230 Organic waste recycling: technology and management
Since about 80% of nitrogen is the waste is recovered in the algal cells (Ca) and
algal cells contain approximately 8% nitrogen (N), a relationship of Ca = 10 N
has been proposed. This relationship suggests that a wastewater containing 20
mg/L of N will yield an algal concentration of about 200 mg/L in HRAP, and
that N in domestic sewage might become a limiting factor for algal growth if Ca
exceeds 200 mg/L. To maintain efficient performance of HRAP in waste
stabilization and algal biomass production, the excess algal biomass has to be
regularly removed or harvested as previously mentioned.
Table 5.3 Reported energy equivalents of illuminance and irradiance in the visible range
of daylight (400-700 nm wavelengths)
Energy equivalents Reference
1 ft-candle = 0.0206 gcal/(cm^2 -day)
1 ft-candle = 0.0936 gcal/(cm^2 -day)
1 ft-candle = 0.0913 gcal/(cm^2 -day)
1 ft-candle = 0.0933 gcal/(cm^2 -day) (daylight, full sun)
1 ft-candle = 0.1119 gcal/(cm^2 -day) (blue sky light)
1 ft-candle = 0.072 gcal/(cm^2 -day)Kreith and Kreider (1978)
Wetzel and Likens (1979)
Golterman (1975)
Coombs and Hall (1982)
Coombs and Hall (1982)
Bickford and Dunn (1972)Table 5.4 Illuminance and irradiance data for Bangkok, Thailand (25th February 2006)
Time of day Temperature
(oC)Illuminance
(ft-candle)Irradiance a
(gcal/(cm^2 -day))
7:00 am 21 360 80
12:00 pm 37 6450 1370
5:00 pm 36 870 230
a the visible light intensities are about 40% of these measured values
Phosphorus (P), important to algal growth, is not expected to become a
limiting factor because of increased use of detergents in the home and
industries. Other nutrients, such as magnesium (Mg) and potassium (K), should
be presented in domestic sewage in sufficient quantities to support the growth of
algae in HRAP. The percent contents of P, Mg, and K in algal cells are 1.5, 1,
and 0.5, respectively (Oswald and Gotaas 1955).
Depth
Pond depth should be selected on the basis of the availability of light to algae.
Oswald and Gotaas (1955) suggested that it might be approximated by Beers-
Lambert law: