Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB

Sludge management in semidecentralized wastewater treatment

Aerobic biological wastewater treatment regularly produces primary sludge from mechanical pretreatment and excess sludge from nitrification and denitrification that has to be removed from the system and disposed of. Removal and reduction of sludge volume determine the sludge storage volume required, and thus the sludge disposal costs, which are a significant factor in the overall operating costs of a wastewater treatment plant. In the semi-decentralized wastewater treatment concept developed by the Fraunhofer IGB, the sludge storage volume can be optimally utilized through the introduction of a sludge management system. This is of especial interest for small sewage plants such as membrane sewage plants. The sludge management system is essentially based on

  • the automated separation of the primary sludge by a solid sensor as well as
  • the automated concentration and separation of the excess sludge with a rotating disk filter.

Experiments using a rotating disk filter for sludge concentration

Change of the transmembrane pressure difference over the sludge concentration during filtration at different temperatures.
Change of the transmembrane pressure difference over the sludge concentration during filtration at different temperatures.

The rotating disk filter is a dynamic membrane filter with ceramic micro-filtration membranes. A stack of membrane disks is mounted within a cylindrical housing on a hollow shaft that discharges the filtrate outside [1]. Experiments in a pilot plant showed that the sludge concentration can be increased up to 3-3.5 percent dry residue without adding chemicals or losing membrane permeability. Figure 1 shows the transmembrane pressure of the membranes over the sludge concentration at constant filtration flux and 10-30 °C. As shown in Figure 2, the sludge is not capable of flowing any more at considerably higher concentrations. Thus, 5-7 times more sludge can be stored in the same volume, assuming suspended solids concentrations between 0.5 and 0.8 percent in the activated sludge reactor.

Experiences at the membrane bioreactor plant Heidelberg-Neurott

Laboratory module of the rotating disk filter with concentrated excess sludge after the end of filtration, which is no longer flowable at 4.5% dry residue.
Figure 2: Laboratory module of the rotating disk filter with concentrated excess sludge at the end of the filtration. The sludge has 4.5 percent dry residue and is no longer flowable.

The sludge management system began testing at the Heidelberg-Neurott membrane bioreactor plant [2, 3] in December 2007. Motor-driven valves were integrated into the filtration stage of the membrane bioreactor that is equipped with Bellmer Fine Filters (industrial application of the rotating disk filter), and the system control has been adjusted accordingly. As a result, the activated sludge can be concentrated automatically in the filter and thence pumped to the sludge tank. Additional volume in the sludge tank is saved by the automated separation of the primary sludge using a solid sensor with scattered light measurement. Up to now, the settled solids in the primary settler were removed by time control. With the installation of the solid sensor, the sludge removal is automatically stopped as soon as the clear water reaches the sensor. Redilution of the removed primary sludge by clear water is thus effectively avoided.

Outlook

Automated filtration stage of the Bellmer Fine Filter (large-scale implementation of the rotating disk filter) at the Heidelberg-Neurott membrane wastewater treatment plant.
Figure 3: Automated filtration stage of the Bellmer Fine Filter (large-scale implementation of the rotating disk filter) at the Heidelberg-Neurott membrane wastewater treatment plant.

Comparable sludge management is not possible with immersed membrane systems, as the concentration of activated sludge within the membrane bioreactor leads to mass transport limitations within the activated sludge tank. Only appropriate filters ensure high filtration flux, high recovery and long operation times. After successful installation at the Heidelberg-Neurott semidecentralized membrane wastewater treatment plant, the sludge management system will prove itself in longterm operation.