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Journal of Environmental Management 90 (2009) 1844–1849
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Journal of Environmental Management
journal homepage: www.elsevier .com/locate/ jenvman
Improvement of fruit and vegetable waste anaerobic digestion performanceand stability with co-substrates addition
H. Bouallagui*, H. Lahdheb, E. Ben Romdan, B. Rachdi, M. HamdiLaboratory of Microbial Ecology and Technology, National Institute of Applied Sciences and Technology, BP 676, 1080, Tunisia
a r t i c l e i n f o
Article history:Received 6 February 2008Received in revised form29 August 2008Accepted 1 December 2008Available online 30 December 2008
Keywords:Anaerobic co-digestionStabilitySequencing batch reactorFruit and vegetable wasteAbattoir wastewaterWaste activated sludgeFish waste
* Corresponding author. Tel.: þ216 22 524406; fax:E-mail address: [email protected] (H. Bou
0301-4797/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.jenvman.2008.12.002
a b s t r a c t
The effect of fish waste (FW), abattoir wastewater (AW) and waste activated sludge (WAS) addition as co-substrates on the fruit and vegetable waste (FVW) anaerobic digestion performance was investigatedunder mesophilic conditions using four anaerobic sequencing batch reactors (ASBR) with the aim offinding the better co-substrate for the enhanced performance of co-digestion. The reactors were oper-ated at an organic loading rate of 2.46–2.51 g volatile solids (VS) l�1 d�1, of which approximately 90%were from FVW, and a hydraulic retention time of 10 days. It was observed that AW and WAS additionswith a ratio of 10% VS enhanced biogas yield by 51.5% and 43.8% and total volatile solids removal by 10%and 11.7%, respectively. However FW addition led to improvement of the process stability, as indicated bythe low VFAs/Alkalinity ratio of 0.28, and permitted anaerobic digestion of FVW without chemical alkaliaddition. Despite a considerable decrease in the C/N ratio from 34.2 to 27.6, the addition of FW slightlyimproved the gas production yield (8.1%) compared to anaerobic digestion of FVW alone. A C/N ratiobetween 22 and 25 seemed to be better for anaerobic co-digestion of FVW with its co-substrates. Themost significant factor for enhanced FVW digestion performance was the improved organic nitrogencontent provided by the additional wastes. Consequently, the occurrence of an imbalance between thedifferent groups of anaerobic bacteria which may take place in unstable anaerobic digestion of FVWcould be prevented.
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1. Introduction
Anaerobic digestion is a well established process for treatingmany types of organic wastes, both solid and liquid (Pain et al.,1988; Ralph and Keith, 1990; Borzacconi et al., 1995; Murto et al.,2004; Neves et al., 2006; Yen and Brune, 2007). This alternativeallows the recovery of energy and a solid product that can be usedas an amendment of soils (Lawson, 1992; Gomez-Lahoz et al.,2007). The nutrient content of the anaerobic compost is favourableand the content of pollutants is low (Krugel et al., 1998; Kubleret al., 2000; Elango et al., 2007).
The easy biodegradable organic matter content of FVW withhigh moisture facilitates their biological treatment and shows thetrend of these wastes for anaerobic digestion (Bouallagui et al.,2003). In general, hydrolysis is the rate limiting step if the substrateis in particulate form (Veeken and Hamelers, 1999). However, theanaerobic degradation of cellulose-poor wastes like FVW is limitedby methanogenesis rather than by the hydrolysis. A major
þ216 71 704329.allagui).
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limitation of anaerobic digestion of FVW is a rapid acidification ofthese wastes decreasing the pH in the reactor, and a larger volatilefatty acids production, which stress and inhibit the activity ofmethanogenic bacteria (Misi and Forster, 2001; Bouallagui et al.,2005).
The addition of co-substrates with high nitrogen content isa solution to adjust nutrient content of FVW. The unbalancednutrients of fish waste (FW), abattoir wastewater (AW) and wasteactivated sludge (WAS) characterised by a low C/N ratio were alsoregarded as an important limitation factor to anaerobic digestion ofthese organic wastes (Mshandete et al., 2004; Gomez et al., 2006;Gannoun et al., 2007). Adding AW, FW and WAS in FVW feedstockto have a balanced C/N ratio was undertaken in this study. Theirgreatest advantage lies in the buffering of the organic loading rate,and anaerobic ammonia production from organic nitrogen, whichreduce the FVW anaerobic digestion limitations.
