Algal production 227Figures 5.5 and 5.6. Depending on the mode of operation, sewage can be fed to
the HRAP continuously or intermittently, i.e. 12 hours per day during sunlight
period. HRAP is not sensitive to daily fluctuations in loading rate. Effluent
overflow from HRAP, containing high algal suspension, normally goes into an
algal separation unit. The effluent obtained, after the algae have been separated,
is expected to have BOD 5 of 20 mg/L and DO of 0.5 mg/L. The effluent may be
used for various purposes such as irrigation, industrial cooling, or recreational
purposes. Because of these advantages, HRAP has in recent years received
increasing attention as a means of both treating wastewater and producing algal
biomass.
Factors affecting the performance of HRAP and algal production include
available carbon and nutrient sources, temperature, light intensity, mixing or
agitation, pond depth, and hydraulic retention time (HRT). It is generally known
that light intensity is the important factor for photosynthesis and therefore algal
production. Temperature influences the biodegradation rate of the organic
matter, and consequently the HRT to be designed for HRAP.
In the context of photosynthesis, illuminance and irradiance are the two terms
commonly used to express light intensity. Illuminance, or luminous intensity, is
defined as luminous flux per unit area and bears a photometric unit of lux
(lumen/m^2 ) or foot-candle (1 ft-candle = 10.764 lux), which can be measured by
a lux meter or foot-candle meter. Irradiance, or radiant intensity, is defined as
quantity of energy that is received on a unit area of surface over time and bears a
radiometric energy unit of calorie per area per time, for example, gcal/cm^2 -day
(also called Langley/day), which can be measured by an actinometer or a
pyranometer. Table 5.2 gives conversion factors for the most commonly used
units of irradiance. It should be noted that illuminance and irradiance may not be
directly correlated or convertible, depending on several factors such as location
on the earth, latitude, season, and other meteorological effects. Some reported
energy equivalents of illuminance and irradiance in the visible range of daylight
(400-700 nm wavelengths) are shown in Table 5.3, where only ft-candle and
gcal/cm^2 -day are used for the convenience of comparison.
Only solar radiation of wavelengths between 400 nm and 700 nm is available
for photosynthesis by green plants and algae, coinciding with the range of
wavelengths visible to the human eye. The daily amount of solar energy that
reaches the earth and water surface depends on astronomical, geographical, and
meteorological factors. The maximum illuminance measured on the earth
surface is about 20,000 ft-candle while the total solar irradiance (TSI) is about
1370 W/m^2. As the earth surface absorbs about 70 % of TSI and the visible light
intensity within the wavelengths of 400-700 nm is about 40 % of TSI, the
maximum light intensity available for algal photosynthesis is approximately 385