clogged packing media and foaming), especially when operated at elevated temperatures. Without additional preventative measures this will result in an increase of operational issues and downtime.

Amine scrubbers are a good choice when surplus heat is available for the regeneration of the washing liquid. The transport and discharge of this washing liquid could however be a burden, as well as the added complexity of operation. With respect to cryogenic Liquefaction (CL) one may conclude that, this technology has a questionable track-record, is highly complex, hard to operate, and should therefore not be selected for small-medium scale applications.

Both PSA and MS provide a “dry” system, both technologies operate without the requirement for a solvent/washing liquid, which significantly simplifies operation and maintenance. Distinctive factor between these technologies is that the membrane based system operates in a continuous mode, while the PSA technology is based on columns filled with absorption materials which operate in a rotating/non-continuous mode.

Moreover, the membrane based system has a more favourable methane slip, energy consumption and turndown ratio. The biggest advantage over PSA however, is that membrane systems do not require any transport of absorbents, its ease of operation and superior up-time.

Main disadvantage of membrane systems are that they are sensitive to pollution by organic compounds, which can decrease efficiency. However, by applying a proper pre-treatment (generally based on activated carbon and condensation) in which these compounds are eliminated, this disadvantage can be relatively easy nullified.

Based on membrane technology, DMT Environmental Technology, developed the Carborex ®MS. A cost-effective plug and play, containerized (and therefore), easy to build in remote locations) biogas upgrading system. The Carborex ®MS membrane system has relatively little mechanical moving components (compared to other upgrading technologies) and therefore, ensures stability of biomethane production, and consequently, the viability of the biogas plant operation.

Moreover, its design for ease of operation and robustness makes this technological platform perfectly suitable for operation at locations with limited experience and expertise on handling of biogas plants.

Impression of a membrane system; Carborex ®MS – by courtesy of DMT

Impression of a membrane system; Carborex ®MS – by courtesy of DMT

Conclusions

Capture of biogas through application of closed ponds or AD’s is not only a necessity for mitigation of greenhouse gas emissions, it is also a method of optimizing liquid waste treatment and methane recovery. Billions of cubic meters of biomethane can be produced on a yearly basis, facilitating a significant reduction of fossil fuel dependency.

Moreover, upgrading of raw biogas-to-biomethane (grid, CNG or LNG quality) provides additional utilization routes that have the extra advantage to be independent of existing infrastructure. To sum up, membrane based technology is the best way forward due to its ease of operation, robustness and the high quality of the end-products.

References

  • Lems R., D. E. (2012). Next generation biogas upgrading using high selective gas separation membranes. 17th European Biosolids Organic Resources Conference. Leeds: Aqua Enviro Technology .
  • Robert Lems, E. D. (2010). Making pressurized water scrubbing the ultimate biogas upgrading technology with the DMT TS-PWS® system. Energy from Biomass and Waste UK . London: EBW-UK .

Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)

Note: This is the final article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland). The first two articles can be viewed at these links

http://www.bioenergyconsult.com/biomethane-utilization/

http://www.bioenergyconsult.com/pome-biogas/

Biomethane Utilization Pathways

biomethane-transportBiogas can be used in raw (without removal of CO2) or in upgraded form. The main function of upgrading biogas is the removal of CO2 (to increase the energy content) and H2S (to reduce risk of corrosion). After upgrading, biogas possesses identical gas quality properties as  natural gas, and can thus be used as natural gas replacement. The main pathways for biomethane utilization are as follows:

  • Production of heat and/or steam
  • Electricity production / combined heat and power production (CHP)
  • Natural gas replacement (gas grid injection)
  • Compressed natural gas (CNG) & diesel replacement – (bio-CNG for transport fuel usage)
  • Liquid natural gas (LNG) replacement – (bio-LNG for transport fuel usage)

Prior to practically all utilization options, the biogas has to be dried (usually through application of a cooling/condensation step). Furthermore, elements such as hydrogen sulphide and other harmful trace elements must be removed (usually trough application of an activated carbon filter) to prevent adverse effects on downstream processing equipment (such as compressors, piping, boilers and CHP systems).

Although biogas is perfectly suitable to be utilized in boilers (as an environmental friendlier source for heat and steam production), this option is rather obsolete due to the abundance of alternative sources from solid waste origin.

Most Palm Oil Mills are already self-reliant with respect to heat and steam production due to the combustion of their solid waste streams (such as EFB and PKS). Consequently, conversion to electricity (by means of a CHP unit) or utilization as natural gas, CNG or LNG replacement, would be a more sensible solution.

