Experimental study on wastewater treatment by biological sand filter

Series of biological sand filter were constructed by river sand of different grain diameters.The obtained filters were applied for the treatment of domestic sewage,and the removal rules of main pollutants were investigated.The results showed that most of COD was removed in the in the middle and upper part of the filter,the removal rate of COD at 1.1 m depth was about 70%.The diameter of river sand had little influence on COD removal,while its influence on ammonia nitrogen removal.The removal rate of ammonia nitrogen in mixed river sand and coarse sand was only 26.91% and 21.28% on the first day,respectively.In the middle and late stage,the diameter of river sand plays an increasingly important role,the effluent concentration of ammonia nitrogen was 3.36 mg/L after 8 days in mixed river sand filter and 2.47 mg/L after 16 days in coarse sand filter,respectively.During the preliminary stage,the biological sand filter presented perfect performance on TP removal with the removal rate of more than 95%.However,the removal rate decreased continuously as the phosphorus adsorption of river sand reached saturation state gradually.Finally,the phosphorus accumulated at the bottom of filter and then discharged from the filter.The organic matters were mainly accumulated at upper of biological sand filter(0-0.2 m).In fine sand filter,he content of organic matters was lowest at depth of 0.2-0.5 m and increased slightly below 0.5 m.