Dr. Riyadh S. Almukhtar
Anaerobic Digestion (AD)
 Anaerobic
Digestion
(AD)
is
the
consequence
of
series
metabolic
interactions among various groups of
microorganisms. It occurs in three stages: Hydrolysis,
 Acidogenesis
 Methanogenesis.
Anaerobic Digestion (AD)
Anaerobic Digestion (AD)
 Energy equivalents
 1 Watt = 1 joule second-1
 1 Wh = 1 x 3600 joules (J)
 1 kWh = 3600000 J
 1 kWh = 3.6 MJ
 22 MJ (1m3 biogas) = 22/3.6 kWh
 = 6.1 kWh
 Electrical conversion efficiency = 35%
 Therefore 1m3 biogas = 2.14 kWh (elec.)
Anaerobic Digestion (AD)
Before Biogas is Used Removal of Contaminants 
 Moisture and particulates in all recovery processes
 • Inefficient combustion, blocked nozzles
 • Reduced by condensers, filters
 Hydrogen sulfide in most recovery processes
 • In presence of moisture, forms sulfuric acid, which
causes corrosion
Municipal Waste Water Treatment
 The sludge that produced in municipal
wastewater treatment plants seems to
be problems in some countries while in
other countries it recognized as a
source of power and they utilized it in
environmental friends rather than
harming the environment
Municipal Waste WaterTreatment Stages
1. primary sedimentation. Solid-water
suspension is partially separated in conical
tanks
2. Secondary Treatment (biological )The
dissolved solids and fine particles that don’t
settle in primary sedimentation are treated by
aerobic bacteria. These microorganisms using
the oxygen for oxidation of solid particles and
convert them to perceptible solids in aeration
tank.
Municipal Waste Water Treatment Stages
 The solids that produced in last step (sludge +
aerobic bacteria) are deposited secondary
sedimentation.
 Some of activated sludge returned to aeration tank
and the waste is mixed with primary sludge then
fed to thickener basins.
 The aim of thickening is decreasing of sludge
volume up to 30% by dewatering.
 Primary sludge
 Primary is also called raw sludge which comes
from the bottom of the primary clarifier. Primary
sludge is easily biodegradable since it consists of
more easily digestible carbohydrates and fats,
compared to activated sludge which consists of
complex carbohydrates, proteins and long chain
hydrocarbons (Gray et al., 2007). So biogas is more
easily produced from primary sludge.
Municipal Waste WaterTreatment Stages
 Activated sludge
 Activated sludge is also called excess sludge
or waste activated sludge which comes from
the secondary treatment. It‘s a result of
overproduction of microorganisms in the
activated sludge process. The content of
activated sludge was just mentioned above.
Activated sludge is more difficult to digest
than primary sludge.
Al-Rustumiya plant
 Al-Rustumiya plant is one of the biggest sewage
treatment plants in Iraq. It is divided to North and
South plant. It had been established different time, the
first plant called Zero established in 1962 and since
then many additional plants had been added through
seventh and eighteenth of last century.
Al-Rustumiya plant
 Al-Rustumiya plant (both North and South) is dealing
with about 650,000 (m3 /day) of wastewater and serve
round about 3,000,000 persons.
 Unfortunately Al-Rustamiyah plant doesn’t use
anaerobic digestion for produced sludge where the
sludge go directly to drying beds to dry and then it
sold as a fertilizer
Accum ilated biogas production (m l)
600
500
400
300
200
100
0
0
2
4
6
8
Tim e, day
10
12
14
16
Pre-treatment of sludge
Chemical: For alkaline pre-treatment 3liters of
sludge sample filled the three beakers (1 liter
capacity) and pH values of samples were set
at 9, 10 and 11 respectively, by addition of 10N
sodium hydroxide (NaOH). The beakers were
mechanically stirred at 250(rpm) for 2(hr).
After alkaline pre-treatment, the hydrochloric
acid (HCl) solution of 10N was added to set
the pH at optimum level (pH of 7).
Pre-treatment of sludge
2200
2000
ch e m ica l p H =9
ch e m ica l p H =1 0
A c cu m ila te d b io g a s p ro d u c tio n (m l)
1800
ch e m ica l p H =1 1
1600
1400
1200
1000
800
600
400
200
0
-2 0 0
0
2
4
6
8
10
12
14
16
Figure: Accumulated biogas after chemical pre-treating. 
T im e , d a y
Pre-treatment of sludge
1000
o
60 C
.
Accum ilated biogas production (m l)
o
80 C
800
600
400
200
0
0
2
4
6
8
Tim e, (day)
10
12
14
16
Pre-treatment of sludge
2400
.
.
Accum ilated biogas production (m l)
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
0
2
4
6
8
Tim e, (day)
10
12
14
16
3500
C ontrol
o
Accumulated Biogas Production, ml
3000
2500
Therm al T= 80 C
U l t r a s o n ic 3 0 k H z
C h e m ica l p H = 1 1
T h e r m o - C h e m ic a l
Figure : Accumulated biogas
after different pre-treatment
process.
2000
.
1500
1000
500
0
Pre-treatment of sludge
According to figure higher biogas volume was achieved 
by 30 KHz pre-treating frequency which was (3209ml)
after 14 days of digestion in the next, thermo-chemical
pre-treating pH (11) +80 accumulate a (2298ml)
volume of biogas followed by chemical pre-treating
pH (11) which was (2028ml) and finally 80 thermally
pre-treated sludge resulted in (949ml) of total biogas
production
Pre-treatment of sludge
while the lower volume of biogas was recorded for 
control (blank test) digestion which was (509ml). The
percentage increasing of biogas by 30 KHz was
(530.45%), pH (11) +80 (351.47%), pH (11) (298.43%)
and 80 (86.45%) comparing to control digestion
(blank test).
Conclusions:Arrangement of improvement efficiencies of on biogas
production
Ultrasonic 30 KHz > Thermo-chemical pH (11) + 80 >
Chemical pH (11) > Ultrasonic 50 KHz > Chemical pH
(10) > Thermal 80 > Thermal 60 > Chemical pH (9) >
Control (blank test)

Conclusions:
Anaerobic digestion was very appropriate to Iraqi
warm weather to reduce as much as possible of daily
accumulated sludge in Al-Rustumiya WWTP in
addition to decreasing of emitted bad odors and to
generate electricity to overcome the shortage
Al-Rustumiya plant
 We believe that it is important to use A D units in Al-
Rustumiya plant for many reasons
1. Power generation from biogas produced can cover
the plant capacity requirements ( less dependency
on national generation system)
2. Sludge without A D still had different types of
pathogens which effects on farmer live or vegetable
consumers
For employees and neighborhoods health because active
organic matter can attract disease carrying vectors
such flies beside the bad odors
energy recovery from sewage sludge decrease the need of
fossil oil and decrease the environmental impact of
global warming

THE END