Before You Purchase an Industrial RO System: Sizing the System Appropriately

If you’re looking to purchase an Industrial Reverse Osmosis (RO) System, there are a number of things you should be aware of.  At Complete Water Solutions, our goal is to come alongside you as a partner and help you make the best decisions for your business when it comes to your RO system.
After you’ve worked through feed water testing, the next step to purchasing an industrial RO system is sizing the system appropriately. This one can be easily overlooked or even missed sometimes. 
Most RO Systems will show you how many gallons per day they will put out. For example, the “57K” in the Osmonics E8-57K RO is actually the amount of water the RO could produce in 24 hours. There are 1,440 minutes
in a 24-hour day so if we take the 57,000 ÷ 1,440 = 40 GPM (39.58 GPM). But it’s essential to look at the fine print as most Reverse Osmosis Systems are designed and run best at 77 Degrees F (25 Degrees C). For every degree below 77 Degree F, the system will lose 1.5% of production.
Let’s look at an example of 55 Degrees F Water Source as an example:
77 Degree – 55 Degree = 22 Degree Difference
22 Degree x 1.5% = 33%
57,000 Gallons Per Day – 33% = 38,190 Gallons Per Day or 26.52 Gallons Per Minute
When you do the math, that’s a huge drop in capacity or RO water production. The drop in capacity simply equals a drop in your production. There are a few ways to approach this issue.

Pre-Heat Feed Water

One of the most common ways to achieve this is using a heat exchanger or blending valve to raise the temperature of the water to 77 Degrees F or the most optimum temperature.

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Let’s look at some estimates of energy usage.
Water weighs 8.33 pounds per gallon so we can calculate that one gallon of water requires 8.33 BTU to raise the temperature 1ºF. If we’re using the example above to calculate how many BTUs it would take to raise the water (RO Output of 57,000 GPD) from 55 Degrees F to 77 Degrees F.

57,000 Gallons Of Water x 22 Degrees x 8.33 BTU (8.33 Pounds Per Gallon) = 10,445,820 BTUs
10,445,820 BTU’s ÷ 24 Hours = 435,242.5 BTU’s Per Hour

In the chart below, we break that down into how much of an energy source you’d need, natural gas coal, fuel oil, or KW electricity.

As you can see pre-heating the water can consume quite a bit of energy, which equals a lot of money. It may not be the most cost-effective for your business.

Using Low-Energy Membranes (Cold Water Membranes)

One of the most common alternatives to pre-heating the water is using Low-Energy membranes (commonly known as Cold Water Membranes). Low-energy membranes will produce the same amount of water as standard membranes but at a lower pressure.

Let’s take a look at a common example of an 8” Standard and Low-Energy Membrane.

At first glance, the AK8040F membrane operates at a lower pressure at 77 Degrees F. This helps save energy as you would not need as large of a pump/motor to deliver the 115 PSI @ 77 Degrees F. Are you wondering how do low energy membranes work in cold water? As the water gets colder, it’s more dense/viscous and thus harder to push through a membrane. So, we use the Low Energy membrane and increase the pressure. This is done by running the AK8040F (Low Energy Membrane) @ 225 PSI vs the 115 PSI. This increase in pressure helps keep the production at the 10,000 GPD output. The energy savings goes away in the form of the smaller pump, but you reclaim that back in fuel savings to pre-heat the water. Low-Energy/Cold Water Membranes are one of the most common options when sizing a RO system for water lower than 77 Degrees F (25 Degrees C).

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Size The System Larger

This is likely the least common option, but still a utilized option. Earlier, we discovered that you can lose up to 1.5% (RO Production) per degree lower than 77 Degrees F (25 Degrees C). Moving from 77 Degrees to 55 Degrees F, we lost 33% of Production or 13 Gallons Per Minute Difference on a 57,000 GPD Machine.

Let’s look at the example below on sizing a system for 55 Degrees F using standard AG8040F Membranes:

57,000 GPD @ 77 Degrees F
38,190 GPD @ 55 Degrees F
18,810 GPD Short
We would need an 86,000 GPD RO Machine
86,000 – 33% = 57,620 GPD (Its pretty close)

Strictly speaking, this option is usually not the most economical. It would require you to size your feed water pumps, pre-treatment, and other ancillary equipment for the larger flow. You would also have more wastewater when you go with a larger system. When you contact Complete Water Systems, we can provide guidance on when a larger system might be beneficial.

Trust CWS When It’s Time to Purchase an RO System

Working with Complete Water Solutions creates a partnership. We want to be there with you for the entire lifecycles of your Industrial Reverse Osmosis system. This sizeable investment is a major part of your production process and it’s something that should always be properly maintained and never left to chance. This is our area of expertise.  We’d be happy to help you throughout the process including the purchase and setting up a maintenance plan.
Contact us today to set up a time and date for us to come out and discuss your Industrial RO System needs.

About The Author

Nathan Olszak
27 Year Of Water Treatment Equipment Experience. I have been blessed with the opportunity to learn from the ground up. Starting in the water treatment field as a service technician. Servicing all major brands of water treatment equipment.

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