Thermal Drying of Sewage Sludge

Abstract

Over the next decade the UK will experience significant, imposed changes in the quantity of sewage sludge produced and in the means available for its disposal. Existing practices are being reappraised and new technologies are being explored to cope with a predicted 40% increase in the quantity for disposal. Thermal drying of sludge undoubtedly has a role to play in coping with this demand.The two principal process types, i.e. direct and indirect heat application, are appraised in depth and a comparison is drawn. A case study is identified to examine the energy demands of the process, including the potential for heat recovery and for resource reuse. The environmental impact of a typical process is also examined.

Immersion frying for the thermal drying of sewage sludge: An economic assessment

“…Comparatively, fry-drying may be expected to be competitive in terms of maintenance and repairs compared with conventional sludge dryers, having less parts in the drying operation, as well as only a small amount of exhaust vapors to manage. Gross (1993), Gru¨ter et al (1990), Hasserbrauck and Ermel (1996) and Ressent (1998Ressent ( , 1999. It is difficult to make a strict comparison of the fixed capital and manufacturing costs of installed dryers with those of the processes simulated here, because of the different aims of each drying facility.…”

“…The annual per capita average requirement is estimated as 20 kg total dry solids (Munck-Kampmann, 2001). This represents a common size for the facilities equipped with a thermal dryer (Gross, 1993;Gru¨ter et al, 1990; Contact (wall-to-particle) 4 Â 10 À3 (Arlabosse, submitted) Fry-dryer Contact (oil-to-film) 3.0-4.4 Â 10 À3 (Peregrina et al, 2006a) Contact (oil-to-particle) 4.5-3.4 Â 10 À3 Thin-layer dryer Heating inertia in the wall 0.5-5 Â 10 À4 (x W 41 kg kg À1 ) ( Carre`re-Ge´e, 1999) Contact (wall-to-particle) 3 Â 10 À3 (x W o1 kg kg À1 ) Hasserbrauck and Ermel, 1996;Ressent, 1999). The facility would consume 1533 tons of oil per year.…”

“…The extent of water evaporation that is needed to prepare fry-dried sludge to the auto-thermal condition, and which is supplied by (Peregrina et al, 2006b) x RCO ¼ 0:35 kg oil kg total dry solid Final temperature of the condensed fry-drying vapors 368 K (Gross, 1993) Overall heat transfer coefficient U 0 ¼ 230 W m À2 K À1 (sludge-steam) (Yamahata and Izawa, 1985) U 0 ¼ 250 W m À2 K À1 (RCO-steam) (Perry and Green, 1984) U 0 ¼ 1250 W m À2 K À1 (steam-water) (Perry and Green, 1984) Medium pressure steam P ¼ 9 atm (abs) (Ressent, 1999, Turton et al, 2003 electricity or natural gas is only 10-14% of C DM . Oil penetration and adhesion to the fried sludge contributes enormously to its energy value, reducing the drying that is needed.…”

Sludge management

“…Thermal drying of wastewater sludge is gaining increased acceptance as a means of reducing the volume of sludge and recycling in the form offuel or biosolids. With direct drying, sludge is brought into direct contact with the heat source, whereas indirect drying separates the sludge from the heat source (Gross, 1993). Wormald et al (1993) compared two heat-drying systems for municipal sludges in terms of performance results, dewatered sludge feed, dried sludge product, air emissions, and condensate characteristics.…”

Green Biotechnology for Municipal and Industrial Wastewater Treatment

“…Dewatered sludge also is used in the feed preparation for ruminants [6], poultries [7], fish [8], worms [9] and aquatic scavengers [10]. Landfilling [12], ocean discharge [13], conversion into useful materials such as oils [14] and building materials [15] are some of the approaches used for sludge disposal and recycling. Also, components such as fats, proteins and vitamins can be extracted from sludge [11].…”