Review
Development of anaerobic digestion methods for palm oil mill effluent (POME)treatment
P.E.Poh,M.F.Chong *
School of Chemical and Environmental Engineering,Faculty of Engineering,The University of Nottingham Malaysia Campus,Jalan Broga,43500Semenyih,Selangor,Malaysia
a r t i c l e i n f o Article history:
Received 10April 2008
Received in revised form 11June 2008Accepted 12June 2008
Available online 25July 2008Keywords:POME
Anaerobic treatment UASB UASFF
a b s t r a c t
Palm oil mill effluent (POME)is a highly polluting wastewater that pollutes the environment if dis-charged directly due to its high chemical oxygen demand (COD)and biochemical oxygen demand (BOD)concentration.Anaerobic digestion has been widely used for POME treatment with large emphasis placed on capturing the methane gas released as a product of this biodegradation treatment method.The anaerobic digestion method is recognized as a clean development mechanism (CDM)under the Kyoto protocol.Certified emission reduction (CER)can be obtained by using methane gas as a renewable energy.This review aims to discuss the various anaerobic treatments of POME and factors that influence the operation of anaerobic treatment.The POME treatment at both mesophilic and thermophilic temperature ranges are also analyzed.
Ó2008Elsevier Ltd.All rights reserved.
1.Introduction
In the process of palm oil milling,POME is generated through sterilization of fresh oil palm fruit bunches,clarification of palm oil and effluent from hydrocyclone operations (Borja et al.,1996a ).POME is a viscous brown liquid with fine suspended solids at pH ranging between 4and 5(Najafpour et al.,2006).The char-acteristics of POME could be referred to the Data for Engineers:POME (2004).Dire
ct discharge of POME into the environment is not encouraged due to the high values of COD and BOD.Further-more,with the introduction of effluent discharge standards im-posed by the Department of Environment in Malaysia,POME has to be treated before being released into the environment (Federal Subsidiary Legislation,1974).
Anaerobic digestion has been employed by most palm oil mills as their primary treatment of POME (Tay,1991).More than 85%of palm oil mills in Malaysia have adopted the ponding system for POME treatment (Ma et al.,1993)while the rest opted for open digesting tank (Yacob et al.,2005).These methods are regarded as conventional POME treatment method whereby long retention times and large treatment areas are required.High-rate anaerobic bioreactors have also been applied in laboratory-scaled POME treatment such as up-flow anaerobic sludge blanket (UASB)reactor (Borja and Banks,1994a );up-flow anaerobic filtration (Borja and Banks,1994b );fluidized bed reactor (Borja and Banks,1995a,b )
and up-flow anaerobic sludge fixed-film (UASFF)reactor (Najafpour et al.,2006).Anaerobic contact digester (Ibrahim et al.,1984)and continuous stirred tank reactor (CSTR)have also been studied for treatment of POME (Chin,1981).
Other than anaerobic digestion,POME has also been treated using membrane technology (Ahmad et al.,2006,2007),aerobic activated sludge reactor (Vijayaraghavan et al.,2007),and evapora-tion method (Ma et al.,1997).
1.1.Clean development mechanism (CDM)
The utilization of methane gas as a renewable energy from the anaerobic digestion can be used to obtain certified emission reduc-tion (CER)credit by clean development mechanism (CDM)under the Kyoto protocol (Tong and Jaafar,2006).Besides helping to re-duce carbon emission to the environment,CDM has the advantage to offer developing countries such as Malaysia to attract foreign investments to sustain renewable energy projects (Menon,2002).Thus,palm oil mills could earn carbon credits as revenue by the utilization of methane gas as renewable energy from anaerobic digestion of palm oil mill effluent.More emphasis has been given to develop anaerobic treatment for POME since the implementa-tion of CDM.Currently,there are two CDM projects that have been registered to recover methane from palm oil mill effluent which are hosted by Kim Loong Power Sdn.Bhd.(Project,0867)and Uni-ted Plantations Bhd.(Project,1153).
