|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|The combination of algal and anaerobic waste treatment in a bioregenerative farm system|
High-Rate Algal Systems for Municipal Waste-Water Treatment
The treatment of municipal waste water in shallow (less than 50 cm deep), mechanically mixed, meandering (folded) channels was first proposed and demonstrated by Oswald and Golueke (9) in California. It was later studied by McGarry et al. (10) in Thailand, by Goldman and Ryther (11) in Woods Hole, Massachusetts, and by Benemann et al. (12) in California, and has been further developed by Shelef et al. (13, 14) in Israel.
Experiments at the Technion Environmental Engineering Research Center ranged from laboratory and pilot plant studies to field-scale studies in ponds of 1,000 m² each, one of which at Haifa Bay is illustrated in Figure 1.
The flow scheme of this system is shown in Figure 2, where raw municipal sewage is continuously introduced to the photosynthetic pond with an average retention time of three days, depending on climatic conditions. The effluent of this pond, which contains between 300 to 500 mg/l suspended matter, mostly algal biomass (approximately 60 per cent) and bacterial biomass (approximately 35 per cent), is treated physico-chemically, using aluminum sulphate (alum) flocculation and dissolved air flotation.
The flotation unit, with a surface flow-through rate of between 4 and 6 m hr(-1), separates the algal bacterial biomass, yielding a treated effluent with average quality characteristics summarized in Table 1 and illustrated in Figure 3. This effluent has a quality adequate for use as irrigation water for most agricultural crops, or for discharge into most receiving bodies of water.
TABLE 1. Effluent Quality from Various Stages of the Accelerated Photosynthetic WasteWater Treatment Process (under Favourable Operational Conditions)
|BOD total, mg/l||330||120||<25||<10|
|BOD dissolved, mg/l||-||-||<10||<10|
|COD total, mg/l||750||450||140||110|
|COD dissolved, mg/l||-||120||90||85|
|Suspended solids, mg/l||260||300||<25||<10|
|Ndissolved (NH3) mg/l||-||20||18||18|
By additional slow sand filtration through a 30 cm-deep sand bed, followed by disinfection by chlorination, the effluent is adequate for irrigation of all agricultural crops, including/hose edible in raw form, for discharge into virtually all surface receiving bodies of water, and for ground-water recharge.
The separated algal-bacterial biomass, in the form of "froth" containing between 4 and 5.5 per cent solids, is skimmed off, thickened by inverted decantation to a slurry of between 7 and 9 per cent solids, further dewatered to a light cake of between 14 and 18 per cent solids, and then steam drum-dried to form dry flakes or powder that can be pelletized with other feed ingredients.
The biomass production, as well as other characteristics of the system, serving, for example, a city of 100,000 inhabitants living in climatic conditions simian to those near Haifa Bay, are given in Table 2.
TABLE 2. Summary of Algae Production Data Based on Technion Pilot Plant and Field-Scale Pond Operation
|Total net biomass yield (dry)||155 tons/ha-year|
|Average daily total biomass production (dry)||42.59/m²-day|
|Per cent algae in biomass||57|
|Algal biomass yield (dry)||88 tons/ha-year|
|Average daily algal biomass production (dry)||24.29/m² day|
requirement for city of 100,000 inhabitants under
Eastern Mediterranean climatic conditions (assuming waste-
water flow of 200 litres per capita per day)
|Total yield (dry) per city||2,060 tons/year|
|Total protein yield (43% dry matter)||886 tons/year|
|Average daily incident irradiance (total)||450 cal/cm²-day|
|Average daily incident photosynthetically available irradiance||202 cal/cm² day|
|Photosynthetic light conversion efficiency (based on
|Photosynthetic light conversion
efficiency (based on photo-
synthetically available irradiance)
|Maximum possible light conversion efficiency (based on
photosynthetic available irradiance)
|Percentage of light conversion attained vs. potential||55%|
The effluent, containing algal-bacterial biomass suspension, can be used directly to feed fish ponds, and yields of Tilapia galilea (St. Peter's fish) of over 7 tons per hectare were achieved in field experiments by Hepher and Sandbank (15). The flotator concentrate, after further thermal dewatering, can be mixed directly with other ingredients and used for semi-wet feeding of cattle and poultry (16). Sun-drying of the dewatered material is practical, preferably when dried over beds containing ground corn, soymeal, and wheat, which are components of the animal feed ration. Because preservation of the wet biomass for more than 48 hours is problematic, wet feeding is possible only when the farm animals are in close proximity to the photo synthetic waste water treatment plant. That is why most of the nutritional and feeding tests were performed with pelletized, drum-dried algal biomass. Extensive feeding experiments with broiler chicks and laying hens (17, 18) showed conclusively that between 25 and 50 per cent of the soymeal portion in the feed rations could be replaced with dried algal biomass, with no change in growth characteristics, weight gain, egg production, or the general welfare of the animals compared to animals fed feedmeal based on soymeal as the principal source of protein. An added benefit of the carotenoid-rich algae feeding was the "healthy" colour of the egg yolk.
