DIY 2 Stage Portable Hobby RO

This post will walk through the basic principles of reverse osmosis for maple sugaring, general design considerations for a DIY hobby unit, and an overview of a DIY 2 stage hobby RO unit.  

General Principles:
  1. Reverse osmosis is a process by which pure water (called permeate) is created through filtration at the molecular level through semi permeable membranes, facilitated by an applied pressure (normally from a booster pump).  This removes unwanted ions, particles, molecules, and even bacteria from the water output (permeate).
  2. The "waste" output (called concentrate) is full of all the items removed from the feed water.  In maple sap applications the concentrate is what we are keeping and leads to less water to boil out of the sap, or, in other words, a higher sugar concentration on a volumetric basis. 
  3. While increasing sugar concentration and reducing your time to boil are great, remember, the bacteria in your sap is being concentrated on a volumetric basis the same way your sugar is. This can be both a time saver, and create a race against the clock. 
  4. The permeate should be kept for uses such as flushing the RO, washing pans, etc.  

Design Considerations:
  • How many gallons of sap do you plan to process on a daily basis? Sap runs can average around 0.5 - 1 gallon/tap/day.  
  • What is your boiling/evaporation rate?
  • Will you process all sap first and then boil or process some to seed the pan and run the RO in parallel with boiling? 
  • You want to match your volumetric output of concentrate to your parameters above.  
  1. RO membranes are rated by GPD (gallons per day) which can be translated down to gpm or lpm requirements to inform proper pump sizing.  These normally operate at approximately 60 - 75% of that rating depending upon fluid temperature.  Safe assumption is 60% at 35 - 40°F of sap. 
  2. Membranes also are given an operating pressure (e.g., 100 psi, 200 psi).
  3. Most home-use RO membranes (good for a hobby RO unit) can be run at a recovery rate of about 50% (i.e., feed water is separated into 50% permeate and 50% concentrate) without damaging the membrane or reducing its life.
  4. Membranes can be combined in parallel, series, or a combination of both to achieve desired outputs.  
  5. Series configurations yield higher concentration (doubling at each membrane) but a smaller volume in a set period of time. Each membrane in series could be downsized appropriately to account for the 50% reduction in feed flow into it (the concentrate output from the upstream membrane) to match operating flow requirements.   PRO: higher concentration levels, CON: slow to get enough volume to boil and low flow rates with higher sugar concentration leads to quicker fouling of membranes. 
  6. Parallel configurations can increase the volume of concentrate created in a set time period (doubling throughput for each additional membrane), but require a higher flow rate pump (N times the flow rate for N membranes in parallel).  PRO: fast to get volumes of concentrate, CON: no additional increase above doubling of concentration levels.  
  7. Combination configurations (e.g., a multi stage setup) can help strike a balance between the parallel and series configurations.  For example the concentrate output of 2 parallel membranes could combine and feed a 3rd membrane which is in series to the first 2.  This is called a 2 stage setup and will create 4 times the concentration in the same volume and approximate time period that a single membrane would.  This not only will reduce boiling time (because of higher concentration) but it will also get you to start boiling faster (and prevent bacteria processes on the sugar and keep pace with your evaporator).  The downside is these are a bit more complex for flushing, but the addition of isolation valves can address that.  
  8. RO booster pumps are normally sized by a pump curve which compares 2 operating parameters such as pressure and flow rate.   You will need to pick a pump that is capable of providing the desired flowrate at the target operating pressure (which is the operating pressure of your membranes).  
  9. A needle valve on the concentrate output line can be adjusted to produce the necessary operating pressure and control the recovery rate of the system.  

My DIY 2 Stage Hobby RO design:
  • 50 taps (25 - 50 gallons of sap per day)
  • Evaporation rate of approximately 4 or 5 gph in batches (draw off and combine batches in finishing pan)
  • Want to run RO in parallel with boiling after I get enough concentrate to start the first boil.
  • GOAL: create 4 to 5 gallons of concentrate per hour and quadruple sugar concentration level. 

  1. Selected three 400 GDP membranes to configure in a 2 stage set-up (2 in parallel feeding the 3rd).  These have operating pressure of 100 psi and an expected flow rate of 0.6 lpm (temperature adjusted to 60% of 1 lpm rating)
  2. Selected Aquatech 8852-2J03-B424 booster pump which is a continuous duty pump capable if delivering 1.2 lpm flow rate at 100 psi.  This flowrate will be evenly split to feed first 2 membranes in parallel. 
  3. Output of those feed into a 3rd membrane which will produce 0.3 lpm of concentrate that is 4 times the raw sap (4.25 gph).
  4. Ball valves were added to allow each membrane to be isolated, as well as a bypass loop to the 3rd membrane.  
  5. Inline pre filter upstream of pump protects the diaphragm from larger particles.  
  6. A 5 to 10 micron filter housing protects the RO membranes from finer sediment/particles.  


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  3. I am wondering if you have ever had a problem with pressure increasing pressure. I have running at about 100 psi, but it keeps increasing and I need to adjust the needle valve. One other question is what is the preservative you use in the final cleaning. I have read some people use hydrogen peroxide.


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