Dewatering Performance of Sewage Sludge during the Thermal Compression Process

Sewage sludge proves to be difficult to dewater due to its composition and biological nature, and it is unsuitable for disposal with high moisture content. In the present work, sewage sludge is dewatered by the thermal compression, which shows unique separation advantages of various materials, and the effects of applied temperature and pressure on the dewatering performance are investigated. The filtrate SCOD reveals that higher temperature benefits the disruption of sludge flocs, resulting in the release of organic content. According to the accumulative filtrate volume, higher temperature and pressure facilitate the dewatering process, obtaining sludge cake with lower moisture content. Introduction At present, million tons of sewage sludge (on a dry basis) is annually generated in China from municipal and industrial wastewater treatment plants. Sludge dewatering is critical to reduce sludge volume and subsequent treatment cost. Sewage sludge is a colloidal system, and it is very difficult to dewater due to the high organic content in the sludge solids [1]. A number of methods have been reported to improve the dewatering efficiency of sludge, including hydrothermal drying [2], ultrasound [3], freezing and thawing [4], chemical oxidation [5] and electrolysis [6]. However, these methods are still difficult to meet the processing of large amounts of sludge. Thermal compression is a cross-technology which combines mechanical filtration and heat drying, offering the possibility of a rapid dewatering with lower energy consumption. The dewatering process is achieved through the vaporization of part of the cake moisture, and the remaining moisture is removed by means of the thrust force produced by the steam pressure inside the cake [7]. The dewatering mechanisms relevant to thermal compression involve: (1) filtration of water under the effect of mechanical pressure and steam pressure, (2) consolidation, and (3) flash evaporation [8]. Thermal compression has been proved to be efficient for dewatering of various materials, such as lignite [9], alfalfa [10], bentonite [11], talc and kaolin [12]. In our previous study, the sludge water could also be effectively removed using thermal compression with the aid of physical conditioner and polymer flocculant [13]. However, the characteristics of sludge flocs in the process of thermal compression remain unclear. This study aims to investigate the effects of operating conditions on the sludge properties and the dewatering process during the thermal compression. Materials and Methods Sludge samples were collected from a sewage treatment plant in Shanghai, China with a moisture content of 92 wt%. The flocculant used to condition the sludge was a cationic polyacrylamide (PAM) with 50% cationic degree and a MW of 15 million obtained from Mitsui. The anthracite coals was used as physical conditioner with a moisture content of 1.8 wt%, carbon content 58.82% and hydrogen content 3.67%. The flocculated sludge is dewatered in a thermal compression cell which consists of a cylindrical vessel (internal diameter 100 mm and height 200 mm) and a piston, both made of stainless steel (Figure 1). Firstly, the sludge sample (with 40 g dry solids) was mixed with 0.2 kg coal/kg dry solid and flocculated with 6 kg PAM/t dry solid. Secondly, equal volume of supernatant was decanted after 10 min precipitation. Thirdly, the precipitate was loaded into the preheated thermal compression cell and treated for 45 min. During the experimental process, the volume and the soluble chemical oxygen demand (SCOD) of the filtrate were measured at regular intervals. Fig.1 Schematic of the thermal compression The moisture content was measured by a fast moisture analyzer (METTLER-TOLEDO HR83P, Switzerland). SCOD was determined by the standard potassium dichromate method after filtering the samples through 0.45 m membrane. Results and Discussion Effects of applied temperature and pressure on the filtrate concentration. Figure 2 shows that the effect of applied temperature on the filtrate concentration is significant. The filtrate SCOD increases as a function of time. After 45 min thermal compression, SCOD increases from 230 mg/L to 4718 mg/L at 120 C. Compared to the operation at room temperature, thermal compression obtained higher SCOD concentration, especially at 100 C and 120 C. The increase of SCOD is mainly due to the release of extracellular polymeric substances (EPS) or cytoplasmic materials of the sludge cell into the medium with the disintegration of the sludge flocs [14,15]. In recent years, many researches have focused on the improvement of sludge dewaterability through the disruption of sludge flocs and breakage of microbial cells [16-18], resulting in the solubilisation of cellular constituents and reduction of sludge viscosity. The water in sewage sludge is classified into four types: free water, interstitial water, surface water and intracellular water [19]. It is reported that thermal treatment of sludge produces disaggregation of sludge flocs, high level of solubilisation, cell lysis and release of intracellular Pressure