IFAS / mbbr media: The most crucial element of MBBR Process and its important properties
As MBBR/IFAS process involves attached growth microbial communities for the biochemical oxidation, the IFAS / MBBR media is the most crucial element of process which allows growth of micro organism in the form of biofilms on it.
Over the years, the entire development of MBBR media has been focused on utilising various plastic materials like (PP,PVC,HDPE,LDPE, Recycled plastic) and having cylindrical shape and structure. The principle assumption in developing this type of carrier materials entirely rely on the hypothesis that more surface area meaning more biofilms and thus more removal rate can be obtained per m3 of MBBR carriers added to the MBBR tank. However, over the years, it has been observed that biofilms from the outer surface of the carriers are being sloughed off or scrapped off due to carrier collision and thus only the inner open surface area within the void is the only available surface area for colonization. Thus, a term called “specific surface area’’ or ‘’protected surface area’’ has been popular for the classification of the conventional plastic MBBR carriers and it has been utilized as sole measure of the carrier efficiency and reliability. Based on this popular notion, various carrier materials having an array of channels and voids have been developed with surface area ranging from 200-1200 m2/m3 of carrier material.
However, surface area or protected surface area alone can not be taken as the sole measure of MBBR/IFAS carrier classification criteria. There are many other factors related to the MBBR/IFAS process design and performance which are directly influenced by the carrier type, its shape, its geometry and structure along with the type of material being used for the manufacturing of carrier material. The heart of the MBBR performance is not the carrier surface area only.
Biofilm Characteristics and its control for efficient MBBR/IFAS Process:
The performance and efficiency of any MBBR/IFAS system is solely dependent on having high amount of active biomass within the carrier element to perform the specific tasks for the biodegradation of target pollutants. Thus, having highly efficient and active biofilms on carrier element is very crucial. The characteristics of biofilms like diffusion and substrate gradient, biomass attachment/detachment, biofilm thickness and its control, its composition, structure and diversity are very much crucial to the efficient attached growth MBBR/IFAS process. The carrier elements, their design, shape, size, structure and various properties are thus of prime importance for the implementation of efficient and robust MBBR/IFAS process.
Important and Crucial aspects of IFAS/MBBR media which are influencing MBBR/IFAS process design and efficacy are:
Hydrophilicity/Hydrophobicity of Carrier material and its density: Influence on Mixing and Mass Transfer
In the Aerobic MBBR/IFAS process, the carriers are kept in suspension by continuous mixing provided with the help of aeration system. As this additional energy is required to keep the carriers in suspension and continuously moving in the reactor, the carrier density and its properties like hydrophobicity are very important so that carrier becomes part of the entire fluidized bed for better mass transfer of substrate across the biofilms developed on the carrier material.
Keeping the density of the material as close as to water will allow better and easier mixing of the IFAS / MBBR media in the reactor making it less prone to float on the reactor surface under varying hydrodynamic conditions within the reactor.
Hydrophobic material tend to repel water from their surface forming a boundary layer which increases the resistance of bacteria to form biofilms on its surface and thus requires significant time for initial colonization and formation of biofilms as bacteria tend to develop biofilms by secreting Extra Cellular Polymeric Substance(EPS).
On the contrary Hydrophilic surface allows for better wetting properties of the carrier material and thus carrier becomes part of the bulk liquid very easily allowing for its easier and smoother mixing. Also due to better water binding ability of hydrophilic material, the resistance of biofilm formation is reduced drastically and thus faster colonization is observed with hydrophilic material.
Most of the carrier materials presently developed and being used are made of hydrophobic plastic materials which requires a significant time for wetting and also due to their density lower than water, they require a lot of additional energy for fluidization and takes a lot time for initial colonization.
Size and Shape of Voids and Inner Porosity:
Large voids tend to cause detachment of biofilms due to high shear forces and thus would increase the biofilm formation time during the initial colonization. During the event of shock loads and toxic loads, a larger fraction of the biofilm is exposed to the toxicity which may cause performance deterioration.
Smaller and deeper voids allow for rapid formation of biofilms within the deeper pores due to lower sloughing off during start up process. Fine pore structure with deeper voids also protects bacteria against toxic shock loads and thus under such dynamic conditions of frequent shock loads, the performance deterioration can be prevented much better compared to carriers with larger and open voids. Also finer and deeper cavities allow for growth of specific type of micro organisms like Anammox bacteria and Denitrifying Bacteria within the inner pores increasing the process efficiency and reliability over the period of operation. However, too fine pore structure may tend to clog the carrier material with excessive biofilm formation within the inner pore and make it heavier. Also due to poor accessibility of the inner pore for substrate, the inner biofilms become less active and redundant over period of operation.
Thus, a carrier material having adequate fine pore structure and inner porosity is very much desirable for MBBR media which allows faster and better colonization, protection of bacteria against toxic events, allow specific type of bacteria to grow on it to perform specific biochemical operations and also at the same time provides better accessibility to the inner biofilms for better substrate penetration in the form of organic matter and Nitrogen and also allows better detachment, shearing and erosion of biofilms on continuous basis so that excessive growth and clogging can be prevented.
Presently much of the development of plastic carriers have been focused on providing larger voids which suffers from poor initial colonization due to shearing of biofilms during the start up, offer minimal protection against toxic shock load except very thick biofilms are formed which embed bacteria within EPS matrix and protect them. The thicker biofilms result in poor diffusion rates and thus removal rates compared to thinner biofilms having better access to substrate present in the bulk liquid.
Surface Roughness and Adsorbing Capacity of the MBBR/IFAS carrier material:
For effective and faster colonization on the MBBR carriers, the surface of the carriers should be rough and must have adsorbing capacity for micro organisms to adsorb on it easily so that microbial adsorption on the carrier surface can be made faster. The rough surface have lower contact angles which allows for faster wetting and fluidization of the carrier material aiding their mixing easier
Degree of Filling and its impact on mixing, Oxygen transfer efficiency:
It has been observed that MBBR reactors with lower degree of carriers filling require lower mixing energy or atleast will be having better mixing pattern due to the free movement of carrier element within the bioreactor resulting in better fluidization. It has been also observed that this free movement at relatively lower degree of filling results in better OTE (Oxygen Transfer Efficiency) improving energy economics in terms of aeration energy.
Thus, a MBBR carrier having lower degree of filling is highly desirable from process point of view so that better mixing and fluidization of the reactor can be achieved for efficient mass transfer of substrate resulting in better removal efficiency.
Important Properties of MBBR Media:
- Hydrophilicity/hydrophobicity and its density
- Size and shape of voids and inner porosity
- Surface Roughness and adsorbing capacity
- Degree of filling
- Biofilm Thickness and its control
- Mixing and Diffusion Gradients