December 04, 2014
In urban environments small scale anaerobic digesters are providing an innovative solution to recycling food and garden waste—with the added benefit of generating energy and producing fertiliser.
Most people are familiar with composting where aerobic bacteria in the heap produce heat, turning organic materials such as manure, sewage, food and garden waste into fertiliser in the form of compost, and carbon dioxide. To go a step further, however, and produce usable biogas, requires the strength and corrosion resistance of nickel-containing stainless steels.
If organic materials are put into an airless tank (instead, for instance, into a garden compost pile exposed to the air), anaerobic bacteria will create fertiliser (technically called ‘digestate’) and combustible biogas, a mixture of carbon dioxide and methane, with trace impurities of othergases such as hydrogen sulphide. The tank and all the equipment needed to feed, mix and heat the material is known as an anaerobic digester and the process is known as anaerobic digestion or AD. AD happens naturally in many places, including the bottoms of ponds and the stomachs of cows—which is why cows belch and pass methane.
Land and energy productivity
The modern, and ultimately unsustainable, ‘linear’ method of food production is underpinned by fossil fuel energy. Food is grown using fossil fertilisers, then processed, transported and, if unused, disposed of to landfill. This results in a flow of fertiliser (nutrients) from rural areas to cities and landfill sites. The challenge for a more sustainable ‘circular’ economy is how to effectively return nutrients to the land in order to grow more food and thus break, or at least minimise, the extraction-to-landfill process. Anaerobic digestion can help do this by effectively recycling the nutrients back to land and reducing fossil fertiliser use, as well as producing renewable biogas for heating, electricity creation and vehicle fuel.
For a number of reasons, including space constraints, it may not be practical to build large anaerobic digesters within many cities. However small on-site AD units can be used to recycle food and non-woody garden waste produced by families, communities, schools, hotels, hospitals, restaurants, food manufacturers and a wide variety of other businesses. The digestate can be used in areas such as allotments, parks, roof gardens, ‘living walls’, office plants, hydroponics and aquaponics—for urban greening and local food production. It also avoids the financial and environmental costs of trucking organic material from urban to rural areas.
A revolutionary project at London’s Camley Street Natural Park near King’s Cross in Central London is doing just that with a 2m3 ‘Biogastronome’ micro-digester. Food waste from local cafes, hotels and food manufacturers is collected using cargo bikes and put into the digester. The biogas is used for heating using a micro-biogas boiler and to make tea with a biogas tea urn. There are plans to convert a natural gas combined heat and power (CHP) engine to run off biogas and heat the buildings.
The fertiliser produced is used in various places on the site, including for vegetables grown on raised beds, shrubs and bushes in the garden and even a ‘floating orchard’—a barge filled with fruit trees and soft fruit bushes, anchored on the canal which runs alongside the park.
Why nickel stainless steel
James Murcott of Methanogen UK Ltd has been designing successful farm, sewage and abattoir digesters for more than 40 years and is responsible for the ‘Biogastronomy’ range of digesters. He engineers for longevity and low operation and maintenance costs.
“My digester designs incorporate both fibreglass (GRP) and stainless steel, two materials I use extensively in order to create robust and long-lived systems. We have even managed to relocate and reuse digester equipment after operating for 20 years and more—both materials withstand the harsh outdoor 24/7 operating conditions extremely well,” he says. Nickel-containing stainless steel is used in a number of places on the Biogastronomes, much of it Type 316 (UNS S31600): on the manual feed handle, the feed auger ends, heat exchangers and on the 12V heater. Stainless steel bolts (generally A2-70, which is the name given to a Type 304 (S30400) bolt with 700 MPa tensile strength) are exclusively used for bolting the two halves of the insulated fibreglass tank together, mounting the motor/gearbox assembly and fixing items such as the viewing port window and micro-switches.
James adds that he “banned any non-stainless bolts from our workshops, as it just wasn’t worth using anything else.” He has also used nickel-containing stainless steel in his micro-biogas boiler design, as the biogas can contain hydrogen sulphide which quickly destroys a lesser alloy.
The Camley Street system was used as a test bed for a very successful micro-hammermill, designed by the Alvan Blanch Group, Wiltshire, UK, which uses Type 304L (S30403) stainless for both the hammer mill body and hopper. This mill macerates the food waste down to 12mm, which helps to maximise the surface area that the bacteria can work on, as well as allowing easy pumping for regular automatic digester feeding.
Boilers as well as digesters
Methanogen UK’s Angela Bywater has been trialling the new automated micro-boiler system on her four year old micro-digester system. The boiler is fabricated from Type 316 stainless and she has found it to be very effective. “I normally cook on the biogas, but am now able to use excess gas through the micro-boiler in order to thermosiphon hot water into a storage tank which raises the temperature of the greenhouse, allowing me to grow winter crops such as lettuces.”
She believes that such local or on-site AD systems have wide applicability, “What we are doing here is taking the concept of many millions of simple digesters which exist in India, China, Nepal, Africa and elsewhere in the world, and engineering it for a colder climate and our own strict regulatory regimes.” And nickel-containing stainless steel has a vital role to play in that engineering.