Anaerobic Wastewater Treatment Using Anaerobic Baffled Bioreactor

Cent. Eur. J. Eng. • 3(3) • 2013 • 389-399 DOI: 10.2478/s13531-013-0107-8

Central European Journal of Engineering

Anaerobic wastewater treatment using anaerobic baﬄed bioreactor: a review

Review Article

Siti Roshayu Hassan1, Irvan Dahlan2∗

1 School of Civil Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia 2 School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia.

Received 31 December 2012; accepted 16 June 2013

Abstract: Anaerobic wastewater treatment is receiving renewed interest because it offers a means to treat wastewater with lower energy investment. Because the microorganisms involved grow more slowly, such systems require clever design so that the microbes have sufficient time with the substrate to complete treatment without requiring enormous reactor volumes. The anaerobic baffled reactor has inherent advantages over single compartment reactors due to its circulation pattern that approaches a plug flow reactor. The physical configuration of the anaerobic baffled reactor enables significant modifications to be made; resulting in a reactor which is proficient of treating complex wastewaters which presently require only one unit, ultimately significant reducing capital costs. This paper also concerns about mechanism, kinetic and hydrodynamic studies of anaerobic digestion for future application of the anaerobic baffled reactor for wastewater treatment. Keywords: Anaerobic baffled reactor • Anaerobic digestion • Industrial wastewater • Wastewater treatment • Mechanism; Microorganisms © Versita sp. z o.o.

1. Background wastewater treatment has gain increasing attention. It has several advantages that have attracted many researchers such as design simplicity, use of non-sophisticated equip- ment, low excess sludge production, high treatment effi- With the rapid development and escalating crisis of wa- ciency and low capital and operating costs [1]. Anaerobic ter pollution, especially discharged by industrial branch, systems can be categorized according to how the biomass it is beginning to take its toll in many regions. Water re- is retained in the system and type of biomass they de- sources are facing increasingly deficient and the value of pend on. Systems where the bacteria suspended and grow environment in the world is persistently becoming worse in the reactor liquid are called suspended-growth pro- in most regions. Therefore, there is a significant need to cesses. Normally, suspended growth systems have sludge develop reliable technologies for wastewater treatment. that is considered to be granular or flocculent in nature- Due to this problem, an anaerobic digestion process for oftentimes both flocculent and granular sludge coexist in a reactor. Granular sludge exhibits high activity rates and settling velocities that reduce required reactor volumes ∗E-mail: [email protected]

