Freeze–thaw sludge conditioning and double layer compression

Abstract

Freeze–thaw conditioning of water and wastewater sludges is known to be an effective and economical means of promoting dewatering when natural freezing is employed. When sludge freezes, both the suspended and dissolved solids are rejected by the growing ice front. Particles trapped in ice are known to have a very thin layer of water surrounding them and this water does not freeze at normal temperatures. Dissolved solids are thought to accumulate in this layer, causing an increase in the ionic strength of the water. This may cause compression of the double layer, leading to neutralization of repulsive forces, thus promoting aggregation. In order to test this hypothesis, ionic strength was increased by adding sodium chloride (NaCl) to water and wastewater sludges and measuring dewaterability (filtration) with the capillary suction time (CST) apparatus. Four different kinds of sludge were used: alum sludge (water treatment), waste-activated sludge, simultaneous precipitation, and anaerobically digested mixed sludge. Salinities of 0–20 000 mg/L as NaCl were tested with every sludge. No enhancement in dewaterability with freeze–thawed sludges of raised ionic strength compared to zero salinity was recorded, therefore the hypothesis of double layer compression being a major factor in freeze–thaw conditioning does not seem to be correct. Key words: sludge, freezing, coagulation, dewatering.

Migration of ions and organic matter during electro-dewatering of anaerobic sludge

“…Water was easier removed from sludge resulting in the high DS content in the final sludge cake, when freeze/thaw conditioning was applied. Indeed, it is well documented that the freeze/thaw conditioning increases sludge dewaterability, whereas a significant decrease in freezing temperature without the subsequent thawing, drastically reduces sludge dewaterability [35,36]. The decrease in freezing temperature did not show any response and did not correlate with any of the parameters (r 2 ranging from 0.12 to 0.25).…”

Volatile Fatty Acids Production from Codigestion of Food Waste and Sewage Sludge Based onβ-Cyclodextrins and Alkaline Treatments

“…And only little of organic carbons can be biodegraded unless the particulate organic matter is significantly solubilized [8, 9]. Aiming at strengthening the performance of anaerobic digestion, some efforts have been developed, such as chemical, mechanical, biological, and thermal cofermentation treatments [10–14]. Alkaline treatment has been selected as a potential way for improving volatile fatty acids (VFAs) production and also affected the composition of hydrolysate, hence leading to distinct VFAs composition in the subsequent fermentation process [15].…”

Characterization of oily sludge from a wastewater treatment plant flocculation-flotation unit in a petroleum refinery and its treatment implications

“…9 A freeze-thaw method known from treatment of wastewater sludge has been found to change the biological nature of the oily sludge, so that after thawing it cannot hold all the initially contained water. 10,11 Using freezing and thawing as pretreatment, a more than 50% release of oil from sludge has been achieved, improving also its fi lterability. 12 The optimal freezing temperature was found to be −40°C with suffi cient freezing time; 13 the best thawing conditions were below +20°C.…”