Sustainable wastewater management in Vienna

Today, Vienna is again at the forefront of wastewater management. Austria’s capital city stands on the threshold of a Sch13Bn ($1Bn) investment programme based on a strategy which starts not at the end-of-pipe but rather at source and is already seeing a number of new approaches to wastewater treatment and disposal.

Developed by the city’s Municipal Departments 30 and 45, and EbS (the company which operates Vienna’s Main STP and incinerator), there are six major elements to the strategy:

1. Waste prevention instead of disposal

The current approach is to use a combination of different systems, allowing the storage of large amounts of stormwater, which can accumulate in very short periods of time. Subsequently, stormwater is conveyed to the STPs for treatment, or allowed to drain off into the rivers when it has been partially treated through storage in outfall ditches within the sewage system.

But this approach is not sustainable. Where stormwater and sewage are discharged into a combined sewer, stormwater can dilute the wastewater. The resulting increased wastewater volume can prove to be an unnecessary strain on STPs as well as requiring an enlargement of sewer profiles.

So, efforts are now focused on re-routing stormwater away from the sewage system by improving drainage in the city.

One idea is to create a series of ‘seepage zones’ by reducing the amount of sealed, tarmacked surfaces in urban areas. This allows relatively unpolluted stormwater – run-off from roofs and gutters – to percolate into the ground naturally. In the city’s ‘seepage zones’, stormwater discharge into the sewers is forbidden, thereby reducing total wastewater volume.

The other part of the plan is to separate natural bodies of water from the sewage system. The Vienna Wood rivers, a series of brooks originating in the Alps, actually flow into parts of the sewer network with the result that clean water can be mixed with wastewater.

By installing ‘parallel river sewers’ – a series of pressure pipelines built on the top of some of the city’s sewers (see schematic illustration, above) – municipal planners hope to allow river water from the Alps to flow directly into the Danube, without being mixed with wastewater.

ii. Industrial discharges

60% of Vienna’s wastewater comes from industry, with 10% alone from the food industry. Raising the price of trade effluent permits has forced Viennese industry to pre-treat its waste before discharge, reducing pollution and preventing non-degradable, toxic substances from entering the sewage system.

2. The immission and emission principles

The emission principle regulates the quality of discharged wastewater by means of limit values. Conversely, the immission principle addresses the pollution absorption capacity of the respective receiving body of water.

EC regulations stipulate permissable emission levels but they fail to take into consideration the capacity of individual water bodies to cope with pollutants.

In Vienna, there are two STPs currently in operation. The city’s Main STP (with a 3.2M p.e. capacity) is located on the River Danube, but wastewater discharges have very little impact. The ratio of the Danube low-water flow volume (850m³/sec) to the wastewater discharged from the STP is about 100:1.

In the case of the city’s Blumental STP (320,000 p.e. capacity), which discharges wastewater into the smaller Liesing River, this proportion is the opposite, i.e. 1:10 (plant discharge 700l/sec, low-water flow volume 70l/sec). Accordingly the immission ratio comes to 1:1000.

The ecological status of Vienna’s receiving water bodies clearly reflects the impact of immission. The biological water quality of the Vienna River, the Liesing River, and several of the brooks originating in the Vienna Woods is heavily compromised due to wastewater discharges but the situation in the Danube and the Danube Canal is the complete reverse as a result of the high dilution effect.

As a consequence, the municipal authorities have decided to close down the Blumental STP in two years time and to treat all of Vienna’s wastewater at the Main STP, saving costs in the process.

3. Integrated sewage system management

An important innovation has resulted from adopting an integrated approach of viewing the city’s network of sewers and STPs as a single system.

The interdependence of the two systems becomes apparent in the wake of heavy rainfall, when solid matter deposited in the sewage system is mobilised, resulting in a greater burden on the STPs.

Put simply, Vienna has large sewers – big enough to walk in. But these only operate at total hydraulic capacity during wet weather flows, on average for 7-8 weeks of the year. During times of dry weather flow, the remaining 45-46 weeks, network capacity is vastly under-used.

This means that combined wastewater, accumulating in the wake of low to medium intensity rainfall, can be collected and stored within the sewage system, and then channelled gradually to the STPs (see cross-sectional schematic, below).

Such a policy permits a substantial reduction in capacity requirements at the STPs – in Vienna from 24m³/sec to 18m³/sec (a reduction of 25%).

