Upflow Anaerobic Sludge Blanket Reactor UASB

The sludge blanket concept was first used in the Reversed Flow Dorr Oliver clarigester which is a modified version of the contact process. Unlike the contact process, it is unmixed and feed flows upward through a dense bed of flocculated bacteria. Flocs collect in the settling compartment and return to the reactor by gravity. Dorr Oliver clarigester improvements due to the biomass loss in effluent led to the upflow sludge blanket process and the UASB process. The UASB has an integrated 3-phase liquid-gas-solids separator to help retain sludge, and mechanical agitation is minimized or omitted.

The UASB reactors have been applied to a wide range of industrial wastewater, including those containing toxic or inhibitory compounds. The process is also feasible to treat domestic wastewater with temperature as low as 14-16°C or even lower.

Since the early 1980s, considerable research and development has occurred in relation to anaerobic wastewater treatment systems and specifically, UASB. Reductions in BOD of 75-90% have been noted in tropical conditions. The UASB technology is feasible in an urban, developing world context because of its high organic removal efficiency, simplicity, low cost, low capital and maintenance costs, and low land requirements. Anaerobic treatment processes are suitable in tropical conditions because anaerobic treatment functions well in temperatures exceeding 20°C.

They are characterized by low sludge production and low energy needs. The UASB is typically constructed with entrance pipes delivering influent to the bottom of the unit and a 3-phase separator at the top of the reactor to separate the biogas from the liquid phase (water and sludge) and of sludge from the water phase; overall this prevents sludge washout.

The UASB process was first described in an international journal with studies on the treatment of methanol with laboratory UASB reactors (Lettinga et al., 1979a), and its potential for dilute wastewaters in general was described (Lettinga et al., 1979b, 1980). There emerged subsequently many reports by Lettinga and his colleagues on application of the UASB process for treatment of a variety of wastewaters including those from sugarbeets (Lettinga et al., 1976, 1977), piggery (van Velsen et al., 1979), alcohols (Lettinga et al., 1979a), fatty acids (Ten Brummeler et al., 1985, Hwu et al., 1997a,b,c), slaughterhouse (Sayed et al., 1987, 1988), potato starch (Field et al., 1987), milk fat (Petruy and Lettinga, 1997; Petruy et al., 1997), and pulp and paper wastes (Sierra-Alvarez et al., 1990; Lettinga et al., 1991; Rintala et al., 1991). A comparison of performance of several full-scale UASB reactors is summarized in Table 3.3.

Of growing interest is the application of the UASB process for treatment of domestic wastewaters, which they clearly demonstrated is feasible (Lettinga et al., 1983, 1993; van der Last and Lettinga, 1992; Bogte et al., 1993; Lettinga, 1995).

The following section highlights the most recent full-scale and pilot-scale findings in the anaerobic treatment of industrial wastewaters.

A control system that is able to manage the start-up of a UASB reactor, using a reduced number of process variables was developed (Punal et al., 2001). Two different start-up strategies were applied: fed-batch and continuous operation. In the fed-batch, results show that starting from an organic loading rate (OLR) of lower than 0.5 kg COD/m3-d, a load of higher than 8 kg COD/m3-d was achieved in only 33 days and the COD removal efficiency was over 90%. In the continuous system, results showed 24 h of an excellent value, and also, starting from an OLR of

Table 3.3. The performance of full-scale UASB reactors (extracted from Liu et al., 2002)

Reactor

Wastewater

Influent

COD

BOD

Biogas production

References

volume (m3)

type

(%)

1500

Juice

22

88

96

0.5 m3/kg COD

Driessen et al. (1994)

1500

Molasses

80-120

65-70

85-90

0.5 m3/kg COD

Driessen et al. (1994)

120

Sewage

0.4

74

80

Vieira et al. (1994)

1400

Brewery

1.7

80

2.0 m3/m3 d

Pereboom and Vereilken (1994)

2200

Paper mill

1.3

70

1.4m3/m3 d

Pereboom and Vereilken (1994)

1700 x 2 unit

Potato processing

12

95

3.0m3/m3 d

Pereboom and Vereilken (1994)

1200

Sewage

0.56

68-74

69-76

0.05-0.1 m3/kg COD

Draaijer et al. (1992)

120

Sewage

0.19-0.46

55-65

60-72

0.09-0.25 m3/kg COD

Vieira and Garcia (1992)

S"

5s-S

a e e a to s lower than 0.5 kg COD/m3-d, a load of 9-12 kg COD/m3-d was achieved in 40 days and the COD removal efficiency was over 95%. Comparing the standard deviation of the process parameters, fed-batch mode has a better process efficiency. However, continuous mode has a better capacity to treat the organic load by enabling the system to operate at a more stable influent OLR. This is especially useful during the first 2 weeks of the start-up phase.

