For more information: AnoxKaldnes AB

www.anoxkaldnes.com

sales@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

sales@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.