389 Anaerobic wastewater treatment using anaerobic baﬄed bioreactor: a review

and increase allowable organic loading rates [2]. Although Figure 1. Vertical Baffled Reactor (W= wastewater, B= Biogas, E= granulation is not necessary in the ABR for optimum per- Effluent) [11]. formance, unlike suspended systems such as the UASB, various reports have noted the appearance of granules in the reactor. Boopathy and Tilche [3] started up HABRs (the inoculum contained 4.01 g VSS/l) with a low initial loading rate (0.97 kg COD/kg VSS d) and liquid upflow velocities below 0.46 m/h, in order to encourage the growth of flocculent and granular biomass. Subsequently, stable granules of 0.5 mm appeared after one month in all chambers of the reactor and they were reported to be growing although no data was given; microscopic studies subsequently showed that the granules were comprised primarily of acetoclastic methanogens. Similarly, Tilche and Yang [4] found Methanosarcina coated flocs held together by fibrous bacteria resembling Methanosaeta. The flocs, which were formed after one month, were smaller with diameters less than 1.5 mm and were weak. Un- higher reaction rate per unit reactor volume (in terms of der the same loading conditions, the authors also found kg COD/m3.d) by retaining the biomass (Solids Reten- densely packed granules typical of a UASB (d <3 mm) tion Time, SRT) in the reactor separately of the incoming formed after 3 months in an anaerobic filter. Boopathy wastewater (Hydraulic Residence Time, HRT), in compar- and Tilche [5] noticed similar particles of both types de- ing to Continually Stirred Tank Reactors (CSTRs), thus scribed above, which grew from 0.5 mm after one month to reducing reactor volume and ultimately allowing applica- 3.5 mm after three months in a hybrid reactor. Granules, tion of high volumetric loading rates, e.g. 10-40 kg COD/ which were made from Methanosarcina clusters, were of m3.d [9]. low density and full of gas cavities and therefore lifted to the surface of the reactor due to higher gas and liquid velocities during high loading. The particle size appeared to be partially dependent on substrate type. In a current de- 2. Reactor development velopment, e.g. hybrid anaerobic baffled reactor (HABR) combines suspended and attached-growth processes in a Anaerobic baffled bioreactor (ABR) can be described as single reactor to take advantage of both biomass types. a series of Upflow Anaerobic Sludge Blanket (UASB) for The hybrid reactor design combines a lower section func- it can be divided into a few compartments [10]. Figure tionally identical to an UASB and an upflow AF on top, 2 shows the original design of baffled bioreactor which the idea being to combine the strengths of each approach is vertical in design. Vertical baffles arranged in series in a single tank [6]. Thus, the lowermost 30 to 50 percent forces the wastewater to flow under and over them as it UASB-like portion of the reactor volume is accountable passes from the inlet to the outlet. Bacteria inside the for flocculant and/or granular sludge formation. The up- reactor gently rise and settle due to flow characteristics per 50% to 70% of the reactor is filled with cross flow and gas production. The main driving force of an ABR is plastic media and also behaves as an anaerobic filter. to enhance the solids retention capacity, however some An anaerobic baffled reactor (ABR) has been developed modification have been made to treat different types of since early 1980s and has numerous advantages over well wastewater. Fannin et al. [11] introduced vertical baffled recognized systems such as the upflow anaerobic sludge to a plug flow to treat high solids slurry in order to en- blanket and the anaerobic filter. Moreover, the over and hance the ability to maintain high populations of slowly underflow of liquid reduces bacterial washout and enables growing methanogens, by replacing with influents solids. it to retain active biological solids without the use of any It proved that by applying a vertical baffled, more biomass fixed media. It is a type of highly efficient, highly promis- can be retained so that slowly growing methanogens can ing anaerobic reactor which is widely used in wastewater actively produce methane gas. From the result, it shows treatment [7, 8]. The successful application of anaerobic that methane levels increased from 30 to over 55% with technology to the treatment of industrial wastewaters is methane yield of about 0.34 m3/kg VSS. In a later study, critically dependent on the development, and employ of Bachmann et al. [12] comparing a baffled reactor before high rate anaerobic bioreactors. These reactors reach a and after narrowing the downflow chambers and slant- ing the baffle edge, it shows that production rates and

