Catchment Water Pre-Filter

[Bob Orts]

I looked through all the past post and didn't really see anything on pre-filtering of catchment water. The systems I'm use to seeing are individual or combo of sand bed or bio-pads. The idea is to keep as much hard and biological debris out of the storage tank to reduce contaminants and organic growth.

So, in addition to the "debris" type filters/diverters/flushers are there any pros or cons to using prefilters from the gutters to the catchment tank? Are there technical reasons it just won't work on a catchment type system?

[Les C]

Bob, I haven't heard of filtering before the tank. What occurs to me is that unless you have enough pressure to push the water through the filtering substrate, you may end up with less water in your tank. In what seems to be a standard filter setup, the filters are placed after the pressure pump, which pushes the water through them.

[Bob Orts]

Thanks Les, I spoke to the manufacture and they said the filters are based not on pressure but on gravity. So long as it's engineered to the downspout's output capacity with a large enough surface, there's no additional resistance on output to the catchment tank. It's a matter of making sure the drop is sufficient to keep a flow over the material and not backup into the gutter.

I guess I was curious why they weren't being used that much? I would think keeping as much foreign matter (solid and biological) out of the catchment tank would be better for the pumps and filters, as well as reducing biological growth.

[Kelena]

I spoke to a very nice and well informed lady at Hawaii Catchment who expressed serious doubts about the efficacy of "First flush diverters". What I took away was that anything that adds to the complexity of the water machinery creates potential problems, and that the most important thing to do is to filter and zap the water, with the recognition that the tank will need to be cleaned from time to time.

We did not discuss pre-filters though. I may have misinterpreted but she seemed skeptical about front end solutions, and more concerned with filtering solutions. Agree or disagree, I have to say she was really, really helpful.

[Bob Orts]

I agree that filtering (filters and UV) is the proper end of the system. What I'm confused about is the practical reasoning.

Let's say you have a jug of water that you run through a filter and UV light before using. You'll end up with a darn good cup of coffee. Now, what if you added a drop of dirt, a few dead insects a live larva, and a piece of leaf debris to that jug? After passing through a filter and UV light, you'll still have a darn good cup of coffee. But add a few things each time to the jug and you have to question what type of contamination your filters must contend with, not to mention the serviceable life span of the filters with that extra stuff in the water. I would think that the biological/vegetative mix isn't stable from the first filling to the cleaning and something biologically is going on in the water storage tank.

As for the mechanics, its a non-mechanical passive system.

[allensylves]

When we stayed with friends in Thailand a few years ago, they had a sand pre-filter, but I do not remember any details.
We had a large in-ground cistern for one of the houses on my parents land in Eastern Washington. Inside there was a large brick 'beehive' filter over the outlet, since it was filled with irrigation water from a ditch coming from a river. The cistern must have been at least 40-50,000 gallons, since there was no irrigation water available over winter.

Allen
Baton Rouge, LA & HPP

[Les C]

Bob, I think that the pre-filter idea is relatively new in the Islands and it seems that most folks don't want to spend the extra money, even if it's to clean up their drinking water. The catchment companies will carry items only if people are willing to buy them.

I think the prefiltering or leaf diversion is an intriguing idea, please share more about it, or email me directly.

I attended the Hawaii Conservation Conference last week in Honolulu. One of the presenters talked about the use of a large copepod (an aquatic organism) that eats mosquito larvae. I am interested in using them in the catchment tank. The presenter said that in SE Asia, they were being used successfully in the the drinking water storage to control malaria by controlling mosquito larvae. It would help to have a screen over your intake pipe so that you don't suck the copepods out of your tank and into your filter system. Or, you can use a floating intake to keep the intake off the bottom of the tank. There is a large native species of copepod that can be used.

Edited by - Les C on 07/30/2007 21:44:10

[Bob Orts]

Les, the system I’m looking at is one of the local built units from a local rancher store. The guy was real open and honest and said a filter can be built for under $100 but he likes to sell them for a few hundred to the “city folks” with cabins in the hills.

The basic concept is nothing but a pond filter without all the stuff to remove nitrates and ammonia from fish waste. He also said for a rainwater collection system, you don’t need any pump; just let gravity do its thing. The unit he was building when I stopped in was nothing but a large 3 X 5 X 3 rectangle plastic box lying on its back with a latching hinged cover. Water from the downspout enters the side pipe at the top, drains over a coarse mat that holds back the leaves, insects, dead mouse carcass, bird feathers, twigs and stuff like that. That drains down through a finer mat that is really to prevent the sand from boiling around and it also catch anything that may have come off the mouse. The water drains down through two feet of sand into the bottom where a pipe takes it to storage. You can add other things if you like but it’s really not necessary.

He said it’s not an attempt to get the water to drinking quality; it’s just to remove as much solid matter from entering your storage tank. The idea is that the less decaying matter in the storage tank, the less the water in the storage tank is breeding and creating its own contamination. Supposedly a decaying caterpillar can cross contaminate a gallon of water in a day and that gallon of water can accelerate the cross contaminate of another few gallons of water that has its own debris floating around. If the caterpillar was never introduced to the storage tank, the degree of contamination and cross contamination is reduced. Now multiply that by all the stuff that’s entering an unfiltered storage tank and you kind of get his point.

Maybe I’m over thinking this, but it just makes sense to me.

[Andrew]

Well Bob you've convinced me. Please provide detail on how I can contact the local rancher store so I can get one of these too.

TIA,
Andrew

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[David M]

Bob

Sure sounds sensible to me. I've sometimes wondered how to incorporate a sand filter prior to tank. Am interested in learning more.

David

Ninole Resident