Organic Waste Recycling

Biofuels production 169

heterogenous substrate comprising of nightsoil, chopped and sun-dried water

hyacinth and rice straw (Polprasert et al. 1982). These digesters were operated

under ambient conditions (mean temperature = 30°C). The characteristics of

these feeding materials are given in Table 4.6; the proportions of these materials

mixed to obtain the desired C/N ratio of 25 are shown in Table 4.7. Among the

three hydraulic retention times (HRT) studied, biogas production was highest

from the digesters operated at an HRT of 30 days (Figure 4.8). A comparison

using volumetric methane production rate per unit volume of digester (Table

4.8) showed the digesters operated at the 30-day HRT to give the highest value

of 0.33 m^3 /(m^3 -day); this parameter is important for operational design because

it relates the size (or capital cost) of digester to gas production.

Polprasert et al. (1982) reported that there was floating scum formed in the

digesters fed with a mixture of nightsoil, water hyacinth and rice straw, causing

operational problems with mixing and feeding/withdrawal of slurry. Table 4.9

shows the characteristics of the digested slurry which still contained high

concentrations of organic matters (based on total volatile solids and chemical

oxygen demand) and nitrogen, and thus need to be treated further prior to

disposal. On the other hand, this slurry is suitable to be reused as fertilizer or

soil conditioner (Figure 4.1).

Table 4.8 Summary of biogas production from the pilot-scale digesters

Hydraulic

retention

time (days)

Organic

loading

rates (kg

TVS/m^3 -

day)

Number of

digesters

Total biogas

production

rate (m^3 /kg

TVS-day)

Volumetric

biogas

production

rate (m^3 /m^3 -

day)

Methane

production

rate (m^3 /kg

TVS-day)

Volumetric

methane

production

rate

(m^3 /m^3 -

day)

30 1.20 4 0.28 0.33 0.18 0.22

50 0.75 4 0.32 0.24 0.21 0.16

70 0.53 3 0.25 0.13 0.16 0.09

Table 4.9 Characteristics of effluents from biogas digesters

HRT TS Reduction TVS Reduction Nonfiltered COD TKN Reduction
(days)

Number
of digesters g/L (%) g/L (%) g/L (%) mg/L (%) pH
30 4 24.7 47 19.1 47.3 24.8 46.7 707.5 20.5 7.4
50 4 18.4 62 13.7 62.7 17.7 63.4 635.8 29.1 7.3
70 3 16.5 68 11.2 70.0 14.9 69.3 619.5 31.4 7.2