MBR operation & maintenance

In operation the operational parameters to be monitored are:

• Biological reactor: DO, MLSS, permeability with filter paper of the activated sludge, pH, conductivity, temperature
• Permeate: water temperature, pH, turbidity, COD, nitrogen…
• Membrane tank; TMP, Return Flow rate, permeate Flow rate, total solids

Trans-membrane pressure difference should be monitored as a useful indicator of fouling

The trans-membrane pressure difference should be continuously monitored so that countermeasures can be immediately implemented if sudden clogging occurs, and also to confirm the effectiveness of the chemical cleaning operations.

When using membrane technology, different types of substances (organic and inorganic) and microorganisms are accumulated on the membrane surface and flow paths over time. It is necessary to conduct regular cleaning operations to minimize fouling and remove the attached substances. It should be considered that a change in the membrane performance may be due to different causes. We can distinguish two types of changes in membrane performance:

• degradation of the membranes: Degradation is an irreversible change in membrane performance caused by a change in the membrane itself: substitution of the damaged membranes would be necessary.
• fouling on the membranes: Fouling refers to an apparent change in membrane performance caused by accumulated substances on the membrane surface or flow paths, including clogging of pores of the membrane capillaries and scaling of salts on the membrane surface. Such changes in performance due to fouling/clogging are often recovered by cleaning the membranes .

Various techniques are used to minimize fouling. Basically fouling of membranes in MBR systems can be minimized by a reduction of flux, promotion of turbulence , air scouring. Membrane cleaning usually includes intermittent physical cleaning (backwash) and maintenance chemical cleaning.

Hollow fiber membrane cleaning methods must include air scour, backpulse/backwash, chemical backwash chemically enhanced backwash. Considering the purpose we can distinguish:

• Preventive cleaning
• Maintenance cleaning
• Recovery cleaning

Types of cleaning methods

Physical cleaning: Physical cleaning includes backwashing and air scouring that can be used independently or in combination.

Chemical cleaning:  As membrane filtration continues, clogged substances that are not removed by physical cleaning accumulate at the surface or inside of the membrane. Then,  chemical cleaning is used to remove such substances in order to recover the membrane performance. When the trans-membrane pressure difference rises during normal operation, oxidants such as sodium hypochlorite are generally used to remove fouling of the adhered organic matter. In cases where fouling is not removed by the oxidant, however, acids, including oxalic, citric, or diluted sulfuric acid, or alkalis, including sodium hydrate, may be effective. It is important to consider the cause of the fouling and the chemical resistance of the membrane material when selecting the type of chemical solutions and determining the time needed for the work, etc..

The different cleaning operations are usually run with a Clean in Place system