Co-digestion is a technology that is increasingly being appliedfor simultaneous treatment of several solid and liquid organicwastes (Poggi-Varaldo et al., 1997; Callaghan et al., 1999; Alatristeet al., 2006; Perez et al., 2006). It combines different organicsubstrates to generate a homogeneous mixture as input to theanaerobic reactor in order to increase process performance
H. Bouallagui et al. / Journal of Environmental Management 90 (2009) 1844–1849 1845
(Hamzawi et al., 1998; Viotti et al., 2004; Zhang and Banks, 2008). Itpermits the exploitation of complementarity in waste characteris-tics e.g. avoidance of nutrients (N, P) addition when a co-digestedwaste contains nutrients in excess (Gavala et al., 1996; Pavan et al.,2005; Neves et al., 2008). Several studies have shown that multi-component mixtures of agro-wastes, rural wastes and industrialwastes can be digested successfully, although with some mixturesa degree of both synergism and antagonism occurred (Misi andForster, 2001, 2002; Cavinato et al., 2008).
The aim of this work was to examine the effect of AW, WAS andFW addition as co-substrates on the FVW anaerobic digestionperformance in mesophilic condition using an ASBR. The criteria forjudging the success of a co-digestion were process stability, VSreduction, biogas production rate, and methane yield.
2. Material and methods
2.1. Reactors design and operational conditions
Four laboratory-scale anaerobic sequencing batch reactors (R1,R2, R3 and R4) of 2 l effective volume were used (Fig. 1). Thetemperature was controlled at 35 �C by a thermostatically regulatedwater bath. Peristaltic pumps were used to fill the reactors and todraw off the effluents after settling. Mixing in the reactors was doneby a system of magnetic stirring. Each digester was initially inocu-lated with anaerobic sludge obtained from an active mesophilicdigester of FVWs treatment plant (Bouallagui et al., 2007).
The ASBR was operated with cycles including the following fourdiscrete steps: (i) fill (15 min): 200 ml of different mixtures ofwastes were added to the reactors at the beginning of a cycle, (ii)react (21 h): during this phase, the reactors were stirred andorganic matter was converted to energy and new cells, (iii) settle(2 h and 30 min): settling started when the react phase wasfinished and (iv) draw off (15 min): at the end of the settling period,the volume of liquid added at the beginning of the cycle was drawnoff from the reactors.
2.2. Wastes sources and characteristics
The fruit, vegetable and fish wastes used in this study werecollected from the group market of Tunis. After shredding to small
(4) (3)
(5)(6)
(1)
(7)
(9)
(2)
(8)
Fig. 1. Schematic of the experimental ASBR system: (1) ASBR, (2) water bath andheating recirculation, (3) magnetic stirrer, (4) feedstock, (5) feeding pump, (6)discharge pump, (7) effluent stock, (8) sampling valve and (9) biogas collector.
particles and homogenizing, they were stored at 4 �C. WAS wascollected from the activated sludge plant (Cherguia, Tunis) treatingdomestic and industrial wastewaters. It is composed of settledsuspended biomass. The AW was collected from an abattoir factory(El Ouardia City, Tunis). Analysis of the raw FVW, FW, AW and WASwere carried out several times and the average compositions areshown in Table 1. The FVW consisted of homogenised courgettes,lettuce, tomatoes, apple, orange, pear, potatoes and carrot to give8.3% TS with VS content of 93%. The feedstock was made up byadding a percentage by volume of water, AW, WAS and FW to FVW.Four feedstocks (W1–W4) (Table 2) which were prepared withaverage TS contents of 2.7%, 2.74%, 2.9% and 2.8%, were used to loadR1–R4. Approximately, 90% of VS in the different feedstocks weregiven from FVWs.
2.3. Technical analysis
The biogas produced was measured daily by gas meter (Ritter –Bochum Langendreer, Germany) and its composition was estimatedusing an ORSAT apparatus (Bouallagui et al., 2003). Total solids (TS),total volatile solids (TVS), total suspended solids (TSS), pH, alka-linity and total volatile fatty acids (VFAs) were determinedaccording to the APHA Standard Methods (1995). Total organiccarbon (TOC) was measured by catalytic oxidation on a TOC Euro-glace analyser. Total nitrogen (TN) was estimated by the Kjeldahlmethod.
2.4. Statistical analysis
In order to determine whether the observed differencesbetween digesters performances were significantly different, datawere subjected to the ANOVA tests (StatSoft Inc, 1997). Differencesbetween co-substrates’ addition effects (p and p1) were comparedwith 0.05.
3. Results and discussion
3.1. Effect of co-substrates addition on the fermentation efficiency
3.1.1. Biogas productionThe biogas production by the digestions of FVW alone and the
co-digested wastes is shown in Fig. 2. Analysis of biogas productionprofiles for the substrates combinations showed that there weresignificant differences among the combinations tested. The resultsfor co-digested substrates are better than those obtained fromdigestion of FVW. The average biogas production rate variedbetween 1.53 l d�1 and 2.53 l d�1, with value being highest for bothmixtures W2 and W3 and lowest for 100% FVW. The specific biogasproductions for the four digestion processes (R1–R4) were 0.403,0.611, 0.580 and 0.436 l g�1 removal VS.
