Transcript Anaerobic treatment of domestic sewage at psychrophilic

ANAEROBIC TREATMENT AS A
SUSTAINABLE TREATMENT OPTION FOR
DOMESTIC WASTEWATER AND ORGANIC
FRACTION OF MUNICIPAL SOLID WASTE
Prof. Dr. Izzet Ozturk
Istanbul Technical University, Environmental Engineering Faculty,
34469, Maslak, Istanbul/TURKEY

High-rate anaerobic treatment has been used widely for the treatment of
many industrial and municipal wastewaters in the last decades
Table 1. The benefits and drawbacks of anaerobic treatment of domestic sewage in the
high-rate anaerobic systems
Benefits
Drawbacks
1. Efficient in the removal of organic material 1. Long start-up period when seed sludge is
especially for tropical regions (developing
not available, as the growth rate of
countries)
methanogenic microorganisms is low
2. Low construction cost and small land 2. Low pathogen removal
requirements as generally at temperatures
> 20˚C and high loading rates can be
applied
3. Low operation and maintenance costs, 3. Requirement for post treatment to reach
energy consumption is low and little
the effluent standarts
equipment is needed
4. Lower sludge production as compared to 4. Low removal efficiently of particulate
aerobic and physical-chemical treatment
organic material at low temperatures
processes
5. Biogas production which can be used for 5. Risk for odour nuisance from the reduction
energy production
of sulphate to sulphide

Under suitable conditions in an anaerobic sewage treatment
system a bacterial population will develop that is compatible with
the applied hydraulic and organic loads. Among the factors that
determine the removal efficiency of biodegradable organic matter,
the following are important:
1) The nature of the anaerobic matter to be removed
2) The suitability of environmental factors for anaerobic digestion
3) The retained amount of viable bacterial matter
4) The intensity of contact between the influent organic matter
and the bacterial populations
5) The design of the anaerobic reactor
6) The retention time of sewage

Important environmental factors affecting anaerobic sewage
digestion are:
- temperature
- pH
- the presence of essential nutrients
- the absence of excessive concentrations of toxic compounds

Nutrients (both macronutrients, nitrogen and phosphorus, and
micronutrients) are abundantly available in sewage

Compounds that could exert a distinct toxic influence on the
bacterial population as well as sulphide are generally absent in
sewage

Besides type of the sewerage system (combined or seperate) also
affects the composition of sewage as well as anaerobic treatment
efficiency

High-rate anaerobic systems are generally applied in the
temperature range of 25-40C

However, many recent researches conducted at all temperature
conditions revealed that temperature is not a limiting factor in
anaerobic treatment applications if the appropriate process design
is chosen

When they are operated in lower temperature ranges (5-20C),
various adaptations of the conventional high-rate reactor design
are needed

The methanogenic biomass and the wastewater should be in
sufficient contact (can be achieved by increased liquid upflow
velocities)
• Psychrophilic anaerobic treatment is an attractive option for
wastewaters which are discharged at moderate to low temperatures
(optimal temperature for psychrophilic microorganisms is around
17C)
• Since domestic sewage has a temperature lower than 35C, heating is
required during the mesophilic anaerobic treatment. Thus, anaerobic
treatment systems allow substantially lower treatment costs due to their
ability to operate at low temperatures (10-20C)
• In recent years anaerobic treatment of wastewaters having low COD
concentrations could be efficient especially with high-rate reactors
such as UASB and fluidized bed reactors
• Since low COD concentration of the influent results in very low
substrate levels, low biogas productions will occur inside the reactor as
well (HRT determines the reactor volume)

The most appropriate anaerobic system to treat domestic wastewater has
been considered as the UASB reactor because of its simplicity, low
investment and operation costs

Particulate organics are physically removed due to settling, adsorption
and entrapment which is the first step in the anaerobic treatment and
conversion of domestic sewage

The rate-limiting step in the overall digestion process is the hydrolysis of
retained particulates which need relatively long retention times,
depending on the applied process temperature

Direct anaerobic treatment of domestic sewage is generally not applied
because of the fact that the high SS concentration in sewage causes
considerable difficulties

Under low temperature conditions, the SS are hydrolysed very slowly
and tend to acccumulate in the reactor (deteriorate the granular sludge)

In order to guarantee an efficient treatment of domestic sewage under
low temperature conditions, at least part of the SS present in the
wastewater should be removed before feeding the wastewater to a sludge
bed reactor

On the other hand, it was reported that two stage systems are more
suitable for anaerobic sewage treatment at low temperatures whereas at
high temperatures single stage systems should be chosen

At two stage reactor approach generally long HRT’s are applied for SS
hydrolysis at the first stage whereas short HRT’s are enough for methane
production at the second stage

