Bioreactor component of MBRs

The bioreactor component of an MBR is designed to remove biologically degradable contaminants of concern. A brief introduction to bioreactor fundamentals is presented below.

2.1.1 Bioreactor fundamentals

A number of bioreactor configurations exist and are commonly used in wastewater treatment applications [7]. All are governed by the kinetics of biomass growth and substrate (i.e., contaminant) consumption (i.e., removal). For a simple, completely mixed flow through bioreactor, the extent of substrate consumption can be estimated using a mass balance approach as depicted verbally and numerically in Eqs. (1) and (2), respectively. Under steady-state conditions, these equations can be simplified to yield Eq. (3).

Change in mass of Mass of substrate Mass of substrate Rate at which substrate substrate in system entering system leaving system is consumed

di where S is the concentration of substrate in the bioreactor (and bioreactor effluent) [M/L ], Vr the volume of the bioreactor [L ], Q the flow rate through the bioreactor [L /T], S0 the concentration of substrate in the influent to the bioreactor [M/L ], and rS the rate at which the substrate is consumed [M/L T]:

As presented in Eq. (3), the size of a bioreactor required to achieve a given amount of substrate consumption (i.e., contaminant removal) is smaller when the rate at which substrate can be consumed is high. The rate at which substrate is consumed can be estimated using Eq. (4).

where k is the maximum rate of substrate consumption per unit mass of

3-i biomass [1/T], X the concentration of biomass in the bioreactor [M/L and kS the half saturation constant for the substrate being consumed by the biomass [M/L3].

Easily biodegradable substrates have a high maximum rate of substrate consumption and are therefore rapidly consumed. Less biodegradable substrates can also be consumed rapidly by maintaining a high biomass concentration in the bioreactor. For this reason, bioreactors used in wastewater treatment applications are not designed as flowthough systems, but rather have some mechanism by which the biomass is retained in the bioreactor. For MBRs, the membrane component of the system retains the biomass within the bioreactor (Fig. 1b and c). Since membranes can retain virtually all of the biomass, relatively high biomass concentrations can be achieved in MBRs, resulting in relatively high substrate consumption rates, and therefore, relatively small bioreactor volumes. Typical biomass concentrations, measured as mixed liquor suspended solids (MLSS), in MBRs range from 8 to 12 g/L [8]. By comparison, the clarifier component ofCAS is not as effective at retaining biomass and therefore typical biomass concentrations that can be maintained in these systems typically range from 2.5 to 4 g/L. As a result, the hydraulic retention time in MBRs, which is the ratio of the volume of the bioreactor to the flow rate through the system, can be as low as 4h [8]. By comparison, the size of the bioreactor component of CASs is typically approximately two times larger than that of MBRs [9,10]. It should be noted that there is a practical upper limit to the biomass concentration that can be maintained in MBRs. At biomass concentrations greater than approximately 12 g/L, oxygen transfer in the bioreactor component of the system is limiting and inhibits the growth of the aerobic biomass [11]. High biomass concentrations can also negatively affect the permeate flux through the membrane component of MBRs as discussed in the next section.

The ability to retain virtually all of the biomass in the bioreactor also enables higher mean cell residence times, also commonly referred to as sludge retention times (SRTs), to be maintained in MBRs. The amount of waste biomass produced during treatment, which can be estimated using Eq. (5), typically decreases as the SRT increases [12-14]. The SRT of CASs typically ranges from 3 to 5 days, while that of MBRs is usually greater than 10 days. As a result, the amount of waste biomass produced by MBRs is typically approximately 15-50% less than that produced by CASs [10,12,15].

where MEB is the mass of excess biomass produced [M/L3], Y the ratio of mass of biomass formed to the mass of substrate consumed [-], Kd the endogenous biomass decay coefficient [1/T], 0 the SRT, which is the ratio of the volume of the bioreactor to the waste sludge flow rate from the bioreactor [T], and (S0— S) the amount of substrate consumed [M/L3].

The ability to rapidly and effectively remove contaminants using relatively small bioreactor volumes and the limited extent of waste sludge production are some of the principal advantages that MBRs have over CASs [16]. It should be noted that the overall amount of waste sludge produced by MBRs can be relatively similar to that produced by CASs since MBRs typically do not have primary clarifiers, which remove the easily settlable contaminants from the waste stream prior to biological treatment (i.e., primary sludge), and because MBRs remove more contaminants than CASs [17].

Although the mass balance equations, rate kinetics, and sludge yields associated with bioreactor configurations used in wastewater treatment and reuse applications are more complex than those presented above for a simple flow through systems, the overall conclusions from the above discussion apply to all biological processes and bioreactor configurations. A more detailed discussion on bioreactor kinetics, beyond the scope of the present discussion, can be found in Metcalf and Eddy [7] and Bailey and Ollis [16]. Most bioreactors are designed to promote the growth of aerobic biomass capable of degrading organic contaminants (Fig. 1). However, bioreactors can also be designed to promote the growth of biomass that is capable of removing nutrients such as nitrogen and/or phosphorus (Fig. 3a and b), or can be designed as anerobic systems. A detailed discussion on these different bioreactor configurations can be found in Metcalf and Eddy [7] and Bailey and Ollis [18].

Treated Treated

Treated Treated

a) Typical MBR configuration for biological b) Typical MBR Configuration for enhanced nutrient removal biological phosphorus removal

Figure 3 Bioreactor system configurations for enhanced nutrient removal.

a) Typical MBR configuration for biological b) Typical MBR Configuration for enhanced nutrient removal biological phosphorus removal

Figure 3 Bioreactor system configurations for enhanced nutrient removal.

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