Study on Performance of a Modified Anaerobic Baffled Reactor to Treat High Strength Wastewater
A.B. Noor Ezlin
The main problem associated with the treatment of high strength materials in an Anaerobic Baffled Reactor (ABR) is difficult breakdown of fat, protein and hydrocarbon molecules at early stage of anaerobic decomposition due to uncertain selection of HydraulicRetention Time (HRT) and therefore organic loading rate (OLR). Thus, in this study a modified four-compartment ABR with a working volume of 50 L was designed to determine the treatment efficiency and methane production rate of high strength wastewater. The first compartment in the ABR was doubled in size to provide longer solid retention time. Based on C/N ratio of 30, a mixture of 62% kitchen waste and 38% sewage sludge was used as substrate and fed to the reactor continuously. Initially the characteristics of kitchen waste were measured and the amounts of fat, protein cellulose, hemicellulose and lignin were found in proper level for anaerobic decomposition. Next the effects of different HRT and OLR were evaluated in the reactor. COD reduction and biogas production were investigated at different HRT (5, 4, 3 and 2 days) and OLR (2, 3, 4, 5 and 6 kg/m3d). Results show that the highest COD removals (74.5 and 75.4%) were observed at 3 days HRT and OLR of 2 kg/m-3day, respectively. While the best production of biogas (7.40 and 9.10 L day-1) was observed at 5 days HRT and OLR of 6 kg/m3 day, respectively.
October 18, 2010; Accepted: December 09, 2010;
Published: March 09, 2011
The successful application of anaerobic technology to the treatment of high
strength wastewater is critically dependent on the development of high rate
anaerobic bioreactors. These reactors achieve a high reaction rate per unit
reactor volume (in terms of kg COD/m3 day) by retaining the biomass
in the reactor. High rate anaerobic biological reactors may be classified into
three broad groups according to the mechanism used to achieve biomass detention
which are, fixed film, suspended growth and hybrid system (Barber
and Stuckey, 1999).
The anaerobic baffled reactor (ABR) was initially developed by McCarty and
coworkers at Stanford University (McCarty, 1981). Then
the process of ABR was used and described by Bachman et
al. (1983, 1985) with strong synthetic wastewater.
The ABR can be described as a series of Upflow Anaerobic Sludge Blanket (UASB)
reactors, which does not require granulation for its operation (Barber
and Stuckey, 1999). In an ABR series of vertical baffles force the wastewater
to flow under and over them as it passes from inlet to outlet. Bacteria within
the reactor gently rise and settle due to flow characteristics and gas production
in each compartment. Each chamber has a vertical baffle to force wastewater
to flow under and over it.
The most significant advantage of the ABR is its ability to separate acidogenesis
and methanogenesis longitudinally down the reactor, allowing the different bacterial
groups to develop under most favorable conditions (Grobicki
and Stuckey, 1992). This study mainly focused on the operational characteristics
of an ABR in COD removal, biogas production and methane content from high strength
An anaerobic baffled reactor operates with a combination of several anaerobic
process principles, the three basic steps involved are: (a) hydrolysis, (b)
fermentation and (c) methanogenesis. Equal inflow distributionand the wide spread
contact between new and old substrate are important process features. It is
known that a three-chamber reactor, together with physical modifications, provided
a longer solid retention time and superior performance than the reactor with
only two compartments (Barber and Stuckey, 1999). Further
analysis shows that despite losing more solids, the three-compartment reactor
is more efficient at converting the trapped solids to methane. Therefore, it
is recommended in many literature sources that the anaerobic baffled reactor
should be equipped with at least 3 chambers (Grobicki and
Stuckey, 1992; Nachaiyasit and Stuckey, 1995; Yang
and Moengangongo, 1987). However, the main problems associated with the
treatment of high strength materials in a baffled reactor is incomplete breakdown
of fat, protein and hydrocarbon molecules at early stage of anaerobic decomposition
due to insufficient HRT and OLR. Therefore, in this study a modified ABR with
four compartments which the first one was double in size, to provide adequate
HRT and therefore OLR, were examined to evaluate the COD reduction and biogas
MATERIALS AND METHODS
Substrate: The kitchen wastes were collected from kitchen refuse of
a canteen located at Universiti Kebangsaan Malaysia. Part and quarterly methods
were used as standard procedures in preparing samples (Tchobanoglous
et al., 1993). The samples were then mixed thoroughly in the laboratory,
shredded and grounded into a size of approximately 1x1x0.5 cm prior to analysis
for chemical composition.
Seed materials: Sewage sludge was used as seed material and brought
from a municipal wastewater treatment plant (Indah Water Konsertium (IWK) Sdn.
Bhd.) in Serdang, Selangor branch. The sewage sludge was collected from the
sewage sludge return pipeline and immediately brought to the laboratory. The
C/N ratio of applied sewage sludge was 6.3/1 with 75% moisture content. Proper
mixture of kitchen waste and sludge was calculated based on Tchobanoglous
et al. (1993) as follow:
Reactor configuration: A laboratory scale Anaerobic Baffled Reactor
(ABR) system used in the study was fabricated using plexiglass. The ABR consisted
of four chambers and a vertical baffle (Fig. 1) separated
each chamber. The working volume of the reactor was 50 L (length, 75 cm; breadth,
27 cm; height, 25 cm). The baffled reactor was modified to reduce up-flow liquid
velocities and to accept the whole substrate.
