Rice Straw: An Alternative for Energy Generation by Anaerobic Co-Digestion to Pig Manure

Abstract

Anaerobic digestion technology (biogas) has been used to treat husbandry wastes in the Mekong Delta of Vietnam for more than 30 years. However, biogas applications are limited. One major barrier is lack of input material for biogas digesters, because husbandry is normally conducted at the household scale. In contrast, rice straw (RS) is available in huge quantities in the Mekong Delta, but it is seldom put to use. This study tested the suitability and efficiency of anaerobic co-digestion of RS and pig manure (PM) to produce biogas. The study used 21-L lab-scale batches of anaerobic co-digestion of PM and RS, testing different sizes of RS. The mixing ratio for co-digestion was 50:50 based on organic dry matter values of input materials. The results showed that temperature, pH, alkalinity, and redox potential of almost all RS sizes were in the suitable range for activities of methanogenic microorganisms. The biogas yields per kilogram of fermented organic dry matter for the treatments were, for 1.0 cm RS, 691.05 L; 10.0 cm, 687.79 L; 20.0 cm, 685.08 L; and original RS size (uncut), 680.44 L. These values did not significantly differ from that of the 100% PM treatment. The methane content tended to increase with time, starting with the second week, and was well qualified for energy use. Our results strongly confirm that RS can be added to PM to produce biogas and that chopping RS is unnecessary.