Oxygen biofilm

As discussed above, deposits can cause accelerated localized corrosion by creating differential aeration cells. This same phenomenon occurs with a biofilm. The nonuniform nature of biofilm formation creates an inherent differential, which is enhanced by the oxygen consumption of organisms in the biofilm. 

SRB, a diverse group of anaerobic bacteria isolated from a variety of environments, use sulfate in the absence of oxygen as the terminal electron acceptor in respiration. During biofilm formation, if the aerobic respiration rate within a biofilm is greater than the oxygen diffusion rate, the metal/biofilm interface can become anaerobic and provide a niche for sulfide production by SRB. The critical thickness of the biofilm required to produce anaerobie conditions depends on the availability of oxygen and the rate of respiration. The corrosion rate of iron and copper alloys in the presence of hydrogen sulfide is accelerated by the formation of iron sulfide minerals that stimulate the cathodic reaction. 

Microsensors have been used to develop profiles in mixed species biofilms. Figure 10 shows concentration profiles of sulfide, oxygen, and pH in a biofilm accumulated on the surface of a mild steel corrosion coupon. The concentration of sulfide is highest near the metal surface, where iron sulfide forms quickly and covers the steel surface if both ferrous and sulfide ions are available. At low ferrous ion concentrations, adherent and temporarily protective films of iron sulfides are formed on the steel surface, with a consequent reduction in corrosion rate. High rates of SRB-induced corrosion of mild steel are maintained only in high concentrations of ferrous ion. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

Mass transfer considerations are critical in any bioprocess. In typical, aerobic, suspended cell fermentations, the major concern is the oxygen transfer rate, determined by the overall mass transfer coefficient, kft, and the driving force. In three-phase biofluidization, in which the cells are immobilized as a biofilm or within carrier particles, the situation is further complicated by possible intraparticle diffusion limitations. Numerous recent studies have addressed these issues. 

The role of cell respiration has been taken into account to interpret the azo-dye conversion by particle-supported biofilm under aerobic conditions [5, 24]. The rapid depletion of oxygen expected/measured as one moves inside the biofilm promotes the establishment of the anoxic conditions needed for azo-dye conversion. 

A dynamic model has been developed to simulate the behavior of a Pseudomonas sp. 0X1 biofilm reactor for phenol and azo-dye conversion during the aerobic-anaerobic cyclic operation. Phenol and oxygen were considered as the limiting substrates for growth kinetics. 

The biofilm thickness (Lf) and density (X = 50 g/L) were assumed uniform and the biofilm treated as a continuum. A substrate diffusion-reaction model assuming spherical particle was used. Diffusion coefficient of phenol and oxygen in the biofilm were assessed according to Fan et al. [64] ... 

Figure 7 shows phenol, dye, oxygen and suspended biomass concentrations and biofilm thickness as a function of time. 

As stated earlier, the biodegradation of azo dyes requires an anaerobic and aerobic phase for the complete mineralization. The required condition can be implemented either by spatial separation of the two sludge using a sequential anaerobic-aerobic reactor system or in one reactor in the so-called integrated anaerobic-aerobic reactor system. In recent years, combined anaerobic-aerobic treatment technologies are extensively applied in the treatment of azo dye-containing wastewaters. Table 1 lists the systems based on combined anaerobic-aerobic treatment in separate reactors. Table 2 lists SBR based on temporal separation of the anaerobic and the aerobic phase. Table 3 lists the other systems, either hybrids with aerated zones or micro-aerobic systems based on the principle of limited oxygen diffuse in microbial biofilms [91]. 

General descriptors may be related to the metabolism responses in the biofilm. Biofilm algae have several mechanisms to counterbalance the damage caused by the toxicants. Environmental stress produces oxidative damage in the cells, which can be tracked down by means of the analysis of many enzymes (superoxide dismutase, catalase, peroxidase, etc.) that function as effective quenchers of reactive oxygen species (ROS). 

Equation (2.19), which concerns a situation without processes in the biofilm, can be extended to include transformation of a substrate, an electron donor (organic matter) or an electron acceptor, e.g., dissolved oxygen. If the reaction rate is limited by j ust one substrate and under steady state conditions, i.e., a fixed concentration profile, the differential equation for the combined transport and substrate utilization following Monod kinetics is shown in Equation (2.20) and is illustrated in Figure 2.8. Equation (2.20) expresses that under steady state conditions, the molecular diffusion determined by Fick s second law is equal to the bacterial uptake of the substrate. 

