Management/Utilization of Wastewater Treatment Sludges

Abstract

Pulp and paper mill industries are always associated with disposal problem of highly contaminated sludge or biosolids. In countries with large-scale pulp and paper production, the huge amount of waste generated has prompted the government and industries to find new use of these biosolids. Paper mill sludges have a net environmental advantage over sewage sludges in that they are nearly pathogen-free; handling and use pose lower health risks. Landfilling, land application, composting, land spreading to improve soil fertility, production of ethanol and animal feed, pelletization of sludge, manufacture of building and ceramic materials and lightweight aggregate (LWA), landfill cover barrier are among the waste management options studied. The challenge to find efficient methods for firing sludge still exists today and is becoming increasingly important as pulp and paper mill strive to be competitive. So far, incineration has been the primary alternative to landfill. However, incineration is associated with environmental pollution problems. The emission of gaseous NO x and SO2 is the major precursor of acid rain. The residue ash contains various toxic metals which need to be landfilled and hence results in ground water contamination. The plastics and glue found in the sludge are the sources of chlorinated compounds such as HCl, dioxins, and furans which are major threat to the environment. Landfilling is becoming less of a viable option as environmental problems and restrictive legislation are making landfills a buried liability. Also, landfill operating cost has increased. Sites and permits for new landfills have become more difficult to obtain in many countries. Incineration and the production of steam and power with sludge will continue to be an option for the foreseeable future. Improvements in sludge drying techniques and boiler configuration are making sludge more of an asset than a liability for heat production. Incineration in beehive burners will soon be eliminated as these burners are being phased out. Land application is gaining momentum in the pulp and paper industry as well as with municipal waste treatment systems. Trials are quickly developing into permanent land spreading programs for forestry application, land reclamation and agricultural application but research is demonstrating that the hurdles of environmental legislation and disposal costs could be overcome. There has been interest in the use of sludge for production of light-weight aggregate and granules to carry agricultural chemicals, in pelletization of sludge for use as a fuel, production of ethanol and single cell protein, use in cement kiln feedstock and as hydraulic barrier material in landfill capping systems. The interest in these particular waste management opportunities probably relates mostly to their potential for using significant amounts of sludge. With the exception of ethanol and light-weight aggregate production from sludge, full-scale operations have successfully demonstrated each of these alternatives.