Sediments biofilms

De Beer D, Kiihl M (1998) Interfacial processes, gradients and metabolic activity in microbial mats and biofilms. In Boudreau B, Jorgensen BB (eds) The benthic boundary layer. Oxford University Press, Oxford De Beer D, Schramm A, Santegoeds CM, Nielsen HK (1998) Anaerobic proasses in activated sludge. Wat Sci Technol 37(4—5) 605-608 De Beer D (1999a) Use of microelectrodes to measure in situ microbial activities in biofilms, sediments and microbial mats. In Akkermans ADL, van Elsas JD, de Bruin FJ (eds) Mol nilar microbial ecology manual. Kluwer, Dordrecht... 

Jones B (1995) Processes associated with microbial biofilms in the twilight zone of caves examples from the Cayman Islands. J Sediment Res A65 552-560... 

The sewer processes take place in a complex system. They proceed in one or more of the five phases the suspended water phase, the biofilm, the sewer sediments, the sewer atmosphere and the sewer walls, and by exchange of relevant substances across the interphases. Processes that proceed in the sewer system affect other parts of the urban system, i.e., the urban atmosphere with malodorous substances. Furthermore, wastewater treatment plants and local receiving waters receive not just those substances discharged into the sewer but also products that are the result of the sewer processes (Figures 1.1 and 1.3). 

The fundamental understanding of the microbial processes in wastewater is based on the fact that substrate utilization for growth of biomass takes place parallel to its removal for energy purposes by an electron acceptor. Figure 2.2 shows the general concept and examples where an external electron acceptor is involved. These fundamental microbial transformations take place in the water phase, in the biofilms and in the sediments of the sewer. 

The microbial processes proceed in different subsystems of the sewer the suspended wastewater phase, the biofilms, the sediments and the solid surfaces in contact with the air phase. 

Fermentation may take place in the three major microbial subsystems of a sewer, i.e., the wastewater, the biofilm and the sediments (Figure 3.2). Sulfate-reducing bacteria are slow growing and are therefore primarily present in the biofilm and in the sediments, where sulfate from the wastewater may penetrate (Nielsen and Hvitved-Jacobsen, 1988 Hvitved-Jacobsen et al., 1998 Bjerre et al., 1998). However, as a result of biofilm detachment, sulfate reduction may, to some minor extent, take place in the wastewater. Methanogenic microbial activity normally requires absence of sulfate — or at least a low... 

As seen from Table 3.5, organic matter constitutes an essential part of sewer sediments, however, generally with a low biodegradability. Class D (sewer biofilm) is included in the taxonomy (Section 3.2.7). Class A sewer sediment material is most commonly found in combined sewer networks. 

Only a few studies have been directly concerned with chemical and biological processes in sewer sediments. However, relatively high anaerobic activity in terms of H2S formation of sediment deposits compared with what is generally observed in sewer biofilms is observed (Section 6.2.5). This activity may indicate H2S formation in the deep parts of the sediment caused by the production... 

Bjerre, H.L., T. Hvitved-Jacobsen, S. Schlegel, and B. Teichgraber (1998), Biological activity of biofilm and sediment in the Emscher river, Germany, Water Sci. Tech., 37(1), 9—16. 

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). 

This chapter deals with the microbial transformations of wastewater under aerobic conditions in a sewer network. It emphasizes the transformations of the organic matter and includes processes in both the water phase and the biofilm. Furthermore, transformations of particles in suspension originating from sewer sediments are included. A concept and a corresponding model for the integration of the major microbial processes, i.e., growth of the heterotrophic biomass, the respiration and the hydrolysis, are also dealt with. The basic chemical and biological aspects of sewer processes are focused on in Chapters 2 and 3. The reaeration process is dealt with in Chapter 4. 

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. 

From a general point of view, but still related to sewer conditions, the anaerobic processes in wastewater are dealt with in Chapter 3, especially in Section 3.2.2. In the following, the sulfur cycle is focused on. A part of this cycle proceeds under anaerobic conditions, and another part is aerobic. In a sewer system with changing aerobic and anaerobic conditions, this combined cycle is of particular interest but, at the same time, also complex to deal with. The nature of the sulfur cycle in a sewer is further complicated because the processes proceed in and between the biofilm, the sewer sediments, the water phase, and the sewer atmosphere. 

Sulfate is typically found in all types of wastewater in concentrations greater than 5-15 gS nr i.e., in concentrations that are not limiting for sulfide formation in relatively thin biofilms (Nielsen and Hvitved-Jacobsen, 1988). In sewer sediments, however, where sulfate may penetrate the deeper sediment layers, the potential for sulfate reduction may increase with increasing sulfate concentration in the bulk water phase. Under specific conditions, e.g., in the case of industrial wastewater, it is important that oxidized sulfur components (e.g., thiosulfate and sulfite) other than sulfate may act as sulfur sources for sulfate-reducing bacteria (Nielsen, 1991). 

The temperature dependency of the sulfate reduction rate for single sulfate-reducing bacteria is high, corresponding to a temperature coefficient, a, of about 1.13, i.e., a change in the rate with a factor Q10 = 3.4 per 10°C of temperature increase. Because diffusion of substrate into biofilms or sediments is typically limiting sulfide formation, the temperature coefficient is reduced to about... 

Focusing on sulfide formation, the sediment is often simply taken into account by considering it covered with a biofilm. The potential for sulfide production in terms of the surface flux will typically exceed what is observed for sewer biofilms, e.g., being 50-100% higher (Schmitt and Seyfried, 1992 Bjerre et al., 1998). 

The first three aspects are related to the release of volatile substances into the gas phase of the sewer and from there into the urban atmosphere. These volatile compounds are H2S and organic odorous compounds produced under anaerobic conditions in the wastewater or associated biofilm and sediment. 

Removal of sewer biofilm and deposits by flushing and use of a cleaning ball for detachment of biofilm and resuspension of sewer sediments are examples of mechanical methods for reducing sulfide occurrence. 

Development of sewer biofilm and sediments thereby an effect on the corresponding processes Sewer upstream quality that affects the in-sewer processes and the downstream quality of the wastewater... 

The text deals with the microbial and chemical process engineering of sewer networks. It emphasizes dry-weather processes and not the wet-weather impacts that are primarily controlled by physical processes. Under such conditions, the physical in-sewer processes in terms of, for example, hydraulics, sediment and biofilm erosion and solids transport are important. A quite different approach must be applied when wet-weather conditions in sewers dominate. However, wet-weather performance of sewers also requires that sediment deposition be dealt with during dry-weather periods. 

For cells that are embedded in macroscopic gel-like structures such as floes, biofilms or other porous media such as soil or sediment aggregates, mass transport to the cells can decrease due to ... 

Because of the similarity of transport in biotilms and in stagnant sediments, information on the parameters that control the conductivity of the biofilm can be obtained from diagenetic models for contaminant diffusion in pore waters. Assuming that molecular diffusion is the dominant transport mechanism, and that instantaneous sorption equilibrium exists between dissolved and particle-bound solutes, the vertical flux ( ) through a stagnant sediment is given by (Berner, 1980)... 

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