PERFORMANCE OF A MBR PILOT PLANT SUBJECT TO A GRADUAL SALINITY INCREASE: ANALYSIS OF BIOKINETIC AND FOULING BEHAVIOR

Abstract

Membrane bioreactors (MBRs) have been widely used for the treatment of various wastewater, including municipal, industrial, landfill leachate (Wintgens et al., 2005). Such systems generally provide decisive advantages compared to conventional activated sludge (CAS) processes: in particular, they feature high quality effluent, small footprint and low sludge production rates (Stephenson et al., 2000). In recent years MBRs have also been used for the treatment of specific saline water, such as wastewater produced from shipboard activities, aquaculture wastewater or wastewater deriving by sediment washing treatment, as an example. However, when subject to salinity, a modification of biomass characteristics may occur. This circumstance is of importance, since it can have a significant impact in terms of biokinetic as well as fouling behaviour. The aim of the study was to evaluate the performance of a pilot plant MBR (fed with synthetic wastewater) in terms of performance, biomass activity and membrane fouling, when subject to a gradual salinity increase. In detail, the experimental campaign was divided in five different phases, each characterized by a different NaCl dosage (1st phase: no salt addition, 2nd phase: 1 g NaCl L-1; 3rd phase: 2.5 g NaCl L-1; 4th phase: 5 g NaCl L-1; 5th phase: 10 g NaCl L-1). The lab scale MBR was built at the Laboratory of Sanitary and Environmental Engineering of Palermo University and consisted in a 17 L bioreactor coupled to a 7 L compartment where a hollow fibre ultrafiltration (UF) module in a submerged configuration was inserted (ZeeWeedTM 01 module; porosity: 0.04 m; surface area: 0.1 m2). Specific respirometric batch tests were conducted in order to characterize the biokinetic behaviour of the MBR pilot plant, using a “flowing-gas/static-liquid” type as batch respirometer (Spanjers et al., 1996). The total Extracellular Polymeric Substances (EPST) concentration was periodically measured by means of the Heating method (Zhang et al., 1999), described in detail in Di Bella et al. (2011). The Soluble Microbial Products (SMPs), that represent the soluble portion of EPST, as well as the bound EPS (EPSbound) were measured through this protocol. The pilot plant showed very high COD removal efficiencies throughout the experimental campaign, with an average value close to 94%, as reported in Figure 1a. As the salinity of the inlet wastewater was increased, it was observed a slight decrease of COD removal efficiency which, however, was always higher than 88%, thus confirming the system robustness. Concerning the nitrification process, the average value was equal to 99% (Figure 1b), highlighting that a gradual salinity increase exert only a moderate influence on nitrifying population.Referring to membrane fouling, the salinity increase exerted a significant effect on the total resistance, in particular starting from the 3rd phase (Figure 2a). Cake resistance represented the majority of total hydraulic resistance; in particular, the irreversible cake deposition was recognised as the predominant fouling mechanism (Figure 2b), likely due to EPS release by the biomass