244 Organic waste recycling: technology and management
To satisfy the minimum time span for algal generation in HRAP and to avoid
cell washout from the system, select an HRT = 1.8 days.
Volume of HRAP = flow x HRT
= 500 × 1.8 = 900 m^3
Surface area of HRAP = pond volume/depth
= 900/0.25 = 3600 m^2
Choose 2 HRAP units, each with the following dimensions: length = 100 m,
width = 18 m with 6 channels (or the width of each channel flow is 3 m).
From the above results, the value of Ca could be re-calculated using Equation
5.7 and taking T = 30°C.
Ca = 0.001475(1.8/0.25)1.71 (400)0.70 (30)1.30
= 238 mg/L which is slightly lower than the theoretical value of 266.7
mg/L as determined from Equation 5.9.
The algal productivity is determined from Equation 5.10.
Pa = Ca (z/HRT) = 266.7 (0.25/1.8)
= 37 g/(m^2 -day)
The above Pa value is within the practical range predicted by Equation 5.5 as
shown in Figure 5.9.
5.3 Algal harvesting technologies
The algal biomass cultured in outdoor HRAP is mainly in the microscopic
unicellular forms with concentrations in the pond water ranging from 200 to 400
mg/L. As shown in Figure 5.2, these algal cells need to be separated from the
pond water prior to being used further. The HRAP effluent to be discharged to a
watercourse may need to have algal cells removed so that the suspended solid
content of the pond effluent is within the regulatory standard for discharge.
Because of their microscopic sizes (generally less than 10 ȝm), the separation or
harvesting of algal cells have been a major challenge and difficulty for
environmental engineers and scientists to develop an efficient harvesting
technology which is economically viable. Some of the technologies that have
been used to separate algal cells from effluents of conventional waste
stabilization ponds and HRAP systems are presented below.
5.3.1 Microstraining
Microstrainers or microscreens are low speed (upto 4 to 7 rpm) rotating drum
filters operating under gravity conditions. The filtering fabrics are normally
made of finely woven stainless steel and are fitted on the drum periphery. Mesh