Process water treatment at afu: Extensive upgrade

afu GmbH Anwendungsgesellschaft für Umweltschutz has upgraded extensively the process water treatment system in its soil washing plant in Berlin. This project was completed successfully a short while ago with the commissioning of a new gravel filter unit. Now it is possible to treat 24,000 litres of process water per hour, removing virtually all suspended particulate matter, and feed the water back into the plant cycle.

In the past, afu used uncovered and unpressurized gravel-packed filters which have now been replaced by two pressure filter units, each consisting of two vessels. As the gravel filter unit had to be modified without disrupting operations first one filter unit was installed provisionally and integrated into operations. Then the afu staff dismantled the uncovered gravel filters including the associated conduits and removed their programming from the plant control system. After the selected areas had been prepared the first filter unit could be erected, installed and commissioned. Then the provisional filters were relocated to their final position and also integrated into operations.

The new gravel filter system consists of four pressure vessels, each having a capacity of 6,000 litres, and a feed tank, two pump units and control and monitoring devices. The gravel filter system is fed from the flocculation line upstream where flocculators and lamella separators are used to remove coarse dirt particles from the water to be treated. A feed tank ensures that each of the filters is constantly supplied with water. Approximately 24.000 litres of process water are thus pumped through the operating filter unit per hour. The filter bed is made of quartz sand and quartz gravel, yielding a filter fineness of up to 80 μm so that virtually all suspended particulate matter will have been removed from the process water when it leaves the filter. This also increases the efficiency of the UV wet oxidation process installed downstream of the filters because almost no radiation energy is lost due to turbid matter.

Pressure and throughput monitoring ensures that the load of the respective filter unit can be determined at any time. If a filter needs to be cleaned – for example when the maximum differential pressure is reached – automatic backwashing of the gravel filter can be initiated at the push of a button. This starts the filter unit that has been in standby mode and completes the cleaning process for the blocked filter unit – automatically triggered by the plant control unit.
In addition two activated carbon filters ensure that regulatory limit values for the discharge of waste water are not exceeded and improve process water quality even further. The activated carbon is used for adsorptive retention of dissolved substances that are hardly biodegradable or even non-biodegradable.