Large-scale feeding experiments in intensive, mechanically aerated fish ponds stocked with carp (Cyprinus carpio) and St. Peter's fish (Tilapia galilea) were performed by Hepher and Sandbank 115) using pelletized feed containing approximately 22 per cent of drum-dried algal biomass, replacing on an equal protein basis all the fish meal in the ration (usually 15 per cent of the commercial ration). Fish growth rate, weight gain, and general welfare have been found conclusively to be equal to, and in most cases significantly better than, in fish fed fishmeal-based ration, with yields of between 15 and 20 tons per hectare per year of fish.
Extensive organoleptic and toxicological tests were performed on this material by Yannai et al. (19), using alum flocculated algae. The algae, although containing aluminum and wastewater-borne heavy metals, showed no adverse effects when fed to chickens or to rats fed on the chicken's meat, and no accumulation of these metals was noticed in the animal's meat or other organs. No abnormal taste or odour was noticed in organoleptic tests of algae-fed chicken or fish.
The economics of the combined waste-water treatment, water reclamation, and protein production photosynthetic system is extremely favourable (14), based on the various benefits simultaneously gained by the system - namely: (a) waste-water treatment with lower energy requirements for mechanical aeration, (b) the value of the high quality effluent for irrigation, and (c) the value of the proteins for animal feeding.
Conservatively, subtracting the benefits of waste-water treatment and reclamation results in a net cost for dried algae of less than US$140 per ton, compared to more than US$200 per ton for soymeal and more than US$380 per ton for fishmeal.
The Growth of Algae on Animal Wastes
Animal wastes constitute an excellent medium for growing algae when diluted with water, and they provide a carbon source (through bacterial aerobic degradation that produces CO2), nitrogen, phosphorus, and trace elements to sustain rich algae production.
On small farms in developing countries, where algae separation and processing are too complicated and costly, the growth of the blue-green Spirulina algae is preferred over green algae for the following reasons: (a) mechanical separation by straining or filtration is possible; (b) protein content is high (55 - 70 per cent); and (c) protein digestibility and availability are high.
Under conditions in small rural communities in developing countries, these advantages surpass the disadvantages of Spirulina compared with green algae (Chlorophytee). The disadvantages include: reduced rate of growth, need for higher temperature, need for relatively high concentrations of bicarbonates or carbonates, and sensitivity to high irradiance levels. Growing Spirulina on animal wastes has been studied experimentally under the conditions prevailing in developing countries by Seshadri (20) in India, Soong (21) in Taiwan, and others Two sets of experiments testing Spirulina growth potential on animal wastes are described herein, one on raw (fresh) cow manure and the other on anaerobically digested cow manure.
Growth potential of Spirulina on raw cow manure
Experiments on the growth potential of Spirulina maxima under outdoor conditions in shallow (20 cm) multiple batch-type 200-litre "mini-ponds" have been performed at the Technion Institute in Haifa.
Five kg of wet, raw (fresh) dairy cow manure from Kibbutz Yagur with a total solids content of 16 per cent were added to each 200-litre mini-pond in order to reach initial solids concentration of 0.4 per cent (25 9/l) based only on the added manure and excluding the initial algae concentration of approximately 200 mg/l.
The composition of the raw manure and the concentration of its various components after the manure was added to the 200-litre mini-ponds are given in Table 3. It should be noted that the composition of the manure in Table 3 includes both the suspended and the dissolved matter of the wet manure fed into the mini-ponds.
TABLE 3. Composition of Raw Cow Manure Used for Growth of Spirulina maxima
|Component||Raw cow manure
|Concentration at 1:40
dilution in mini-pond'
|Total volatile solids||135||3,380|
|Total Kjeldahl nitrogen||5.3||130|
|Total phosphate as PO4||4.1||100|
|pH||7.2**||9 - 11**|
* Components of stock algae culture are excluded.
Two weeks before the addition of the manure, the mini-ponds were inoculated with Spirulina maxima that had been isolated from domestic waste-water treatment ponds (the latter ponds were inoculated with Spirulina a few years ago). The initial medium was a synthetic one enriched with 16 g/l of sodium bicarbonate. The temperature of the mini-ponds was maintained at 30 C, and the ponds were continuously stirred by a long-winged rotor (6). The concentration of Spirulina before the addition of the manure was approximately 200 mg/l, and the chlorophyll-a concentration was approximately 3.5 mg/l. The changes in suspended solids concentration and in chlorophyll-a are given in Figure 4.
