Abstract

THE USE OF BIOREACTORS COUPLED with MEMBRANES FOR THE TREATMENT OF EFFLUENTS

R. Bergamasco ¹ , F.R. Lapolli ² , C.R. Granhen ¹ and A. Grasmick ³

¹Departamento de Engenharia Química, Universidade Estadual de Maringá - Av. Colombo, 5790 - Bloco D-90m Maringá, PR - Brazil - 87020-900 - Telephone: (044) 226-2727 - Fax: (044) 226-1180

²Departamento de Engenharia Sanitária e Ambiental, Universidade Federal de Santa Catarina - SC - Brazil

³Laboratoire des Materiaux et Procédés Membranaires, Université de Montpellier II - Montpellier - France

(Received: March 5, 1997; Accepted: August 5, 1997)

Abstract: The objectives of this paper are to verify the viability of operating a bioreactor coupled with a membrane, and to analyze the global mechanisms witch need to be considered in the bioreactional concept in the separation by membrane. In order to meet the proposed objectives, a culture with a synthetic substratum (ethanol) was utilized. A mineral membrane with the following characteristics was used: a pore diameter of 0.2 m m, 19 channels of a 4 mm diameter, a width of 0.85 m, a filtering surface area of 0.2 m², a pressure of 2 bar and a tangential velocity of 2 m/s. The experiments consisted of modifying the residence time of the substratum within the reactor. The following measurements were taken: chemical oxygen demand (COD), concentration of biomass and filtered flow. The results show a treated effluent of good quality, indicating that the time of hydraulic residence time influences the efficiency of the system and is influenced by the restriction of the filtered flow by a fast fouling of the membrane.

Keywords: Membranes, bioreactors, effluent treatment.

INTRODUCTION

Studies on the processes of separation by membrane have led to the development of industrial technologies, mainly in the areas of chemical biopurification, pharmaceuticals and food processing.

The used of biological processes associated with membranes and their technologies to treat wastewaters has shown a great potential for substituting the conventional techniques (coagulation and separation of solid and liquid) that normally demand large tanks.

In the recent research on wastewater treatment, using techniques of separation by membranes, the attainment of a good quality effluent, as well as the filtration capacity of these membranes is emphasized.

Thus, the purpose of this study is to verify the viability of operating a bioreactor coupled with a membrane, as well as to analyze the global mechanisms to be considered in the bioreactional concept and in separation by membrane.

THEORETICAL FOUNDATION

Currently, the treatment of the organic pollution in wastewaters is being done by anaerobic biological processes or by aerobic processes with activated sludge.

Different systems using mainly the organic membranes have been developed. Most of these processes consist of secondary treatments which filter waters from decanters or biofilter effluents. Examples are the "Ohte Center Building" treatment plant in Tokyo (Audic et. al, 1986) and the "UBIS" system (Ultra Biological System) which associates an activated sludge reactor with an ultrafiltration module.

The "PLEIADE" system (Roullet, 1987 and Lambert, 1983) treats and recuperates used waters from kitchens, wash basins and toilets. The water is reused in the toilets of buildings. These systems treat 100m³/day with 34m² of membranes. Most viruses and bacterias are blocked by the membranes, where the analyses show volatile solids in suspension (VSS) at a value inferior to 1 mg/l and a COD reduction of 89 to 12 ppm.

The Activated Sludge and Membrane Complex System permits water treatment with a biochemical oxygen demand (BOD₅) of around 13,000 mg/l and functions under the same principle as that adopted by Roullet (1987). The velocity of circulation through the membranes is 2.5 m/s.

A treatment plant established in Japan (Tokyo) by Degrémont enterprise, whose operating principle is similar to that of Audic et. al (1986), uses organic membranes (polyvinyl

acetate) whose pores have a diameter of 0.04m m. In that plant, microfiltration constitutes, solely, the final treatment before water recycling to the toilets. The system obtains a 99% return with a COD less than 2 mg/l .

Research developed in the field has presented problems related to rapid membrane fouling and the economic profitability of the process. Some of the factors responsible for fouling the are the presence of concentrated biomass, which suggests the need for a comparison of treatments with total biomass recycling and with no recycling at all. For that reason, the need for "purges" of filtered sludges is emphasized (Bouillot, 1988).

The role of tangential velocity in the fouling was observed by several authors (Lacoste, 1992; Marsigny, 1990) who agree on the fact that an increase in the tangential velocity allows an increase in permeation.

For the process to be economically profitable, the problem is the high cost of membranes, especially the mineral ones, which are often recommended for presenting better resistances.