Subsequent to this,investigation by Yacob et al.(2006a)on the methane emission from anaerobic pond shows that 1043.1kg/day/pond of methane gas is emitted.Based on the ponding system
0960-8524/$-see front matter Ó2008Elsevier Ltd.All rights reserved.doi:10.1016/j.biortech.2008.06.022
*Corresponding author.Tel.:+60389248347;fax:+60389248017.
E-mail addresses:MeiFong.Chong@ ,chong_mei_fong@yahoo (M.F.Chong).
Bioresource Technology 100(2009)
1–9
Contents lists available at ScienceDirect
Bioresource Technology
j o u r n a l h o m e p a g e :w w w.e l s e v ie r.c o m /l o c a t e /b i o r t e c
h
investigated,there were4anaerobic ponds,which in total pro-duced4172.4kg/day of methane gas.It is estimated that approxi-mately RM1,027,975per year(€228,438.9)can be generated as revenue if methane gas emitted from the anaerobic ponds are cap-tured as renewable energy.Calculations are based on300working days and carbon credit price of€10per ton of carbon quoted by Menon(2007).
Payback period for investment on anaerobic bioreactors can be short if carbon credit prices remain high(Menon,2007).Consider-ing the revenue and advantages achieved through capturing meth-ane gas,palm oil mills could switch to anaerobic bioreactor for POME treatment.
2.Anaerobic digestion
Anaerobic digestion is the degradation of complex organic mat-ters under the absence of oxygen.This process is time consuming as bacterial consortia responsible for the degradation process re-quires time to adapt to the new environment before they start to consume on organic matters to grow.
In the process of degrading POME into methane,carbon diox-ide and water,there is a sequence of reactions involved;hydroly-sis,acidogenesis(including acetogenesis)and methanogenesis (Gerardi,2003).Hydrolysis is where complex ,car-bohydrates,lipids,proteins)are converted into sugar,amino acid and etc.In the step of acidogenesis,acidogenic bacteria will break down these sugar,fatty acids and amino acids into organic acids which mainly consist of acetic acid(from acetogenesis)together with hydrogen and carbon dioxide.Hydrogen and carbon dioxide will be utilized by hydrogenotropic methanogens while acetic acid and carbon dioxide will be utilized by acetoclastic methanogens to give methane as afinal product.
Methanogenesis is the rate limiting step in anaerobic digestion of POME(Ibrahim et al.,1984).As such,conventional anaerobic digesters require large reactors and long retention time to ensure complete digestion of treated influent.Nonetheless,high-rate anaerobic bioreactors have been proposed(Borja and Banks, 1994a,b,1995a,b;Najafpour et al.,2006;Ibrahim et al.,1984)to re-duce reactor volume,shorten retention time as well as capture methane gas for utilization.
2.1.Anaerobic and alternative POME treatment methods
Aerobic treatment,membrane treatment system and evapora-tion method are the currently available alternative methods for POME treatment.The advantages and disadvantages for anaerobic and alternative treatment methods are shown in Table1.In terms of energy requirement for POME treatment operation,anaerobic digestion has a stronger advantage over other alternative methods as it does not require energy for aeration.Furthermore,anaerobic POME treatment produces methane gas which is a value-added product of digestion that can be utilized in the mill to gain more revenue in terms of CER.