Following the addition of the manure, chlorophyll-a concentration was reduced by 20 per cent, and for approximately 20 days algal growth was almost negligible. It should be noted that the addition of 5 kg wet, raw manure to the 200-litre mini-ponds created highly stressed conditions as far as organic loading and turbidity were concerned, and the ponds became turbid and brown. During the initial 20 days, intensive degradation of the manureborne organic matter and solids occurred, evident in the reduction of solids concentration.
Spirulina grew rapidly between the twentieth and thirtieth days, as revealed by rises both in chlorophyll-a and suspended solids concentration. After 30 days, algal concentration stabilized and a small reduction in suspended solids was observed after 40 days; this reduction was not followed by a reduction in chlorophyll concentration, which might have been due to the continuing increase in chlorophyll-a content of the alga.
No direct measurement of Spirulina biomass production was possible because of the presence of manure-borne suspended matter and the bacterial biomass developed during the degradation of the manure. Algal growth was therefore estimated by the increase in concentration of chlorophyll-a, which is a rather poor indicator of algal biomass because the chlorophyll content varies according to growth conditions, light availability, etc. Depending on conditions, chlorophyll-a content can range between 0.5 and 2.5 per cent in blue-green algae (22), and between 1 and 2.5 per cent in Spirulina grown in outdoor mass cultures (23). The chlorophyll-a content of the Spirulina maxima prior to the addition of the cow manure was approximately 1.75 per cent, and assuming that its percentage remained the same, Spirulina concentration reached 370 mg/l at the end of the experiment.
The growth of Spirulina following the addition of cow manure was therefore quite slow, but this can be explained by the high initial dose of raw manure. During the manure degradation period, algal growth was minimal, indicating the advantage of using treated manure (composted or digested) as an algae growth medium.
Growth potential of Spirulina on digested manure
Following the experiments described above, anaerobically digested cow manure from mesophilic (35°C) and from thermophilic (55°C) mixed, semi-continuously fed digesters with a retention time of eight days was used as a medium in batch, continuously illuminated, 500 ml laboratory algae growth units. The production of biogas (approximately 65 per cent methane) during the thermophilic and mesophilic anaerobic digestion of the cow manure (24) as a function of retention time and at various concentrations of total solids is given in Figure 5.
The digested matter, with approximately 16 per cent total solids, of which approximately 30 per cent of volatile solids had been destroyed during the anaerobic digestion, was mixed with the sodium bicarbonate-enriched medium at dosages of 10 and 25 g of wet, digested manure per litre of medium. The medium was inoculated with 5 ml of Spirulina maxima concentrated suspension.
Enrichments with 0.5, 1.0, and 1.5 per cent NaHCO3 in the medium were compared to growth units with no enrichment. The concentration of chlorophyll-a after ten days of algal growth, fed with 10 9/l of both mesophilically and thermophilically digested manures, is given in Figure 6.
No marked effect on Spirulina growth was observed by increasing NaHCO3 enrichment above 0.5 per cent, nor have the temperatures of the anaerobic digestion (55 C vs. 35 C) of the feed had a significant effect on Spirulina growth, as evident from the concentration of chlorophyll-a.
The effect of the dosage of digested manure is given in Figure 7, where dosages of 10 and 25 9 of wet, digested manure per litre of alga culture were compared.
The higher digested manure dosage, which was similar to the raw manure dosage described earlier, gave the highest increase (up to 9.3 mg/l after 12 days) in chlorophyll-a concentration. Assuming average concentrations of 1.75 per cent of chlorophyll-a in Spirulina (dry basis), an algal concentration of over 510 mg/l was achieved in 12 days. The limitations of chlorophyll-a as a reliable quantitative measurement of algal biomass production have been discussed previously.
No appreciable lag time was observed in these experiments, and no time requirement for organic matter degradation (which was particularly evident in the experiments with raw manure) was found. Obviously, the production of biogas during anaerobic digestion is an important added benefit to the production of Spirulina Combined System for Algae Production and Anaerobic Digestion.
Following the previously discussed experiments showing the potential of algal growth on anaerobically digested manure, a combined "sandwich" system was developed whereby anaerobic digestion of farm organic residues takes place at the bottom part (gas is collected by a bell-shaped dome), while the digester's supernatant directly feeds the algal pond at the upper part of the system. A small pilot plant outdoor system with a 100-litre digester and a 2.7 m² algal pond (35 cm deep) was recently set up at the Technion Institute and is now under study. Preliminary results show a production of 50 litres of biogas (STP) per day and a total biomass production (approximately 67 per cent algae) of 130 g/day (dry basis). A solar panel has recently been installed to heat the digester part of the system to approximately 37°C (mesophilic) to increase gas production rates.
A small windmill is under design to provide driving force for digester and pond mixing and for the outflow pump.