Nielsen, P.H., K. Raunkjaer, N.H. Norsker, N.Aa. Jensen, and T. Hvitved-Jacobsen (1992), Transformation of wastewater in sewer systems —Areview, Water Sci. Tech., 25(6), 17-31. Norsker, N.-H., P.H. Nielsen, and T. Hvitved-Jacobsen (1995), Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm, Water Sci. Tech., 31(7),... 

Only the formulas for KLa by Parkhurst and Pomeroy (1972), Taghizadeh-Nasser (1986) and Jensen (1994) have been developed for sewer pipes. Taghizadeh-Nasser (1986) performed the investigation in a pilot sewer, whereas the formulas developed by Parkhurst and Pomeroy (1972) and Jensen (1994) were based on measurements in real sewers. Parkhurst and Pomeroy (1972) made investigations based on an oxygen mass balance in sewers that were cleaned for sediments and biofilm. Jensen (1994) based his formula on the one developed by Pomeroy and Parkhurst (1972) and measurements of the reaeration by a direct methodology using krypton-85 as radiotracer (cf. Chapter 7). 

Basically, a concept for microbial transformations in sewer networks should cover soluble and particulate components and relevant processes in the water phase, in the biofilm and in the sewer sediments. In addition, mass transfer between these phases and an air-water transfer of oxygen should be taken into account (Figures 1.3 and 5.2). Although only the aerobic microbial activity will be focused on in the concept presented in this chapter, anoxic and anaerobic processes should be considered possible extensions (cf. Chapter 6). 

The microbial transformations of the wastewater described in the concept shown in Figure 5.5 deal with the COD components defined in Section 3.2.6. The figure also depicts the major processes that include the transformations of the organic matter (the electron donors) in the two subsystems of the sewer the suspended wastewater phase and the sewer biofilm. The air-water oxygen transfer (the reaeration) provides the aerobic microbial processes with the electron acceptor (cf. Section 4.4). Sediment processes are omitted in the concept but are indirectly taken into account in terms of a biofilm at the sediment surface. Water phase/biofilm exchange of electron donors and dissolved oxygen is included in the description. 

Norsker, N.-H., P.H. Nielsen, and T. Hvitved-Jacobsen (1995), Influence of oxygen on biofilm growth and potential sulfate reduction in gravity sewer biofilm, Water Sci. Tech., 31(7), 159-167. 

Injection of air the oxygen in the injected air will prevent sulfate-reducing conditions in the sewer. The DO concentration in the wastewater establishes an aerobic upper layer in the biofilm, and sulfide produced in the deeper part of the biofilm or the deposits that may diffuse into the water phase will be oxidized (cf. Figure 6.2). The oxidation of sulfide will mainly proceed as a chemical process, although microbial oxidation may also take place (Chen and Morris, 1972). Factors that affect the oxidation rate of sulfide include pH, temperature and presence of catalysts, e.g., heavy metals. 

It is important to note that the amount of oxygen needed to avoid sulfate-reducing conditions is determined by the aerobic respiration rate of the wastewater and the biofilm and not the potential amount of total sulfide production in the sewer. The relatively low solubility of oxygen (9-11 g02 m-3) in wastewater compared with the DO consumption rate typically requires that oxygen must be injected at several points of a sewer pipe to ensure aerobic conditions. This is, of course, expensive and requires manpower in terms of operation and maintenance. Furthermore, the readily biodegradable and fast hydrolyzable fractions of the organic matter may be depleted (Tanaka et al., 2000b). In the case of requirement for mechanical treatment, this is positive ... 

A reliable measurement of DO in a wastewater system requires that the surface of the sensor be regularly cleaned to avoid the development of a biofilm that otherwise will consume oxygen and disturb the measurement. 

Measurement of biofilm activity can be performed based on laboratory reactor experiments or with a technique combining biofilm growth taking place in a sewer followed by measurements in laboratory scale (Raunkjaer et al., 1997 Bjerre et al., 1998). Huisman et al. (1999) developed a sewer in situ biofilm respiration chamber. It includes a DO sensor and a chamber that can be pressed onto the sewer wall. It is designed to achieve an even and unidirectional flow distribution over the entire measurement area. Pure oxygen is injected for oxygenation. 

In Figure 8.3, the oxygen transfer coefficient, KLa, the flow velocity, u, the bulk water DO concentration and the DO consumption rate of the biofilm, ry, are all plotted versus the flow, Q, under steady state conditions in a gravity sewer pipe under the conditions given. 

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