No pictures today - my camera-phone had a completely flat battery and I didn't have a charger with me, but I can finally report a stunning success on the filter front.
More research into the finer points of slow sand filtering eventually led me to throw away the rule-book (which is mostly American) to some extent, and to pick up on some research conducted here in the UK. Several factors have come to light as a result of my web trawling.
The sand - forget common or garden builder's sand. What I needed was a fairly specialist type of sand called 16/30 silica sand. This is very clean and has a carefully graded particle size of 0.5mm to 1mm. It's much coarser than builder's sand, but has a very uniform spread of particle sizes and is the 'right stuff for the job'. Used in swimming pool filters and aquarium filters, it has exactly the right properties. Builder's sand was fine in my larger butt, and it had been well flushed and was filtering ok, but the much larger surface area allowed a reasonable flow. In my 4" filter, there was nowhere near enough surface area to allow more than half a cup an hour of flow, and the almost clay-like minute silt particles just clogged the outlet up.
Depth of sand - conventional wisdom, he say a minimum of 40-60cm of media depth...more if you can afford the space, and a good 50cm or more of head at the inlet. My new findings say I can get away with much less...as low as 15cm potentially. I settled for 25-30cm of sand as that works well in my filter column. With a proviso...and here is the real deviation from the rules.
Reticulated PE foam - filter foam in other words. A very low resistance to flow, but a large surface area on which the active biolayer (schmutzdecke) can form, without forming a homogenous layer on the surface of the sand and slowing the filter down too much and causing head loss.
Apparently, 25mm-50mm of foam sat on top of the sand media does exactly the same job as an extra foot of sand and a schmutzdecke. It also means that the effective run-time of the filter is increased several-fold, the head loss is reduced to almost nothing, and the sand only has to act as a mechanical filter, rather than a mechanical and biological filter. Foam filters have been used for some time in the fish-keeping game. I once had tropical fish and used a bio-foam filter in my aquarium, and this foam on the top of the sand media works in exactly the same way. The bioactive layer forms on the foam, rather than the sand itself....it has a much larger surface area because of the many walls between the pores of the foam, and better still, the foam just needs to be removed and cleaned gently under the tap every so often to wash out any sediment, leaving behind the majority of the active biolayer.
I've read the in-depth analysis, reports of field trials and results of water quality tests on the system, and I'm totally swayed to the new concept. So, a couple of purchases on ebay...one sheet of 50cm x 50cm x 2.5cm reticulated PE filter foam, and one 25kg bag of 16/30 silica pool filter sand.
As luck would have it, today was not only a nice day and I managed to get a bit more painting done, but both my deliveries turned up by mid-afternoon.
I rebuilt the filter and output hose (with washed gravel over the collector tube), set the output hose at about 20cm from the top of the filter column and flushed through three buckets of water. With the hose blocked off so the water stayed in the pipe, I then slowly added the silica sand to the right final level. I ran the three buckets through again, catching the outfall in an empty bucket, and passing it back through the system. The silica sand was pretty clean and dry to start with, but this flushing through was designed to rinse out any 'dust' and to ensure the sand bedded in and formed a nice air-free column. After several runs, and pretty fast runs too, the water was coming out almost clear.
Compared to the original sand butt which produced a very small trickle, dying back to a few drops as the head in the butt got lower, this new filter runs at something like a half a litre to a litre a minute. A little too 'fast' for a slow filter, but with zero head loss and no bio-layer, I'm rather pleased at the speed. The filter was then replaced in the facilities and the outlet pipe coupled back up to the clearwell tank. With a circular piece of the new PE foam in place on top of the sand media, I then filled the column with fresh water (pre-filtered through the Pozanni) to the point where it just began to flow (head height=outlet height). At the point it stopped, the height of the water (supernatent water) above the foam filter (soon-to-be 'schmutzdecke') was checked and found to be a sliver under 5cm, which is apparently the optimum height to allow oxygenation of the biolayer through the water, and to prevent the biolayer drying out. To be fair, I had already marked the heights I wanted to be at, and this was close enough for me. A centimetre or so either way, and I could have added or removed a little sand from the column to optimise my supernatent water level. As it turned out, it was almost spot-on at the first attempt. Sometimes it's luck...sometimes it's luck!
Now, in theory, I should have no leaks, a well-proportioned and effective filter and a rapidly establishing biological filter and a unit that will require a lot less maintenance. Perhaps a weekly or fortnightly inspection and rinse of the foam filter, and then a media (or possibly even a complete column) change perhaps as infrequently as 3-6 months or so.
All I need now is some rain to check that it can cope with the flow-rate from the gutter without complaining to much. I think it will handsomely.
I think I'll let the system establish itself over the next week or so, drain off some of the existing clearwell stock and run it back through the gutter a few times, and then get a chemical/biological test kit and analyse the water produced. If the tests look really good, I may even be able to bypass the Pozanni cartridge altogether and improve the overall flow rate from the storage tank. For now I'll accept a trickle from the tap knowing that my as yet 'unknown quality' of water is going through a triple stage ion-exchange and activated carbon 'backup'.
I did make three cups of coffee today with water from the tap. As yet...no ill effects :o)
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