Biology Reference
In-Depth Information
safe treatments. However, UBA identifi ed a number of limitations. These are lack of suffi cient light
and/nutrients, low temperature, low clay or silt, anoxic conditions and absene of clogging layer, i.e.
indicative of the absence of bacteria that bring out decomposition (Gruetzmacher, 2003). Membrane
fi ltration involved physical removal of bloom material via a semi-permeable membrane. It may
cause some damage to cells leading to release of toxins into waters. Although the content of toxins
in waters is not increased (Chorus and Bartram, 1999), fi ltration can not remove extracellular toxins.
Coagulation/fl occulation and clarifi cation are the other conventional treatment methods that are
practised to remove cyanobacterial cells but are ineffective in removing extracellular toxins. Ferric
chloride and aluminium sulphate are used as coagulating/fl occulating agents. Optimization of doses
and the pH are important (James and Fawell, 1991; Mouchet and Bonnelye, 1998). It has generally
been observed that at times coagulation may also cause the release of toxins from the cyanobacterial
cells (James and Fawell, 1991). The introduction of air bubbles into the system following a fl occulation
stage makes the fl ocs to fl oat to the surface. This is termed as dissolved air fl oatation (DAF). This
has an added avantage because cells can be removed easily and there is no danger of the cells
getting lysed. Different cyanobacterial species behave differently for DAF treatment. As for example,
Microcystis
was removed by 40-80%,
Anabaena
by 90-100% but
Planktothrix
only by 30% in a Belgian
DAF plant (Drikas and Hrudey, 1994). A combination of these methods with other techniques seems
to be important in the conventional treatments.
ii) Extracellular toxin removal
:
A number of physicochemical methods have been proposed for the
removal of extracellular toxins from time to time. Adsorption to activated carbon, electrochemical
degradation, photolysis, reverse osmosis and ultrasonication constitute the physical methods.
Chemical treatment methods involve the use of chlorination, ozonation, permanganate and hydrogen
peroxide.
A) Physical treatment methods for the removal of toxins:
(i) Activated carbon
:
Carbon industry
utilizes a number of carbonaceous precursors such as peat, bituminous and lignite coal, wood
and coconut shell for preparing activated carbon (Wigmans, 1989). Apart from these, some other
precursors include almond shells, olive stones, apricot stones, rice husks, cedar nutshells and
corncobs. The purpose of activation is to create a porous structure that can readily adsorb pollutants.
Three types of pores are generally recognized on the basis of their diameter, i.e. micropores (<20 Å),
mesopores (20-500 Å) and macropores (>500 Å). Activated carbon is prepared by either physical
activation or chemical activation. Physical activation involves two steps. In the fi rst step pyrolysis
is allowed in an inert environment at elevated temperatures of 650 to 850ºC followed by oxidation
with steam and/or carbon dioxide at silimar temperatures (Wigmans, 1989; Mazyck and Cannon,
2000). According to Wigmans (1989), carbon dioxide increases mesopores whereas steam creates
a microporous structure. On the other hand, chemical activation does not require intensive heat
and energy treatment but activation is achieved by treatment with zinc chloride, phosphoric acid
or potassium hydroxide. Though chemical activation renders impressive porous structures with
large surface areas, the resulting carbon has an acidic surface that hinders adsorption (Karanfi l
et
al
., 1999). Powdered activated carbon (PAC) and granulated activated carbon (GAC) are the two
sources generally employed.
In order to create a desired adsorbent, carbon activation is tailored by adding a catalyst like
calcium. But in the long run, the deposition of calcium within the pores leads to a detrimental effect
during reactivation procedures by leading to pore widening. To overcome the problems of calcium
deposition, a reactivation process by the use of steam at 375ºC followed by ramped-nitrogen treatment
was advocatd by Mazyck and Cannon (2000, 2002) that can yield reactivated GAC similar to that