Take for instance the open digesting tank for POME treatment without land application,capital cost quoted by Gopal and Ma (1986)for a palm oil mill processing30tons FFB/h is RM 750,000.Based on the Chemical Engineering Plant Cost Index in 2006(Ullrich and Vasudevan,2004)the capital cost for this system is estimated to be RM1,147,842in2006.Comparing this to the capital cost for a membrane system in POME treatment for a palm oil mill processing36tons FFB/h at RM3,950,000(Chong,2007),it is obvious that the former anaerobic treatment has better advan-tage over other treatment methods in terms of capital cost.The only two significant drawbacks of anaerobic treatment are long retention times and long start-up period.However,the problem of long retention times can be rectified by using hi
gh-rate anaero-bic bioreactors while the long start-up period can be shortened by using granulated seed sludge(McHugh et al.,2003),utilizing seed sludge from same process(Yacob et al.,2006b)or maintaining suit-able pH and temperature in the high-rate anaerobic bioreactor for growth of bacteria consortia(Liu et al.,2002).
Untreated wastewater with BOD/COD ratio of0.5and greater can be treated easily by biological means(Metcalf and Eddy, 2003).With reference to the published values of BOD and COD in Data for Engineers:POME(2004),aerobic and anaerobic treatment is suitable for POME treatment since the BOD/COD ratio is of0.5.In comparison of these two treatment methods,the anaerobic treat-ment can be regarded to be more suitable for POME treatment due to its lower energy consumption while producing methane as a value-added product in the process.
2.2.Anaerobic treatment methods
2.2.1.Conventional treatment systems
Ponding system is the most common treatment system that is employed in palm oil mills to treat POME with more than85%of the mills having adopted this method.Ponding system comprises of de-oiling tank,acidification ponds,anaerobic ponds and faculta-tive or aerobic ponds(Chan and Chooi,198
4).Number of ponds varies according to the capacity of the palm oil mill.Facultative or aerobic ponds are necessary to further reduce BOD concentra-tion in order to produce effluent that complies with Federal Sub-sidiary Legislation,1974effluent discharge standards.
A typical size of an anaerobic pond in a palm oil mill which has a processing capacity of54tons per hour is60.0Â29.6Â5.8m (lengthÂwidthÂdepth)(Yacob et al.,2006a)which is approxi-mately equivalent to half the size of a soccerfield.Size of pond de-pends on the capacity of the palm oil mill as well as the area available for ponds.Anaerobic ponds have the longest retention time in ponding system which is around20–200days(Chan and
Table1
Advantages and disadvantages between anaerobic and alternative treatment methods
Treatment
types
Advantages Disadvantages Reference
Anaerobic Low energy requirements(no aeration),producing methane gas as a valuable end product,generated sludge from process
could be used for land applications Long retention time,slow start-up(granulating reactors),large
area required for conventional digesters
Metcalf and Eddy
(2003),Borja et al.
(1996a)
Aerobic Shorter retention time,more effective in handling toxic wastes High energy requirement(aeration),rate of pathogen
inactivation is lower in aerobic sludge compared to anaerobic
sludge,thus unsuitable for land applications Leslie Grady et al. (1999),Doble and Kumar(2005)
Membrane Produce consistent and good water quality after treatment, smaller space required for mem
brane treatment plants,can
disinfect treated water Short membrane life,membrane fouling,expensive compared to
conventional treatment
Ahmad et al.(2006),
Metcalf and Eddy
(2003)
Evaporation Solid concentrate from process can be utilized as feed material for fertilizer manufacturing High energy consumption Ma et al.(1997)
2P.E.Poh,M.F.Chong/Bioresource Technology100(2009)1–9
Chooi,1984).Investigations by Yacob et al.(2006a)showed that anaerobic pond had a higher emission of methane with an average methane composition of54.4%compared to open digester tank.In addition to that,the methane composition from anaerobic ponds was also found to be more consistent in the ga
seous mixture. Methane emission in anaerobic ponds is influenced by mill activi-ties and seasonal cropping of oil palm(Yacob et al.,2006a).
