2. Literature Review
2.4 Development of Anaerobic Bio-Reactors
2.4.2 High Rate Immobilized Biomass Reactors
Apart from energy production from wastewater, the second most economically advantageous attribute of anaerobic biomass is its ability to immobilize - to form fixed biomass on packing media or fast settling biomass pellets - under certain conditions. This immobilization meant that biomass retention inside the reactor (sludge age - Rs) is significantly longer than that of the wastewater to be treated (hydraulic retention time - HRT) resulting in small reactor volumes and high OLRs.
The immobilized anaerobic biomass reactors was developed in response to the difficulties experienced with the treatment of soluble or relatively dilute organic industrial wastes. Maintenance of high treatment rates over extended periods with such wastes using dispersed biomass systems proved problematic. It was found however that when biomass immobilization could be induced, high organic removal rates (> 5 kgCOD/m3Vr/d) could be maintained continuously (McCarty, 1974).
Major process configurations developed for high rate anaerobic reactors over the last 5 decades include the fixed film systems such as the Anaerobic Packed Bed Reactor (AnPBR) and the Anaerobic Fluidized Bed Reactor (AnFBR). A third generation of reactors that have become popular in the past 3 decades are the granulating anaerobic bioreactors such as the Up-flow Anaerobic Sludge Bed Reactor (UASB), the Extended Granular Sludge Bed Reactor (EGSB) and the Internal Circulation (IC) ultra high rate reactors. The design philosophy of these systems is biomass retention to increase OLR and reduce reactor volume. The biomass retention is accomplished by immobilizing the anaerobic biomass as a biofilm on support media surfaces as in the AnPBR and AnFBR or by granulation, which is the spontaneous aggregation of bacteria to form granular sludge. This granular sludge usually shows a high level of activity and good settling properties as is observed in the UASB, EGSB and IC systems.
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Fixed Film Anaerobic Bio-Reactors
Fixed film anaerobic bio-reactors are systems in which biomass is immobilized on a solid material and retained in the reactor volume. They offer distinct advantages such as (i) simplicity of construction, (ii) elimination of mechanical mixing, (iii) elimination of external solid-liquid- separation systems such as SSTs, (iv) better stability at higher loading rates and they are (iii) generally less sensitive to organic shock loads (Kansal et al., 1998)
In the AnPBR – also known as the Anaerobic Filter - biomass immobilizes on some sort of packing media to form a thick biofilm. This biofilm contains the predominant active mass fraction in the reactor (Show & Tay, 1999). The packing is typically a large sized solid support optimized for cell immobilization. In the case of the AnPBR, the packing is situated in a fixed bed, submerged in the reactor liquid volume through which the wastewater passes. Mixing in the AnPBR can be enhanced by including an effluent recycle in the system. This recycle has advantages in situations where (i) alkalinity needs to be recycled to avoid low pH conditions at the entrance of the reactor (McCarty, 1974, Moosbrugger et al., 1991) and (ii) to maintain settlable solids in suspension in the reactor volume (Kansal et al., 1998).
During the initial research conducted on the AnPBR, it was suggested that a staged process, in which the acidogenic (acidification) and methanogenic steps are separated, will enhance the reactor performance (Alves et al., 2000). However, in studies comparing multi-staged to non-staged AnPBRs under laboratory conditions, no conclusive evidence for enhanced performance of the multi-staged system could be found (Kansal et al., 1998).
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Figure 2.7, Anaerobic Packed Bed Reactor (Anaerobic Filter)
Various configurations of AnPBR systems exist including up and down flow with and without recycle (Figure 2.7). It has been proven extensively that a recycle significantly increases the mixing in the AnPBR and decreases the negative effect of channeling and dead zones in the packed bed. The recycle can also act as a method of backwashing to remove inactive biomass after long periods of operation.
From most studies conducted on AnPBR systems, the ability of the packing media to retain high concentrations of immobilized biomass, dictates the reactor performance. It has been found that the surface properties of the packing material have a significant effect on the rate of biomass immobilization, especially during the start-up stages. However, under steady state operating conditions, media surface area appears to only have only a minor effect on performance. A less than 5% improvement in COD removal performance has been documented with a twofold increase in media surface (Show & Tay, 1998).
