Wastewater treatment plants have become a focal point in protecting our future water supply by reducing the toxic impacts of wastewater and its use in daily life.
Several wastewater treatment procedures use physical and chemical pollutants removal.
These techniques have varying degrees of effectiveness, as well as environmental and economic limitations.
Biological wastewater treatment technologies, on the other hand, have received a lot of attention in recent years.
They have cheap operational expenses, are simple to use, and have comparatively fewer negative consequences on the environment.
How does biofilm work in water treatment?
Biofilms are small ecosystems usually consisting of three layers of differing thickness, which change in thickness and composition with location and over time.
In the first phase of colonization, macromolecules are adsorbed at clean solid surfaces (proteins, polysaccharides, lignin), because they are transported from the bulk liquid to the solid surface faster than the microorganisms.
As a consequence of this adsorption, the coverage of the solid surface with water is reduced.
During the second phase, microbial cells attach to this prepared surface.
Frequently, they do not form closed layers of uniform thickness, rather they form small attached colonies, which may spread by growth and further attachment.
Usually, these cells are supplied with substrate and oxygen and can grow at their maximum rate.
During this process, they produce organic molecules, which diffuse through the cell wall and to extracellular polymeric substances (EPS) catalyzed by exoenzymes.
These EPS molecules are necessary for the formation of a stable biofilm.
In the third phase, the biofilm may consist of bacteria and EPS, the thickness of which is a function of growth rate and depends on the stability of the biofilm and the shear stress of the flowing water.
At lower shear stresses, eukaryotic organisms (protozoa, insects, their eggs, and larvae) typically establish themselves. All these organisms live in a community.
Materials such as substrates and oxygen are transported into the biofilm by diffusion and convection and the products are transported out of the biofilm.
Oxygen may reach only into the exterior part of the biofilm, resulting in the growth of aerobic microorganisms such as nitrifying bacteria and protozoa.
Nitrate and nitrite produced in this layer are reduced by anoxic metabolism within a middle layer, resulting in an anaerobic interior layer directly at the solid surface, where acetic acid and sulfate may be reduced.
Heterogeneous biofilms grow on the sides of ships and buildings near the water’s edge, inside human and animal mouths, and within inner organs.
They frequently cause damage to these surfaces (biocorrosion) and must be removed.
In the area of environmental biotechnology, however, they can be utilized to advantage in certain bioreactors, such as:
-Trickling filters.
-Submerged, aerated fixed bed reactors.
-Rotating disc reactors.
The formation of biofilms is a requirement for their effectiveness.
Trickling Filters
A trickling filter consists of a layer of solid particles or bundles of synthetic material inside a cylindrical or prismoid container.
Wastewater must be distributed uniformly at the top of the fixed bed – frequently by a rotating system of two or four horizontal tubes equipped with many nozzles.
To compensate for the fact that the area of a circular section of the reactor increases with distance from the center, the distance between nozzles must decrease the further they are away from the center to have an even distribution of water over the surface.
Furthermore, the changes in available pressure in the rotating tubes must be considered as a function of the flow rate.
Uniform distribution of the wastewater and uniform packing of the reactor with solid substances are of high importance for a high loading and removal rate.
It is critical to ensure that two conditions are met:
The downward flowing liquid films must be in direct contact with the biofilm (i.e., the biofilm has to be trickled overall places and at all times) and must be in contact with the upward or downward flowing air (i.e., the trickling filter should not be flooded at any location or time).
The wastewater must be practically free of solids. The wastewater must pass a primary settler under controlled conditions and is never overloaded.
distinguish between:
Natural aeration is a result of density differences between the air saturated with moisture inside the trickling filter and the air outside the trickling filter, and
Forced aeration by a ventilator at the top of the trickling filter. In this case, the reactor may have a height of up to 12 m and is filled with packages of synthetic supporting material.
Submerged and Aerated Fixed Bed Reactors
In circumstances of heavy hydraulic loading, the trickling filter may be operated as a flooded bed, increasing the pressure differential required for downward flow.
The amount of wastewater required to overcome flow resistance is determined by the type of support material utilized and the thickness of the biofilm.
Aerobic fixed beds must be aerated near the bottom of the system, resulting in a two-phase flow in a three-phase system with an upward airflow.
Water is moved uphill in the reactor’s center and flows downwards near its borders due to friction forces.
Biomass is attached to the support material’s surface and is also suspended as flocs.
It is difficult to avoid clogs in biofilm regions with a higher thickness and a lower local flow rate.
The fixed bed must be cleaned regularly by significantly raising the wastewater flow rate.
Synthetic support materials, such as BIOPAC, have been employed successfully, particularly when immobilizing nitrifying bacteria with slower growth rates.
In contrast to fixed beds with solid particles, the flow of water and air in reactors with suspended particles is more readily controlled, and obstructions can be avoided.
Unlike trickling filters, their airflow rates can be changed to fit the organics and ammonia loading.
The particular surface area can be expanded to 400 m2 m-3.
Fine bubbles are created using membrane-type tubular aerators, and the mass transfer rate is significantly boosted.
As a result of the flow’s friction forces, the suspended biological sludge detaches from the surfaces and is carried to the secondary settler. There are no blockages.
Rotating Disc Reactors
In rotating disc reactors (RDR), the principle behind the intense transport of substrates and oxygen to the biofilm is different.
In trickling filters and fixed bed reactors, water and air are moved; here, the support material with the biofilm is moved.
In rotating disc reactors, circular plates with diameters of 1–2 m are fitted to a horizontal shaft with a spacing of a few centimeters.
The system of parallel plates is submerged nearly halfway in a cylindrical tank through which wastewater flows.
The packet of plates rotates at a speed of 0.5–5.0 rpm. Bacteria grow on both surfaces of the circular discs.
During the portion of the rotation where the biofilm travels through the air, wastewater drips down and oxygen is taken up by convection and diffusion.
Parts of the biofilm rinse off from the discs from time to time.
Larger pieces settle in the tank and must be removed as surplus sludge, while smaller parts are suspended and involved in aerobic substrate degradation and further growth (carbon removal and nitrification).
References
[1] Marshall, K.C.; Blainey, B. 1991, Role of bacterial adhesion in biofilm formation and biocorrosion, in Biofouling and Biocorrosion in Industrial Water Systems, ed. Flemming, H.-C.; Geesey, G.G., Springer-Verlag, Heidelberg, p. 8–45.
[2] Metcalf, Eddy 1991, Wastewater Engineering: Treatment, Disposal, And Reuse, 3rd and, McGraw-Hill, New York.
[3] Meyer-Reil, L.-A. 1996, kologie mikrobieller Biofilme, in: kologie der Abwasserorganismen, ed. Lemmer, H.; Griebe, T.; Flemming, H.-C., Springer-Verlag, Berlin, p. 24–42.
[4] Wingender, J.; Flemming, H.-C. 1999, Autoaggregation of microorganisms: flocs and biofilms, in Environmental Processes I, (Biotechnology, Vol. 11a), ed. J. Winter, Wiley-VCH, Weinheim, p. 65–83.
