CASE STUDY: The Bromma Biogas Plant, Stockholm

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SECTOR - Water/Sewerage COUNTRY - Sweden

BACKGROUND
In the waste water plant of Bromma, Stockholm, about 10000 tons of sewage sludge (dry matter) is produced annually. Before the 1970's this sludge used to be deposited on waste deposits. However, the sludge contains a large amount of organic matter which can be used as an energy source. Since the 1970's the matter has been treated by anaerobic digestion producing biogas. In such a plant the sludge is biologically treated under anaerobic conditions. The product is biogas and a solid organic rest product. The biogas was earlier for interal heating at the WWTP, and also sold for external use. In 1996-1997 a pilot biogas upgrading plant was installed, and in 2001 a full-scale upgrading plant is under construction. The upgrading plant separate methane from carbon dioxide and other contaminants in order to produce a vehicle fuel of natural gas quality. 1.5 million m3 gas will be produced annually, which corresponds to 1.5 million litres of petrol. Today (2004) there are four fuel stations in Stockholm providing gas as fuel. Biogas is one of the least environmentally harmful fuels economically available today. Because it is based on biologically produced substances it is part of recirculation, and does not contribute to the CO2 content in the atmosphere. Emissions of hydrocarbons, CO and nitric oxides from production and use are small. The rest product has to a large extent been used as soil improvement and fertilizer thanks to the decreasing amounts of pollutants e.g. heavy metals. However, the use in agriculture is nowadays limited. In the future, with still better control of the quality of the sewage water input, the rest product can be utilized. This is important from a From the recycling point-of-view, because the rest product contains phosphorus, which is a limited natural resource.

Filling up bio gas at one of four gas fuel stations in Stockholm
Photo: Catharina Wikstrom/Stockholm Water

INDICATORS

• Sewage sludge treatment
• Bio gas production
• Renewable energy sources
• Recirculation of nutrients
• Vehicular emission

EVALUATION
The Bromma biogas plant will be continuously evaluated. The first evaluation will be presented in 2001. However, a test plant in Stockholm is evaluated. Digestion plants in Kristianstad and Linkoping for treatment of sewage sludge and organic waster are evaluated

Generally, digesters can be used for treatment of urban sewage sludge, manure from farms, and food industry waste. After digestiopn, hygienisation is made at a temperature of 700 C. Before use as fertililzer the rest product shall be treated in 15-30 days and nights at a temperature of 370 C.
When mainly farm manure and food industry waste are used, the amount of pollutants such as heavy metals can be controlled. The rest product can be transported back to the farmers for use as fertilizer and soil improvement. The same lorries as those transporting the manure can be used. The product is better than ordinary manure because the nitrogen is more available for plants. Problems with odour are less than when ordinary manure is used.
If more than 10-20% sewage water is added, the amount of pollutants will increase to a level when the rest product is not well suited to put on arable land.

Economically, the gas produced at Bromma is a little cheaper than petrol. On the other hand, the cars have to have a special gas combustion equipment which costs 2500-5000 E. If manure or industrial organic matter is used, it is of crucial economic importance that the same lorries can be used for transportation of the rest product and the organic matter. Yet, in this case the gas is somewhat more expensive than petrol. However, the commercial interest depends on the energy taxation systems.

BENCHMARK DATA
One of the National Swedish Environmental Goal Committee goals is that 75% of the phosphorus in sewage sludge and in organic waste shall be offered on the market as commercial products in the year 2010.

DRIVERS
Formally, for the biogas technology there are driving forces on several levels:

• · Agenda 21 - UN Rio Conference ·
• "The Swedish national environmental goals" - 15 main environmental goals. 1998. http://www.environ.se/milmalnet98/ ·
• "The Swedish generation goal" - "To the next generation leave an environment where the main environmental problems are solved". 1996 ·
• Green Key Notes (Swedish governmental key notes for Energy use, Electricity for heating, CO2-emissions, Amount of benzol in air, Acid emmissions, Environmentally adapted transportation, Number of environmentally certified companies, Depositing of waste, Nitrogenous and phosphorous deposits to the sea, Recycling of phosphorus to agriculture, Area of protected forests). 1998 ·
• Governmental strategy for recycling (Swedish Government Bill). 1997 ·
• Factor 10 (Swedish official report). 1997
• A Sustainable Society (Swedish depart-mental guidelines for the environment). 1993
• Environment Protection Act. 1999

However, there is no doubt that the main driving forces for the Bromma biogas plant are of a different kind. Stockholm Water company running the Bromma plant, developed a policy regarding long term sustainability for its total area of operations. There are also other market drivers. To some part the Bromma plant was financially supported by governmental funds for sustainable development. The City of Stockholm has taken the decision to convert their 1600 vehicles to non-fossil fuel. The private market for biogas-fuelled cars is also increasing. These drivers should be seen as the most powerful ones.