Another way to have a fast start-up is to adjust the microbial load index (MLI) values (Tay and Yan, 1997). Findings show that under high MLIs of 0.8 and 0.6, granulation developed well in 3 to 4 months of operation, making a fast start-up. However, with low MLIs of 0.3 and 0.2, there was still no granulation after 6 months. It was observed that there are 3 phases during the process of granulation, namely acclimation, granulation, and maturation. A stepwise and gradual increment in the sludge loading rate (SLR) must be followed too, to avoid the scenario of overloading or starving at different stages.

Show et al. (2004a) had developed an unconventional approach to accelerate start-up and granulation processes in UASB reactors by stressing the organic loading rate (OLR) without having to reach steady-state conditions. Three UASB reactors treating a synthetic feed with chemical oxygen demand (COD) of 2500 mg/L, at a mesophilic temperature of 35°C were studied. The results indicate that the start-up of reactors could be significantly accelerated under stressed loading conditions. Start-up times of the moderately and severely stressed reactors for operating at OLRs of 1 to 16 g COD/Ld ranged from 10 to 80 days and 13 to 90 days, respectively. Comparing with 17 to 120 days needed in the control reactor to reach the same OLRs, the start-up times were shortened by 25 to 41%. The extent of acceleration depends on the level at which the reactor are stressed. Applying stress and the extent of stress level in starting up the reactors did not reduce the reactor loading capacity, as all the reactors reached a similar maximum OLR of 16 g/L-d at the end of operation.

Show et al. (2004a) explored further into the possibility of accelerating development of granulation with the unconventional approach of stressed loadings. The researchers found that under stressed loading conditions, the sludge particles began to form granules earlier in both the stressed reactors after 24 and 30 days of start-up operation. Comparing with the control reactor without applying stress, the time taken to form granule was reduced by 45 and 32% in the severely and moderately stressed units, respectively. The granule formation occurred earlier in the severely stressed reactor than the moderately stressed unit.

While the results obtained had established significant acceleration in start-up and granulation processes, the characteriztics of granules developed were greatly influenced by the level of stress exerted. Characterization of bioparticles revealed that the granules developed in the moderately stressed reactor exhibited superior characteristics in terms of settleability, strength, microbial activity and morphology, and granular sludge growth, as compared with both the control reactor operated without stress and the unit which was over-stressed (Show et al., 2004a). The results presented indicate that the unconventional start-up approach could offer a practical solution for the inherent long start-up in UASB systems with concomitant saving in time and cost.

The microbial mechanism of thermophilic granulation and sludge retention during start-up was studied (Syutsubo et al., 1998). Development of well-settleable granular sludge is the key factor of successful operation of the UASB process. The inoculum was taken from thermophilic digested sewage sludge. Operating temperature was 55°C. The feed solution consisted of sucrose, yeast, and volatile fatty acids which are acetate and propionate. Results showed that the granule's sludge volume index (SVI) finally settled at about 13mL/g volatile suspended solid (VSS) upon maturation of the thermophilic granules. As a result of establishment of the entire granulated sludge bed, the reactor allowed a maximum volumetric COD loading of 45 kg COD/m3-d with a COD removal efficiency of 90%. The maximum sludge loading achieved was 3.7 g COD/g VSS-d, which was two to three times larger than that of sludge grown under mesophilic conditions.

Both acetate and hydrogen utilizing methanogenic activities exhibited their optima at 65°C, while that of propionate-fed methanogenic activity was at 50°C. Methanogenic activities of the retained sludge increased finally up to 110 times for acetate, 25 times for propionate, and 3.6 times for hydrogen, when compared with those of the seeded sludge. This relatively low value for propionate implies that the propionate degradation was most likely to be a rate-limiting step in the thermophilic anaerobic process.

