Biogas Production

The food waste from the kitchen and the excrements of 750 students are used to produce biogas that supplies the thermal energy for cooking.

Why Biogas?

Biogas involves the generation of a combustible gas from biomass digestion in the absence of oxygen. The waste is mixed with water to create the right environment for the bacteria to decompose the biomass.

A biogas installation can convert any biodegradable material into biogas, which can be used in similar ways as natural gas for cooking, space heating, or water heating. The residue of the digestion can be used as a fertiliser, replacing industrial fertilisers made from fossil fuels.

Humans are an excellent source of low temperature heat, especially when they are exercising. However, they are much less efficient in generating the higher temperatures that are required for cooking.

Producing electricity for an electric cookstove involves dozens of students generating power. Converting human waste and food waste into biogas promises to be a more energy efficient way.

How Much Biogas can 750 Students Produce?

It’s surprisingly difficult to find reliable data about the quantity of feces that humans produce – estimates vary by an order of magnitude. Our calculation is based on a conservative 25 litres of biogas production per person per day. 

Assuming 280 grams of food waste per student per day (which is the figure for the UK according to a recent study), we obtain another 100 litres of biogas per student.

Combining food waste and excrements, each student thus provides on average 125 litres of biogas per day, which corresponds to the thermal energy of 608 grams of firewood.

At first sight, this seems to be hardly sufficient for cooking. Boiling one litre of water requires 30-40 litres of biogas, while cooking half a kilogram of rice requires 120-140 litres of biogas.

A biogas cookstove consumes 200 to 450 litres of biogas per hour and the biogas consumption for cooking lies between 300 and 900 litres per person per day (assuming two warm meals per day).

Fireless Cookers

However, energy use for cooking can be reduced substantially. First of all, communal cooking is more energy efficient than individual or family cooking, therefore we assume that we need only 300 litres of biogas per person per day.

Second, the communal kitchens are equipped with fireless cookers, which can halve the energy use of the cooking process. After food is brought to a boil on a biogas fire, cooking pots are placed in a heavy insulated container, which keeps the cooking process going without the need for additional energy.

A fireless cooker with associated cooking pots. Image: Wikipedia Commons.

Thanks to the large-scale use of fireless cookers, the thermal energy needs in the kitchen can be entirely supplied by what the students provide themselves, in terms of both human waste and food waste.

There are multiple variables that can influence this calculation, both for better or for worse. Further reductions in energy use could be obtained by low temperature cooking methods, by eating fewer warm meals and more raw foods, salads and sandwiches (very common in the Netherlands), or by making cold soups.

On the other hand, the students could reduce their food waste, which would lower the biogas production. Pets could also influence the calculation. Their excrements could be used for biogas production, but if they eat all the food waste, biogas production would decrease substantially.

Vacuum Toilets

The use of excrements for biogas production requires the installation of vacuum toilets. These toilets separate the solid waste from the urine and transport it to the biogas installation in the basement with the use of very little water.

The vacuum toilet was developed in 1866 by Dutch engineer Charles Liernur. In the 1870s, several Dutch cities were equipped with a city-wide vacuum sewer system. This avoided the dilution of human waste by the admixture of water, thus preserving its value as a fertiliser. Today, the technology is still used on planes, trains and boats.

The human powered student building has shared toilet facilities throughout the building, all with vacuum toilets. A vacuum toilet requires roughly 2 watt-hour per flush. The energy use for flushing is generated by the communal power generating floors.

Assuming 5 flushes per day per person, this comes down to 10 watt-hour per person per day, or 7,500 Wh per day for 750 students. This means we need only 7.5 people exercising for 10 hours to power the toilet flushing system.

The Biogas Digester

We also need to take into account the inefficiencies of the system. Biogas production is most efficient in the (sub)tropics, because the ideal process temperature for the fermentation process is at about 35 degrees Celsius.

In countries like the Netherlands, this requires additional heating, insulation of the digester, and/or building a larger digester with a longer digestion (or “retention”) period.

Obviously, we don’t want to spend more human power for heating the digester so we build a heavily insulated digester in the basement of the building, and we make it as large as possible.

There’s ample space for that, because with a retention time of 60 days, the digester only needs 50 to 110 m3 – the volume of a large shipping container. We have a lot more space in the basement so we can make it much larger.

In principle, a larger biogas installation could also provide the energy for space heating and water heating, but that would require food waste, agricultural waste, green waste or excrements from outside the student building. Since we want to be self-sufficient in terms of energy, this is not an option.

Operating the Biogas Digester

Biogas is not a “set-and-forget” technology. Operating a biogas digester is labour-intensive: food waste needs to be collected, and all biodegradable material needs to be mixed with water, fed in the digester, and stirred. The slurry that can be used as a fertiliser must be removed every few days.

Skills, know-how, discipline and routine are required both to maintain a high gas production and to ensure a long lifespan of the biogas unit – which is essentially a living thing.

The biogas organisations we consulted all discouraged the use of biogas in the context of a student community, mainly because of safety issues. Biogas hazards are fire and explosion, asphyxiation, and disease. To address some of these issues, we have encapsulated the digester to limit the damage in the case of an explosion.

A biogas installation can be completely automated, but in our case humans would need to generate the electricity for the machines and computers anyways. This is why we follow the manual approach to biogas, which is common in developing countries. This involves the manual labour of students, who all have to operate the biogas plant during certain periods of the year.

The biogas installation remains the biggest technical challenge for the human powered student community -- all advice is welcome.

Thanks to biogas-e.

Comments

[Thomas]

In Austria you need 13kwh/Cubic Meter for the wastewater treatment. Ordered yesterday a Membrane Biogas 1m^3 with Dehydrator and Desulfurizer for about 500 EUR with a small light mobile Glasshouse as Mini Family Biogas Plant. maybe would be interesting if harvesting the Sun Energy for a Biogas Reactor on the roof of this Building is more efficient than a PV Plant - or a combination known from Italy: Glasshouse PV Hybrid...?

[Nikolay Ivankov]

I'm just curious: why using wind power is not an option? In such a windy place as Utrecht?

What I also could not have gotten from any of the articles: are you supposing to have water pumped to each floor? Or to each floor with a toilet if they are not on every floor? Or to the kitchens only?

[kris de decker]

@ Nikolaj

Indeed, students also need to pump water. And we do make use of wind energy. Both topics will be dealt with in future articles.