Reverse Osmosis Water Management
 

Created to help WaterWitchin, but I thought I'd Nye on a bit, so here's my thoughts:

Pure water is best for orchids. They evolved getting very pure water, whether that be heavy, tropical rains or high-altitude dew.

Reverse osmosis (RO) is a reliable, economical way to provide that, if you do not have sufficient rain collection or snow melt capabilities.

RO, unless you go for a VERY large, costly system, is not an on-demand water supply. A "100 gallon per day" system sounds like a lot, but that's around a cup per minute, and you're not watering much at that rate. Storage of accumulated, purified water is the way around that.

My volume demand is low, so I can produce it a few hours in advance. Most residential systems are supplied with a bladder tank to hold about 3 gallons, but in greenhouse applications where the usage can be quite high on a per-watering basis, it is more common to have an open-air storage tank, connected to the RO system through a float valve.

Small RO systems, like the counter-top one I use, are manually operated. Turn the feed water on, collect the pure volume needed, then turn it off. Residential and grower's systems are typically automated, via the use of a hydraulic cutoff device.

A little bit about RO functioning: water entering the device passes through sediment and carbon filters, then goes into a membrane housing. There, the water flow splits in two: some of the water is pushed through the membrane - only pure water passes, and the rejected dissolved solids are flushed away by the other stream.

Back to the hydraulic cutoff: before water enters the membrane it passes through one side of the cutoff, which is about the size of an egg. The pure water exiting the membrane housing passes through the other side before going to the storage tank.

http://firstrays.com/Pictures/RO/cutoff.png

Assuming the pure water usage is stopped, the bladder tank fills, or in the case of an open-air tank, that fills until the float valve closes. When the back-pressure in that pure water line reaches 2/3 of the incoming water pressure, the cutoff snaps shut, stopping all flow of water to the membrane.

My setup depends on 1 HP Shallow Well pumps... I have a 150-gal-per-day unit, which feeds into a 55 gallon drum (with a float-switch cutoff) The pump turns on when there is a pressure drop (like opening a hose nozzle or running it through sprinklers) The effluent (which is still decent water for most purposes) goes int two 55-gallon drums (the unit produces 1 gallon of pure water to 2 gallons of effluent) The effluent is delivered to the lawn sprinklers - on a timer, again pressure comes from one of those pumps. It is important to have safety switches on the pumps so that when the tank is drained the pump is turned off... running dry will destroy them in short order. The lawn is happy, and the orchids are even happier.

This is a very helpful thread

Thanks for mentioning the flush water ratio..

Off-the-shelf units typically produce 3 or 4 gallons of flush water for every gallon of pure water produced. That can be reduced by changing the flow restrictor on the flush water line. The 1:2 (pure:flush) ratio Roberta mentioned is pretty much the “tightest” I recommend, as reducing the flush water further results in faster fouling of the membrane.

One should do a financial and environmental calculation on that. Membranes cost roughly $50 and typically last about 2 years, but it may be worth tightening down on the flush water more and replacing them more often.

Boosting the pressure and/or temperature of the incoming water also enhances the efficiency. Membranes are rated at 65 psi & 77F right at the membrane. Increasing them enhances the membrane throughput while the flush/effluent flow is unchanged.

As to what to do with the flush water, I like the idea of using it for less-sensitive plants. In her case, the TDS of the effluent will be 1.5x that of the incoming water (TDS in x input/flush = TDS of flush).

When I had my greenhouse in PA, mine went to a small, artificial pond outside. Frogs loved it, and the local birds, deer, etc. - and our dogs - drank from it.

Most folks just let it run onto the greenhouse floor where it helps keep the humidity up.

If you are using RO in your home, there are kits available that pump the effluent into your home’s hot water lines, making the water usage 100% efficient.

Get an inline TDS meter to monitor inlet and outlet water so there's no guessing as to when to replace the membrane.

My head hurts. I'm gonna study this a while, and get back with you.

Okay, is this kit I found on Amazon what you're talking about:

Malida 1/4" Tube Float Valve Kit for RO Water Reverse Osmosis System water filter Push to Connect Pipe Hose Tube Fitting

Then this TDS thing...

HM Digital DM-1 In-Line Dual TDS Monitor, 0-9990 ppm Range, +/- 2% Readout Accuracy

I think that's overkill... I just check the TDS with my cheapie meter, I'm not looking for great accuracy, just change. Actually, the output rate on mine drops very noticeably before the TDS is affected very much. It still works, but slowly until I start running out of day to refill.

If the output rate drops while the TDS is still acceptable, and assuming that the inlet temperature and pressure haven't dropped, your sediment and/or carbon filters are clogged.