390 S. R. Hassan, I. Dahlan

reactor efficiency were improved in the modified design. mances of ABR were further discussed throughout this pa- The first several hybrid designs were introduced by Tilche per. and Yang [4]. The idea to modify the existing design was to enhance solids retention for high strength wastewater treatments. The solid settling chambers were incorporated 3. Applications of ABR in industrial after the last compartment and the solids washed out were collected and recycled to the first compartment. Packed wastewater treatment Pall rings is randomly positioned at the liquid surface of the first two phase chambers and a deeper structured Many researchers nowadays focus one anaerobic reac- modular corrugated block which has a high voidage is po- tor to treat wastewater. An overview of previous re- sitioned at the third chamber. Higher loading rates are search done was summarized in this review to illustrate possible for this structure due to minimal solids washouts the application of ABR where an extensive list of recent during elevated gas mixing. The separation of the gas can projects has been compiled. Application of ABR in differ- enhance reactor stability by shielding entrophic bacteria ent wastewater industry has been presented and discuss. from elevated levels of hydrogen which found in the front The study of performance of an ABR is done by treating compartment. In addition, Boopathy and Sievers [13] fur- dilute wastewater by Langenhoff and Stuckey [14]. The ther modified the baffled reactor to treat swine wastewater studies showed that more than 80% COD removals were containing high content of small particulate material. The achieved in this work. An ABR with total working vol- baffled reactor was modified to reduce upflow liquid veloc- ume of 10 L and with eight compartments were used to ities and to accept the whole waste. The first compartment treat dilute wastewater of 500 mg COD/L. The reactor was double in size to 10L followed by 5L of second com- started with a hydraulic retention time (HRT) of 80 h at partment. It is found that the larger compartment acted as 35◦C. It has been shown that low strength wastewaters a natural filter and provided superior solids retention for can be treated successfully at 35◦C with about 95% re- small particles. The reactor collected double the amount moval. In another study, Hui-Ting and Yong-Feng [15] of solid material (20.9 g/L) than the reactor with three investigated the performance of ABR in treating brewery chambers. This was further substantiated in the solid wastewater. This ABR is used to create an anaerobic washout data, which was lower in the two-compartment system with plug flow, multi-phase granular bed charac- reactor despite showing lower treatment efficiency. Fur- teristics. The reactor is operated continuously with an ther analysis showed that despite losing more solids, the organic loading rate (OLR) increased from 1.2 to 5.6 kg three-compartment reactor was more efficient at convert- COD/ m3 d. The results verified that the system is very ing the trapped solids to methane [13]. The compartmen- effective in treating the high strength wastewater where talization ABR bioreactor results in a buffering zone be- up to 92% COD removal was observed for an organic load- tween the primary acidification zone (downflow chamber) ing of 5.6 kg COD/ m3 d. The biogas production rate was and active methanogenesis zone (upflow chamber). ABR increased with increasing load, from 6.09 to 24.01 L/day bioreactor will behave partly as a fluidized bed reactor for loading rates of 1.20 and 5.60 kg COD/ m3 d. Methane similar to up-flow anaerobic sludge blanket (UASB) re- content in biogas increased gradually from compartment actor and it also acts as an activated sludge reactor as 1 to 5, which indicated methane-producing capacity of well as trickling submerged fixed film reactor. This biore- the anaerobic sludge increased. The successful applica- actor combines suspended- growth and attached-growth tion of ABR technology is known worldwide. Chang et al. processes in a single reactor to take advantage of both [16] perform the anaerobic digestion using ABR to treat biomass types which theoretically advantageous to reac- soybean wastewater. The reactor contained four compart- tor kinetics and process optimization. The most signif- ments of the same size with a total effective volume of 28 icant advantages of this design was its ability to nearly L. It shows that when the organic loading rate raised to perfectly realize the staged multi-phase anaerobic theory, 6.0 kg COD/ m3 d from 1.2 kg COD/ m3 d step by step allowing different bacterial groups to develop under more under the same hydraulic retention time, HRT 39.5 hrs favorable conditions, low costs and without the associ- and temperature 35◦C, the ABR could perform excellent ated control problems. Other advantages include reduced stability with a COD removal as high as 97%. In a more sludge bed expansion, no special gas or sludge separa- recent work, Ji et al. [17] used ABR for treating heavy oil tion required and high stability to organic and hydraulic produced water with high concentrations of salt and poor and toxic shock loads [2]. Modification of the basic con- nutrient which become key problems for petrochemical in- figuration of an ABR contributes to the development and dustrial regional environmental protection. A 105 L total improved treatment efficiency of wastewater. The perfor- volume ABR with an available volume of 75 L, consist- ing of six equal compartments was used for this purpose.

391 Anaerobic wastewater treatment using anaerobic baﬄed bioreactor: a review

The results show that the ABR can achieve high average addition, it has been shown to be stable to shock loading COD and oil removals of 65% and 88% for heavy oil pro- and capable of achieving high volumetric rates [? ]. duced water with poor nutrient and high salt concentration (1.15-1.46%) respectively. Additionally, it can keep stable Kinetic Model: Kinetic models were developed for sim- during 2.5 times the COD level shock load. The spherical ulating the anaerobic digestion process when treating granule sludge in the bioreactor was compact and con- varies wastewater. Four basic types of model are identi- tained large quantity of organics, amorphous materials, fied: Monod, Modified Monod, First Order and Modified

and crystals of Fe2O3, FeS, and CaCO3, whereas the rod- Contois. The Monod model, based upon the principles of shaped granules sludge was incompact without crystals of saturation kinetics, is the model most extensively used.

