Dual media sand filters at Henriksdal WWTP began to show high head loss after five years of operation, especially during the fluidization for sorting the filter bed after backwashing. At that time, the filters were still clean and the dynamic head loss, due to clogging of the filters with suspended solids during a cycle of operation, had its lowest value. Dynamic head loss over the lower sand layer surface in the dual media sand filter was detected in some filter cycles of operation. The initial head loss in the filters has increased over the years. This type of head loss is constant during a cycle of operation and increases only slowly with time. It is due to different factors, as precipitation of ferric oxide hydroxide in the nozzle slots and on the grains in the filter bed, and accumulation of filter bed grains below the filter bottom. Different measures were tested to reduce or eliminate the initial head loss in the filters. Result from frequency diagrams showed that changing nozzles and removing of filter bed material from below the filter bottom were the most favourable actions. Backwashing more frequently and with more sequences also reduced the initial head loss.
Initial head loss from inorganic clogging developed by filtration of wastewater.
Measures taken decreased the initial head loss by 0.10–0.31 mH2O.
Sand filter nozzles were changed every 12th year in average.
Dynamic head loss developed from organic clogging during each filtration cycle.
Initial head losses were zero–0.67 mH2O before undertaken measures.