Optimization of Processing , Managing and Disposing of Sewage Sludge

Summary

Optimization of systems for sludge management requires preliminary investigations on the disposal options as a function of sludge characteristics, acceptability limits and environmental conditions. Once the possible disposal alternatives are identified, the processes, which allow the sludge with the desidered characteristics to be obtained, can be selected. Optimization criteria based solely on cost analysis, are however negatively affected by local conditions and by design and operating parameters, which generally do not take into account the interactions among the treatment processes. If models of operations, describing how the input sludge characteristics are modified by the process operating variables, are known, then rational optimization criteria can be developed.

Sludge disposal options are practically restricted to spreading it on the topsoil or disposing of it underground. The practice of dumping, which is still carried out in some countries, is subjected to very stringent limits because of the risks it entails for the environment; incineration and the subsequent disposal of the ashes and composting, with relevant use of the product obtained, are valid alternatives to the disposal of more or less dewatered sludge. In some cases it may be economical to combine disposal of sludges and urban solid waste (USW) or fractions thereof, either by means of incineration or composting processes or by discharge in landfill. In these cases there is a possible additional advantage owing to the fact that each component has a beneficial effect in that: a) USW helps to reduce the auxiliary fuel requirements in sludge incineration; b) USW acts as a supporting agent and source of carbon for the sludge composting, which provides nutrients lacking in the USW; c) USW give the mass greater consistency in the landfill area, thus enhancing its handlability by the vehicles used for packing and covering various layers.

The considerable difficulties encountered in optimizing the treatment — disposal system are due to the numerous factors that can affect the choices to be made and to problems involving the a priori determination of sludge characteristics during the various stages of the process and the definition of the capital and operating costs of each single operation. The present paper has been subdivided into two parts: Part I deals with the models of the most commonly used treatment operations and Part II with the main disposal alternatives.