Physicochemical and biochemical changes during composting of different mixing ratios of biogas sludge with palm oil mill wastes and biogas effluent

Abstract

Prior to composting, the composition of palm oil mill wastes were analyzed. Palm empty fruit bunches (PEFB) contained the highest total organic carbon (52.83 % dry weight) while palm oil mill biogas sludge (POMS) and decanter cake (DC) contained higher total nitrogen (3.6 and 2.37 % dry weight, respectively) than the others. In addition, palm oil fuel ash (POFA) had a high amount of phosphorus and potassium (2.17 and 1.93 % dry weight, respectively). The effect of mixture ratio of POMS and other palm oil mill wastes for composting was studied using the mixed culture Super LDD1 as an inoculum. All compost piles turned dark brown and attained an ambient temperature after 40 days incubation. The pH values were stable in the range of 6.9–7.8 throughout the process whereas the moisture content tended to decrease till the end with the final value around 30 %. After 60 days incubation, the mixture ratio of POMS:PEFB:DC at 2:1:1 with the addition of biogas effluent gave the highest quality of the compost. Its nitrogen content was 31.75 % higher than the other treatments that may be a result of growth of ink cap mushroom (Coprinus sp.). This is the first report on the occurrence of this mushroom during composting. In addition, its nutrients (3.26 % N, 0.84 % P and 2.03 % K) were higher than the level of the Organic Fertilizer Standard. The mixed culture Super LDD1 produced the highest activity of CMCase (6.18 Unit/g) and xylanase (11.68 Unit/g) at 9 days fermentation. Therefore, this solid-state fermentation could be employed for production of compost as well as enzymes.