The use of compressive rheology tools to determine the dewatering properties of selected sewage sludges

Abstract

Wastewater treatment produces sludge which is usually dewatered prior to disposal to reduce disposal costs. This work used compressive rheology tools to accurately measure the dewatering properties of selected sewage sludges with a focus on sludge produced by Autothermal Thermophilic Aerobic Digestion (ATAD). Sludges were sampled from a 500 litre ATAD pilot-plant at Killarney Co. Kerry, and from wastewater treatment plants in Australia, Ireland, Germany and Luxembourg. The project aims to outline the dewatering properties of these sludges and to determine some of the factors which govern wastewater sludge dewatering. The theoretical basis of this work was the phenomenonological solid-liquid separation theory of Buscall and White (1987). The gel-point, compressibility, and permeability have been established as the fundamental physical properties that determine suspension dewaterability. However, until recently, the experimental determination of these properties for wastewater sludges (which exhibit atypical behaviour) was limited due to theoretical and experimental constraints. Recently, new qualitative and quantitative methods based upon the Buscall and White (1987) approach to dewatering characterisation have been developed, and have been successfully applied to wastewater sludges (Stickland, 2005). In this study these methods were applied to ATAD sludges for the first time. Filtration, settling and centrifugation tests were used to measure the material characteristics of several ATAD sludges, some of which were conditioned using polymeric flocculants and inorganic coagulants. All wastewater treatment sludges which were investigated in this work exhibited nontraditional filtration characteristics, forming highly networked suspension at low solids concentrations. Such sludges are impermeable and highly compressible. A 500 litre ATAD pilot-plant was commissioned as part of this research. Thickened waste activated sludge was used as the feedstock for the ATAD pilot-plant. In retention time trials, temperatures of 60 °C were achieved during digestion. The pilot-plant achieved 47% volatile solids removal on a 10 day retention time. Digested sludges obtained from the ATAD pilot-plant had very poor dewatering properties at 7 day and 10 day retention times, and it also had an exceptionally high optimum polymer dose rendering it unsuitable for dewatering. Furthermore, ATAD sludge was obtained from the post-process storage tanks at Killamey WWTP, and from treatment plants in Germany and Luxembourg. In pressurefiltration tests, these ATAD sludges were shown to dewater to similar solids concentrations to anaerobic sludges for pressures ranging from 2kPa to 400kpA. However, in the case of the ATAD sludges obtained from Killamey WWTP the quantities of polymeric flocculant required to achieve satisfactory dewatering were high. The sludges obtained from treatment plants in Germany and Luxembourg showed markedly better dewatering properties than the Killamey ATAD, and this may have been due to the feed sludge to these treatment plants having a higher proportion of primary sludge than at Killamey WWTP which was operated solely on a feed of waste-activated sludge. iv Investigations into the factors influencing ATAD dewatering showed that the optimum polymer dose correlated strongly with negatively charged biopolymeric material in the solution phase. A proportion of this material consisted of proteins and polysaccharides which were released as a function of digestion time during batch trials. These substances may accumulate in solution because of cell lysis effects and the erosion of extracellular polymers from sludge floes during digestion. Finally, the conditioning of ATAD sludge with ferric sulphate was shown to improve dewatering properties in terms of permeability but reduced the compressibility of the sludge.