04-30-2019, 10:43 AM
Interesting reading. The 10 micron filter let through more larvae than the 20 micron filter did. The researchers note that the construction of the 10 micron filters was different, having smaller diameter yarn and they seemed more loosely woven. The study acknowledged that the filter ratings were nominal meaning they would stop a range of particle sizes centered around the nominal value. They said more testing would be required to see the performance of filters with absolute ratings. The L3 larvae that infect people can swim but 95% of the larvae were detected in the bottom samples out of 3 samples taken from 50ml water columns. Whole drowned slugs yielded more larvae than minced slugs. The larvae didn't start jumping ship in significant numbers the first 2 days after drowning, peaked during the 3rd and 4th days and tapered off quickly after that. The L3 larvae lasted "at least" 21 days and the L1 larvae lasted at least 56 days. Not sure what at least means. I take it to mean that the dangerous L3 larvae remain viable about a month. All filters stopped most of the larvae and the 5 and 1 micron filters tested let hardly any through. They disinfected their equipment between test runs with 10% bleach solution so I assume that concentration of bleach will reliably kill them.
Having read the report it is clear to me that sediment filters do filter the larvae out to some degree. Getting high quality filters is important. Filling from the bottom and drawing from the top can potentially make a huge difference. A 5 micron carbon block filter stopped all larvae in this study. I think a safe catchment system is possible and it should have the following features:
1. The tank design should exclude slugs and snails.
2. Water from roof should enter bottom of tank and pump pickup should draw from the top level. Avoid excessive mixing. You want the tank to stratify.
3. Multiple filters of progressively smaller filtration size. The 20 micron filters let through less than 10% of the larvae. The 5 micron filters basically let through no larvae except one run showed 1.2% transmission. 10% x 1% is already a very tiny percentage getting through. Finally use a 5 micron carbon block filter and you are down to basically complete removal of larvae.
I think the real threat would be in getting a bad filter with a leak in it, screwing up when changing filters so as to compromise function, or just general poor habits that let lots of unfiltered water through during filter changes. However even unfiltered water is unlikely to have enough larvae to really hurt you though.
Having read the report it is clear to me that sediment filters do filter the larvae out to some degree. Getting high quality filters is important. Filling from the bottom and drawing from the top can potentially make a huge difference. A 5 micron carbon block filter stopped all larvae in this study. I think a safe catchment system is possible and it should have the following features:
1. The tank design should exclude slugs and snails.
2. Water from roof should enter bottom of tank and pump pickup should draw from the top level. Avoid excessive mixing. You want the tank to stratify.
3. Multiple filters of progressively smaller filtration size. The 20 micron filters let through less than 10% of the larvae. The 5 micron filters basically let through no larvae except one run showed 1.2% transmission. 10% x 1% is already a very tiny percentage getting through. Finally use a 5 micron carbon block filter and you are down to basically complete removal of larvae.
I think the real threat would be in getting a bad filter with a leak in it, screwing up when changing filters so as to compromise function, or just general poor habits that let lots of unfiltered water through during filter changes. However even unfiltered water is unlikely to have enough larvae to really hurt you though.