Anaerobic co-digestion of dairy raw by-products and Ulva sp. macroalgae: effect of organic and inorganic additives

Abstract

An investigation has been carried out for biogas production from dairy raw materials (DRM) and Ulva sp. macroalgae as a co-substrate in order to find a usefulness for these species. Some nutrient media have been selected to optimize methane yield. The effect of the mineral additive medium (medium I), based on bicarbonate compound, and the organic additive one (medium II), based on glucose, on the anaerobic digestibility of DRM has been assessed by using a single-stage anaerobic process. Four batch experiments (batch 1, batch 2, batch 3 and batch 4) have been performed and correspond respectively to DRM without inoculum, DRM with additive medium I, DRM with additive medium II and DRM and Ulva sp. with additive medium I. Measurements of methane produced, chemical oxygen demand (COD), proteins, lipids, carbohydrates and volatile fatty acids have been carried out. It has been found that the mineral additive medium provided a higher specific methane yield compared to the organic additive one, the methane yield was about 0.208 $\mathrm{L}_{\mathrm{CH}_4}/\mathrm{g}_{\mathrm{VS}}$. The co-digestion of DRM with Ulva sp. showed that the methane yield reached was about 0.118 $\mathrm{L}_{\mathrm{CH}_4}/\mathrm{g}_{\mathrm{VS}}$, and revealed that the use of Ulva sp. as co-substrate reduces the concentration of volatile fatty acids, and enhances the biogas quality which contains 96% of methane and only 0.5 to 2% of hydrogen sulphide. Furthermore, a first-order kinetic model has been applied to describe proteins, lipids and carbohydrates biodegradation and showed that these three compounds follow a different kinetics according to the composition of the medium and the presence or not of additive medium.

Keywords

• Dairy raw materials
• Additives
• <i>Ulva</i> sp. macroalgae
• Biogas
• Volatile fatty acids