Anaerobic membrane bio-reactors for severe industrial effluents and urban
spill waters; the AMBROSIUS project
Jules B. van Lier1, Hale Ozgun1,2, Mustafa Evren Ersahin1,2, Recep Kaan Dereli1,2
1
Delft University of Technology, Faculty of Civil Engineering and Geosciences, Department of Watermanagement, Sanitary Engineering Section, Stevinweg 1, 2628 CN Delft, the Netherlands
2
Istanbul Technical University, Faculty of Civil Engineering, Department of Environmental Engineering, Maslak, 34469 Istanbul, Turkey
Introduction
With growing application experiences from aerobic membrane bioreactors, combination of membrane and anaerobic processes become more and more attractive and feasible. In anaerobic membrane bioreactors (AnMBRs), biomass and particulate organic matter are physically retained inside the reactor, providing optimal conditions for organic matter degradation. AnMBRs offer high quality effluents free of solids and complete retention of biomass, regardless its settling and/or granulation properties. Therefore, this technology may present an attractive option for treating industrial wastewaters and/or slurries at extreme conditions, such as high salinity, high temperature, high concentrations of suspended solids (SS), high lipid concentrations and presence of toxicity, that hamper granulation and biomass retention or reduce the biological activity (van Lier et al., 2001). Owing to the high influent flows and low organic matter content, municipal wastewater is treated at ambient temperatures in both aerobic and anaerobic applications worldwide, e.g. (van Lier et al., 2010), especially in developing countries with (semi) tropical climates such as Brazil, India and Colombia. Under low temperature (<20 ºC) conditions, hydrolysis of particulate matter into dissolved molecules becomes the rate-limiting step, which results in the accumulation of suspended solids (SS) in the reactor and a decrease in organic matter conversion efficiency together with a decrease in methanogenic activity (van Lier et al., 1997; Lettinga et al., 2011). AnMBRs provide a possibility for the agricultural use of the treated effluent for non-potable purposes in many regions suffering from water shortage (Martinez-Sosa et al., 2011). Agricultural use of treated effluents generally demands extensive pathogen removal along with the availability of macronutrients. Since macronutrients such as ammonium and orthophosphates are not removed by anaerobic bioprocesses and pathogens can be retained by the membrane unit (Saddoud et al., 2006), permeates of AnMBRs are certainly of interest for agricultural use.
Despite the mentioned advantages, there are still critical obstacles such as low flux, membrane fouling, high capital and operational costs that limit the extensive use of AnMBRs. Among these disadvantages, membrane fouling is the main bottlenecks limiting their application. Membrane fouling is the reduction in flux due to the accumulation of organic and inorganic particles in/on the membrane pores or on the surface. Membrane fouling is a multivariable process that is affected by the influent characteristics, reactor operation, membrane features and biomass properties. Cake layer formation was identified as the most important fouling mechanism in AnMBRs (Choo and Lee, 1998; Jeison and van Lier, 2007). Although membrane fouling is inevitable at least on the long term, efforts focus on how to reduce its build up with time by efficient operation and control measures (Jeison and van Lier, 2006).
The major constraints of MBR processes are related to membrane costs, energy demand, fouling control, and low flux. Dynamic membrane (DM) technology may be a promising approach to resolve problems encountered in MBR processes (Fan and Huang, 2002; Ye et al., 2006). A DM, which is also called secondary membrane, is formed on an underlying support material, e.g. a membrane, mesh, or a filter cloth, when the filtered solution contains suspended solid particles such as microbial cells and flocs. Organics and colloidal particles which normally result in fouling of the membrane will be entrapped in the biomass filtration layer, preventing fouling of the support material (Kiso et al., 2005; Jeison and van Lier, 2007).
This study summarizes the recent findings of the ongoing research about AnMBRs in Delft University of Technology, Sanitary Engineering laboratories.