Open digesting tanks are used for POME treatment when lim-ited land area is available for ponding system.Yacob et al.(2005) investigated on the methane emission from open digesting tanks where each tanks was half the capacity of anaerobic ponds (3600m3)with retention time of20days.Emission of methane gas from open digesting tank was found to be less than anaerobic pond with an average methane composition of36.0%.Lower meth-ane composition is due to the transfer of oxygen into the tank when feed is induced into the tank.Mixing in digesting tanks im-proves the digestion process as bacteria consortia are brought into more contact with food(Leslie Grady et al.,1999).Nevertheless, mixing in open digesting tank only depends on slow bubbling and eruption of biogas which causes low conversion of methane gas.
2.2.2.Anaerobicfiltration
Anaerobicfilter has been applied to treat various types of wastewater including soybean processing wastewater(Yu et al., 2002a),wine vinases(Nebot et al.,1995;Pérez et al.,1998),landfill leachate(Wang and Banks,2007),municipal wastewater(Bodkhe, 2008),brewery wastewater(Leal et al.,1998),slaughter
house wastewater(Ruiz et al.,1997),drug wastewater(Gangagni Rao et al.,2005),distillery wastewater(Acharya et al.,2008),beet sugar water(Farhadian et al.,2007)and wastewater from ice-cream manufacture(Hawkes et al.,1995;Monroy et al.,1994).Borja and Banks(1994b,1995b)have also utilized anaerobicfilter for POME treatment.The packing allows biomass to attach on the sur-face when raw POME feed enters from the bottom of the bioreactor while treated effluent together with generated biogas will leave from the top of the bioreactor.
Anaerobicfilter is selected for wastewater treatment because(i) it requires a smaller reactor volume which operates on a shorter hydraulic retention times(HRTs)(ii)high substrate removal effi-ciency(Borja and Banks,1994b),(iii)the ability to maintain high concentration of biomass in contact with the wastewater without affecting treatment efficiency(Reyes et al.,1999;Wang and Banks, 2007),and(iv)tolerance to shock loadings(Reyes et al.,1999;Van Der Merwe and Britz,1993).Besides,construction and operation of anaerobicfilter is less expensive and small amount of suspended solids in the effluent eliminates the need for solid separation or re-cycle(Russo et al.,1985).
However,filter clogging is a major problem in the continuous operation of anaerobicfilters(Bodkhe,2008;Jawed and Tare, 2000;Parawira et al.,2006).So far,clogging of anaerobic
filter has only been reported in the treatment of POME at an organic loading rate(OLR)of20g COD/l/day(Borja and Banks,1995b)and also in the treatment of slaughterhouse wastewater at6g COD/l/day.This is due to the fact that other studies were con-ducted at lower OLRs which had lower suspended solid content compared to POME.
In general,anaerobicfilter is capable of treating wastewaters to give good effluent quality with at least70%of COD removal effi-ciency with methane composition of more than50%.Table2indi-cates the COD removal efficiency of some treated wastewater using anaerobicfiltration based on highest achievable percentage of methane in the generated biogas.In terms of POME treatment, the highest COD removal efficiency recorded was94%with63% of methane at an OLR of4.5kg COD/m3/day,while overall COD re-moval efficiency was up to90%with an average methane gas com-position of60%(Borja and Banks,1994b).
Investigations have been done to improve the efficiency of anaerobicfiltration in wastewater treatment.For instance,Yu et al.(2002a)found that operating at an optimal recycle ratio which varies depending on OLR will enhance COD removal.How-ever,methane percentage will be compromised with increase in optimal recycle ratio.Higher retention of biomass in thefilter will also lead to a better COD removal efficiency.In order to optimize the retention of biomass on thefilter media surface and tra
pped suspended biomass within the interstitial void spaces,Show and Tay(1999)suggested the use of support media with high porosity or open-pored surfaces.It was also suggested that continuously fed system gives better stability and greater degradation efficiency in anaerobicfilters(Nebot et al.,1995).
2.2.
