Activated carbon in conjunction with biological treatment has been widely used in water treatment and water reclamation applications for the removal of organic contaminants for over three decades. Such integrated adsorption-biodegradation treatment systems involve either addition of powdered activated carbon (PAC) to activated sludge or by granular activated carbon (GAC) adsorbers with immobilized biofilms. The main disadvantage of such processes is the expense associated with thermal regeneration (particularly relevant to GAC), as well spent adsorbent replenishment and disposal. Bioregeneration presents a cost-effective alternative to the above as it enhances the operational life-span of activated carbon. Bioregeneration is dependent on the biodegradability of the adsorbed compound as well as the adsorbability of the pollutant and reversibility of adsorption. It can only occur with adsorbates that are desorbabale, and is not observed in the case of activated carbon loaded with non-desorbable compounds. Two mechanisms are generally proposed to explain bioregeneration of activated carbon. The first involves the biodegradation of organic compounds released from the activated carbon into the liquid phase. The adsorbed organics are desorbed due to the concentration gradient between the carbon surface and the bulk liquid. This mechanism also involves the difference in Gibbs free energy between the molecules in solution and those modified in the porous adsorbent. The second mechanism involves extracellular enzymes excreted by microorganisms that diffuse into the activated carbon pores and react with the adsorbed substrates. The decay of the adsorbent might occur, or the enzyme metabolite may undergo desorption due to weaker adsorbability.
The effectiveness of activated carbon bioregeneration depends on numerous factors besides reversibility of adsorption and biodegradability of the adsorbate(s). These factors include the microbial pouplation's ability to biodegrade the substrate(s), optimal microbial conditions (temperature, dissolved oxygen levels, nitrogen and phosphorus levels, and microorganisms-substrate stoichiometric ratios), reactor residence time for activated carbon particles, and spatial distribution of adsorbate(s) within adsorbent pores. Also important are the activated carbon particle size, porosity, pore size distributions, type of activation, and physico-chemical surface properties including functional group characteristics and charge distributions, because they influence the desorption and biodegradation kinetics of substrates.
Integrated processes involving multi-component mixtures and competitive adsorption present more complexity with regard to evaluating adsorbent bioregeneration for each component. The presence of natural organic matter (NOM) in aqueous systems not only affect the adsorption of organic contaminants, but also influence the bioregenerabilty of activated carbon. The NOM are predominantly comprised of humic and fulvic substances besides proteins, carbohydrates and fatty acids, and have the ability to form complexes with organic constituents and inorganic ions. These organic complexes can be either be strongly or weakly adsorbed on activated carbon, and so the binding of organic constituents with NOM can affect their desorbability. Additionally, the NOM molecules can compete for adsorbent sites and can promote desorption. Furthermore, the NOM adsorbed on the activated carbon pores could also affect the enzyme activity and adsorbent bioregeneration. The presence of inorganic species such as calcium and magnesium ions can significantly affect the adsorption of NOM and other organic substrates. Calcium and magnesium ions also form complexes with macromolecules of humic and fulvic acids (constituting NOM), and these complexates are compacted molecules that are better adsorbed on activated carbon. The increase in NOM adsorption may result in lower and weaker adsorption of organic constituents due to displacement mechanisms, thereby influencing their desorption and bioregeneration. Additionally, inorganic species may form precipitates such as calcium carbonate, that might clog the adsorbent pores and reduce its capacity.
An important aspect of the present study is the qualitative and quantitative evaluation of the extent of activated carbon bioregeneration in bioactive PAC or GAC employed in integrated adsorption-biodegradation processes. This is performed by using an adsorption-biodegradation model used for predicting the dynamics of bioactive carbon systems. The model compounds chosen for testing the proposed model and evaluation of bioregeneration are phenol, para-nitrophenol and toluene, besides trichloroethyelene. The choice of these compounds is based on the gradation in adsorption and biodegradabaility characteristics. While phenol is moderately adsorbable on activated carbon and easily biodegradable, para-nitrophenol is strongly adsorbable but is less biodegradable than phenol. On the other hand, toluene is less adsorbable than phenol and more biodegradable than para-nitrophenol. Trichloroethylene is less adsorbable than more resistant to microbial degradation than these compounds. The governing equations of the adsorption-biodegradation model will be analyzed from the standpoint of adsorbent bioregeneration, taking into consideration the important factors affecting the phenomenon. The modeling scenario involves only adsorption with biofilm degradation, wherein biological activity in the suspension phase shall be suppressed. The net substrate flux into the adsorbent is an important variable that can provide the magnitude and direction of substrate transport into the adsorbent, and its estimation is incorporated into the computational algorithm. A three-dimensional perspective of simulated concentration profiles at node points in the carbon particle, biofilm phase, and bulk solution shall be presented to gain more insight into bioregeneration. The magnitude and direction of the net contaminant transport flux into activated carbon at the biofilm-adsorbent interface will provide a more in-depth understanding of bioregeneration dynamics. Additionally, the the role of bioregeneration will be highlighted in comparison with pore and/or surface diffusion of adsorbate(s) into activated carbon, as well the biofilm diffusivity and fluid-film transfer from the solid phase to the bulk liquid-phase. Furthermore, the effect of background NOM and divalent cations (such as calcium and magnesium) on the bioregeneration of activated carbon will be addressed.