Waste disposal 10 Sewage sludge 9 Sewage sludge digestion 9 Palm oil industry 5 Purification 5 Paper mills 4 Sewage 4 Activated sludge process 3 Anaerobic bacteria 3 Biological treatment 3 Sewage sludge as fertilizer 3 Environmental aspects 2 Hazardous wastes 2 Heavy metals 2 Industrial applications 2 Absorption and adsorption 1 Accounting 1 Analysis 1 Biodegradation 1 Biomass energy 1 Characterization 1 Clay 1 Costs 1 Economic aspects 1 Electroplating 1 Environmental auditing 1 Environmental impact analysis 1 Food industry and trade 1 Inductively coupled plasma atomic emission spectrometry 1 Management 1