The biogas masterplan as drafted by the Asia Pacific Biogas Alliance foresees a distribution in which 30% of the biomethane is used for power generation, 40% for grid injection and 30% as compressed/liquefied fuel for transportation purpose (Asian Pacific Biogas Alliance, 2015).

For each project, the most optimal option has to be evaluated on a case to case basis. Main decision-making factors will be local energy prices and requirements, available infrastructure (for gas and electricity), incentives and funding.

For the locations where local demand is exceeded, and no electricity or gas infrastructure is available within a reasonable distance (<5-10 km, due to investment cost and power loss), production of CNG could offer a good solution.

Moreover, during the utilization of biogas within a CHP unit only 40-50% of the energetic content of the gas is converted into electricity. The rest of the energy is transformed into heat. For those locations where an abundance of heat is available, such as Palm Oil Mills, this effectively means that 50-60% of the energetic content of the biogas is not utilized. Converting the biogas into biomethane (of gas grid or CNG quality) through upgrading, would facilitate the transportation and commercialisation of over 95%  of the energetic content of the biogas.

Within the CNG utilization route, the raw biogas will be upgraded to a methane content of >96%, compressed to 250 bar and stored in racks with gas bottles. The buffered gas (bottles) will be suitable for transportation by truck or ship. For transportation over large distances (>200km), it will be advised to further reduce the gas volume by converting the gas to LNG (trough liquefaction).

Overall the effects and benefits from anaerobic digestion of POME and utilization of biomethane can be summarized as follows:

  • Reduction of emissions i.e. GHG methane and CO2
  • Reduced land use for POME treatment
  • Enhanced self-sufficiency trough availability of on-site diesel replacement (CNG)
  • Expansion of economic activities/generation of additional revenues
    • Sales of surplus electricity (local or to the grid)
    • Sales of biomethane (injection into the natural gas grid)
    • Replacement of on-site diesel usage by CNG
    • Sales of bottled CNG
  • Reducing global and local environmental impact (through fuel replacement)
  • Reducing dependence on fossil fuel, and enhances fuel diversity and security of energy supply
  • Enhancement of local infrastructure and employment
    • Through electrical and gas supply
    • Through Fuel (CNG) supply

Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)

Note: This is the second article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland). The first article can be viewed at this link

Biogas Upgradation Methods

Upgradation of biogas is primarily achieved by carbon dioxide removal which then enhances the energy value of the gas to give longer, driving distances with a fixed gas storage volume. Removal of carbon dioxide also provides a consistent gas quality with respect to energy value. The latter is regarded to be of great importance from the vehicle manufacturers in order to reach low emissions of nitrogen oxide.

At present four different methods are commercially used for removal of carbon dioxide from biogas, either to reach vehicle fuel standard or to reach natural gas quality for injection to the natural gas grid. These methods are:

  • Water absorption
  • Polyethylene glycol absorption
  • Carbon molecular sieves
  • Membrane separation

Water Scrubbing

Water scrubbing is used to remove carbon dioxide but also hydrogen sulphide from biogas since these gases is more soluble in water than methane. The absorption process is purely physical. Usually the biogas is pressurized and fed to the bottom of a packed column where water is fed on the top and so the absorption process is operated counter-currently.

Polyethylene Glycol Scrubbing

Polyethylene glycol scrubbing is a physical absorption process. Selexol is one of the trade names used for a solvent. In this solvent, like in water, both carbon dioxide and hydrogen sulphide are more soluble than methane.

The big difference between water and Selexol is that carbon dioxide and hydrogen sulphide are more soluble in Selexol which results in a lower solvent demand and reduced pumping. In addition, water and halogenated hydrocarbons (contaminants in biogas from landfills) are removed when scrubbing biogas with Selexol.

Carbon Molecular Sieves

Molecular sieves are excellent products to separate specifically a number of different gaseous compounds in biogas. Thereby the molecules are usually loosely adsorbed in the cavities of the carbon sieve but not irreversibly bound. The selectivity of adsorption is achieved by different mesh sizes and/or application of different gas pressures.

When the pressure is released the compounds extracted from the biogas are desorbed. The process is therefore often called “pressure swing adsorption” (PSA). To enrich methane from biogas the molecular sieve is applied which is produced from coke rich in pores in the micrometer range. The pores are then further reduced by cracking of the hydrocarbons. In order to reduce the energy consumption for gas compression, a series of vessels are linked together.

