This part of making the bio-sand filter is a labourious task, but must be done correctly to ensure good quality water at the end of the day.
Sifting the sand
As I mentioned on the last post, we used crushed rock from a quarry (they called it blue rock). The quarry was very kind and allowed me to sit and sieve the stone to the correct sizes and only pay for what I needed. In total I think it took me 8 hours, sore back and a little frazzled by the sun.
My sifting setup at the quarry.
For the bottom of the sand filter where your outlet pipe is going to be fitted, you need stone that measures between 9mm-6mm in size. You should sieve enough to cover the outlet grid by 3 to 5cm. I did not measure, but as far as I can remember it was 2x 20l plastic buckets full of stone. Use a sieve that will allow 9mm stone to pass through, the stone too big will be discarded. Then sieve the stone through a sieve that will allow 6mm stone through. What is caught above the sieve is kept for this drainage layer.
The 9 to 6mm stone around the outlet PVC pipe - it needed another 3 cm above the pipe
The next size down in the stone are the 6mm-1mm sizes, you need enough to fill the drum by 10cm – at a guess it would be 2 full 20l buckets as well. Use a sieve small enough to allow 1mm stone to pass through, the bigger particles left on the top will be used for this layer.
The next size down is the 1mm to 0.7mm stone/sand, again enough to fill up 10cm in the drum. 2x20l buckets. Use sieve to allow 0.7mm through, the bigger particles will be used for this layer. The smaller grains of sand will be used for the filter sand as below.
The 1mm-0.7mm sand before loading the actual filter sand
Then comes the smaller than 0.7mm sand, which is used for the actual sand filtering. The above stone sizes basically stop the sand from passing through the outlet pipes at the bottom of the filter. The amount you need here will be the balance of the drum up to 5cm below the water level – you want to always have water above the top of the sand to preserve the micro-organisms that clean the water – this top layer of sand is known as the schmutzedecke or bio layer. Estimated 6 to 8x 20l buckets of sand will be needed.
An idea of the levels of the stone used in the drum
For the stones – 9mm to 6mm and 6mm to 1mm you need to wash the stones so that when you pour the water out of the container you are using to wash the stone, it runs clear. This check is easy.
Make sure you don’t have too much in each batch, and then pour water double the amount of the stone and swirl it around. You can use your hands, but maybe a clean spade would be better.
Washing the stone and letting it dry
If you are not sure of the cleanliness of the stone, after washing put it out flat in the sun to dry and sterilise for a day or two.
Not as easy for the sand, you need to wash it but not as well as the stone where the water runs clear.
Getting ready to pour the water and start stiring the filter sand for the cleaning process
Here are the steps I took to wash the sand…
- Use 5 buckets the same size – I used old HTH pool chlorine buckets – 10kg
- Use a scoop of about 400g – I used the blue chlorine scoop from the chlorine bucket (these were thoroughly washed first)
- Use harvested water if you have, otherwise municipal water will be fine
- Put 5 scoops of sand into each of the 4 buckets (it makes it easier to swirl and wash when the quantity is small)
- Use the other bucket to fill with water and share amongst the buckets each time (basically a quarter bucket for each)
- For the first rinse, double the amount of water and once you have all the sand moving with the water - swirl for 20 revolutions/swirls – I used my hands, which now have no fingernails and are rather tender – but a wooden paddle may be a good idea
- After you finish swirling, wait for 16 seconds for the sand to settle and pour the water out – this is too dirty to reuse, so just dump it out
- Obviously repeat this for all 4 buckets
- From the second wash through to the end, use a quarter bucket of water for each and swirl for 10 swirls after the sand has mixed with the water – in other words, when the water and sand are all moving together – make sure you get to the bottom corners of the buckets
- Also from the second wash, wait 4 seconds after you finish stirring before pouring the dirty water out – you want to wait the shortest amount of time to allow the sand to settle
- Keep track of how many times you wash, as this will be your standard number of washes you will need going forward – my washing took 7 washes and I kept pegs on a piece of wire to keep track – otherwise you lose track
- For the first to third wash, discard the water – it will be too dirty to reuse
- After the third wash, decanter the dirty water into a container to use for the next wash sequence – you will be lucky to get three washes out of that water, but it will save water
- When the water you pour into the buckets settles on top of the sand and remains quite clear, your sand will probably need one or maximum two washes and it will be done
- To test the sand, take a small hand full of sand and put it into a clear container, put more than double the amount of water in the container and shake it up, then stop and let the sand settle – leave it for 4 seconds and you should be able to make out the surface of the sand – note that the water will not be clear – if it is you have over-washed the sand and the sand will not filter properly.
Checking the filter sand so that after 4 seconds you can see the surface of the sand.
If you are not sure that the sand is sterile, leave it in the sun to dry for two days.
Next we will look at the construction of the filter drum.
Other related articles in this series:
- Harvesting Drinking Water - Preparation
- Sorting and Cleaning the Sand
- Making the bio-sand filter drum
- Lessons Learnt