Central storage canal Graz

Mur downstream of the Radetzky Bridge. On the left bank building road for the construction of the storage canal.

The Central Storage Canal (ZSK) is a project by Holding Graz Wasserwirtschaft, which will be built in the Styrian capital together with the City of Graz from 2017 to 2022 . The first (lowest, southernmost, left bank) section of the Central Storage Canal with a length of 3.2 kilometers was built between 2009 and 2012. This is located in the area between the sewage treatment plant in Gössendorf (Graz area) and the Hortgasse in Graz. The construction of the main project, from Hortgasse to the Radetzky Bridge in Graz, will take place from autumn 2017 to 2021. The commissioning of the completed canal is planned for 2022. The cost of the central storage channel is around € 81.4 million, plus the funds from the Murmasterplan Graz-Mitte at € 84.45 million. On September 22, 2016, the municipal council decided to release these funds. Template: future / in 2 years

Graz sewer network

Construction work below the Radetzky Bridge in the bed of the Mur to build a canal crossing.

The degree of connection of buildings and facilities used by people to the sewer system is between 95 and 99%. Only occasionally - especially small, old - houses discharge sewage into the Mur, flowing streams, the mill tunnel or cesspools. Faecal and other waste water is almost entirely collected in the public sewer system. But not everything ends up in the sewage treatment plant.

In Graz, as in most of the larger cities, combined sewerage is predominant. This means that both rainwater and wastewater are fed to the sewage treatment plant in a common pipe. Rainwater from street and roof surfaces mixes in the sewer with the wastewater from Graz households. In the event of heavy rain, however, not all of the water can be forwarded to the sewage treatment plant in Gössendorf. To relieve the sewage treatment plant, so-called mixed water reliefs were built, which discharge mixed water from the canal into the Mur . At the moment, when it rains heavily - an average of 50 times a year in Graz - the Mur is polluted with all kinds of pollutants, as the rain masses, mixed with the dirt from the streets, in addition to the wastewater from Graz households, to a greater or lesser extent, depending on the volume flow over the reliefs into the Mur. The pollution of the Mur with pollution does not only take place on these around 50 days a year. The pollution loads that flow unfiltered into the river have a lasting effect on the ecological system of the Mur, as some substances are deposited in the river bed and can thus develop long-lasting toxic effects.

Decentralized rainwater management

In order not to overload the sewer system, Graz Wasserwirtschaft has been pursuing the strategy of decentralized rainwater management for years. The aim is to reduce the amount of mixed water that collects in the sewer by infiltrating rainwater close to the volume. The decentralized rainwater management in Graz is mainly used for new buildings and new roads. In the densely built-up inner city area, many existing buildings and streets have already been connected to the canal - some for decades. A change in this situation is not foreseeable on the part of Graz Wasserwirtschaft in the long term, as this is usually associated with extreme technical and financial outlay, and changes cannot be legally enforced against the homeowner and there are hardly any new buildings to be built in this area.

Another problem with decentralized systems is the pollution of rainwater. Rainwater from streets in particular cannot seep away without cleaning due to this pollution (rubber abrasion, heavy metals, salt, etc.), since unfiltered this water would endanger the groundwater and thus the drinking water. For this reason, Graz Wasserwirtschaft has been pursuing a dual strategy for years: Wherever possible, attempts are made to keep rainwater away from the sewer and to dispose of it locally. On the other hand, these decentralized measures can only have a clear effect in the very long term. Mixed water management is pursued for the existing stock and for heavily polluted areas, which subsequently requires sensible storage, such as the ZSK. This combination of measures corresponds to the state of the art and the latest scientific findings.

How the ZSK works

The finished ZSK will have a storage volume of 94,000 m³, which will more than double the storage space in the Graz sewer network. Cascade structures are built at intervals of around one kilometer, in which the storage of the mixed water is controlled and this is gradually passed on to the sewage treatment plant. As soon as the water rises, so-called hydraulic weirs lock the storage sections and enable the canal to activate its full storage volume. After the rain has stopped, when resources in the sewage treatment plant are free again, the water is slowly forwarded directly to the sewage treatment plant.

The ZSK reduces the number of days on which mixed water is discharged into the Mur by 80 percent. In concrete terms, this means that this will only happen on around 10 days a year. The amount of pollution that is routed from the sewer network into the Mur is reduced by 50%.

