Membrane Characteristics and Definitions

Membranes are flat, semi-permeable structures that are permeable for at least one component and are impermeable for others. According to the nomenclature of membrane technology, various membrane processes are characterized according to the molar mass or diameter of the transported component, the aggregate state on the two sides of the membrane as well as the separation principle. The transport can be caused by gradients of concentration or pressure. Pressure-driven membrane processes like micro-, ultra- and nanofiltration as well as reverse osmosis are used in wastewater treatment.

Figure 12.1 shows the classification of membrane processes based on the average particle diameter or molar mass, with a few examples of wastewater components. Note that the ranges of the separation processes overlap with respect to the particle diameter and the driving pressure.

The functional principle behind membrane processes used in wastewater treatment are filtration or sorption and diffusion, whereby the wastewater feed is divided into a cleaned part, i.e. the filtrate or permeate, and a concentrated part, i.e. the concentrate or retentate (Fig. 12.2).

Fig. 12.1 Classification of membrane processes used in wastewater treatment (MUNLV 2003; Rautenbach 1997).

294 12 Membrane Technology in Biological Wastewater Treatment membrane process

Fig. 12.2 Scheme showing the principle of the membrane process.

The performance of a membrane filtration unit is determined by the following main parameters:

• The selectivity of the membrane is the capability to separate between components like oil and water or salt and water. Low selectivity can only be compensated with an expensive multi-stage process. For aqueous systems of a solvent and a solute the retention coefficient or retention R is a measure of the selectivity. The solute is retained while the solvent, most often water, passes through the membrane; the retention R is given by:

where c0 is the concentration of the pollutant in the feed and c2 is the concentration of the pollutant in the permeate.

The true retention achieved with the membrane is higher because the concentration of the retained component increases at the surface of membrane c3 as a result of concentration polarization (Section 12.3).

In the field of biological wastewater treatment, one main component often has to be eliminated; and the feed and permeate concentrations are given, for example, as suspended solids in g L-1 MLSS.

The relative volume flux Jp0 characterizes the hydrodynamic permeability: Qp

where Qp is the permeate volume flow rate, ApTM is the transmembrane pressure and Am is the membrane area.

12.2 Mass Transport Mechanism | 295 The gradient of trans-membrane pressure, i.e. the driving force, is given by:

which takes into account the pressure drop along the cross-section of the membrane p0 - p1.

• Mechanical stability and resistance to fouling and scaling must be considered as other important factors.

Low permeability of a given membrane can be compensated by increasing the membrane surface area. The permeate flux Jp or the permeate velocity wp is given by:

The flux and the retention coefficients R and Rt are not constant along the surface area of a membrane, even if there is no variability in the quality of the membrane material. The concentration of the retained component increases continuously and affects the flux and retention coefficients.

In wastewater treatment, transmembrane pressure ApTM varies from 0.1 bar up to 120 bar. The characteristic cut-off of a membrane corresponds either to the particle diameter (in microns) or to the molar mass (measured in Dalton) of the largest retained substance.

Fig. 12.3 Concentration and pressure gradients through a solution-diffusion membrane. We have to distinguish Ap' of Eq. (12.4) from Ap of Figure 12.3.

The cut-off of a membrane is determined as the molar mass of macro-molecules and dissolved substances with a retention coefficient of 90% or 95%. It is determined experimentally by the fractional separation curves for ultrafiltration membranes with different substances (Rautenbach 1997) and is often used for the characterization of membrane processes with the exception of microfiltration.

Various transport models are employed when studying the selectivity of different membranes and their transport mechanisms (Rautenbach 1997):

• A black-box model resulting from a large base of experimental results from real systems of combinations of treated fluids and membranes.

• Semi-empirical models for the real system with regard to physical and chemical parameters (solution-diffusion and pore model).

• Structural models in fundamental research.

Here we use semi-empirical models because it is common engineering practice to utilize the understanding of physical properties together with results of investigating process parameters. The solution-diffusion model (reverse osmosis and partly nanofiltration) and the pore model (ultra- and microfiltration) can be used with the physical and chemical background information to reduce the required number of experiments for studies and to quantitatively optimize membrane filtration operation in the area of wastewater treatment.

The design and layout of biological wastewater treatment plants with membrane bioreactor (MBR) technology has to focus on the requirements of the activated sludge process. The first guidelines were formulated by ATV-DVWK (2000b) with special regard to reactor volume, oxygen transfer rate, pre-treatment of wastewater, sludge disposal and, of course, membrane performance and cleaning (Section 12.3).

First we will focus on the most common mechanistic models of mass transport through membranes. They are based on diffusion and convection. Then we will consider resistances to mass transfer, like concentration polarization as well as the combination of transport mechanisms and resistances. A further combination of models, e.g. the solution-diffusion model and the pore model, is necessary when using membranes with an active, deep layer, a porous carrier layer or if a gel-layer is formed.

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