Effect Of Agitation On Performance And Flow Patterns Of High-solid Sludge Anaerobic Digestion

Sludge produced during the wastewater treatment process is an environmental problem that must be solved.Anaerobic digestion technology is a very promising sludge treatment technology since the sludge is usually in rich in organic matters which can be recovered and produce biomass energy.It is worth noting that the high-solid sludge（Total solid,TS≥10%）anaerobic digestion technology has the following advantages:improving the organic loading,reducing the occupied area of the infrastructure,reducing the maintenance cost and so on.However,due to the restriction of high-viscosity materials,high-solid sludge anaerobic digestion is faced with problems such as difficulties of mass and heat transfer.Therefore,agitation is important in the large-scale application of high-solid sludge anaerobic digestion technology since agitation can effectively improve anaerobic digestion performance of high-solid sludge.The purpose of this paper is to explore the effect of agitation on the high-solid sludge anaerobic digestion performance and flow patterns through the high-solid sludge anaerobic digestion experiments and Computational fluid dynamics（CFD）simulation.Furthermore,the economic feasibility of high-solid sludge anaerobic digestion technology was evaluated combined with energy balance calculation.The main research contents and results are as follows:1.Through the start of high-solid sludge anaerobic digestion reactor in the sequencing batch,low-load semi-continuous and high-load semi-continuous operation mode,the experiment explored the evolution of biogas production performance,reactor stability,and agitation power during the start-up process and realized the rapidly start-up and stable operation of reactor.The study provided guidance for the start-up of high-solid sludge anaerobic digestion technology.From sequencing batch operation to semi-continuous operation with low load,then to semi-continuous operation with load-increasing,the anaerobic digestion system achieved transformation rapidly and stably.During the stable period of anaerobic digestion,the average volume biogas yield was 0.3592±0.011 m³/（m³·d）,the methane content was stable at 60.14±2.45%,and the sludge organic degradation rate was22.05±0.52%.The volume mean diameter,surface mean diameter and median diameter of the sludge were reduced to 78.43%,74.08%and 70.52%respectively.The pH value was suitable and stable at 7.5-7.8,the concentration of volatile fatty acids（VFAs）,ammonia nitrogen and free ammonia were lower than the inhibition level,the residual energy of anaerobic digestion reached 2.61 MJ/（m³·d）.The biogas production and stability of the high-solid sludge anaerobic digestion were good,and the economy was viable.2.The effect of agitation intensities（2 h/d-50 rpm,2 h/d-25 rpm,2 h/d-10 rpm,6 h/d-50rpm,6 h/d-25 rpm and 6 h/d-10 rpm）of double-bladed impeller on the biogas production and the stability performance were explored,and CFD simulation was used to analyze the flow pattern of reactor and achieve the visualization of the mixing performance.In the anaerobic digestion reactor with double-bladed impeller,the increase of agitation intensity was beneficial to the improvement of biogas production performance of high-solid sludge,and the effect of agitation speed was greater than the agitation time.Under the agitation condition of 6h/d-50 rpm,the agitation intensity was the highest and the biogas yield was the highest(188.29±23.28 mL/g VS _added).Agitation condition of 2 h/d-25 rpm was the most economical strategy,with a residual energy of 7.25 MJ/（m³·d）.The pH value was stable at 7.4-7.8,the highest ammonia nitrogen and free ammonia concentration were 1431.89 mg/L and 84.53mg/L,which were lower than inhibition level.The CFD simulation results showed that the increasing of the effective speed region and the decreasing of apparent viscosity were positively related to the improvement of the high-solid sludge anaerobic digestion performance.3.The influence of the agitation speed（10,25,and 50 rpm）of ribbon impeller on the anaerobic digestion performance and the flow patterns of digested sludge was explored.The results showed that the increase of agitation speed was conducive to the increase of biogas production.At agitation speeds of 10,25 and 50 rpm,the biogas yields of the high-solid sludge anaerobic digestion were 243.99±42.11,314.18±34.53 and 340.38±15.09 mL/gVS_added,and the methane content remained 61.55±1.47%.The pH value of the digestive system was maintained at 7.51-8.10.The ammonia nitrogen and free ammonia concentrations increased with the increase of agitation speeds,but did not cause the inhibition of anaerobic digestion system.The results of CFD simulation showed that the increase of agitation speeds could improve the distribution of sludge velocity field,reduce the apparent viscosity of sludge,which is beneficial to the increase of anaerobic digestion performance of high-solid sludge.4.By comparing the performance of the double-bladed impeller and the ribbon impeller,it was found that the agitation power of the double-bladed impeller was higher than that of the ribbon impeller at the same agitation speed,but the mixing time of the ribbon impeller was much less than that of double-bladed impeller.From the point of view of mixing performance and energy consumption,ribbon impeller was more suitable for the high-solid sludge anaerobic digestion system with high-viscosity.The relative abundance of microorganisms showed that the relative abundance of Methanosarcina in both ribbon impeller and double-bladed impeller increased with the increasing of agitation speed.The relative abundance of Methanosaeta was high at low agitation speed but decreased at high agitation speed,indicating that the distribution of high shear rate was conducive to the growth and reproduction of Methanosarcina,but had an adverse effect on the growth of Methanosaeta.