Article Review:
Topic: Review on Effect of Operational Parameters in UASB Reactor Performance for
the case of Moderate Temperature in Domestic Wastewater Treatment Plant
Johnny Girma1*, Assie Kemal 2
1
2
School of Civil and Environmental Engineering, Addis Ababa University, Ethiopia
School of Civil and Environmental Engineering, Addis Ababa University, Ethiopia
Correspondence: johnny.girma@aait.edu.et; Tel: +251913338233;
2
Abstract
One of several anaerobic technologies used to treat household wastewater is the up flow Anaerobic
Sludge Blanket (UASB) reactor. The UASB reactor is today's best and most popular solution for
removing organic materials from domestic wastewater. Temperatures between 30°C and 35°C are
ideal for UASB reactor operation in tropical climates. If the UASB reactor is at mesophlic condition,
complex organic compounds should go through the following anaerobic breakdown processes:
hydrolysis, acidogenesis, acetogeneous, and methanogenesis. The question is what if the UASB
reactor is installed at moderate temperature? UASB Operational parameters like Temperature,
hydraulic retention time (HRT), organic loading rate (OLR), up flow velocity (Vup), and PH are the
main operational parameters to be considered in order to improve the performance efficiency of the
UASB reactor for domestic wastewater treatment. The purpose of this review is to emphasize that
combining operational parameters such as HRT, temperature, PH, up flow velocity, organic loading,
and so on will improve the treatment efficiency of the UASB reactor more than just one or two
parameters. This article review shall help Academia, Policy makers /regulatory bodies, and
Industrialists/operators to use the combination of all operational parameters to enhance the
performance efficiency of UASB reactor working at moderate temperature.
Keywords:
Concentration, Toxic Heavy Metal, Kality Wastewater Treatment Plant, UASB, Trickling Filter, EPA
Key Words: UASB, HRT, PH, WWTP, OLR, Temperature, Up flow velocity
1. Introduction
Aerobic and anaerobic biological processes with a high content of organic materials are used in
wastewater treatment plants. Aerobic processes have the disadvantage of producing a large amount
of cell mass from organic matter, whereas anaerobic processes produce only about 3% of the organic
matter in the wastewater feed. The researcher went on to describe anaerobic processes, including
how they use less energy and produce methane, which can be used as a source of energy, how they
do not require any support media for the development of microorganisms because the hydraulics in
the reactor induce the formation of dense granules, which are kept in the lower part of the reactor,
and how they are simple to maintain. When compared to aerobic processes, operating costs are
relatively low. However, it has some drawbacks, including a long startup time, the need for
additional alkali to control the pH, and they are more susceptible to toxic substances [1].The Up
Flow Anaerobic Sludge Blanket (UASB) reactor is one of several anaerobic wastewater treatment
processes. For removing organic matter from wastewater, the UASB reactor is a popular option. It is
an anaerobic process that uses microorganisms to remove organic matter. Because the UASB reactor
performs better at high temperatures, it can be used in tropical countries to treat both industrial and
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domestic wastewater containing high levels of organic material. The flow of wastewater drags the
sludge to the reactor's outlet. There are, however, numerous publications that describe methods for
achieving a successful reactor start-up [1], and this technology is constantly being improved. The
UASB reactor has been used to produce biogas with different compositions, such as methane or
hydrogen [2]. The formation of granules is critical to the successful operation of a UASB reactor
[2].Granules are formed by fine particles containing microorganisms, and the flow of water causes
these fine particles to move, according to physical theory. Two or more particles will inevitably
collide and attach, forming the granule's core. This process is repeated several times, resulting in the
formation of granules as the aggregate's size increases. As a result, the sum of the hydraulic load and
the load caused by gas generation has been reported to play an important role in granulation in the
UASB reactor [3]. Furthermore, the performance of the UASB reactor is evaluated using the removal
efficiencies of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total
Suspended Solid (TSS), Total Dissolved Solid (TDS), sulphates, and other pollutants [ 4].
