Optimum Mesophilic Temperature of Biogas Production from Blends of Agro-Based Wastes

MATERIALS AND METHODS

The Cassava waste water used for this study was obtained from one of the local processors of Garri (a staple food in Eastern part of Nigeria), while the powdered rice husk was procured from a local rice mill in the same area. The cow dung liquor was obtained from an abattoir. The swine dung was collected from the vertinary farm, University of Nigeria Nsukka. The Brewery spent grain was collected from Nigerian breweries Limited, Enugu. Nigeria. The study was carried out at the National Center For Energy Research and development, University of Nigeria, Nsukka in 2006. The digesters used were 1liter Buckner flak constructed as micro digesters. Other materials used are 2 L glass measuring cylinders (Pyrex) for measuring biogas production, plastic water troughs, metal stand and clamp, hose pipe, thermostatically controlled water bath (Gallenkamp), thermocouple, digital weighing balance and digital pH meter (unified National inventory database).

Charging of Digester
The waste blends were charged using 1 L micro digesters. A 150 g of each of the waste blends were charged into the different digesters. CW: SD (2:3), BS: PR (1:1) and BS: CD (1:3) were mixed with water in the ratio of 1:1, 1:2 and 1:1, respectively and were subsequently charged in the different digesters. The moisture contents of these wastes determined the waste to water ratios used. The experiment was carried out in the laboratory using thermostatically controlled water bath as the source of heat. The digestion was batch operated. Volume of gas production was taken daily using downward displacement of air over water at temperatures of 25, 30, 35 and 40°C. The slurry temperature of the systems were recorded using thermocouple. Ice block was occasionally used to lower the temperature of the water bath to the desired mesophilc temperature of 25°C. pH of the systems were also monitored throughout the period of gas production.

The ambient temperature of the laboratory was 28°C throughout the period of the experiment while that of the water bath ranged between 29 and 30°C.

Analysis of Waste Blends
Total and volatile solids were determined using Meynell (1976) method. Crude nitrogen content was determined using the Micro-Kjedahl method (Pearson, 1976) while carbon content was determined using Walkey and Black (1934) method.

RESULTS AND DISCUSSION

A retention period of seven days was used for the experiment. Gas production commenced within 24 h post charging period (Fig. 1-3). The results in these figures also indicate that volume of gas production was generally low at 25°C for all the waste blends. This may be attributed to the low temperature because at that temperature, the methanogens are not sufficiently activated for enhanced biogas production. This will consequently lead to low biodegradation of the organic wastes and poor yield of gas production. At the temperatures of 35 and 40°C, there was higher biogas yield as a result of higher rates of biological degradation (Fig. 1-3) (Huan et al., 1982). The cumulative biogas yield for each of the waste blends, show the highest performance at 40°C (Table 1). The BS:CD blend had the highest cumulative biogas yield of 2.51 L. This observation underscores the superiority of cow dung to other wastes in quantitative and qualitative biogas production (Odeyemi, 1987; Dioha et al., 2005; Ofoefule and Uzodinma, 2006).

Fig. 1:	Daily biogas production for the CW: SD blend. (Cassava waste water (CW): Swine dung (SD)

Fig. 2:	Daily biogas production for the BS: PR blend. (Brewery spent grain (BS): Powdered rice husk (PR)

Fig. 3:	Daily biogas production for the BS: CD blend. (Brewery spent grain (BS): Cow dung (CD)

Table 1:	Cumulative biogas yield for the waste blends. Yield at various temperatures (L/150 g of feedstock) for seven days

In addition to temperature, Biogas production is dependent on so many other factors working together to ensure effective biogas production. These factors include biodegradability of the feedstock, pH, carbon to nitrogen ratio etc. The relative low volume of gas production observed for the CW: SD blend at the various mesophilic temperatures when compared with the other waste blends could be attributed to the low C:N ratio which was the least and below the acceptable limit (26:1-30:1) (Kanu, 1988; Garba et al., 1996). Anaerobic bacteria use up carbon 30 times faster than nitrogen. Apart from the increased temperature at 40°C, the result of the physicochemical properties of the waste blends indicates that the BS:CD blend had high enough volatile solids and the pH which fell within the acceptable limit (Table 2). It also contained relatively adequate C: N ratio.

Table 2:	Physicochemical properties of the undigested waste blends

CONCLUSION

The results show that the upper limit of the mesophilic range gives a higher biogas yield. The Optimum temperature observed from the experiment is at 40°C. Therefore to achieve enhanced and constant generation of biogas for effective distribution and end use, it is expected that a constant temperature be maintained through the use of insulations, heating elements, heat exchangers, steam injections or water baths. Secondly, maintaining the pH of agro-based waste blends at optimum for biogas production is important in achieving the optimum mesophilic temperature for biogas production.