Thermophilic anaerobic digestion of sugars in upflow anaerobic sludge blanket reactors

Abstract

In recent years numerous designs have been developed for the high-rate anaerobic treatment of wastewater.'-6 All these processes have their advantages and drawbacks, which have been discussed repeatedly (see, for example, ref. 7). Of these processes, the upflow anaerobic sludge blanket (UASB) process is currently the most widely applied, at least in Western Europe. It is successfully applied in the treatment of wastewaters from maize and potato starch industries and sugar beet and potato processing industries.' Perhaps the main advantage of the UASB process is that no specific support material is required. However, this main advantage may sometimes be a serious drawback: the absence of support material necessitates the availability of a sludge with very good settling properties. Whenever such a sludge is obtained, or can be maintained, successful operation of the UASB process is readily achieved. Under certain circumstances the settleability of the sludge in UASB reactors is greatly improved because bacterial growth is in the form of spherical flocs with a quite consistent structure, normally referred to as granular sludge.6 Although already considerable research has been performed on factors considered to be important in the phenomenon of gran~lation,~- the knowledge on this matter is still relatively poor. Thermophilic anaerobic digestion may become an attractive alternative for mesophilic digestion because of the higher growth rates of the bacteria involved and, consequently, the higher maximum activities per unit of biomass. As the specific growth rates of the methane-forming bacteria of the genus Methanothrix, which are believed to be the predominant acetoclastic methanogens in methane digestion processes with a high biomass retention time,L2313 is ca. 2.5 times as high at 55°C as at 30°C,'2.'3 also a 2.5 times higher methanogenic acitivity per unit of biomass may be expected. Also, in thermophilic sewage sludge digestion a Methanosarcina was found to be the predominant acetoclastic methanogen, which even has a higher specific growth rate than the thermophilic Methanothrix. l 4 The successful operation of high-rate thermophilic systems requires a high activity per unit of volume, and therefore a high biomass retention is a prerequisite. In view of the successful use under mesophilic conditions, we investigated the thermophilic UASB process for this purpose. Glucose was chosen as the substrate because granulation is reported to occur in UASB reactors when it is the substrate for a acid-formation process.