10th April 2012 at 1:56 pm #793
I’d say that if flow rates are still acceptable to users, then go for it. As a guide for continually-operated slow sand filters, flow rate should be somewhere between 0.1 to 0.4 m/hr, which is 0.1 to 0.4 m3/m2/hr, which is the same as 100 to 400 litres per m2 sand surface per hour. So I guess have a look at the flow rate equivalent that you have, and aim to not have slower than 100 litres/m2/hr. I don’t know what the impact would be on a slower flow rate, but possibly reductions in oxygen and food transported to the biological layer if it’s too slow?
In general with slow sand filters, I think we struggle to get sand that is fine enough. Studies have shown that (for intermittent filters) a finer sand (nearer the 0.15mm end of the range) is preferable as it gives better pathogen removal results in combination with a lower hydraulic loading above the sand. So normally a fine sand shouldn’t be a problem, but maybe see if you can keep it around or above the low end of the flow rate?
Maybe you can report back with what flow rate you have in practice?