Pressure swing adsoprtion process for biogas upgradation

The gas pressure released from one vessel is subsequently used by the others. Usually four vessels in a row are used filled with molecular sieve which removes at the same time CO2 and water vapour.

Membrane Purification

There are two basic systems of biogas purification with membranes: a high pressure gas separation with gas phases on both sides of the membrane, and a low-pressure gas liquid absorption separation where a liquid absorbs the molecules diffusing through the membrane.

  • High pressure gas separation

Pressurized gas (36 bar) is first cleaned over for example an activated carbon bed to remove (halogenated) hydrocarbons and hydrogen sulphide from the raw gas as well as oil vapour from the compressors. The carbon bed is followed by a particle filter and a heater. The raw gas is upgraded in 3 stages to a clean gas with 96 % methane or more.

The waste gas from the first two stages is recycled and the methane can be recovered. The waste gas from stage 3 (and in part of stage 2) is flared or used in a steam boiler as it still contains 10 to 20 % methane.

  • Gas-liquid absorption membranes

Gas-liquid absorption using membranes is a separation technique which was developed for biogas upgrading in the recent past. The essential element is a micro-porous hydrophobic membrane separating the gaseous from the liquid phase. The molecules from the gas stream, flowing in one direction, which are able to diffuse through the membrane will be absorbed on the other side by the liquid flowing in counter current.

The absorption membranes work at approx. atmospheric pressure (1 bar) which allows low-cost construction. The removal of gaseous components is very efficient. At a temperature of 25 to 35°C the H2S concentration in the raw gas of 2 % is reduced to less than 250 ppm.

POME as a Source of Biomethane

POME-BiogasDuring the production of crude palm oil, large amount of waste and by-products are generated. The solid waste streams consist of empty fruit bunch (EFB), mesocarp fruit fibers (MF) and palm kernel shells (PKS). Reuse of these waste streams in applications for heat, steam, compost and to lesser extent power generation are practised widely across Southeast Asia. POME or Palm Oil Mill Effluent is an underutilized liquid waste stream from palm oil mills which is generated during the palm oil extraction/decanting process and often seen as a serious environmental issue but it is a very good source for biomethane production. Therefore, discharge of POME is subject to increasingly stringent regulations in many palm oil-producing nations.

Anaerobic Digestion of POME

POME is an attractive feedstock for biomethane production and is abundantly available in all palm oil mills. Hence, it ensures continuous supply of substrates at no or low cost for biogas production, positioning it as a great potential source for biomethane production. (Chin May Ji, 2013).

POME is a colloidal suspension containing 95-96% water, 0.6-0.7% oil and 4-5% total solids, which include 2-4% suspended solids. Biological Oxygen Demand (BOD) generally ranges between 25,000 and 65,714 mg/L, Chemical Oxygen Demand (COD) ranges between 44,300 and 102,696 mg/L.

Most palm oil mills and refineries have their own treatment systems for POME, which is easily amenable to biodegradation due to its high organic content. The treatment system usually consists of anaerobic and aerobic ponds. (Sulaiman, 2013).

Open pond systems are still commonly applied. Although relatively cheap to install, these system often fail to meet discharge requirements (due to lack of operational control, long retention time, silting and short circuiting issues).

Moreover, the biogas produced during the anaerobic decomposition of POME in open pond systems is not recovered for utilization. The produced gas dissipates into the atmosphere where it causes adverse environment effects (due to the fact that CH4 is a twenty times stronger greenhouse gas then CO2 (Chin May Ji, 2013).

Biogas capture from POME can be carried out using a number of various technologies ranging in cost and complexity. The closed-tank anaerobic digester system with continuous stirred-tank reactor (CSTR), the methane fermentation system employing special microorganisms and the reversible flow anaerobic baffled reactor (RABR) system are among the technologies offered by technology providers. (Malaysian Palm Oil Board, 2015).

Biogas production largely depends on the method deployed for biomass conversion and capture of the biogas, and can, therefore, approximately range from 5.8 to 12.75 kg of CH4 per cubic meter of POME. Application of enclosed anaerobic digestion will significantly increase the quality of the effluent/ discharge stream as well as the biogas composition, as mentioned in table below.

 Table: Performance comparison between open and closed digester systems

Parameters Open digester system Closed anaerobic digester
COD removal efficiency (%) 81% 97%
HRT (days) 20 10
Methane utilization Released to atmosphere Recoverable
Methane yield (kg CH4/kg COD removed) 0.11 0.2
Methane content (%) 36 55
Solid discharge (g/L) 20 8

*This table has been reproduced from (Alawi Sulaiman, 2007)

A closed anaerobic system is capable of producing and collecting consistently high quality of methane rich biogas from POME. Typical raw biogas composition will be: 50-60 % CH4, 40-50 % CO2, saturated with water and with trace amounts of contaminants (H2S, NH3, volatiles, etc.).

