Research On The Flow Patterns Of Mechanical Agitation Anaerobic Digestor

Solid waste treatment and energy recovery can be achieved synchronous by anaerobic digestion. Anaerobic digestion has been used with a wide range of applications in sewage sludge, municipal solid waste, food waste, agricultural waste, livestock manure and other solid waste treatment. As a complex biological reaction, the performance of anaerobic digestion is affect by mixing significantly. Currently, research on anaerobic digestion is focus on substance pretreatment, co-digestion, temperature staged and biologically phased(TSBP), and other process to improve the efficiency of anaerobic digestion. Study on flow field pattern in anaerobic digestor is deficient. Design of digestion tank and stirring intensity is usually depends on design experience.In this topic, RTD and flow field pattern was study by tracer test and CFD simulation in pilot reactor and full scale digestor. Through a variety of sludge concentration, stirring method and digestor shape, flow pattern character of mechanical stirred anaerobic digestor was study in this paper. The variation of flow pattern character and influencing factors was revealed. Design basis and guidance was provided. The medium in CFD simulation is water, so the result of CFD simulation only could be used in low concentration substrate digestor.Tracer test was carry out on small scale reactor at the sludge concentration 2.5, 5.0 and 10.0 g/L. Sludge concentration and the use of continuous or intermittent stirring, didn’t affect the operation profermance and flow pattern in digestor.CFD simulation was conduct on cylinder and egg shaped digestor in different ratio of height to diameter(H/D). The results showed that agitator type and H/D ratio are significant factor for the configuration of digestor’s flow field. For the cylinder shaped digestor use top-entry central agitator, average velocity become lower with growth of H/D ratio. For side-entry agitation digestor, the average velocity becomes higher with increase of H/D ratio. Higher H/D ratio is appropriate for the side-entry agitation digestor. Flow field in egg shaped digestor is not affect by H/D ratio.Tracer test for full scale digestor was carrying out in four large or medium size anaerobic digestors. The tracer curve is accordancy with ideal CSTR curve; flow pattern in digestor is completely mixed. Dead zone was found in each digestor. However, degree of dispersion of material is high, substantially no short-circuiting. Top-entry central agitator digestor is similar with CSTR reactor, but side-entry agitation digestor have a high degree departure from CSTR. The result of CFD simulation show that at middle of egg shaped digestor, the mixing is weak. flow field distribution in cylinder shaped digestor is more uniform. Slow speed and high torque agitator is better than high speed agitator for large size full scale digestor.