Analysis of Drying Technologies for Wastewater Treatment Plant Sludge as an Alternative Source of Energy

摘要

Wastewater treatment plant sludge (WWTPS) is used as fertilizer or disposed in landfills. Potential hazards related to the existence of pathogens and heavy metals along with the disposal cost, have made current applications less accepted. Recently, high-energy consumption industries have considered WWTPS as an alternative fuel. Basic requirements for WWTPS as fuel are heating value and water content. Typical WWTPS have 80% water, and drying down to 10% is required for fuel purpose. The objective of this study was to analyze energy balance and feasibility of commercially available WWTPS dryers. Energy demands for all dryers ranged from 0.82 to 1.1 kWh/kg evaporated water. Total drying energy demand was calculated and compared to the energy value of the dry sludge. Average drying energy was calculated at 2514.70 kWh /ton dry sludge. Anaerobic digested sludge energy value was measured at 2075.50 kWh/ton, while non-anaerobic digested sludge was 3590.60 kWh/ton. Most facilities produce anaerobic digested sludge, since biogas is an attractive byproduct for electricity purposes. In this scenario, the production of dry sludge for alternative fuel is not feasible, since the net energy gain is negative. Solar dryers appeared to be the best drying option for the production of alternative fuel from WWTPS. The solar drying technology allows the use of dried sludge as a fuel in high energy consumption industries, such as concrete, asphalt and chemical and also generates Certified Emissions Reductions (CER's) according to Clean Development Mechanisms (CDM) in the frame of the Kyoto Protocol.