Dresden Stadtentwaesserung GmbH operates a number of sewage treatment works with different capacities in order to treat the waste water from the catchment area (refer to map).

The most modern and largest plant is the sewage treatment works in Dresden Kaditz. The catchment area of this works extends far beyond the city limits. In a number of smaller communities there is still small treatment works applying various processes which have limited operating licences.

Sewage Treatment Works Dresden-Kaditz
From 2nd to 4th June the STW Dresden-Kaditz will be in the spotlight of public interest. That weekend we have good reason to celebrate as we will officially commission the new biological treatment plant. This will be the final stage of a 14 year scheme to upgrade the works and make it fit to meet the latest legislative requirements. And it continues the tradition of the works which was first commissioned in this location on 15th July 1910. The installations of the primary treatment are still in the historical part of the works, only the two-chamber sand trap was added in 1954.

The buildings and plants belonging to that part of the treatment process were completely refurbished in 1990 and 1991. The biological treatment plant and sludge treatment facilities were mostly new constructions. Only the first part of the biological stage was modified. As a consequence of this upgrade the final effluent will eventually have a quality from which it could easily be purified to obtain drinking water.

At the same time as the existing works was refurbished and extended a new stormwater tank facility was built. This new facility ensures a controlled flow to the works and prevents the discharge of untreated water into the river Elbe. The investment for both projects is the largest since the establishment of the company and amounts to more than €100m.

1st Stage: Primary treatment
In order to remove foreign matter from the incoming waste water flow 4No. coarse screens (65mm rack width), 3No. fine screens (15mm rack width) and 2No. grit chambers are being used. In this preliminary plant paper, fabrics, floating wood and plastics are taken out of the waste water flow. The screenings are cleaned and re-used if possible or properly disposed of.

The 2No. grit chambers reduce the velocity of the flow to 0,3 m/sec and thereby force heavier particles like sand, small pebbles etc. to settle out. Although more automated plants are on the market today there was no immediate requirement to replace or modify it. The very simple removal mechanism with crane and attached lug is very cost efficient. The remains from the grit chamber and sand from sewer cleaning is treated in a purification plant and can afterwards be re-used as a filler or additive in the construction industry for instance.

From there the waste water is transferred with 6No. pumps to primary settlement tanks with a total capacity of 4,800m3. In these tanks floating particles and settled out sludge is removed.

2nd Stage: Secondary treatment
In the aeration process soluble organic matter in the waste water is degraded by micro-organisms under aerobic conditions, i.e. while adding air or oxygen respectively.

To facilitate this process 8No. aeration tanks with a total capacity of 28,800m³ are available. These aeration tanks originate like the primary settlement tanks from the year 1991 and were modified. Added to these tanks were 6No. aeration tanks with a total capacity of 84,000m³ and secondary settlement tanks with a capacity of 9,360m³. A special feature of this extension was that the old system could be operated without any interruption as the new plant was built on a previously unused area. The arrangement is based on an optimum hydraulic flow. In this process the phosphate is removed by precipitation.

Sludge treatment
The sludge treatment at the works comprises the following processes:

• Sludge thickening
• Anaerobic digestion (planned)
• Dewatering with centrifuges
• Sludge Drying / Liming

These processes reduce the moisture content of the sludge to less than 10%.

The digestion of the sludge in three closed tanks with a capacity of 8,000m³ each reduce the amount of sludge (by weight) by approximately 30% and produce 12,000m³ of biogas.  This gas is stored in a vessel with a capacity of 5,000m³ and used for the thermal drying of the sludge. The produced biogas provides 90% of the required energy for drying. Before drying the sludge is dewatered with centrifuges. The dried product may be composted and further used for land reclamation, for instance the recultivation of open pits from coal mining.

The end product from the sludge treatment process at the Sewage Treatment Works in Dresden Kaditz meets the requirements to be used as fertiliser in agriculture, for land reclamation or further processing in composting plants.

The sludge treatment facility can also efficiently treat other bioproducts (e.g. sewage sludge from other works, food waste, animal fats etc.). This combined treatment of indigenous sludge and bio-products from other facilities is called co-digestion and contributes to increase the biogas yield and thereby the overall efficiency of the plant.

After having restored the areas for intermediate storage of sludge these are now transferred to become ponds. Flora and fauna will then once again take over this area of the natural reserve.