Reverse osmosis: a reliable technology

Reverse osmosis is achieved by exerting a pressure on a semi-permeable membrane greater than the osmotic pressure on the concentrated solution of total dissolved solids. This semi-permeable membrane is made of non-cellulosic polymer, which is resistant to chemicals and impermeable to larger molecules. However, it does allow water molecules to pass.

The reverse osmosis filtration process is a water treatment technology and can be applied in many areas. These include drinking water production, treatment of process water for discharge into the environment, water recycling, and food and pharmaceutical applications.

Some uses:

• Desalination of sea water to produce drinking water
• Concentration of cane, beet and maple water sugars
• Production of ultrapure water for the pharmaceutical industry
• Treatment of saline water and saline liquid waste
• Concentration of whey from the dairy industry
• Removal of per- and polyfluoroalkyl substances (PFAS) from drinking water
• Maintenance of boiler and cooling tower waters
• Reuse of industrial wastewater

Before reverse osmosis filtration, you need to pre-treat using 5-micron (or less) filtration or ultrafiltration, where necessary. However, this membrane technology is sensitive to clogging by biofilm, fine particles, colloids and organic and mineral compounds.

Reverse osmosis membranes cannot be backwashed like other filtration processes (sand filters or ultrafiltration). The membrane feed is tangential to the filtration flow, so chemical washing must be performed periodically by an automated clean-in-place (CIP) system. Membrane washing must be performed as soon as permeate flows decrease, conductivity varies or when pressure drop is greater than normal.

The advantages of using reverse osmosis filtration are remarkable; the process produces very high-quality treated water, which can be reused in the process where water is in short supply. Filtration produces very clean water as well as a concentrated discharge of salts and dissolved metals. This waste can also be filtered a second time to recover the water from the first discharge and produce an even more concentrated discharge. If the economy is favourable, these highly concentrated discharges can then be evaporated or crystallized to add value to the discharges or provide a by-product in solid form. Filtered water from reverse osmosis is known to be of very high quality; however, it must be remineralized for human consumption.

When choosing to implement reverse osmosis filtration in a process, you must always consider the capital and energy costs required to operate it as well as the savings achieved to manage concentrate. For example, a first filtration stage followed by a second stage of reverse osmosis filtration will significantly reduce the capital and energy costs of managing the concentrate. It should be noted that any water filtered by reverse osmosis is an opportunity to recycle water back into the process.

Reverse osmosis filtration is one of the technologies used for lithium concentration, which requires managing large volumes of water. Reusing the treated water is an innovative concept that can minimize discharges into the environment (minimal liquid discharge, or MLD) or achieve zero liquid discharge (ZLD). Consequently, reverse osmosis can be used to maximize the use of the extracted resources but can also minimize the environmental impact of discharges on the receiving environment.

To meet the needs of the mining sector, particularly with respect to future minerals, referred to as critical and strategic minerals by the Québec government, BBA has a team of engineers and chemists who are specialized in water treatment and have participated in several projects involving reverse osmosis membrane technology. The expertise our teams have acquired over the years allows BBA to offer solutions that meet the needs of our clients while respecting environmental requirements.