Technical regulations and legal situation

The concept of the ZSK is based on EU requirements for the protection of water bodies, the so-called Water Framework Directive (WFD, Directive 2000/60 / EC). There is u. a. the reduction of the entry of pollutants into rivers is required. In Austria, the regulations are implemented, among other things, via the Water Law Act (WRG). At the technical level, the technical experts of the Austrian Water and Waste Management Association laid down §12a in more concrete terms for the dimensioning of mixed water discharges in the ÖWAV (Austrian Water and Waste Association). According to this rule sheet, the state of the art is specified by the achievement of a minimum degree of efficiency for the forwarding of the rainwater to be discharged in the mixed water sewer system to the sewage treatment plant in rainy weather and also to be treated there. For Graz this would have to be around 55% hydraulically on an annual average. In concrete terms, this means: on an annual average, 55% of all rainwater discharged into the sewer system in rainy weather would have to be cleaned in the sewage treatment plant. For the substances that can be filtered off (AFS) inevitably in the wastewater, this value should even be 70%.

In Graz, hydraulics and the AFS are kept below 30%, as the Graz sewer system currently has very little intermediate storage space. The Graz sewer system is therefore not yet state-of-the-art, which is why the ZSK is necessary because it creates exactly this intermediate storage space.

natural reserve

The central storage channel has come under fire from nature conservationists and activists, especially because of the clearing that has to be carried out on the mud bank in the course of the construction work. A total of 839 trees have to be removed during the construction phase, the majority of them in the 2018/19 and 2019/20 winter seasons. In the north, a five-meter-wide strip is being cleared along the waterfront along the promenade on the left bank of the Mur between the soap factory and the Radetzky bridge, two areas between the Bertha von Suttner and the railway bridge and downstream from the Radetzky bridge. In the south, the existing embankment between Hortgasse and Puntigamerbrücke will be used in the course of the construction work. The Augarten of Graz remains untouched. In addition, the builder assured that after the construction work was completed, all trees would be replanted to the exact number, i.e. in a ratio of 1: 1, and that trees of any size could grow above the ZSK. This means that no tree will be lost permanently and that the mud slope will be similar to its previous state after the ZSK is completed and the tree has grown for a few decades.

With regard to animal welfare, ecological measures were prepared and implemented in order to take care of both reptiles and protected animal species with comprehensive protective measures or to relocate them before the construction work. The measures were coordinated with the nature conservation authorities and are requirements of the official permits. A total of around four million euros will be invested in accompanying animal and nature conservation.

Graz Mur power plant

In January 2017, construction of the Mur power plant began in Graz ; a river power plant with a reservoir several meters high, some water subsidence in the underwater, the interruption of bed load transport in the river bed and a significant change in the navigability of the river with boats. With the damming up, the water becomes deeper over longer stretches, if the flow speed drops here, there is increased deposition of fine sediment, which also contains organic substances, such as leaf material from the bank vegetation. The biological degradation of organic material consumes oxygen, which can only be supplied from the air from the surface of the water.

The entry of atmospheric oxygen into the water is high with a comparatively fast flowing river like the Mur in Graz, and will be significantly reduced by the future sharp reduction in the water speed in the reservoir. Rapidly flowing water tends to mix right up to the river bed, but in slow-moving stagnation, both in summer and in sub-zero temperatures, thermal stratification can occur in the reservoir, which greatly reduces the transport of oxygen-rich water to the bottom.

The damming up in the storage area makes the Mur more sensitive to oxygen-consuming pollution.

The construction of the ZSK is therefore at least gradually becoming more necessary due to an impoundment for a power plant. The environmental impact of the power plant is lower in terms of oxygen depletion in the stowage, if at the same time a storage sewer collects a significant part of the rainwater mixed water to be expected per year and thus the Mur saves a lot of pollution from sewer overflow.

The authorities have approved the Murkraftwerk and ZSK projects under nature conservation and building law by way of exception, because the public interest in building them outweighs the certain impairment of landscape and nature that this entails.

The task of the central storage canal, however, is to reduce the water pollution of the Mur through the mixed water overflows, which exist independently of a hydropower plant in Graz. In concrete terms, this means that the central storage sewer significantly reduces the amount of uncleaned mixed water from the sewer that is currently being discharged into the Mur. For this reason, the ZSK would have to be implemented completely independently of a Mur power plant in the medium term.

However, due to the resulting synergies with the Mur power plant, it is advantageous both financially and in terms of construction and permits to build the central storage channel at the same time as the power plant.