Fouling and Fouling Control
The MBR filtration performance inevitably decreases with filtration time. This is due to the deposition of soluble and particulate materials onto and into the membrane, attributed to the interactions between activated sludge components and the membrane. This major drawback and process limitation has been under investigation since the early MBRs, and remains one of the most challenging issues facing further MBR development.
In recent reviews covering membrane applications to bioreactors, it has been shown that, as with other membrane separation processes, membrane fouling is the most serious problem affecting system performance. Fouling leads to a significant increase in hydraulic resistance, manifested as permeate flux decline or transmembrane pressure (TMP) increase when the process is operated under constant-TMP or constant-flux conditions respectively. In systems where flux is maintained by increasing TMP, the energy required to achieve filtration increases. Alternatively frequent membrane cleaning is therefore required, increasing significantly the operating costs as a result of cleaning agents and production downtime. More frequent membrane replacement is also expected.
Membrane fouling results from interaction between the membrane material and the components of the activated sludge liquor, which include biological flocs formed by a large range of living or dead microorganisms along with soluble and colloidal compounds. The suspended biomass has no fixed composition and varies both with feed water composition and MBR operating conditions employed. Thus though many investigations of membrane fouling have been published, the diverse range of operating conditions and feedwater matrices employed, the different analytical methods used and the limited information reported in most studies on the suspended biomass composition, has made it difficult to establish any generic behaviour pertaining to membrane fouling in MBRs specifically.
The air-induced cross flow obtained in submerged MBR can efficiently remove or at least reduce the fouling layer on the membrane surface. A recent review reports the latest findings on applications of aeration in submerged membrane configuration and describes the enhancement of performances offered by gas bubbling. As an optimal air flow-rate has been identified behind which further increases in aeration have no effect on fouling removal, the choice of aeration rate is a key parameter in MBR design.
Many other anti-fouling strategies can be applied to MBR applications. They comprise, for example:
- Intermittent permeation, where the filtration is stopped at regular time interval for a couple of minutes before being resumed. Particles deposited on the membrane surface tend to diffuse back to the reactor; this phenomenon being increased by the continuous aeration applied during this resting period.
- Membrane backwashing, where permeate water is pumped back to the membrane, and flow through the pores to the feed channel, dislodging internal and external foulants.
- Air backwashing, where pressurized air in the permeate side of the membrane build up and release a significant pressure within a very short period of time. Membrane modules therefore need to be in a pressurised vessel coupled to a vent system. Air usually does not go through the membrane. If it did, the air would dry the membrane and a rewet step would be necessary, by pressurizing the feed side of the membrane.
- Proprietary anti-fouling products, such as Nalco's Membrane Performance Enhancer Technology.
In addition, different types/intensities of chemical cleaning may also be recommended:
- Chemically enhanced backwash (daily);
- Maintenance cleaning with higher chemical concentration (weekly);
- Intensive chemical cleaning (once or twice a year).
Intensive cleaning is also carried out when further filtration cannot be sustained because of an elevated transmembrane pressure (TMP). Each of the four main MBR suppliers (Kubota, Memcor, Mitsubishi and Zenon) have their own chemical cleaning recipes, which differ mainly in terms of concentration and methods (see Table 1). Under normal conditions, the prevalent cleaning agents remain NaOCl (sodium hypochlorite) and citric acid. It is common for MBR suppliers to adapt specific protocols for chemical cleanings (i.e. chemical concentrations and cleaning frequencies) for individual facilities.
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