Design And Analysis Of Process And Device For Continuous Pretreating Sludge By Microwave

In recent years,the output of domestic sludge from municipal sewage treatment plants has increased significantly.The sludge disposal cost accounts for about 50% of the operating cost of the sewage treatment plant.Dewatering can reduce the amount of sludge and reduce transportation costs,which is a key link in sludge disposal.However,the large amount of water and microorganisms in the sludge will form a special gel structure,which makes deep dehydration difficult.Traditional mechanical dewatering can reduce the moisture content of sludge to about 80%,which still cannot meet the requirements.Therefore,it is necessary to study appropriate sludge pretreatment processes and devices.Microwave treatment technology is an effective method to improve sludge dewatering performance.The traditional microwave pretreatment technology has the problems of high processing cost and low heating uniformity.In order to solve the above problems,the process and equipment of microwave sludge treatment were studied by means of process test and computer simulation.The influence of microwave and biomass ash on the deep dewatering performance of mechanical dewatering sludge was studied by orthogonal experiment,and the optimum process conditions were determined.The continuous microwave sludge pretreatment device was designed according to the optimum process conditions.The structure size of the device was optimized by electromagnetic simulation,and the sludge heating test process was simulated.The influence of the key parameters on the sludge heating effect was analyzed,which provides a reference for the process design and equipment development suitable for the application of microwave on sludge treatment.The results show that:(1)After microwave conditioned under the optimum conditions,the sludge moisture content(MC)and specific resistance to filtration(SRF)decreases by 2.9%and 61.8%,respectively.According to the dosage of biomass ash,the above parameters decreases by about 14.4% and 29.4% by using biomass ash alone.The microwave radiation and biomass ash combined treatment is more effective to improve the deep dewatering performance of sludge compared with conditioning alone.The optimal treatment condition is as follows: the microwave treatment is carried out at the microwave power of 260 W and the radiation time of 80s(microwave specific power consumption of 333J/g),and then biomass ash with 10% of the sludge volume is added for full mixing.Under the optimal pretreatment conditions,the sludge MC decrease by 21.5% and SRF decrease by 85.6%.The study of conditioning mechanism shows that microwave radiation will destroy the original stable structure of sludge and promote the loss of water.Then the biomass ash rebuild the porous skeleton structure which is easier to remove water.Therefore,the combined conditioning greatly improves the sludge dewatering performance.(2)Based on the theoretical calculation of the resonator,the main dimension parameters of the cylindrical microwave resonator is φ580mm×1440mm.The special structure of the heating and insulation chamber is designed.At the same time,the structure of the main components,such as the waveguide mouth,the leakage energy suppression module,the sludge flow channel,the spiral stirring shaft,the feed bin and the discharge bin are designed.(3)The results of electromagnetic simulation show that,the distribution of electric field in the cavity is the most uniform when the direction of the microwave feed is vertical.At the same time,the heating efficiency also is the highest,which is in the high value of 98.7%-99.4%.Under certain conditions,the larger ceramic medium tube radius will occupy a large amount of space in the resonator,which will hinder the propagation of electromagnetic wave to a certain extend.Furthermore,there is obvious strong and weak region of electric field in the resonant cavity,and the heating efficiency decrease to different degrees.(4)The simulation results of sludge heating process show that,the rotation of ceramic medium tube can obviously improve the uniformity of sludge heating.The temperature uniformity of the sludge section increased with the increase of rotating speed,and the optimum rotating speed is RO =0.3r/s.Under the same microwave power,the faster the sludge flow rate is,the greater the temperature variation coefficient of the sludge section will be,which indicates that the heating is not uniform,and the average temperature of the sludge will also be reduced,so the flow rate needs to match the microwave power.The uniformity of sludge heating can also be improved by increasing the rotating speed of the screw stirring shaft.The optimal working conditions of the screw shaft is as follows: the radial movement direction of the sludge is the same as the rotation direction of the medium pipe,and the optimal speed is Vr=0.15 r /s.At this time,the heating of sludge is uniform enough,and the problem of energy consumption increased and wear caused by high rotation speed will be avoided.In summary,the sludge will reach the specified temperature and the thermal uniformity of the sludge will be significantly improved when the appropriate microwave power ratio is adopted for the heating chamber and the thermal insulation chamber.