Biological Drinking Water Treatment: Microbiological Considerations For The Operation And Control Of Biofilters
By Alejandro Ureta, PhD, Molecular Biology, Biochemist, State Sewage & Water Works, Uruguay
It’s well known the beneficial role that particular groups of microorganisms have in the food and beverage industry. Similarly are the beneficial aspects that either engineered or non-engineered biological treatment systems have in the drinking water production process. In this context, biological treatments rely on the metabolic ability of microbial communities or particular groups of microorganisms to efficiently catalyze or facilitate the removal or inactivation of contaminants potentially present in surface or groundwater sources. Among the contaminants targeted in this way we find natural organic matter (NOM), including biodegradable organic carbon (BDOC)/assimilable organic carbon (AOC); disinfection byproducts (DBPs) and their precursors; toxins; taste/odor causing molecules; pesticides; herbicides; and inorganic contaminants, such as ammonia, nitrates, perchlorate, and metals such as iron, manganese or arsenic.
The biological treatment in the production of drinking water occurs through the process of biofiltration in which microbial communities are immobilized in a support bed material in contact with the water phase. The most commonly designed configuration for this process is the fixed-bed system, although fluidized-bed and membrane systems are also used. Fixed-bed biofilters can use different support or attachment materials that can be classified as non-sorptive (e.g., sand and glass beads) or sorptive such as granular activated carbon (GAC). Different fixed-bed filtration processes or procedures have been described for the biological treatment in the drinking water production industry. Examples include:
Slow Sand Filtration (SSF): This process involves the filtration of water at rates that determine the accumulation of organic matter at the surface of the filter media. This layer is able to support the colonization and maintenance of microorganisms that in turn become the predominant filtering mechanism of the system. This biologically active layer is called the “schmutzdecke” and works as the primary biofiltration component to remove potential contaminants present in the incoming water.
Rapid Sand Filtration (RSF): RSF uses a relatively higher surface loading rate compared to the SSF process. In this configuration microorganisms are able to proliferate deeper in the filter media compared to the SSF configuration. In order to maintain biological activity, disinfectants should be added downstream of the filter media or if added upstream their levels should not allow the maintenance of the disinfectant residual through the filter bed.
Biological Activated Carbon Filtration (BAC Filtration): In the water industry granular activated carbon (GAC) is widely used for the removal of NOM and specific organic contaminants. This removal relies both in the physical adsorption and/or biotransformation (assimilation, redox modifications) of contaminants. Compared to sand or anthracite filters, GAC can maintain higher microbial concentrations.
Several modifications from the strategies referred above are commonly used in the process of drinking water production. Examples include:
Ozone-Enhanced Biological Filtration (OEBF): To facilitate the removal or inactivation of several contaminants and/or improve aesthetic aspects of the influent water, several drinking water production facilities pre-oxidize influent water with ozone. This treatment could determine a significant increase in its BDOC/AOC levels. In such systems, a biofiltration step (either sand and/or GAC biofiltration) downstream ozonation is usually required in order to decrease the concentration of BDOC/AOC in the corresponding effluent decreasing the risk of microbial regrowth in distribution systems
Biological Reductive Removal of Perchlorate or Nitrate: These processes are based in the reduction of these contaminants into chloride or nitrogen gas respectively. In this context both perchlorate and nitrate behave as terminal electron acceptors for the respiratory chain of particular groups of bacteria. For these activities to work efficiently an adequate anoxic or anaerobic (micro) environment is required as well as the presence of nutrients (e.g., phosphate) and electron donors, usually added exogenously.
The reductive removal of these contaminants can occur in various system configurations including fixed-beds (e.g., BAC), fluidized beds, and membrane systems and is usually followed by an aeration step in order to facilitate the removal of excess BDOC/AOC caused for instance by the upstream addition of organic compounds as electron donors. Beside the use of organic electron donors, the addition of inorganics such as hydrogen gas or elemental sulfur have been described for the reductive treatment of these contaminants.
Irrespective of the biofilter configuration or process followed, the filter’s biomass resides primarily in biofilms, a complex microbiological community in which microorganisms are encased and immobilized in a self-secreted matrix of hydrated extracellular polymeric substances (EPS). In itself, and by allowing the accumulation and use of environmental organic and inorganic molecules by the resident microbial community, this matrix is a source of nutrients (carbon, nitrogen, phosphorous, etc.) for their microbial residents. The EPS matrix, that can have depths of several tens or hundreds of micrometers, behaves as a physical barrier for antimicrobials or as an ion exchange media for mineral precipitation and accumulation of toxic metals. In this regard, chlorine concentration profile gradients obtained as a function of biofilm depth shows a sigmoidal shape where the inflection point is located at the biofilm surface, indicating a decrease in chlorine concentration as a function of the matrix depth.
