B br Brs X
The sludge age © [d] is calculated from VR, X, the volumetric flow of excess sludge Vs, [m3/d] and the biomass concentration in the excess sludge cS [kgCOD/m3].
Table 4.18 summarizes all available findings for sludge- bed reactors with pellet-ized and flocculated sludge.
For only slightly contaminated sewage (COD value <1000 mg L-1, $ > 25 °C), the required reactor volume is derived from the required residence time.
The surface load (wet surface of the separators) can also be decisive for the calculation apart from the volume load. The residence time is varied by the reactor height.
For UASB-Reactors the maximum permitted area load wS amounts to:
• wS = 3 mh-1 for water containing sewage a dissolved state with sludge
• wS = 1-1.25 mh-1 for only partly dissolved sewage with granulated sludge
• wS = 0.5 mh-1 for voluminous, flaky, activated sludge.
For the fermentation of sewage waters for which it is certain that solids are always previously separated and therefore all degradable substances are dissolved, the bioreactor height can reach up to 10 m and more (Table 4.19). For all other sewage waters a lower bioreactor height has to be chosen. For slightly contaminated sewage, the reactor height should be 3-5 m. A height of more than 7 m for sewage water without dissolved components leads to sludge losses.
Table 4.18 Applicable volume load for UASB reactors depending on the waste water composition.
Waste water Undissolved Applicable volume load BR at 30 °C in kgCOD/m3 -d concentration COD (%) -
(mgcoDL 1) Granulated sludge Flaky sludge
High DM Low DM
decomposition decomposition
|
Up to 2000 |
10-30 |
2-4 |
8-12 |
2-4 |
|
30-60 |
2-4 |
8-14 |
2-4 | |
|
60-100 |
* |
* |
* | |
|
2000-6000 |
10-30 |
3-5 |
12 - 18 |
3 - 5 |
|
30-60 |
4 - 6 |
12 - 24 |
2-6 | |
|
60-100 |
4 - 8 |
* |
2-6 | |
|
6000-9000 |
10-30 |
4 - 6 |
15 - 20 |
4-6 |
|
30-60 |
5-7 |
15 - 24 |
3 - 7 | |
|
60-100 |
6 - 8 |
* |
3 - 8 | |
|
9000-18000 |
10-30 |
5 - 8 |
15 - 24 |
4-6 |
|
30-60 |
Uncertain at DM >6-8 g/l |
Uncertain at DM > 6 - 8 g/l |
3 - 7 | |
|
60-100 |
* |
* |
3 - 7 |
* Application of the UASB process cannot be recommended under these conditions.
* Application of the UASB process cannot be recommended under these conditions.
Table 4.19 Required residence time in h at different reactor heights at maximum tolerable average feed per area of wS = 3 m/h and 1 m/h, under the assumption that only the hydraulic load is correct.
HBR tmin
HBR tmin
|
3 |
1 |
3 |
|
4 |
1.3 |
4.9 |
|
5 |
1.7 |
5.1 |
|
6 |
2 |
6 |
|
9 |
3 |
9 |
|
12 |
4 |
12 |
The distribution system, characterized by the number of inlets per reactor cross section or its inverse value, is adequate for the contact between sewage water and sludge (Table 4.20 ).
For slightly contaminated sewage waters, the area per inlet should be 0.5-2 m2; for heavily contaminated ones 2- 5 m2.
Table 4.20 Reference values for the number of feeding points of UASB reactors.
Features of the sludge BR Area perfeeding point
Features of the sludge BR Area perfeeding point
|
Thick flaky sludge (>40kgDM/m3) |
<1 |
0.5 - 1 |
|
1-2 |
1 - 2 | |
|
>2 |
2-3 | |
|
Medium thick flaky sludge (20-40kgDM/m3) |
<1-2 |
1 - 2 |
|
>3 |
2-5 | |
|
Pellets |
Up to 2 |
0.5 - 1 |
|
2 - 4 |
0.5 - 2 | |
|
>4kg |
> 2 |
- | | Biogas |
Figure 4.30 Three-phase separator in a UASB-reactor.
Figure 4.30 Three-phase separator in a UASB-reactor.
