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

Activated Carbon Adsorption
Granular activated carbon in combination with biological pretreatment is the leading
technology for the treatment of landfill leachate for the removal of chemical oxygen
demand (COD), adsorbable organic halogens (AOX) and other toxic substances. More than
130 different types of organics have been identified on spent carbon from leachate
treatment plants. Granular activated carbon is used to remove AOX and COD, both of
which are not primary focus of biological treatment systems and therefore, the effluent
quality may be found above discharge consent levels from such treatment systems. With
particularly dilute leachate, it may be operated with a plate separator or pressurized sand
filter removing suspended solids from the flow, in order to ensure that the carbon filter is
not blocked with solids. It is necessary to ensure that there are no substances in the
leachate which will damage the carbon prior to selecting such a system.
When Fettig (1996) studied the treatment of landfill leachate by preozonation and
adsorption in activated carbon columns, the data evaluation revealed that degradation took
place inside the activated carbon beds. Therefore, the total removal efficiency of ozonated
leachate in activated carbon columns was found to be higher than the removal efficiency
due to adsorption processes. A review of physical-chemical processes done by Qasim and
Chiang (1994) indicated that adsorption by activated carbon was more effective in organic
removal from raw leachate than chemical precipitation with COD removal efficiencies of
59 to 94 %. The humic substances remains unaffected by activated carbon treatment while,
1,000 MW fluvic substances could be easily removed by activated carbon.
Membrane Filtration
A membrane is defined as a material that forms a thin wall capable of selectively
resisting the transfer of different constituents of a fluid and thus affecting separation of the
constituents. The principle objective of membrane manufacture is to produce a material of
reasonable mechanical strength that can maintain a high throughput of a desired permeate
with a high degree of selectivity (Visvanathan, et al., 2000). The optimal physical structure
of the membrane material is based on a thin layer of material with a narrow range of pore
size and a high surface porosity. This concept is extended to include the separation of
dissolved solutes in liquid streams and the separation of gas mixtures for membrane
filtration.
The classification of membrane separation processes are based on particle and
molecular size. The processes such as reverse osmosis (RO), nanofiltration (NF),
ultrafiltration (UF) and microfiltration (MF) do not generally require the addition of
aggressive chemicals and can be operated at ambient temperature making these processes
both an environmentally and economically attractive alternative to the conventional
operating units. Table 2.12 summarizes the various membrane processes and its separation
potential. RO membranes can remove more than 99 % of organic macromolecules and
colloids from feed-water and up to 99 % of the inorganic ions.
27