O A

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Advances in Environmental Biology, 6(7): 2143-2150, 2012
ISSN 1995-0756
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLE
A Review on Anaerobic Digestion, Bio-reactor and Nitrogen Removal from Wastewater
and Landfill Leachate by Bio-reactor
1
Amin Mojiri, 1Hamidi Abdul Aziz, 1Nastaein Qamaruz Zaman and 2Shuokr Qarani Aziz
1
School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang,
Malaysia
2
Department of Civil Engineering, College of Engineering, University of Salahaddin–Erbil, Iraq
Amin Mojiri, Hamidi Abdul Aziz, Nastaein Qamaruz Zaman and Shuokr Qarani Aziz; A Review on
Anaerobic Digestion, Bio-reactor and Nitrogen Removal from Wastewater and Landfill Leachate by
Bio-reactor
ABSTRACT
The nitrogen removal from wastewater and landfill leachate has become a vital part of the overall treatment
process because nitrogen compounds have the significant impact on the environment. Generally, biological
nitrogen removal is used for nitrogen elimination from wastewater. In the last years, newly advanced anaerobic
reactor systems such as up flow anaerobic sludge blanket (UASB), anaerobic filter (AF), anaerobic fluidized bed
(FB), anaerobic sequencing batch reactor (AnSBR) and other anaerobic reactors have been used for the
treatment of low strength wastewater, landfill leachate and, etc. The aims of this study were the review on the
anaerobic digestion, some kinds of bio-reactors and nitrogen removal from wastewater and landfill leachate by
bio-reactor. Many investigations showed that the bio-reactor and anaerobic treatments could be reducing
nitrogen from wastewater and landfill leachate. Additionally, many studies also were reported some
environmental factors such as pH, temperature, ammonia, alkalinity and nutrients affecting on anaerobic
digestion.
Key words: Anaerobic digestion, bio-reactor, nitrogen removal, wastewater, landfill leachate
Introduction
One of the elements of concern in wastewater is
nitrogen, especially since the use of synthetic
nitrogen fertilizers produced from atmospheric N2 by
the Haber-Bosch process has increased tenfold over
the last 40 years. The human contribution to nitrogen
pollutions, especially in the form of urine, is ever
increasing in view of the growing world populations.
Discharge of this nitrogen into the natural waters can
lead to eutrophication and oxygen depletion [11].
The nitrogen removal from wastewater has
become a vital part of the overall treatment process
because nitrogen compounds have significant
impacts on the environment. Conventionally,
removal of nitrogen is performed by means of a two
step process. First, NH4+ is oxidized to nitrite (NO2-)
or nitrate (NO3-) by an autotrophic nitrification
process, and this is subsequently reduced to nitrogen
gas by a heterotrophic nitrification process. However,
some wastewaters are rich in NH4+ but poor in
biodegradable organic carbon; these wastewaters are
less suitable for biological nitrogen removal via the
conventional nitrification-denitrification processes
[18].
One of the common and economical methods in
wastewater treatment is biological processes. The
main objective of biological treatment is to convert
organic materials into other products using
microorganisms.
Generally, biological nitrogen removal is used
for nitrogen elimination from wastewaters. Although
NO2- or NO3- can be present, ammonia (NH3-N) is
abundant in many wastewater streams. NH3-N
removal
is
often
achieved
using
nitrification/denitrification processes. In such
systems, nitrifying bacteria oxidize NH3-N to NO3under oxic conditions, and NO3- is subsequently or
simultaneously reduced to nitrogen gas under anoxic
conditions [20].
Nitrogen removal is normally realized by
sequentially alternating between oxic and anoxic
situations or by the creation of separated zones with
suitable
conditions
for
nitrification
and
denitrification, respectively. Alternatively, high rates
of simultaneous nitrification and denitrification
(SND) can be achieved, in activated sludge and
biofilm type systems alike, at operational conditions
where both oxic and anoxic micro-environments are
present. Nitrification can occur at the liquid/biomass
Corresponding Author
Amin Mojiri, School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300
Nibong Tebal, Penang, Malaysia
E-mail: [email protected]
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Adv. Environ. Biol., 6(7): 2143-2150, 2012
interface, while denitrification of NO3- (or NO2-) may
be found in deeper sub-surface biomass zones [4].