The three-phase separator in UASB, EGSB, and IC reactors is a rooflike installation (Figure 4.30). Plate inclination:
a > 50 to 60° for flocky sludge a > 45° for pelletized sludge
Velocity of inflow:
VG = 3-5mh-1 for high-load sewage water VG = 1m h-1 for low-load sewage water
Overlapping:
The area of the openings between the three- phase separators has to be between 15 and 20% of the bioreactor surface.
|
1 | |||||||||
|
Phase 1 1 |
Phase II |
Phase III | |||||||
|
1 1 |
CSB | ||||||||
|
1 | |||||||||
|
S 1 | |||||||||
|
1 | |||||||||
|
1 | |||||||||
|
1 o[ |
DM in the d |
ischarge | |||||||
0 10 20 30 40 50 60 70 Time in days — Figure 4.31 Starting phases of a UASB-reactor.
80 90
0 10 20 30 40 50 60 70 Time in days — Figure 4.31 Starting phases of a UASB-reactor.
80 90
The height between the three-phase separator and the sludge -bed should be Hbs = 1.5-2 m at an overall reactor height of HBR = 5-7 m.
If sludge formation is expected, e.g., with sewage containing fats or proteins, spraying devices have to be placed above the water level.
Operation of a sludge-bed reactor
The start-up operation is divided into 3 phases, shown in Figure 4.31.
Phase I: Volume load BR < 2kgCOD/m3-d
The sludge-bed rises as a consequence of the starting gas formation. Very fine material is washed out with the sewage. Because of the growth of thread-like microorganisms, the sludge sediments less well. Scum develops at the end of phase I.
Phase II: Volume load BR = 2-5kgCOD/m3-d
As a consequence of the increasing volume load and gas formation, more material is washed out. Only the heavy parts of the sludge sink downwards. After ca. 40 days, the first sludge grains form (diameter up to 5 mm). Despite the heavy sludge losses caused by the washing out, the sludge load increases to 2 kgCOD/m3-d at constant volume load, because the specific sludge activity rises.
Phase III: Volume load BR = 2-50kgCOD/m3-d
If granulates are able to form better and as such the sedimentation ability of the sludge is improved, less material can be washed out. The volume load increases to its maximum value of BR = 50 kgCOD/m3-d.
The granules can develop rapidly (6 months) especially when the sewage composition promotes the formation of natural biopolymers (extra-cellular polymers, polysaccharides, and proteins) or if residues of synthetic polymers are in the sewage.
The start-up phases can be shortened ifthe sludge-bed reactor is inoculated with sludge. The activation is carried out with 10-20 kgoDM/m3 activated sludge from another plant with low methane activity and low sludge index ISV at a sludge load of Brs = 0.05-0.1kgCOD/kgDM-d in phase I and a sludge load of0.6kgCOD/kgDM-d during phase II for the formation of pellets. The volume load should not be increased before all volatile fatty acids are degraded. Sludge difficult to sediment should be washed out and heavier sludge particles should be retained.
If pelletized excess sludge from another reactor operating for a long time is used, the sludge index ISV of the residual sludge should amount to approx 50 mLg-1 after washing out the colloidal components during the first days of operation. The pelletized excess sludge should contain 75 kgDM/m3. Its ratio oDM/DM should be as high as possible.
The specific methane formation rate should be kept low at the beginning (below 0.6kg CH4 per kgCOD/m3-d), so that no washout due to intense gas development takes place.
Pelletized sludge (DM « 100gL-1) is available on the market at a price ofapprox 5000 US$/MgDM. During the starting phase, it is common to load the reactor up to approx Hbp = 1 m with pelletized sludge.
In the outflow of sludge-bed reactors, the dry matter content is DMR e = 0.051.1 gL-1 or oDMRe = 0.02-0.6gL-1- This biomass is tolerated and even wanted, because flocky sludge has a negative influence on the formation of pellets.
Reactors with immobilized microorganisms All slowly growing microrganisms78) of the anaerobic degradation process tend to immobilization. Microorganisms of the genus Methanosaeta grow particularly well on hydrophobic surfaces, because they are not covered with a barrier of adhesively bonded water molecules that prevent attachment to surfaces.
For this reason, reactors with immobilized microorganisms are equipped with packing material on which the microorganisms can grow as a thin layer, the so-called biofilm.