Fe2O3, FeS, and CaCO3. The Modified Monod model incorporates a modified form of saturation kinetics allowing for end product inhibition effects. The First Order model is the simplest type produced. The Modified Contois model is based upon Contois Table 1 summarized the performance data of the recent microbial kinetics and is effective at predicting process works. Based from significant number of relevant refer- performance during steady state. None of the models are ences reviewed, it shows that ABR are capable to be used adequate for the simulation of dynamic state processes. to treat various wastewaters with satisfactory results if Moreover with all four types of model it is assumed that incorporated with proper technology. the primary microbial substrate is soluble, whereas in fact, many agricultural wastes contain high concentrations of suspended insoluble organics. Some recent developments 4. ABR characteristics and perfor- in modeling, applicable to anaerobic digestion, allow bet- mance ter simulations but there is still considerable potential for improvement. Methanogenic fermentation involves a Design: Taking into consideration the slow growth bacterial fermentation of complex organic biopolymers to rate of many anaerobic microorganisms, particularly simpler compounds which can be further metabolized by methanogenics. The main objectives of the efficient reactor methanogenic bacterial species to produce the mixture of design must be a high retention time of bacterial cells with gases, primarily methane and carbon dioxide, known as very little loss of bacteria from the bioreactor. The tech- biogas. It is a mixed culture [35] decomposition process nical challenge to improve the anaerobic digestion lies in which has been widely applied by the waste treatment enhancing the bacterial activity together with good mix- industries for reducing the pollution strength of effluent ing to ensure a high rate of contact between the cells streams, with the added benefit of methane gas produc- and their substrate. The anaerobic baffled reactor was tion which can be used as a source of energy on site. designed primarily for water-soluble waste [31]. Original Design and development to optimize the process require design of Bachmann et al. [31] was modified in order to an understanding of the process kinetics describing micro- improve the efficiency of the reactor. The baffle design was bial growth and enzyme action [36]. Anaerobic digestion modified in both the height of bioreactor and the volume of can be regarded as a self-regulating mixed culture fer- the compartments, which enabled to increase the ability of mentation followed by a methanogenic step. Owing to the entrapping microbe-rich small particles in the reactor. The inherent complexity of the process it is difficult to develop baffles were angled at 45◦ to the horizontal and making dynamic mathematical models which accurately simulate upcomer and downcomer ratio of 4:1 to reduce entrance process performance. Attempts to simplify the analysis velocities on the upcomer and direct incoming wastewater can be made by concentrating on the rate limiting step to the center of the chambers. A series of vertical baffles which is hydrolysis [37]. The development of kinetic mod- which force the wastewater to flow under and over them els to describe the anaerobic treatment process has con- as it passes from inlet to outlet. The wastewater can, tinued over the last thirty years. Although early work con- therefore come into intimate contact with a large amount sisted mainly of "black box" (descriptive) modeling, it was of active biomass as it passes through the bioreactor. This quickly realised that a greater sophistication was needed new configuration has been shown that modified anaero- if the models were to be further developed into better pre- bic baffled bioreactor (MABR) is capable of holding a high dictors of process performance. The predictions of unstruc- retention time of cells in bioreactor and proving efficient tured models such as the "black box" cannot meaningfully treatment of varying wastewater. The anaerobic baffled be extrapolated. Each type of model has major advan- reactor (ABR) showed promise for industrial wastewater tages and disadvantages and some are better suited than treatment, i.e. simple and inexpensive to construct, since others for particular applications. Dynamic models are there is no moving parts or mechanical mixing device. In recognized as being the most useful type [? ]. They can