3.Fluidized bed reactor
Fluidized bed reactor exhibits several advantages that make it useful for treatment of high-strength wastewaters.It has very large surface areas for biomass attachment(Borja et al.,2001;Toldráet al.,1987),enabling high OLR and short HRTs during operation (Garcia-Calderon et al.,1998;Sowmeyan and Swaminathan, 2008).Furthermore,fluidized bed has minimal problems of chan-neling,plugging or gas hold-up(Borja et al.,2001;Toldráet al., 1987).Higher up-flow velocity of raw POME is maintained forflu-idized bed reactor to enable expansion of the support material bed. Biomass will then attach and grow on the support material.In this way,biomass can be retained in the reactor.Investigations have been done on the application offluidized bed to treat cutting-oil wastewater(Perez et al.,2007);real textile wastewater(Sßen and Demirer,2003);wine and distillery wastewater(Garcia-Calderon et al.,1998;Sowmeyan and Swaminathan,2008);brewery waste-water(Alvarado-Lassman et al.,2008);ice-
cream wastewater (Borja and Banks,1995a;Hawkes et al.,1995);slaughterhouse wastewater(Toldráet al.,1987);pharmaceutical effluent(Saravan-ane et al.,2001)and POME(Borja and Banks,1995b).
OLR ranges and COD removal efficiencies of various wastewater treatments usingfluidized bed is tabulated in Table3.Based on Table3,it can be concluded that anaerobicfluidized bed can typi-cally remove at least65%and up to more than90%of COD.Inverse flow anaerobicfluidized bed is capable of tolerating higher OLRs
Table2
Operating OLR range;COD removal efficiency in various wastewater treatments using anaerobicfiltration based on highest%of methane production
Types of Wastewater Operating OLR range
(kg COD/m3/day)COD removal efficiency
(%)
Highest methane composition
(%)
Reference
Slaughterhouse wastewater  1.0–6.579.9(91.5)51.1Ruiz et al.(1997)
POME  1.2–11.494.0(94.0)63.0Borja and Banks(1994b) Baker’s yeast factory effluent  1.8–10.069.0(74.0)65.0Van Der Merwe and Britz
(1993)
Distillery wastewaters0.42–3.491.0(93.0)63.0Russo et al.(1985)
Landfill leachate0.76–7.6390.8(90.8)N/A Wang and Banks(2007)
()–number in bracket denotes highest COD removal efficiency.N/A–data unavailable.
P.E.Poh,M.F.Chong/Bioresource Technology100(2009)1–93
compared to up-flow configuration.Alvarado-Lassman et al.(2008) showed that inverseflowfluidized bed shows excellent stability when overload is applied.By making a comparison between the operating
OLR ranges for POME from Tables2and4,it was found that in general,anaerobicfluidized bed is able to operate at higher OLRs,implying that less reactor volume will be required to operate at lower OLRs.
The type of support material in thefluidized bed plays an important role to determine the efficiency of the entire treatment system(Garcia-Calderon et al.,1998;Sowmeyan and Swamina-than,2008)for both inverseflow and up-flow systems.Studies usingfluidized bed to treat ice-cream wastewater showed different COD removal efficiencies when different support materials were used.Hawkes et al.(1995)found thatfluidized bed using granular activated carbon(GAC)gave about60%COD removal while Borja and Banks(1995a)obtained94.4%of COD removal using ovoid saponite.Thus suitable support material needs to be selected to obtain high COD removal efficiency in the system.
In POME treatment,fluidized bed was found to be a better treat-ment method compared to anaerobicfilter due to its ability to tol-erate higher OLRs and its better methane gas production.Shorter HRT(6h)also proved to be an advantage offluidized bed over anaerobicfilter(1.5–4.5days)in POME treatment.