Biofilm
The thinner the biofilm on the packing material, the more effectively the plant works, because only the topmost, approx 1 mm thick biofilm layer is actively involved in the degradation of sewage water components. After a residence time of t = 154 days in an anaerobic reactor, a dense biofilm has grown on the carrier material. Tests with different plastic materials of the specific surface O-pec. = 98-138m2/m3 for the time specified above led to film thicknesses of 2- 3 mm when synthetic, easily methanizable sewage consisting of 30% sugar and 65% ethanol was used.
The biofilm grows continuously (Figure 4.32). After about 11/2 years ofoperation, perhaps only ca. 65-70% of the total volume flows through or is intermixed, and 30-35% of the reactor - s volume belongs to the stagnation space (skip zone). The biofilm has to be removed no later than this.
Figure 4.32 Biofilm after 44 days (left) and after 154 days (right) of anaerobic fermentation. After 44 days some filamentous growing Methanosaeta can be observed. After 154 days the typical bamboo-like segmented cell formation of Methanosaeta can clearly be seen. (SEM-photo: the length of a white bar equals 10pm).79)
Figure 4.32 Biofilm after 44 days (left) and after 154 days (right) of anaerobic fermentation. After 44 days some filamentous growing Methanosaeta can be observed. After 154 days the typical bamboo-like segmented cell formation of Methanosaeta can clearly be seen. (SEM-photo: the length of a white bar equals 10pm).79)
Should primarily carbohydrates (oligo- and polysaccharides) be degraded, microorganisms can rapidly grow and develop a slime shell guaranteeing good adherence. Other non- slime -developing microorganisms can then stick to this slime shell and so form a heterogeneous biofilm.
The form, surface, horizontal areas, and cavity volumes of the packing material determine the constitution of the biomass in the reactor. Randomly inserted material grows faster than axially flowed - through, packed packing material, but also forms easier skip zones with short-circuit currents and is more prone to clogging, leading to a lower elimination of organic matter as COD. By addition of floccula-tion agents (polyacrylamides) and calcium, the biofilm develops faster.
The packing material can be
• Random-fill packings or cubes.
To this category belong limestones, quartz, lava slag, activated coke, corals, shells, clay, expanded clay, plastics (polyurethane), ceramics, pumice stone, small stones, ordered and packed or filled plastic elements with various shapes, sizes, and surface structure (specific surface 70-400 m2/m3, porosity >95%), foils, glass marbles, glass ceramic, wire and plastic braidings, natural sponges, etc.
PVC is less suitable for the adhesion of microorganisms.
The following materials can also be used: carrier materials from brewer's barley, etc., (e.g., in the form of compressed plates), native fibers, e.g., hemp, flax, sisal, cotton, jute or cocoa, bamboo stick, wood plates, burr walnut, used tires. A natural fixed bed of cocoa fibers has proven to be superior to other carrier materials from the point of view of bacteria adhesion, degradation performance, homogeneity of the outflow, concentration, and costs.
Well established is also packing material of SiranTM, with density pFS = 1gcm-3, open pore volume eFS = 55-70%, adjustable pores diameter dFS = 10-400 pm, and usable colonization surface up to OFSspec = 90 000 m2/m3 fill.
Figure 4.33 Packing material in the form of cord before installation in a bioreactor.
In a fluidized bed, spherical inert material with a large surface (Ospec = 3000m2/m3, £ = 80%), e.g., made ofsand, activated coke, foamed plastic flakes, or plastics is used.
• Inserts in the form of webs or blocks.
Vertically placed strings (Figure 4.33) of hydrophobic plastic strings are not only recommended for proofness against interlocking, large specific surface area (As > 100m2/m3), and a cavity volume of93%, but also because Methanosaeta settling is promoted by the hydrophobic surfaces.80)
Fixed-bed reactor, filter reactor,fixedfilm reactor In fixed-bed reactors such as anaerobic filters, PCR (Polyurethane-assisted Carrier Reactor) packing material is accumulated. The packing material takes up to 2/3 of the bioreactor height according to 10-20% of the bioreactor volume and its layer should be minimum 2 m thick.
In the fixed- bed reactor, packing materials with a coarse surface are provided for the microorganisms, on which they can immobilize particularly well. Packing material in general is characterized by its very large specific surface and very small flow resistance, so that clogging is prevented.
Fixed-bed reactors are particularly well suited for waste water, which contains few or no larger solid lumps.
