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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderAnalysis of bioconversion systems at the village level
View the document(introductory text...)
View the documentIntroduction
View the documentApproach to bioconversion analysis
View the documentSome results and costs from integrated systems
View the documentFuture development possibilities
View the documentConclusions
View the documentSummary
View the documentReferences

Some results and costs from integrated systems

Figure 2 shows a system of algal ponds with a biogas system in actual operation in a village. This system has been in operation since September 1978 and is run by local people. Table 4 shows the physical data and some of the costs associated with this system. The technical details are essentially as described in MCRC Notes (3).

TABLE 4. Costs and Other Data for an Integrated Algal Pond System

Location: Injambakkam Village        
No. Item and Description Cost Rs Depreciation Rs Remarks: US$1 = Rs 8.00
      (per cent/yr)  
1. Digester 878 31.00 (5) Includes labour, depreciation on materials
  MCRC design 4 - 5 cattle      
2. Geodesic support (MCRC) 322 14.60 (5) Same as above
3. Gas container 175 88.00 (50) Depreciation on total
  Transparent PVC + coconut thatch      
4. Piping and burner bought off the shelf 100 10.00 (10)  
5. Algal ponds 618 207.00 (50) Price/m' = 34.34
  Claylsand bund lined with 1,000 g      
  (HDPE)     Depreciation on materials
  Exposed area: 3 m² + 6 m² + 9 m²      
6. Solar driers 100 40.00 (50) Price/m² = 33.00
  (MCRC)     Depreciation on materials
7. Buckets, screens, etc., bought off the      
  shelf 50 50.00 (100)  
  Totals 2,243 440,60  

8. Interest on borrowing 4 per cent/year = Rs 92.00 (4 per cent rates available for poorer sections)

I merest plus depreciation Rs 532.00

Working days/year 300

Average yield 10 g/m² /day or 54 kg/yr

Credits: biogas plus slurry as manure (when not used for algae)

Estimated share of capital towards algae: Rs 6.001kg

Labour: 1/2 man day/day for operation; labour component of capital: 25 per cent

Table 5 shows some actual data from a biogas effluent-fed algal pond growing Spirulina. The second column shows that a medium consisting of one-half Zarrouk's formula (2) plus 2 1 of unfiltered biogas effluent every other day gives satisfactory results. The culture volume was, on average, 150 I; the area exposed was 2 m² /pond; and an initial start of 51 of biogas effluent was added to the culture. Harvesting the culture every other day yielded more than harvesting every day. Occasionally, a bicarbonate boost was given to the ponds to keep up the pH. Small amounts of HPO4² and NO3 were added primarily to the pure synthetic medium culture, but also occasionally to the other cultures.

TABLE 5. Spirulina Growth on Biogas Effluent - Yield and Other Details

Yield dry weight in gas


Number Date Pond PC2 (2m²) - initial dose
full Zarrouk's
Pond PC3
initial dose
1/2 Zarrouk's+
5% v/v
Pond PC4
-initial dose
1/4 Zarrouk's +
5% v/v biogas
1 5 Sept. 1978 72 110 102 HCO3, no3 PO4
2 7 Sept. 60 45 35 "boost" TO REPLACE
3 9 Sept. 50 33 47 carbon uptake by
4 11 Sept. 20 45 52 algae:
5 13 Sept. 35 50 40 pond PC2, EVERY
6 17 Sept. 45 65 - 2nd day after
7 19 Sept. 50 90 75 pond PC3, every
8 21 Sept. 47 40 65 25th day, plus
9 23 Sept. 30 - _ 21 biogas efl, 2nd day after harvest;
10 27 Sept. 28 25 27  
11 29 Sept. 25 33 45 pond PC4, every
12 3 Oct. 20 52 42 21 biogas efl.
13 5 Oct. 20 45 35 2nd day after harvest.
  Total 502 693 565  
Yield in gas/m/day 8.36 10.88 9.41  
Cost of initial chemicals

per kg of algae

Rs 3.05 Rs 1.17 Rs 0.67 Based on 300 days/year
Cost of "bost" chemicals

per kg of algae

Rs 20.15 Rs 2.48 Rs 5.98  


The average culture temperatures varied between 27 C at 0800 hours and 34 C at 1600 hours. The lux readings were averaged at 20,000 lux (0800 hours), 80,000 lux (1200 hours), and 16,000 lux (1600 hours). To prevent photo-oxidation, coconut thatch covers were used for the first three days and between 1100 hours and 1500 hours every day. The pH ranged between 9.5 and 10.5.

Based on the data obtained here (work is continuing), some calculations are presented to evaluate and compare different bioconversion modes. As pointed out earlier, this kind of evaluation has to remain subjective until more quantitative yardsticks are evolved,

Consider a family with five cows, and assume one year of operation. Then assume:

  1. 80 per cent collection efficiency (7),
  2. 10 kg wet dung/day of 18 per cent dry solids (8),
  3. carbon in dung= 30 per cent by weight of dry solids C/N ratio = 18 (8), and
  4. gas yield of 0.067 m³ /kg wet dung of 65 per cent CH4, 30 per cent CO2 (8), and the remainder H2 O.

Calculation of carbon balance:

5 x 10 x 0.18 x 365 x 0.30

cows kg/cow dry days C/dung

= 788 kg C/yr, entering the system

(Mol. wt. of gas = 24.5, with no correction for normal conditions)

0.067/22.4 x 365 x 50 x 0.8 x (0.65x12+0.30x12)

moles of gas days kg collect C/CH4 C/CO2

= 498 kg C/yr, leaving as biogas 788 - 498 = 290 kg C/yr, leaving in slurry

Based on MCRC's experience, if the slurry is to be fed into algal ponds every other day, approximately 4,0001 of culture ponds are needed. This can be accommodated in ponds of about 14 m² with a depth of about 0.3 m. The yield of algae over 300 days of pond operation can be expected to be 42 kg (at 10 g/m² /day). If two ponds are used, the yield is doubled by feeding each pond alternately.

If carbon comprises 50 per cent of the algal biomass and nitrogen 9 per cent, then C utilization is 17 per cent based on the carbon in the slurry, and nitrogen utilization is about 21 per cent. This is for 150 days of feeding slurry into one pond.

Table 6 shows a projected comparison of five modes of dung use without reference to cost. It must be emphasized that where no bibliographic references are given, the data were obtained or estimated by MCRC. They have to be checked again; an attempt, however, has been made to be conservative.

The first three items in the table are self-explanatory. The fourth and fifth items involve one use of dung as compost. This is to grow Sesbania grandiflora (agathi) trees, highyielding leguminous trees, growing indigenously all over South India. They are used for fodder, fuel, and building wood. Our experience is that 9 to 12 months after planting, the trees grow to 6 m and weigh an average of 16 kg. However, the yield given in the table is yield over unfertilized land. T.M. Paul (13) has demonstrated that barren, rocky land can be used to grow trees. If such land is used, the cultivation of tree crops becomes worthwhile. If land has to be paid for, the cost goes up sharply; in fact, up to 80 per cent of the final value of the crop can be ascribed to land value (12).

Comparison of end-products from bioconversion steps seems to favour a conventional agricultural crop until it is realized that in most villages, land and water are at a premium. However, each situation is different and the analysis here and in Part I may determine the best usage of the residues.