Study On Influencing Factors And Mechanisms Of Low Pressure Membrane Fouling Controlled Via Adsorption

Water is the source of life,which plays an irreplaceable role in both daily life and industrial production.Water purification technology plays an important role in the recycling of water resources.Among them,low-pressure membrane technology is an efficient and green physical separation technology,which can effectively remove colloidal particles,suspended particles and other pollutants in water.Adsorbent has a large specific surface area,and it has a significant removal effect on organic pollutants and inorganic pollutants.Using adsorbent for wastewater pretreatment is an effective pretreatment method of low-pressure membrane technology,but there are still many unsolved problems.Adsorbents with different dosing modes have different effects on membrane fouling and foulant removal.However,there is little research on the influencing factors and control mechanism of adsorbent dosing modes on low-pressure membrane fouling control.How to select the appropriate adsorbent dosing mode under different conditions has become a subject to be further studied.This study is divided into two parts.Firstly,in the conventional constant flux operation mode,three kinds of powdered activated carbon（PAC）and three kinds of kaolin,a total of six adsorbents,were used for the preparation of algae water in the laboratory.One microfiltration membrane type and four ultrafiltration membrane types were used for experimental comparison to explore the key factors and control mechanism of adsorbent dosing mode on membrane fouling.In the second part,the gravity-driven constant pressure operation mode was adopted.PAC¹² and polyvinylidene fluoride microfiltration membrane with aperture of 200 k Da were used to test the effluent of sewage treatment plant.The effect of adsorbent dosing mode on foulant removal was compared,and the influence of biological action on pollutant removal effect under different adsorbent dosing modes was explored.Adsorption experiments were carried out for different experimental water,and the adsorption effects of six different adsorbents on extracellular organic matter and PAC on foulants from wastewater treatment plant were compared.The results show that the larger the specific surface area of adsorbent is,the stronger the adsorption capacity is,while the adsorption capacity of adsorbent for foulants is inversely proportional to the pore size,that is,the larger the pore size is,the weaker the adsorption capacity is;In terms of the size of pollutants,high hydrophobic small molecular substances are easier to be adsorbed by adsorbents,while large molecular pollutants are relatively difficult to be removed by adsorbents.In the conventional constant flux fouling experiment,the effect of adsorbent dosage on algae source membrane pollution was studied.The experimental raw water was algae water prepared in the laboratory,and the concentration of algae cells was 3×10⁸ cell/L.The results show that for the pre-mixed PAC experiment,with the increase of PAC dosage,the pollution rate presents a linear downward trend,while under the same PAC dosage,the pollution rate is PAC²⁵>PAC¹²>PAC³⁰.In the pre-deposited PAC experiment,the increase of PAC dosage promoted the fouling rate to increase gradually;However,with the increase of PAC particle size,the increase of fouling rate showed a gradual downward trend.It can be proved that the change of PAC particle size will produce different feedback to different adsorbent dosing methods.In the pre mixing experiment,the increase of PAC particle size can significantly increase the porosity of cake layer,thus reducing the pollution resistance;In the pre deposition experiment,when the PAC dosage exceeds 200 g/m²,the larger the PAC particle size is,the faster the PAC particle binding space-time gap increases,and the more significant the pollution rate decreases.In the conventional constant flux fouling experiment,whether using pre-mixed or pre-deposited dosing mode,kaolin makes the fouling rate show a downward trend,and the performance of pre-mixed dosing mode is better than that of pre-deposited dosing mode.The reasons are as follows:（1）the formation of new substances by chemical reaction makes the cake layer loose（2）compared with PAC（12μm、25μm and 30μm）,the particle size of kaolin was smaller（2.5μm、4μm and 7μm）.It is difficult for algae cells to enter into the pre-deposited layer,and there is a delay in the contact between them.However,the pre-mixed dosing mode can make the contact between the algae cells immediately and form a porous filter cake layer.In addition to the characteristics of the adsorbent,the pore size and flux of the membrane are also important factors affecting the control of algal membrane fouling.The experimental results show that reducing the membrane pore size and increasing the membrane flux can improve the control ability of pre-deposited PAC on membrane fouling,and in the pre-mixed PAC dosing mode,PAC also shows more and more positive effect on small pore size membrane.In the gravity-driven constant pressure fouling experiment,the effect of adsorbent on the treatment of wastewater by gravity driven membrane was studied.The experimental raw water is the effluent from the sewage treatment plant.The structure of cake layer was observed by Scanning Electron Microscope（SEM）;Excitation-Emission-Matrix（EEM）Spectra was used to analyze the change of foulants content in the inlet and outlet water;ATP assay was used to analyze the effect of microorganism on membrane fouling under different adsorbent dosing mode.It is found that different adsorbent dosing mode have different effect on membrane fouling.Pre-deposited PAC aggravated membrane fouling,while pre-mixed PAC and intermittent PAC had the opposite effect.SEM images showed that the structure of cake layer was different under different feeding modes.The cake layer was obviously stratified under pre-deposited dosing mode,while the cake layer in the other three groups of reactors was uniform structure.EEM images showed that different organic compounds were affected by PAC dosing modes.Humic acid or fulvic acid small molecule organic matter is mainly removed by PAC and microfiltration membrane adsorption,so its concentration in the reactor concentrate is obviously lower than that of protein,while the molecular weight of protein is larger,which is mainly removed by cake layer interception,so the concentration in the reactor concentrate is higher.By detecting the ATP content of the reactor concentrate and cake layer extract,it can be shown that the spatial distribution of PAC will affect the microorganisms in the reactor,and the metabolites of microorganisms will in turn affect the structure of PAC layer,thereby affecting the removal efficiency of PAC for foulants.To sum up,the change of adsorbent dosing mode has different effects on the development of low-pressure membrane fouling.For different pollutants in water,it is necessary to select the appropriate dosing mode to slow down membrane fouling and reduce the possible negative impact of adsorbent on low-pressure membrane process.