2.2.4.Up-flow anaerobic sludge blanket(UASB)reactor
UASB was developed by Lettinga et al.(1980)whereby this sys-tem has been successful in treating a w
ide range of industrial efflu-ents including those with inhibitory compounds.The underlying principle of the UASB operation is to have an anaerobic sludge which exhibits good settling properties(Lettinga,1995).So far, UASB has been applied for the treatment of potato wastewater (Kalyuzhnyi et al.,1998;Lettinga et al.,1980;Parawira et al., 2006);domestic wastewater(Barbosa and Sant’Anna,1989;Beh-ling et al.,1997);slaughterhouse wastewater(Sayed et al.,1984); ice-cream wastewater(Hawkes et al.,1995);POME(Borja and Banks,1994c);pharmaceutical wastewater(Stronach et al., 1987);instant coffee wastewater(Dinsdale et al.,1997);sugar-beet wastewater(Lettinga et al.,1980)and etc.UASB has a relatively simple design where sludge from organic matter degra-dation and biomass settles in the reactor.Organic matter from wastewater that comes in contact with sludge will be digested by the biomass granules.
Table4indicates some performances of wastewater treatment using UASB system.For potato wastewater treatment,Kalyuzhnyi et al.(1998)and Parawira et al.(2006)both observed foaming and sludgefloatation in the UASB reactor when operating at higher OLRs(>6.1kg COD/m3day).The ability of UASB to tolerate higher OLR for potato wastewater investigated by Lettinga et al.(1980) compared to Kalyuzhnyi et al.(1998)and Parawira et al.,2006is due to the fact that the latter two studies were conducted at labo-ratory scale.In general,UASB is successful in COD removal of more than60%for mo
st wastewater types except for ice-cream waste-water.Hawkes et al.(1995)suggested that the lower COD removal percentage from ice-cream wastewater was due to design faults in the reactor’s three phase separator and high contents of milk fat that were hard to degrade.
POME treatment has been successful with UASB reactor,achiev-ing COD removal efficiency up to98.4%with the highest operating OLR of10.63kg COD/m3day(Borja and Banks,1994c).However, reactor operated under overload conditions with high volatile fatty acid content became unstable after15days.Due to high amount of POME discharge daily from milling process,it is necessary to oper-ate treatment system at higher OLR.Borja et al.(1996a)imple-mented a two-stage UASB system for POME treatment with the objective of preventing inhibition of granule formation at higher OLRs without having to remove solids from POME prior to treat-ment.This method is desirable since suspended solids in POME have high potential for gas production while extra costs from sludge disposal can be avoided.Results from this study showed the feasibility of separating anaerobic digestion into two-stages
Table3
Operating OLR range,COD removal efficiency of various wastewater treatments usingfluidized bed reactor
Types of wastewater Operating OLR range
(kg COD/m3/day)COD removal efficiency
(%)
Reactor
configuration
Reference
POME10.0–40.078.0–94.0UF Borja and Banks(1995b) Protein production from extracted sunflower
flour effluent
0.6–9.380.0–98.3UF Borja et al.(2001)
Ice-cream wastewater  3.2–15.694.4UF Borja and Banks(1995a) Cutting-oil wastewater11.9–51.367.1–95.9UF Perez et al.(2007)
Distillery effluent  6.11–35.0980.0–92.0DF Sowmeyan and Swaminathan
(2008)
Brewery wastewater0.5–70.080.0–90.0DF Alvarado-Lassman et al.(2008) Real textile wastewater0.4–5.078.0–89.0UF Sßen and Demirer(2003)
UF–upwardflow;DF–downward/inverseflow.reactor technology
Table4
Performance of UASB in various wastewater treatments
Types of wastewater Operating OLR range
(kgCOD/m3/day)COD removal efficiency
(%)
Methane composition
(%)
Reference
Potato wastewater  1.8–13.963.0–81.054.0–67.0Kalyuzhnyi et al.(1998)
1.5–6.19
2.0–98.059.0–70.0Parawira et al.(2006)
25.0–45.093.0N/A Lettinga et al.(1980)
POME single-stage two-stage
(based on methanogenic reactor)1.27–10.6396.7–98.454.2–62.0Borja and Banks(1994c) 1.1–60.0>90.060.0–83.0Beccari et al.(1996)
Ice-cream wastewater0.5–5.050.069.6Hawkes et al.(1995) Sugar-beet  4.0–5.095.0N/A Lettinga et al.(1980) Confectionary wastewater  1.25–2.2566.0N/A Forster and Wase(1983) Pharmaceutical wastewater0.27–2.0026.0–69.0N/A Stronach et al.(1987) Domestic sewage  3.7674.069.0Barbosa and Sant’Anna
(1989)
Slaughterhouse wastewater7.0–11.055.0–85.065.0–75.0Sayed et al.(1984)
N/A–data unavailable.