In comparative studies involving the anaerobic contact process and the AnPBR under identical conditions, it was found that steady state was reached quicker in the contact process than in the AnPBR. However, the AnPBR proved more stable and the daily methane production and corresponding COD removal was higher. TSS and SCFAs in the effluent also were far lower in the AnPBR (Hamdi & Garcia, 1991).
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Effluents treated at full scale with the AnPBR include; pharmaceutical, olive mill (Hamdi & Garcia, 1991), industrial (Perez et al., 1998), potato waste (Parawira et al., 2005) and sewage (Reyes et al., 1999). It has been proven extensively that start-up after a period of starvation is comparatively quick (Kansal et al., 1998). This makes the AnPBR ideal for the treatment of intermittent charges and seasonal operations – such as experienced in the fruit canning industry – where rapid secondary start-ups are required (Van Zyl et al., 2002; Reyes et al., 1999). In addition AnPBRs can tolerate sudden organic shock loads and recover to normal performance within a few days if the alkalinity is sufficient and the pH is maintained above 6.2 (Kansal et al., 1998).
Accumulation of high concentrations of active solids permits the treatment of dilute wastewaters at low temperatures (<35 oC) with little sludge production and an effluent substantially free of TSS compared to the anaerobic contact process. Removal efficiencies of 90% at 4 kgCOD/m3/d and 75%
at 16 kgCOD/m3/d are typically observed. It was found that about 55% of the COD removal and resultant methane production was done by the suspended biomass in the AnPBR. Hence system failure at OLRs higher than the maximum (~16 kgCOD/m3/d) was attributed to un-immobilized (planktonic) micro-organism washout, rather than short circuiting and dead zones (Show & Tay, 1998).
It was shown with tracer tests that mixing can be problematic in the AnPBR. This was verified in comparative studies done on the mixing regimes in the AnPBR and the Anaerobic Fluidized Bed Reactor (AnFBR). The steady state OLR of the AnPBR was only 60% of that of the AnFBR.
Insufficient mixing – even with a large recycle – and dead space in the reactor volume due to the accumulation of biomass clogging media porosity are the two major disadvantages of the AnPBR (Show & Tay, 1998).
In response to the performance limiting problems generally experienced with the AnPBR, the AnFBR was developed (Figure 2.8). In this reactor, the water to be treated is pumped through an expanded aggregate of the appropriate medium (typically coal, sand or PVC) on which an anaerobic biofilm has developed. Effluent is recycled to dilute the incoming waste and to provide adequate flow to maintain the bed in the expanded condition.
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Figure 2.8, Anaerobic Fluidized Bed Reactor
To maintain the sludge bed fluidized in the AnFBR, an up-flow velocity ~ 5 times higher than the AnPBR (~1m/h) is required. Because of the good settling properties of the biomass and media, biomass concentrations in the order of 15 to 40 gTSS/L can be maintained and because of the high organic loading capabilities (8 – 15 kgCOD/m3/d), the AnFBR can treat municipal sewage – in conjunction with chemical nutrient precipitation – at very short HRTs (Metcalf & Eddy, 1991). Even though the AnFBR addresses the issues around mixing experienced in the AnPBR, the packing material that is used takes up a significant fraction of the reactor volume (up to 40%). This large fraction of ‘dead’ space in the reactor volume is the primary disadvantage of the AnFBR.
Granulating Anaerobic Bio-Reactors
The development of the third generation, self granulating anaerobic bio-reactors was a direct response to the mixing and dead volume issues that were experienced in the fixed media reactors.
The exact start of research into this field is not clear, but it would appear that the UASB concept was developed from the combination of two up-flow systems namely; (i) the reverse flow clarigester and (ii) the up-flow anaerobic filter.
