ANAEROBIC TREATMENT AS A SUSTAINABLE TREATMENT OPTION FOR DOMESTIC WASTEWATER AND ORGANIC FRACTION OF MUNICIPAL SOLID WASTE

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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
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• 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 especially for tropical regions (developing countries) 1. Long start-up period when seed sludge is not available, as the growth rate of methanogenic microorganisms is low
2. Low construction cost and small land requirements as generally at temperatures gt 20C and high loading rates can be applied 2. Low pathogen removal
3. Low operation and maintenance costs, energy consumption is low and little equipment is needed 3. Requirement for post treatment to reach the effluent standarts
4. Lower sludge production as compared to aerobic and physical-chemical treatment processes 4. Low removal efficiently of particulate organic material at low temperatures
5. Biogas production which can be used for energy production 5. Risk for odour nuisance from the reduction of sulphate to sulphide
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• 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

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• 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

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• 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)

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• 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)

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• 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

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• 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
  HRTs are applied for SS hydrolysis at the first
  stage whereas short HRTs are enough for methane
  production at the second stage

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• 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

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Table 2. Pilot and full scale UASB reactor
applications for domestic sewage
Country Volume (m3) C Influent (mg/L) Influent (mg/L) Influent (mg/L) Seed ?h (hr) Removal () Removal () Removal () Reference
Country Volume (m3) C COD BOD SS Seed ?h (hr) COD BOD SS Reference
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 gt13 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 Lettinga et al., 1987
Colombia 3360 24 380 160 240 None 5 45-60 64-78 ?60 Schellinkhout and Osorio, 1994
Italy 336 7-27 205 -326 55-153 100 -250 None 12-42 31-56 40-70 55-80 Collivignarelli et al., 1991 Maaskant et al., 1991
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 Non- adapted 13 68 (-) 76 Chernicharo and Borges, 1997
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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

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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
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Figure 2. Flow diagram for an anaerobic on-site
treatment
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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

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Figure 3. Sectional view of a UASB duckweed
ponds fish pond system
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Biogas
Stabilization Pond
To land
UASB
(a)
Sludge to drying beds
Biogas
Facultative Aerated Lagoon
To river or land
UASB
(b)
Sludge to drying beds
Biogas
Three Chamber Oxidation Pond
To land
UASB
(c)
Sludge to drying beds
Figure 4. Treatment alternatives following UASB
reactor (a) Stabilization pond (b) Facultative
aerated lagoon (c) Oxidation pond
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Anaerobic Digestion of OFMSW

• Anaerobic digestion for the treatment of OFMSW
  was devoloped in the 1980s and early 1990s.
• 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 ?0,1/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).

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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.

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Waste Characteristics and Biogas Potential
Parameter 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
Substrate Type B0 (m3 CH4/kg TVS) G0 (m3/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)
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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)

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Seperate Collected Organic Fraction of Municipal
Solid Waste (SC-OFMSW)
Cake to Post Aerobic Composting
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
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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 CN 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.

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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 CN 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).
  

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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.

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