Vienna's main wastewater treatment plant in Simmering will suspend 504 solar panels from steel cables above a single primary clarifier, with the 63-ton array reeling itself in over about 30 seconds when wind, snow or hail arrives, according to a report published 6 September 2026 (Autonocion.com). The design is the first Austrian deployment of dhp Technology's "solar folding roof," a cable-and-carriage system the Swiss firm has already used over clarifiers in Chur and near Lake Thun (EcoPortal.net).
Key takeaways
- 504 modules rated at 262 kW, expected to deliver about 260 MWh per year, framed as a pilot rather than a flagship (Autonocion.com).
- Roof footprint 115 by 164 feet over a 108-by-230-foot basin, with 14.3 feet of clearance under the modules (Autonocion.com).
- City-owned operator ebswien already covered 110% of the plant's electricity and 173% of its heat in 2025 from sludge-derived gas (Autonocion.com).
- Drive pulls the array in within roughly 30 seconds (around 60 seconds on a German site), with stowing losses already inside the published yield (EcoPortal.net).
- Commissioning is set for June 2027 on a single basin; the rest of the plant, about 11.4 acres of tank surface, is not yet covered (EcoPortal.net).
What happened
The Simmering plant handles all of Vienna's wastewater, taking in about 1,600 gallons a second, or roughly 53 billion gallons a year, and is run around the clock by about 170 staff (Autonocion.com). Ebswien, the city-owned operator, met 110% of the plant's electricity demand and 173% of its heat demand in 2025 from its own generation, mostly gas squeezed from sewage sludge (Autonocion.com). The folding roof, by contrast, is a small 262 kW pilot aimed at protecting a surplus that the operator says future load growth will otherwise eat into.
"The roof can be retracted or extended within 30 seconds," Vienna climate councilor Jürgen Czernohorszky said in the City of Vienna's announcement, and a fixed canopy was never an option because the basin has to stay reachable for maintenance (Autonocion.com). A primary clarifier needs the water surface to stay open because slow-moving scrapers push settled sludge one way and floating scum the other, and crews have to reach the mechanism whenever something jams (Autonocion.com). The folding concept hangs modules from steel cables strung between slim masts at the edges of the basin; each row rides the cables on a carriage, so pulling the rows toward one end concertinas them into a stack and leaves the water surface open again (EcoPortal.net).
Hardware is supplied by dhp Technology of Switzerland, which built a 643 kW pilot at Chur in 2017 and the 3.6 MW roof inaugurated on 31 October 2025 at the ARA Thunersee plant near Bern, described by the company as the largest folding solar roof in the world (Autonocion.com). A 1.37 MW, 2,640-module version is also running over stormwater basins at a plant in southern Dusseldorf (EcoPortal.net). Vienna's pilot is the second folding-roof installation in Austria, after a roughly 1.85 MW system in Styria went up over basins earlier in 2026 (EcoPortal.net). Neither the City of Vienna nor ebswien has published a price for the Vienna pilot (Autonocion.com).
| Site | Capacity | Modules | Footprint | Notes |
|---|---|---|---|---|
| Vienna Simmering (pilot) | 262 kW | 504 | ~18,800 sq ft | First folding roof in Vienna, completion set for June 2027 |
| ARA Thunersee, near Bern | 3.6 MW | — | 215,000+ sq ft | Inaugurated 31 Oct 2025; largest folding solar roof, per dhp |
| Chur, Switzerland | 643 kW | — | — | First dhp folding roof, 2017 |
| Southern Dusseldorf, Germany | 1.37 MW | 2,640 | — | Over stormwater basins; ~60 s retraction |
| Styria, Austria | ~1.85 MW | — | — | Earlier 2026, makes Vienna the second Austrian mover |
Specification read
The reported hydraulic load is 1,600 gallons per second, or about 523,000 m³/day (general conversion from the 1,600 gal/s figure reported; the plant is described as the third-largest in the European Union, per the City of Vienna cited in Autonocion.com). The class of project this bears on is a municipal primary settling stage inside a full train of screening, grit removal, primary clarification, biological treatment (typically A2O, MBBR or MBR at this scale), and tertiary or disinfection polishing, with sludge digestion already feeding combined heat and power on site (EcoPortal.net). The folding roof is a retrofitted cover on the primary clarifier, not a new treatment stage, so it does not change the process train; it changes the energy balance and the surface chemistry by shading the water, slowing algae, cutting evaporation and holding down some of the odor (EcoPortal.net).
For a procurement lead, the headline unit is kW installed on usable basin area, not on greenfield land, and the cost premium is paid in cables, carriages, drives, sensors and corrosion-grade hardware rather than in extra kilowatt-hours (EcoPortal.net). If your plant looks like a 100,000 to 600,000 m³/day municipal works with an open primary clarifier, an existing CHP surplus, and a tightening European effluent-quality rule that will force a new treatment stage, the case is to cover a basin the operator already owns before that rule lands, and the supplier shortlist is currently dominated by one Swiss vendor, dhp Technology, with around 20 reference roofs running by late 2025 and roughly 30 more in planning or construction per figures released at the Thunersee opening (Autonocion.com). Buyers scoping the underlying clarification step can also review Integrated Sewage Treatment packages and Daf Flotation Systems for the polishing or side-stream stage, and benchmark clarifier selection against recent plant choices such as DAF vs Clarifier for Mining Wastewater in Fairbanks, 2026: Which Should Factories Choose? and DAF vs Clarifier for Petroleum Wastewater in Selmer, TN: 2026 Factory Selection Guide, or open the broader Municipal hub.
FAQ
How much does a folding solar roof over a clarifier cost?
Neither the City of Vienna nor ebswien has published a figure for the 262 kW Vienna pilot (Autonocion.com). Cables, carriages, drives, sensors and corrosion-grade materials all sit on top of the module price, and none of it buys a single extra kilowatt-hour, so the premium over a fixed ground-mount array is meaningful (EcoPortal.net). (General industry range, not from the sources: cable-suspended, retractable PV covers typically run several times the per-kW cost of a fixed ground array; no source number was reported.)
What is the realistic lead time and when does Vienna's roof switch on?
Completion on the Vienna basin is set for June 2027 (EcoPortal.net). Around 20 folding roofs were running by late 2025, with roughly 30 more in planning or construction, per figures released at the Thunersee opening (Autonocion.com).
Who is the supplier, and are there alternatives?
The Vienna, Chur, Thunersee and Dusseldorf roofs are all dhp Technology systems, described as a "solar folding roof" built on ropeway-style cable hardware (Autonocion.com). With one firm dominating the reference list at the time of writing, buyers should expect a single-source tender risk and plan for it (EcoPortal.net).
Why bother if the plant is already a net exporter?
Ebswien director Günther Schmalzer said the plant's own power demand is expected to climb and the surplus only survives if new generation comes online alongside it, while the city also flags new European water-quality rules that will force an additional treatment stage onto plants like this one (Autonocion.com; EcoPortal.net). The roof is sized to cover a future bill, not today's.
