Project Description

International role model in the disposal of sewage sludge

Harnessing the potential of waste products to produce energy is an integral part of a functioning and environmentally-friendly circular economy. From 2021, Wien Energie will process 140,000 tonnes of digested sewage sludge every year by means of a newly developed process. The digested sewage sludge, a by-product of the main sewage treatment plants in Vienna, can be used to produce some 120 GWh of district heating. Wien Energie has invested EUR 36 million in this innovative approach.

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Project details

The drying process at Simmeringer Haide creates the conditions for subsequently converting the digested sewage sludge into environmentally-friendly district heating without using primary energy sources in the fluidised bed furnaces. With this process, which is unique in Austria, Wien Energie has positioned itself as a technical pioneer when it comes to the disposal of sewage sludge in the future.

The sewage sludge first passes through a digestion process which reduces the organic content of the sewage sludge. The main sewage treatment plant in Vienna produces methane gas, which is used to generate environmentally-friendly electricity and heating, thereby enabling it to produce all the power it requires itself.

After digestion, the water is removed from the sewage sludge in mechanical centrifuges at the Simmeringer Haide plant. Some of the sludge is then dried in the dryer section currently under construction. Once it has been dried, it is mixed with the remaining undried sewage sludge, a process which is needed to achieve the necessary calorific value for combustion.

The resulting sewage sludge mixture is then incinerated at temperatures of at least 850°C in fluidised bed furnaces at the Simmeringer Haide power plant, with the energy created here being used to generate steam with a temperature of 350°C, which in turn is converted into electricity for the site and hot water for Vienna’s district heating network.

The remaining ash from the fluidised bed furnaces is cooled and will undergo a process to recover phosphorus from 2021 onwards. The phosphorus has a high degree of purity and can therefore be used to make fertiliser, enabling the City of Vienna to create a full circle for the key raw material phosphorus, which is becoming increasingly scarce.

The previous approach to disposing of sewage sludge did not involve any prior digestion and subsequent drying.

The addition of the digestion step means that the organic content of the sewage sludge can now be significantly reduced. However, the organic parts of the sewage sludge are very important when it comes to the calorific value. A higher calorific value is needed for the subsequent incineration of the sewage sludge to be able to thermally process it without having to add primary energy sources such as gas or oil. This means that a mixture of undried and dried sewage sludge is created for the most efficient calorific value to be ensured.

This new process significantly reduces the volume of sewage sludge while, at the same time, mixing undried and dried sewage sludge increases the calorific value, thereby enabling larger quantities of sewage sludge to be processed with fewer CO2 emissions. This was not possible in the past. It was only by combining dried and undried sewage sludge in this new way that it was possible to achieve such an improvement in efficiency.

All environmental standards are comfortably met throughout the entire process, from digestion and dehydration to drying and incineration. A closed system for transporting the sludge along with the system of returning the cooling water and condensates ensure that no pollution escapes into the air, water and soil at any time.

The overall process increases the degree of energy utilisation of sewage sludge as a green fuel. By eliminating the power needs of the main sewage treatment plant, an additional 40,000 tonnes of CO2 per year are saved. Another environmentally-friendly advantage of the process is that phosphorus, a known scarce resource, can be recovered from the ashes.

In addition to the positive features outlined above, bringing the technical facilities close together at ‘one’ location avoids the need for costly transport by rail or truck, which in turn leads to sustainable relief for the environment.

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The Vienna Model

The model combines the use of waste heat from power generation (cogeneration), the generation of energy from waste, the utilisation of industrial heat and the use of renewable energy sources. This saves three million tonnes of CO2 every year.