THE EVALUATION OF CLOTH MEDIA FILTRATION AS PRETREATMENT TO ULTRAFILTRATION IN WASTEWATER REUSE APPLICATIONS

摘要

As requirements for wastewater reuse quality are becoming increasingly more difficult across theUnited States, conventional tertiary filtration is often unable to deliver the required performanceand reliability necessary to meet the new objectives. As a result, tertiary filtration is oftensupplemented or replaced with microfiltration (MF) or ultrafiltration (UF) membranes in thetechnology selection process. While some approaches couple the membranes with existingtertiary filtration systems, many applications directly apply clarified secondary effluent to themembranes. Unless the tertiary filters exist, a common perception holds that the requiredeffluent quality can be achieved with membranes at a lower cost by eliminating the selection ofintermediate tertiary filters. However, a strategy which employs tertiary filtration to preconditionthe water prior to membrane treatment offers distinct advantages, such as reducedoperating costs, improved flexibility and increased reliability. Under most circumstances, theoperation and maintenance (O&M) savings will offset the higher initial investment and yield alower life cycle cost. To help determine the extent of the O&M reduction, this paper willcompare operating parameters of a UF pilot system treating the same clarified secondarywastewater with and without the benefit of tertiary filtration.Under the first treatment scenario, domestic wastewater is screened and treated with aconventional activated sludge system. The clarified water is then directly applied to the UF pilotwithout pre-conditioning by the tertiary filter. This approach eliminates equipment and O&Mcosts associated with the filter. However, the lack of pre-filtration results in lower permeabilityand reduced recovery through the UF membranes. In addition, this arrangement potentiallyincreases the membrane area required to treat the same flow at the same pressure, which actuallyincreases the overall O&M costs.Under the second treatment scenario, the same clarified wastewater is sent through a full scalecloth media filtration (CMF) pilot system providing 5 m~2 of effective filtration area and featuringpolyester cloth media. The filter uses an outside-to-inside, vertically mounted configuration thatallows heavier solids to settle to the bottom of the filter tank, where they are periodicallyvacuumed to drain. As a filtration layer develops on the cloth, the tank level rises until a highsetpoint is reached, which initiates a backwash step. The high quality filtered water that passesthrough the cloth represents the influent to the subsequent UF treatment system.The UF system used in the evaluation of both scenarios consists of a single vertical 60 m2module provided by inge GmbH, a subsidiary of BASF. The module contains multibore fibersconstructed of polyethersulfone (PES) that filter the wastewater using an inside-to-outside flowpath that eliminates the need for cleaning with an air-scour system. Wastewater flows alternatelyinto the top and bottom of the fibers in order to achieve an even layer of solids on the membranesurface, which improves the effectiveness of the periodic backwash events. The UF systemintegrates a flow-paced coagulant feed system, which can be set to inject metal salts into the feedwater to optimize permeability. To maintain low trans-membrane pressures (TMP), thebackwash flow is routinely injected with one of three cleaning chemicals: caustic for removal oforganic foulants, hydrochloric acid to control inorganic fouling, or sodium hypochlorite fordisinfection.Each scenario consists of duplicate pilot runs to benchmark chemical dosage, flux, TMP,permeability and effluent quality to establish the optimum system O&M requirements. Thispaper will compare and contrast the operating data acquired during the pilot runs in order toassist in the life-cycle evaluation of each scenario.