The volume load of the reactors amounts to BR = 5-100 kgCOD/(m3-d). It should be below BR = 12kgCOD/m3-d. The degradation oforganic substances in the waste water to COD values < 1 kgCOD /m3 is achieved. Without recycling of biomass, the normal DM concentration reaches oDM = 10-20 gL-1. and with recycling it rises to 50 g L- 1.
Fixed-bed reactors can be operated both in upflow, i.e. substrate and developed biogas move parallelly from the bottom upward, and in downflow, i.e. substrate
- Figure 4.34 Technologies of fixed-bed reactor.
Table 4.21 Characteristic values for bioreactors with immobilized microorganisms.
Upstream velocity (superficial velocity) [m s-1]
Expansion factor
Energy load [W/m3]
Sludge floc reactor 1
Expanded-bed reactor 2-10
Fluidized-bed reactor with Biolith81) 10-30 Fluidized-bed reactor with sand
5-15 20-40
flows from the top downward and the biogas flows upward (Figure 4.34). For waste water with high organic load (DMcod = 30-60 g L-1) but low solids, upflow fixed-bed reactors (UAF = Upflow Anaerobic Filter) are used. It may happen that dead biomass tears away and contaminates the clear water. Fixed-bed reactors in down-flow operation (DSFF reactor = Downflow Stationary Fixed Film Reactor) in contrast do not block so easily and therefore are used for solid-rich waste water.
Because of the large surface which can be settled by microorganisms, the residence time can be short. With a continuous or semi-continuous supply of waste water, the microorganisms in the fixed-bed reactor are held back for the most part and are not washed out.
Characteristic values for fixed-bed reactors are given in Table 4.21. During startup, the velocity of the substrate should be below vA = 0.4 mh-1 in the beginning and should rise slowly to ca. vA = 1mh-1. In industrial scale plants the velocity of the upflow reachs vA = 2mh-1. Higher velocities can cause losses ofsludge. A circulation pump with adjustable drive is recommended, which is able to recycle 15-20 times the feed, in order to improve the flow through the entire reactor, to prevent blockage, and to keep the best biological conditions over the reactor height. The higher the concentration of the waste water, the higher is the circulation rate that should be chosen.
- Waste water Figure 4.35 Fluidized-bed reactor.
By circulation of substrates with a concentration higher than oDMCOD = 8000 mgL-1, the following can be achieved:
• The demand on alkalis and trace elements is reduced.
• The concentration of organic acids in the zone where the substrate meets the reactor wall is decreased.
• A slightly better cleaning is achieved.
If the biofilm is washed out or the supplied sludge is too strongly whirled up and thus biomass passes through the reactor and is to be found in the clear water, the waste water must later be cleaned and possibly retreated.
Expanded-bed reactor, fluidized-bed reactor
In the fluidized-bed reactor (Figure 4.35) and anaerobic attached film expanded-bed reactor, spherical packing materials are preferably used, covered over with microorganisms.
Well-approved materials especially suitable for growing microorganisms include Al2O3, PVC, clay, activated charcoal, bentonite, and sand with a specific surface of 1000-3000 m2/m3.
The packing materials are used on a perforated plate. Through the plate, substrate is pumped with such a velocity that the packing materials hover in the fluid. The high velocity is reached by the fact that substrate is circulated by a pump and the reactor is tall and narrow. Reactors are classified according to the upstream flow velocity.
In order to ensure sufficient matter transfer, the following power supply rates are necessary for continuous operation (according to experience). The installed capacity should be about twice as large, however.
Sludge floc reactors and expended-bed reactors are more suitable for weak to highly concentrated sludge (5-80 gL-1). Without biomass feedback in the reactor, solid concentrations can be reached up to oDMCOD = 20-70 kgCOD/(m3-d).
Often fluidized-bed reactors are built in two stages in order to ensure a constant supply of acidified water (oDMCOD = 1500-3600 mgL-1) into the second reactor.
Water recirculation
Figure 4.36 Contact process.
Water recirculation
Figure 4.36 Contact process.
The contact between biomass and waste water is particularly intensive in the fluidized-bed reactor. There is no risk of clogging. Because of the high liquid flow locally, particularly high concentrations of poisoning materials are impossible.
Fluidized-bed and expanded-bed reactors are started up either with slowly added packing material and constant substrate flow rate or with maximum packing material and slowly increasing substrate flow rate. The smaller the amount of packing material, the shorter is the start- up period.
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