STUDY: The Bromma Biogas
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SECTOR - Water/Sewerage
COUNTRY - Sweden
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
- Bio gas production
- Renewable energy sources
- Recirculation of nutrients
- Vehicular emission
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
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
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.
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.
Formally, for the biogas technology
there are driving forces on several levels:
- ˇ Agenda 21 - UN Rio Conference
- "The Swedish national environmental goals" - 15 main environmental
goals. 1998. http://www.environ.se/milmalnet98/ ˇ
- "The Swedish generation
goal" - "To the next generation leave an environment where the main environmental
problems are solved". 1996 ˇ
- 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).
- Governmental strategy for recycling (Swedish Government Bill).
- Factor 10 (Swedish official report). 1997
- A Sustainable
Society (Swedish depart-mental guidelines for the environment). 1993
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.
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.
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.
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
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
e) The safety of gas systems is much greater than that of petrol systems.
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
* European project
PR Manager Brita Forssberg
Tel +46 (0)8 52212023; fax +46 (0)8 52212002;
Techn. Lic. Anna
Tel +46 8 4756030; fax +46 8 4756900;
IEA Bioenergy, Task 37 Energy
from biogas and landfill gas.