Study On Intensified Removal Of Fine Silt In Primary Sludge In WWTP

Due to the unsound urban drainage system in our country and the construction intensity of the city,the content of silt in the inlet water of the municipal wastewater treatment plant is relatively high,and more follow-up issues caused by fine silt and clay with relatively small granularity.In the municipal wastewater treatment plant in Chongqing,the fine silt with a particle size of 50¹00?m couldn't be effectively removed by the grit chamber.For a WWPT with a primary settling sink,most of the silt entered the primary settling tank,and minority of them still accessed to follow-up biological treatment units.When without a primary settling sink,the silt entered the biological treatment units immediately.Once the fine silt in the primary sludge entered the digestive system in the large-scale WWPT,they gradually deposited and hardened in the digester,which reduced the effective volume of the tank,shorten the cleaning cycle,exacerbated equipment wear,seriously decreased efficiency of the sludge dewatering and reduced the service life of the transport equipment,or caused a breakdown due to blocking.So it increased the processing costs and difficultys.In order to avoid this problem,wastewater treatment plants had to transport sludge after dewatering.However,organic matter in primary sludge is an important source of organic matter in the digestive system,usually accounting for 40%⁵0%,and organic matter in excess activated sludge was insufficient to maintain normal operation of digestive system.The ratio of organic matter to total suspended solids?VSS/TSS?in digested sludge was usually between 0.25 and 0.4,which was far below the normal level of 0.6 of the design value and the rest of the country,and It makes it difficult for the digestive system to a long-term run.It can be seen that the crux of the problem was silt in the primary sludge.Therefore,in the prerequisite of retaining organic matter.it was a good way of removing the fine silt in the primary sludge.This could not only save the cost of treatment of primary sludge,but also solve the key problem of the digestive operation.This paper proposed to use special equipment for cyclone separation of fine mineral particles to remove the fine silt in the primary sludge.And then the organic matter in the sludge from the bottom flow of hydrocyclone separator was selected by the fluidized bed separator.So as to achieve the purposes of effective silt controlling and resource utilization.In order to obtain accurate fine silt removal efficiency,this paper established a complete set of testing methods for particle size distribution analysis.And it also explored the characteristics of silt in the primary sludge and the particle size distribution characteristics of fine silt.Through the pilot test on the fine silt removal performance with the hydrocyclone separato,and organic flotation performance test with the fluidized bed separator,a productive scheme for fine silt removal from primary sludge was proposed.The conclusions drawn are as follows:?1?Through the comparative analysis of the sieving,mechanical analysis-laser particle size measurement and high-temperature buining-laser particle size measurement,it was found that the mechanical analysis method would not change the structure of mud silt particles and the loss of mud and silt was less after pre-treament by oxidation and acid washing.The accuracy within the whole grain size distribution was better than the other two methods.The measurement accuracy and repeatability were higher.The result was closer to the true distribution of the particle size.Therefore,the mechanical analysis method was used as the measurement method of the particle size distribution of fine silts.?2?Through the continuous monitoring of the sediment concentration in a sewage treatment plant in Chongqing,it was found that the sediment content in the primary sludge had a certain degree of volatility,while the fluctuation range was not large.And the concentration of mud and sand was greater than the concentration of organic matter and they were the main component of the sludge.Within one day,the peak of the mud and silt content alternated with the valley were appeared due to the peak period of urban residents'water using.It was found that the average particle size of the sediment particles in the primary sludge was small based on the further study on the characteristics of the silt in the primary sludge of the plant,and the particle size of 90%of the silt particles was below86.66?m.And the particles withthe size less than 30?m accounted for approximately70%of the total amount of silt,fine silt particles with a particle size of between 30-200?m accounted for approximately 28.5%and coarse particles with a particle size in the range of 200-450?m accounted for only 1.5%.It was also found that the majority of coarse silt particles>0.2 mm were removed by the grit chamber before the sewage entered the primary sedimentation tank.The silt particles entering the primary sedimentation tank were mostly fine silt and particle size was almost below 0.2 mm.The mineral composition of silt is mainly quartz,plagioclase and calcite.And the ratio of the relative contents of the three is 5:1.8:0.75,which is similar to the main mineral composition and content of surface sediments and hawthorn rocks in the main urban area of Chongqing.?3?Since the bottom flow's orifice diameter Du and the working pressure P are the main factors affecting the separation performance of the hydrocyclone,the single factor for removing silt from the FX100-PU-B and FX50-PU-II hydrocyclone separators was adopted.The test found that the FX100 and FX50 hydrocyclones had the best de-silting effect at pressure of 0.17 Mpa,diameter of the overflow mouth of 18mm and at pressure of 0.20 Mpa,diameter of the overflow mouth of 6mm,respectively.And the FX50-type hydrocyclone had the better ability to remove the fine silt And the highest efficiency of which was 71.39%.But the effect of organic matter enrichment was poor and the removal rate of organic matter reaches 17.38%.Although the total removal rate of fine silt was only 6.89%,the removal rate of organic matter by FX100-type was only 6.89%.Considering factors such as the amount of processing per unit time and operating power,the FX100 model was suitable for industrial applications in terms of production operations.The classification efficiency for fine silts of the FX100-type in particle sizes of 200-100?m,100-75?m,75-50?m and 50-30?m was 89.13%,84.38%,69.47%and28.89%,respectively.Through serial tests on the FX100 and FX50 hydrocyclones,it was found that the silt removal effect of the series operation was slightly higher than that of the single-stage operation of the FX100 type hydrocyclone,but the superposition effect of the removal rate was not achieved.The operating power was approximately was the 3.5times of single-stage FX100.Furthermore,running in series added extra runtime and workload.Therefore,the series operation was not recommended.Through the pilot test of the FX100 hydrocyclone,it was found that the fluctuation of the silt content in the primary sludge did not affect the separation performance of the hydrocyclone,and the FX100 hydrocyclone had a high operating stability.?4?Considering to the existing process of the wastewater treatment plant,the productive scheme of removing the fine sediment in the primary sedimentation tank was put forward.According to the treatment capacity of the WWTP,a linear parallel hydrohydrocyclone separator group was set up to remove fine silt in the primary sludge,and a fluidized bed sorter was used to determine the organic matter in the sludge.FBS500 fluidized bed separator when operating at feed concentration of 46.48g/L,feed flow rate of 1m³/h and opening degree of bottom flow valve of 20%,the flotation yield r was approximately 60%at the rising water flow rate of 10.90 mm/s,the organic component recovery rate?_orgrg was about 50%,the VSS/TSS value of the overflow sludge increased from 0.34 to 0.41,and the removal rate of fine silt was approximately 60%.Fluidized bed separator achieved the goal of reducing fine silt content into the sludge digester.After treatment,the VSS/TSS value in the sludge was close to that of the digested sludge,which effectively solved the problem of disposal of primary sludge.