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Treatment of municipal secondary effluent using polyolefin microfiltration membranes in vacuum-driven submerged membrane configuration to remove pathogens

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The objective of this study was to characterize the application of HYDRAsub™ hollow fiber polyolefin (polypropylene/polyethylene) microfiltration membranes in the submerged membrane configuration in the treatment of municipal secondary effluent. Filtration was effected by applying vacuum while the membranes are immersed in the feed to be filtered. The effect of operating parameters such as filtrate flux and cleaning method (aeration and filtrate backwashing) on membrane fouling rate was studied. The feed and filtrate quality was monitored over the course of the testing by measuring the turbidity and total suspended solids.

Testing was conducted on secondary effluent feed at the La Salina wastewater treatment plant in Oceanside, California. The tests were conducted with a polyolefin hollow fiber membrane bundle (20 mm diameter) submerged in a tank containing the feed water. Outside-in filtrate flow was effected by applying suction on the lumen side of the membranes. Operating parameters were varied and their effect on membrane fouling rate was monitored. The permeability of fouled membranes was restored by chemical cleaning.

In the bench-scale test, the polyolefin membranes proved to be quite effective in providing excellent filtrate quality and maintaining a low fouling rate. Filtrate turbidity values were less than 1 NTU. No fiber clogging or breakage was observed during the course of the testing. The membranes showed complete recovery of permeability after chemical cleaning.

A second phase in the same pilot test was to test membranes in the commercial HYDRAsub™ membrane module configuration on the same feed source. The impact of scaling-up the membrane module on system parameters was studied. Some of the parameters studied were operating flux, fouling rate, backwash frequency, chemically enhanced backwash procedure, and CIP cleaning procedure.

The HYDRAsub™ commercial module prototype was operated for more than three months. It showed a lower value for stable operating flux compared to the bench-scale module. Feed conditioning with ferric chloride and chlorine addition and daily chemical backwashing were essential for stable operation. The addition of chlorine resulted in chloramine formation which was helpful in assuring disinfection in the membrane system. Membrane cleaning interval was 60 days. The filtrate quality indicated excellent turbidity reduction with filtrate turbidity <0.1 NTU and TSS removal. It was shown to be suitable for operation of a downstream reverse osmosis system at low fouling rates.

The study provided a basic understanding of the operating characteristics of polyolefin hollow fiber membranes in the treatment of municipal secondary effluent. It provided a basis for designing full-scale systems and drafting operational guidelines for them.
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Document Type: Research Article

Publication date: 01 January 2005

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