392 S. R. Hassan, I. Dahlan

Table 1. Summary of performance data

Wastewater HRT (h) Influent COD COD Re- OLR Gas pro- References (mg/L) moval (%) (kg/m3.d) duced (L/day) Dilute Wastewater 80 50 80 - [4] Brewery Wastewater 15 Varies 92 5.6 24.01 [13] Soybean Wastewater 39.5 2000 97 1.2 Varies [14] Heavy Oil Produced Water 60 50 65 Varies - [15] Domestic Wastewater 48 305.18 74 - - [18] Municipal Wastewater 6 350 86 2.62 0.34 [19] Palm Oil Mill Wastewater 3 16000 77.31 1.60 27.4 [20] Indiluted Brewery 15 5500 31.82 2.75 55.7 [21] Wastewater Synthetic Substrate 24 3000 82 3.00 - [22] Wastewater Pulp and Paper Mill Black 48 4020 68 5.00 2.95 [23] Liquor Penicillin Reduction 64 8.00 65 2.64 - [24] Wastewater Domestic Wastewater 22 716 72 - - [25] High Sulphate Wastewater 240 6.6 82.71 0.66 0.29 [26] Low Strenght Wastewater 12 550 89 1.69 - [27] Soybean protein process- 39.5 10000 97 6 - [28] ing wastewater Complexed Wastewater 8 500 88 2 0.31 [29] Synthetic Wastewater 10 501 90.7 1.2 0.36 [30] be used for control purposes and to predict process failure, although the Monod type model has been applied exten- unlike steady state models which are only useful for pro- sively, there are still major problems when using it for the cess design applications. The most popular models can be dynamic modelling of the digestion of wastewaters. For divided into four groups, depending on the type of kinet- modified Monod kinetics, a controversial point is the incor- ics assumed: Monod, Modified Monod, First Order and poration of a substrate inhibition function in these models. Modified Contois. All of the published dynamic modelling Some workers feel this is an unjustified complexity which work uses Monod kinetics. In all cases the models are does not improve the predictive capabilities of the basic based on microbial kinetics, it being implicitly assumed Monod model. However, it is important not to generalize, that rates of substrate utilization and product yield can as the effects of substrate inhibition depend greatly on the be predicted on this basis alone. Enzyme kinetics and type and concentration of substrates in the waste streams. variations in enzyme concentration are rarely considered. To summarize, the substrate inhibition kinetics model is a Furthermore monoculture kinetics are assumed as a basis modified form of the saturation kinetics model, allowing for predicting specific population growth. The Monod ex- for end product inhibition effects. However, although it pression has been used primarily for dynamic modelling may be an improvement over conventional Monod kinetics work, regardless of the limitations outlined, and especially in predicting process performances at high organic con- where a computed solution is necessary. Slight modifica- centrations, it still suffers from the major drawbacks of tions have been made to the models in some cases, to im- Monod models as many of the assumptions used in the prove predicting power and to reduce the number of input model formulation are the same. Pfeffer [42] and Grady et parameters. These modified models are the most accurate al.[43] have proposed first order kinetic models for the pre- of the models for predicting dynamic process performance, diction of digester performance under steady state condi- particularly of gas yields. However, experimental results, tions. Their simulation results compared well with experi- mainly with laboratory scale digesters, have shown that mental data for volatile solids destruction at low substrate the models are poor at predicting a volatile solids reduc- concentrations, where a first order model approximates to tion, especially at high solids loadings. To summarize, Monod kinetics. Pfeffer [42] found it is difficult to quantify

