Two-stage nanofiltration for purification of membrane bioreactor treated municipal wastewater – Minimization of concentrate volume and simultaneous recovery of phosphorus
Introduction
Decreasing natural resources, such as fresh water and phosphorus, as well as the need to protect the environment, is increasingly leading to stricter legislation concerning the effluents discharged from wastewater treatment plants (WWTPs). Phosphorus is an important, but unfortunately depleting natural resource. The global production from phosphate rock is estimated to peak around 2033, after which the production will gradually decrease. Therefore, it is important to enhance the nutrient removal and recovery in municipal wastewater treatment in addition to the enhanced purification to enable water reuse. One very promising method for improving effluent quality as well as nutrient removal and recovery is membrane filtration, and especially nanofiltration (NF) and reverse osmosis (RO), which enable the production of highly purified water from municipal wastewater, but also enhanced phosphorus removal. Typically, these processes are carried out in spiral-wound modules that have a high packing density and good cost competitiveness. However, these modules are sensitive to plugging and have limitations concerning their achievable water recovery. Nevertheless, they have been applied in many processes as tertiary treatments, such as in water reclamation, e.g. in a membrane process branded as NEWater process by Singapore’s Public Utilities Board, where drinking water is produced from municipal wastewater. The challenge of using spiral-wound modules is that only 50–85% of the feed flow is purified as membrane permeate. The rest of the feed forms a concentrate, which is a voluminous waste stream containing elevated amounts of nutrients and various impurities, such as low molecular weight refractory organic compounds and inorganic salts. In addition, this concentrate often has limited biodegradability. Further treatment of this concentrate can also be demanding due to its very high volume and relatively diluted nature [1], [2], [3], [4], [5], [6], [7].
From an environmental and resource recovery point of view, an efficient membrane concentrate management is a major issue of membrane-based wastewater treatment [6], [8], [9], [10]. Membrane concentrate disposal strategies and regulations have been extensively studied [11], [12]. According to Van der Bruggen et al. [12], the traditional concentrate disposal options are incineration or direct or indirect discharge to surface water, ground water, or a landfill, whereas a more beneficial option is recovery of valuable substances from the concentrates before disposal. A relatively concentrated stream would be highly recommended for efficient recovery of valuable substances from concentrates. However, a low shear rate, thin spacers, and narrow feed channels increase the propensity of spiral-wound modules to clogging by suspended solids or precipitates formed during filtration. Therefore, high concentrations of valuable substances (e.g., nutrients) cannot be achieved using the spiral-wound modules that are traditionally used in tertiary municipal wastewater treatment.
The volume of membrane concentrates can be decreased by shear-enhanced membrane filtration, which simultaneously increases the concentration of valuable substances. Shear-enhanced membrane filtration creates a high turbulence at the membrane surface, which allows a decrease in the concentration polarization, an increase in the permeate flux, and a potential inhibition of membrane fouling [13], [14]. A high shear rate at the membrane surface can be generated with moving parts such as rotating discs, rotors, or rotating membranes, or by vibrating the membrane with, for instance, vibratory shear enhanced processing (VSEP) technology [13], [14], [15], [16].
VSEP technology has been utilized for several applications, such as landfill leachate treatment [17], [18] or reverse osmosis treatment of brackish water concentrate [16]. According to Subramani et al. [16], the water recovery in from brackish water with a high silica content could be increased from 75% (volume reduction factor, VRF, 4) to 94% (VRF 16) using VSEP technology for RO concentrate treatment. They also reported an inhibition of deposition of colloidal silica to the TMG10 Toray membrane by acid treatment of the RO concentrate (pH from 7.4 to 5.0) during VSEP filtration. However, the barium sulfate precipitation still presented a problem and the membrane required frequent cleaning [16].
Another high shear rate technology applied for the treatment of challenging effluents (e.g. from the pulp and paper industry) is cross-rotational (CR) filtration technology [19], [20], [21], [22]. The CR nanofiltration process was chosen for use in the present study. A rotating blade in the CR filter provides an extremely high shear rate inside the module; therefore, high VRFs can also be achieved even without the recirculation of concentrate back to the feed required with spiral-wound modules. This means that the concentrate flow rate can be extremely low when the filter is operated at high VRFs.
The high turbulence created by rotors was assumed to prevent the formation of precipitates on the membrane structure and/or to facilitate the removal of possible precipitates from the module by centrifugal forces. In addition, a low flow rate of concentrate would favor a precipitation process outside the module. These advantages were expected to enable extremely high water recovery without significant membrane fouling and module plugging. The feasibility of a novel process concept for further purification of the real MBR effluent by applying a two-stage NF process combined with a settling tank to enable precipitation of phosphorus was evaluated in the present study from the viewpoint of permeate flux and purity, residual concentrate amount, and phosphorus recovery. The membrane concentrate after phosphorus recovery was further purified in our previous study [23] by pulsed corona discharge (PCD) oxidation. The aim of that study was to decrease the amount of micropollutants and to increase biodegradability of the concentrate. In the present study we present an option to minimize the wastewater volume in membrane based tertiary treatment with a simultaneous recovery of phosphorus.
Section snippets
Effluents and filtration experiments
The MBR permeate from municipal wastewater treatment (Table 1) was used as feed effluent for the tertiary nanofiltration. The MBR permeate originated from an MBR-activated sludge process operated without chemical phosphorus removal, thereby enabling potential phosphorus recovery from the final NF concentrate (Fig. 1). The MBR pilot provided by Alfa Laval had a polyvinylinedifluoride (PVDF) microfiltration membrane (MFP2) with a pore size of 0.2 µm, membrane area of 8.5 m2, and average
The first stage nanofiltration with a spiral-wound module
The NF270 membrane was chosen as it is a high permeability nanofiltration membrane that has shown a low fouling tendency in wastewater applications. The low fouling tendency often observed for the NF270 membrane is mainly attributed to its low surface roughness and hydrophilic surface (contact angle around 30°) [24], [27], [28], [29].
An average permeability of 4.3 L/(m2 hbar) (flux 22 L/(m2 h), TMP 5.1 bar, 14 °C) was obtained for the first stage nanofiltration with the 2.5″ spiral-wound module
Conclusions
Municipal wastewater treated with an MBR process without addition of phosphorus precipitation chemicals was further purified in a novel two-stage NF process using spiral-wound NF and high shear-rate CR-nanofiltration to minimize the amount of final concentrate and to recover phosphorus. The CR -module construction prevents precipitation of phosphorus compounds inside the module due to extremely high shear rate but favors precipitation of phosphorus outside the module in the settling tank, where
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the registered association Maa- ja vesitekniikan tuki ry for financial support. Ramboll Analytics Oy is also appreciated for conducting the micropollutant analysis and the accredited environmental laboratory Saimaan Vesi- ja Ympäristötutkimus Oy for conducting the BOD7 analyses. Doctor of Science Liisa Puro and laboratory technician Toni Väkiparta are appreciated for conducting the ion and EDS analyses.
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