Reverse Osmosis Design for the Food & Beverage Industry - Designing Reverse Osmosis Equipment

Use of Reverse Osmosis (RO) in Food and Beverage Plants

In a Food or Beverage plant, Reverse Osmosis (RO) is often used for plant service water and boiler water pre-treatment. More recently, RO is finding increasing use in the processing of food and beverage products, for example concentrating fruit juices.

The end use of the permeate (or reject water) will generally dictate the design of the Reverse Osmosis (RO) system. Since most boilers in a food plant tend to require low hardness and solids feed water, RO systems in this application are invariably followed by some type of further purification treatment, such as softening (if low pressure boilers are present), or demineralization (if higher pressure boilers are present). Reverse Osmosis equipment, by itself, is incapable of providing the boiler feed water quality demanded even by lower pressure boilers.

If Reverse Osmosis (RO) water (either permeate or reject) is used in other than boiler feed water applications, further purification of the fluid is generally not required.

If an RO system is directly involved in the processing of foods or beverages, RO performance (permeate or reject quality and flow) must be maintained at expected levels. If issues occur with the RO equipment, such as fouling, then the plant products’ quality or quantity is directly affected. This can have a drastic effect on plant profitability.

Understanding How Reverse Osmosis Works

In order to understand how RO works, one must look into the physics of osmotic pressure and semipermeable membranes.

A semipermeable membrane allows the passage of specific molecules through it. If a concentrated aqueous solution exists on one side of a semipermeable membrane, pure water molecules tend to spontaneously diffuse from the more dilute side of the membrane to the more concentrated side. This is called Osmosis.

As water molecules continue to flow across the membrane, the amount of water increases on the concentrated side of the membrane, as does its pressure, called the head pressure. Once this head pressure increases to a given level such that further water flow can no longer occur across the membrane, the system is said to be in equilibrium. The pressure at this point is called the Osmotic Pressure. It is proportional to the dissolved solids concentration in the more concentrated solution.

According to the Van’t Hoff equation for the calculation of osmotic pressure
(symbol P)...
PV = nRT = (g/m)RT or
P = (g/m)RT/V, where

R = universal gas constant, 0.0821 Litre•atm/(mol•K)
T = absolute temperature, K (degrees Kelvin)
g = solute weight, grams
V = volume of solution, Litres
m = molecular weight of solute, if non-ionic
n = moles
P = osmotic pressure, atmospheres

Using this equation, and applying it to an aqueous solution of 1,000 mg/L. of dissolved ionic solids, as CaCO3, we arrive at an osmotic pressure of 7.2 psi [50 kPa] at 77° F.

In general terms, the osmotic pressure averages about 1 psi [6.9 kPa] for every 100 mg/L. of dissolved solids.

By applying a pressure on the concentrated side of this membrane, we can cause this process to reverse. Pure water molecules (and dissolved gas molecules) can be forced to flow from the concentrated side to the dilute side.

This is the entire Reverse Osmosis or “RO” process in a nutshell. Water purification occurs when water molecules are forced to flow from a concentrated solution through a semipermeable membrane to the dilute side.

To overcome the osmotic pressure, and force water molecules to reverse flow, one must apply a pressure. The Net Driving Pressure needed is defined as:

NDP = Feed Pressure + Permeate O. P. (usually negligible) – Permeate Pressure – Feed O. P.
O. P. = Osmotic Pressure

The flow through an RO membrane is proportional to the NDP.

In order to obtain reasonable permeate flow rates, and to minimize membrane fouling, the applied feed pressure must be very much greater than the calculated P. It is generally in the range of 200 – 450 psi [1.4 – 3.2 MPa]. This high pressure requires specific design considerations of RO trains, and elements.

An in-depth analysis of Reverse Osmosis design for the Food and Beverage Industry, including tables and drawings can be downloaded in the free Layne Christensen white paper titled REVERSE OSMOSIS DESIGN FOR THE FOOD AND BEVERAGE INDUSTRY- WHAT YOU NEED TO KNOW.

As a leader in the development of reverse osmosis (RO) systems, Layne Christensen Company has the technical expertise to design and build reverse osmosis systems for all of your plant water needs.
Beyond RO, Layne Christensen’s Water Treatment Division Research & Development team focuses on refining and expanding the water treatment methods we currently employ so we can meet the most demanding challenges head-on with innovation.