Impact of High Lipid Containing Wastewaters on Biological and Filtration Performance
The potential of anaerobic membrane bioreactors (AnMBRs) for the treatment of lipid rich corn-to-ethanol thin stillage was investigated at three different sludge retention times (SRT), i.e. 20, 30 and 50 days (Dereli et al.,2013). The membrane assisted biomass retention in AnMBRs provided an excellent solution to sludge washout problems reported for the treatment of lipid rich wastewaters by granular sludge bed reactors. The AnMBRs achieved high COD removal efficiencies up to 99% and reliable effluent quality. Although higher organic loading rates (OLRs) up to 8.0 kg COD/m3.d could be applied to the reactors operated at shorter SRTs, better biological degradation efficiencies, i.e. up to 83%, was achieved at increased SRTs. Severe long chain fatty acid (LCFA) inhibition was observed at 50 days SRT, possibly caused by the extensive dissolution of LCFA in the reactor broth, inhibiting the methanogenic biomass. Physicochemical mechanisms such as precipitation with divalent cations and adsorption on the sludge played an important role in the occurrence of LCFA removal, conversion, and inhibition. Good membrane fluxes between 10 to 14 L/m2.h was obtained at 0.5 m/s cross-flow velocity and the long term fouling was controlled by using frequent filtration and backwash cycles. Cake layer formation was found as the most important contributor to membrane filtration resistance whereas a low degree of organic fouling was detected. Moreover, inorganic fouling in the form of phosphate precipitates was observed on the permeate side of the membrane. The sludge relative hydrophobicity, which was altered by LCFA adsorption on biomass, affected the fouling propensity of sludge.
Dynamic Membrane Technology in Anaerobic Membrane Bioreactor Systems
One of the most important potential benefits of DM technology is that the membrane itself may be no longer necessary, since solids rejection is accomplished by the secondary membrane layer which can form and re-form as a self-forming DM in situ. Repeated processes of DM formation and removal may reduce membrane permeability losses as encountered in conventional MBRs (Ersahin et al., 2012). Applicability of DM technology in AnMBRs for the treatment of high strength wastewaters was investigated (Ersahin et al., 2013). A monofilament woven fabric was used as support material for the DM formation. An anaerobic dynamic membrane bioreactor (AnDMBR) was operated under a variety of operational conditions, including different SRTs of 20 and 40 days in order to determine the effect of SRT on both biological performance and DM filtration characteristics. High COD removal efficiencies exceeding 99% were achieved during the operation at both SRTs. Higher filtration resistances were measured during the operation at SRT of 40 days in comparison to SRT of 20 days, applying a stable flux of 2.6 L/m2.h. The higher filtration resistances coincided with lower extracellular polymeric substances (EPS) concentration in the bulk sludge at SRT of 40 days, likely resulting in a decreased particle flocculation. Results showed that DM technology achieved a stable and high quality permeate and AnDMBRs can be used as a reliable and satisfactory technology for treatment of high strength wastewaters. As an alternative to microfiltration or ultrafiltration membranes, polypropylene monofilament filter cloth can be used to form a DM (cake) layer and to provide high quality filtration by this self-forming layer.
Coupling Membranes with Upflow Anaerobic Sludge Blanket Reactors: An Alternative
Approach in Anaerobic Membrane Bioreactors
Membranes can be coupled to miscellaneous anaerobic reactor types such as completely stirred tank reactors (CSTR), upflow anaerobic sludge blanket (UASB) and expanded granular sludge bed (EGSB) reactors, etc. in different configurations. Among these, the need to treat high volumetric flow rates of municipal wastewater puts membrane-coupled UASB reactors forward since membrane is challenged by solely sludge supernatant due to the entrapment of most of the particulate matter by adsorption and biodegradation in the sludge bed. Therefore, membrane flux may become less dependent on the reactor mixed liquor suspended solid (MLSS) concentration, possibly leading to high membrane fluxes (Ozgun et al., 2013). The impact of membrane incorporation on the performance of a UASB reactor in terms of biological treatment performance and sludge characteristics was investigated.
Adding membrane to a UASB reactor resulted in significant changes in both physical and biological aspects, mainly due to the elimination of hydraulic shear as bacterial selection criterion. Membrane incorporation induced an accumulation of fine particles and a decrease in EPS concentration of the sludge, which are partially responsible for decreasing the particle size distribution and thus, affecting sludge settleability. Deterioration in sludge settleability led to an increase in sludge washout, with a resultant increase in chemical oxygen demand (COD), total suspended solids (TSS) and soluble microbial products (SMP) concentrations in the effluent of UASB reactor. Despite the increases in COD, TSS and SMP concentrations in the UASB effluent, suspended solids-free permeate with an average COD of 42 mg/L was obtained in the AnMBR system after membrane incorporation due to the complete retention of all particulate and colloidal matter inside the reactor by the membrane. Studying the response of UASB reactor to membrane incorporation, the overall results put UASB reactor forward as a suitable alternative for coupling membranes in AnMBR systems.
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