Quantifying the Effect of COD to TN Ratio, DO Concentration and Temperature on Filamentous Microorganisms’ Population and Trans-Membrane Pressure (TMP) in Membrane Bio-Reactors (MBR)

Petros Gkotsis,Manassis Mitrakas,Alexandros Pentedimos,Giannis Lemonidis,Margaritis Kostoglou,Anastasios Zouboulis

doi:10.3390/pr8111514

Abstract

Using moderate populations of filaments in the biomass of Membrane Bio-Reactors (MBRs) is a biological anti-fouling method which has been increasingly applied over the last few years. This study aims to quantify the effect of COD to TN ratio, Dissolved Oxygen (DO) concentration and temperature on filaments’ population and Trans-Membrane Pressure (TMP) in a pilot-scale MBR, with a view to reducing membrane fouling. The novelty of the present work concerns the development of a mathematical equation that correlates fouling rate (dTMP/dt) with the population of filamentous microorganisms, assessed by the Filament Index (FI), and with the concentration of the carbohydrate fraction of Soluble Microbial Products (SMPc). Apart from TMP and SMPc, other fouling-related biomass characteristics, such as sludge filterability and settleability, were also examined. It was shown that at high COD to TN ratio (10:1), low DO concentration in the filaments’ tank (0.5 ± 0.3 mg/L) and high temperature (24–30 °C), a moderate population of filaments is developed (FI = 1–2), which delays the TMP rise. Under these conditions, sludge filterability and settleability were also enhanced. Finally, TMP data analysis showed that the fouling rate is affected by FI and SMPc concentration mainly in the long-term fouling stage and increases exponentially with their increase.

Highlights

The Membrane Bio-Reactor (MBR) is a state-of-the-art technology which has been increasingly employed in the field of wastewater treatment over the last decade, due to its significant advantages over the conventional Activated Sludge Process (ASP), such as high quality of treated water, low space requirement and small footprint
Biological methods for fouling control in MBRs are usually classified into the following two groups: (i) inhibition and structure optimization of the bio-cake layer that is formed on the membrane surface, which mainly aim at preventing cell–cell or membrane–cell interactions or at decreasing cell activity without killing the deposited cells, and (ii) enzymatic or bacterial degradation of biopolymers, which employ enzymes or bacteria in order to achieve efficient degradation of foulants, such as Extracellular Polymeric Substances (EPS) or Soluble Microbial Products (SMP) [6,7]
Excessive growth of filamentous strains can be triggered by the operating conditions which are applied in the wastewater treatment plants (WWTP), such as Solids Retention Time (SRT), temperature, pH, dissolved oxygen (DO), etc., and/or by the influent wastewater characteristics, such as feed flow rate, COD to TN ratio (i.e., Chemical Oxygen Demand to Total Nitrogen ratio), etc

Summary

Introduction

The Membrane Bio-Reactor (MBR) is a state-of-the-art technology which has been increasingly employed in the field of wastewater treatment over the last decade, due to its significant advantages over the conventional Activated Sludge Process (ASP), such as high quality of treated water, low space requirement and small footprint. Deterioration of the sedimentation process due to the presence of filamentous microorganisms ( known as filaments) is avoided, since sedimentation is replaced by membrane filtration in the sedimentation stage. Membrane fouling, which is the undesirable deposition of suspended solids, colloids and/or soluble substances onto the membrane surface or inside its pores, is regarded as the main drawback of the MBR technology, limiting the widespread application of membrane bioreactors [1,2,3,4,5].

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