Numerous reactor designs for biological sulfate reduction have been reported, such as batch reactors, sequencing batch reactors, continuously stirred tank reactors, anaerobic contact processes, anaerobic baffled reactors, anaerobic filters, fluidized-bed reactors, gas lift reactors, upflow anaerobic sludge blanket reactors and anaerobic hybrid reactors.
The reactor configuration has implications for the ratio of sludge retention time/hydraulic retention time (SRT/HRT) in continuous flow reactors. The loading rates of a process are largely dictated by the biomass retention in the reactor. Maximal sludge retention or biomass retention is desirable for process stability and minimal sludge production. Minimal HRT minimizes the reactor volume and thus reduces capital costs. Continuously stirred tank reactors (CSTR) are subjected to washout of active biomass (Figure 1). Biomass retention has been enhanced by employing internal sedimentation systems and cationic flocculants. Anaerobic contact process (ACP) relies on biomass separation and recycling to increase the SRT/HRT. Several methods have been suggested for recovering biomass from the reactor effluent, including sedimentation, flocculation, centrifugation and magnetic separation of sulfate-reducing bacteria.
Figure 1. Continuously stirred tank reactor (CSTR) and anaerobic contact process (ACP).
Due to the slow growth rate and low biomass yield of SRB, various immobilized biomass reactors have gained increasing attention. In anaerobic filter reactors (AFR) (or packed bed reactors, PBR) biomass is retained as a biofilm on packing material as well as unattached in the packing interstices. AFRs have been operated in horizontal, upflow or downflow modes (Figure 2). The downflow AFR allows the utilization of gravity and, thus, passive operation. Packing materials used in AFRs include cobbles, polypropylene pall rings, glass beads and alkaline minerals. Biological sulfate reduction has been enhanced with solid organic materials as well as liquid substrates. Solid substrates have a limited lifetime and have to be replaced or supplemented with liquid substrates once the original substrate is depleted. The main shortcomings of AFRs are the channeling of the flow and clogging of the bed by precipitates.
Figure 2. Anaerobic filter reactors (AFR) used in horizontal, upflow and downfow modes.
In the fluidized-bed reactor (FBR), channeling and clogging are avoided by fluidizing the inert biomass carrier (Figure 3). Fluidization can be carried out with recycle water or using a gas stream in which case the reactor is called a gas lift reactor. Carrier materials used include iron chips, synthetic polymeric granules covered with iron dust, pumice particles, porous glass beads, and carbon dust. The fluidized carrier enables efficient mass transfer and provides a large surface area for biofilm formation.
Figure 3. A schematic diagram and a photo of a laboratory scale fluidized-bed reactor (FBR).
In upflow anaerobic sludge blanket (UASB) reactors, biomass retention is based on good settling characteristics of granular sludge (Figure 4). The presence of methanogens in the biomass can enhance granulation. The produced biogas is trapped by a hood located below the water surface and can be periodically burned in a flare. Due to the biomass granulation, no packing or carrier material is needed which reduces the start-up costs of the UASB compared to AFR and FBR. However, extensive biogas production may require extra instrumentation which increases capital costs. Moreover, methanogens compete for substrates (acetate and H2 + CO2) with sulfate reducers, resulting in decrease in the yields of H2S and alkalinity per amount of substrate added. Other problems encountered with UASB reactors are poor or slow granulation and the rapid disintegration of the granular sludge under certain conditions.
Figure 4. A schematic diagram and a photo of a laboratory scale upflow anaerobic sludge blanket reactor (UASB).
The anaerobic hybrid reactor (AHR) is a combination of UASB and AFR, where the granular sludge bed is in the lower section of the reactor and packing material in the upper section (Figure 5). The packing material improved the separation of solids from the reactor effluent. Another modification of the UASB reactor is an anaerobic baffled reactor (ABR) which is a staged reactor where biomass retention is enhanced by forcing the water through several compartments (Figure 5).
Figure 5. Anaerobic hybrid reactor (AHR) and anaerobic baffled reactor (ABR)
Depending on the reactor type and process configuration, the metal sulfide sludge can be recovered from the bottom of the bioreactor, such as AHR, by back washing the AFR at regular intervals or with a clarifier downstream of the precipitation unit.