General introduction to the selective bio-treatment process design

The term of bio-augmentation is a generic name for the addition of microorganisms to bioreactors in order to improve the existing biomass activity and performance within the reactor. Bio-augmentation is used for:

  • Preferential degradation of specific compounds (targeting hard-to-biodegrade contaminants and toxic contaminants)
  • Enhancing BOD removal, with emphasis on polymeric BOD
  • Enhancing oil and grease removal, which are considered hard-to-biodegrade COD
  • Enhancing sludge degradation, reducing sludge release
  • Improving settling of solids (sludge) – SSV
  • Enhancing nitrification/de-nitrification
  • Odor reduction
  • Rapid bioreactor start-up
  • Rapid recovery of biomass in the reactor after mortality due to toxicity shocks or other causes
  • Lowering the intensity of bio-process stress episodes

One of the greatest benefits of using a bio-augmentation treatment approach is the ability to target selective contaminants.

However, achieving bio-augmentation treatment success is a challenge, due to several factors that prevent the accomplishment of a sufficient selective biomass that is necessary for performing the desired bio-chemical catabolic process. Selective culture accumulation failure is mainly caused by negative interactions of bio-augmented cultures with the natural local micro-flora (i.e., predators and toxicant secretion) and by continuous dilution of the culture due to constant flow of influents into the bioreactors, which leads to the washing out of the introduced culture.

Bacterial immobilization methods have been developed for overcoming most of the abovementioned obstacles. Encapsulation of bio-augmented bacterial cultures and their immobilization in beds overcome the continuous dilution and also provide some protection against predators found in the mixed liquor. These immobilization methods therefore enable the accumulation and achievement of a sufficient biomass for the degradation of selective contaminants.

Small particles, such as bacteria, present some challenges for their implementation. These include bacterial viability, transportation and site control. The SBP (Small Bioreactor Platform) technology presents a successful macro-encapsulation (2.5 cm long) method for microorganism growth and administration. The key component of the SBP capsules is a structural micro-filtration membrane that enables the generation of a confined aquatic environment for microorganism growth and prosperity. The capsule membrane acts as a physical barrier between the internal medium that holds the particle and the external medium. This membrane enables holding the encapsulated microorganisms inside an aquatic environment in a suspended state, while preventing external predators and other microorganisms from infiltrating into the internal medium of the capsule. The membrane allows water-dissolved molecules to traffic across at an unexpected relatively fast diffusion rate, almost at the same rate as without a membrane. This enables application of the technology to the field of water and wastewater treatment. To the best of our knowledge, the SBP technology presents the fastest biodegradation kinetic rate among current encapsulation formulas. Furthermore, to date no-one has succeeded in developing and manufacturing a large and stable particle (bigger than 10 mm) that can resist hostile environments (high shear forces environment) such as those that exist in the mixed liquor of domestic and industrial bioreactors. The SBP capsule retains its activity for at least two months, after which it degrades into a sugar, and thus leaves no environmental fingerprint. The technology allows us, for the first time, to achieve control of three important microorganism parameters: 1. The culture type that will be introduced. 2. The amount of culture (biomass) that will be introduced. 3. Culture controlling site implementation. We have already demonstrated that the SBP technology enables the targeting of selective contaminants, while avoiding the need for removal of other organic matter that can affect treatment cost.

 

SBP capsule illustration

The SBP capsule contains several elements: the external barrier made of cellulose acetate microfiltration membrane, aquatic interface which include inside the exogenous inoculums and nutrients supplemental agar. The SBP capsules are marketed in a dry state (inactive product), and need to be activated by hydration, prior the capsule activity. Each host bioreactor can have several thousands of capsules which encase in cartridges or perforated cages. The ratio of SBP capsules numbers to wastewater volume, depends mainly on the target contaminants (type & concentration), and the hydraulic time retention.

 

Illustration of SBP capsule microfiltration membrane

 Microfiltration membrane encasing the selective bacterial culture within a given medium (mixed liquor). The SBP capsule's microfiltration membrane physically separates the introduced microbial culture inside the capsule from the outer microorganisms (i.e predators) while enabling the trafficking across the membrane of the dissolved molecules and organic matter (i.e phenols, hydrocarbons, nitrogen compounds ect.).

 Mechanisms of action

The SBP technology is an innovative technology integrating engineering and microbiology into a solution that enables us to adapt and accomplish a sufficient biomass of unique bacterial cultures. The SBP capsules encase specific microorganism cultures which specialize in the biodegradation of various contaminants.

Role of the micro-filtration membrane: The SBP capsules are coated with a semi-permeable membrane (micro-filtration membrane) which allows only dissolved molecules and compounds to traverse across the membrane, while the microorganisms are kept inside under favorable microcosmos conditions. The physical barrier provides the exogenous bacterial culture protection against preying and negative interactions associated with wastewater microorganisms (i.e., protozoa, bacteria). This leads to a significant reduction in the natural selection forces against the introduced culture.

