Membrane cleaning is an extremely important procedure if you want the water treatment process in your system to be smooth, quick and trouble-free.
You should not expect that fouling will not occur just because you have provided the right operating conditions, pre-treated the membrane or have applied high quality anti-scaling products.
All of these measures are definitely important, but they are not enough to prevent fouling, so in practice, you can only delay the process and only to a certain extent. What, on the other hand, is necessary for proper membrane operation? - It is regular cleaning .
Why does fouling occur on the membrane surface? Fouling is the outcome of many factors: decreasing quality of so-called permeate, i.e. filtrate and, consequently, decreasing flow rate (stream), pressure changes (which have a direct impact on the efficiency of a reverse osmosis process) or fluctuations in salt level and temperature.
When to clean a membrane?
As a general rule, membranes require cleaning when: normalized pressure drops by 10-15%; filtrate flow decreases by 10%; salt concentration increases by 5-10%.
Sometimes it is hard to tell what is the actual level of foulants accumulated on the membrane because usually they are not evenly distributed. On the other hand, you should never postpone the next scheduled cleaning because it is very likely that the layer of foulants on the membrane surface has significantly increased in volume since the last cleaning and can be difficult to remove.
As a result, you will reduce the capacity of the membrane and will have to clean it even more often.
Cleaning done properly and as frequently as needed has numerous advantages: it maximizes membrane performance, extends membrane life, provides better permeate quality, which in turn minimizes energy consumption and reduces overall membrane operating costs.
How to select the best cleaning methods for a given membrane?
When selecting the optimum cleaning method (and agent) for a membrane, the following factors should be considered: type of membrane, type of foulants, bed material, but also fluctuations in the pH value of feed water.
When cleaning the membrane, each step of the process should be carried out separately (this prevents possible cross-contamination).
The cleaning process is more effective when performed at a fairly high temperature, preferably about 15° C. This temperature increases the kinetics of the chemical reactions desired in the cleaning process.
Membrane cleaning methods
Membrane cleaning is related to such processes as ultrafiltration and reverse osmosis (see a separate section for information on these topics).
Ultrafiltration is nothing more than filtration with the use of special membranes, screens and other materials. The filtration process is achieved by applying specific hydraulic pressure.
Ultrafiltration can efficiently separate different particles (elements, ions, chemical compounds, etc.).
If membranes with very small pores (less than 2 nm) need cleaning, a reverse osmosis process is performed to release the particles.
The most popular membrane cleaning methods are: forward flushing, back flushing, air-water flushing (with air) and also chemical cleaning.
- forward flushing
As the name indicates, with this technique membranes are flushed with water (or filtrate) that is supplied in the direction of the flow. The particles of contaminants are captured on the membrane due to a more rapid water flow and turbulence occurring at the same time. Though, the particles caught on the membrane (or more precisely - in its pores) cannot be removed. For this purpose, backward flushing is necessary.
- back flushing
It is a reversed process during which the filtrate passes to the side of feed water. The filtrate is forced under pressure twice as high as the pressure applied during the filtration process (reverse to back flushing). Back flushing is washing the membrane from the inside to the outside. When using this type of flushing, it is always necessary to use the filtrate, because apart from many other reasons, the collection tank must remain perfectly clean.
Sometimes this type of flushing is not effective enough and then a chemical cleaning procedure must be performed.
- chemical cleaning
In this process, a special chemical agent is used to clean the membranes.
Nowadays, advanced chemicals for membrane elements are becoming increasingly popular as they can be used to remove deposited contaminants (both organic and mineral compounds) in an easy, fast and safe way.
These agents include water-based chemicals, polymer-based chemicals or comprise phosphoric acid ester. They are excellent for cleaning various types of membranes used in reverse osmosis plants. With these agents, the membrane is not exposed to any damage, and after all foulants are removed, it is possible to bring every membrane up to a nearly new condition and restore its original performance.
Finally, there are many other chemicals designed to remove microbiological contaminants from membranes effectively. The agents are formulated with polymers and phosphoric acid esters. It is an excellent choice for those who want to quickly and effectively get rid of inorganic residue (formed on the membrane surface). They are ideal for cleaning membranes used in the reverse osmosis process.
The advanced membrane chemical cleaners do not contain chlorine compounds.
They all ensure not only cleanliness, but also longer, trouble-free membrane operation.
- Air cleaning (air-water flushing)
This type of membrane cleaning is gaining great popularity. Air cleaning proves to be very effective if you want to flush a membrane. The membrane can be cleaned very thoroughly with the stream of air (actually this stream is a combination of air and water).
The air causes bubbles to form (forward flush). The air bubbles, in turn, create more turbulence, which results in more efficient removal of foulants.
Which method is best for cleaning? There is no direct answer to this question. It all depends on the parameters of the water, the system, the level and type of contamination, etc. Only after a detailed assessment of the system is carried out, the most suited membrane flushing method can be chosen.
In this case, the decision must be based on experience. Only professional, well-trained technicians relaying on their knowledge can offer the appropriate solutions.
Choosing to cooperate with Granimex, a company operating on the water treatment market for over twenty years, you can be sure not only of the experience of our team of experts, but also of the commitment to implement projects based on cutting-edge solutions.