Biological nitrogen removal proceeds slowly
because the microorganisms responsible for the
elimination reactions grow slowly. In addition, the
operational control of aerobic and anaerobic
conditions needed for nitrification and denitrification,
respectively, can be difficult. To cope with these
problems, various kinds of bioreactors have been
studied for enhancing the efficiency of nitrogen
removal. Examples of enhanced processes include
the combined nitrification and denitrification,
immobilization of bacteria on polymeric gel beads in
a moving bed biofilm reactor, and the formation of
bacterial film on the surface of rotating disks or other
packing in a moving bed biofilm reactor or aeration
tank [12].
The objectives of this study were the review of
the some kinds of bio-reactor and nitrogen removal
from wastewater and landfill leachate by bio-reactor.
Anaerobic digestion (AD):
Anaerobic treatment represents a sustainable and
appropriate wastewater treatment system for
developing countries and already is becoming an
accepted simple and cost-effective technology for the
treatment of a variety of wastewaters [1]. AD is a
biological process that happens naturally when
bacteria breaks down organic matter in environments
with little or no oxygen. It is effectively a controlled
and enclosed version of the anaerobic breakdown of
organic waste in landfill which releases methane [6].
AD of industrial wastewater is commonly used all
over the world, not only as a pollution control tool
but also for energy recovery as methane [22].
The AD process takes place in an airtight
container, known as a digester. The first stage of
AD is a chemical reaction called hydrolysis, where
complex organic molecules are broken down into
simple sugars, amino acids, and fatty acids with the
addition of hydroxyl groups. This is followed by
three biological processes:
Fig. 1: The three distinct temperature ranges [25].
 Acidogenesis - further broken down by
acidogenic bacteria by into simpler molecules,
volatile fatty acids (VFAs) occurs, producing
ammonia, CO2 and hydrogen sulfide as byproducts.
 Acetogenesis - the simple molecules from
acidogenesis are further digested by bacteria called
acetogens to produce CO2, hydrogen and mainly
acetic acid.
 Methanogenesis - methane, CO2 and water are
produced by bacteria called methanogens.
The pH level should be kept between 5.5-8.5 and
the temperature between 30-60°C, in order to
maximize digestion rates [6].
Some environmental factors affecting on AD:
Temperature:
There are three distinct temperature ranges
associated with microbial growth in most of the
biological processes namely: psycrophilic range
between 4 and approximately 15 °C, Mesophilic
range between 20 and approximately 40 °C and
thermophilic range between 45 and 70 °C and above.
In each of these ranges, where microbial growth is
possible, within these ranges three temperatures
values are usually used to characterize the growth of
the microorganism species namely [7];
 Minimum temperature, below which growth is
not possible
 Optimum temperature, in which growth is
maximum
 Maximum temperature, above which growth is
not possible
Temperature effects on kinetic formulation:
specific microbial growth rate, half-saturation
constant, and inhibition constants were presumed to
be exponential equation [3]:
F (T) = eθ (T-To)
(1)
Where θ = temperature coefficient in the mesophilic
(30−40°C) and thermophilic (50−60°C) temperature
range.
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pH and alkalinity requirements:
The optimum pH for methane fermentation is
between 7 and 8, but the methane bacterio generally
are not harmed unless the pH drops below about 6,
the lower the pH the shorter the time for a given
decrease in activity. For this reason, it is essential
that the pH be maintained above 6 at all time. When
the bicarbonate alkalinity drops below about 500
mg.L-1, and with the normal percentage of carbon
dioxide in the digester gas, the pH will drop
dangerously close to 6. When digesters become
unbalanced, the volatile acid concentration increases
destroying the bicarbonate alkalinity [8].
Low pH inhibits the process, for an enhanced
degradation rate, a neutral pH is recommended [25].
The pH is an important factor for keeping functional
anaerobic digestion. A typical pH is in the range of
6.5-7.6. The accumulation of intermediate acids leads
to pH drop during fermentation. In order to maintain
stable operation, it is necessary to add bicarbonate or
carbonate as an alkalinity buffer to neutralize volatile
fatty acids and carbon dioxide [3].