393 Anaerobic wastewater treatment using anaerobic baﬄed bioreactor: a review

concentrations of biomass and available substrate for use while hydraulic dead space is a function of flow rate and in the modelling of municipal digesters assuming Monod the number of compartments in the reactor. A decreased kinetics, and he therefore used the first order expression in HRT cause an increased in the hydraulic dead space for the rate of substrate utilization. As it was difficult while creating less biological dead space [46]. Several au- to measure the substrate concentration directly, he used thors have been studying on the effect of residence time the ultimate gas yield as an indicator of substrate con- distribution (RTD) of an ABR. Langenhoff and Stucky [14] centration. They concluded that the model could account studied the effect of RTD at different temperatures to de- successfully for degradation rates falling much faster than termine whether viscosity and the rate of gas production the substrate concentration, and for gas production rates affecting the hydrodynamics of the ABR. From the studies, not proportional to the substrate utilization rate. This im- it shown that the volumes of the dead space were constant provement to the basic first order model is well justified. at the temperatures used, although the gas produced was However, the limitations of the first order model are still low due to the lower COD removals. The average dead inherent and restrict the application. Contois [44] pro- space noted between 25-30%, and the flow pattern showed posed a bacterial growth model in an attempt to simulate an intermediate between plug-flow and ideally mixed. The the effects on specific growth rates of mass transfer limi- mixing characteristics of the reactor can be modeled by tations due to variations in population density. The model two phase dispersion model. The decrease in temperature is a better predictor of volatile solids reduction than is the result in increased viscosity and decreased gas produc- Monod model, as it takes account of the relative recalci- tion, but it seems not to influence the amount of dead trance of the various types of waste. However, its predic- space in the reactors. The modification of an ABR is done tions of gas yield must be used with care, particularly at by splitting the feed between the individual compartments low substrate concentrations, owing to the inherent lim- [47]. It shows that the flow pattern slightly changed due itations of the Contois model. Four basic types of mi- to the higher degree of mixing within the reactor. The crobial kinetics have been used for modelling the volatile effect of splitting the feed on mixing pattern is analyzed solids reductions and gas yields produced by the anaero- by trace analysis. Result shows that when the split-feed bic treatments, Each type of model has major advantages regimes were applied, the pattern changes due to a higher and disadvantages. None of the models so far developed degree of mixing in the reactor. The mixing pattern pro- accurately simulates a dynamic state. For the most basic duced represent intermediate between plug flow and com- simulations, e.g. those assuming first order kinetics, this pletely mixed reactors. These split feed reactors achieved is due to the way the model is constructed, and although an overall performance of 95% COD removal at only or- Monod kinetics have been extended for modelling dynamic ganic loading rate of 10.5 kg COD/ m3 d. In another study, conditions, the basic assumptions used suggest an inad- Liu et al. [48] studied the hydrodynamic characteristics of equate understanding of the kinetic principles involved. a four compartment periodic ABR. The studies were done Further developments have attempted to account for the to investigate the dead spaces and mixing patterns in peri- inhibition and solids solubilization effects, but with only odic ABR by manipulating different organic loading rates limited success. As steady state simulations are of only (OLRs) in switching manners and frequencies. Residence limited value for process control and performance predic- time distribution (RTD) studies were done on both clean tions, a dynamic model based on an appreciation of the and working reactors at same hydraulic residence time basic process kinetics is required [45]. Information should (HRT) of 2 days. The results indicate that the fraction of be gathered from experimental studies, particularly of the dead space is similar to that in ABR, compared to other rate limiting hydrolysis step, and used to propose a suit- reactor design. The flow patterns show intermediate be- able dynamic model. Acceptable assumptions about, and tween plug flow and perfectly mixed under all conditions approximations of, the process kinetics can then be made tested. In addition, other studies on hydrodynamic char- to ensure the resulting model can be extensively used for acteristics of the split feed ABR were done by Ma et al practical control purposes. [21]. The research focuses on tracer tests of hydrodynamic characteristics and the impacts of the characteris- Hydrodynamics: The hydrodynamics or mixing occurs tics section to the split ABR. The dead spaces and mixing within an ABR has strong influence the extent contact pattern under different effluent recycling ratios and inlet between the bacteria and substrate which will affect the proportions were investigated. From the obtain results, it whole treatment efficiency of the bioreactor. An ABR shows that the hydrodynamics of the reactor were influ- shows a low levels of dead space compared to other anaer- enced by both the changing in effluent recycling ratio and obic designs where which the dead space consists of both inlet proportion. The dead space volume and tank in se- hydraulic and biological dead spaces. Biological dead ries (N) values were tends to decreased with the increas- space is a function of biomass activity and concentration

