A continuous flow bioreactor is a well-stirred vessel containing microorganisms (X) through which a substrate (S) flows at a continuous rate. The microorganisms grow in the vessel through consumption of the substrate to produce more microorganisms and a product (P). Unused substrate, microorganisms, and the product flow out of the reactor. In the treatment of industrial wastewaters a continuous flow bioreactor of this description is also known as an `aeration only complete mixing activated sludge system' or a `conventional sewage sludge digester'.
In a bioreactor with recycle the effluent emerging from the reactor is fed into a settling unit. Microorganisms settle to the bottom of the tank, from where they are recycled into the reactor vessel. As a consequence of settling the concentration of microorganisms leaving the settling unit in the recycle stream is higher than that entering it from the biological reactor. The wastewater is removed as the effluent from the settling tank. The settling of the microorganisms greatly reduces their concentration in the effluent leaving the settling unit, producing a cleaner effluent stream. Recycle enables a higher concentration of microorganisms to be maintained in the bioreactor, which allows the reactor to run at much greater flow-rates and increases its efficiency. This process is illustrated in Figure 1.
|Figure 1. A bioreactor with recycle and separate wasting of biomass|
In the treatment of industrial wastewaters recycle is also known as `sludge return' and a continuous flow bioreactor operating with recycle is known as the `activated sludge process'. Settling units can be divided into two types, depending upon whether all the microorganisms is discharged in the effluent stream, w=0 in figure 1, or if there is separate wasting of microorganisms after the reaction mixture has passed through the settling unit, 0<w<1 in figure 1. The latter produces an effluent with lower suspended solids.
Some advantages of investigating biological processes using continuous flow bioreactors were identified, independently, by a number of researchers in the 1920s (Felton et al, 1924; Haddon, 1928; Moyer, 1929; Rogers & Whittier, 1930). Experiments in which microorganisms are constrained to remain in the reactor date back at least to 1930 Rogers & Whittier, 1930). Continuous flow reactors, and continuous flow reactor models, came to prominence in the study of the growth of pure microbial systems (Monod. 1950; Novick & Szilard, 1950; Herbert et al, 1956; Herbert, 1958; Marr et al; 1963; Dawes & Ribbons, 1964; Schulze & Lipe, 1964; Pirt, 1965; Wase & Hough, 1966)
Flow reactors have long been used in the treatment of industrial wastewaters, where the objective is to reduce the concentration of a soluble organic substrate. One advantage that they offer over other types of reactors is that they produce a greater operational stability in response to toxic or shock loads (McKinney, 1962). This is because mixing dilutes spikes in toxicity levels (`shock loads') across the whole of the reactor volume. Starting in the 1960s the models developed for pure microbial systems, (described elsewhere), were used to model waste treatment facilities (McKinney, 1962 McCarty, 1966) Wastewater is a complex mixture of many substrates and microorganisms. Formally, the use of a model containing a single substrate and a single microorganism can be justified if the overall process kinetics are controlled by a process-rate limiting step . A flow-reactor with recycle is the simplest representation for the biological oxidation of wastewaters from industrial processes (McCarty, 1966).
Bioreactors sometimes employ a permeable membrane, such as a microfiltration membrane, to physically retain microorganisms inside the reactor. The higher concentrations of microorganisms obtained leads to greater pollutant removal, allowing for a more rapid and efficient process. In an ultrafiltration membrane reactor the membrane also retains solids and high-molecular weight compounds that are found in the effluent from a conventional activated sludge reactor. Thus the quality of the water delivered by a membrane reactor can be significantly cleaner than that emerging from conventional reactors. Due to these advantages membrane reactors have increasingly been used as key elements of advanced wastewater processing schemes. The higher quality water that they can produce and their compactness compared to conventional reactors make membrane reactors particularly suitable for the development of domestic wastewater treatment facilities in urban areas (Wisniewski, 2007). Membrane reactors are also finding increasing use in many practical applications across the fields of bioprocessing engineering and chemical engineering.