While the dewatering of wastewater sludges has received considerable research attention, it has been assumed that the water con tinuum 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 deter mination of the various fractions of these waters may lead to a better understanding of mechanical sludge dewatering.zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Water Environ. Res., zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF 66,4(1994).
By studying the drying characteristic curve, the moisture in sludge is classified into four categories: free moisture, interstitial moisture, surface moisture and bound moisture. The effects of three dewatering procedures: gravity drainage, vacuum filtration, and centri-fugation, as well as chemical conditioning by a cationic polymer, and physical conditioning by freeze-thaw on the moisture distribution, are investigated. It is found that the dewatering procedures studied can only remove part of the free and interstitial moisture; surface and bound moisture are not affected. Part of the free and interstitial moisture also remains with the sludge cake, which can be considered as the inherent inefficiency of the dewatering process. The complete removal of all the free moisture can be considered as the theoretical best performance any mechanical dewatering device can achieve. Polymer addition is found to improve the dewaterability of the sludge by reducing the inherent inefficiency, primarily by reducing the amount of free moisture retained in the cake. Freeze-thaw is found to effect the removal of free, interstitial and surface moisture, bringing about a dramatic increase in dewaterability. The treated sludge cake drains readily and retains no free moisture.
We postulate a mechanism for floc formation and demonstrate its feasibility by building synthetic sludge flocs. In this work, polystyrene latex particles of a size similar to bacteria are used to simulate individual bacteria; alginate, a polysaccharide, is used to simulate microbial extracellular polymers; and calcium ions are used as the bridging cations. Calcium and alginate are added at typical concentration ranges relative to the amount of microorganisms existing in wastewater treatment biological sludges. Flocs are formed almost instantly by the addition of calcium ions and are observed to be remarkably similar to the activated sludge flocs in physical appearance. Floc formation is faster at higher calcium concentrations. Higher alginate and calcium concentrations always ensure lower turbidities indicating better floc forming abilities.The results of this study indicate that formation of metal-polymer complexes and polymer gelation are important means of flocculation. Because of similar concentration ranges of alginate and calcium in this study to extracellular polymers and calcium in activated sludge, these mechanisms of flocculation are suggested also to be the main mechanism of interaction during biotlocculation in activated sludge systems. Water Environ. Res" 68,927 (1996).
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