Experimental Study On Inhibiting Membrane Fouling Of Alginate Ultrafiltration By Adding Metal Ions

Alginate recovery from aerobic granular sludge?AGS?has become an emerging research direction for wastewater treatment and resource utilization.Alginate is a special extracellular polymer substances?EPS?with a dry sludge content of up to 25%.Following the recycling of alginate,the amount of sludge is reduced and its dewaterability is enhanced.However,the solutions of alginates recycled from AGS afford the moisture contents of?100%and therefore the dehydration of these solutions is an urgent problem.The conventional dewatering method consumes many chemical agents and causes secondary pollution;alternately,ultrafiltration?UF?,generally used for polymer concentration,can avoid these defects.Membrane fouling is the bottleneck of ultrafiltration concentration applications.Based on the previous results that multivalent metal ions can mitigate membrane fouling,this study further explored the roles and mechanisms of adding metal ions to mitigate membrane fouling,and the properties of recovered alginate were analyzed.For four typical metal ions such as Ca²⁺,Mg²⁺,Fe³⁺,and Al³⁺individually and in combination,the interaction mechanism between metal ions and sodium alginate?SA?and membrane fouling mitigation behaviors were discussed in detail.Experiments showed that after adding various concentrations of Ca²⁺,the filtration resistance decreases in the order of 1.75 M?2.8mL?>1 M?5 mL?>0.5M?10 mL?.Ca²⁺with high concentration were wrapped in the gel capsules and were not effectively used,resulting in larger filtration resistance.For different flow rates,the filtration resistance decreased sequentially,arranged in the order of 5 mL/min>15 mL/min>25 mL/min.For high flow rate,more content of Ca²⁺was pumped,and more calcium alginate was formed at the initial stage,therefore filtration resistance decreased.The difference between the dosing order of SA and single metal ion resulted in different filtration behaviors,which is caused by the formation of substances with different structures.For the combined metal ions,the SA action is more complete,resuting in the mitigation of membrane fouling arragned in the order of Ca²⁺?Fe³⁺(Ca²⁺first and then Fe³⁺)>Fe³⁺?Ca²⁺(Fe³⁺first and then Ca²⁺)>Fe³⁺+Ca²⁺(Fe³⁺and Ca²⁺are mixed).The lower the molecular weight and concentration of SA,the lower the filtration resistance.The properties of the recovered alginate materials were studied via optical microscope observation,dynamic light scattering,X-ray photoelectron spectroscopy,and scanning electron microscope.The addition of metal ions can not only mitigate membrane fouling,but also make the recovered products with high added value.Under the action of multivalent metal ions,it can promote the dehydration of alginate and reduce the water content of the filter cake.The microstructure of the formed alginate varied greatly,which changes the chain structure of SA,and formed larger floc.The particle size of alginate decreased in the order of Al-SA>Fe-SA>Ca-SA>Mg-SA.The filter cake formed by adding metal ions has more pores,the order of porosity is Ca+Fe-SA>Fe-SA>Ca-SA>SA,and stripping occurs near the surface of membrane.This study provides a new idea for the recovery of alginate by ultrafiltration in aerobic granular sludge.Especially,the introduction of multivalent metal ions into the alginate recovery process is expected to provide a novel method for the development of new alginate nanomaterials.