A biography of biogas

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As energy prices rise, so does the status of a humble gas. Gord Cope finds that in the current environment, biogas comes up smelling of roses.

It is fair to say that visions of financial gain and environmental acclaim do not immediately pop into the heads of most observers standing downwind of a biological waste treatment plant.

But for movers and shakers in large sectors of the global economy, this is indeed the case, for the biogas that wafts in the breeze is not only proving its potential to lower operational costs for a wide array of municipal, industrial and agricultural concerns – it is also helping to mitigate climate change.

Biogas is a mixture containing 60% methane, with carbon dioxide, water vapour and trace elements (such as hydrogen sulphide, which contributes the eye-watering element to emanations) composing the rest. It is formed naturally in swamps in the absence of oxygen, where anaerobic bacteria decompose organic compounds.

It also occurs in wastewater treatment plants, where organic matter from humans, animals and industrial processes (think potato peelings from chip manufacturers) are placed in specially designed anaerobic digesters, where it decomposes under controlled conditions.

For much of civilisation, human-made biogas was allowed to disperse into the atmosphere, much to the annoyance of anyone standing nearby. The Chinese were the first to capture the gas for cooking purposes, although the emergence of widespread, commercial-scale facilities did not occur until the 20th century. Today, a multi-million dollar sector has evolved to engineer, build, operate and maintain biogas facilities.

CH2M Hill, Black & Veatch and Synagro are major engineering firms involved in the sector. System vendors include Biothane and Siemens of the US, Paques of the Netherlands, GWE of Germany, and ADI of Canada. A multitude of further vendors supply storage tanks (for example EIMCO), and internal combustion combined heat and power (IC CHP) engines that run on biogas (Caterpillar, Waukesha).

Paques has been designing anaerobic digesters for almost four decades. “We are one of the innovators,” says Aafko Scheringa, sales manager for North and South America and South Africa. Paques, which is privately owned, does not reveal annual revenues, but its 400 global staff oversee the installation of a dozen new facilities every year (a further dozen or so facilities are installed through licensees of their patented line of digesters). “We now have 1,000 units all over the world,” he states.

Why bother with biogas?
The reasons behind the growth in the biogas sector have evolved over time. “When Paques started in the Netherlands, space was a driver,” says Scheringa. “Aerobic digestion takes a lot of space in comparison to anaerobic digestion.” Traditionally, most municipal wastewater treatment plants have used aerobic digestion, which relies on large, open tanks into which air is injected as huge paddles churn biological sludge in order for bacteria to break down larger chain molecules into smaller, less volatile solids.

Unfortunately, it smells, and generates 50% more bulk solids than enter the system, creating disposal headaches. Anaerobic digesters, on the other hand, have a much smaller footprint and the end product is less bulky. To sewage system designers, anaerobic digestion offers a much more elegant and efficient solution to waste.

Anaerobic digestion also presents many environmental advantages. “You can generate energy from the breakdown of biosolids, and that energy is becoming more and more valuable,” says Paul Sellew, a principal at anaerobic digestion consultants Sellew & Associates, based in Massachusetts. “You are also reducing greenhouse gases and benefiting climate change, and because it doesn’t generate odours, you are being a good neighbour.”

In addition, international agreements on climate change, such as the Kyoto Protocol, contain incentives that reward those who capture fugitive emissions of methane (a very potent greenhouse gas), and turn it into CO2 (which is more benign). “There are many types of engines that burn biogas to make electricity, and boilers and turbines also accept it,” says Gary Martin, a product manager for anaerobic solutions with Siemens Water Technologies. “Biogas is eligible for renewable energy credits and carbon credits.” Aerobic digestion also requires twice as much energy as anaerobic digestion. “You have two bonuses there; you use less energy and get some biogas back.”

Big belchers
There are three main sectors that account for the vast majority of anaerobic digestion installations: municipal, dairy and industrial. About one-third are associated with municipal sewage treatment plants. In a typical set-up, wastewater passes through screens to remove larger objects, and is then pumped to primary treatment tanks.

As suspended solids settle, they are transferred to sludge tanks, where they are treated in anaerobic digesters for 25 days at 35ºC.

Anaerobic digestion reduces the weight of the resultant sludge by 50%, which is a distinct advantage when disposing of the solids in a landfill, or by incineration. Because it has been largely neutralised of harmful bacteria and molecules, the discharge can also be distributed to farmers as a soil conditioner and fertiliser.

While European countries generally have more anaerobic digesters installed at municipal sites than North America, the overall number is only a small fraction of aerobic digesters. The reason is primarily due to economies of scale. The supply and installation of a modern anaerobic digestion module can easily exceed USD10 million, and one therefore needs a sufficiently large population within the plant’s catchment basin to justify the cost.

