Experimental Study On Optimization Of Filler Layers Structure In Bioretention

As one of the widely used low-impact development technologies,bioretention has outstanding storage and purification capabilities.When the bioretention is constructed,it should be filled,compacted,leveled and controlled according to the drainage layer,transition layer and filtration layer.The construction process is complex,the requirements are high,and the time is long.It is an important development direction of bioretention technology to research and develop bioretention with simple structure and improve construction convenience,but without reducing efficiency and increasing cost.The hydrological and water quality effects of each structural layer of conventional biological detention were analyzed.Three forms of structural optimization were proposed.The comprehensive performances of various biodetention are compared through field pilot tests.The better structures of bio-retention are screened out.According to the conventional practice of 50cm filter layer,10cm transition layer and 20cm drainage layer,three devices of“all three layers”,“without transition layer”and“without drainage layer”were designed to evaluate hydrological effects of various structural layers under the conditions of different rainfall intensities（0.5a,2a,10a,50a）,different rainfall durations（15min,30min,60min）,and the results showed that the transition layer and drainage layer played an important role in the runoff control.Compared with the“all three layers”,the runoff control rate of“without the transition layer”and“without the drainage layer”is reduced by 0-7.4%,0-10.1%,and contributing time is advanced by 6-8min,1.5-4.5min.Raw water(COD_cr200mg/l,NH₄⁺-N5mg/l,TN7mg/l,TP0.5mg/l)prepared according to the initial runoff of urban roads.The distribution of pollutant concentration in the vertical direction during the operation of the conventional structural bioretention was analyzed.It showed that the average removal rates of COD_cr,NH₄⁺-N,TN and TP are 86.0%,85.4%,71.8%and 68.0%respectively.The pollutants are effectively removed in the filter layer.The average removal amount of each pollution factor in the filter layer is accounted for 95.3%,93.0%,95.8%,and 79.4%of the total removal respectively.In order to investigate the role of the transition layer in the conventional structural bioretention,the particle size distribution of the fillers in each structural layer during the operation is tested and analyzed.The results show that during the operation,the particle size of the downward moving packing in the filter layer is mainly 0.16-0.63mm.After 40 days’operation,the filler in the particle size range of the transition layer and the drainage layer is increased by 5.0%and 1.3%;the transition layer has the function of preventing the loss of the filter material of the filter layer.Using the conventional size of the packing layer height of 80cm,according to the test results of the functions of each structural layer,three kinds of new structure bioretention are designed:the first type replace the transition with permeable geotextile and 10cm thick permeable concrete prefabricated block;the second type replace the transition layer with a permeable geotextile;the third type eliminate the transition layer and the drainage layer,and only the C-shaped permeable concrete prefabricated block wrapped by the permeable geotextile is arranged at the water outlet,and the perforated diversion tube is arranged in the raft.Using these three new types of structures and conventional structural bioretention（1m*1m*1m）,field pilot tests were carried out to compare the hydrological effects,water quality effects and plant growth of four structural bioretention.The results show that:under different rainfall intensities（0.5a,2a,10a）and different rainfall durations（15min,30min,60min）,“the third type（perforated geotextile wrapped C-shaped permeable concrete prefabricated block with perforated diversion tube）”has the highest runoff control rate,and the runoff control rate is 0.0-2.6%higher than“the conventional structure”.The contributing time of the three new structures is shorter than“the conventional structure”,and“the first type（permeable geotextile replaces the transition layer）”has the shortest contributing time;for 27-29min,it is shortened by 6.5-7min.“The third type”CODcr,NH₄⁺-N,TN,and TP have the highest average removal rates of 87.8%,87.5%,80.1%,and 75.3%respectively,which are 2.1%,1.9%,3.3%,and 9.3%higher than the conventional structure.Photinia frondosa,calamus and ryegrass commonly used in bioretention can survive and grow normally in each bioretention.Comparing the construction complexity of three new structures and conventional structural bioretention and the engineering cost under the current market conditions,the results show that“the third type”construction is the simplest,reducing the working time by 12 hours compared with the conventional structure.The conventional structural engineering cost is about 4500 RMB,"the first type"and"the third type"are reduced by about 1.8%and 1.7%respectively,and"the second type"increased by about 6.6%because of the high price of water permeable concrete.According to the results of the small tests and the field tests,“water-permeable geotextile replaces the transition layer”and“C-shaped water-permeable concrete and perforated diversion tube replace the transition layer and drainage layer”are two new,economically viable,and effective structural bioretention.