|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|The combination of algal and anaerobic waste treatment in a bioregenerative farm system|
Following the experiments described above, a conceptual design of a bioregenerative farm based on the utilization of wastes from 5 adult humans, 12 cows, 30 pigs, and 2,000 hens was developed. The quantities of wastes, volatile matter, expected biogas production, gross energy output, and nitrogen and phosphorus content are given in Table 4. It should be noted that the human wastes contain both liquid (sewage) and solids (garbage). Conditions in both developing countries (a) and developed countries (b) were assumed regarding the quantities and composition of wastes as well as the degree of volatile solids destruction (25 per cent and 30 per cent, respectively). Mesophilic digestion with 12 days average retention time was assumed for developing countries, yielding a digester with a net liquid volume of 17.5 m³, while a thermophilic digester with 8 days retention and 28 m³ (liquid) volume is assunied for developed countries. Waste matter mixture fed into the digesters contains 6.6 per cent volatile solids in the developing countries, and 4 per cent in the developed, with gross energy outputs of 0.1 and 0.2 million kcal per day in the developing and developed countries, respectively.
TABLE 4. Material and Energy Balances of the Anaerobic Digestion Part of a Model Bioregenerative Farm under Conditions of Developing Countries (a) and Developed Countries (b)
|Source of wastes||Volume per day*||Volatile solids kg (dry) per day||Biogas production m³ (STP) per day**||Energy production (gross) 10³ kcal/day***||Nitrogen kg-N/day range||Phosphorus content kg-P/day range|
|5 humans (liquid and solid waste)||0.56||1.00||2.8||5.6||0.58||1.40||3.3||7.9||0.1 - 0.3||0.013 - 0.05|
|12 cows||0.10||0.50||19.2||60.0||3.95||14.80||22.5||84.4||0.5 - 2.7||0.09 - 0.20|
|30 pigs (75 kg each)||0.71||1.84||35.4||35.4||7.28||8.74||41 5||49.8||1.9 - 3.2||0.2 - 0.6|
|2,000 hens (2 kg each)||0.08||0.12||38.0||38.0||7.82||9.38||44.6||53.5||1.4 - 2.9||0.4 - 0.9|
|TOTAL||1.45||3.46||95.4||139.0||19.63||34.32||111.9||195.6||3.9 - 9.1||0.70 - 1.75|
* Average data for developing countries la) and developed countries (b).
** Assuming 25 per cent of volatile solids converted to biogas in developing countries and 30 per cent conversion in developed countries with 0.823 litres (STP) of gas produced per gram of volatile solids destroyed.
*** Assuming 60 per cent methane in biogas with an energy content of 5700 kcal per m³ (STP) of biogas.
Not only will this energy production enable the farm household to become self-sufficient in energy, but surplus energy is provided for such purposes as water pumping, algal thermal treatment, digester heating, crop drying, small agro-industry, etc. Solar heating of the digester and wind-driven mixing and pumping can further reduce energy requirements and increase net energy surplus.
The digester's supernatant is diluted to provide an effluent with a concentration of 750 mg/l volatile solids to be fed to algal photosynthetic ponds. This diluted liquid medium contains the bulk of the nutrients (primarily nitrogen and phosphorus) as well as an ample carbon source in the form of organic carbon, dissolved CO2, and bicarbonates.
Table 5 summarizes the data of an algal pond model under conditions of both developing and developed countries under proper climatic conditions. It can be seen that the area for the ponds is 1,200 m² in developing countries and 1,620 m² in developed countries, yielding 45.4 and 61.6 kg per day of dry biomass, respectively, with an average protein content of 42 per cent. This yield of biomass can provide the major source of the farm animals' protein requirements through recycling. The pond effluent, following algal biomass separation, can be used for fish ponds, crop irrigation, or diluting the digester's supernatant.
TABLE 5. Summary of Parameters for the Biomass Production Part of the Model Bioregenerative Farm
|Daily quantity of digested volatile solids||71.5 kg||97 kg|
|Daily volume of digested material||1 45 m³||3.46 m³|
|Volume of wastes diluted to 750 mg/l volatile solids||95 m³/day||130 m³/day|
|Photosynthetic (algal) pond retention time||5 days||5 days|
|Pond's depth||0.4 m||0.4 m|
|Pond's area||1,200 m²||1,620 m²|
|Algal biomass production (net)||22 g/m²/day||22 g/m²/day|
|Total daily net algal biomass production (dry)||26.4 kg/day||35.6 kg/day|
|Daily production of non-algal biomass (200 mg/l)||19.0 kg||26.0 kg|
|Total daily biomass production (dry) (algal and non-algal)||45.4 kg||61.6 kg|
|Total daily protein production(dry) (assuming 42% in biomass)||19 kg||36 kg|
Separation of the algal-bacterial biomass can be done by auto-flocculation (increasing pH by intensive exposure to sun-light in a shallow pond) or by alum flocculation-flotation. Thermal treatment of the algae slurry is recommended to provide pasteurization and partial dewatering.
Figure 8 illustrates schematically a conceptual bioregenerative farm where animal and domestic wastes are treated in a solar-heated, windmill-driven mixed biogas digester. The digester's supernatant feeds an algal photosynthetic pond from which proteinaceous biomass is produced for animal feeding and treated effluent is reclaimed for irrigation. A cycle combining the algal pond with a fish and duck pond is proposed as well. The scheme of Figure 8 shows a physical separation between the anaerobic digester and the photosynthetic pond; however, the combination of the two as in the "sandwich" design is possible and even advantageous.
Past experience with both anaerobic digestion and algal biomass production can provide the necessary design criteria and the data for material and energy balances of such a combined system. Nevertheless, actual demonstrations in various climates, using different farming practices and in various socio-economic settings in both developing and developed countries, under full-scale farm unit conditions, are required and recommended in order to prove and establish the complete design criteria for such a bioregenerative system, thus bringing about maximum self-reliance and self-sufficiency of the farming communities in the developed and developing countries.