Currently, the threshold population size is a function of the amount of biogas produced (which offsets energy purchases and creates a payback time on the original investment). One pound of solid human waste produces about 8 cubic feet of biogas. Because biogas is only 60% methane, it produces less energy (about 600 BTU per cubic foot) than pure natural gas (1,000 BTU per cubic foot) when burned.

Nonetheless, the amount of solid waste flowing through a treatment plant is large (about 70 lbs per capita per annum). Currently, anaerobic digesters pay back the original investment within a reasonable timeframe when the facility’s input population exceeds approximately 200,000; while this means that most rural systems (the majority by number) do not qualify, most urban systems (which handle the vast majority of wastewater in North America), do.

Calgary, Canada, with a population of one million, is a typical example of where anaerobic digestion works well. The city’s Bonnybrook wastewater treatment plant handles most municipal sewage, receiving some 600,000m3 every day. For the last 27 years, Bonnybrook has been capturing biogas and converting it to energy. Currently, the plant has a bank of 12 anaerobic digesters that produce almost 17 million m3 of gas annually; this is then converted into heat and electricity using an IC CHP.

The engine generators can cost up to USD1 million and generate 1MW from 160 cubic feet per minute of biogas. In 2007, Bonnybrook produced 9.4 million kWh of electricity and recovered 13 million kWh of heat energy.

“All the power is used on-site,” says Andy Dutton, Bonnybrook’s head of engineering. “We don’t have methane storage, so we sometimes have to flare the excess gas. We are looking at the latest equipment.”

EIMCO Water Technologies manufactures covers that fit over anaerobic digesters and capture excess biogas. The float covers are prefabricated from steel and assembled on site to create a large, upside down canister. As the biogas is produced, the canister rises on external, vertical rails from a surrounding ditch. The covers can be up to 80 feet in diameter, and cost approximately USD200,000. “We have hundreds of digester float covers in North America,” says Donna Morano, sales manager for EIMCO. “They are mostly for municipal plants.”

Industrial innovation
According to a survey conducted by Applied Technologies, a Wisconsinbased engineering consultancy, there are approximately 2,000 large-scale anaerobic digesters installed in breweries, beverage manufacturers, food processors and pulp and paper mills around the world. “Industrial plants like soft drinks and breweries have high strength output,” says Dennis Totzke, vice president of Applied Technologies. “They’ve been on the (anaerobic digester) bandwagon for 20 years.”

When it comes to anaerobic digestion, industrial plants have numerous advantages over sewage facilities. Unlike municipal treatment plants, many industrial plants have fluid streams in which the BOD (biological oxygen demand, an indicator of biological load), is dissolved; concentrations of BOD can often exceed 5,000 ppm, 25 times that encountered in sewage wastewater.

Industrial plants often have a choice in the way they dispose of their wastewater streams. If they discharge to municipal waste systems, they pay a surcharge based on flow and BOD. If the surcharge is low, it is more economical to send it untreated to the sewage plant. If the surcharge is high, then onsite treatment is more economical. Industrial plants can also exploit the energy and environmental advantages.

“They are looking for a reduction in operating costs, with energy payback,” says Totzke. “Also, they get carbon credits that other people will buy. Companies realise that they will someday face CO2 reduction costs, and these credits are a lot cheaper now.”

A big brewery may generate up to 5 million gallons per day (18,925m3/d) of wastewater, with 3,000 ppm of BOD. The fluid goes to a clarifier for pretreatment, and then into a continuous flow anaerobic digestion system.

Each digester unit produces 500-700 cubic feet per minute. The biogas can be sent to boilers and burned there directly or to IC CHP engines that power generators to produce electricity. Electrical generation can range from 500 KW up to 10MW per installation. A large digester/power system can cost USD10 million, but it can produce as much as USD2.5 million per year of power, as well as an 80% reduction in municipal sewage treatment surcharges, which can more than offset the cost of operating and maintaining the plant.

“You do a feasibility study and look at the volume of sludge and the biogas potential,” says Totzke. “The payback time is site-specific, depending on costs of construction and local codes.”

Bovine bowels
Anaerobic digestion is also making major inroads into a third sector: dairy farming.

Paul Sellew recently helped design and install an anaerobic digester at a 15,000-cow dairy operation in California. “A cow produces about 120 lb of liquid waste a day, 10% of which is solids,” he explains. “That’s about 180,000 lbs of solid waste a day for the farm.”

Each cow’s waste produces about 100 cubic feet of biogas per day, for a total of 150,000 cubic feet per day, or around USD500,000 worth of gas annually. The gas is run through an IC CHP to create heat and electricity. “The result is they generate all their own power, and even have some electricity to sell back to the grid,” says Sellew. “They can also add vegetable oils and greases from food processing residuals to increase the volatile solids and the methane output.”

