Experimental Study On The Treatment Of Simulated Tunnel Construction Wastewater By The Combined Process Of Clarifying Filter Cloth

With the proposal of the transportation power strategy,the proportion of tunnel engineering in mountainous and hilly areas is increasing day by day.The environmental pollution and ecological damage caused by tunnel construction have attracted people’s attention,especially the serious impact of tunnel construction wastewater on the water environment and aquatic ecology.The amount of wastewater during tunnel construction is greatly affected by hydrogeological conditions,making it difficult to choose a reasonable equipment design load.In addition,conventional treatment processes have problems such as lengthy treatment processes and large land occupation.Therefore,it is urgent to find a process that can overcome large fluctuations in water quality and quantity,ensure the accuracy of tunnel construction wastewater discharge,and save land occupation to cope with poor on-site construction conditions.This article proposes a combination process of"clarification+filter cloth filtration",which utilizes the characteristics of the clarification tank that integrates coagulation and sedimentation.A filter cloth filtration component is added in the clarification zone to resist the rapid deterioration of the effluent caused by the expansion of sludge due to water expression.At the same time,the water purification process is further shortened,reducing land occupation and saving construction costs.When the amount of wastewater is small,simply start the clarification tank;When the wastewater volume fluctuates violently,causing the suspended sludge layer in the clarification tank to expand upwards,the high concentration of suspended sludge water is filtered through an additional filter cloth to ensure the effluent quality.This not only saves land area,but also achieves the goal of responding to water quality fluctuations.The experiment studied two filtration methods,cross flow filtration and vertical upward flow,to treat high concentration tunnel construction wastewater.The main conclusions are as follows:（1）Under constant pressure conditions,the filtration characteristics within 600 minutes were analyzed using a cross flow filtration method,including changes in effluent turbidity,filtration flux,and filtration resistance.On the premise of ensuring that the effluent turbidity does not exceed the standard（20 NTU）,the maximum filtration pressure is 196 pa（2 cm water head）,the maximum stable flux is 91 L/（m²·h）the effluent turbidity is always maintained within10 NTU,the total filtration resistance increases from to,and the cake layer resistance increases from to;Through mechanical analysis of the filter cake model,an approximate theoretical formula for the density of the filter cake layer under cross flow filtration was obtained.The theoretical maximum stacking coefficient（0.740）and maximum stacking density（0.755-0.784mg/L）of the filter cake layer were analogized using the particle stacking model.Based on literature analysis,the pollution mechanism of filter cloth filtration was proposed,mainly the displacement and deformation of particles.The pores within the framework of the filter cake layer are the main flow passage;The failure of the filter cake layer is mainly divided into three stages:the overall strength failure of the filter cake layer,the expansion of the failure channel（crack）,and the penetration and erosion of the failure channel.（2）Under the same constant pressure conditions,the filtration characteristics within 600minutes were analyzed using a vertical upward flow filtration method.On the premise of ensuring that the effluent turbidity does not exceed the standard,the maximum filtration pressure is 539 Pa（5.5 cm water head）,the maximum stable flux is 650 L/（m²·h）,the effluent turbidity is always maintained within 20 NTU,the total filtration resistance increases from to,and the cake layer resistance increases from to;The approximate theoretical formula for the density of the filter cake layer under the vertical upward flow filtration method was obtained through the mechanical analysis of the filter cake model,and the three important positive effects of gravity on alleviating filter cloth pollution under this filtration method were explained.Subsequently,by assigning values to the parameters of the filter cake layer at different times,it was found that the most significant impact on filter cloth pollution was the upward drag force caused by water flow velocity.（3）According to the curve of filtration flux over time,it can be seen that the blockage of filter cloth and the formation process of filter cake layer in cross flow filtration follow the empirical exponential model,while in vertical upward experiments,it follows the linear attenuation model.（4）Under the same backwashing intensity,the flux recovery rate of vertical upward flow is higher than that of cross flow filtration.When the backwashing intensity is 3.67 L/（s·m²）,the flux recovery rates of cross flow filtration and vertical upward flow filtration after backwashing for 30 minutes are between 87.14%-97.84%and 91.68%-99.87%,respectively.This also indicates that the pollution level of the filter cloth in vertical upward flow filtration is lower than that in cross flow filtration.（5）The flocculation conditions indirectly have a significant impact on filtration by affecting the characteristics of the flocs.Too short or too long hydraulic retention time will have adverse effects on filtration.When the inflow flow is 80L/h,the hydraulic retention time is14.25min,at this time,the floc particle size and fractal dimension are the largest,and the filtration effect is the best;The filtration effect of PAC as a flocculant is better than that of Al₂（SO₄）₃ and Fe Cl₃;Adding too little flocculant can cause many smaller colloidal particles to suspend in water,while adding too much flocculant can cause colloidal protection and instability.In this study,the optimal condition was when PAC dosage was 40 mg/L.（6）The type and pore size of filter cloth will have a significant impact on filtration.Fiber filter cloth can adsorb particles to form a filter cake layer due to its presence of cilia,thereby exhibiting better filtration performance than monofilament nylon filter cloth;Single filament nylon filter cloth with smaller pore size will make it difficult for wastewater to pass through the filter cloth due to high filtration resistance.If the pore size is too large,it will be difficult to form a filter cake layer,with only a pore size of 30μm.The monofilament nylon filter cloth of m will show good filtering effect.（7）The Analytic Hierarchy Process（AHP）was used to comprehensively evaluate the combination processes of"clarification+filter cloth"and"mixing+grid flocculation+inclined plate sedimentation+valveless filter tank"from three aspects:economic criteria,technical criteria,and management criteria.The calculation results showed that the combined process of clarification and filter cloth in this paper is more suitable for treating tunnel construction wastewater with frequent hydraulic impact loads.