Early development of UASB in the 1980s had been used in treating food industry wastewaters, such as beet sugar, corn, and potato starch processing. Recent studies showed that UASB can be applied in treating wastewaters containing concentrated proteins (Fang, 1994) and aromatic compounds such as phenol (Wen et al., 1995). Changing the rate of effluent recirculation is widely used to avoid toxic impact to the microorganisms. Recirculation, together with biogas production, results in higher superficial upflow velocity that causes washout of biomass. Low hydraulic loading rate on treatment of wastewater containing high concentration of phenol using a Re-circulated UASB (RUASB) operating under mesophilic condition is encouraged (Lay and Cheng, 1998). As the hydraulic loading rates decreased from 2.5 to 1.6 m/h, the relative bacterial activity also decreased from 80 to 50%.

Granulation is generally a slow process, but it is the pre-requisite of the optimum performance of UASB-like reactors. Use of polymers enhanced the anaerobic granulation process (Mamouni et al., 1998). Chitosan, natural polymer, out-performed Percol 763, a synthetic polymer in terms of granules formation rate. Chitosan yielded a granulation rate as high as 56 m/d, compared to 35 m/d with Percol 763 in acidic pH. Under alkaline conditions, chitosan is progressively neutralized, thus resulting in a less effective flocculation of suspended sludge. The high granulation yield of chitosan was most probably attributed to its polysaccharides structure, acting similarly to the extracellular polymeric substances (EPS) in aggregating anaerobic sludge.

Recently, Show et al. (2004b) investigated the effects of cationic polymers on reactor start-up and granule development. The experimental results demonstrated that adding the polymer in the seeding stage markedly accelerated the start-up time by as much as 50% and the granule formation by 30% through the use of an appropriate dosage of polymer. In addition, granules developed in polymer-assisted reactor exhibited better settleabil-ity, strength, and methanogenic activity at all organic loading rates tested. Organic loading capacities of polymer-assisted reactor were also increased significantly from 24 to 40 g COD/Ld. It was hypothesized that the cationic polymer is able to form bridges among the negatively charged bacterial cells. The bridging enables greater interaction between biosolids resulting in preferential development and enhancement of biogranulation in UASB reactors.

Competition between methanogenesis and sulfidogenesis in anaerobic wastewater treatment exists (Zhou and Fang, 1998). High concentrations of sulfate in wastewater can adversely affect the methane production in anaerobic treatment processes. Sulfidogens degrade substrate into bicarbonates and intermediates in the reduction process of sulfate to sulfide. Sulfidogens and methanogens coexist in many anaerobic ecosystems as they have similar physiologies. They are strictly anaerobes, and in favor of similar optimum temperature and pH. Results showed that after acclimation, a benzoate removal efficiency of 99.5% was consistent regardless of the sulfate concentrations. Sulfidogenesis slowly out-competed methano-genesis during the acclimation phase. This was indicated by the increased sulfate reducing efficiency from 48 to 99% while it was accompanied by the decrease in methane production from 1.02 to 0.39 L methane/L-d.

Supplement glucose improves the anaerobic degradation of phenol (Tay et al., 2001). Phenol is present in the wastewater of some industries, like coal gasification, coke production, pharmaceutical, pesticide, fertilizer, dye manufacturing, synthetic chemical, and pulp and paper. The maximum concentration of phenol could go as high as 6000 mg/L and this is far too toxic to living aquatic organisms. Glucose is used as a co-substrate to achieve effective and constant anaerobic biodegradation of phenol. Phenol can be degraded to methane and carbon dioxide through phenol metabo-lizers and hydrogen utilizing and acetotrophic methanogens. The phenol removal efficiency was also the best at 98%, compared with 88% without glucose supplement. Moreover, it also exhibited greater resistance to those adverse conditions and the system recovered faster than the other system without the glucose supplement.

The results of the pilot study together with the results from the intensive laboratory studies suggest the feasibility of thermophilic anaerobic treatment for the food industry wastewaters (Rintala and Lepisto, 1997). The reactor was operated at 55°C and placed on the premises of a factory manufacturing deep-frozen goods from vegetables. The hot (greater than 80-90°C) and concentrated (14-79 g COD/L) wastewater streams, deriving from steam peeling and blanching of carrot and potato were used. More than 80% COD removal was reported.

