Finer sand still possible to use?

[Guest]

#792

I’m currently attempting to set up some slow sand filters in a little more remote places in India. The readily available sand appears to be a little finer (.05-.15). I understand the impact on flow rates, but if the water flows enough for a family in the container we are using, are there any negative side effects in using the finer sand…other than the possibility of having it clog sooner and require cleaning more often disturbing the shmutzdecke layer, of course.

The water I’ve seen in the villages is predominately well water and is very clear, just bacteria and virus filled.

We’ve been working to produce a sand that is within the typical (.15-.35), but that can be quite a challenge when the available materials here don’t produce within that range. Our only other alternative is a much harder process to get .3-1mm sand.

Thanks!

[Eric]

#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?

[gsgriffin]

#794

Thanks. That is helpful way to look at it. The challenge in the area I am planting this is communication and then quality control. I’ve written a complete guide with photos on how to build, start, and maintain the system. It is being translated into other local languages now. Without the backing of a university or foundation or NGO, the availability of equipment to precisely control the sand is really not feasible. If this system is going to be really appropriate, it must be replicable locally by the people here with what they have available. Those are my personal constraints I’m working with.

I’ll try to keep tabs on the flow rates, but that is a challenge due to locations and the people monitoring it.

[Eric]

#795

Yes I totally agree you need to use what sand you have locally. Monitoring shouldn’t be so hard – just go to site once and measure the flow rate when the filter is full – that will give you an idea. Doesn’t need to be a constant measurement I don’t think.