Rebuild for world’s largest underground WWTP

Plans to expand residential, commercial and business faciliites near to the world's largest underground wastewater treatment plant in Stockholm, Sweden, necessitated rebuilding the plant. Johnny Stohne, senior engineer for SWECO, reports on the £30 million project, which includes new mechanical treatment, transportation and ventilation facilities.

Henriksdal Wastewater Treatment Plant (WWTP) in Stockholm, Sweden, is the largest underground plant in the world. The plant was put into operation in 1941 and has since been extended several times to increase the capacity and to enable the operator, Stockholm Vatten, to meet increasingly stringent requirements.

Today the plant meets the requirements for tertiary treatment with far-reaching removal of phosphorus and nitrogen. The current load corresponds to 700,000 p.e. and the hydraulic flow to 280,000m3/d.

The whole of the basin block lies inside chambers blasted into the rock. The surface facilities include the control centre building, mechanical treatment, the sludge tanks, the upper parts of the sludge thickeners, the digesters and the gas holder.

The ventilation chimney is 80m tall and 7m in diameter. Sludge treatment takes place in a separate facility at Sickla, located about 500m from Henriksdal.

The main part of the plant occupies an area of almost 300,000m2 in underground rock chambers, this includes tanks and other treatment facilities as well as different service areas. The total length of transport tunnels is about 18km.

In a couple of years, about 7000 residential flats will be built very close to the plant, along with a new business and shopping centre including improved infrastructure for trams and buses. This means that a lot more people will live and work near the plant and that noise and bad odour will be unacceptable.

For this reason, the plant has to be rebuilt and the environment improved yet again. The mechanical treatment stages, consisting of screens, grit chambers and a reception station for external organic waste must be moved from the central building into a new space in the rock, well-hidden from the external environment.

For the same reason, the transport routes of the screenings and sand from the plant must also be changed. A new tunnel will be constructed to carry waste directly out onto a highway, without passing the residential area.

These measures necessitate the extraction of another 80,000m3 of rock from the plant but will have the additional result of considerably improved working conditions for the staff at the plant.

Mechanical treatment

The new screening and grit chamber stages in the rock will be designed for a maximum hydraulic load of 12m3/s. The screening stage will consist of 24 screens with a distance of 3mm between the bars.

The installation will be divided into two groups of screens, one for each inlet tunnel to the plant, but they will be placed close to and in reverse to each other. A channel between the two groups of screens will level out the difference in the hydraulic flow to the two inlet tunnels.

The screens will be covered by hoods and connected to the system of channels and the ventilation system with an underpressure, to evacuate the bad-smelling air. Each screen will also be separated from the others by adjustable gates to secure maintenance of individual units to eliminate the problem of bad odour in the immediate surroundings.

The screenings will be collected by screw conveyors and transported via compressed-air conveyors and a levelling storage system to two dispersion units. In the dispersion stage, the screenings will be disintegrated to fibre-size in the same way as recycled paper usually is recovered.

The dispersed product will be mixed with treated wastewater to reach a desired Dry Solid (DS) content of 2-7% before being heated in a reactor to make it easier to separate it from sand. The sand will later be treated together with the separated sand from the grit chamber.

Finally, the dispersed product may be treated in one of the following ways:

  • Digested in the existing digesters
  • Dewatered together with the digested sludge
  • Pumped back to the grit chamber and taken out as primary sludge.

The grit chambers will be designed as a system of six aerated units. The sand will be evacuated by means of mammoth or airlift pumps installed on mobile wagons running along the tanks. The sand will be lifted to channels from where it will be pumped by ejectors to sand dewater and washing units.

The grit chambers will be decked with concrete beams and the total system will, in principle, be covered in the same way as the screens to prevent the surroundings to be exposed to bad odour.

Organic waste station

A new station for the reception of 50,000 tonnes/year of external organic waste from the catering and food industries will be designed for a DS content of about 10%. The receiving station will be placed close to the new mechanical treatment stage where four reception silos will be constructed with equipment for prevention of bad odour.

The external organic waste will be dispersed in the same way as the screenings and also in the same dispersion units. It will be stored in two storage silos from where it will be pumped into the digesters during periods of low sludge charging for levelling of the load.

Expansion of the reception of external organic waste will mean an increased load on the digesters of only 4-5% and a corresponding increase of the amount of digested sludge. The total amount of digested sludge will thus amount to maximum 60 tonnes/day and gas from the digesters will be used by a local traffic company as fuel for its buses.


Screenings, sand and organic waste from the plant will be transported by lorries via a new transport tunnel directly out onto a highway adjacent to the plant. Transportation of chemicals to the plant will also be diverted through this tunnel in the future; currently, chemicals are transported through local streets.

Odour and ventilation

A new fresh-air inlet will be built on top of the bedrock and connected to the new treatment stages with a new shaft. Equipment for odour treatment will be installed in connection with the new shaft.

The fresh air will be pre-heated by heat from the sewage and process equipment. The exhaust air will be treated in a filter with a media consisting of expanded clay.

Contact: Sweco Viak

Tel: +46 8 695 60 00

Email: [email protected]


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