Case

Complete replacement of sand filter system at Danish wastewater treatment plant

A complete replacement of the sand filter system at Herning Renseanlæg has been carried out, and VATECH 2000 has supplied 120 AVK butterfly valves fitted with pneumatic actuators for the project.

At Herning wastewater treatment plant, 25 years of operation of the sand filters had set they marks, and a complete replacement of the filters was necessary.

Increasing maintenance and decreasing performance were signs that the sand filters needed to be replaced. The valves were no longer closing properly and other parts of the system were worn out. For example, the slots at the bottom of the sand filter did not only pull water but also sand and stones into the system.

Sand filtration is one of the final steps in the water treatment process before the water is discharged into nature. The taxes for discharge depend on the quality of the water purification. New sand filters to improve performance of the treatment process were thus not only better for the environment but also for the economy, and in addition to the heavy requirements for maintenance, which took a lot of man hours, this also gave a financial incentive to replace the filters.

As a first step of the renovation, knife gate valves were installed to section the sand filter system, and allow each section to be individually disconnected for e.g. maintenance or other conditions that require the system to be taken out of service. Then one of the sections was shut down and inspected in details to get an overview of the conditions, and it became clear that the whole system needed to be replaced.

The wastewater to be cleaned in the sand filters is pumped into a trench, and from there it is distributed into the 24 tanks in the system. The actual flow of wastewater into the system determines how many tanks are in use, and the amount of wastewater pumped into the trench and into each of the individual tanks are regulated by butterfly valves. Normal operation of the sand filter system has a capacity of approximately 300 l/s and a maximum capacity of approximately 1,500 l/s.

Part of the cleaning process in the filters is handled by biofilm, which is built up over time, and therefore it typically takes some time to reach the optimal performance of a new sand filter. However, shortly after the replacement, the new filters in Herning showed good results on the quality of water purification. Also, the sand filters became basically maintenance-free and immediately saved a lot of man hours.