Thesis - faculty.ait.ac.th - Asian Institute of Technology

through the effluent. Different operational conditions for the application of MBR in
wastewater treatment are presented in Table 2.19.
When the performance of MBR was evaluated, the removal efficiencies was found to
be between 78 to 94 % for young leachate with COD > 10,000 mg/L, 60 to 65 % for
intermediate leachate with COD ranging from 5,000 to 7,000 mg/L and 23 to 46 % in the
case of stabilized leachate with COD < 2,500 mg/L. However, the membrane bioreactor
alone was not able to treat the pollutant to meet effluent discharge as it was unable to
reduce chlorides, sulphates, ammonia-nitrogen and refractory organic compounds.
When combined reverse osmosis-nanofiltration system has been operated at a landfill
site in Halle-Lochau, Germany, consistency with a permeate recovery rate of 95 to 97.5 %
could be achieved. The primary disadvantages of membrane bioreactors include capital
costs for the membranes and operating costs associated with routine membrane cleaning.
However, one of the major disadvantages of reverse osmosis and membrane
processes is membrane fouling, more especially biofouling. Biofouling is a serious
problem for the operation of membrane bioreactor systems because it results in decreased
trans-membrane fluxes. Biofouling involves the combined effects of biological, physical,
and chemical clogging of membrane pores. Clogged pores result in: (a) reduced transmembrane
fluxes, (b) a need for higher operating pressures, and (c) deterioration of the
membrane. To eliminate the problems associated with biofouling, it is necessary to study
biofilm attachment and formation on membrane surfaces. By understanding the
mechanisms of biofilm formation, the initiation of biofouling formation can be eliminated.
If the initiation of biofouling is eliminated, the costs associated with cleaning the
membranes could be dramatically reduced.
Membrane bioreactors are further able to pre-treat leachate more successfully than
SBR processes prior to disposal in the sewers. Due to the presence of membrane; a
complete retention of solids is still possible to maintain. However, membrane systems are
susceptible to shock loading of ammonia. When this occurs, biomass may be affected.
2.11.3 Sludge Characteristics
In the membrane bioreactor (MBR) system, membrane fouling is attributed to the
physico-chemical interaction between the biofilm and membrane. When the biofilm gets
deposited on the membrane surface, this leads to decline in the permeate flux. This cake
layer can be removed from the membrane through a suitable washing protocol. On the
other hand, internal fouling caused by the adsorption of dissolved matter into the
membrane pores could be generally removed by chemical cleaning.
The phenomenon of membrane fouling in the MBR system is very complex and
difficult to understand. The sludge characteristic is one of main factors influencing the
membrane fouling which includes mixed liquor suspended solids (MLSS), dissolved
substances, floc size and extracellular polymeric substances (EPS). The components of the
mixed liquor, ranging from flocculant solids to dissolved polymers such as extracellular
polymeric substances (EPS) can lead to membrane fouling.
45