Regardless of what material a membrane is made of and which dimension it has, the main problem in membrane filtration processes is fouling. Therefore, cleaning is essential to reduce fouling issues and restore permeability and selectivity. Cleaning is typically performed with mechanical forces as a physical cleaning or with chemicals as a chemical cleaning.
As feedwater is fed to the ceramic membranes, it is separated into permeate, which is filtered liquid, and concentrate, which is concentrated feedwater. During operation, both these elements are removed from the membranes. However, visible and invisible objects such as particles, oil, and biomass from the feedwater will eventually foul the membranes. Such fouling is unavoidable, so when the fouling reaches a specific level, these objects must be removed utilizing a rinsing mechanism. Such cleaning procedures should therefore be considered preventative maintenance of the membranes of a liquid filtration system. The cleaning procedure can occur either manually, semi-automatically, or automatically. If this process is semi-automated or automated, a set time interval will generate the cleaning process when the filter turbidity exceeds a defined filtration guideline or when the differential pressure exceeds a specified value.
Regardless of whether the cleaning procedure occurs manually, semi-automatically, or automatically, there are three membrane cleaning processes. From what happens most frequently to most infrequently, these processes are 1) Crossflow, 2) Backwash, and 3) Cleaning-in-Place (CIP).
Process 1) Crossflow
In the crossflow rinsing mechanism, the surface of the membranes is continually rinsed with feedwater. By doing so, fouling consisting of small filter cakes of particles or bio-fouling is continuously removed due to the shear force on the membrane surface created by the crossflow. This process increases the time interval that a filtration unit can be operational without further cleaning.
Crossflow filtration with a tubular ceramic membrane
Process 2) Backwash
A backwash is initiated via a backwash pump, which will reverse the liquid filtration system's flow. This means that water is flushed back through the pores of the membrane from the opposite direction to mitigate and remove fouling. A backwash will jog the particles, oil droplets, and biomasses around the membrane's flow channels. A backwash may also include sporadically use of compressed air. A backwash typically takes a few minutes, most frequently approximately 10 minutes, which will ensure that all the fouling is sufficiently removed to continue the operation without further cleaning. The water consumption for backwash is reduced through modern technology, which provides a more sustainable solution by protecting scarce water resources.
Backwash in a tubular ceramic membrane
Process 3) Cleaning-in-Place (CIP)
Until the 1950s, closed industrial applications were dismantled completely to conduct manual cleaning. However, with chemical cleaning-in-Place, we can clean filtration systems without utilizing time to dismantle them. CIP includes the combination of chemicals, heat, and water, which will cause targeted fouling to be dissolved due to the exploited chemicals. However, a membrane needs high chemical resistance to withstand periodic chemical cleaning. As ceramic membranes possess high chemical resistance, CIP is a very efficient method to clean ceramic membranes of a liquid filtration application while maintaining high hygiene standards, which is essential in most fluid purification processes.
A water filtration system often has two attached CIP tanks, ensuring that targeted fouling can be removed. One tank contains a chemical alkaline, which is used to remove objects such as grease and other organic compounds. The other tank contains a chemical acid, which is used to remove objects such as minerals. Thus, the membranes can be cleaned in a wide range of pH.
During CIP, the whole filtration system shuts down, and the required chemicals are dosed into the system. The substances will then be circulated in the ceramic membranes with a crossflow pump to secure sufficient cleaning of all surfaces. The circulation time can be installed based upon specific requirements. To boost the effectiveness of the chemicals, the temperature is increased to approximately 40-60 ˚C. Depending on the water filtration system, the utilized CIP process, and the cleaning requirements, CIP typically takes 60-90 minutes. When the CIP process is done, the system is emptied using a substantial cleaning dissolution, and everything is led back to the water system’s process tank. Thereby, CIP does not produce any chemical waste, which is not filtered.
Once a CIP is performed, the TMP and the permeability are restored.
CIP in a tubular ceramic membrane
Flat Sheet Membrane Cleaning
As a flat sheet membrane has another dimension and uses outside-in filtration, cleaning is performed differently from the tubular membrane. Flat sheet membranes are cleaned with a combination of air-scoring underneath the modules. Still, backwashing and chemical cleaning, as described above, are also used for this type of membrane.
As CIP, air scoring can happen as an automated process. In air scouring, pipework distributes a combination of air and water evenly across the base of a membrane. Combined with high velocity, the water and air remove filter cakes consisting of biomass, enabling a water treatment unit with flat sheet membranes to continue operation. The air scouring intensity highly depends on the feedwater.
Efficient Cleaning Ensures Efficient Operations
It is essential to obtain the perfect balance between the operation and the cleaning procedures. On the one hand, if a water filtration application is not cleaned often enough, the ceramic membranes will foul, which will result in downtime. On the other hand, if a water filtration application is cleaned too often, it will also result in downtime. At LiqTech, we have more than 20 years of experience in membrane filtration systems, and we are ready to support you with expert guidance in terms of preventive membrane cleaning. Identifying the most efficient balance ensures the most efficient and cost-effective operation. Read more about how ceramic membranes work here.
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