Algal production 235of stay in over-exposed layer where irradiance might be too high for the algal
cells (Soeder and Stengel 1974).
On the other hand, mixing results in the suspension of sediments and reduces
light penetration. Too much mixing is also not economic for HRAP operation.
Moraine et al. (1979) found the intensified mixing to adversely affect the algal
population stability and suggests a flow velocity of algal suspension in HRAP to
be 5 cm/sec. Because the surface water in HRAPs is fairly homogeneous due to
paddle wheel mixing, Green and Oswald (1995) suggested that the mixing linear
velocity should be maintained near 15 cm/sec due to the following reasons:
- There are two distinct biological portions in a high-rate algal pond,
an oxidative bacterial floc portion and the photosynthetic algal
portion. At the flow velocity of 15 cm/sec, the algal portion is
suspended but the bacterial portion, being stickier and hence heavier
and more flocculent, stays near the bottom where its optimal pH is
about 7.0 and it is protected from the higher pH surface water.
Being near the bottom, the bacterial floc does not interfere with the
penetration of light into the algal portion, permitting photosynthesis
to proceed. - Maintaining a velocity of 15 cm/sec requires only 1/8 as much
energy as a velocity of 30 cm/sec and only 1/64 the energy of a
velocity of 60 cm/sec - Delicate algal flocs that tend to form are not disrupted at 15 cm/sec
and hence are more settleable when transferred to the settling pond.
Based on the information given above, design criteria for HRAPs are given in
Table 5.5. Since the algal-bacteria symbiotic activities are greatly dependent on
temperature, under tropical conditions, low HRTs and/or high organic loading
rates can be employed for HRAP design and operation.
The principal advantages of pond recirculation are the maintenance of active
algal and bacterial cells in the HRAP system and aeration of the influent
wastewater. Most HRAP have configuration similar to that shown in Figure 5.5
in which recirculation is normally practiced in pond operation.
Although the major factors affecting HRAP performance are light intensity
and temperature, the engineering parameters, which can be manipulated to
produce optimum HRT in year-round operation, are pond dimensions, namely
area and depth (Azov and Shelef 1982). Based on their HRAP research in Israel,
they have proposed three modes of pond operation in which a comparison of
estimated pond dimensions and productivity based on a community of 50,000
people is given in Figure 5.7.