2. The working principle of UASB reactor
Wastewater from the grit chamber enters the UASB reactor at the bottom and flows up through the
anaerobic sludge blanket. The organic compounds then come into contact with the microorganisms in
the sludge blanket. When organic compounds are broken down by anaerobic digestion, biogas, such
as methane, is removed, and the liquid-solid separator separates the liquid and solid. UASB works
best at mesophilic temperatures (30-35oC) because methanogenosis results in the greatest removal of
COD and BOD; however, lowering the temperature results in poor soluble COD and BOD removal
and deterioration of sludge stability.
3. Factors Affecting the Performance of UASB reactor
3.1. Temperature
Microorganisms are important in the decomposition of organic compounds found in wastewater
treatment plants. There are three different temperature ranges. Psychrophilic (15-30oC), Mesophilic
(30-45oC), and Thermophilic (45-70oC) are the three types. In these three temperature ranges,
anaerobic treatment is possible, and the removal efficiency of the UASB reactor increases as the
temperature rises from Psychrophilic to Thermophilic [5, 6, 7, 8].
The temperature of the UASB reactor is directly related to microorganism growth and death.
Microorganisms have four distinct growth phases, as is well known. These are the lag, exponential,
stationary, and death phases [9].When the working temperature of UASB reactor decreases
methanogenic activity, viscosity, diffusivity, hydrolysis, maximum specific growth rate, and substrate
utilization rate, resulting in a decrease in UASB reactor performance [10].
3.2.
Organic Loading Rate(OLR)
Volumetric hydraulic loading is the amount of wastewater applied to the reactor per unit volume on a
daily basis. Volumetric organic load, on the other hand, is the amount of organic matter applied to the
reactor on a daily basis per unit volume [11].
L = Q/V
Where L-is volumetric hydraulic loading (m3/m3.d)
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Q- is flow rate(m3/d)
V-is volume of the reactor (m3)
𝑂𝐿𝑅 = 𝑄. 𝑆𝑜/𝑉 =So / HRT
Where OLR –is Organic Loading Rate (Kg/m3.d)
Q- is flow rate(m3/d)
V-is volume of the reactor (m3)
HRT-is Hydraulic Retention Time (d)
Microorganisms use organic compounds as food to generate heat and energy.
The food aids in reproduction and locomotion. The rate of organic loading influences parameters
such as COD, methane gas, volatile fatty acid, and bio-gas production. In the UASB reactor, an
appropriate organic loading rate should be maintained to improve COD removal efficiency, bio gas,
methane gas removal efficiency, and process stability. A decrease in the OLR would result in a
gradual increase in the PH. On the other hand, an increase in OLR increases the likelihood of contact
between biomass and substrate, as well as poor COD degradation [12]. The other researcher put
forward that OLR should be in the range of 1-3.5kgCOD/m3.d for proper UASB reactor performance
[13, 14 ]
3.3.
Hydraulic Retention Time (HRT)
Hydraulic Retention Time (HRT) is the average number of hours it takes for wastewater to pass
through the UASB reactor. It is one of the most important parameters influencing the performance of
the UASB reactor [15].
The hydraulic detention time can be calculated by dividing the reactor volume by the
wastewater discharge. The fluctuation of HRT is well known to affect the biological activities of the
UASB reactor. If the HRT is extended beyond the limit, granular sludge will form or granules will be
re-suspended, but this may be beneficial in shortening the reactor's start-up time. If the HRT is too
short, the biomass will be washed away [16].The HRT of the reactor should be between 4 and 12
hours for the UASB reactor to function properly [17. However, there is a formula in the research
done by [18] to calculate the HRT of the UASB reactor, but this would result in a single HRT value,
which would not give the optimal HRT.
3.4.
PH
Because municipal wastewater contains laboratory waste, its PH is higher than that of domestic
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wastewater. To maximize methanogenesis, the PH in the UASB reactor should be kept between 6.3
and 7.8.Because of the buffering capacity of the acid-base system or carbonate system, the PH
should be maintained in this range, and chemical addition is not required [19, 20].
When the OLR in the UASB reactor is low, the PH of the reactor rises. To reduce the PH of the
reactor, the temperature should be changed from mesophilic to thermophilic. This phenomenon
demonstrates that PH influences the performance of the UASB reactor in terms of COD removal,
TSS, VFA concentration, and biogas production [ 21]. [22] investigated how temperature changes
from mesophilic to thermophilic cause severe biomass wash out, lowering PH and fatty acid
accumulation.
