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Ind. Eng. Chem. Res. 2007, 46, 6716-6723
Micropollutants Removal and Operating Strategies in Ultrafiltration Membrane Systems for Municipal Wastewater Treatment: Preliminary Results Paolo Battistoni,*,† Emanuela Cola,† Francesco Fatone,‡ David Bolzonella,‡ and Anna Laura Eusebi† Institute of Hydraulics and Transportation Infrastructures, Marche Polytechnic UniVersity, Via Brecce Bianche, 60131 Ancona, Italy, and Department of Science and Technology, UniVersity of Verona, Strada Le Grazie 15, Ca` Vignal 37134 Verona, Italy
Membrane systems are reported to enhance the removal of micropollutants (heavy metals and organic persistent compounds) from wastewaters. However, with regard to real municipal wastewater, where the micropollutants are present at very low concentrations, the debate on the real convenience of operating membrane systems is still ongoing. This paper presents the preliminary results from a pilot study where the removal of several micropollutants (80 compounds, grouped in the families of metals and metalloids, polynuclear aromatic hydrocarbons (PAH), volatile organic compounds (VOC), halogenated volatile organic compounds (HVOC)) from real municipal wastewater was studied using an ultrafiltration membrane system. With the purpose to optimize the removal performances, the prime objective was to determine the best plant configuration, tertiary filtration, or membrane bioreactor, as well as the best operating parameters, with particular concern to the activated sludge concentration. To expand the practical interest of the results for application in real plants, the sludge filterability also was studied according to different activated sludge concentrations and permeate fluxes. The objective was to estimate operating parameters able to enhance the removal of micropollutants and optimize the ultrafiltration process. After one year of experimentation, the results that were obtained gave important indications about the real role of the membrane system. The membrane not only demonstrated that it could be a simple barrier against the particulate pollutants, but it also demonstrated that it can enhance the removal of dissolved micropollutants, thanks to the “layer effect”. Introduction The necessity to produce treated wastewaters with highquality standards for discharge or reuse implies the adoption of very effective processes in the field of wastewater treatment. Among the best-available techniques (BAT), the membrane bioreactor (MBR) is supposed to be also the best choice for enhancing the biochemical process performances.1 Recently, many researchers have studied different peculiarities of MBRs and their applications,2 with reference to different topics such as the filterability of sludge,3 the overall feasibility of the widespread application of MBRs,4 or parameters linked to the critical flux.5 Numerous studies have showed that the MBRs have the capacity to efficiently remove conventional pollutants (carbon, nutrients, suspended solids, pathogens), as well as heavy metals6 and organic persistent compounds, to obtain reusable water and high quality standards.7-9 However, in regard to treating real municipal wastewater, the actual benefits of adopting membrane technology are still controversial.6 Besides the performance, the process configuration also should be considered (i.e., the choice between tertiary filtration (TF) or MBR processes), because these two schemes involve almost different capital and operation and maintenance (O&M) costs.10,11 In light of the scenario just outlined, this paper examines the treatment of real municipal wastewaters by ultrafiltration (UF) membranes, operating both as TF and as a filter submerged in the activated sludge, and it focuses on numerous micropol* To whom correspondence should be addressed. Tel.: +39 071 2204530. Fax: +39 071 2204528. E-mail:
[email protected]. † Institute of Hydraulics and Transportation Infrastructures, Marche Polytechnic University. ‡ Department of Science and Technology, University of Verona.
