(110g) A Sustainable Process For Conversion Of Sludge To Power

Sewage sludge is solid, semi-solid, or liquid residue generated during the treatment of domestic sewage in a treatment facility. United States alone produces approximately 8 Million tons of sludge per year. Hence, requirements concerning the sludge neutralization and disposal are of importance. Moisture content of the sludge varies with type of method used for dewatering the sludge, and might vary from as low as 75% up to 95%. Drying the sludge will reduce volume and weight, and drying allows for subsequent energy conversion by thermochemical means.

We have evaluated the energy content of the dried sludge from three distinct treatment plants by bomb calorimetric method. The energy content of the sludge is found to be comparable to that of other fuels such as wood and coal.

Knowledge of the relationship between air relative humidity and moisture content of sludge is essential for its drying. The equilibrium moisture content (EMC) of sludge was measured using static method, where samples were kept inside sealed containers which were placed inside of a constant temperature water bath. Inside the containers, supersaturated salt solutions kept the ambient relative humidity at a constant value, depending on the temperature. Equilibrium moisture content is determined at relative humidities and temperature ranging from 11% to 85% and from30 to 60˚C, respectively. The experimental results were compared with five different models available in literature ? Modified Chung-Pfost, Modified Halsey, Modified Oswin, Modified Henderson, and Modified BET. The experimental data were best described by the Modified Halsey equation.

We have developed a novel fluidized bed technology to dry the municipal sludge, with a goal of converting it to fuel for electricity generation. A fluidized bed is an ideal method for drying the sludge because of uniformity of the temperature in the bed, its superior heat and mass transfer properties, high efficiency, large capacity and operating flexibility. We have evaluated the effects of retention time, air velocity, temperature, and loading rate on drying characteristics of the sludge and optimized the complete process.

We have developed a process flow diagram (PFD) for a process which takes as its raw materials sludge, and produces electricity and co-generated heat. The economics of the integrated process are shown to depend on scale and on the water content of the sludge.