All membrane systems will produce some amount of waste solution. This is necessary in order to carry away the dissolved or particulate components that have been separated from the solution. The proportion of product solution is known as recovery. It is calculated with a simple formula:
Where QF is feed flow or quantity and QC is concentrate flow or quantity.
It is generally desirable to maximise the recovery (i.e. minimise waste) for a variety of reasons. However, there are a lot of things that limit how high it can be. A few of these are:
- scaling or fouling tendency of the water. The more scaling salts or foulants present, the lower the recovery will be. Different salts will scale differently. Antiscalant chemicals are generally used to suppress scaling and enable much higher values.
- A certain minimum amount of concentrate is needed to keep surfaces flushed. In small systems, hydraulics can be improved by recycling a portion of the concentrate, although care must be taken to avoid biological or particulate fouling.
- In the case of reverse osmosis, osmotic limits apply. e. the higher the salt concentration in the water, the higher the pressure needed to drive the process.
Something to be careful of when comparing recovery values, is whether the quoted value is an instantaneous recovery across the membrane itself, or if it takes into account other losses in the overall system (known as “system recovery”). These losses may be from prescreening/prefiltration and chemical cleans or from a few other minor sources such as analytical intruments.
Reverse Osmosis
When designing reverse osmosis systems there is no “normal” recovery value. For example, sea water RO systems may operate at recovery anywhere between 20 and 50%, although 35-40% is most common. In low salinity feed waters, recovery may be 75 to 85%. Take care if someone is telling you that higher than 85% is achievable. Above 85%, a lot of challenges will arise and the designer must be very competent to give you an operable system. Typically, the system cost increases quickly as design recovery goes above 85%, because the complexity and operating pressures increase a lot. In recent times, RO recovery has become much more important due to the challenges of disposing of the concentrate. Unfortunately, even when we maximise recovery, this leads to concentration of the salts. Handling and disposal of these salts can be a challenge regardless of how high the recovery is.
Membrane Filtration
Similarly for membrane filtration systems (MF/UF), different systems can have widely differing recoveries. The recovery will be impacted by feed solids (particulates), membrane flux, process type (i.e. if and how the system uses air for backwashing), frequency of chemical backwashes and cleans. Complexity of connecting pipe work can also impact the achievable recovery, which is generally within 90-95%.