Alkalinity is an important parameter in
anaerobic digestion; it is a measure of chemical
buffering capacity of the aqueous solution.
Bicarbonate, hydrogen sulphide, dihydrogen
phosphate and ammonia are the compounds that
provide a significant buffering capacity in the useful
region around pH 7. The other compounds such as
VFAs and ammonia also contribute to the alkalinity.
The alkalinity is generated from the decomposition
of organic compounds during the anaerobic digestion
process [7].
High VFA concentration:
Fermentative reactions stopped at a VFA
concentration of 13,000 mg/L accompanied by a low
pH of 5. Propionic acid at a concentration of more
than 1000 mg.L-1 is considered as toxic and can
cause digester failure [25].
Microbiology of AD:
Although anaerobic digestion is generally
considered a two-phase process, it can be subdivided
into various metabolic pathway, with the
participation of several microbial groups is illustrated
in Fig. 1 [25].
Fig. 2: Metabolic pathways and microbial groups involved in AD [16 & 7].
concentration of 1500 mg/L which leads to increase
the pH up to 8.5 which is toxic to methanogens [25].
NH3-N:
Ammonia
inhibition
occurred
at
NH3-N
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Table 1: Effect of NH3-N on AD process [25].
NH4-N (mg L-1)
50-200
200-1000
1500-3000
Above 3000
EFFECT
Beneficial
No adverse effect
Inhibitory at pH over 7.4-7.6
Toxic
Nutrients requirements:
All organisms need essential ingredients for their
growth, viz. macronutrients and micronutrients. The
deficiency of these nutrients will negatively affect
the growth of these organisms. Even though the
requirement of these elements is in very low
concentration, but play very important role in the
growth and performance of the microorganisms [7].
 Nitrogen and Phosphorus, The nitrogen
requirement for anaerobic treatment is only a small
fraction of that required by aerobic process and the
phosphorus requirement is approximately 15% of
nitrogen requirement.
 Trace nutrient requirements, the lack of
understanding of trace nutrients requirements of
methanogens has been a serious hindrance to the
commercialization of anaerobic biotechnology. Four
elements including iron, cobalt, nickel and sulfide
have been shown to be essential for methanogens to
convert acetate to methane [8].
Some Important
treatment:
calculations
for
while anaerobic digestion of which the liquid has to
stay within the digester until degradation. The HRT
can be calculated as follows [15].
(3)
Where,
HRT= Hydraulic retention time (days)
OLR= Organic loading rate (kg-COD/L.day)
CODin= Influent COD (kg-COD/L)
The flow rate:
The HRT and flow rate examine the exact
influent stream from feed inlet to the outlet.
Normally, flow rate is controlled by means of a
peristaltic pump with corresponding tube hosing of
different diameter. The flow rate is designed
according to the working volume of the reactor. The
flow rate can be calculated as follows [15],
(4)
anaerobic
The F/M ratio:
The F/M ratio would simply be the digester
loading divided by the concentration of volatile
suspended solid (biomass) in the digester
(kg-COD/kg-VSS.day). For any given loading,
efficiency can be improved by lowering the F/M
ratio and increasing the concentration of biomass in
the digester. Also for given biomass concentration
within the digester, the efficiency can be improved
by decreasing the loading.
The F/M can be
calculated as follows [15],
(2)
Where,
Organic loading rate= COD of the influent stream
(kg-COD/L.day)
Volatile
solid=
Volatile
suspended
solid
concentration in the reactor (kg-VSS/L)
F/M= kg-COD/kg-VSS.day
The hydraulic retention time (HRT):
The HRT calculation before proceeding
experiments is also an important process control
parameters. It shows the total time required by the
liquid to degrade. The HRT plays an important role
Where,
Q= Flow rate of influence stream (L/day)
Vw= Working volume of the reactor (L)
HRT= Hydraulic retention time (days)
A new and promising trend in wastewater and
solid waste management is using a bio-reactor. In the
last few years, newly advanced anaerobic reactor
systems, such as UASB, anaerobic filter (AF),
anaerobic fluidised bed (FB), anaerobic sequencing
batch reactor (AnSBR) and other modifications of
anaerobic reactors have also been used for the
treatment of low strength wastewater [Bodik et al.,
2002] landfill leachate and etc.
