GeneralPotable WaterWater

Water Treatment for Off-the-Grid Rainwater Harvesting Systems – Water Organizing Modules (WOMs)

After implementing a rainwater harvesting system, you still need to distribute the water throughout your house. What other steps need to occur before it is safe for human consumption? There’s a lot to consider when building an off-the-grid water filtration system. Luckily there is an ample supply of education from pioneers who have already solved this challenge. This article focuses on the system developed by the Earthship folks in Taos, NM, called a “Water Organizing Module” or WOM. It’s been constantly evolving for 40 years and has proven itself as a reliable model.

A Water Organizing Module serves these primary functions:

  • Connects outside water storage to the interior plumbing and hot water heater
  • Pumps and pressurizes your home’s water supply lines
  • Filters water through reverse osmosis
  • Supplies valves, gauges, and switches for monitoring and controlling your home’s system
  • Acts as the “plumbing command center”


Aside from the autonomy and security that comes with rainwater harvesting and filtration, there are many other advantages that are worth highlighting:

  • Healthy drinkable water without ANY chemicals
  • Water harvested from local systems is used to produce and sustain life before being returned to that exact same system. (i.e. No mass water displacement offsetting natural ecosystems, which is what’s happening with the water grid)
  • Billions of dollars are spent annually every year on maintaining and expanding the water grid, which is incrementally polluting our water supply lines from coast to coast. If the solutions proposed here are applied on a large scale, it would eliminate those costs and mitigate the poisoning of our earth. (and our bodies when we drink tainted water)
  • If there is an incident with water contamination, the problems are isolated and not dispersed over an entire region.
  • If applied on a large scale, it would also return tons of water back into the ecosystems. The need to have tens-of-thousands of miles worth of large pipes filled with pressurized water would no longer be necessary – that’s a significant net gain in available water.

Core Components for Off-the-Grid Water Systems

These are the main elements in a Water Organizing Module (WOM). When you size your system, be as consistent as possible. We ended up choosing 3/4″ pipe sizes for everything, except the pump which is 1/2″. Most Water Organization Modules use either 1/2″ or 3/4″.

Rusco spin down filters arriving in the mail.
Rusco spin down filters arriving in the mail.

Water is filtered through the process of reverse osmosis. By using pressure to force water through a semipermeable membrane, we can separate out the unwanted solutes from our water. In a WOM, this membrane is a Rusco spin down mesh filter.The first one in the manifold is 100 mesh, intended to stop large particles from entering that could damage your pump. Being the first line of defense, this filter is also the one that requires the most frequent cleaning.

The second spin down filter is rated at 500 mesh, which takes out any tiny particles we wouldn’t want to shower in. At this point, the water is ready to enter into our supply lines.

Drinking Water

The drinking water line has two more filters to go through. A 1000 mesh filter, then a 6-cone ceramic filter that will remove 99.9% of bacteria and any class I particles. Instead of a ceramic filter (or as a supplement), some people integrate a UV light which is equally effective. However, there is the additional electricity cost to consider.


The distribution of water throughout the system starts with the pump. Most home lines are pressurized between 40 – 80 psi. Finding the right pump means weighing in the size of your home, the number of lines, and the number of faucets you anticipate being operated simultaneously. There is no magic formula that I know of, and we largely ended up guessing and over-sizing our pump. Worst case scenario – I’d rather have a high pressure than not enough.


Boat and RV pumps are great as they are often built to run on direct current. If your off-the-grid electrical system runs on 12 or 24 volts, you can purchase a pump formatted for those specs. Otherwise, there are plenty of pumps to choose from that operate on standard 110v AC. Ultimately if you’re able, DC is the more energy efficient choice.

An automatic pressure switch will also be necessary. Whenever someone turns on the sink and water begins to flow, that loss in pressure in the supply lines will be detected by an automatic pressure switch – telling the pump it’s time to turn on. To save on parts, we ended up purchasing a pump that has an automatic pressure switch already integrated. The downside to this is that we cannot wire up additional components to that pressure switch. (such as a supplemental UV light)

If a pump had to turn on every single time someone turned a sink knob, you can imagine that would burn up your pump’s life quickly. A pressure tank is a great resource to extend the life of your pump and minimize the noise. Be sure to size your tank according to your pump stats and home needs.

Assembly Parts

The water organizing module manifold is assembled using schedule 40 PVC pipe, glue, thread tape, valves, clamps, bibs, tubes, adapters and fittings. PVC valves tend to leak, so we integrated brass valves throughout as much as possible. The Rusco spin down filters came with PVC flush valves, so we kept those as is.

