This study proposed the recycling of sewage sludge (SS) and red gypsum (RG) as potential temporary landfill cover materials. Mixtures with different SS and RG compositions were prepared and tested in determining the most suitable design mix based on the resulting physical, mechanical, and geotechnical properties, namely the hydraulic conductivity, compressive strength, and plasticity. A maximum compressive strength of 524 kPa was achieved for the optimum SS:RG composition of 1:1, corresponding to Ca:Si composition of 2.5:1, which was appropriate to form the calcium silicate hydrate (CSH) gel. The SS and RG compositions did not affect the hydraulic conductivity, which was in the order 10-5 cm/s for all mixtures. Mixtures with RG greater than SS in composition exhibited plastic behaviour due to the Fe content in the RG, which helped minimize the risk of cracking. The optimum mixture had compressive strength greater than the specified minimum of 345 kPa, medium hydraulic conductivity, and moderate plasticity, thus appropriate for application as an alternative material for the temporary landfill cover in the tropics.
Nutrient leaching and volatilization cause environmental pollution, thus the pursuit of developing controlled-release fertilizer formulation is necessary. Biochar-based fertilizer exhibits slow-release characteristic, however the nutrient release mechanism needs to be improved. To overcome this limitation, the approach of applying encapsulation technology with biochar-based fertilizer has been implemented in this study. Black peppercorn waste was used to synthesize urea-impregnated biochar (UIB). Central composite design was used to investigate the effects of pyrolysis temperature, residence time and urea:biochar ratio on nitrogen content of UIB. The optimum condition to synthesize UIB was at 400 °C pyrolysis temperature, 120 min residence time and 0.6:1 urea:biochar ratio, which resulted in 16.07% nitrogen content. The tapioca starch/palm oil (PO) biofilm formulated using 8 g of tapioca starch and 0.12 µL of PO was coated on the UIB to produce encapsulated urea-impregnated biochar (EUIB). The UIB and EUIB pellets achieved complete release of nitrogen in water after 90 min and 330 min, respectively. The nutrient release mechanism of UIB and EUIB was best described by the Higuchi model and Korsmeyer-Peppas model, respectively. The improvement of water retention ratio of UIB and EUIB pellets was more significant in sandy-textural soil as compared to clayey-textural soil. The EUIB derived from peppercorn waste has the potential to be utilized as a sustainable controlled-release fertilizer for agriculture.