The upflow anaerobic sludge blanket (UASB):
The UASB reactor was developed in the
Netherlands in the early 1970s [16]. This reactor, as
originally proposed by Lettinga, was one of the
earliest systems in which development of a granular
biomass was observed. As the result of the excellent
settling characteristics of this granular biomass and
the presence of a specially designed three-phase
(biogas, water and biomass) separator device in the
upper part of the UASB-reactor, excellent sludge
retention is assured in this reactor system. The major
disadvantage of the UASB process comprises,
although merely in those cases where proper seed
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sludge is not available in sufficient quantities, the
relatively long start-up period. During the initial
phase of the (first) start-up process of a
UASB-reactor, inoculated with flocculent seed
sludge, a significant washout of sludge generally will
manifest and therefore the first reactor start-up would
highly benefit from skilled operation [19].
The UASB reactor is the most successfully used
as high rate anaerobic treatment system and it
becomes possible high volumetric loadings at short
retention times and at high temperatures. The
performance of the UASB system is limited by slow
hydrolysis of entrapped solids especially at low
temperatures and a two-phase system can be operated
to provide optimal conditions for the microorganisms
for greater efficiency in digestion in the treatment of
domestic wastewater [1]. The UASB reactor (in
sequel as UASBR) consists of a sludge bed in the
lower part and a three phase separator (g-l-s
separator) in the upper part of the reactor [9].
Fig. 3: UASB Reactor Schematic
The Upflow anaerobic filter (UAF):
Anaerobic hybrid reactor (AHR):
Anaerobic filters have been used successfully for
the treatment of dilute wastewater with chemical
oxygen demand (COD) removal efficiencies between
60 and 80% depending on the HRT used [1].
Padilla-Gasca and López reported the UAF presents
advantages over other anaerobic treatment systems
such as the UASB or the expanded bed reactor. The
UAF maintains the biomass immobilized over a fixed
support, which allows the maintenance of high
Cellular Retention Times (CRT) in spite of operating
at low HRT. Also, the UAF tolerates small variable
Organic Loading Rate (OLR) and pH. These features
enable a reduction in the reactor’s dimensions and, as
a consequence, capital investment, but above all,
make it easier to operate.
The essential features of the anaerobic filter
design was listed by Switzenbaum [21] as: a
distributor in the bottom of the column, a media
support structure, inert packing material, a free board
above the packing material, effluent draw-off and
optional features such as, recycle facilities,
backwashing facilities or a sedimentation zone below
the packing material [1].
The anaerobic hybrid reactor combining the
sludge blanket in the lower part and filter in the
upper part has been reported to promote the
advantages of both UASB and upflow filter, while
minimizing their limitations [13].
The AHR is a combination of the UASBR and
the anaerobic filter. A layer of biomass carrier is
situated in the upper part of the AHR instead of the
g-l-s separator. This layer separates the bubbles of
the biogas from the biomass and acts as a support
material for the biomass growth as well. The layer
even has a notable efficiency as a suspended solids
(SS) separator [9].
AnSBR (Anaerobic sequencing batch reactor):
An AnSBR process is one of the novel and
promising high-rate anaerobic processes and has
been used for treating organic wastewaters.7–10
Sequencing batch reactor processes offer distinct
advantages when compared with continuous
processes, including a high degree of process
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flexibility and no requirement for a separate clarifier.
This process repeats a cycle that includes feeding,
reaction, settling and withdrawal steps in a single
reactor [17].
Dague et al. [5] studied anaerobic treatment of
dilute wastewater using the laboratory AnSBR. The
results showed that the AnSBR process was capable
of achieving in excess of 90% soluble COD and
5-day biochemical oxygen demand (BOD5) removal
at temperatures of 25 °C and 20 °C at HRT in range
6–24 h. At the low temperature of 5 °C and the 6 h
HRT, soluble COD and BOD5 removals were 62%
and75%, respectively [2].
Fig. 4: Schematic diagram of the UAF-reactor [19].
Nitrogen removal from wastewater by bio-reactor:
Lv et al. [18] studied autotrophic nitrogen
removal discovered in suspended nitritation system.
