250 Organic waste recycling: technology and management
(1975) studied algal removal using dissolved-air flotation process, with and
without coagulant aids. They found that at 25% recycle and 3 atmospheric
recycle pressure the algal removal was 35% in batch process without coagulants.
The percent removal increased to about 80 when ferric sulphate was used at 85
mg/L. The same result could be obtained when 75 mg/L of alum was used at the
same operating conditions.
Autoflocculation
Autoflocculation refers to the precipitation of algae and other particulate matters
in a pond when the pH rises to a highly alkaline level. This phenomenon is
related to the chemical make-up of the water, and, in particular, the presence of
calcium and magnesium carbonates (Middlebrooks et al. 1974). As the algae
remove CO 2 , the pH rises to a point at which precipitation of magnesium
hydroxides and calcium carbonate along with algae occurs, causing removal of
the particulate matters.
A recent study by Sukenik and Shelef (1984) found the proper concentrations
of calcium and orthophosphate ions in the medium to be important for
autoflocculation. To attain autoflocculation within the pH range of 8.5-9.0, the
culture medium (or HRAP water) should contain 0.1-0.2 mM orthophosphate
and between 1.0-2.5 mM calcium. Calcium phosphate precipitates are
considered as the flocculating agent, which reacts with the negatively charged
surface of the algae and promote aggregation and flocculation.
Removal of the precipitated algal cells require another non-agitated basin for
the cells to settle, or mechanical aeration has to cease for a few hours daily if
cell precipitation is to occur in the HRAP. Removal of the precipitated cells may
be conducted during nighttime or early morning when the photosynthetic
activity is non-existent and the algal cells do not tend to rise to the water
surface.
Autoflocculation appears to be a rather simple method of algal harvesting
because it does not require sophisticated mechanical equipments and operation.
However its efficiency in algal removal is generally less than the
aforementioned methods, and requiring a large area for construction of a settling
basin. In addition a very warm and cloudless day is required to attain the high
pH values (greater than 9) in HRAP water; this condition does not occur all year
round in most locations.
5.3.4 Biological harvesting
The unicellular algae grown in HRAP are food for herbivorous fish (fish feeding
on phytoplankton) and other macro-invertebrates. The culture of herbivorous
fish in algal pond water to graze on the algae is an attractive means to produce