Organic Waste Recycling

Biofuels production 149

in Table 4.1). Other limitations include high capital cost, seasonal variations in
gas production, operation and maintenance problems.
Because pathogen inactivation in anaerobic digesters is generally incomplete
and the digested slurry is in liquid form, special care has to be taken in the
handling and reuse of the digested slurry. Perhaps, this is a reason for the
negative attitude towards the use of human nightsoil for biogas production.
Table 4.2 summarizes some of the advantages and disadvantages of the use
of biogas technology as a method of processing biodegradable organic wastes.

Table 4.1 Comparative analysis of biogas technology and composting (Adapted from
Tam and Thanh 1982)

Operating conditions Composting

(aerobic/anaerobic)

Biogas technology

Materials added to

nightsoil or animal

manure for C/N ratio

and moisture

adjustments.

Vegetation

Water + vegetation

Temperatures 50 - 70°C Ambient

Period of operation 6-8 weeks (including

maturation and curing)

4-8 weeks

Nitrogen loss Low to high Low to medium

Space required Same Same

Modes of operation Range from traditional to

complicated

Complicated

Odor Problem for open system Less problem due to closed

system

End products Composted material Digested slurry (and CHgas)^4

Weight Reduced due to water loss Increased in density due to

biomass production

Water content 40 - 50 % 88 - 92 %

Humus content Abundant Less than composted

products

Pathogen destruction Good Moderate

Transport Easier (solid matter) More difficult (liquid

matter)

Further handling Not necessary Drying usually needed

Storage Easy, little loss of

nitrogen

Difficult. With possible

loss of nitrogen