Comparative evaluation and kinetics of biogas yield from duckweed (Lemna minor) co-digested with water hyacinth (Eichhornia crassipes)

Abstract

Anaerobic co-digestion of duckweed (DW) with water hyacinth (WH) at five DW: WH ratios (1:0, 7:3, 1:1, 3:7 and 0:1 w: w dry basis) was carried out with a view to comparing and evaluating the effect on biogas yield. Fixed quantity of cow dung slurry was added to each treatment as inoculum to seed the digesters before digestion for seventeen weeks in batch type digesters. Biodegradation and maximum biogas yield models based on first-order kinetics were fitted to the experimental biogas yields to describe the cumulative and predict maximum biogas yields, respectively from each treatment. The results indicated that DW was viable for biogas production and more prolific than WH. Co-digestion did not affect (p > 0.05) temperature and pH but affected (p ≤ 0.05) total bacterial count and biogas yield. The high R² values obtained from the biodegradation model fit showed that the model described the experimental yields satisfactorily. Furthermore, the high R² values and percentages of predicted maximum yield/observed maximum yield showed that the maximum biogas yield model predicted the maximum yields satisfactorily. The study concluded that co-digestion of DW and WH was best at ratio 7:3 while ratio 1:1 was the best described and predicted by the biodegradation and maximum biogas yield models.