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The Importance of Biological Treatment and Secondary Effluent Quality on Tertiary Membrane Filtration Design

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For successful design and operation of a low pressure membrane filtration (LPMF) system that can reliably meet treatment objectives, whether it be either non-potable reclaimed water production and quality requirements or feedwater to downstream reverse osmosis, the fouling characteristics of the LPMF feed water must be well understood. This understanding is derived from two primary factors: (1) the systems and processes utilized to convert the wastewater into secondary effluent and (2) the type of membrane system used to treat the effluent. In focusing on the factor (1), this paper will present and compare the performance of a specific type of pressurized LPMF technology when operated on effluent produced by different full-scale wastewater treatment plants utilizing a range of secondary treatment processes and will describe the interactions between secondary effluent treatment method, effluent quality, foulant characteristics and LPMF system performance. The findings are also important to elucidate factors that are important in translating both bench and pilot testing results into full-scale plant design in order to achieve reliable performance.

The evaluation includes review of design and operational data as well as effluent quality from five different wastewater treatment plants (WWTPs), representing a range in secondary treatment methods and including one facility where chemical clarification was employed to condition the effluent prior to LPMF. Bench, pilot and full scale unit LPMF operational data were correlated with the WWTP data, to develop key relationships between effluent treatment, quality and LPMF performance. Pilot plant protocols were structured to evaluate such impacts in order to determine optimized operation conditions.

The outcome of this comprehensive evaluation was to show that WWTP operational parameters, in particular solids retention time (SRT), correlated well with the level of colloidal (filterable) TOC measured in the LPMF feed. Shorter SRT and higher levels of colloidal TOC resulted in lower sustainable LPMF flux rates and greater frequency of chemically-enhanced backwash and clean-in-place (CIP). The use of chemical coagulation and coagulation and sedimentation were found to be beneficial in reducing the fouling properties of the colloidal TOC and in providing increased flux through removal of dissolved and colloidal TOC. Analytical results showed that TOC characteristics varied based on the secondary treatment system employed and in most instances approximately half of the total TOC and COD were readily available soluble fraction, and remaining half could be removed with chemical addition and filtration. Turbidity and total suspended solids (TSS) levels were determined to have less impact on membrane fouling and sustainable production than TOC. For WWTPs treating colder wastewater, temperature was also determined to impact sustainable flux and or CIP frequency. Chemically enhanced backwash and CIP regimes and their ability to manage fouling were also evaluated based on the membrane feed water quality. Where the TOC levels were high, due to the passage of less readily biodegradable organic matter through the treatment system, membrane fouling and associated TMP increases were rapid. The high fouling rate necessitated the use of reduced membrane flux and implementation of one or more chemically enhanced backwashes in order to achieve sustainable performance at each facility. SRT directly correlates with the amount of EPS, exocellular proteins and polysaccharides. It was shown that certain percent of SMPs and EPS are retained and lead to fouling. The correlation between the TOC and the membrane fouling rate was consistent with the outlined fouling mechanisms. The greater amount of colloidal TOC present in the secondary effluent resulted in the need to design the LPMF system at a significantly lower flux rate than is employed in drinking water applications having similar (naturally occurring) TOC levels, reflecting the greater fouling potential of the effluent organic matter. The manuscript will present considerations necessary for reliable LMPF design in addition to the feed quality control parameters based on actual findings from the evaluated facilities.
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Keywords: Membrane filtration; organic fouling; secondary effluent treatment; tertiary filtration

Document Type: Research Article

Publication date: 01 January 2010

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