Sludge Dewatering by Impulse Drying

An innovative and energy efficient sludge dewatering technology has been developed. The technology, impulse drying, involves briefly contacting the sludge under pressure with a heated surface. A unique feature of impulse drying is that the majority of moisture removal occurs as lipid water. A pilotand laboratory-scale demonstration was performed on low ash primary paper mill sludge. The pilot-scale impulse dryer was able to increase the sludge solids level 23.2% greater than belt pressing alone. The pilot scale equipment was not optimized, and fiuther gains are expected. Laboratory-scale demonstrations suggest that impulse drylng is effective on a wide variety of sludge types: primary (high and low ash), secondary, and mixed sludges. Impulse drying efficiency depends upon the presence of sufficient moisture in the sludge. Preliminary results indicate that the process is effective for sludges with initial solids content in the range of 15 to 50%. Since, a sludge solids content of 30% is typical of the performance of an inexpensive belt press, these experiments demonstrate the potential of retrofitting existing belt presses with an add-on impulse dryer. A high percentage of liquid water removal is achieved by the impulse drying process, which gives the technology an economic advantage relative to thermal drying. The liquid water removal has major cost reduction potential for either sludge burning or lan-g. JNTRODUCTION Since the 198Os, impulse drying has been under extensive development in the field of paper drying (Lavery 1987). Essentially, the impulse drying process involves pressing in a nip between heated and unheated rolls. Paper impulse drying involves pressing a moist paper sheet with a felt against a heated roll (200 to 350°C). Nip dwells of less than one second are generally used. Impulse drying induces an intense dewatering process that gives a higher dryness level than (wet) pressing, and it is more energy efficient than thermal drymg. Unfortunately, for paper impulse drying, the usefid range of operating conditions is limited by sheet delamination and product quality concerns (Crouse, Woo, and Sprague 1989; Orloff 1992). Sludge dewatering is an ideal application for the impulse drying technology because sludge delamination is not a concern. This paper describes recent work being done at the Institute of Paper Science and Technology (IPST) on the application of impulse drying to the field of sludge dewatering. The water removal thermal drying or pressing. mechanism of impulse drymg is different fiom that involved in either A combination of related phenomena likely contributes to dewatering.