Good water quality for the "Old Danube"
Clean-up and management of the "Old Danube" lake
The Alte Donau ("Old Danube") is a flat, crescent-shaped lake of considerable size within the city limits of Vienna which has traditionally been used for a wide range of recreational activities. In the 1990s, its water became so nutrient-rich that an "algal bloom" of cyanobacteria occurred, greatly impairing water quality. Together with other measures, application of the so-called Riplox process helped to redress the situation quickly. A comprehensive water quality management regime was set up to ensure long-term good water quality for the Old Danube.
The "Old Danube" lake is one of the former channels of the Danube river system which was cut off from the main water stream as a result of river regulation works in the late 19th century. With a surface of some 1.5 square kilometres and an average depth of 2.3 metres, it is a popular recreation area. As the Old Danube is a stagnant lake, its water is replenished only by subterranean aquifers as well as precipitation and evaporation. The fluctuations of the water level are much smaller today than in the past, with the upper impoundment of the New Danube canal as the main influencing factor.
In the 1990s, a combination of inefficient wastewater disposal techniques (septic tanks and cesspits, inadequate sewers), polluted street runoff, groundwater contamination caused by a nearby former landfill and intensive use of the lake resulted in high nutrient loads which caused massive propagation of cyanobacteria ("algal bloom"). With the disappearance of aquatic plants, the situation deteriorated further, and the Old Danube eutrophicated.
In an effort to reverse this situation, the Municipal Department for Water Management (MA 45) took immediate action to restore the lake to its former equilibrium. Plans were developed to improve sanitation in the surrounding area, and the "Riplox" process was applied to treat the lake water. In this process, ferric chloride and calcium nitrate are injected in the water. The iron chloride precipitates the phosphorus that is responsible for algal growth. During the precipitation process, the phosphorus sinks to the bottom of the lake where the nitrate then helps its biodegradation in the sediment. Remediation of the nearby contaminated site, as well as extension and improvement of the sewer system in the area also contributed to the Old Danube clean-up.
To ensure sustainable high water quality, MA 45 applies a sophisticated water management regime. Every spring, the water level of the lake is lowered to improve the growth conditions for aquatic plants and draw a greater inflow of fresh groundwater into the lake. The aquatic plants are direct competitors of the planktonic algae and are important habitat features, for example for young fish. Tall plants are regularly mown, and nutrients removed from the water with the cut-off plant material. The condition of the Old Danube lake is continually monitored. This enables a swift response in the event of changes, in order to lastingly secure the good state of the water.
Facts & Figures
Old Danube lake:
- Water surface: 1.5km²
- Length of shoreline: approx. 17km
- Average water depth: 2.3m
- Deepest point: approx. 7m (at "Birner Loch")
- Water volume: approx. 3.5 million cu.m.
- Water level is lowered every spring (March-May)
- Extensive monitoring to ensure good water quality
DI (FH) Volker Schaffler
Tel. +43 1 4000 84269
Fax +43 1 4000 7997
- Municipal Department 45 – Water Management
- Water Framework Directive
- Information on the Old Danube lake