394 S. R. Hassan, I. Dahlan

ing of effluent recycling ratio and hydraulic flow of split Faisal and Unno [20] has been studied the kinetic anal- ABR tended to be completely mixed pattern. The result ysis of palm oil mill wastewater treatment by a modified from RTD curves also proved that both the effluent recy- anaerobic baffled reactor. Results showed that value of cling ratio and inlet proportion had affected the hydraulic kinetic parameters, (A) and refractory coefficient (R) cal- characteristics of split ABR. When the effluent recycling culated according to the model are 0.329 and 0.119 re- ratio increased from 0 to 100%, it shows a decreased of spectively. Smaller R value means higher digestibility, volume of dead space from 0.349 to 0.114. On the other which indicates that palm oil wastewater is an appropri- hand, the treatment of low strength wastewater (500 mg ate substrate for an anaerobic digestion to obtain methane COD/L) was studied by Krishna and Kumar [49] to ana- gas. The meaning of kinetic parameter is made clear by lyze the hydrodynamics of the reactor at 8 and 10 h by the following equation: adding a pulse of an inert tracer (Lithium chloride) to the reactor. Residence time distribution (RTD) studies show that the dead space increased with a decrease in HRT µ S 1 = = 1−A (1) due to more channeling in the reactor bed. However, in µm A(So − S) + Ks + S ASo + KS )/(1 − A)S this particular work, it shows that at low HRT, higher gas production was noted due to an increased in the organic As shown in the above equation, the variable group loading rate. Hence, it creates less biological dead space A(So − S) + Ks can be observed as an apparent half sat- due to more mixing occur in the reactor. For that reason, uration concentration. The maximum achievable specific the results give decreased in volume of dead spaces as growth rate will be µm/(1 + Ks/So) . It can be concluded the HRT decreased. This result seems contradictory to that the kinetic parameters, A = KskY /Kh reflect solely the theory that dead space should increase as the HRT the reaction characteristics, independent of the concen- decreased. This is mainly due to the biological dead space tration of substrate. Similar to that, Zinatizadeh et al. that contributes to overall dead space. It can be concluded [51] studies the kinetic evaluation of palm oil mill ef- that variation of HRT had no significant effect on hydraulic fluent (POME) digestion in high rate up-flow anaerobic dead space. In a more recent work, Yuttachai et al. [50] sludge fixed film bioreactor. Chen and Hashimoto kinetic studied the hydraulic characteristics of an ABR as on-site equation and simplified Monod’s model were employed wastewater treatment system. The ABR used consists of in the study to explain the kinetics of POME anaero- one sedimentation chamber and three up-flow chambers bic digestion at organic loading rates between a range in series under different peak flow factors (PFF), super- of 0.88-34.73 g COD/ L.day. From the result calculated ficial gas velocities and hydraulic retention times (HRT). by simplified Monod model, the apparent rate constants, The hydraulic characteristics were determined based on K calculated were in the range of 2.9-7.41 CH4/g COD the RTD curves. From the research done, it is found that day. K values show a linear relationship with the vari- dead spaces of an ABR under non steady flow conditions ations of the VSS content of the reactor at different in- do not exceed 13% when PFF < 4, there 2-fold increase fluent COD concentrations. It is clear that the proposed (26%) at PFF of 6, which is relatively lower than any other kinetic equations are applicable to anaerobic treatment high rate anaerobic systems. It shows that ABR is "inter- of POME. The biokinetic coefficients A, Ks, µm, were mediate" between plug flows and completely mixed. The found to be 0.738, 0.982 g COD/L, and 0.207/day re- hydraulic efficiency reflects the ability to distribute the in- spectively. Methane production rate was between 0.287 flow evenly across the system and also the amount of mix- and 0.348 l/g COD removed day. The biomass yield (Yx) ing. The hydraulic efficiency was affected by the amount and methane yield (YM) coefficients were 0.174 g VSS/g of mixing patterns. Indeed there are lots of research have CODremovedand CH4 STP/g COD removed respectively. In been done regarding the hydrodynamic characteristics of another research work, Ghaniyari et al. [52] investigated ABR, but investigation not taken into account other fac- the kinetic and performance of a hybrid anaerobic baf- tors, which are biogas mixing properties, viscosity changes fled reactor in treating synthetic wastewater at mesophilic due to extracellular polymer production and biomass par- temperature. The aim is to study the kinetics behaviors of ticle size within the reactor. Further studies should be two different models which are axial diffusion or disper- carried out. sion model and completely stirred tank reactor (CSTR), assessed and compared to simulate the organic matter re- Mechanism and Kinetics: Kinetic studies of an anaerobic moval or fractional conversion. The results obtain shows digestion of ABR have received more attention recently. that kinetic constant (k) using CSTR in series model was Many papers have been established in this field. Here 0.60 ± 0.07 h−1, while for dispersion model was 0.67 are some reviews of kinetics studies involving the anaer- ±0.06 h−1, the dispersion coefficient (D) being 46. It obic digestion of ABR in treating different wastewater. can be concluded that dispersion model allowed a slightly