4P.E.Poh,M.F.Chong/Bioresource Technology100(2009)1–9
(acidogensis and methanogenesis)using a pair of UASB reactors. The methanogenic reactor was found to adapt quickly with the feed from the acidogenic reactor and also tolerate higher OLRs.It was suggested that OLR of30kg COD/m3day could ensure an over-all of90%COD reduction and efficient methane conversion.
UASB reactor is advantageous for its ability to treat wastewater with high suspended solid content(Fang and Chui,1994;Kal-yuzhnyi et al.,1998)that may clog reactors with packing material and also provide higher methane production(Kalyuzhnyi et al., 1996;Stronach et al.,1987).However,this reactor might face long start-up periods if seeded sludge is not granulated.A study by Goodwin et al.(1992)has proved that reactors seeded with granu-lated sludge achieved high performance levels within a shorter start-up period.It could also adapt quickly to gradual increase of OLR(Kalyuzhnyi et al.,1996).
2.2.5.Up-flow anaerobic sludgefixed-film(UASFF)reactor
UASB and anaerobicfilter has been integrated to form a hybrid bioreactor–UASFF.This hybrid reactor combines the advantages of both reactors while eliminating their respective drawbacks.As such,UASFF is superior in terms of biomass retention,reactor sta-bility at shock loadings and operation at high OLRs while eliminat-ing the problems of clogging and biomass washout in anaerobic filter and UASB.Ayati and Ganjidoust(2006)has proven that UASFF is more efficient compared to UASB and anaerobicfilter in the treatment of woodfiber wastewater.
Other investigations of wastewater treatments using UASFF in-cludes sugar wastewater(Guiot and van den Berg,1985);dairy wastewater(Córdoba et al.,1995);slaughterhouse wastewater (Borja et al.,1995c,1998;Lo et al.,1994);wash waters from puri-fication of virgin olive oil(Borja et al.,1996b);coffee wastewater (Bello-Mendoza and Castillo-Rivera,1998);brewery wastewater (Yu and Gu,1996)and POME(Najafpour et al.,2006).Performances of UASFF for wastewater treatments are tabulated in Table5.
This hybrid reactor is generally capable of tolerating OLRs high-er than UASB and anaerobicfilter.Clogging is not reported in stud-ies on the performance of hybrid reactor.UASFF is also able to achieve COD removal efficiency of at least70%and above except for woodfiber wastewater as woodfiber is harder to degrade. Methane production for UASFF is also at a satisfactory level.In the tre
atment of POME,Najafpour et al.(2006)found that internal packing and high ratio of effluent recycle are both vital to control the stability of the UASFF reactor.Internal packing effectively re-tained biomass in the column while effluent recycle produced internal dilution to eliminate effects of high OLR.
2.2.6.Continuous stirred tank reactor(CSTR)
CSTR is equivalent to a closed-tank digester with mixer.The mechanical agitator provides more area of contact with the biomass thus improving gas production.In POME treatment,CSTR has been applied by a mill under Keck Seng(Malaysia)Berhad in Masai,Johor and it is apparently the only one which has been oper-ating continuously since early1980s(Tong and Jaafar,2006).Other applications of CSTR on wastewater treatment include dilute dairy wastewater(Chen and Shyu,1996);jam wastewater(Mohan and Sunny,2008)and coke wastewater(Vázquez et al.,2006)where coke wastewater was treated in aerobic conditions.
