Physical Properties and Nutrient Release of Biogas Sludge and Boiler Ash from Palm Oil Industry

The palm oil industry generates significant waste each year which can be recycled as energy source. However, after recycling, other types of waste generated, largely in the form of biogas sludge and boiler ash that require treatment prior to disposal. This research studies the physical properties and nutrient release of biogas sludge and boiler ash. Waste samples were collected three times at three-month intervals (29 November 2019, 29 February 2020, and 29 May 2020) and were analysed for physical properties. The first biogas sludge sample had the highest macropore, micropore, bulk density, water absorption, and porosity values of 30.56%, 83.65%, 0.8516 g cm−3, 138.82%, and 114.22%, respectively. The micropore of boiler ash was the highest in the second sample with 69.69%, but the other parameters were not statistically different. The two materials were incubated under 60% moisture content at 25 ± 2 °C for 60 days. The biogas sludge had higher available nitrogen than boiler ash and was the highest after 60 days of incubation with 0.42 g kg−1 and 0.76 g kg−1 of NH4 and NO3, respectively. However, boiler ash released higher available P and extractable K and Mg than biogas sludge and was also the highest after 60 days of incubation with 6.40 g kg−1, 17.95 cmolc kg−1, and 50 cmolc kg−1 of available P and extractable K and Mg, respectively. Therefore, boiler ash and biogas sludge may prove to be a good source of macronutrients. Besides, some physical properties were not at appropriate levels so the material requires treatment or mixing with other materials to improve its physical properties before use as growing media.