Despite biofilms can be formed by single microbial species, in most natural or artificial systems they are composed by mixed species consortia of either a restricted group of microorganisms (even though a few species can be numerically and/or functionally dominant) or by groups of phylogenetically highly diverse microorganisms. As the microbial cells adapt to grow within biofilms, they express phenotypic traits that are often distinct from those that are expressed during planktonic or free-living growth. Depending on the species of bacteria, these phenotypic differences can be manifested as variations in colony morphology under culture conditions, intrinsic resistance of “released” cells to antimicrobials (disinfectants or antibiotics), modified rate for the consumption of organic or inorganic compounds, facilitated horizontal gene transfer, etc.
Biofilm development has three distinct stages: 1- attachment of cells to a surface, 2- growth of the cells into a sessile biofilm community, and 3- detachment of mass of cells from the community into the surrounding medium. The surface attachments of bacterial cells involves an initial weak, reversible interaction between a bacterial cell and the corresponding surface that is later reinforced by the strong and irreversible adhesion mediated by cellular components located on the bacterial cell surface or on cellular appendages such as pili and fimbriae. The second stage of biofilm development involves the multiplication of bacteria on the surface and the concomitant synthesis of an extracellular polymeric matrix. As mentioned before the matrix holds the bacterial cells together in the EPS mass and firmly attaches the bacterial community to the underlying surface. Biofilm detachment and the consequent microbial dispersal can be triggered by biological signals (referred as active or regulated dispersal) or non-biological cues (e.g., water flow regimes). Active dispersal from biofilms can be precisely regulated and it’s typically (but not in all species) preceded by localized death and lysis of cells in the center of mature biofilm structures. Because of the heterogeneous nature of the cells in the mature biofilm, only a subpopulation of cells will undergo lysis. These killed cells provide nutrients for the bacteria that will become the dispersal cells. In turn, the dispersal cells “escape” by coordinated evacuation from breakup points, leading to the characteristic hollowing of biofilm micro-colonies that are observed during the dispersal stage of many biofilms. The signals that trigger active dispersal include alterations in the availability of nutrients, sudden changes in oxygen levels, temperature, or metals such as iron, and determine the induction of effectors such as polysaccharide-degrading enzymes, chitinases, proteases, and nucleases. Additionally, there are several endogenous signals that induce biofilm dispersion such as acyl-homoserine lactones, autoinducing peptides, diffusible fatty acids, and D-amino acids.
Operational and Design Considerations
By definition, biofilters operational and control procedures are based in the maintenance of its bacterial community, in the conservation of its associated (desired) activity (e.g., removal of organics/inorganics, etc.) and in the assurance of the physical, chemical, and biological quality of the corresponding effluent. Among others, important factors in developing and controlling biofilter performance include biofilter maturation time, operational temperature, adequate oxygen levels, redox control, appropriate contact time contact time between the fluid and fixed phases (empty bed contact time - EBCT), and proper backwash conditions.
The timeframe for a biofilter to reach steady state biological activity is an important design parameter. For instance the acclimation period in sand filters to achieve consistent manganese removal can be up to two months. EBCT refers to the time required for the influent water to move through the media and is considered a vital component for effective biofiltration. EBCT is based on the loading rate and the volume of the filter media. As an example, an EBCT of 10 to 20 minutes has been shown to remove 90 percent of biofilter influent organic matter. However optimal EBCTs may be determined by the composition of the biofilm microbial consortia as well as the nature and levels of contaminants to be removed. Concerning temperature, it has been shown for instance that while ammonia removal efficiency by biofiltration can reach up to 90 percent at temperatures ranging between 4 to 10º C, it decreases to about 30 percent at temperatures below 4º C. Regular backwashing is necessary for the continuous operation of fixed-bed bioreactors, the operating procedure for backwashing may affect reactor performance. For example, if backwash water contains chlorine, when other operating parameters are not optimal, the reactor performance will be impaired, similarly, backwash intensity and frequency can also affect reactor performance as well as the distribution and/or activity of the microbial communities present in the bioreactor.
Questions still remain regarding the capacity and safety of engineered biofiltration in the context of drinking water production. In this regard, while studies on biological drinking water treatment have mainly focused on removing the target of interest, they have not extensively addressed other characteristics of effluent quality that might be affected by the biological treatment processes. For instance, considering that either microbes or adsorbed contaminants from the biological treatment process could end up in the reactor’s effluent, more data is needed about its microbiological quality.
As referred before, the addition of external electron donors is required for many biological drinking water treatment processes. Considering they may promote the overproduction of EPS (a major contributor to biofilter clogging either by directly filling binding media and filter void or indirectly by accelerating the deposition of minerals into biofilms), and/or reach bioreactor’s effluents, more studies addressing the optimization of their addition in these treatment systems are needed.
In conclusion, in view of its low operating cost, high water recovery and a potentially efficient simultaneous multiple contaminant removal, a better understanding and control of particular biofiltration processes would be necessary for a more widespread acceptance and use of engineered biofiltration steps in the production of drinking water.