Specific biogas production rate is relatively low under psychrophilic
conditions:
- At low temperatures, the increase of CO2 dissolution in water might
cause a decrease in the pH of the reactor because the solubility of gaseous
compounds present in biogas increases with decrease in temperature
- Low biodegradable organic matter concentration in the influent

Pilot and full-scale UASB reactor applications for domestic sewage are
given in Table 2
Table 2. Pilot and full scale UASB reactor applications for domestic sewage”
Country
Volume
(m3)
˚C
Influent
(mg/L)
Seed
COD
BOD
SS
h
(hr)
Removal
(%)
Reference
COD
BOD
SS
Holland
6
10-18
100
-900
53-474
10
-700*
Granular
9-16
46-60
42-48
55-75
de Man et al., 1986
Holland
20
11-19
150
-550
43-157
50
-400*
Granular
6.2
-18
31-49
23-46
(-)
de Man et al., 1986
Holland
120
>13
391
291
(-)
Granular
2-7
16-34
20-51
(-)
van der Last
and Lettinga, 1992
Colombia
64
25
267
95
(-)
Digested
cow
manure
6-8
75-82
75-93
70-80
Colombia
3360
24
380
160
240
None
5
45-60
64-78
60
Italy
336
7-27
205
-326
55-153
100
-250
None
12-42
31-56
40-70+
55-80+
India
1200
20-30
563
214
418
None
6
74
75
75
Draaijer et al., 1992
India
12000
18-32
1183
484
1000
(-)
8
51-63
53-69
46-64
Haskoning, 1996a;
Tare et al., 1997
India
6000
18-32
404
205
362
(-)
8
62-72
65-71
70-78
Haskoning, 1996b;
Tare et al., 1997
Brasil
120
18-28
188
-459
104
-255
67
-236
Granular
5-15
60
70
70
Vieira and Garcia,
1992
Brasil
477
(-)
600
(-)
303
Nonadapted
13
68
(-)
76
Chernicharo and
Borges, 1997
Lettinga et al.,
1987
Schellinkhout
and Osorio, 1994
Collivignarelli
et al., 1991;
Maaskant et al., 1991
On-site Anaerobic Treatment

Wastewaters are usually transported to centralised treatment plants
through extended sewage networks however, decentralised wastewater
treatment, i.e. community – or house- on-site treatment, may be more
sustainable in some cases

Anaerobic on-site treatment is considered sustainable with its simple,
thus cost-effective reactor design, small space requirement, low sludge
production, low energy and nutrient demand, potential for energy
production, high loading capacity, efficient removal of organic matter,
possibility for nutrient recycling and suitability for small houses

The produced biogas is collected and utilised as renewable energy

Simple and easy-to-use anaerobic processes suitable for on-site treatment
are septic tank, UASB-septic tank, and accumulation system
Figure 1. Flow scheme for a decentralised integrated system. 1. Pre-sedimentation tank, 2. UASB reactor,
3. RBC = Rotating Biological Contact Reactor, 4. UV/O3 = ultraviolet-ozone generator
Figure 2. Flow diagram for an anaerobic on-site treatment
Post-Treatment Alternatives

Anaerobic treatment is effective in removing biodegradable
organic compounds, leaving mineralized compounds like
ammonium, phosphate and sulfur in the solution.

These compounds therefore have to be removed by an
additional post-treatment step to meet sufficiently the criteria
for a sustainable environmental protection.

Besides it was reported that no pathogen removal could be
achieved at low temperatures. Thus, anaerobic treatment of
low strength wastewaters by UASB reactor should be
considered as a pre-treatment alternative
Figure 3. Sectional view of a UASB + duckweed ponds
+ fish pond system
Biogas
(a)
UASB
Stabilization
Pond
To land
Facultative
Aerated
Lagoon
To river or
land
Sludge to
drying beds
Biogas
(b)
UASB
Sludge to
drying beds
Biogas
(c)
Three Chamber
Oxidation
Pond
UASB
To land
Sludge to
drying beds
Figure 4. Treatment alternatives following UASB reactor (a) Stabilization pond
(b) Facultative aerated lagoon (c) Oxidation pond
Anaerobic Digestion of OFMSW

Anaerobic digestion for the treatment of OFMSW was devoloped in the
1980’s and early 1990’s.

The biomethanization of OFMSW will become a very feasible option by
applying subsidies to electricity production from wastes.

In most of the EU member countries the electricity generated from the
reneweable sources is subsided with an additional Є0,1/kWh

Since 1990, more than 120 waste treatment facilities have been constructed in
Europe.

In most of these plants, the anaerobic digestion is followed by an aerobic
phase, for the additional pathogen removal, so that not only biogas but also
compost is produced.

