236 Organic waste recycling: technology and management
Table 5.6 Design criteria for high-rate algal ponds
Pond depth (z), m
HRT, days
HRT/z, day/m
BOD loading, kg/(ha-day)
Mixing linear velocity, cm/sec
Channel length/width ratio0.3-0.6
1.5-8
6-12
75-300
5-15
> 2- Constant-HRT operation, which is appropriate for tropical climates
where seasonal variations in solar radiation and temperature are
minimal. This method requires the least area, but also produces the
least biomass. - Variable-HRT or -depth operation, which is recommended for
moderate climates and can be economically achieved by varying
pond depth at constant area using a variable-level overflow weir.
During summer period when temperature is high, the required HRT
and consequently pond depth should be less; and vice versa during
the winter period. Azov and Shelef (1982) stated that determining
the required changes in pond depth to produce optimal HRT is a
matter of 'trial and error', depending on operational experiences and
wastewater characteristics.
This method of pond operation has land area requirement 25%
greater than method 1, but the pond productivity is highest. - Variable-HRT operation using dual function ponds, which might be
of interest in agricultural locations, but it has double the land
requirement of method 1. The ponds are operated solely for
wastewater treatment during winter, while some can be converted
into fish-rearing ponds during summer.
A recent development on HRAPs by Green and Oswald (1995), called
“Advanced Integrated Wastewater Pond System” (AIWPS, Figure 5.8), was
found very effective in wastewater treatment and algal production. The system
consists of four units in series: a primary pond (advanced facultative pond, or
AFP) with internal fermentation pits, a secondary shallow continuously mixed
pond (high-rate algal pond, or HRAP), a tertiary settling pond (advanced settling
pond, or ASP), and a quaternary holding pond (advanced maturation pond, or
AMP), which can be used as aquaculture tanks.