Biomethane Potential in Southeast Asia

The amount of biomethane (defined as methane produced from biomass, with properties close to natural gas) that can be potentially produced from POME (within the Southeast Asian region) exceeds 2.25 billion cubic meter of biomethane (on a yearly basis).

Especially Indonesia and Malaysia, as key producers within the palm oil industry, could generate significant quantities of biomethane. An impression of the biomethane potential of these countries including other feedstock sources is being highlighted below (VIV Asia, 2015).

Indonesia (4.35 billion m3 of biomethane):

  • 25 billion m3 of biomethane from Palm Oil Mill Effluent (POME).
  • 2 billion m3 of bio-methane from Sewage Treatment Plant (STP).
  • 9 billion m3 of bio-methane from Municipal Solid Waste (MSW).

Malaysia (3 billion m3 of biomethane):

  • 1 billion m3 of biomethane from Palm Oil Mill Effluent (POME).
  • 2 billion m3 of biomethane from Sewage Treatment Plant (STP).
  • 8 billion m3 of biomethane from Municipal Solid Waste (MSW).

The Asian Pacific Biogas Alliance estimates that the potential of conversion of biomass to biomethane is sufficient to replace 25 percent of the natural gas demand by renewable biogas (Asian Pacific Biogas Alliance, 2015).

To sum up, due to the high fraction of organic materials, POME has a large energetic potential. By unlocking the energetic potential of these streams through conversion/ digesting and capture of biomethane, plant owners have the opportunity to combine waste management with a profitable business model.

Co-Authors: H. Dekker and E.H.M. Dirkse (DMT Environmental Technology)

References

Alawi Sulaiman, Z. B. (2007). Biomethane production from pal oil mill effluent (POME) in a semi-commercial closed anaerobic digester. Seminar on Sustainable Palm Biomass initiatives. Japan Society on Promotion of Science (JSPS).

Asia Biogas Group. (2015, 08 15). Retrieved from Asia Biogas : http://www.asiabiogas.com

Asian Pacific Biogas Alliance. (2015). Biogas Opportunities in South East Asia. Asian Pacific Biogas Alliance/ICESN.

Chin May Ji, P. P. (2013). Biogas from palm oil mill effluent (POME): Opportunities and challenges from Malysia’s perspective. Renewable and Sustainable Energy Reviews , 717-726.

Malaysian Palm Oil Board. (2015, 08 26). Biogas capture and CMD project implementation for palm oil mills. Retrieved from Official Portal Of Malaysian Palm Oild Board:

Sulaiman, N. A. (2013). The Oil Palm Wastes in Malaysia. In M. D. Matovic, “Biomass Now – Sustainable Growth and Use”. InTech.

VIV Asia. (2015, 08 26). The international platform from feed to food in Asia. Retrieved from http://www.vivasia.nl

Note: This is the first article in the special series on ‘Sustainable Utilization of POME-based Biomethane’ by Langerak et al of DMT Environmental Technology (Holland)

Food Waste Management

The waste management hierarchy suggests that reduce, reuse and recycling should always be given preference in a typical waste management system. However, these options cannot be applied uniformly for all kinds of wastes. For examples, food waste is quite difficult to deal with using the conventional 3R strategy.

Of the different types of organic wastes available, food waste holds the highest potential in terms of economic exploitation as it contains high amount of carbon and can be efficiently converted into biogas and organic fertilizer.

There are numerous places which are the sources of large amounts of food waste and hence a proper food waste management strategy needs to be devised for them to make sure that either they are disposed off in a safe manner or utilized efficiently. These places include hotels, restaurants, malls, residential societies, college/school/office canteens, religious mass cooking places, communal kitchens, airline caterers, food and meat processing industries and vegetable markets which generate food residuals of considerable quantum on a daily basis.

anaerobic_digestion_plant

The anaerobic digestion technology is highly apt in dealing with the chronic problem of food waste management in urban societies. Although the technology is commercially viable in the longer run, the high initial capital cost is a major hurdle towards its proliferation.

The onus is on the governments to create awareness and promote such technologies in a sustainable manner. At the same time, entrepreneurs, non-governmental organizations and environmental agencies should also take inspiration from successful food waste-to-energy projects in Western countries and try to set up such facilities in cities and towns.