L. M. Freitas dos Santos

Novel membrane bioreactor for detoxification of VOC wastewaters : biodegradation of 1,2-dichloroethane

In this work a novel extractive membrane bioreactor has been used to overcome air-stripping problems which occur during aerobic treatment of 1,2-dichloroethane (DCE) contaminated wastewaters. The operation of a conventional air-lift bioreactor at a wastewater residence time of 11.6 h led to 25–34% of the DCE supplied to the reactor being lost to the exit gas stream. In contrast, employing the novel membrane bioreactor resulted in negligible (1.5%) air stripping at the same wastewater flowrate. The membrane bioreactor operates by separating the DCE containing wastewater from the aerated biomedium. This is achieved by means of a silicone rubber membrane which is coiled around a perspex draft tube. DCE diffuses across the silicone rubber membrane and into a biofilm growing attached to the surface of the membrane, while oxygen diffuses into the biofilm from the biomedium side. Oxygen and DCE meet in the biofilm and degradation occurs without the DCE being directly exposed to the aerating gas stream. Of the DCE supplied to the membrane bioreactor, 94.5% was biodegraded during operation of this system, and approximately 65% of the carbon entering the system was evolved as CO2. A mathematical model has been used to describe the transfer of DCE across the membrane and subsequent diffusion and reaction of DCE and O2 in the biofilm attached to the membrane tubes. Parameters describing microbial growth kinetics on DCE were determined using a CSTR bioreactor. The results of the mathematical analysis confirmed that the biofilm has two major effects on system performance: (a) it prevents direct contact between DCE and the aerating gas, thus avoiding air stripping; and (b) it limits the flux of DCE across the membrane with consequent accumulation of DCE at the membrane-biofilm interface, which reduces the mass transfer driving force for DCE extraction from the wastewater.

Prediction of optimal biofilm thickness for membrane-attached biofilms growing in an extractive membrane bioreactor

This article presents a mathematical model of membrane‐attached biofilm (MAB) behavior in a single‐tube extractive membrane bioreactor (STEMB). MABs can be used for treatment of wastewaters containing VOCs, treatment of saline wastewaters, and nitrification processes. Extractive membrane bioreactors (EMBs) are employed to prevent the direct contact between a toxic volatile pollutant and the aerated gas by allowing counterdiffusion of substrates; i.e., pollutant diffuses from the tube side into the biofilm, whereas oxygen diffuses from the shell side into the biofilm. This reduces the air stripping problems usually found in conventional bioreactors. In this study, the biodegradation of a toxic VOC (1,2‐dichloroethane, DCE) present in a synthetic wastewater has been investigated. An unstructured model is used to describe cell growth and cell decay in the MAB. The model has been verified by comparing model predicted trends with experimental data collected over 5 to 20‐day periods, and has subsequently been used to model steady states in biofilm behavior over longer time scales. The model is capable of predicting the correct trends in system variables such as biofilm thickness, DCE flux across the membrane, carbon dioxide evolution, and suspended biomass. Steady states (constant biofilm thickness and DCE flux) are predicted, and factors that affect these steady states, i.e., cell endogeneous decay rate, and biofilm attrition, are investigated. Biofilm attrition does not have a great influence on biofilm behavior at low values of detachment coefficient close to those typically reported in the literature. Steady‐state biofilm thickness is found to be an important variable; a thin biofilm results in a high DCE flux across the membrane, but with the penalty of a high loss of DCE via air stripping. The optimal biofilm thickness at steady state can be determined by trading off the decrease in air stripping (desirable) and the decrease in DCE flux (undesirable) which occur simultaneously as the thickness increases.

Determination of pollutant diffusion coefficients in naturally formed biofilms using a single tube extractive membrane bioreactor

A novel technique has been used to determine the effective diffusion coefficients for 1,1,2-trichloroethane (TCE), a nonreacting tracer, in biofilms growing on the external surface of a silicone rubber membrane tube during degradation of 1,2-dichloroethane (DCE) by Xanthobacter autotrophicus GJ10 and monochlorobenzene (MCB) by Pseudomonas JS150. Experiments were carried out in a single tube extractive membrane bioreactor (STEMB), whose configuration makes it possible to measure the transmembrane flux of substrates. A video imaging technique (VIT) was employed for in situ biofilm thickness measurement and recording. Diffusion coefficients of TCE in the biofilms and TCE mass transfer coefficients in the liquid films adjacent to the biofilms were determined simultaneously using a resistances-in-series diffusion model. It was found that the flux and overall mass transfer coefficient of TCE decrease with increasing biofilm thickness, showing the importance of biofilm diffusion on the mass transfer process. Similar fluxes were observed for the nonreacting tracer (TCE) and the reactive substrates (MCB or DCE), suggesting that membrane-attached biofilm systems can be rate controlled primarily by substrate diffusion. The TCE diffusion coefficient in the JS150 biofilm appeared to be dependent on biofilm thickness, decreasing markedly for biofilm thicknesses of >1 mm. The values of the TCE diffusion coefficients in the JS150 biofilms <1-mm thick are approximately twice those in water and fall to around 30% of the water value for biofilms >1-mm thick. The TCE diffusion coefficients in the GJ10 biofilms were apparently constant at about the water value. The change in the diffusion coefficient for the JS150 biofilms is attributed to the influence of eddy diffusion and convective flow on transport in the thinner (<1-mm thick) biofilms.