Because the origin is 100% manure, it also qualifies for a further financial benefit. “If you have organic waste that is source-separated, then subjected to anaerobic digestion and then aerobic digestion, you end up with a clean source of organic fertiliser,” says Sellew. “It has been naturally pasteurised and is much less odorous. It’s clean and pesticide free. Depending on the location, it can sell for up to USD100 per ton.”

Gunning for growth
Growth rates within the anaerobic digestion sector vary, depending on the target market. Although municipalities represent the largest producers of organic waste, most potential is concentrated in larger urban centres, where demand is already well served.

“Our business is expanding at the rate of population growth, as well as the movement of people into big cities,” says Jan Brandkvist, a project manager at CH2M Hill’s Toronto office.

“We see growth in Vancouver, Toronto and Calgary. Europe is not so busy right now, but Asia is very busy.”

“We will see steady progress,” agrees Siemens’ Martin. “Anyone building a municipal plant right now is looking at anaerobic digestion as an option.”

With 40 million cattle in North America alone, much potential lies within the food animal industry. “The agricultural market is still wide open,” says Totzke. “Any operator with over 2,000 head of cattle can benefit.”

The industrial market varies quite a bit between different sectors. “Anheuser Busch (a major brewer) has anaerobic digestion at eight of its nine plants,” says Totzke. “Nestlé, on the other hand, may have 10 at 5,000 plants.”

The industrial market also varies by region. “South America is very active these days,” says Totzke. “You open a soda plant or a brewery, and they have anaerobic digestion.”

Surging industrialisation in Asia is also having an impact. “We have 200 staff in Shanghai,” says Scheringa. “China is a large consumer of food and beverages and pulp and paper products.”

In addition to the economic and environmental advantages, manufacturers in developing countries also see a benefit in energy reliability. “In many countries, there is a shortage of electricity production, and factories are often cut off from it,” says Martin. “There are opportunities out there to make 5-10MW plants – plenty to cover much of their needs.”

Climate change
The future of anaerobic digestion could be guided to a large extent by legislation. Most current laws regarding municipal and industrial wastewater treatment focus on fluid discharge, ensuring that water re-entering rivers and lakes is sufficiently clean. Laws governing gaseous emanations, on the other hand, are less stringent.

“Open lagoons of organic waste are allowable in the US,” says Sellew. Few expect this state of affairs to remain for long, however; both major candidates in the US presidential election favour some form of control over GHG emissions. Rather than impose restrictive rules, however, participants within the sector expect that a cap-and-trade system, similar to that in place within the EU, will spur the development of anaerobic digestion systems in order to create valuable carbon credits.

“Already, credits are being marketed on the voluntary CO2 market in Chicago, and we have clients doing so,” says Totzke. “If you see GHG legislation emerge, you could see a boom.”

In conclusion, most participants in the sector are optimistic about the future of anaerobic digestion. “I’m bullish,” says Sellew. “We’re seeing increasing energy costs, increasing concern for GHGs and an increasing desire to be a good neighbour. Everyone is adopting sustainable business practices.”

Biogas – the facts

* Biogas is a mixture of methane (60%), carbon dioxide, water vapour and trace elements. It occurs in waste treatment plants where organic matter from humans, animals and industrial processes is placed in specially designed anaerobic digesters, where it decomposes under controlled conditions.

* Anaerobic digestion is a multi-million-dollar global market. Major participants include engineering firms CH2M Hill, Black & Veatch and Synagro. System vendors include Biothane and Siemens of the US, Paques of the Netherlands, GWE of Germany, and ADI of Canada. Caterpillar and Waukesha build engines that run on biogas.

* Municipal wastewater, dairy farms and industrial plants with large biological waste loads (breweries, food manufacturers, pulp & paper) account for the majority of anaerobic installations worldwide.

* The cost of a large anaerobic digester and associated power modules can exceed USD10 million. The biogas produced can offset up to USD2.5 million in energy costs annually.

* The solid waste from one cow can produce 100 cubic feet of biogas a day; a human can produce about 1.5 cubic feet.

* In North America and Europe, the sector is growing at the same rate as the population is increasing. China, South-East Asia and South America are growing more rapidly as the food and beverage industries expand and the population moves to large urban centres.

* Germany leads all countries in biogas generation, with 3,750 gas plants capable of generating up to 1,280MW of electricity. The Könnern plant in East Germany will be the world’s largest biogas plant when it comes onstream in 2009, producing 30 million m3 of biogas a year (which will be purified and injected into the natural gas distribution grid).

* Climate change legislation in various jurisdictions rewards the capture and energy conversion of biogas with carbon credits. Adoption of similar legislation in North America is expected to add momentum to the sector.