Biofilm detachment

Aerobic phase Steady state values of phenol concentration (40 mg/L) and biofilm thickness (170 pm) were approached after a 5 h transient period, which reproduces fairly well the experimental dynamical patterns reported in Fig. 6. However, biomass was present also in the liquid phase as a consequence of biofilm detachment. 

Biomass containment in continuously operated bioreactors is an essential prerequisite for the feasibility of practical industrial-scale dye biodegradation. Biofilm airlift reactors have demonstrated excellent performance for their ability to control mixing, interphase mass transfer and biofilm detachment rate. Further studies are required to further exploit the potential of this type of reactors with either aggregated cells or biofilm supported on granular carriers. 

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... 

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... 

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. 

In biochemical processes biofilm have strong adsorption ability, and can be absorbed most of organic suspend solid which can be degraded by bacteria. Some dead bacteria and old biofilm detached from activated biofilm consisted of main suspend solids, and most of suspend solids can settle easily. Fig. 5 show that content of suspend solids is decreased obviously from more than 30 mg in biochemical infow to lower than 15 mg in settled biochemical outflow. Activated carbon adsorption can ensure the content of suspend solids less than 5 mg ... 

In water at pH values between 4 and 7, chlorine predominantly exists as hypochlorous acid (HOCl) it is in equilibrium with the hypochlorite ion (OCf), which predominates above pH 9. Hypochlorous acid is the rapidly acting component, and is mainly responsible for the microbicidal activity of chlorine. Thus it is suggested that low pH values enhance the biocidal activity, whereas high pH values ( > 7) favour hypochlorite ion-mediated disruption and detachment of mature biofilms (Characklis, 1990). In this respect, data from the literature for chlorine efficacy on biofilms are difficult to compare because either different pH values were used or pH values are not mentioned. Moreover, most studies focus on the killing of biofilm organisms, while a limited number of studies also include the aspect of biofilm detachment from surfaces (Table 1). 

QUAG Effective in low concentrations Surface activity supports biofilm detachment Relatively nontoxic Adsorb to surfaces and prevent biofilm growth Inactivation at low pH or in the presence ofCa orMg " Development of resistance... 

Biodispersants are usually nonionic molecules which adsorb to metal surfaces more readily than biofilm polymers. By reducing the size of the biofilm points of contact with the surface, these materials cause biofilm to detach from the surface. In practice, since these materials do not actually destroy biofilm, the biofilm detaches from high-flow areas and accumulates in low-flow areas. The low solubility of these materials can also lead to fouling by the biodispersants themselves. 

Detachment includes two processes erosion and sloughing. Sloughing is a process in which large pieces of biofilm are rapidly removed, frequently exposing the surface. The causes are not well understood. Biofilm erosion is defined as continuous removal of single cells or small groups of cells from the biofilm surface and is related to shear stress at the biofilm/fluid interface. An increase in shear stress increases the erosion rate and decreases the biofilm accumulation rate. Empirical observations indicate that the erosion rate is related to biofilm thickness and density. 

The continuous exchange of cells between the biofilm (X) and the liquid phase (Xl) was described by means of a combined attachment/detachment mechanism. The net rate of detachment balances biofilm growth under steady state conditions. 

Gjaltema A, Tijhuis L, van Loosdrecht MCM et al (1995) Detachment of biomass from suspended nongrowing spherical biofilms in airlift reactors. Biotechnol Bioeng 46 258-269... 

Mechanical methods Flushing Ball for detachment of biofilm... 

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. 

A four-stage model has been used to describe the development of a bacterial biofilm. The stages comprise, first, primary attachment of cells to the underlying biomaterial, second, accumulation in multiple bacterial layers, third, maturation of the biofilm, and fourth, detachment of planktonic cells from the biofilm, which may then initiate a new cycle of biofilm formation elsewhere [11], In the staphylococci clear evidence exists for the attachment and accumulation/maturation stages of the biofilm model [12] (Fig. 1), while active detachment mechanisms are not well... 

It can be supposed that biofdms grew under higher flow velocities were more rigid and strongly attached because of shear stress and as a result bacteria detachment was reduced. Increasing impact of mature biofilms under higher flow velocities 0.7 m/s or 1 m/s found 175 day after the beginning of the process can be related to the critical value of biofdm thickness reached to and shear stress effect on the biofdms surface layer. 

Overall, it appears that the effects of electric fields on detachment, adhesion, and biofilm formation on electrodes are a result from electric current exchanged rather than the electric potential applied (Fig. 18.1b). 

Conditioning iayer Baoteriai attachment Biofiim formation Exopoiysaooharide Biosynthesis I Biofilm maturation t i r j i Disiodgement Detachment... 

The most difficult type of microbiological deposit is in the form which is reinforced in some way, as this is difficult to re-disperse. The most common form of microbiological reinforcement is the presence of either filamentous (thread-forming) bacteria or fungi growing in the microbiological deposit. However, reinforced or not, eventually when a biofilm becomes too thick sections will detach and contaminate the rest of the system and cause defects in the paper sheet or cause breaks. 

Fig. 1 Biofilm development and regeneration illustrated in six steps. Step 1 initial attachment to substratum Step 2 synthesis of EPS to establish more secure association with substratum Step 3 bacterial replication on the substratum and microcolony formation Step 4 formation of mature biofilm Step 5 sloughing and detachment of portions of biofilm Step 6 recolonization of areas of substratum exposed by sloughing events [5].

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