The CSTR in Keck Seng’s palm oil mill has COD removal effi-ciency of approximately83%and CSTR treating dairy wastewater has COD removal efficiency of60%.In terms of methane composi-tion in generated biogas,it was found to be62.5%for POME treat-ment and22.5–76.9%for dairy wastewater treatment.Another study on POME treatment using CSTR has been investigated by Ugoji(1997)where
results indicated that COD removal efficiency is between93.6–97.7%.The difference of COD removal efficiency between the two published results by Keck Seng and Ugoji is due to the different operating conditions where the latter study was done in laboratory scale.In the plant scale POME treatment at Keck Seng’s palm oil mill,the treated wastewater could not be assumed to be well mixed due to the large volume of feed which might af-fect the overall efficiency of the COD removal.
Ramasamy and Abbasi(2000)attempted to upgrade the perfor-mance of CSTR by incorporating a biofilm support system(BSS) within the existing reactor.Low-density nylon mesh were rolled into cylinders and inserted into the CSTR.This BSS functions as a support media for growth of biomass.From this study,it was found that efficiency of CSTRs can be improved without biomass recy-cling.The implementation of BSS into CSTR can be useful to in-crease COD removal efficiency as well as biogas production in POME treatment.
2.2.7.Anaerobic contact digestion
Contact process involves a digester and a sedimentation tank where sludge from digester effluent is left to settle and the effluent is recycled back into the digester.This process has been imple-mented in POME(Ibrahim et al.,1984);ice-cream wastewater, alcohol distillery wastewater(Vlissidis and Zouboulis,1993)and fermented olive mill wastewater treatment(Hamdi and Garcia, 1991).
Concentrated wastewaters are suitable to be treated by anaer-obic contact digestion since relatively high quality effluent can be achieved(Leslie Grady et al.,1999).In the study of fermented olive mill wastewater treatment,anaerobic contact was capable of reaching steady state more quickly compared to anaerobicfil-ter;however,more oxygen transfer in the digester(due to mix-ing)causes this process to be less stable.While scum formation was reported in POME treatment pilot plant(Ibrahim et al., 1984),instability was not reported in other treatment systems. Despite the problems that might be encountered in anaerobic contact,this system has been able to remove COD efficiently, achieving up to80%removal efficiency(Vlissidis and Zouboulis, 1993)
2.3.Comparison of various anaerobic treatment methods in POME treatment
Table6lists the performance of various anaerobic treatment methods of POME.Although thefluidized bed reactor has the
Table5
Performance of UASFF in various wastewater treatments
Types of wastewater OLR(kg COD/m3day)COD removal efficiency(%)Methane composition(%)Reference
Sugar wastewater  5.0–51.063.0–96.0N/A Guiot and van den Berg(1985)
Wash waters from olive oil purification  2.6–17.875.7–90.869.0–75.0Borja et al.(1996b)
POME  1.75–23.1589.5–97.562.0–84.0Najafpour et al.(2006)
Slaughterhouse wastewater  2.49–20.8290.2–93.456.0–74.0Borja et al.(1998)
Dairy wastewater  1.8–8.490.1–92.065.3Córdoba et al.(1995)
Woodfiber wastewater  1.0–15.052.0–72.5N/A Ayati and Ganjidoust(2006)
Coffee wastewater  1.06–6.022.4–88.6N/A Bello-Mendoza and Castillo-Rivera(1998) N/A:data unavailable.
P.E.Poh,M.F.Chong/Bioresource Technology100(2009)1–95

版权声明：本站内容均来自互联网，仅供演示用，请勿用于商业和其他非法用途。如果侵犯了您的权益请与我们联系QQ:729038198，我们将在24小时内删除。