Chapter 3 Anaerobic Digestion and Reactors
3.6.5. The Anaerobic Baffled Reactor
The anaerobic baffled reactor (ABR) is a reactor designwhichuses a series of baffles to force the waste water containing organic pollutants to flow under and over (or through) the baffles as it passes from the inlet to the outlet (Figure 3-6).Bacteria within the reactor gently rise and settle due to flow characteristics and gas production but move down the reactor at a slow rate (Nachaiyasit and Stuckey, 1997). The main driving force behind the reactor design has been to enhance the solids retention capacity and treat difficult wastewaters. The ABR is simple and inexpensive to construct because there are no moving parts or mechanicalmixing (Polprasertetaf.,1992).
Gas mete, Feed mtet
Recycle
Eltluentcenecucn
Figure 3-6: The most common design oftbe ABR (Boopatbyet al., 1988)
Probably the most significant advantage of the ABR is its ability to separate acidogenesis and methanogenesis longitudinally down the reactor. It behaves as a two-phase system but without the associated control problems and highcost (Barber and Stuckey, 1999).Two-phase operation can increase acidogenic and methanogenic activity bya factor of up to four,as differentbacterial groups develop under more favourable conditions. Having a continuous gas space above the chambers enhances reactor stability
Chapter 3 Anaerobic Digestion and Reactors by shielding syntrophic bacteria from elevated levels of hydrogen, which are found in the front compartments of the reactor.
3.6.5.1. Bacterial populations under phase separation
In the ABR various profilesof microbial communitiesmay develop within each compartment. The ecology of each chamber will depend on the substrate andthe amount of it present. Other factors such as pH and temperature also have an effect. The most common observation in the population shift is that of the acetoclastic methanogensMethanosarcina sp.Methanosaeta sp.;Methanosarcina has a doubling time of 1.5 days compared to 4 for Methanosaeta. At high acetate concentrations Methanosarcina outgrows Methanosaeta; however at low concentrations Methanosaeta is dominant because of its scavenging capability (Ks=30 mg/L compared with 400 mg/L forMethanosarcina) (Barber and Stuckey, 1999).Other observations that have been made are summarisedinAppendix II.
3.6.5.2. Hydrodynamics
In 1992,Grobicki and Stuckey conducted a series of hydrodynamic studies on the ABR. Theyfound low levels of dead space, less than 8% for an empty reactor; other designs have between 50 to 90% in an anaerobic filter and 80% for a CSTR. The presence of biomass had no significant effect on hydraulic dead- space, which was found to be function of flowrate and the number of baffles. Biological dead-space was established as the major contributorto the overall dead space at high HRT.Its effect decreased at low HRT because gas production prevented channelling within the biomass bed (Grobicki A. and Stuckey D.C., 1992)
3.6.5.3. Solids retention
The main driving force behind the ABR design has been to enhance the solids retention capacity.The longer the solids stay in the reactor the longer the time available for biodegradation to occur.Boopathy and Sievers managed to measure the solids retentiontime for two hybrid reactors running at a retention time of 15 d (Barber and Stuckey, 1999). The three-compartment reactor had a solids retention-time of 25 d compared to 22 d for a two-compartment reactor. If the reactor manages to develop a sludge blanket its capacity to trap particles increases.
3.6.5.4. Treating low strength wastewater
Low strength wastewater can be described as those wastewaters with COD less than 2 OOOmg/L,which contain a variety of biodegradable compounds such as short chain fatty acids, alcohols, VFA, carbohydrates,lipids and proteins.Low strength wastewaters inherently provide a low mass transfer driving force between biomass and substrate (Katoetat.,1997). As a result these waters encourage the dominance
Chapter 3 Anaerobic Digestion and Reactors
of biomass in the later compartments.Data on the performance of the ABR on low strength wastewaters is shown in Table 3-3.
Table
3-3:Performance of the ABR on low strength wastewater (Barber and Stuckey,
1999)Temperatureat 25 C. bTemperature lowerthan 16 C. All otherwork at performed at mesophilictemperature range.
