Two new HUBER Rotary Drum Fine Screens ROTAMAT® Ro2 2600 XL for Salzburg-Siggerwiesen WWTP

The wastewater treatment plant in Siggerwiesen in the municipality of Bergheim, which borders the city of Salzburg to the north, has been in operation since 1986 and treats approx. 40% of the total wastewater generated in the city and province of Salzburg. The first expansion of the sewage treatment plant took place between 1994 and 1998, followed by further adaptation measures between 2003 and 2004. With a capacity of 680,000 PE60 and a dry weather inflow of 103,600 m³ per day, the Siggerwiesen municipal sewage treatment plant is the largest in the province of Salzburg.

The catchment area of the sewage board Reinhalteverband RHV Großraum Salzburg covers a total area of 185 km² in twelve member municipalities. The wastewater is channelled to the sewage treatment plant via 567 km of local networks and 143 km of collectors. The inflow volume from the sewer network with a 70% mixed system changes proportionally with the amount of precipitation over the course of the year.

Circulation screens were installed when the wastewater treatment plant was built; the second generation (approx. 1995 to 2009) were step screens with a bar spacing of 5 mm. In 2009, perforated plate circulation screens with a diameter of 6 mm were installed (2009 to 2023).

The fourth generation now consists of two HUBER ROTAMAT^® Ro2 2600 XL screens with 3 mm bar spacing, the largest screens in southern Germany/Austria.

The general conditions:

• Maximum inflow per line 2000 l/s
• Two 3 m wide channels
• Channel depth 2.45 m

Under discussion were revolving chain screens with 2.5 or 3 mm bar spacing or the HUBER Rotary Drum Fine Screen ROTAMAT^® Ro2 2600 XL with 3 mm bar spacing. At first, the screen did not seem to be the machine of choice because it caused higher investment costs than a chain screen. However, performance data, calculations and their detailed comparison as well as international references caused the price to recede into the background as a criterion.

The main difference between the ROTAMAT^® screen and screening systems with a flat bar screen with rake or scraper bar is, on the one hand, the size of the area through which the screen flows and, on the other, the clearing speed. Compared to a rectangular bar rack with an installation angle of 55°, the flow area of the Ro2 screen is significantly increased. This is due to the flatter installation angle of 30 degrees of the screen and the cylindrical shape of the screen basket. In addition, the rotating drum offers a significantly higher speed for cleaning the screen surface compared to other screens.

A third major advantage of the ROTAMAT^® screen is the way in which the screenings are removed. While chain screens with scraper bars set the screenings in motion, this is not the case with systems such as the previously installed perforated plate screen or the screens that are now installed. This is the reason for the high separation performance. In combination with the high clearing speed, this leads to a lower upstream head, less grit deposits in the channel and reduced operating hours. This can also be seen at the Siggerwiesen wastewater treatment plant. The operating hours there have been reduced by 60% since the screens were installed.

Raw screenings volume

The diagram below shows the amount of screenings produced by different types of wedge wire screens. It can be seen that, for example, with a bar spacing of 3 mm, rotating drum screens generate significantly more screenings than wedge wire screens with scraper bars, which push rollers of screenings over a bar rack, or step screens, which set the retained screenings below the waterline in motion over the entire surface.

The high separation efficiency is also due to the fact that the direction in which the screenings are detached from the screen surface and discharged (from outside to inside) is opposite to the direction of wastewater flow (from inside to outside). As a result, fibres that are already hanging through the slotted screen behind the screening surface are conveyed by the cleaning system (water or air) back in front of the screening surface and thus back into the screenings removal process. Any solid matter that does not enter the wastewater treatment plant minimises clogging and increases operational safety. The operational and technical managers of the Salzburg wastewater treatment plant followed this idea.

The previously installed perforated plate screens with a 6 mm hole diameter already had a very good separation performance as they also did not move the screenings on the screen surface. With the installation of HUBER Rotary Drum Fine Screen ROTAMAT^® Ro2 units with 3 mm wedge wire, the monthly production of dewatered screenings increased further from 140 to 160 tonnes.

The cleaning speed of the drum screen makes the ROTAMAT^® Ro2 screen insensitive to changes in the load, e.g. in case of flushing surges during rain events or during the final phase of emptying stormwater retention basins.

Wear and screen surface cleaning

The ROTAMAT^® Ro2 screen has no chain joints, sprocket bearings, bar rack surfaces, scrapers, etc. that can wear out. There is no wear on the screening surface because the screened material is not moved on it. The machine has a single bearing point: the so-called "foot bearing", which supports the weight of the rotating parts of the worm shaft and screen drum.

Screen surface cleaning

Efficient screen surface cleaning on these two HUBER Rotary Drum Fine Screen ROTAMAT^® Ro2 units is achieved by three screen surface cleaning systems:

1. Water spray nozzle bar with a pressure of 5 bar effectively at the nozzles (standard, here as backup)
2. Air jet cleaning for cleaning the screen without using water
3. High-pressure cleaning with 120 bar working pressure for intensive basic cleaning after every 3 hours of operation

Summary

The two screens discharge washed and dewatered screenings into the two downstream spiral conveyors. A screw conveyor transports the screenings into the container room, where they are transported fully automatically from an existing distribution system into three large-capacity containers.

The installation in Salzburg shows that the HUBER ROTAMAT^® Ro2 screen is superior to other screening systems when considering criteria such as separation performance, combined screenings treatment, service life, maintenance intervals, spare parts requirements, in short the lifecycle costs.

The reliability of the ROTAMAT^® Ro2 screen must also be taken into account. Of the large ROTAMAT^® Ro2 screens in Austria, the plant in St. Jakob in Defereggen (East Tyrol), size 1500 with 3 mm gap width, is the oldest. It has been in operation since 1992 – a working life long.