At the hardware store every part was laid out on the floor so we could be everything mapped correctly.
At the hardware store every part was laid out on the floor so we could be everything mapped correctly.

Designing the system was extremely difficult because I had no experience with plumbing or anything really relevant. My cousin Dan and I built a PVC version of “Redneck Golf” once, that’s about it. It took several weeks of researching all this stuff before I could really wrap my head around it. Some of the most challenging parts were learning what things were called, what they were for, where to find these items, how to assemble them together, etc. In the hardware store, I physically laid out all the parts on the floor just to be sure it was mapped correctly. Some employees and strangers there were happy to offer helpful advise too!

If you’re a total noob like me, here are some basic tips:

  • PVC threading is different than hose threading. There are different types of tubing as well that require adapters or clamps to connect to PVC.
  • CPVC is best for hot water applications or dealing with chlorinated water.
  • MPT and FPT are standard pipe threading. The “M” stands for male, the “F” for female. NPT (National Pipe Thread) is exactly the same, just a different name. MIP and FIP are the same as MPT and FPT, the “I” just stands for “iron”.
  • Schedule 40 PVC is great for handling the Water Organizing Module pressure. If you ended up getting a different size like schedule 80, know that it only effects the inner diameter size.
  • If you’re outside of the US, you’re dealing with entirely different acronyms – good luck.
We iterated through several ideas and drawings. The final product ended up being slightly different than this design. You have a lot of creativity and flexibility with exactly how to build a water organizing module.
We iterated through several ideas and drawings. The final product ended up being slightly different than this design. You have a lot of creativity and flexibility with exactly how to build a water organizing module.

Mounting The Assembly

You have a lot of flexibility in choosing how to connect each core component. We chose a long horizontal setup to create a left-to-right flow for simplicity’s sake. There are plenty of other designs, what’s important is that it works!

To attach the plumbing to the plywood, we used a bunch of hooks that could be tightened using bolts. Hardly any nails or screws were used. The reason for this is because if a problem emerged, we could easily detach it from the board for maintenance.

The DIY Water Organizing Module Costs

The total cost was around $1,000. Water Organizing Modules reportedly last for 10 years before major replacements if you are responsible with upkeep and maintenance. That means if the average American family’s water bill is $30/month, it would take 3 years to break even. However this does not factor in time, labor, or additional rainwater harvesting and storage equipment costs.

Ultimately, the real benefits of water autonomy transcend economics. No toxic chemical consumption. Water freedom and security. Eco-friendly. And if you factor in the billions of dollars spent annually on water-grid upkeep, paid for by us tax payers, then it is significantly more economical to filter our own water.

Cost Breakdown

  • Pump = $170
  • Pressure Tank = $45
  • Ceramic Filters = $135
  • Ceramic Filter Housing, Bracket & Wrench = $49
  • Pressure Gauge = $9
  • Rusco Spin Down Filter 100 Mesh = $65
  • Rusco Spin Down Filter 500 Mesh = $65
  • Rusco Spin Down Filter 1000 Mesh = $65
  • Brass Valves = $52
  • Reinforced Tubing = $30
  • Hooks ≈ $40
  • Plywood = $15
  • Wood Stain & Protective Coating – $23
  • Adjustable Ropes = $18
  • PVC Glue, Primer, & Tape = $30
  • 3/4″ PVC Pipe = $4
  • PVC Joints, Tees, & Fittings ≈ $100
  • Nuts, Bolts, Washers & misc ≈ $60

Testing and Home Integration

Depending on where you live, you may not be allowed to utilize such a system. Or it may require additional permits, fees, and inspections. Be sure to check out our collection of states and counties that are off-the-grid friendly.

Testing all the connections for leaks.
Testing all the connections for leaks.

Originally Published Here:


  1. Seriously? I built a system that will produce over 1,000 GPH of drinking water, for under $500 (110 volts). The filter will need to be changed based on the water quality in, and the actual amount of use it. It takes about 5 minutes and costs about $3. I use rain water and change every year just for the hell of it.

    DC motors are LESS efficient than AC and the further from the power source, the more dramatic the efficiency loss. If our power grid were DC, we would have to have a generating station in every other block! That’s why our grid is AC. I vote for nuke plants in my neighborhood.

    If someone builds a system like this it would save a lot of time, space and money to just go filter, filter, filter, filter, pump. They could also put a pressure gauge between each filter to see which one needs replacing. But that’s silly. They could just build a system like mine for less time and money with a simple water in, water out and power connection.

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Articles

Back to top button