The SBP membrane contains two dimensions of molecule-penetration mechanisms: fast flow in the funnel-like pores (large molecules and colloidal particles as well as small molecules) and slower diffusion through the nano-mesh structure, due to the concentration gradient which enables trafficking of small molecules, thus enabling the achievement of a relatively high biodegradation rate. As the bacteria consume the contaminant and mineralize it into carbon dioxide, the internal medium concentration of the contaminant decreases and fresh contaminant molecules diffuse from the outer medium into the the internal medium of the SBP capsules. Consequently, there is no need to expend energy in bringing the contaminants into the SBP capsules. They will automatically traffic into the capsule in accordance with the basic physical law of diffusion. Small molecules will diffuse through the large pores and through the nano-mesh membrane wall which enable a rapid molecules trafficking rate across the SBP membrane.

The SBP capsule membrane has a very low affinity for biofilm formation on its surface, and the membrane channels thus do not become blocked. Our experience has shown only a negligible amount of attached biofilm on the SBP membrane after its immersion in a sanitary mixed liquid for one month.

 

High resolution scanning electron microscope (HRSEM) images of the capsule's outer surface. Based on the images, it is suggested that the cellulose acetate (CA) membrane exhibits two types of channels: Constructed large pores varying in size from 1 µm to 10 µm in diameter (average ~2 µm). These large pores are present mainly on the surface of the membrane. Since the membrane wall thickness is approximately 400-500 µm, these pores are funnel-shaped and their inner hole has a diameter of approximately 0.8 µm. A higher magnification (X50,000-100,000) reveals that the membrane is constructed in a mesh-like packed structure, with nano-metric channels (~50-150 nm) which are critical for the filtration process.

HRSEM images of the outer surface of the CA SBP capsule: A) X1,000, B) X10,000, C) X10,000, and D) X100,000.

Bacterial culture growth state inside the SBP capsule: To the best of our knowledge, the SBP capsule is the only technology that allows the introduced bacterial culture to grow and proliferate in a suspended/planktonic growth state. This affords several advantages:

  1. Fast metabolic rate.
  2. No physiological alteration of the bacterial cell.
  3. Rapid digestion rate.
  4. Achievement of a high bacterial culture concentration within the SBP capsule.

Advantages 1 and 4 enable us to achieve a rapid biodegradation rate of hours that is sufficient for considering the SBP technology as a practical solution in DWWTPs/SWWTPs/STPs.

Once the bacterial culture reaches a certain concentration (approximately log 9 to log 10 CFU/mL), the bacterial culture enters a stationery growth state where the proliferation rate is approximately equal to the death rate. This mechanism allows long-term (months) activity of the bacterial culture in the presence of nutrients. After a period of a few months (depending on medium type and shear forces intensity), the cellulose membranes degrade into sugar and the SBP capsule vanishes, leaving no environmental fingerprint. Thus, it is not necessary to remove the old SBP capsules from the treated medium. This simplifies the implementation of the SBP treatment. All that is needed is a periodical (once a month) addition of new SBP capsules into the treated medium in order to replenish the encapsulated biomass.

Confined environment: The SBP structural micro-filtration membrane creates a confined aquatic environment for the introduced culture. The confined aquatic environment enables the provision of preferable conditions to the introduced culture. This results in better culture adaptation and persistence within the bioreactor medium and allows rapid achievement of a high concentration of the viable biomass (within hours). Approximately 2 mL of concentrated suspended bacterial culture medium are encased by the micro-filtration membrane, creating a basic unit of activity. The SBP capsules encase an internal water body that is connected to the host aquatic medium by water channeling (i.e., capsule membrane pores). These connecting channels enable the transport of outer nutrients into the confined aquatic medium that encases the introduced bacterial culture. The same mechanism serves for the evacuation of the SBP’s internal waste into the outer medium. The bacterial cells are not immobilized to a polymer, or to some other biological or chemicals matrix. They are suspended within the internal aquatic medium of the SBP capsule. The use of a protected suspended bacterial cell culture is assumed to preserve normal cell physiology and activity.

Dormant bacterial culture activated upon water exposure: The SBP capsules are provided in a dry state with a dormant bacterial culture. Once the SBP capsules are introduced into the water which penetrates into the capsules, the bacterial cells become active. It is assumed that the confined water body within the SBP capsules is similar to the host aquatic medium. The internal biomass therefore has an initial adaptation period (few hours), prior to entering the proliferation stage (log phase).

Nutrient supplementation: The SBP capsules contain some nutrients in order to increase the internal biomass recovery and proliferation rate.

SBP technology implementation: The SBP technology is usually implemented in aerobic bioreactors. Introduction devices are used to introduce several thousands of SBP capsules into the WWTP host bioreactor. The introduction devices are composed of two components: a crane and a perforated cage that encases the SBP capsules. Different wastewater sources require specialized products, tailor-made to treat specific contaminants.

 

Read more about the comparison between the SBP technology encapsulation method and other macro and micro encapsulation methods.

Read more about the SBP technology in scientific publications in peer-reviewed journals.