Several factors need to be studied to achieve the optimization of the process. However, the use of bioreactors coupled with membranes for the treatment of wastewaters is now, proving itself to be a promising alternative to the classic pattern utilized.

MATERIALS AND METHODS

The materials and methods used in the development of this study, which contributed to the success of the proposed objectives, will now be described.

The simplified diagram of the pilot plant used is shown in Figure l.

Ethanol (C₂H₅OH) was used as a source of carbon for the synthetic substratum. Potassium nitrate (KNO₃) was used as a source of N(NO₃). The substratum was prepared with COD/N = 6 in such a way that the reaction of the reduction would show an excess source of carbon. The substratum referred to was stored in a feeding reservoir.

A mixed culture taken from an aeration tank of the Wastewaters Treatment Plant of Montpellier - France was utilized as a bacterial source.

Figure 1: Simplified diagram of the pilot plant.

A recirculation pump assured the flow of the suspension (biomass). One part of the permeate was recirculated to the reactor, and the other part was taken out to maintain the flow rate of the system. A mineral membrane with the following characteristics was utilized: a pore diameter of 0.2 m m, 19 channels with a diameter of 4 mm, a width of 0.85 m , a filtering surface area of 0.2 mof 2 bar and a tangential velocity of 2 m/s. Regular ², a pressure washing of the membrane with basic detergent (30%) and acid (50%) and rinsing with plain water was performed to avoid excessive fouling. The system was kept at a steady temperature of 25° C during the experiments.

The analysis of the chemical oxygen demand (COD) and the volatile solids in suspension (VSS) was carried out according to standard methods (APHA, 1980).

RESULTS AND DISCUSSION

The performance of the bioreactor-membrane was evaluated, taking into consideration the reaction kinetics and filtration in a continuous flow rate, by varying the residence time (t ) inside the reactor.

Four experiments were performed and the results are presented in Table 1.

The quantity of biomass was maintained constant during the experiments.

The influence of the residence time is observed in Figure 2, where it is possible to verify a reduction in COD efficiency (Table 1) when t is reduced.

Figure 2: COD concentration of permeate versus residence time (t ).

In Figure 3, the amount of COD eliminated from the system raises linearly when the residence time is low, yet after a residence time of over five hours, the curve presents a slight decline. This indicates that, for high residence times, the amount of eliminated COD could possibly be almost constant.

In the course of the four experiments, the permeation (J) versus time was measured (Figure 4). The measurement procedure for the curves was repeated throughout all the experiments. This is considered to be a classic reaction, where in the beginning we find a very fast change in volume (flow or membrane resistance) and later on we find a slower change. Lacoste (1992) observed the same reaction in their studies on the microfiltration of biological suspension. Such findings are usually explained by progressive deposition during the initial phase and later, in a second phase, layer thickness is limited due to the local shearing. For this reason, slower filtration will be shown as a consequence of the particles already deposited on the membrane.

Figure 3: Relation between the amount of COD eliminated versus residence time (t ).

Figure 4: Flow permeate of (J) versus time.

In the present study, the hypothesis is that pore obstruction seems to be caused by microorganisms. However, the microscopic observation of this biomass complemented by a granulometric analysis, will allow us to discover the size of the particles in suspension, and provide evidence on the main fouling substances. This is shown by Lacoste (1992) and Duclert (1990) in their studies, where they affirm that fouling depends in part on particles whose size is close to the diameter of the membrane pores.

It is also possible to take into consideration that the membrane structure, the porosity, and the distribution of pore sizes are the key parameters of fouling.

Due to the complexity of biological fouling and the categorization of fouling particles, several studies will be developed for the (bioreactor membranes) BRM process to be considered economically viable for the treatment of effluents.

FINAL CONSIDERATIONS

This study shows the initial results of research being developed at the Laboratoire Génie des Procédés Membranaires de L’Université de Montpellier II.

Considering the experiments performed, it can be concluded that:

- the functioning of a bioreactor coupled with a membrane is viable for obtaining a good quality effluent, as well as good filtration capacity;

- residence time showed no influence on membrane filtrability, which is explained by the continuos maintenance of the quantity of biomass in the experiments;

- the amount of COD eliminated tends to be a constant for high values of residence time, while a reduction in the COD output is noted for low values;

- the microorganisms, the membrane structure, the porosity and the distribution of pore size are responsible for the fouling;

- this study requires further research in order to obtain new results to attest to the technical and economic viability of the use of a (Bioreactor Membrane) BRM.

NOMENCLATURE

COD Chemical oxygen demand, mg/l

J Permeation, l/h/m²

VSS Volatile solids suspension, g/l

t Residence time, h

REFERENCES

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