Effects Of Hydrodynamic Conditions On The Stability Of Biofilm In A Bioreactor Packed With Semi-suspended Bio-carriers

A new typed of self-designed semi-suspended bio-carrier was adopted to construct a biofilm reactor called the packed semi-suspended bio-carriers bioreactor for the treatment of simulated domestic sewage.In the past works,it only studied how to construct biofilm reactors rather than study on the stability of biofilms.This research mainly explored the characterization of biofilm stability and its influencing factors under the condition that the water quality of the reactor permeate was stabilized providing a basis for improving the operation efficiency and treatment effect of the biofilm reactor.Two experiments were carried out in this research,the first one was a trial run,with running time from October 8,2016 to November 17,2016 for 42 days.The main purpose of this experiment was to investigate the proposal operation conditions that bio-reactor can be of great capacity to treat watewater.The effect of the reactor operation and the removal mechanism were monitored by measuring the regular water-quality indices(CODcr,total nitrogen,and total phosphorus)of the effulents every day.During the trail run,the waterquality indices were not measured for seven days.The second experiment was a formal operation,it began on November 27,2016 and it ran for 50 days.In this experiment,atomic force microscope(AFM)was used to determind the adhesion force performance and microscopic morphology of the biofilm during the entire operation under the condition that improved according to the first test.At the same time,calculation formulas of hydraulic parameters were used to caculate the shear stress force of water flow and its effect on biofilm stability.The main findings are as follows:In both experiments,the active sludge from the secondary sedimentation tank of the wastewater treatment plant was aerated for 48 hours and then was put into the reactor rapidly for biofilm formation.Intermittent water intake was used,the hydraulic retention time was 24 hours and the temperature inside the reactor was maintained in the range of 15~20°C.The reactor was designed to have two zones with different aeration rates with bottom connecting.During the operation of the reactor,two dissolved oxygen instruments were used to automatically monitor the dissolved oxygen values of two reactor zones for 24 hours and connected to a computer to automatically record the DO value.On the first trial run experiment,during the stabilized operation,the CODcr removal rate in the I and II zones maintained above 90% and even reached to 100% and the effluent concentration was around 10 to 20 mg/L.The removal rate of ammonia nitrogen in zone I reached above 90%,and the effluent concentration remained below 1.0 mg/L;although on the last stage where the biofilm began to slough the ammonia nitrogen concentration of effluent in zone II reached to 17.2 mg/L and remained below 20 mg/L.However,the removal rate of total phosphorus in the two regions averaged about 65%.The first experiment confirmed that under the operating conditions,as the main pollutants,CODcr and ammonia nitrogen could be degraded and removed,and the reactor permeate could achieve superior quality.In the second experiment,atomic force microscope(AFM)was used to determine the change of the biofilm adhesive force during the entire reactor operating cycle and calculate the Young's modulus that characterized its mechanical properties.At the same time,the microscopic surface roughness of the biofilm was measured by AFM.Samples were taken at the same time at the same spot to measure the content of biofilm extracellular polymers(EPS)(including polysaccharides and proteins).Results showed that these parameters all exhibited the same changing rule with different biofilm growth cycle characteristics,so the parameters could be used to characterize the biofilm stability.And further,the Reynolds number and flow shear force in the reactor were calculated.The results showed that the stability of biofilms in the area with large shearing forces was not excellent;EPS had a active correlation with biofilm adhesion performance,the more the EPS content,the stronger the adhesion force,the content of polysaccharide in biofilm had a significant influence on the stability of biofilm.