Symptoms
Sand presence in the collected screenings
Water level increase in the screening channel
Sand reduction collected form the grit chamber
Main causes
Reduced approach velocity
Obstruction occurrence in the screening channel
Investigations and analyses
Confine the area where sand accumulates
Measure the approach velocity corresponding to the different wastewater flow rate;
Examine the as-built drawings to check the presence of some shape irregularity in the approaching channel
Remedial actions
If the approaching velocity is less than 0.5 m s-1 then an increase is needed:
a temporary solution could be the flow rate increase through a recycle flow, a reduction of the screening channels (if there are more than two working in parallel) a water level reduction modifying the out flow weir
Empty the screen channel and remove all the bottom irregularities