Synthetic sludge as a physical and chemical analogue of real sludge in the activated sludge process

The most widespread process in use today for the biological treatment of municipal and industrial wastewaters is the activated sludge process (ASP). The ASP is an aerobic bioreactor process, in which micro-organisms are grown for the digestion and removal of soluble organic matter to low levels. It is flexible and reliable, capable of producing a high quality effluent which is low in suspended solids due to the tendency of the biomass to flocculate. The ASP consists of two liquid-stream unit operations: the biological reactor for pollutant digestion; and a secondary clarifier, in which solid flocs are separated (usually by gravity) from the effluent. The formation of stable biological flocs is essential for the successful operation of the process. Poorly flocculated sludge can also have an adverse effect on sludge dewatering. The thickened sludge that is produced as waste from the process is often dewatered to reduce its handling costs; a well-flocculating sludge translates into one that will dewater easily Activated sludge flocculation is a very complex physical, chemical and biological process, in which many factors interact and have an influence. Since many of these influences are poorly understood, the characteristics of floc-formation remain difficult to predict and control. Yet an improved knowledge of the flocculation step is an essential requirement for optimal biological wastewater treatment. The living consortium of micro-organisms in activated sludge is dynamic, complex and unstable, making it extremely difficult to carry out controlled experiments during biological sludge studies. Fortunately, Sanin and Vesilind (1996, 1999) developed a novel physical and chemical surrogate for activated sludge, which they termed synthetic sludge, to study sludge de-watering, settling and conditioning characteristics. It is made out of non-living components that resemble activated sludge components physically and chemically, if not biologically. The components of synthetic sludge include: polystyrene latex particles of bacterial size, to simulate individual bacteria; alginate, to simulate extracellular polymeric substances (EPS) such as polysaccharides; fibrous cellulose, to simulate the filamentous micro-organisms found in activated sludge and calcium ions, as bridging cations for the EPS. All components are present at representative quantities. The early work of Sanin and Vesilind showed that it is possible to create a chemical sludge in this fashion, having close resemblance to biological sludge. Such a stable, physically and chemically well-defined system was used here as a surrogate for sludge during tests. The current article aims to present the results of an extensive and ongoing study to link the properties of synthetic and activated sludge, including floc formation, floc structure and stability, settling, dewatering and conditioning. Cations, biopolymer, filamentous organisms and shear all play important roles in the flocculation of both synthetic and activated sludge; calcium (and aluminium) plays a role in forming bridges between alginate or biopolymer adsorbed on the particles or bacteria. Calcium also has a major impact on settling and dewatering, while polymer conditioning has a major impact on the latter. Our work has shown that the stable and well-defined nature of synthetic sludge makes it very useful as a simple, non-biological and non-complex surrogate for studying the physical and chemical properties of biological activated sludge, including polymer conditioning. Nonetheless, some quantitative discrepancies are seen to arise in settling and sludge volume index, supernatant turbidity, sludge dewatering, and floc strength. For such studies, the inclusion of a filamentous material in synthetic sludge, such as cellulose, is recommended.