Conventional sand- and two-layer filtration is frequently put into action for the purpose of advanced wastewater treatment after low-loaded activated sludge systems. However, the effectiveness of this conventional filter concerning the biological content which is difficult to degrade (measured by COD) is extremely low - and the costs too high. In direct comparison, modifying the filters to a biologically intensified system, brings about a substantial increase in efficiency while costing very little more.

The investigations of the authors for the optimization of the whole system, consisting of an activated sludge process and a biologically intensified filter, led to the fact that for the requirements of advanced treatment, the size of the activated sludge stage can be substantially reduced, without the effectiveness of the system as a whole being affected.

In the field of normal requirements (national effluent standards) and comparable low influx concentrations, biologically intensified filtration can be combined with a high-loaded activated sludge process of SLR up to 5 kg BOD5/(kg MLSS.d). The area required for the construction of the system is under 50% compared to the conventional one-stage activated sludge system.

At higher influx concentrations the simultaneous addition of precipitants/flocculants to the high-loaded activated sludge stage, leads to substantial improvement of the effluent of the whole system and simultaneously to a distinct phosphorus elimination. This process modification is greatly advantageous, especially for densely populated lake areas.

The oxygen supply system of the filter has a key function in connection with the influent concentration and the loading of the biological stage.

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