|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|Indian experience with algal ponds|
While Spirulina platensis is grown at Delhi (Indian Agricultural Research Institute) and Nagpur (National Environmental Engineering Research Institute), Chlorella is being grown at Pondicherry (Auroville Centre for Environmental Studies). Spirulina, besides its rapid growth rate, high protein content, and lack of a thick cell wall (3), is amenable to simple filtration, giving it an economic advantage over such algae as Scenedesmus and Chlorella.
At Delhi, the algal production unit is a 30 m² cement tank with a partition in the middle to facilitate circulation of the algal suspension by means of a hand-operated paddle wheel turned for 30 minutes twice a day (Figure 1). For harvesting, the algal suspension is pumped out by a hand-pump onto a series of cloth filters fitted to wire mesh baskets suspended in a frame. The filtered algal slurry is scooped out of the cloth and sun-dried. The filtrate is then recycled into the production unit. The average yield of algae amounts to about 15 - 20 g/day/m².
Tables 1 and 2 show the growth potential of Spirulina in the digested slurry effluent from the cow dung gas plant and in cattle urine, respectively, with and without bicarbonate fortification. The slurry effluent supported algal growth at all dilutions even in the absence of added bicarbonate, although addition of bicarbonate (18 9 NaHCO3/l) stimulated algal growth to the level of algae grown in synthetic inorganic nutrient medium. In contrast, pure cattle urine failed to support algal growth in the absence of bicarbonate, presumably because the urine lacks an available carbon source. Supplementation of cattle urine with bicarbonate supported the growth of the algae up to a level of 3 per cent urine, beyond which the urine per se seemed to inhibit algal growth even with addition of bicarbonate.
TABLE 1. Growth Potential of Spirulina platensis in Digested Cow Dung Slurry Effluent, with and without Added Bicarbonate (18 9 NaHCO3/l)
|Dry wt. alga
|1% + bicarbonate||9.2||0.63|
|2% + bicarbonate||9.3||0.74|
|3% + bicarbonate||9.1||0.87|
|5% + bicarbonate||9.1||1.08|
|7% + bicarbonate||9.2||0.84|
|10% + bicarbonate||9.1||0.94|
|Control (synthetic medium)||9.2||-||400||0.9|
Source: Rao and Venkataraman, unpublished data.
TABLE 2. Growth Potential of Spirulina platensis in Cattle Urine with and without Added Bicarbonate (18 9 NaHCO3/l)
|Concentration of urine||pH||Dry wt. Alga(g/l)|
|1% + bicarbonate||8.8||1.33|
|3% + bicarbonate||8.8||0.8|
|5% + bicarbonate||8.8||-|
|7% + bicarbonate||8.8||-|
|Control (synthetic medium)||9.2||0.91|
Source: Rao and Venkataraman, unpublished data.
Spirulina has a 50 - 60 per cent protein content with a well balanced amino acid pattern except for a deficiency of sulphur amino acids. The PER is higher than that in Chlorella and Scenedesmus (4) (Table 3). The BV, TD*, and NPU values are 68, 75.5, and 52.7, respectively. Cereals like rice, wheat, and ragi fortified with the alga were used in our experimental diet. Because lysine content is higher in the alga (4.34 9/16 9 N) than in the cereals, an increasing proportion of algal protein in the supplemented diets progressively improved the PER. The best growth pattern was obtained in diets containing alga and rice, each of which contributed 50 per cent of the protein.
TABLE 3. PER, NPU, and BV Values of Different Micro-algae.
|Spirulina maxim*||2.30||45.6 - 49.8||60 - 65|
|Spirulina platensis||2.07||52 7||68|
* Clement and Van Landeghem (3)
** Becker et al. (4)
Two types of integrated recycling systems are being developed at the Nagpur centre. In one, domestic sewage is utilized to produce an alga which is then used for fish culture. To adapt Spirulina to raw and settled sewage, a system has been developed in which the alga is initially grown in a synthetic medium that is progressively diluted at regular intervals with raw-plus-settled sewage, and finally with raw sewage alone. By this means, a population of alga that grow profusely in raw sewage has been selected. As sufficient phosphate is present in sewage, no phosphate fortification has been found necessary. However, 2 - 3 g NO3/l are required under such conditions for optimum growth of the alga. In contrast to a high requirement for bicarbonate (18 9 NaHCO3/l) by the alga growing in the synthetic nutrient medium, only a little bicarbonate (2 - 4 9 NaHCO3/l) is required in the sewage medium.
The other approach involves an integrated system of a night soil gas plant, algal culture, and pisciculture. The digesters have a capacity of about 18 m³ and yield about 25 m³ gas per day. The volatile solids loading is kept at about 2.5 kg/m3 /day. Destruction of volatile solids varies from 40 to 50 per cent. The oxidation pond (36' x 18'x 4') is made of earthen embankments with inlet and outlet structures and normally holds about 25 m³ (825 cubic fee feet) of effluent. After digestion, the sludge filtrate is added to the pond. The biological oxygen demand (BOD), chemical oxygen demand (COD), suspended solids, pH, and alkalinity are being studied to arrive at the optimum load for a pond of this size. The efficiency with which this system kills helminths and other parasites will also be determined. The sludge is dried on special drying beds and carted off as manure.
At Pondicherry, the major emphasis is on Chlorella production and harvesting. This requires aeration and rain water collection, the latter achieved by pumping and water delivery systems. The circular algal production unit of about 200 m² illuminated surface directs the circulating culture alternatively in a thin moving sheet and in cool, deeper sections to optimize utilization of light and CO2 as well as to control temperature. Three pre-existing concrete slopes have been joined to the pond to give a total of more than 100 m² to be used for the moving sheet illumination and aeration.
Four venturi devices have been installed in one compartment of the underground reservoir to increase aeration in the circulation pattern. The suction produced evacuates the CO2enriched air from a system of three inter-connected fermentation tanks of 25,0001 total capacity as well as from two composting chambers (each 5 m x 0.75 m x 1 m). The fermentation tanks are heated by the composting chambers as well as by the solar collectors that form the top of the tanks. The complete circuit for the circulation of the culture thus includes an aeration chamber drawing warm, CO2-enriched air from the fermentation tanks, a dark retention period in the underground reservoir, passing through the pump into a thin, rapidly moving sheet over the sloping roof, a distribution channel jetting into a cool, growing ring of 20,0001 capacity that overflows into a thin sheet and collects in the cool central growing basin before again passing into the venturi aerators by gravity.
This circulation can be timed at various speeds up to 30,000 I/sec. The average yield of the alga in this system, which uses 3 per cent cattle urine, amounts to 10 - 15 9/ m² /day. Infusion of small quantities of animal blood from a slaughter house has been found to stimulate algal growth considerably.
The alga is harvested from the sedimentation tanks and sundried.