lutants. Furthermore, the filterability of the activated sludge was also investigated. This 2-fold approach could allow one to evaluate whether an operating strategy that optimizes the removal performances can be also industrially sustainable for the membrane system. Therefore, the final intention of this study was to draw useful considerations about the design and operation of the membrane plants for municipal wastewater treatment. Materials and Methods The Municipal Wastewater Treatment Plant (WWTP) and the Membrane Pilot Plant. The study was conducted using a large pilot plant hosted in a full-scale municipal wastewater treatment plant (WWTP) that had a treatment capacity of 85 000 population equivalent (PE). Its operation units were fairly conventional, as shown in Figure 1 and detailed in Table 1. The membrane pilot plant adopted a ZeeWeed (GEZenon) hollow fiber module; the main characteristics of this pilot plant are reported in Table 2, together with the main features of the filtration chamber. As shown in Figure 2, the pilot plant was equipped with several on-line probes and meters that measured properties such as the following: transmembrane pressure (TMP), permeated/backwashed flow rates, soluble chemical oxygen demand (COD), conductivity, turbidity in the permeate, temperature, and suspended solids in the filtration chamber. The pilot plant could operate at fixed permeate fluxes (J), set by a program line controller (Figure 2) and a frequency regulator for the process pump. The filtration cycle was composed of suction (300 s) and backwashing (30 s). At the end of each test, the original conditions of the membrane were re-established by submerging the module in hypochlorite
10.1021/ie070017r CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2007
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Figure 1. Full-scale wastewater treatment plant (WWTP) and location of the membrane pilot plant. Table 1. Main Features of a Full-Scale Wastewater Treatment Plant (WWTP) parameter
value
primary circular settler number volume biological process number of lines total volume, pre-denitrification total volume, nitrification air blowers total power secondary settler with radial flow number volume
2 1325 m3 2 3500 m3 2450 m3 4 240 kW 2 1608 m3
Sampling and Analysis Methodology. Two types of automatic samplers were used to collect composite samples of wastewaters over a period of 24 h: the first was conventional and the second was expressly designed for the experimentation (it was equipped with a ZeeWeed10 membrane module). The two samplers allowed one to obtain samples, averaged over 24 h, for the determination of the total concentration of pollutants and their distribution in the liquid and solid phases. The samples were collected from the main streams of both the full-scale WWTP and the pilot plant. In particular, daily averaged samples were taken from the influent sewage, the effluent from the primary treatment, the waste-activated sludge (WAS), the effluent from the full-scale secondary clarifier (which was the influent to the pilot plant), and the membrane permeate. The first screening for the characterization of the wastewater considered the determination of some 80 parameters, according to the United States Environmental Protection Agency (USEPA) methods.12 Among the compounds, heavy metals, volatile organic compounds (VOCs), halogen volatile organic compounds (HVOCs), and polynuclear aromatic hydrocarbons (PAHs) were detected. Parallel and Contemporary Tests. Two types of parallel and contemporary tests were performed. The pilot plant that was operating as a MBR-like reactor at 5 g MLSS/L was used in parallel to (i) the previously mentioned UF sampling system in TF modality, in the Type 1 test; and (ii) a laboratory-scale filtering system that was equipped with a cellulose membrane filter (with nominal pore size of 0.45 µm), in the Type 2 test.
Table 2. Main Characteristics of the Membrane Pilot Plant characteristic
value
membrane model module dimensions nominal pore size filtration type module configuration membrane type membrane area filtration chamber volume analysis tank volume permeate tank volume
ZeeWeed 500 1000 mm × 700 mm × 200 mm 0.04 µm out to in submerged hollow fiber 21.6 m2 1400 L 210 L 145 L
solution (200-600 mg Cl/L) for 4-12 h, always keeping the scouring aeration switched to “on” mode. The membrane pilot plant was located between the secondary clarifier and the disinfection contact tank (see Figure 1). This choice allowed one to (a) easily feed the membrane system with fresh effluents from the secondary clarifier, and (b) use NaClO from the disinfection section for the chemical cleanings of the UF membrane. Membrane Pilot System: Tertiary Filtration (TF) and the Membrane Bioreactor-Like (MBR-like) Configuration. The experimental tests were performed by feeding the filtration chamber with both secondary clarified effluent (to obtain the TF modality) and activated sludge that was thickened differently via the addition of secondary effluent, according to the experimental test (to obtain a form of MBR-like modality). In regard to the MBR-like configuration, the objective of the experimentation was to determine the pure effect of the membrane system, under different concentrations of mixed liqour suspended solids (MLSS), on the removal of micropollutants. Hence, at the beginning of each test, the filtration chamber was fed with activated sludge that was taken from the full-scale plant and the test lasted for 4 days. According to this approach, the characteristics of the activated sludge in the pilot plant and the full-scale WWTP were always similar.
Results and Discussions Micropollutants in the Main Stream of the Full-Scale WWTP. To compare the performances of the membrane system and the conventional WWTP, the micropollutant concentrations were determined in the main streams of both processes. Great variability of the influent concentrations of hazardous compounds was observed, as a consequence of nonidentifiable reasons. Therefore, Table 3 reports the ranges of concentration grouped by the family of compounds that may well show the basic roles of the different operation units. Generally, the occurrence and removal of metals was consistent with the data reported for other municipal WWTPs that treat municipal or municipal/industrial wastewaters.13-15 In regard to the PAHs, the influent wastewater showed contents that were lower than those of other cases;16,17 nevertheless, the concentrations were on the order of micrograms per liter (µg/L) and allowed us to suppose the presence of industrial discharges in the catchments area.18 The volatile compounds were almost organic solvents that had been probably discharged from local factories into the sewer system; their occurrence were very variable but always 5.0 halogen volatile organic compounds, HVOCs
percentage of compounds in the liquid phase >99
