The CSTR is also commonly called a completely mixed reactor. It is described here because it is so commonly used around the world. In the CSTR, the liquid or slurry stream is continuously introduced, and liquid contents are continuously removed from the reactor.
Anaerobic treatment using a CSTR normally consists of a well-mixed reactor without solids recycling because all solids are in suspension and exit with the effluent. The SRT is equal to the HRT in this type of reactor. If operated properly, biomass that grows within the reactor continuously replaces the biomass removed from the reactor in the effluent stream (Droste, 1997). The basic characteristic of the ideal CSTR is that the concentration of substrate and biomass are the same everywhere throughout the reactor. In addition, the concentrations leaving in the effluent are the same as those in the reactor. This uniformity of concentration makes analysis of a CSTR comparatively simple. The CSTR is commonly used for aerobic or anaerobic treatment of highly concentrated organic mixtures, such as waste primary and biological sludge, as well as high-strength industrial wastes (Rittmann and McCarty, 2001). Since the biomass in the CSTR configuration is not retained by settling or attachment, the percentage of COD removal tends to be limited. The reactor must provide a minimum HRT of about 10 days at 35 °C depending on the waste; 10-30 days is a reasonable HRT range. Nevertheless, these vary with wastewater composition and degree of agitation (Stronach et al., 1986; Mulligan, 2002).
A disadvantage of the CSTR system is that it is susceptible to malfunction upon shock loading or subsequent introduction of a variety of toxic substances. Malfunction manifests itself in terms of reduced gas production, reduced degradation of organic materials and simultaneous increase in acidity. Improved digestion of animal waste by simple gravitational settling of the contents of a conventional mixed digester prior to effluent removal has been demonstrated. However, the time required for this additional treatment step is the major drawback, and widespread adoption of the technique seems unlikely (Stronach et al., 1986).
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