Improving Membrane Treatment Energy Efficiency Through Monitoring the Removal of Colloidal Particle Foulants

There is an increased emphasis on indirect and direct potable reuse of water in recent years to meet future water demand and increase water security. Low-pressure membranes, consisting of microfiltration and ultrafiltration, are integral components of most indirect and direct potable reuse processes and provide reliable water quality for reuse applications. However, fouling of these membranes increases the need for membrane cleaning and replacements as well as energy. Studies attribute fouling to deposits of feed water colloidal particles smaller than 200 nanomicrons inside the membrane pores.

No online techniques are available to directly measure colloidal particles in feed water, a process that would aid in appropriate pretreatment and real-time control to remove colloidal particles and prevent deposition in the membrane pores. In this study, the research team implemented a real-time monitoring technology to track colloidal particle concentration and size distribution in feed water to assist targeted removal by coagulation. This new strategy used nanoparticles tracking analysis (NanoSight NS500 by Malvern Instruments) and was tested at Orange County Water District and West Basin Municipal Water District. Although the team conducted similar demonstration tests at both sites, there were differences in type of coagulants tested, membrane type, feed water quality, and the absence of reverse osmosis testing at West Basin.

Results showed that targeted polyaluminum chloride coagulation mitigated fouling potential for the microfiltration membranes tested at Orange County, with transmembrane pressure values reduced by 60 percent after six weeks compared to not adding coagulant. For reverse osmosis operations, researchers observed no negative impact related to coagulation of the microfiltration feed water but do recommend longer testing. Energy and economic evaluation at the Orange County Advanced Water Purification Facility indicated that the proposed approach can reduce energy consumption from microfiltration by 28-35 percent with potential savings of 2,940 megawatt-hours (MWh) per year and $610,000 per year in membrane cleaning, replacement, and operating costs for a 100-million gallon per day (GPD) plant.

Ultrafiltration membrane testing at the West Basin demonstration site indicated that targeted polyaluminum chloride coagulation can improve membrane filtration performance and energy efficiency compared to no coagulant. However, coagulant choice is important to the success of the targeted pretreatment strategy. Targeted coagulation with polyaluminum chloride slowed the rate of transmembrane pressure increase by 43 percent compared to the control and reduced energy consumption by 63 percent. Based on West Basin’s 14.4-million GPD ultrafiltration facility, these results could provide savings of more than 600 MWhs/year and $110,000 per year in overall operating and maintenance costs.