The nutrient rich supernatant from these digesters can be treated with MAP
and struvite is produced which has a marketing value as a fertilizer (Also the
supernetant from digesters has a potential of agricultural use).
Waste Characteristics

The organic fraction of municipal solid waste is rather a heterogenous
substrate and the biogas yield in anaerobic treatment of OFMSW is
dependent not only the the proces configuration, but also on the waste
characteristics.

The waste characteristics is highly dependent on the collection system.

Source sorting of MSW generaly provides OFMSW of higher quality in terms
of biogas yield and smaller quantities of non-biodegradable contaminants like
plastics.

Mechanically seperated OFMSW which has a lower biogas potential is more
contaminated, which leads to persistent handling problems and lower
acceptability of the effluent product of the treatment process as fertilizer on
agricultural land.
Waste Characteristics and Biogas Potential
Mechanicaly Sorted OFMSW
(MS-OFMSW)
Source Sorted OFMSW
(SS-OFMSW)
TS (g/kg)
647,2
163,9
TVS (%TKM)
46,5
90,6
TCOD
(kgO2/kg)
0,5
1,1
TKN (%TS)
1,4
2,1
P (%TS)
1,9
2,1
Parameter
Substrate Type
(m3
B0
CH4/kg TVS)
G0
3
(m /kg TVS)
Mechanicaly Sorted OFMSW
(MS-OFMSW)
0,16 - 0,37
0,29 - 0,66
Separetely Collected OFMSW
(SC-OFMSW)
0,45 - 0,49
0,81 - 0,89
Source Sorted OFMSW
(SS-OFMSW)
0,37 - 0,40
0,67 - 0,72
B0 : Maximum Methane Potential, G0 : Maximum Biogas Potential (%55 CH4)
Co-digestion Approach

The co-digestion concept involves the treatment of several
waste types in a single treatment facility.

The profit of co-digestion in the anaerobic degredation proces
is mainly within the folloving areas:





Increasing the methane yield
Improving the process stability
Achieving a better management of waste
The economical benefits of the collection and treatment of different
types of wastes in a single treatment facility (Figure 5)
The fermentation products of OFMSW which is mainly VFA can be
used as external carbon source for biological nutrient removal plants
which suffers from organic carbon deficiency (Figure 5)
3
Biogas I Qgas=0.1-0.3 m /kg CODremoved
(80% CH4)
Solar Energy Assisted Heating
(if required!)
Aerated or SBR
Lagoon
Pump UASB
Reactor
Domestic
Wastewater
Screen
Agricultural Use
Grit
Chamber
Liquid Fermentation
Products (High VFA)
Qgas=0.30 m3/kg VSfed
(65-70% CH4)
Biogas II
Seperate
Collected
Organic Fraction
of Municipal
Solid Waste
(SC-OFMSW)
Pulper
Sludge
Dewatering
Plastics & Paper
Size
Reduction
Heavy
Fractions
Cake to Post
Aerobic
Composting
OFMSW ~ 8 to 10% TS
Anaerobic
Hdrolysis & Acid
Fermentation
Methane Reactor
Filtrate
Liquid Fertilizer
A general flow scheme for the implementation of co-digestion aproach and
anaerobic treatment of municipal wastewater in a single treatment facility
Co-digestion Approach

The application of anaerobic co-digestion process may face problems due to some
substrate characteristics.





High total solids content of typically 30 – 50%
High C:N ratio
Deficiecy in macro- and micronutrients
Content of toxic compounds (heavy metals, phthalates)
The key for codigestion lies in balancing several parametres in the co-substrate
mixture.
Waste
A
Makro and micro nutrients
C:N ratio
pH
Inhibitors/toxic compounds
Biodegradable organic matter
Dry matter
Waste B
Co-digestion with Sewage Sludge

The codigestion can be applied at existing facilities without great investments
and it combines the treatment of the two largest municipal waste streams.

The addition of high solids concentration of OFMSW digester operated with
sludge having a low TS concentration will be possible even in rather high
concentrations

The stabilizing effect of sludge on the digestion of OFMSW has been
confirmed with sludge doses between 8 – 20% of feedstock volatile solids
(Kayhanian and Rich, 1996; Rivard et al., 1990).

For the codigestion of OFMSW with sewage sludge, the optimum C:N ratio
of the feedstock was found to be in the range of 25 to 30 based on
biodegradable carbon (Kayhanian and Tchobanoglolous, 1992; Kayhanian
and Rich, 1996).
Co-digestion with Other Waste Types

Other organic waste types such as;




Livestock waste (manure)
Olive mill effluents
Macroalgae
Wastes generated by agro industries such as slaughterhouses,
meat-processing industries
can also be used as a co-substrate with OFMSW.
THANK YOU FOR YOUR
ATTENTION!