Evaluation of the potential use of thermotolerant lignocellulolytic Streptomyces thermocarboxydus ME742 and the mutant MEEMS15-16 as inoculum for effective composting of wastes from palm oil mill

Abstract

Composting is a potential option for managing the large amount of waste from a palm oil mill, including empty fruit bunches (EFB) and decanter cake (DC). In this study, Streptomyces thermocarboxydus ME742 (WTME742) and its mutant MEEMS15-16 were used as lignocellulotytic inoculum to compost mixed EFB and DC (1:1) in a vertical cylindrical tank with moisture content maintained in the range of 50–70% for 40 days. Their effects were compared to microbial activators (LDD.1). In composting system, MEEMS15-16 showed rapid production for high cellulase (3.86 U/g) on day 3 with ability to sustain high lignin peroxidase over mesophilic stage, while WTME742 produced highest xylanase (11.07 U/g) and manganese peroxidase (100 U/g). After day 40, nutrient contents in all the composts met quality standard requirements of compost, except for the un-inoculated control. WTME742 provided a high nitrogen content (N:P:K was 1.72:1.21:2.35) with a considerably low C/N ratio (12.19) whereas MEEMS15-16 gave higher potassium content (N:P:K was 1.46:1.10:2.52) with a lower C/N ratio (11.09). Co-inoculum with LDD.1 also showed the promising results. Regarding spectral analyses (FTIR and XRD), MEEMS15-16 showed the greater ability for lignocellulosic matter transformation and composting using co-inoculum of MEEMS15-16 or WTME742 with LDD.1 provided high humification degree. The findings demonstrated the potential for use of WTME742 and MEEMS15-16 as inoculant in either single culture or the co-culture with LDD.1. These microbial inoculants would serve as the driving forces in facilitating degradation of palm oil mill wastes, and consequently enhancing the feasibility of compost production at industrial scale.