Development of chitosan/ alginate/ AgNP membrane for decentralized greywater treatment

Rapid development and worldwide industrialization have caused severe impact to the fresh water bodies. Cases of water scarcity and pollutions reported worldwide drew attention of researchers to reduce water stress and seek for alternative water sources. Thus, greywater recycling is gaining impetus as the solution to curb this issue. However, greywater treatment is essential to remove pollutants and pathogens prior reuse as these will pose health risks to those in contact with greywater. In this study, membrane technology that is simple and compact was investigated as a process for application in a decentralized greywater recycling system to encourage wider implantation in Malaysia. A membrane comprising of anti-microbial biopolymers (chitosan and alginate) and a heavy metal biocide (AgNP) was developed and tested to conduct greywater treatment and disinfection in a single membrane filtration unit. The formation of polyelectrolyte complex (PEC) between the two biopolymers reduced the membrane molecular weight cut-off (MWCO) of a 242 kDa chitosan membrane to 3800 Da in the 2A1CP polyelectrolyte bilayer membrane (PCBM). However, the dense layer of alginate formed on the chitosan membrane and decline of MWCO caused the ultrapure water flux of the dense PCBM to be severely reduced. Therefore, 2A1CP was further modified with porogen on the alginate layer to improve the water flux. The modification caused the water flux of porous 2AP1CP PCBM to increase by 60% as compared to the dense 2A1CP PCBM. The 2AP1CP PCBM was capable of removing 99.8% turbidity, 99.5% total suspended solid (TSS), 81.5% chemical oxygen demand (COD), 96.9% 5-days biological oxygen demand (BOD₅), 2.6-log of Escherichia coli (E. coli) and 2.93–log of other coliforms in greywater. In addition, the disinfection efficiency of 2AP1CP membrane was enhanced when increasing concentration of AgNP was incorporated into the membrane. The membrane can remove up to 3.6-log Escherichia coli (E. coli) and 3.7-log other coliforms with 1.5 ppm AgNP loaded into the alginate layer. The increase in the bacteria removal efficiency was found to be attributed to the additional contact-killing mechanism of AgNP in the membrane structure. The 1.5 ppm AgNP PCBM was subsequently installed in a decentralized greywater treatment system to evaluate its long-term treatment performance in a greywater treatment system. The fresh 1.5 AgNP PCBM could produce 1125 L m-²day^-1 at 3 bar (g) and 446 L m^-2day^-1 without pumping. The membrane flux reduced over two weeks of operating the system. The analysis on flux profiles showed that flux decline was mainly due to intermediate pore blocking and cake formation mechanisms. However, flux decline could be resolved with proper maintenance and membrane cleaning. In conclusion, this research contributes to the development of a membrane specialized for greywater treatment and the decentralized greywater treatment system could help to conserve freshwater, especially in arid countries. Furthermore, development of a biopolymeric membrane helps to reduce secondary waste generation from the disposal of used membranes from greywater treatment. In the future, the application of such system could be extended to treat other sources of wastewater to further increase the recycling capacity and reduce freshwater consumption.