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For more information: AnoxKaldnes AB
www.anoxkaldnes.com
Tel: +46 (0)46 182150
MUNICIPAL
MBBR
Visby WWTP, Gotland, Sweden
The client Visby WWTP is a municipal
wastewater treatment plant
located on Gotland, an island in
the Baltic Sea about 90 km from
the main land of Sweden. Visby is
and old medieval town and the
main town on the island. Gotland
is a popular tourist destination
during summer which is also when
the plant receives its maximum
loads.
Key information > Plant capacity: 10 500 m³/day,
max 1000 m³/h
> MBBR with BOD and nitrogen
removal
> Start-up year: 2001 and 2007
The needs Visby WWTP was built in 1967 with mechanical treatment.
Ten years later biological treatment in a trickling filter,
chemical precipitation and secondary clarifiers were
installed. In 1991 a digester was installed, and in 2001 the
first steps for nitrogen removal; an SBR treating the reject
water and an MBBR for post-denitrification with methanol
as a carbon source, were put into operation. In 2006 the
plant had to be upgraded oncea again, in order to meet a
new effluent requirement of 15 mg/l TN on an annual
average.
AnoxKaldnes’ solution The biological treatment was completely rebuilt in 2006
and commissioned in 2007. The old trickling filter was
refurbished into two pre-denitrification volumes and five
new volumes were constructed for BOD removal and
nitrification. Together with the already installed post-
denitrification the biological treatment line was now
complete, with BOD and total nitrogen removal in a a pure
MBBR process.
For more information : AnoxKaldnes AB
www.anoxkaldnes.com
Tel: +46 (0)46 182150
MUNICIPAL
MBBR
0102030405060708090
100110120130140150
2005 2006 2007 2008 2009 2010 2011 2013 2014
mg
/l
TN in (mg/l) TN out (mg/l) requirement (15 mg/l)
Technical data Design load
Flow 10 500 m3/d
BOD7 4 200 kg/d
TN 720 kg/d
Effluent requirements
BOD7 10 mg/l
TN 15 mg/l
TP 0.3 mg/l
The process Visby WWTP now consists of a primary settler, a MBBR process
with pre-denitrification, BOD removal, nitrification and post-
denitrification with methanol dosage as well as a secondary
settler. Phosphorus is precipitated with ferric chloride in the
secondary settler. Due to seasonal load variations to the plant a
"Swing" MBBR reactor was installed between the second pre-
denitrification reactor and the BOD reactor. The "Swing" reactor
can be operated either as a pre-denitrification or BOD removal
stage. This gives extra flexibility to optimize the operation of the
plant depending on season.
Results The plant has shown excellent results since commissioning and
has a great flexibility to handle major influent load variations and
get down to very low values in the effluent if necessary. The
figure below shows the incoming and effluent total nitrogen
concentration starting from 2005 (before commissioning of the
complete MBBR plant) until the beginning of 2014. As seen in the
figure, the plant has stable results that meet the requirement of
maximum 15 mg/l total nitrogen on an annual average.
