Biofilm Formation in drinking water distribution

Biofilm Formation in drinking water distribution

1. Formation of a surface conditioning film

It is a thin layer of organic molecules and ions covering the adhesion surface and is formed before attachment of microorganisms. Physical or chemical adsorption helps these processes. The former is a reversible process and includes nonspecific bonds such as van der Waals with low adsorption energies. Whereas latter is a nonreversible and includes specific chemical bonds such as electrostatic, covalent and hydrogen bonds, dipole interactions, and hydrophobic interactions with higher adsorption energies. In this process several adsorbed molecular layers are formed which determines the strength of the biofilm adhesion. Besides, the conditioning film helps bacteria to attach on the surface of pipeline by neutralizing flow velocity and do also provide nutrient for bacteria (Marshall, 1996).

2. Initial adhesion of “pioneer” microorganisms to the surface;

Planktonic (free floating) microorganisms are transported towards the surface either by fluid dynamics, gravitational forces and Brownian motion, or by migration through active cell motility (e.g. flagella). The surface electrostatic charge and hydrophobic interactions are also found to affect this approaching and adhesion (Mueller, 1996).

The first arriving free floating bacteria adhere to the surface initially through weak, reversible electrostatic attraction and van der Waals forces. If not immediately separated from the surface, the bacteria are anchored more permanently by developing stronger bonds for the attachment to the surface for example, by active cell biosynthesis of EPS and by chemical forces. These cells become irreversibly adsorbed. Different mechanisms had been proposed to explain the bacteria attachment to surfaces with different hydrophobicity character (Marshall, 1996).

3. Biofilm growth

The attached bacteria start to grow, excrete organic polymers, and initiate the formation of the biofilm matrix. Tolker-Nielsen et al., (2000) observed that the first micro-colonies formed were mono-species. Further attachment of planktonic bacteria and of inorganic particles will contribute to a structurally heterogeneous biofilm growth, as well as, migration of attached bacteria between and inside of micro-colonies. During this phase, bacteria detachment events occur although at a lower extent compared to the growth rate.

4. Biofilm maturation - equilibrium between accumulation and detachment.

Mature biofilm are composed of an organized consortia of microorganisms embedded in an organic matrix that protects the bacteria. The structure of a mature biofilm depends on the microbial composition, EPS production, the nutrient availability, hydrodynamic conditions and temperature. In a biofilm several processes may occur simultaneously: bacterial detachment into water, attachment of planktonic bacteria, growth, death etc. In a mature biofilm these processes are at equilibrium and the attached cells per unit surface area are constant with time although with periodic fluctuations. At this phase, the biofilm reach the highest thickness that does depends on the hydraulic conditions, the mass transport and the biofilm cohesion (Momba et al., 2002).

According to Momba et al., (2002) several factors were observed to promote detachment of biofilm portions or of isolated bacteria. These factors are the following:

Sloughing off: The increase of shear stress, alternating flow conditions and abrasion due to particle collisions promote sloughing off of biofilm pieces that were not well cohesive.

Starvations of bacteria: It does promote size reduction, and increase of bacteria fragmentation and of motility which will increase biofilm detachment.

The increase of nutrients: it promoted the release of bacterial cells up to 80 % the total attached ones. This biofilm dispersion phenomenon was associated with increased expression of flagella genes.

Chelating agents: A chemical change in the EPS due to presence of chelating agents (Ca²⁺) that will reduce the cohesive strength of the attached cells.

Surfactants: The excretion of surface-modified products (surfactants) by certain bacteria may promote detachment.

Signaling molecules: In biofilms the excretion of certain signaling molecules induced the detachment events.