Removal of chlorinated phenols (CP) is possible in UASB reactors (Droste et al., 1998). Halogenated organic pollutants are labeled as toxic and recalcitrant in the environment. Effluents containing CPs and related compounds are especially problematic to treat due to their persistence and their high solubility in fat. Once introduced into water ecosystems, accumulation within river sediments and bioaccumulation within the tissues of organism are possible. CP compounds were able to be metabolized to mineral end products to a large extent at loading rates where the reactor's performance was not hindered. There was no accumulation of phenol in any of the reactors in the experimental conditions.

Treatment of polyethylene terephthalate (PET) wastewater with UASB is proven feasible in full-scale application (Polanco et al., 1999). PET is generated by direct esterification of terephthalic acid (TPA) with ethylene glycol. The raw material of highly purified TPA is easily available in the market. So the wastewater from the esterification process consists of mainly unreacted raw material, largely ethylene glycol, and products of the secondary or degradation reactions, such as terephthalic acid esters, methanol, acetaldehyde, and crotonaldehyde being the major part. There is also another wastewater stream, called the second stream, from the melt spinning process where a bath of chemicals is showered to improve the physical characteristics of the fiber. It was reported that the anaerobic biodegradability was 90 and 75% for esterification wastewater and second stream wastewater respectively.

Anaerobic treatment of wastewater from a fish-canning factory is also proven feasible in a full-scale UASB reactor (Punal and Lema, 1999). The wastewater comes from two main streams, mussel cooking which is seasonal, and tuna cooking. Most of the organic load from mussel cooking wastewater consisted of carbohydrate (74.5%) while that of tuna cooking wastewater has a significant percentage of fat (23.5%) and protein (73.0%). So, the high fluctuation in wastewater characteristics caused high variance in the reactor's efficiency. However, the performance was better when a mixture of both streams was treated due to the high degradable carbohydrate content of the mussel cooking wastewater. Through alkalinity control, it was possible to operate the system properly with a COD removal between 70-90% for influent ranging from 2 to 8 kg COD/m3-d.

The UASB technology can also treat crab-processing wastewaters (Boardman and McVeigh, 1997). Crab cooker wastewater contains high concentrations of COD, total suspended solids (TSS) and total Kjeldahl nitrogen (TKN). With UASB, the BOD5 and COD removal efficiency was over 90%. Acidification of the feed wastewater improved treatment as it reduced the concentrations of the feed suspended solids.

It is feasible for UASB to treat tapioca starch industry wastewater effectively (Annachhatre and Amatya, 2000). After removal of suspended solids by simple gravity settling, starch wastewater was used as a feed. COD conversion efficiency was greater than 95% and gas productivity of

5-8 m3 biogas/m3-d was obtained. Removal of starch solids from waste-water by a simple gravity settling was sufficient to obtain satisfactory performance of the UASB process.

The application of UASB reactor in the world shows an increasing trend. However, the start-up and control of those anaerobic systems are complex due to the low methanogenic activity of microorganisms. Although a number of mathematical models for anaerobic processes are available in the literature (Harper and Suidan, 1991), it seems still difficult to quantitatively describe the anaerobic processes because of the biological nature of degradation mechanisms involved. Recently, two powerful mathematical tools, namely fuzzy pattern and neural network have been successfully introduced into the anaerobic systems, such as anaerobic filters, UASB, and expanded granular sludge bed reactor (EGSB) (Marsili-Libelli and Muller, 1996; Guwy et al., 1997; Tay and Zhang, 1999, 2000).

A database containing system performance information is a prerequisite for training the neural fuzzy model, and the performance of the neural fuzzy model is highly dependent on the quality of training data although the training data collection sometimes is quite difficult, especially for a novel system. On the other hand, the input and output parameters need to be carefully selected or generated from the parameters commonly used for anaerobic system description in order to compute the neural fuzzy model. Tay and Zhang (2000) suggested that for the liquid phase, information on pH, total and specific volatile fatty acids (VFA), alkalinity, COD or total organic carbon (TOC) concentration, COD or TOC reduction, and redox potential (ORP) must be provided; as for the gas phase, the parameters include gas production rates and methane, carbon dioxide, hydrogen, and monoxide concentrations. The advantages of the neural fuzzy model include high adaptability to the variation of system configurations. This mathematical methodology has a prospective industrial application potential for the simulation and real-time control of complex anaerobic systems.

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