This study revealed that nitritation, Anammox and
autotrophic denitrification were responsible for the
nitrogen removal. The NO3- production was caused
by the coaction of nitrite-oxidizing bacteria and
AnAOB (anaerobic ammonium oxidationbacteria).
Lackner et al. [14] investigated Heterotrophic
activity compromises autotrophic nitrogen removal
in membrane-aerated biofilms: Results of a modeling
study. These results clearly indicate the importance
of heterotrophic activity in autotrophic N removal
biofilms, especially in counter-diffusion systems
where they may compromise N removal capacity.
Clifford et al. [4] investigated Nitrogen
dynamics and removal in a horizontal flow biofilm
reactor for wastewater treatment. These results can
be used to optimise horizontal flow biofilm reactor
(HFBR) reactor design. The HFBR technology could
provide
an
alternative,
low
maintenance,
economically efficient system for carbon and
nitrogen removal for low flow wastewater
discharges.
Yu and Zhou [24] studied nitrogen removal
efficiency of an A2/O bio-reactor treating domestic
sewage mixed with landfill leachate and fecal sewage.
These results showed that: the optimal volume ratio
of landfill leachate, fecal sewage and urban
wastewater in the A2/O process was 1:3.75:1000. The
average removal efficiency of NH3-N, total nitrogen
and COD can reach 96%, 61% and 85% respectively
under the conditions of HRT of 11h, dissolved
oxygen of 3 mgL-1 the mixed-liquid return ratio (r)
of 200% and sludge return ratio (r) of 80%.
Sliekers et al. [20] studied completely
autotrophic nitrogen removal over NO2- in one single
reactor. These methods showed that during steady
state, anaerobic ammonium-oxidizing bacteria
remained present and active. In the reactor, no
aerobic nitrite-oxidizers were detected. The
denitrifying potential of the biomass was below the
detection limit. NH3-N was mainly converted to N2
(85%) and the remainder (15%) was recovered as
NO3-.N2O- production was negligible (less than
0.1%). Addition of an external carbon source was not
needed to realize the autotrophic denitrification to
N2.
Nitrogen removal
bio-reactor:
from
landfill
leachate
by
He et al. [10] investigated characteristics of the
bioreactor landfill system using an anaerobic–aerobic
process for nitrogen removal. After 56 days
operation, the leachate total nitrogen and NH4 -N
concentrations decreased to less than 200 mg/l in the
bioreactor landfill system. The COD concentration
was about 200 mg/l with less than 8 mg/l BOD5 in
recycled leachate at the late stage.
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Tengrui et al. [23] investigated characteristics of
nitrogen removal from old landfill leachate by
sequencing batch biofilm reactor (SBBR). The
results showed that after two months long period of
domestication and one month period of stability, the
NH3-N removal efficiency reached to 99% in the
SBBR, at nitrogen loading rate 0.51 kg total nitrogen
m-3 per day and HRT was 9 hrs, met to Chinese
standards for discharge.
Visvanthan et al. [26] investigated landfill
leachate treatment using thermophilic membrane
bioreactor. This result indicates that thermophilic
system is highly suitable for COD and BOD5
removal especially at elevated organic loading.
5.
6.
7.
8.
Conclusions:
9.
AD plays an important role in wastewater
treatment processes. It includes a series of
biochemical processes by different microorganisms
to degrade organic matter under anaerobic conditions.
Methane, the digestion byproduct, is a rich source of
renewable energy, which can help to replace fossil
fuel to contribute to environmental conservation and
sustainability. Therefore, AD is widely used as an
attractive means for wastewater treatment around the
world while more and more new process
configurations are continuously being developed.
Many studies were reported some environmental
factors affecting anaerobic digestion such as pH,
temperature, NH3-N, alkalinity and nutrients. Newly
advanced anaerobic reactor systems, such as UASB,
anaerobic filter AF, FB, AnSBR and other
modifications of anaerobic reactors have also been
used for the treatment of low strength wastewater. In
literature, many investigations showed that the
bio-reactor could be reducing nitrogen from
wastewater and landfill leachate.
10.
11.
12.
13.
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