MBBR process is based on attached growth microbial communities growing as biofilm on specially designed MBBR media. These microbial communities developed on MBBR media are playing major role in the biochemical oxidation of pollutants present in the wastewater as BOD, COD and Ammonia Nitrogen. Because of this reason MBBR media having properties which allows efficient and optimum growth of these biofilms are of prime importance for any MBBR and IFAS based attached growth process for biological wastewater treatment.
Since the invention of MBBR technology, the development of MBBR media has been focused solely on utilizing various plastic materials like Poly Propylene (PP), High Density Polyethylene (HDPE), Low Density Polyethylene (LDPE) and other suitable plastic materials. By and large plastic MBBR media have cylindrical or circular shape providing varying levels of surface area for biofilm development on it.
The primary assumption of this development is based on the hypothesis that more surface area provides more biofilms which eventually results in more reduction efficiency and higher process economy.
However, it has been observed with these plastic MBBR media that biofilms grow on the inner void surface while the biomass from outer surface area of the MBBR media is sloughed due to various reasons. To account this phenomenon, specific surface area (SSA) and protected surface area (PSA) per m3 volume of MBBR media provided are extensively used to estimate the efficacy of plastic material based MBBR Media. At present different plastic MBBR media having different shapes and size are offering SSA of 500-4000 m2/m3.
However total surface area or protected surface area alone can not be taken as sole measure of MBBR media efficiency for its better efficiency. Apart from surface area other important properties such as media surface material, its texture, porosity, geometry, orientation of media plays crucial role. These properties greatly affect the MBBR process design and efficiency to a greater extent.
Biofilm Characteristics and its control for efficient MBBR Process
The efficiency of any MBBR process is solely dependent on retaining high amount of active biomass on the MBBR media performing specific tasks for the biodegradation of target pollutants. Thus, having highly efficient and active biofilms on MBBR media is most critical. Characteristics of biofilms such as diffusion and substrate gradient, biomass attachment and detachment, biofilm thickness and its control, its composition, structure, and diversity of microbial population are very much crucial to the efficient attached growth MBBR process.
Most important and crucial aspects of MBBR media design which influence MBBR process and are summarized as below:
Hydrophilicity, Hydrophobicity, density of MBBR media: Influence on Mixing and Mass Transfer
In the aerobic MBBR process, the media elements are kept in suspension by continuous mixing and agitation provided using aeration system. It is imperative to develop a MBBR media which can be kept in suspension with minimal additional energy requirement mixing and agitation. MBBR media specific gravity, density, hydrophobicity plays crucial role for mixing and agitation of media in the MBBR reactor.
Keeping specific gravity of MBBR media slightly higher than that of water will allow better and easier mixing in the MBBR reactor making it less prone to floating on the reactor surface. Plastic MBBR media have specific gravity in the range of 0.95-0.98 which may tend to float on the MBBR surface under suboptimal aeration mixing and under varying hydrodynamic conditions.
Hydrophobic plastic material tends to repel water from its surface forming a boundary layer. This layer increases resistance to biofilm formation and requires significant time for initial colonization and biofilm formation by bacteria on the MBBR element.
On the contrary hydrophilic surface allows for better wetting of the MBBR element. In combination with higher specific gravity, this hydrophilic surface of the MBBR element helps in easier and smoother mixing of the MBBR media in the reactor. Due to this combination of properties, better fluidization of MBBR media can be achieved at much lower energy expenditure compared to hydrophobic materials having specific gravity lower than water. Hydrophilic surface also has better water binding ability which allows quick wetting of the surface. This in turn reduces the boundary layer resistance for biofilm formation drastically making faster colonization and biofilm formation feasible.
MBBR Media size, 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 toxic shock 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 during start up process. Fine pore structure with deeper voids also protects bacteria against toxic shock loads. So MBBR media with fine pore structure provide much better protection against shock loads and quicker start up compared to the element having large and exposed voids.
Finer voids also allow for growth of thinner biofilms which keeps a large fraction of biomass present in the biofilm exposed to the bulk liquid of the reactor for substrate uptake. This provide better diffusion gradients allowing better reduction efficiencies.
Finer and deeper cavities allow for growth of specific type of microorganisms such as 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. Further due to poor accessibility of the inner pore for substrate, the inner biofilms become less active and redundant over period of operation.
Thus, a MBBR media material having adequate fine pore structure and inner porosity is very much desirable as it provides numerous benefits such as:
- Faster and better colonization
- Protection of bacteria against shock loads at the same time provides adequate accessibility to inner biofilms for better substrate diffusion.
- Adequate detachment, shearing and erosion of biofilms on continuous basis to prevent excessive growth and clogging.
- Thinner biofilm growth with better diffusion gradients allowing better substrate uptake and reduction efficiencies.
On the contrary present plastic material based MBBR media element have larger voids which has many disadvantages such as:
- Poor initial colonization due to shearing of biomass during start up.
- Minimal protection against toxic shock loads before thick biofilms are formed with EPS to protect them.
Poor diffusion gradient with less accessibility to inner biofilms under thick biofilm formation.
Surface Roughness and Adsorbing Capacity of the MBBR carrier material:
For effective and faster colonization on the MBBR media, the surface of the media should be rough and must have adsorbing capacity for microorganisms to adsorb on it easily so that biofilm formation on the media surface can be made faster. Rough surfaces have lower contact angle which allows for faster wetting and fluidization of the carrier material making their mixing and fluidization much easier.
Degree of Filling: Its impact on mixing and Oxygen transfer efficiency
It has been observed that MBBR reactors with lower degree of carriers filling require lower mixing energy and have better mixing pattern due to the movement of carrier element within the bioreactor resulting in better fluidization. It has been also observed that this 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 media 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 reduction efficiency.
Based on our above critical observations, we can summarize that an ideal MBBR media element should have following important properties for its efficient functioning.
Important Properties of MBBR Media and its advantages:
|MBBR Media Property
|Higher Specific Gravity
|Easier mixing and fluidization with lower energy requirement
|Quicker colonization and biofilm formation, Better wetting and fluidization of media, and Lower energy consumption for mixing
|Fine pores and higher porosity
|Faster colonization, Protection against shock loads, Better diffusion gradients and Development of specific biomass
|Surface Roughness and adsorbing capacity
|Lower degree of filling
|Better mixing, Better mass transfer and Better fluidization.