Carbohydrates
Sugar
carbonic Acid,
Fats
Fatty Acids
hydrogen,
Proteins
Amino Acids
CO2, Ammina
hydrogen,
CH4
Acetic Acid,
CO2
CO2
Hydrolysis
Acidogenesis
Acetogenesis
Methanogenesis
Fig. 1: Anaerobic degradation of complex polymers in UASB reactor [ 8]
3.5.
Up Flow Velocity
Up flow velocity is the velocity of wastewater crossing a transverse cross section of the UASB
reactor. It’s unit is m3/m2.d.
𝑉=
𝑄
𝐴
Where V-is the up flow velocity (m3/m2.d)
Q-is flow rate (m3/d)
A- Cross - sectional area for the reactor (m2)
In the UASB reactor, up flow velocity helps to disperse the stagnant film around the granules, which increases the
degradation of the substrate granules and facilitates the growth of microorganisms [23]. The up flow velocity in the
UASB reactor should be in the range of 0.5m/hr to 1.5m/hr.If the velocity of flow is less than 0.5m/hr, a stagnant
film will accumulate at the bottom of the reactor; on the other hand, if the velocity of the reactor is greater than
1.5m/hr, microorganisms will be washed out, reducing the efficiency of the UASB reactor.
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4. Removal Efficiency of the reactor
The removal efficiency of the UASB reactor can be calculated by multiplying the difference between
influent and effluent concentrations of the measured parameters by 100.
𝐶𝑖𝑛𝑓.−𝐶𝑒𝑓𝑓.
R=(
𝐶𝑖𝑛𝑓.
) ∗ 100
Where R-is removal efficiency
Cinf.- is concentration of influent
Ceff. –is concentration of effluent
The removal efficiency of the UASB reactor can be determined by measuring the concentrations of
COD,TSS,VSS, NH4-N,VFA,SO4-2, and other contaminants in the influent and effluent .
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5. Conclusions
Temperature, Organic loading, hydraulic retention time, up flow velocity, PH, and other operational
parameters should be used in conjunction to improve the performance of the UASB reactor for
domestic wastewater treatment. Reduced temperature reduces the activities of microorganisms,
thereby inhibiting methanogenesis [24].Temperatures that exceed the range, on the other hand, have
an impact on the health of microorganisms. Another major factor influencing the performance of the
UASB reactor is organic loading. The OLR should be in the range of 1kgCOD/m3.d to
3CODkg/m3.d to improve reactor performance. A decrease in OLR would result in less microbial
activity in the UASB reactor, whereas an increase in OLR would result in more food in the UASB
reactor, resulting in more microorganism activity. The other factor to consider is HRT (4hrs.
to12hrs.).An increase in HRT increases the likelihood of microorganisms coming into contact with
organic compounds, while a decrease in HRT causes organic compounds to be washed out. Up flow
velocity (0.5m/hr. to 1.5m/hr.) is another parameter to consider when improving the UASB reactor's
performance. An increase in up flow velocity above the range washes out microorganisms, whereas a
decrease in up flow velocity allows microorganisms to decompose carbohydrates, fats, and protein.
To facilitate methanogenesis, the PH of the UASB reactor should be in the range of 6.3 to 7.8 for
optimal performance. However, if it is outside the range, H2S will be produced as an effluent gas
rather than CH4 gas. In general, the operational parameters of the UASB reactor section B process
design calculation 2013 should be within the following ranges for the best performance of the UASB
reactor. Temperature (35°C to 45°C), OLR (1kgCOD/m3.d to 3kgCOD/m3.d), HRT (4hrs. to 12hrs. ),
up flow velocity (0.5m/hr. to 1.5m/hr.), PH (6.3 to 7.8), and other parameters.
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Acknowledgments (All sources of funding of the study must be disclosed)
First and foremost, the researchers express gratitude to the Almighty GOD, who gave us the
commitment and tolerance to overcome several obstacles and complete this research in the presence
of many challenges. Last but not least, the researchers want to express their gratitude to Addis Ababa
University, School of Civil and Environmental Engineering which gave the opportunity to pursue
PhD.
Conflict of interest
No Conflict of Interest
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