Extraction and biodegradation of a toxic volatile organic compound (1,2-dichloroethane) from waste-water in a membrane bioreactor

An extractive membrane bioreactor has been used to treat a synthetic waste-water containing a toxic volatile organic compound, 1,2-dichloroethane (DCE). Biofilms growing on the surface of the membrane tubes biodegrade DCE while avoiding direct contact between the DCE and the aerating gas. This reduces air stripping by more than an order of magnitude (from 30–35% of the DCE entering the system to less than 1%) relative to conventional aerated bioreactors. Over 99% removal of DCE from a waste-water containing 1600 mg l−1 of DCE was achieved at waste-water residence times of 0.75 h. Biodegradation was verified as the removal mechanism through measurements of CO2 and chloride ion evolution in the bioreactor. No DCE was detected in the biomedium over the operating period. The diffusion-reaction phenomena occurring in the biofilm have been described by a mathematical model, which provides calculated solutions that support the experimental results by predicting that all DCE is biodegraded within the biofilm. Experimentally, however, the rate of DCE degradation in the biofilm was found to be independent of O2 concentration, while the model predictions point to O2 being limiting.

A novel bioreactor system for the destruction of volatile organic compounds

The biological treatment of waste-waters containing 1,2-dichloroethane (DCE) in conventional bioreactors results in air-stripping of DCE. In the present work, a novel bioreactor system intended to overcome this problem has been developed for the treatment of a synthetically concocted DCE-containing waste-water (1000 mg DCE l−1). The operation of a conventional air-lift bioreactor at a waste-water flow rate of 0.24 l h−1 led to 33% of the DCE supplied to the reactor being lost to the exit gas stream. The use of the novel enclosed system, operated with a recycling O2 sparge instead of air, resulted in negligible air-stripping at the same waste-water flow rate. A control system was implemented to add O2 as required to maintain the pressure of the recycle gas stream, and a scrubber removed the CO2 produced. Over 99% of DCE supplied was biodegraded during operation of this system, and virtually all carbon entering the system was evolved as CO2.

Membrane attached biofilms for waste treatment- fundamentals and applications

Membrane Attached Biofilms (MABs) are being used in an increasing variety of bioreactors. Extractive Membrane Bioreactors (EMB) have been developed at Imperial College (1,2) for the aerobic biotreatment of toxic organics which employ MABs for treating Volatile Organic Compounds (V0Cs)-containing wastewaters without incurring air-stripping problems. Investigations of the key factors controlling the optimal operating conditions for the EMB system have shown that process efficiency is highly dependent on the development of these MABs. Therefore MAB development and its influence on the flux across the membrane over time has been studied and is presented here. Two MAB model systems have been studied; Xanthobacter autotrophicus GJ10 growing on 1,2-Dichloroethane (DCE) and Pseudomonas JS150 growing on Monochlorobenzene (MCB). The results show that there is a problem in this system with excess biofilm growth on the membrane surface, resulting in reduced flux of organic substrate across the membrane. At the same time, a diffusion-reaction model has been developed to explain the experimental results, and to describe the behaviour of the EMB. It was theoretically concluded that an optimal biofilm thickness could be found from a compromise between the level of air- stripping and flux of pollutant across the membrane, and that cell endogenous decay could be used to manipulate the biofilm thickness. Methods of controlling excessive growth of biomass have been investigated, and the addition of sodium chloride to the biomedium to control excessive biofilm development has been shown to be effective.

Minimisation of biomass in an extractive membrane bioreactor

Many traditional biological methods for the treatment of wastewater cope poorly with toxic, volatile organic compounds. The extractive membrane bioreactor is a novel process for the treatment of industrial wastewaters containing such compounds which combines extraction across a silicone rubber membrane with biodegradation. Previous work has shown that there is a problem in this system with excess biofilm growth on the membrane surface, resulting in reduced flux of organic substrate across the membrane. The work presented here shows that addition of sodium chloride to the biomedium increases the maintenance energy requirement of the degradative microorganisms and results, in a carbon-limited situation, in a reduction in biofilm growth. Flux of organic substrate was shown to remain high under reduced biofilm growth conditions.

Growth of immobilised cells: Results and predictions for membrane-attached biofilms using a novel in situ biofilm thickness measurement technique

Abstract This paper describes results from a novel technique developed for measuring the rate at which the thickness of biofilms growing attached to the surfaces of tubular membranes increase over time. Two biofilm/immobilised cell model systems have been studied; Xanthobacter autotrophicus GJ10 growing on 1,2-dichloroethane and Pseudomonas sp. strain JS150 growing on monochlorobenzene. A dynamic mathematical model has been developed to explain the experimental results, treating the expansion of the biofilm as a moving boundary problem. This model is able to predict accurately the rate of biofilm growth and the overall system performance.