i i i i i
* * i + *
Figure 13.13 Downflow stationary fixed film process.
Figure 13.13 Downflow stationary fixed film process.
storage and/or use. The media used in DSFF systems is similar to that used in AF systems, although higher specific surface area media (on the order of 140 nr/m') may be advantageous because of the importance of attached growth. As with many other high-rate anaerobic treatment processes, loadings are generally expressed in terms of both HRT and VOL. Typical HRTs for the DSFF process range from 0.5 to 4 days and VOLs range from 5 to 15 kg COD/(m !-day), both of which are similar to the AF process.
Fluidized Bed and Expanded Bed. Fluidized bed and expanded bed systems differ from those previously considered in that they are essentially attached growth systems with little or no suspended growth/'" As illustrated in Figure 13.14, FB/' EB systems use upflow bioreactors, just like the UASB, AF, and hybrid UASB/AF processes, but the upflow velocities are much higher, resulting in minimal retention of suspended biomass. Instead, the biomass grows attached to granular carrier particles that are fluidized by the upflow of influent wastewater and recirculated effluent. Fluidization is discussed in Section 18.2. The carrier particles are often silica sand with a diameter in the 0.2 to 0.5 mm range and a specific gravity of 2.65 or granular
Fluid ized/Expanded Media Bed
Media and Biomass to Separation
Cleaned/ Recycled Media
Figure 13.14 Fluidized bed and expanded bed process.
activated carbon. In FB/EB systems upward flow rates are sufficient to support the individual carrier particles with their attached biomass (bioparticles), resulting in expansion of the bed volume in comparison to its resting volume. In expanded beds the upflow velocity is sufficient to expand the bed by 15 to 30%. At this degree of expansion the bioparticles are supported partly by the fluid and partly by contact with adjacent bioparticles and, consequently, they tend to remain in the same relative positions within the bed. In fluidized beds a higher upflow velocity is used, resulting in further expansion of the bed to between 25 and 300% of its resting volume. Under these conditions the bioparticles are fully supported by the upward flowing fluid and move freely in the bed. Gas production also results in turbulence that tends to mix the bioreactor contents. Gas-solids separation devices are provided in some cases to allow the bioparticles to be retained in the bioreactor.
The turbulence created by the upward fluid flow and the gas production allows small carrier particles to be used without bioreactor plugging. It also encourages high mass transfer rates. The use of small carrier particles results in a high specific surface area and a high active biomass concentration. For expanded bed bioreactors, the specific surface area of the carrier particles is in the 9,000 to 11,000 m:/m' range, with a void volume of 45 to 55%.2,'M For fluidized bed systems the specific surface area is in the 4,000 to 10,000 nr/m' range, with void volumes of 50 to 90%, depending on the degree of expansion. These specific surface areas are approximately two orders of magnitude greater than those provided in AF or DSFF bioreactors. The high specific surface areas allow high biomass concentrations to develop, on the order of 15 to 35 g/L as VSS, which are similar to those achieved with the UASB process.2111 The high biomass concentrations allow operation at relatively low HRTs and high VOLs while maintaining adequate SRTs for efficient treatment. HRTs in the 0.2 to 2 day range arc used, depending on the concentration of the wastewater. Volumetric organic loadings over 20 kg COD/im'day) are common with FB/EB systems.
As with any other biological process, excess biomass must be removed from the bioreactor to control the biomass inventory. As discussed in Section 18.2.2, accumulation of biomass on the carrier particles increases the bioparticle diameter and decreases its density. The result of these two contrasting factors is a decrease in the settling velocity of the bioparticles and the tendency for bioparticles with more biomass to accumulate in the upper portion of the bioreactor. As a consequence, solids wasting is generally from there. The bioparticles are conveyed to a device where biomass is sheared from the carrier particles. The carrier particles are then returned to the bioreactor and the biomass is taken to further processing or ultimate disposal. Several devices are available for this purpose, but they often consist of a rubber lined pump where the biomass is sheared from the carrier particles and a centrifugal separation device where the lighter biomass is separated from the denser media particles. Many of the commercially available devices are proprietary in nature. Fluidized beds are discussed in detail in Chapters 18 and 21.
The basic process options described in this section can also be combined in a variety of ways to produce a wide range of additional anaerobic treatment systems. For example, interest currently exists in the use of membranes as a means of further separating the SRT and the HRT, thereby producing an even more compact anaerobic process.2"2"
Solids fermentation processes are used to solubilize particulate organic matter in primary solids and ferment the soluble products to VFAs, particularly acetic and propionic acid, for use in BNR processes.""7" The objectives of solids fermentation processes are different from those of the anaerobic stabilization processes discussed previously. They are to maximize the production of VFAs and recover them in a stream that can be delivered to a BNR system. The first objective is achieved by controlling the SRT to a value that allows the growth of hydrolytic and fermentative bacteria but prevents the growth of aceticlastic methanogens, which would consume the VFAs.,xAs indicated in Figure 9.5, at 35°C this requires an SRT in the 2 to 3 day range. In general, the feed solids and bioreactor contents are not heated, so the SRT must be increased to compensate for the lower temperature. Some methane will be produced as a result of the growth of H,-utilizing methanogens, but the amount will be small. The second objective is achieved when the VFAs are separated from the residual primary solids by passing the bioreactor effluent through a liquid-solids separation step.
Figure 13.15 illustrates schematically the concepts of fermentation systems. Feed-solids are fed to a mechanically mixed bioreactor where fermentation occurs. The SRT is controlled by adding dilution water in sufficient quantities so that the HRT, which equals the SRT, is maintained at the desired value. The use of gravity sedimentation to achieve liquid-solids separation is illustrated in Figure 13.15. The option of adding elutriation flow to the bioreactor effluent is provided to ensure sufficient supernatant to effectively recover the produced VFAs. Typically, the settled solids are removed from the settler and taken to further processing. However, the capability to recycle a portion of those solids to the bioreactor may be provided to increase its SRT above its HRT.
Figure 13.16 illustrates how the concepts in Figure 13.15 have been implemented at several full-scale wastewater treatment plants. In an activated primary clarifier, primary solids are accumulated in a sludge blanket where fermentation occurs. The settled solids are then recycled to an upstream mixing/elutriation tank where the soluble VFAs are washed from the fermented primary solids and into the
MIXING/ ELUTRIATION R,w TANK influent _
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