Wastewater HRT COD (mgIL) COD Removal OLR Gas
(h)
(%)
(Kg mJ/d) ProducedIN OUT
(v/v.d)
Greywater 84 438 109 75 0.13 0.025
Greywater 48 492 143 71 0.25 0.05
Greywater" 84 445 72 84 0.13 0.025
Sucrose" 6.8 47 74 74 1.67 0.49
Sucrose" 8 473 66 86 1.42 0.43
Sucrose" 11 441 33 93 0.96 0.31
Slaughterhouse 26.4 730 80 89 0.67 0.72
Slaughterhouse 7.2 550 110 80 1.82 0.33
Slaughterhouse 2.5 510 130 75 4.73 0.43
a u. U, ..
The results show that the amount of gas produced is proportional to the organic loading rate,COD removal and hydraulic retention-time. The hydraulic retention-time is dependent on the temperature and type of substrate. Sucrose had the shortest retention time because it soluble and readily hydrolysable . Greywater had the longest retention time because it is a complex substrate, a mixture of soluble,readily-hydrolysable, slowly hydrolysable and particulate substrate. The particulate and the slowly-hydrolysable substrates need more time to be treated.
3.6.5.5. Recovery of reactor from shock loads
At high loading rates,imbalances between acidogens and methanogens may lead to the accumulation of intermediate acid products thereby exceeding the buffering capacity of the environment and causing the pH to drop to a level that inhibits methanogens (Cohen et a/.,1981).
The variable nature of wastewaters requires the reactor to be stable to shock loads. Shock loads can manifest themselves in two ways:either as a short term transient slug which lasts a few hours,or as a long term step change lasting for days or weeks before reversing back to the original operating condition.The microbial response to both these shock loads are identical,however the long-term shock leads to a new steady state.~erformance of the reactor in the new steady state may not be the same as the previous one (Nachaiyasit and Stuckey,1997).
The hydraulic flow pattern in the ABR causes the bacteria to move horizontally down the reactor very slowly giving rise to cell retention time (CRT) of 100 d at 20 h HRT (Nachaiyasit and Stuckey, 1997).
Chapter 3 Anaerobic Digestion and Reactors
Systems with high CRT such as the ABR in contrast to CSTR' require a considerably longer time to establish a new steady-state. The accepted norm is three HRTs for a CSTR (Nachaiyasit and Stuckey, 1997).
ilXX>r---.
\00 n.ne{bls)
I~
____ 1 ... 2-.-3 ...4-o-5~6_o_7~8
Figure 3-8: COD profile of each compartment after the shock load with a readily hydrolysable substrate at an HRTof20h (Nachaiyasit and Stuckey, 1997)
Asthe shock wave moves down the reactor,the size of the COD peaks decreased. Two days after the shock the peaks flattened out but at a higher COD level than at time zero (Figure 3-8). Itwas concluded that the reactor was stable to high shock loads and responded quickly. The pH initially rose and dropped dramatically in compartment 1 and 2.Itstayed constant in compartment 3 and increased in compartments 4 to 8 (Figure 3-9).The decrease in pH in compartments 1 and 2 was the result of increased VFA production leading to a build up.The increases in pH in compartments 4 to 8 were due to increased buffering capacity from increased feed.
8,---,
7.8 7.6 7.4 7.2
6.8 6.6 6.4
6.2
6.L~---'o so 100 ISO zoo
nme(Jus)
Figure 3-9: pH profile of each compartment after the shock load with a readily hydrolysable substrate at anHRTof20h (Nachaiyasit and Stuckey, 1997)
Chapter 3 Anaerobic Digestion and Reactors
Table 3-4: Table summarising advantages ofthe ABR (Barber and Stuckey, 1999) ADVANTAGES
CONSTRUCTION OPERATION
I Simple design and inexpensiveto construct I LowHRT
2 No moving parts 2 Intermittent operation possible
3 No mechanical mixing 3 Extremely stable to hydraulic shock loads
4 Highvoid volume 4 Protection from toxins in influent
5 Reduced clogging 5 Long operation times without de-sludging
6 Reduced sludge bed expansion 6 High stability to organic shocks 7 Low capital and operating costs
BIOMASS
I No requirements for biomass with unusual settling properties 2 Low sludge generation
3 High solids retention times
4 Retention of biomass without fixed media or solids settling chamber