The role of water in sludge dewatering

ABSTRACT: While the dewatering of wastewater sludges has received considerable research attention, it has been assumed that the water continuum surrounding the sludge particles has the physical properties of bulk water. This paper suggests that such an assumption may not be valid. The existence of vicinal water, consisting of ordered and layered water molecules on the surfaces of submerged solids, is demonstrated using research from various disciplines. The meaning of “bound water” as used in sludge technology is discussed, and it is shown that bound water can be defined in several ways, depending on the test used. It seems reasonable that bound water as it is usually defined actually consists of three different types of water—water captured in the interstitial spaces within floes and within cells, vicinal water on the surfaces of solids, and water of hydration. Only some of the interstitial water can be removed by mechanical dewatering. Because of the large surface area of sludge particles, including living cells, it is likely that much of the bound water is vicinal water, and this cannot be removed mechanically. The determination of the various fractions of these waters may lead to a better understanding of mechanical sludge dewatering.