Kemira Oyj/Kemira M&I ABSTRACT Vesa Kettunen ... - Svenskt Vatten

Jukka Yli-Kuivila, project manager, investments for water services
Helsinki Region Environmental Services Authority HSY
jukka.yli-kuivila@hsy.fi
NORDIWA 2013 13th Nordic Wastewater Conference, platform presentation, English
No prolonged maintenance breaks in a sewer rocktunnel
A general plan for a new 150 000 m 3 /d wastewater treatment plant in Espoo, Finland was completed in
October 2011. Purified wastewater should be transferred 8,5 km via a rocktunnel to the outlet tunnel of the
present WWTP. The tunnel would be below a river at -20 m depth and further below a metro at -60 m. If
water could not pass through the tunnel, a six months period was estimated to be needed for fixing it.
Meanwhile the water would be discharged to the crossing river. This was estimated to happen once within a
hundred years.
The river leads to the Gulf of Espoo (Baltic Sea), where lives a population of Macroplea pubipennis– very
rare water insect, protected by a national law and Natura-2000 program. Regional environmental authorities
wrote in November 2012, that no major risk for that insect population could be allowed. This meant a delay
risk for the whole 286 million euros WWTP project, and possible 20 million euros extra investment costs for
an emergency outlet tunnel. Therefore the outlet tunnel plans are updated as follows and extra costs limited
to about 2 million euros.
The part of the outlet tunnel from the new WWTP to the river will be blasted above the sea level. This part
can be reached by men and vehicles whenever needed. Most inspection and maintenance can take place,
while the WWTP and the outlet tunnel are operated normally. If those duties can’t be completed when water
is flowing on the bottom of the tunnel, treated wastewater can be turned back to the inlet tunnel to avoid a
total break in the WWTP. The storage capacity of the inlet tunnel enables one to two days break for the
WWTP use, or running it with diluted wastewater. Excellent purification results are easier to maintain, if the
exception period doesn’t last longer than 8 h/d. That enables repeatedly one working shift for tunnel
maintenance and two shifts for emptying the inlet tunnel storage. The fixing of the tunnel bottom can be
done part by part while the flow is isolated from the part under construction. Therefore no prolonged
maintenance breaks are needed for this part of the tunnel.
Before the tunnel would cross the river, a 20 m vertical shaft is needed to remain the tunnel in a solid rock.
After that the slope of the tunnel is towards the seashore, where another vertical shaft is needed from -60 m
to the present outlet tunnel. Normally this 7,5 km part of the tunnel is full of water. A pumping station will
be built beside the tunnel before it dives deed because of the metro. This pumping station can raise water
about 30 m, from tunnel bottom level to sea level behind a pressure wall in the tunnel. If the tunnel from the
pumping station to the present outlet tunnel should be emptied, water would be pumped via another pipe to
the street level above and from there further to a nearby ditch and seashore. When inspections or
maintenance is needed, water level could be kept low in the tunnel and vehicles driven in.
The normal outlet tunnel discharge is 7.5 km to deeper sea and a discharge to seashore should also be
avoided. Therefore the last tunnel part will be inspected by a diving robot and kept empty only during
essential maintenances.
Another benefit of the pumping station is the adaptation to climate change. The WWTP can be built to lower
position and booster pumps used only, when the sea level or water flow is too high for gravity discharge.