3- The biological effects
It is evident that infiltration of nutrient-rich wastewater into the soil is increasing microbial activity. When the soil layer is influenced by steady state fluid and gradually become saturated then the microbial activity is immersed under the liquid. Initially, a microorganism cell needs to attach on the solid surface in order to reduce mobilization. The immobilize matrix of microbial cells could offers a protective and nutrient rich environment for microbial growth. The adhesion between a microbial cell and a surface particularly is the result of electrostatic attraction and hydrophobic interaction (Daeschel and McGuire, 1998). An electrical gradient between exterior and interior cell membrane is forming a voltage across the cell membrane. This electrical charge is usually negative for a microbial cell (James, 1991). In a higher distance from cell membrane (50 nm) hydrophobic interactions is particular force of adhesion while in a lower distance (20 nm) electrical attraction dominants of adhesion progression (Daesche and McGuire, 1998).
The microbial cells behave in liquid are very similar as particles (Donlan, 2002). The adsorption of surfactant monomers on microbial cell (Chen et al, 2000, Kaczorek et al.2011) can increase the mobility of cell as well (Streger et al, 2002; Powelson and Mills, 1998). This mechanisms may is changing the surface electrical charge of bacterial cell and increasing its hydrophobicity. Then, by increasing the repulsive force between microbial cell and liquid the mobility of cell become easier. Additionally, it seems that with the increase of surfactant concentration in liquid more simplicity in bacterial transportation inside the porous media can be provided. This is because the adsorption of surfactant monomers at the interface reduces the attractive forces inside the pores. In this regard, Stelmack et al. (1999) studied the biodegradation of NAPL in aqueous phase. The results showed that after the increase in the concentration of surfactant the bacteria will not able to adhere to the NAPL and solid surfaces. Furthermore, the increased cell mobility is enhancing bio-effect dimension in porous media by more journeys of individual cells. Jackson et al, 1994, was found that applying anionic surfactant sodium dodecylbenzene sulfonate (DDBS) is increasing the bacterial transport in saturated porous media. Moreover, the same effect is reducing ability of cellular aggregation and effective contribution to form micro-colonies.
In contrast, surfactant adsorption on the microbial cells can make it easier to adhere to solid surfaces with increasing the cell’s hydrophobicity. This effect may cause change of activity and environment of microorganisms (Vanloosdrecht et al, 1990) and thus reducing the microbial population (Stelmack et...