Microalgae treatment of low-turbidity, soft, highly coloured waters

Microalgae have a great industrial and economic
potential as a wide range applications and one of them is an alternative
biofuel production (Mata, 2010). The harvesting and dewatering operations as a
key challenge for economic algal biofuel processing. Harvesting is a crucial
step in biofuel production from microalgae. Different method exists for
microalgae biomass harvesting such as centrifugation, gravity sedimentation,
filtration and flotation have been put in practice but one of the most effective
method is dissolved air flotation (Margarida et al., 2006).

 

 

Primary clarification involves either
sedimentation or flotation of flocculated water. Flotation has several
process advantages over
sedimentation. It can produce better water quality, it can be operated at high
surface loadings, resulting in relatively small, shallow plants and it can be started up quickly
with a steady water quality being achieved within shorter time than
sedimentation. The solids concentration of the sludge produced is significantly
higher than
that of sludge produced by sedimentation (Margarida et al., 2006).

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Sedimentation is the most widely used primary clarification stage before rapid
gravity filtration for the
production of potable water. However, many raw water sources contain low-density
particles such
as algae because of their tendency to float can cause problems in the sedimentation stage. In particular,
nutrient-rich stored waters that may contain heavy algae blooms are difficult to
treat by sedimentation. In
addition, treatment
of low-turbidity, soft, highly coloured waters produces very light flocs
that settle
very slowly. Therefore,
sedimentation plants can operate only at relatively low surface loadings, and
costly flocculation aids such as
polyelectrolytes are needed to increase the settling rates. Thus due to these
problems, dissolved-air
flotation has attracted considerable interest in recent years as an alternative to sedimentation (Margarida et
al., 2006). Dissolved air flotation is generally considered more effective than
sedimentation in the treatment of algal-rich water. However, the type and dosage
of coagulant and flocculation and air flow rate requirement are key parameters
for removing green algae (Margarida et al., 2006).

 

There
are several flocculants that can be applied to the treatment of a lot of
microalgae. Inorganic coagulant such as aluminum sulfate (alum), ferric sulfate, ferric
chloride, ferrous sulfate, sodium aluminate, iron salts has been used for the
recovery of microalgae via coagulation-flocculation and is demonstrated
successfully to
achieve the goal (Molina Grima et al., 2003). However, coagulation-flocculation
by metal salts may be unacceptable if harvested biomass is to be used for
aquaculture purposes, animal feed or organic fertilizer. High aluminum
concentration not only bring negative effects to aquatic life, but also birds
and other higher animals in the
food chain that consumed the contaminated fish and insects. Another negative
environmental effect of aluminum is that its ions can react with phosphates,
which causes total phosphate to be less available to water organisms.

 

It was reported that the
major component of alum and acrylamide could lead to human health implications,
such as involvement in Alzheimer’s disease and the cause of cancers (Ahmad et
al., 2011; Hamid et al., 2014). Therefore, an alternative of
environmentally friendly harvesting approach need to be developed completely
not only to ease microalgae biomass recovery but also to preserve our natural
environment The harvesting method that
used in this study is by using dissolved air flotation (DAF) and natural
bioflocculant, Moringa oleifera.