4. Technical solutions

Narrowly barred screens (with a 6mm gap between bars) have been installed at the mouth of some of the city’s sewers.

These grates detain much of the solid matter accumulating in the wake of heavy rainfall and storms. With this method, the pollution load on receiving water bodies can be reduced by as much as 8-10%. Moreover, screening serves to avoid visible pollution in water bodies.

ii. Sewage treatment

Austrian standards for large STPs call for a minimum efficiency of 95% BOD removal and 85% COD and TOC removal. The specified minimum for nitrogen removal is an annual average of 70% at a temperature of more than 12oC. Compliance with these standards, as well as the need to improve phosphorus removal, requires changes in process technology and major structural expansions at Vienna’s Main STP.

The two-stage activated sludge process chosen as part of the expansion programme is an advanced technology characterised by high process stability and, compared to single stage processes, low space requirements. With the expanded system, the Main STP will be equipped for biological treatment of 680,000m³/d in dry weather periods and 18m³/sec during storm flows.

5. Cost-benefit optimisation/ process optimisation

In the face of huge investment requirements (Sch13Bn in total) and restricted budgets, municipal planners had to apply objective criteria in prioritising individual needs. Priorities differ depending on whether measures are considered on the basis of the immission or emission principle. In Vienna, the municipal authorities decided to give equal weight to both principles.

The ranking of emission values was based on the investment cost required to avoid the annual discharge of 1kg of BOD into receiving bodies of water. To determine immission values, the figure was considered in relation to the flow of the respective receiving water body.

Furthermore, by determining priorities in this way, city planners were able to better spread out the required investments over a longer period, while at the same time obtaining rapid improvements in water quality.

Part of the funds saved by changing from the old end-of-pipe approach to the new sustainable wastewater management strategy (due to the reduced capacity of STPs and the redimensioning of sewers) will be set aside for ‘renaturalisation’ projects – restoring water bodies to a near-natural state.

6. Integration of water bodies into a near-natural system: the Vienna and Liesing Rivers

The sewers to the right and the left of the Vienna River were built in 1831 after a prolonged cholera epidemic had gripped the city. Even though they are under-sized and in a bad state-of-repair, they are still in use today.

To cope with stormwater overflows, the municipal authorities have decided to build a relief sewer in the bed of the Vienna River.

By keeping the river free of wastewater, it becomes more accessible to aquatic life. The bottom of the river (currently paved or covered with concrete) will then be renaturalised to allow fish from the Danube to return to their traditional spawning grounds in the Vienna Woods.

The ‘natural recovery’ of the Liesing River is likely to prove more difficult. Situated in the south of Vienna, the Liesing is the most polluted body of water in the city. Downstream of the Blumental STP, the river mainly consists of treated wastewater, which has all but destroyed the natural habitat of plants and animals.

Closing the Blumental STP will improve the situation but not cure it. So, in order to reduce pollution, a sewer for the Main STP (taking flows away from the Liesing) will be built. The Blumental STP will then be used as a settling tank for stormwater treatment.

As with the Vienna River, the banks of the Liesing will be ‘recovered’ and the river allowed to return to its original course.

Vienna’s sewage system

The sewage system in the Austrian capital is made up of 2100km of main sewers and about 5400km of household connections. Five large collector sewers, which divide Vienna into five large catchment areas, form the main arteries of the network. These are:

the right and left main collector along the Danube Canal

the right and left Vienna River collectors

the left River Danube collector

the right River Danube collector

the Liesing Valley collector

Because of Vienna’s favourable topographical location, natural inclinations lead most of the wastewater downwards to the STPs. In most cases, a combined system is used to channel domestic sewage, stormwater and industrial wastewater.

The Vienna and Liesing Rivers can only be properly protected if wastewater from combined sewers does not flow into the rivers. So the existing collector sewers, built more than 100 years ago, will be complemented by deep-lying relief sewers to permit the temporary storage and subsequent flow of stormwater to the Main STP.

Redistributing wastewater loads between the left and right main collectors along the Danube Canal will help reduce pollution levels too. The construction of connecting pipes leading to the pumping stations below the bridges across the Danube Canal has solved the problem of mechanical cleaning – a process which, although necessary at regular intervals, leads to massive local pollution and the release of unpleasant odours along the Canal. This has made it possible to avoid a billion-schilling ($80M) investment in a new collector sewer on the left bank of the Canal.