Biogas production from raw and oil-spent date palm seeds mixed with wastewater treatment sludge

ABSTRACT

The objective of this study was to evaluate biogas production from raw and oil-spent date palm (Phoenix dactylifera) seeds mixed with wastewater treatment sludge. Two types of date seeds were ground to three size ranges (1.18–3.75, 0.6–1.18, and 0.425–0.6 mm) and co-digested with wastewater sludge at seed/sludge total solids (TS) ratios of 0% (sludge alone), 2.5, 5, 7.5, 10, 20 and 40% (i.e. 10% ratio results from adding 0.2 g dry seeds to 100 ml 2% sludge containing 2 g dry solids). After 14 weeks of incubation, biogas production, expressed in terms of the date seed/sludge ratios, was in the following order: 10% > 7.5% ≈ 5% ≈ 2.5% ≈ 0% > 20% > 40%. The size of date seed particles did not significantly affect biogas production. The specific biogas production was in the range of 370–390 ml g^–1 volatile solids (VS) for the 0–10% seed/sludge ratios, 245 ml g^–1 at 20% and 120 ml g^–1 VS at 40%. The relatively low biogas production from the 20 and 40% seed/sludge mixtures indicated inhibition, which was also indicated by the low pH in the mixtures following digestion. Oil extraction from the date seeds reduced, but did not fully overcome, inhibition of biogas production from the 20 and 40% mixtures. The inhibition reduction due to oil-extraction was indicated by observed discrepancy between biogas production and solids digestion. The discrepancy between biogas production and solids digestion suggested inhibition of the acid forming but not the more sensitive methane forming microorganisms.

KEYWORDS:

• Inhibition of biogas production
• methane production
• wastewater treatment sludge
• oil-spent date seeds
• Khalas date palm seeds
• Khudari date palm seeds

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Research Institute for Sciences and Engineering (RISE), University of Sharjah, UAE [project number: 15020401002-P]