Characteristics And Control Of Membrane Fouling And Wetting In High Salinity Wastewater Treatment By Membrane Distillation

Under the context of the global shortage of freshwater resources,desalination technology has experienced a rapid development,which induces discharging a large volume of high salinity wastewater and brings serious environmental pollution problems to the surrounding environment.Membrane distillation technology has great advantages in small-scale high salinity wastewater treatment since it can not only break through the salinity limitation and using the waste heat but also has a compact structure and can be easily modularized.However,membrane fouling and membrane wetting problems significantly limit its application through increasing operation energy consumption and reducing system stability.In this paper,the membrane fouling and wetting characteristics during high salinity wastewater treatment were studied and the membrane fouling and wetting control techniques applicable to high salinity wastewater treatment were explored.The paper first discussed the use of reverse osmosis concentrate as typical high salinity wastewater.By comparing the effects of different membrane materials and system operating conditions on the membrane distillation system,specific flux reduction,increasing of feed pressure and permeate conductivity were adopted to evaluate the membrane fouling and wetting characteristics of membrane distillation in the treatment of high salinity wastewater.The results showed that membrane distillation was able to recover water resources for drinking from high salinity wastewater(46.5 g/L)before membrane wetting.As the wastewater concentrates over a certain concentration ratio,the slightly soluble calcium sulfate crystallized on the surface of the membrane,causing a sharp drop in membrane flux.Membrane scaling reduced the hydrophobicity of the membrane,which resulted in membrane wetting at a running pressure that was much lower than the liquid entry pressure of the membrane.Membrane with different characteristics experienced the membranes scaling at the same salinity,and membrane scaling was mainly affected by the concentration of the solution.By reducing the inlet water temperature from 60 °C to 40 °C,the membrane fouling could be effectively retarded,and the salinity of the membrane wetting was increased from 153 g/L to 186 g/L.Increasing the influent flow rate could delay membrane fouling to a certain extent,but it led to an increase in operating pressure and resulted in a risk increase of local wetting of the membrane.Although the membrane fouling layer was reversible and the flux recovery rate was greater than 88% by pure water flushing,the salt rejection rate of the contaminated regeneration membrane decreased.To fundamentally solve the problem of membrane fouling,optimizing the system operation condition is not enough.To clarify the influence of organic matter on the membrane distillation system,the combination of model organic matter simulation water and real high organic matter wastewater were studied.The results showed that bovine serum albumin and sodium alginate caused crystal morphological aberrations and impeded membrane crystallization,but sodium alginate reacted with calcium ions to form large crystals,which led to accelerated pollution in the later stage.Humic acid could induce crystallization after the concentration of the solution exceeded a certain limit,resulting in homogeneous nucleation of the solution,thereby slowing the contamination of the membrane surface.All of the three organic substances in the solution showed a certain degree of scale inhibition,delaying the decrease of the flux of the membrane.The organic matter on the surface of the membrane could combine with calcium ions to delay the process of membrane crystallization,but the humic acid and sodium alginate deposited membrane increased the interaction between calcium sulfate and membrane,which accelerated the membrane fouling process.During the real wastewater treatment,membrane distillation achieved excellent retention efficiency,DOC removal rate was higher than 99%,phosphorus was not detected in the effluent,ammonia nitrogen removal rate exceeded 98% when acidified to p H=5,and effluent meets industry cooling water standard.Although a high concentration of organic matter deposition caused a decrease in the contact angle of the membrane,no membrane wetting occurred during the operation.When organic and inorganic pollution coexisted in the system,the inorganic scale may enhance organic pollution through adsorption,and acidification could effectively control membrane crystallization pollution caused by inorganic salts such as Ca,Mg and Si,thereby reducing membrane organic pollution.Based on the membrane fouling and wetting characteristics of high salinity wastewater treatment by membrane distillation,this paper further carried out inducing crystallization to control membrane fouling and wetting.The results showed that the heating induced crystallization can promote the homogeneous nucleation crystallization of the solution,thus delaying the contamination of the membrane surface,and the recovery rate was increased from 26% to 45% with an elevated crystallization temperature of 10 °C.Adding calcium sulfate seed crystals to the crystallization tank could accelerate 37% production rate of crystals by 5 g/L seeding dose.The crystallization rate increased with the increase of the seeding dosage,but high seeding dosage induced deposit of seed on the membrane surface.Deposition induced crystallization,which deteriorated the control effect of membrane fouling.Membrane fouling and membrane wetting effect were best controlled by adding 1 g/L seed crystal.The induced crystallization was further applied to the reverse osmosis concentrate treatment.Under the seeding condition,the critical salinity for maintaining 80% membrane flux was increased from 159 g/L to 189 g/L,and the salinity was further increased to 200 g/L by increasing the crystallization temperature by 10 °C.This paper started with the experiment of study membrane fouling and wetting characteristics during the high salinity wastewater treatment and found that membrane scaling was the main factor leading to membrane fouling and wetting.The effects of three typical organics on membrane crystallization were investigated using simulated water and the effects of actual wastewater were explored as well.Based on the membrane fouling and wetting characteristics,the methods of heating and seeding were proposed to induce crystallization outside membrane and control membrane fouling and membrane wetting during the high salinity wastewater treatment by membrane distillation.This work provides guidance for practical high salinity wastewater treatment.