Sewer biofilms

The biological processes in biofilms are either described by 1-order or 0-order kinetics. However, the 0-order reaction is of specific importance for sewer biofilms as is also the case for treatment processes of wastewater in biofilters. The saturation constant, Ks, is normally insignificant compared with the substrate concentration, and the biofilm kinetics [cf. Equation (2.20)], is therefore 0-order. As shown in Figure 2.8, two different conditions exist the biofilm is either fully penetrated or partly penetrated, corresponding to either a fully effective or a partly effective biofilm. The distinction between these two situations can be expressed by means of a dimensionless constant, P, called the penetration ratio (Harremoes, 1978). For each of these two situations, the flux of substrate across the biofilm surface can neglect the stagnant liquid film being calculated [Equations (2.23) and (2.25)] ... 

The redox reactions taking place in a sewer biofilm require that diffusion of both electron donor and electron acceptor be considered. The steady state mass balance for these two components is [cf. Equation (2.20)] ... 

FIGURE 2.9. Biofilm kinetics with respect to the bulk water phase. Typically, only conditions corresponding to 1/2-order kinetics are observed in a sewer biofilm. 

All types of sewer biofilms are produced at surfaces exposed to the water phase and also, to some extent, at the sewer air surfaces where aerosols are present and the humidity is high. Biofilms in sewers are often referred to as slimes and consist mainly of microorganisms, extracellular polymeric substances (EPS) produced by the microorganisms and adsorbed organic and inorganic compounds from the wastewater. 

The microorganisms in sewer biofilms are embedded in a matrix of EPS that consists mostly of polysaccharides produced by the bacteria (Characklis and Marshall, 1990). The EPS fraction is the largest organic fraction in the biofilm, i.e., up to about 90% of the total organic content. Only limited studies of the total composition of sewer biofilms in terms of carbohydrates, proteins and humic substances have been undertaken (Figure 3.12). Corresponding information on the composition of anaerobic biofilms in pressure mains does not yet exist. 

Significant biomass production can take place in a gravity sewer biofilm. The biomass generated in the biofilm detaches and is, together with the biomass produced in the water phase, transported to the treatment plant or via overflow structures into receiving waters. A simple method to assess the amount of... 

FIGURE 3.12. Typical composition of a gravity sewer biofilm (Jahn and Nielsen, 1998). 

A 4 km intercepting sewer with diameter D = 0.5 m is flowing half full. The DO consumption rate, rf, of the sewer biofilm is measured, and an average value of 0.6 g02 m-2 h-1 was estimated. The biofilm yield constant of the heterotrophic biomass was not measured but was estimated as Yf= 0.55 gCOD biomass produced per gCOD substrate consumed. Only aerobic heterotrophic transformations in the biofilm are expected to proceed. 

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... 

Jahn, A. and P.H. Nielsen (1998), Cell biomass and exopolymer composition in sewer biofilms, Water Sci. Tech., 37(1), 17-24. 

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),... 

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. 

Laboratory and mixed field/laboratory studies have confirmed that half-order kinetics for DO surface removal rates may be a reasonable approximation for sewer biofilm (Raunkjaeretal., 1997 Bjerre etal., 1998b). These results also showed the influence of readily biodegradable substrate. Furthermore, temperature dependency limited by diffusion is included (Nielsen et al., 1998). The following equation for the aerobic growth rate was therefore used ... 

This growth expression requires a minimum of kinetics and stoichiometric coefficients to be determined, and no hydraulic details are included. The dynamics of sewer biofilm detachment are not quantitatively known, and a steady state biofilm with a biomass release to the bulk water phase, equal to the biomass growth within the biofilm, is therefore an estimate. 

TABLE 5.2. DO Surface Removal Rates for Sewer Biofilms. The Biofilms were Grown by Continuous Supply of Wastewater from the Sewer, and the Experiments were Performed under Organic Substrate Nonlimiting Conditions (Bjerre etal., 1998b). 