395 Anaerobic wastewater treatment using anaerobic baﬄed bioreactor: a review

better ﬁt of experimental results of fractional conversions which are lower than 8% between the experimental and theoretical values. In addition, Yunqin et al. [53] studied on the kinetics of mesophilic anaerobic digestion of pulp and paper sludge. The sludge retention time (RTD) and reaction rate constant, k is described on the basis of a mass balance in a CSTR by following the ﬁrst order ki-

netics equation. From the studies, the values of ym and k obtained were 0.733 m3kg−1 of remove VS and 0.07 d−1 respectively. The created kinetic model equation can be used to dimension completely stirred tank digesting organic waste from different waste. In a more recent work, Luo et al. [54] reported the kinetics in anaerobic digestion of waste activated sludge enhanced by α-amylase. From the kinetic analysis results, it shows that enzymatic hydrolysis well fitted first order kinetic model at 50◦C. The conversion coefficient (α) of VSS to soluble chemical + oxygen demand, NH4 −N and carbohydrates were found to be 0.266, 0.038 and 0.043 respectively. Result from the first order kinetics fitted the experimental data rea- sonably well (R2= 0.985). The corresponding substance conversion equation was considered as

−0:442t X = Xo + 0:266So(1 − e ) (2)

From the given equation, the conversion coefficient, α predicated from the model was 0.266 ± 0.012 g SCOD/g VSS. Based on recent established work, it shows that the anaerobic digestion of ABR is kinetically applicable to be used to treat different types of wastewater. Figure 2. Three Main Categories of Anaerobic Bacteria in the ABR System (a) Hydrolytic Bacteria, (b) Acetogenic Bacteria, Microorganism: Activities of various kinds of mi- (c) Methanogenic Bacteria [55] croorganisms are the main factor for anaerobic digestion which produces methane gas. There are many recent studies are done to investigate anaerobic microorganisms which converting the waste into biogas. Sallis and tain result, it shows that there are small amount of fungi Uyanik [47] investigated the effect of split-feeding on the (2%), protozoa (5%) and almost 93% of the microorgan- distribution of bacterial populations within the granule ism present consists of bacteria. It is obviously proved architecture. Results obtained indicate that filamentous that bacteria are responsible for anaerobic biodegrada- bacteria developed in the latter stages of granulation. tion of kitchen waste. Methane gas were derived two After 68 days, the lowest and highest value of bacterial third from acetate conversion by methanogenic bacteria population was observed in compartment 2 (6.5% of and other third result from carbon dioxide reduction by hy- total count) and compartment 4 (11% of total count) drogen. Methanosarcina, methanococcus, methanospril- respectively. It confirms that split feeding gave support to ium, methanotrix and methanobacterium were found in the methanogenic populations in the initial compartments. the biodegradation of kitchen waste. Methanosarcina, The microorganism present includes methanosaeta sp., methanococcus and methanotrix were higher in percent- methanobrevibacter sp., and methanococcus sp. age compared to other methane formers due to ability of In addition, Malakahmad et al [55] identify anaerobic mi- activity in acetate environment in the ABR. In another croorganisms that responsible for converting kitchen waste study, Ji et al [15] investigated the presence of microor- to biogas. The mixtures of 25% sewage sludge with 75% ganism in heavy oil produced water with high concentra- kitchen waste were used as substrate. Figure 2 illus- tion of salt and poor nutrients. The result shows that rod- trated hydrolytic, acetogenic and methanogenic bacteria shaped and spherical granules in which Methanosarcina, in the ABR system from the research done. From the ob- Clostridia, and Methanothrx sp. were main populations,

396 S. R. Hassan, I. Dahlan

to digest the wastewater to produce biogas, however there Figure 3. Diﬀerent types of bacteria present [49]. are still a few of them. Intensive research must be done to investigate the microbial involved and how it evolved throughout the system.

5. Future research directions

Overall analysis of the ABR for treatment of industrial wastewater leads to the conclusion that there is no ideal system applicable to all conditions. Each situation must be analyzed individually, with the constant concern of in- corporating the local speciﬁcities in the stage of investigation and decision. Only through the opening of multi- ple avenues one can really reach an eﬃcient, economical and adequate solution, not only in the design stage; but mainly throughout the operational life of the treatment system. Indeed, to enhance maximum commercial application of ABR, more work need to be done in the following area: reaction mechanism of anaerobic digestion, the fate of solids, intermediate products, COD removals, nutrient requirements, and an improved understanding of the factor that controlled the ecology of the bacteria.

6. Acknowledgments

The authors wish to acknowledge the financial support from the Universiti Sains Malaysia (RU-I Grant A/C.1001/PJKIMIA/814148). found in each compartments of ABR. Rhodopseudomonas with the activity of lipase and halotolerants were also observed in the fist five compartments. This photosynthetic bacterium is beneficial to acidogenesis for hydrocarbons References (HCs) and recalcitrant organics under the condition of high strength of salt of heavy oil produced water. On the other [1] Abdullah A.G.L., Idris A., Ahmadun F.R., Baharin B.S., hand, Krishna and Kumar [32] reveal the microbial popu- Emby F., et al., A Kinetic Study of a Membrane lation distribution in the reactor system of a low strength Anaerobic Reactor (MAR) For Treatment of Sewage soluble wastewater. From the results, VFA profiles and Sludge. Desalination, 2005. 183(1-3): p. 439-445 SEM images tend to indicate compartmentalization in the [2] Hassan S.R., Zwain H.M., Dahlan I., Development of ABR serves to separate acidogenic and methanogenic ac- Anaerobic Reactor for Industrial Wastewater Treat- tivities longitudinally through the reactor. The highest ment: An Overview, Present Stage and Future acidogenic activity was found in the first compartment. Prospects. Journal Advance Science Research, 2013. Two types of cocci were observed within the reactor which 4(1): p. 07-12 are small and large, resembles species of methanococ- [3] Boopathy R., Tilche A., Anaerobic Digestion of High cos genus. Amount of small cocci decreased from the first Strength Molasses Wastewater Using Hybrid Anaer- compartment to compartment 8 while large cocci increased obic Baffled Reactor. Water Research, 1991. 25(7): p. from compartment 5 to 8. Other morphotype found were 785-790 bamboo-shaped rods, a typical characteristics of aceto- [4] Tilche A., Yang X., Light and Scanning Electron Mi- clastics methanogen. Figure 4 shows different types of croscope Observations on The Granular Biomass of bacteria found in their study. Although there are recent Experimental SBAF and HABR Reactors. in Proceed- papers discuss on microbial identification that responsible ings of Gasmat Workshop. 1987. Netherlands.

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