Journal of Cold Regions Engineering

Sludge dewatering processes are usually preceded by sludge conditioning, which improves the sludge dewaterability. Although sludge conditioning is most commonly achieved by the addition of chemicals, freeze‐thaw conditioning can also be beneficial. The most practical way of using freeze‐thaw conditioning is to dewater the sludge on a freezing bed. When sludge is pumped from its sources to the freezing bed, it is subjected to high shear force induced by the pump and turbulent pipe flow. This shear may affect the effectiveness of freeze‐thaw conditioning. The objectives of this study are to investigate the effects of preagitation on freeze‐thaw conditioning of anaerobically digested sludge. The results indicate that preagitation has positive effects on freeze‐thaw conditioning of anaerobically digested sludge. Agitation intensity was found to correlate with sludge dewaterability, with increased agitation leading to enhanced dewaterability of freeze‐thawed sludge.

The magnitude of the hydrostatic pressure that results from the growth of an ice layer at the top of a water‐filled manhole is determined using both analytical (for a thin ice layer) and finite‐element methods (for a thick ice layer). The ice‐layer freeze/fracture cycle during the growth of the ice layer is predicted analytically and subsequently verified experimentally. A general expression for the hydrostatic pressure as a function of ice thickness and cross‐sectional area $({l^2}_i/a^2)$ for a square cross section is developed. Experimental results for an ice layer of a circular cross section show that up to ${l^2}_i/r^2\simeq 0.5$ the trend in pressure versus ice thickness results is predicted by the finite‐element method although in some cases the actual experimental values are approximately twice the predicted values, probably due to thickening of the ice disk around the periphery. For the ${l^2}_i/r^2\simeq 0.5$ case, hydrostatic pressures resulting from ice formation at the top of the manhole can be two orders of magnitude greater than the pressure due to the static head in a water‐filled manhole.