Membrane Distillation Bioreactor (MDBR) – A lower Green-House-Gas (GHG) option for industrial wastewater reclamation
Introduction
Freshwater makes up 2.5% of the earth’s total water supply and of this, less than 1% is available for use by humans and the ecosystems (UNEP, 2008). With industrialization, twenty percent of the world’s freshwater had already been extracted for industrial use by 2010 and many countries are extracting groundwater faster than it can be replenished (WWAP, 2012). Industries require different grades of water from low grade applications such as cleaning to high grade water required for heating and cooling applications (Leverenz and Asano, 2011). Industrial wastewater reclamation typically includes an advanced water treatment process (eg. reverse osmosis (RO)) as a polishing step after the wastewater treatment process (eg. membrane bioreactor (MBR)) to recover high grade water (Fane et al., 2011). Life-cycle analyses have shown that in most cases, wastewater reclamation requires less energy than desalination of locally available resources (WEF, 2011). The reduction in energy consumption results in cost savings, which makes wastewater reclamation a more economically-feasible option for water production especially in cases where surface or ground water are not readily-available (Durham and Mierzelewski, 2003). In addition, increasing environmental awareness of the toxic and recalcitrant nature of industrial effluents has resulted in stringent trade discharge requirements in many countries. Advanced treatment technology is often required to remove emerging micropollutants from an effluent prior to discharge, leading to increased cost for effluent disposal and preference for wastewater reclamation (Tambosi et al., 2010).
Membrane processes have been used in industries to reclaim different grades of water for reuse (Leverenz and Asano, 2011). High quality water is often required for cooling and heating applications in industries (Table A.1 in Supplementary information). To achieve the water quality suitable for cooling and heating applications, some industries have already started to use a combination of MBR and RO processes for simultaneous wastewater biodegradation and ions removal (Durham and Mierzelewski, 2003). Like the MBR–RO process, the high-retention Membrane Distillation Bioreactor (MDBR) system is able to produce high quality product water with simultaneous biodegradation of organics (Phattaranawik et al., 2008). The MDBR combines a thermophilic bioprocess with the membrane distillation (MD) process, which works by transferring water vapor across a thermal gradient through a hydrophobic, microporous membrane to produce water. As such, only volatiles such as water vapor would be able to diffuse across the membrane. There is a growing interest in tapping of waste heat for MD applications in industrial wastewater reclamation (Drioli et al., 2012) and the MDBR may find application in this niche area.
By reviewing the MDBR studies conducted to date, this paper aims to evaluate the potential of the MDBR as a lower Green-House-Gas (GHG) emission option for industrial wastewater reclamation in terms of its GHG emission potential and wastewater reclamation performance (in terms of the quality and quantity of produced water, as well as efficiency of biotreatment process) and sludge production. In this paper, the MBR–RO has been used as a baseline for comparison.
Section snippets
Overview of MDBR
The MDBR (Phattaranawik et al., 2008) is a system which couples the thermophilic bioprocess with the MD process (Fig. 1). The basic principle behind the MD process is the creation of a vapor–liquid interface using a microporous hydrophobic membrane; water in the hot feed vaporizes on the membrane surface and diffuses down the vapor pressure gradient before it is condensed/removed (depending on the MD configuration) at the permeate/distillate side (Lawson and Lloyd, 1997). MDBR systems typically
Electrical requirement and GHG emission potential
Inevitably, all thermal processes are energy-intensive but the MD/MDBR uses less thermal energy than its conventional thermal counterparts. In addition, instead of requiring high-grade electrical energy like pressurized membrane systems, it can utilize low grade waste heat (typically <200 °C) which is usually lost to the environment. The challenge is capturing this waste heat in industry and using it effectively. On-going efforts to reduce heat loss by membrane conduction in the MD system has
Possible configurations for MDBR application
Without optimizing the existing MDBR system for biological nutrient removal, there are three possible configurations for the MDBR system (Fig. 4). Waste heat input and heat recovery processes would be required to reduce the electrical energy requirement for the system. In the first configuration, the MDBR system can be used in its current form to treat industrial wastewater with high carbon and low nitrogen content. Depending on the quality of the permeate, the permeate could be used to close
Conclusions
With the move toward zero liquid discharge, industrial interest in advanced wastewater treatment to reclaim high quality product water for reuse is increasing. Like the RO system, the MDBR is a high retention system and has the potential to produce high quality water. The GHG emission from the MDBR system is likely to be lower than the MBR–RO in situations where waste heat is available and a non-renewable energy (such as coal) is used to generate electricity. In terms of wastewater reclamation
Acknowledgement
The authors would like to acknowledge the Singapore Economic Development Board (EDB) for funding the Singapore Membrane Technology Centre (SMTC).
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4.12 - Wastewater as a Source of Energy, Nutrients, and Service Water