Analysis Of Removal Mechanisms Of E. Coli

The mechanisms of E. coli inactivation or removal in woodchip bioreactors have not been fully explored. But, many removal mechanisms have been suggested and these involve chemical, physical, and biological interactions, alone or in combination.

Physical mechanism may involve the filtration of micro-organisms through the attachment to the substrate or sedimentation. Filtration of drainage water has this potential to keep bacteria on the surfaces of the woodchips, as it has been shown in sand-based filters in the previous studies). There is less likely to have a high E. coli inactivation by sedimentation inside the woodchip bioreactors, as a small percentage of E. coli can be settle out by simple gravity sedimentation inside the system, but it is still possible. Interaction of E. coli with woodchip surfaces through cellular properties including cell surface charge, surface structures (e.g. fimbriae and flagella), hydrophobicity and extracellular polymeric substances would be among another physical mechanisms. As an example, bacteria (E. coli also included) are able to attach to the surface by excreting a sugary substance (which it is called extracellular polymeric substances) holding the cells together and attaching them to the surface. This may help biofilms be made up on the wood surfaces and build a biological active layer on the woodchip surface. Organisms on this active layer may consume pathogens and have an important role onE. coli inactivation in the system.

Electrostatic interactions may also effect on E. coli inactivation inside the woodchip bioreactors when E. coli with a negative net surface charge comes into contact with a surface with a positive charge. Adsorption of the pathogens to organic matter and oxidation may be considered as chemical mechanisms in the woodchip bioreactors for E. coli inactivation. Oxidation of organic matter may lower oxygen concentration and create toxic forms of oxygen molecules which may be injurious to E. coli cells. Predation by organisms, natural die-off, and competition for resources may constitute the biological factors for E. coli inactivation in the woodchips bioreactors. Retention in the woodchip bioreactor may also enhance E. coli inactivation by natural decay or predation.

In our laboratory experiment, as a natural E. coli die-off was found in the influent containers before they were pumped into the columns, it may be possible that any decay that occurred in the influent container could be occurred throughout the woodchip bioreactors. Moreover, we found that higher microbial communities in the reactors may lead to have higher predation and competition between micro-organisms and finally led to higher E. coli removal. As it is also proved by another study that presence and diversity of an indigenous microflora (such as protozoa) had a negative effect on E. coli survival as a result of predation, substrate competition and antagonism.

Further research on removal mechanisms of E. coli is needed to evaluate how these mechanisms change with bioreactor age, and how they would be affected by factors such as seasonality, loading rate, inflow concentration, microbial community, flow rate, and hydraulic retention time (HRT).