FIGURE 5.6. DO surface removal rates for sewer biofilms, of. Table 5.2. 

Equation (5.12) shows a linear dependency in the DO concentration that is not in agreement with the results shown in Figure 5.6. Matos (1992) also found a discrepancy between Equation (5.12) and experimental results and substituted the expression 5.3 S0 in Equation (5.12) with a constant equal to 10.9. This constant depends on biofilm and wastewater characteristics and should be determined from local measurements. In addition to the information given by Bjerre et al. (1998b) in Example 5.2, values of respiration rate measurements for sewer biofilms are shown in Table 5.5. 

TABLE 5.5. Experimentally Determined Values of the DO Consumption Rate, h for Different Sewer Biofilms. 

Poulsen (1997) investigated the anoxic transformations of wastewater in biofilms originating from a biofilter and found maximum NUR values of 0.025-0.055 gN03-N nr2 h-1 at 20°C. The change from 0-order to 1/2-order kinetics was found to be about 3 gN03-N m-3. Aesoey et al. (1997) found an NUR from a 1-2 mm thick sewer biofilm to be 0.15-0.18 gN03-N m-2 h-1 at... 

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. 

Raunkjaer, K., P.H. Nielsen, andT. Hvitved-Jacobsen (1997), Acetate removal in sewer biofilms under aerobic conditions, Water Res., 31, 2727—2736. 

Referring to Figure 6.2, it is important and interesting to note that a DO consumption that takes place in a gravity sewer biofilm may proceed with relations to aerobic and anaerobic processes. The anaerobic microbial processes produce... 

FIGURE 6.2. Aerobic and anaerobic process interactions in a gravity sewer biofilm. 

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). 

When designing sewer networks, particularly gravity sewers, reaeration is the major process that should be focused on to reduce sulfide formation and the formation of organic odorous substances (cf. Section 4.4). A number of hydraulic and systems characteristics can be managed to increase the reaeration rate and avoid or reduce sulfide-related problems. The hydraulic mean depth, the hydraulic radius, the wastewater flow velocity and the slope of the sewer pipe are, in this respect, important factors that are dealt with in Section 4.4. It should be stressed that it is not necessarily the objective to avoid sulfide formation (in the sewer biofilm), but the sulfide that occurs in the bulk water phase should be at a low concentration level. Therefore, the DO concentration in the bulk water phase should not be lower than about 0.2-0.5 g02 m-3, sufficiently high to oxidize sulfide before a considerable amount is emitted to the sewer atmosphere. 

The thickness of the sewer biofilm affects sulfide formation. Reduction of the biofilm thickness by increasing the wastewater velocity may lead to reduced sulfide problems. At very low velocities in an arerobic gravity sewer, a biofilm thickness may be more than 50 mm however, it may be substantially reduced to typically 1-5 mm when the velocity is increased. The thickness of... 

It is important to design the sewer system to avoid permanent solids accumulation, as the development of deposits may cause an enhanced sulfide formation rate compared with a sewer biofilm. Solids deposition and resuspension in sewers will not be dealt with in this text. An overview of these physical properties is found in Ashley and Verbanck (1998). 

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. 

As shown in Figure 6.8, the most important part of the anaerobic sulfur cycle in terms of the sulfate respiration process can be integrated with the anaerobic carbon cycle. A fractionation of the readily biodegradable substrate (Ss) into SF and SA fits well to the anticipation that mainly SF is used by the sulfate-reducing biomass in sewer biofilms. By integrating the sulfide formation in this way, a simple conceptual approach is obtained instead of the traditional empirical descriptions as depicted in Table 6.1. 

These four procedures are all recommended to be performed in the order shown to achieve optimal parameter estimation followed by a final validation of the gravity sewer process model (Figure 7.7). In the case of design of a new sewer system, procedure number 4 is, of course, not relevant and kinetic parameters for the sewer biofilm must be evaluated and selected based on information from comparative systems. 

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... 

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