Two tanks as influent tank and effluent tank were designed for feeding and
removing the materials to and from the reactor.
|| Anaerobic baffled reactor configuration
A gas collector was also provided for collection, calculation and analysis
s on the amount of biogas. The down-flow chambers were 3 cm above the reactor
s bottom to route the flow to the center of the up-flow chamber to achieve
better contact and greater mixing the feedstock and solids. The first compartment
was bigger in size, which was 34 liters, while the following compartments were
17 L. This physical modification provided longer solids retention time and superior
performance as compared to the reactor with similar size compartments. The edges
on baffles slanted on 45° to route the flow towards the center of the compartment
and, hence, encourage mixing.
Analytical methods: The characteristics of kitchen waste were initially
determined by analyzing the samples using analytical methods given by USEPA
(2004). Samples were analyzed for moisture content, total solids, total
volatile solids, ash content, total organic carbon, Kjeldahl nitrogen, fat,
protein, cellulose, hemicellulose and lignin. A total of 40 samples were analyzed
and mean value for each parameter was calculated. Gas samples were collected
by gas sampling injectors and a sample of 100 ML was used for each run. The
biogas composition (CH4+CO2) was determined by using a
Gas Chromatograph (NUCON 5700) equipped with a thermal conductivity detector
and stainless steel column of length 6 ft, OD 1/4 inch, ID 2 mm, containing
Porapak Q 100 having mesh range 80-100. The carrier gas used was H2
and the analysis was carried out at a carrier gas flow rate of 30 mL min-1
with the injector, column and detector temperatures maintained at 120, 90 and
120°C, respectively. Gas volume was measured using water displacement method.
According to the APHA-AWWA (1992) Dichromate reflux method
was selected for determining Chemical Oxygen Demand (COD).
RESULTS AND DISCUSSION
Characteristics of kitchen waste: Table 1 shows the
characteristics of periodically collected kitchen waste. The amounts for organic
polymer such as fat, protein, cellulose, hemicellulose and lignin was found
in desirable level as these organic polymers have very important role in the
first stage of anaerobic digestion of organic compounds and their present is
vital, because these organic polymers are broken down by extra-cellular enzymes
produced by hydrolytic bacteriaand dissolves in the water. C/N ratio for kitchen
waste was found 38.2/1, which was higher than appropriate amount. Bacteria normally
use up carbon 25-30 times faster than they use nitrogen (Polprasert,
1996). Therefore, at this ratio of C/N (25-30/1) the digester is expected
to operate at its best performance.
Therefore, as C/N ratio of shredded kitchen waste is not sufficient for full
decomposition, other sources of N such as human nightsoil, animal manures or
sewage sludge are needed to be added to the kitchen waste (Vesilind
et al., 2002). Thus, sewage sludge as a source of nitrogen was added
to kitchen waste based on mentioned calculation. The result shows that the composition
of 62 and 38% of kitchen waste and sludge has the C/N ratio of 30 and is suitable
to undergo anaerobic digestion.
Start-up of ABR at different hydraulic retention times: The continuous
operation of the ABR was started using an initial COD concentration of 25 g
L-1 at HRT of 5 days. The ABR was run continuously and observations
were made at particular HRT. When there were no more variations in different
parameters such as COD removal, biogas production and methane content then the
HRT was decreased. The results in Table 2 show that there
was little variation in the percent COD removal with decrease in retention time,
but there was an increase in methane content and biogas production. This suggested
that at higher retention times there was limited availability of substrate for
methanogenesis as most of the COD was utilized for new cell synthesis. However,
the decrease in retention time resulted in more availability of substrate, which
could be converted to biogas. The increase in methane content may be attributed
to the increase in the growth of anaerobes particularly methanogens in the newly
Effects of organic loading rate in the ABR system: The effect of different
OLR was studied in a continuous system by varying the COD of the influent substrate
and the result are illustrated in Table 3.
||Chemical composition of kitchen waste
||Performance of anaerobic baffled reactor at different hydraulic
||Performance of anaerobic baffled reactor at different organic
The best COD removal efficiency of 75.4% was observed when the OLR was 2 kg/m3d.
Low organic loading rates have a better efficiency in COD removal (Grover
et al., 1999). When the OLR was higher the COD removal efficiency
was decreased. The result shows COD removal efficiency of 72.3, 64.3, 63.3 and
57.3% at OLR of 3, 4, 5 and 6 kg/m3d, respectively. This observation
is consistent with results from other researches where they found that the better
efficiency of COD removal occurs at an organic loading rate below 3 kg m-3
day (Bae et al., 1997; Boopathy
and Tilche, 1991; Nachaiyasit and Stuckey, 1995).
On the other hand, an increase in biogas production wa s observed with an increase
in organic loading rate despite the decrease in percent COD removal. According
to the kinetic considerations, high substrate concentrations will encourage
both fast bacteria and organisms growth (Barber and
Stuckey, 1999). This may be attributed to the fact that although there was
a decrease in COD removal at higher loading rates; even then the biogas production
was more at higher OLR than at lower OLR. The increase in OLR resulted in a
decrease in methane content and this resulted in an increasing rate of acidogenesis
and non proportional growth of methanogens, which consumes CO2 as
substrate to produce methane (Grover et al., 1999).
Treatment efficiency of the modified ABR shows that the larger compartment
in the modified ABR acted as a natural filter and provided superior solids retention
for the small particles. The modified reactor can collect twice the amount of
solid material than a reactor with three equal chambers. Observation in different
HRTs showed that there was little variation in the percent of COD removal with
decrease in retention time, but there was an increase in methane content and
biogas production due to more availability of substrate which could be converted
to biogas. When the OLR was higher than 2 kg m-3day COD removal efficiency
was decreased, but an increase in biogas production was observed with an increase
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