The reverse flow clarigester was developed by Hemes and others in South Africa in the 1960’s. The feed point was moved from the normal middle inlet of the clarigester to the bottom of the reactor,
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creating an enhanced mixing due to the up-flow regime in the reactor. It was found that the reverse flow clarigester could treat influent strengths of 10 gCOD/L at an organic loading rate of 3 kgCOD/m3Vr/d. Unknowingly the system was designed to select biomass prone to aggregation and granule formation. Over time the system developed a good settling sludge and granular sludge developed at the bottom of the reactor. No significance was attached to this phenomenon at the time (Ross, 1984)
McCarthy (1974) in the USA investigated various means of separating the solid and liquid retention times in anaerobic digestion so that low strength wastewaters could be treated anaerobically without requiring long HRTs and excessively large reactor volumes or sludge recycling (anaerobic contact process). So the up-flow anaerobic filter (AnPBR) was born. It was found that low strength wastes (1500 – 6000 mg/L) could be treated without solids recycling at OLRs of up to 4 kgCOD/m3/d. A secondary observation was that flocculated solids were suspended in the voids between the packing along with granules of ±3 mm in diameter. The formation of these granules was attributed to the rolling action induced by rising biogas bubbles (McCarthy, 1967).
The water research group at the University of Wageningen in the Netherlands lead by Lettinga, took cognizance of the experience on the reverse flow clarigester and the up-flow anaerobic filter and developed the Up-flow Anaerobic Sludge Bed Reactor (UASB) as a combination of the two. The UASB consists of a tall cylindrical reactor with the influent entering the reactor at the bottom, where it comes into contact with the granulated anaerobic sludge (Figure 2.9, without recycle). A 3 phase separator at the top of the UASB separates the produced biogas, granulated biomass and the treated effluent.
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Figure 2.9, Up-flow Anaerobic Sludge Blanket (UASB) and Extended Granular Sludge Bed Reactor (EGSB) (Lim, 2007)
The UASB typically operates at an up-flow liquid velocity of ± 1 m/h and OLRs in the order of 10 kgCOD/m3/d can be maintained. A further modification to the UASB is the Extended Granular Sludge Bed Reactor (EGSB). In the EGSB an external effluent recycle increases the up-flow velocity of the sludge bed to ~6 m/h, thus significantly increasing the mixing in the reactor. Design loading rates in the order of 20 kgCOD/m3/d are typical for the EGSB configuration. Such high OLRs are possible due to the very fast settling rate of the anaerobic granules (2-4 mm diameter) (Versprille, 2001).
Apart from effluent recycle, increased up-flow velocities and mixing in granular sludge beds can also be attained with a biogas recycle. This is the case with the Internal Circulation (IC) reactor.
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Figure 2.10, Internal Circulation (IC) Reactor (Driesen et al., 2000)
The IC reactor is also tall cylindrical reactor such as the UASB and EGSB systems. The internal biogas recirculation ensures rapid dispersion of the raw wastewater and also ensures optimal mixing conditions for the anaerobic granules and wastewater (Figure 2.10). Optimal biomass retention is provided by a two stage of solid liquid separator. The first stage separates the majority of the biogas from the liquid so that the second stage mainly separates the biomass from the effluent (Driesen et al., 1999).
Currently more than 70% of the world’s full scale anaerobic bioreactors are self granulating systems.
These reactors are especially suitable for the treatment of sugar, carbohydrate and protein rich wastewaters, provided there is a high level of solubility and biodegradability. However, the granulation process on SCFA streams appears to be slower and more unreliable than if compared to granules cultivated on more complex soluble substrates. It was found that the granules that were cultivated, were weak and filamentous and tended to break apart easily (De Zeew & Lettinga, 1980;
Hulshof Pol et al., 1982). This lead to poor settling properties, biomass washout and the eventual failure of UASB-type systems treating pure SCFA streams (Sam-Soon et al., 1989). For FTRW it has been extensively proven (1986 to present) that the start-up of granulating bioreactors on
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domestic sludge and FTRW is not economically viable and if these systems are seeded with granules from other UASB-type systems, stable operation cannot be maintained for extended periods of time.
(Rossouw & Van Zyl., 2008).
Although a major development over the first and second generation digesters, the UASB-type reactors still have two major disadvantages; (i) granulation papers to be problematic on high SCFA wastewater streams (especially for FTRW) (ii) the total COD particulate concentration in the effluent is typically in the order of 500 mgCOD/L and 200 mgTSS/L respectively, which implies that aerobic post treatment (effluent polishing) is still required. These are the two major issues which led to the development of the fourth generation anaerobic treatment systems; the Anaerobic Membrane Bio-Reactor (AnMBR).