Due to fast growth of industrial sector, the volume of sludge from wastewater treatment systems increases, and this becomes a big problem to the environment. The cost for treatment of sludge is high and requires typical landfill for disposal. This study suggested recovering energy from the sludge of wastewater treatment plants through an anaerobic fermentation process. Sludge from wastewater treatment plants of fish processing factory (C) and shrimp processing factory (T) were collected for the testing of this study. The 20 L lab-scale batch anaerobic digesters were used to ferment sludge which has different humidity of 85 - 90 % and of 95 - 98 %. The biogas yield of the treatment C 85-90 is 305.3 L, C 95-98 is 444.2 L, T 85-90 is 144.4 L, and T 95-98 is 171.1 L/kg ODM_fermented. After 10 days, the percentage of CH₄ in produced biogas was around 50 % which is high enough for burning. The result clearly confirms that sludge from wastewater treatment plants not only is possible treated in anaerobic digesters but also is able to be applied as input material to produce biogas for energy consumption.