Abstract:
Drying beds are a simple and economical means to dewater municipal sludge and are
widely used in places with a suitable climate for air-drying. However, research-based information
on drying thickness/drying depth effects on nutrient content and land size requirements for sludge
drying is scarce. In this study, aerobically digested (AeD), and anaerobically digested without
polymer (AnDP0) and with polymer (AnDP1) sludge types were dried in sand drying beds at 5, 10,
15, 20 and 25 cm depths in South Africa. Measured nitrogen (N) fractions and other parameters were
more strongly influenced by sludge types than by drying depth. Total N content followed the order
of AeD > AnDP1 > AnDP0. Polymeric material addition tended to increase total and inorganic N
content and reduce the length of sludge drying period and land size requirement. The study showed
that larger land size is required to dry sludge at shallower depths, even though the sludge dried more
quickly. Drying sludge at 15 cm was the best option across sludge types in winter, taking an average
land area between 261 and 383 m2 over the fewest days of sludge drying, whereas in spring, drying
at 20 to 25 cm depth was most favorable. The findings suggest that drying bed management can be
based on land area requirements with little concern for biosolid quality changes. For wastewater
treatment plants relying on drying in beds, adding polymeric materials may be beneficial where land
area for drying beds is limited, but not otherwise. Therefore, land availability is critical in decision
making for sustainable sludge drying thickness.
