Principal Researchers: Steven McGee, Rajeshkumar Patel, Adrian C. Penisson, Robert D. Hill and Mark R. Wiesner
This pilot study was designed to evaluate the efficacy of membrane microfiltration in treating secondary wastewater effluent conditioned with various inorganic coagulants. A 8 liter per minute pilot (Memcore) was operated at the City of Houston's Southwest Wastewater Treatment Plant. Membrane permeates were of similar or better quality than filtrates from plant's conventional sand filters. Good removal of particulate contaminants, including coliform bacteria, was observed. In comparison with the plant's filtrate, permeate quality was also much less variable. Operating the pilot in a crossflow configuration did not improve the permeate quality and did not retard the increase in transmembrane pressure in comparison with dead-end operation. Chemical cleaning of the membrane with caustic solution was required approximately once every 5 days. As anticipated, results indicate that coagulation pretreatment in conjunction with membrane microfiltration can also be used to remove phosphorus from wastewater.
Only a few studies have evaluated membrane microfiltration of secondary wastewater effluent. Microfiltration membranes might be used to achieve very low turbidy effluents with very little variance in treated water quality. Because bacteria and many other microorganisms are also removed, such membrane disinfection might avoid the need for chlorine and subsequent dechlorination. Metal salts of iron or aluminum may also be added to enhance membrane performance. For example, iron or aluminum coagulants may be added to precipitate otherwise soluble species such as phosphorus and arsenic as well as improving the removal of viral particles. Coagulation of colloidal materials may also increase the effective size of particles applied to membranes and increase permeate flux by 1) reducing foulant penetration into membrane pores, 2) forming a more porous cake on the membrane surface, 3) decreasing the accumulation of materials on the membrane due to particle size effects of particle transport, and 4) improving the backflushing characteristics of the membrane. We report on pilot experiments in which secondary wastewater effluent was filtered using membrane microfiltration. Some preliminary results using a commercial polyaluminum coagulant (Kemira Water Treatment, Inc.)4 to pretreat the feed water are also reported.
Materials and Methods
A skid-mounted microfiltration membrane pilot5 with a nominal capacity of 8 liters/minute was used in this study. This pilot uses 4 membrane modules, each with 1 m2 of outside-in hollow fiber membrane area with an effective pore size of 0.2 µm. Feed water to the pilot was drawn from a secondary clarifier at the Houston Southwest Wastewater Treatment Plant (WWTP). Clarified water at the WWTP is subsequently chlorinated and filtered using conventional sand filters before being dechlorinated. The dechlorinated effluent is discharged into Brays Bayou, a river which runs through residential portions of the City of Houston and is bordered with parks, jogging paths and bike trails.
The membrane unit was operated in a constant permeate flux mode in which the transmembrane pressure (TMP) increases over time as materials deposit on and in the membrane. Permeate flux was set at 340 L/hr and readjusted as needed to maintain this flow. The unit was operated in both dead-end and crossflow filtration modes. During dead-end filtration, all water entering the membrane module exits as permeate. In the crossflow mode, a portion of the water entering the module flows across the membrane fibers and exits the module as concentrate. In both modes of operation, the pilot was automatically backflushed every 20 minutes, resulting in a recovery (permeate flow divided by feed flow) for the unit of 82 %. One run was also performed with backflushing once every 5 hours corresponding to a recovery of 98%.
Samples from the feed and permeate flows were analyzed for turbidity, total organic carbon (TOC), phosphorous concentration, and particle size distribution. TOC was measured using a Shimadzu TOC 500 total organic carbon analyzer. Phosphorus concentrations were measured colorimetrically. The particle size distributions of the feed and permeates were measured using an electronic particle counter (Coulter Multisizer) which operates on the electrical sensing zone principle. Samples were also analyzed for total and fecal coliforms. Similar analyses were performed on samples of influent and effluent water from the packed bed filters at the treatment plant during the same period of operation.
Membrane microfiltration produced a permeate of similar or better quality than that produced by conventional filtration. Good removal of particulate contaminants, including coliform bacteria, was observed. In this regard, the process appears to be as effective as chlorination for the removal of coliforms from secondary waste effluent. A key advantage is the ability to filter and disinfect in a single step without the need for subsequent dechlorination. Preliminary results indicate that coagulation pretreatment in conjunction with membrane microfiltration can be used to reduce phosphorus concentrations as well. There did not appear to be any advantage in running the microfiltration unit in a crossflow mode and there may even be some disadvantages. The permeate quality and evolution of pressure drop obtained from the membrane operated in the dead-end mode was similar or superior to that obtained under crossflow conditions.