The principle of the bioreactor is, under controlled conditions, to increase the contact surface between the contaminants present in the target matrix and the microorganisms responsible for their biodegradation present in the system.
Two biological reactor processes are primarily used for the treatment of contaminated water, the suspended or fixed film systems. In both cases, the water extracted from the site is brought into contact with microorganisms adapted for the biodegradation of the targeted contaminants, circulating in an aeration pond or on a biofilm. Microorganisms may be native to the site or from an inoculum.
Water is the main matrix to be treated, but slurry phase bioreactors can also be used for the treatment of contaminated soil, sediment, sludge or other solids. In these cases, the contaminated soils or other solids are first excavated and sieved to retain only the fine fractions. These are mixed with water and other substrates to keep them in suspension during the treatment and in contact with microorganisms to promote biodegradation.
Other types of reactors are also under development, depending on the type of contaminant to be treated.
In a suspended film system, the contaminated water or that containing the fine fraction, circulates in a pond or aeration tank containing suspended microorganisms. The microorganisms are kept in suspension by means of pneumatic aeration or mechanical agitation. Biodegradation of contaminants usually occurs during aerobic processes, although anaerobic processes are also possible. The biomass resulting from the process can be clarified and returned to the system or accumulated as sludge for disposal.
These systems can take different configurations:
The activated sludge reactor includes a sludge of organic material and a population of microorganisms that can treat contaminated water under aerobic conditions.
The fluidized bed reactor uses granular or solid materials (sand, activated carbon or pearls) on which microorganisms are present which are suspended in the water to be treated in order to ensure contact between the latter and the contaminants. Effluents from this system are continuously recycled into the system.
In a sequential biological reactor, treatment is based on time rather than flow in terms of contact between microorganisms and contaminants. In this system, all contaminated water is included from the beginning (batch), treated for the removal of contaminants and rejected. Microorganisms are included in the system during the process. The sludge created is then aerated until the reaction is completed.
Membrane bioreactors make it possible to combine the clarification, aeration and filtration steps within the same process. This technology is considered very powerful, leaving a minimal footprint and having a simplified operation.
Fixed film bioreactors are available in several configurations such as rotary filter bioreactors, where microorganisms are attached to a circular disk that rotates in the contaminated affluent to initiate treatment. Bacterial bed reactors include a permeable matrix (rock, plastic or wood), a water distribution system and a drainage system. Contaminated water flows at a constant rate to promote contact with the matrix and promote the development of a biofilm. Fixed film bioreactors are characterized by large areas that promote microbial colonization. This support is generally adsorbent, which retains the contaminants while releasing them slowly so that the microorganisms can degrade them.
The size of the bioreactors can vary by several orders of magnitude depending on the volume of water or solids to be treated. In all cases, the environmental conditions within the bioreactor must be controlled to optimize the biochemical activity levels of the microorganisms.
The implementation of a bioreactor may include:
- Mobilization, access to the site and setting up temporary facilities.
- Establishment of a system for pumping contaminated groundwater, including wells, collection trenches and/or permeable drainage as well as pumping equipment.
- Excavation of contaminated soils or sediments and the installation of equipment necessary to separate coarse and fine fractions.
- Establishment of the necessary equipment for the mixing of fine particles with water.
- The installation of bioreactor equipment (may require the construction of a building or container).
- The establishment of tanks for the recovery of sludge resulting from treatment or treated soils.
- Vapour treatment equipment resulting from the treatment process as required.
- Discharge equipment for treated water (for reinjection into the ground using wells or trenches for the connection to a sewer system or into surface water).
This type of system should generally be piloted tested prior to full-scale implementation.
Materials and Storage
- The development of wells, collection trenches and/or permeable drains is performed using traditional/readily available methods and equipment readily available for installation of wells, drainage systems, water supply or public utility.
- The bioreactor can be built on-site or pre-assembled and delivered to the site inside a container, trailer or truck.
- The operation of the bioreactor requires energy and products such as substrates, water, nutrients, inoculum, etc.
- Installation activities for this type of system generally have little impact but may require on-site storage, notably for residues produced (treated soil, biomass, sludge, etc.).
Residues and Rejects
In general, bioreactors destroy contaminants. Some residues such as sludge may be produced during this treatment.
Installing systems for pumping water or excavating contaminated soil to be treated can lead to the handling of contaminated soils that may need to be disposed of off-site.
Soils resulting from the treatment and sludge from the reactor must be recovered, returned to the site or disposed of off-site. The nature of these rejects will have to be determined in order to make an adequate disposition.
Treated groundwater must meet the applicable criteria for its discharge. However, releases may contain by-products or an unacceptable pH that may be hazardous to the receptors and require a polishing step.
Potential vapours from the treatment may also constitute an effluent from the bioreactor.
Composed by : Claudie Bonnet, M. Sc. , National Research Council
Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada
Updated Date : April 12, 2016
Latest update provided by : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée
Updated Date : March 22, 2019