LESSONS LEARNT
The anaerobic digestions technology for sewage sludge and organic waster treatment is well established. The digesters in Kristianstad and Linkoping are evaluated. The plants as such work well. The rest product is sanitised. If the sewage mainly comes from controlled sources as food industries and manure from farms, the rest substance is well suited as soil improvement for agriculture, and the phosphorus is recycled. The rest product is better as fertiliser than ordinary manure because of better nitrogen availability and less odour. If farm manure is used in the digester, it is transported to the plant with trucks. When the trucks go back, they bring the sanitised fertilisers back to the farms. Problems occur when more than 10-20% of the sludge is household waste and/or waste water sewage sludge. Then there is a risk that it will contain too much pollutants to be accepted in agriculture. The quality of the sewage water must be controlled for recirculation of plant nutrients.
Bromma vehicle fuel plant has not yet been evaluated, but a test plant is Stockholm for 200 cars is. No problems seem to have occurred regarding the use of gas as fuel. A plant is under construction for the Henriksdal WWTP in Stockholm.

APPLICATION
The method anaerobic digestion of sewage sludge has been in use for decades. Generally, the gas produced was used as fuel for local engines, producing electricity for the plant itself. Plants adapted to a wider range of waste substances have been developed during the 90`s. The latest and most modern plants are Kristianstad, Linkoping and Bromma (Stockholm), of which Kristianstad and Linkoping mostly uses manure and food industry waste. All sewage sludge produced in Stockholm produces biogas. The gas is used for heating, for electricity generation (Henriksdal) and in purified and compressed form as car fuel.

TRANSFERABILITY
The process is controlled, and can be used anywhere. In areas with a large production of e.g. waste from food industries, the technology is well suited for combination of solid waste and sewage sludge, and the gas as well as the rest product can be utilized. If the origin of the main part of the water treated is urban sewage sludge, methods must be developed to control, limit and decrease pollutants. However, in any case the gas produced can be utilized.

SUMMARY OF IMPACT ON SUSTAINABILITY

Ecology		Economy		Social Aspects
Emissions? a)		Cost effective? b)		Participation? d)	-
Use of natural resources? a)		Willing to pay? c)		Transparency?
Bio-diversity?	-	Effective organisation?		Safety? e)
Total		Total		Total

a) Emissions and use of natural resources are reduced, but because of the very limited number of cars driven by gas the effects are limited.
b) The Bromma biogas is a little cheaper than petrol, but gas produced of manure or organic waste is a little more expensive. Cost reasonable compared to other renewable fuels.
c) Assumption; gas equipment in cars cost about 2500-5000 E.
d)The biogas plants are built because of the public environment discourse; however, the plants as such are built by private contractors with no public participation.
e) The safety of gas systems is much greater than that of petrol systems.

REFERENCES

Biogas Upgrading and Utilisation. IEA Bioenergy, Task 24: Energy from biological conversion of organic waste

Dalén, Asa. 1999. Biogas as vehicle fuel at Stockholm Vatten AB

http://www.stockholmvatten.se/

www.zeus-europé.org * European project

http://www.tekniskaverken.se/teknisaverken/foretag/biogas

http://www.kristianstad.se/templates_custom/Page_5036.aspx

PROJECT CONTACT
PR Manager Brita Forssberg
Stockholm Water
SE-10636 Stockholm,
Sweden
Tel +46 (0)8 52212023; fax +46 (0)8 52212002;
e-mail: brita.forssberg@stockholmvatten.se

Techn. Lic. Anna Lindberg
ITT Flygt
Box 1309
SE-17125 Solna
Sweden
Tel +46 8 4756030; fax +46 8 4756900;
e-mail: anna.lindberg@flygt.com

IEA Bioenergy, Task 37 Energy from biogas and landfill gas.