The methane yields from FVW, which have been reportedpreviously, are variable depending on the waste composition and
Table 1Characteristics of raw wastes.
FVW AW WAS FW
TS (%) 8.3 0.35 0.6 19.6VS (% of TS) 93 60.3 81.5 60.2TSS (g/l) 46.3 0.46 4.9 53.7tCOD (g/l) 162 3.8 11.7 188.8tCOD/VS 2.1 1.8 2.4 1.6pH 4.2 7.2 6.93 6.1Total Nitrogen (% of TS) 2.1 12.1 5.4 5.4Total Carbon (% of TS) 72 60.2 52.5 48.2C/N 34.2 5.14 10.48 8.8
Table 2Characteristics of mixtures wastes.
W1 W2 W3 W4
TS (%) 2.7 2.74 2.9 2.8VS (% of TS) 92 90.1 86.5 89.5TSS (g/l) 14.1 14.8 18.7 22.6tCOD (g/l) 52.2 50.6 56.4 48.9tCOD/VS 2.1 2.05 2.25 1.95pH 4.3 5.4 4.94 5.1Total Nitrogen (% of TS) 2.1 3.1 2.6 2.5Total Carbon (% of TS) 72 70.2 64.4 69.1C/N 34.2 22.58 24.76 27.6
H. Bouallagui et al. / Journal of Environmental Management 90 (2009) 1844–18491846
the used reactor design. The reported range was from 0.16 to0.4 m3 kg�1 VS added (Bouallagui et al., 2005). The results pre-sented in this paper for FVW digestion are therefore comparablewith these earlier results.
The data for the co-digestion may be also compared with earlierworks. Callaghan et al. (2002) examined the co-digestion of FVWwith cattle slurry and chicken manure. The methane yields theyobtained of 0.35–0.4 l g�1 VS added were very similar to those inthe current study for FVW–AW co-digestion. Gomez et al. (2006)have also reported similar results for the anaerobic co-digestion ofFVW and primary sludge. However the methane yield obtainedfrom FVW and WAS anaerobic co-digestion in two stages tubular
0
0,5
1
1,5
2
2,5
3
3,5
Bio
gas rate (l d
-1)
0 5 10 15 20 25 30 35 40 45Time (d)
0
0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
0,45
0 5 10 15 20 25 30 35 40 45Time (day)
Me
th
an
e y
ie
ld
(l g
-1 V
S rem
oved
)
Fig. 2. Biogas rate and methane yield variation during anaerobic digestion of W1:30%FVW/70%Water (-), W2: 30%FVW/70%AW (:), W3: 30%FVW/70%WAS (,) andW4: 30%FVW/1.4%FW/68.6Water (�), under mesophilic condition and an HRT of 10days.
digesters of 0.25 l g�1 VS added (Dinsdale et al., 2000) was lowerthan that is presented in this work.
The addition of AW, WAS and FW enhanced the biogas yield by51.5%, 43.8% and 8.1%, respectively. Biogas yields for FVW:AW andFVW:WAS co-digestions are much greater thanks to the better C/Nratio of these feedstocks (Fig. 3). The ANOVA of the data indicatedthat digesters (R2 and R3) performance enhancement was statis-tically significant (p< 0.05) (Table 3). Despite a considerabledecrease of C/N ratio from 34.2 to 27.6, the addition of FW slightlyimproves the gas production rate and biogas yield compared toanaerobic digestion of FVW alone (p1>0.05). The C/N ratios of theco-digested FVW:AW and FVW:WAS which ranged between 22 and25 were within the C/N ratio (20–25) required for stable and betterbiological conversions reported by others on anaerobic digestion oforganic wastes (Parkin and Owen, 1986; Mshandete et al., 2004;Yen and Brune, 2007). Kayhanian and Hardy (1994) reported C/Nratios between 25 and 30 as being optimal. However, Kivaisi andMtila (1998) argue that the C/N of approximately between 16 and19 is optimal for methanogenic performance.
3.1.2. Organic matter removalThe total volatile solids destruction for the various co-digested
substrates combinations is given in Table 3. The higher degradationefficiencies were obtained for the digesters treating FVW:AW andFVW:WAS and operated at an organic loading rate of2.46 gTVS l�1 d�1 and 2.51 gTVS l�1 d�1, respectively. They wereassociated with the higher specific biogas production and a lowercontent of volatile solids in the digested effluent, which representsa lesser amount of output stabilised effluent with a better dew-atering properties. The data of Table 3 showed that about 84–85.4%of TVS were degraded to methane and carbon dioxide with the co-digestion of organic wastes. These results are in agreement withthose obtained by Fernandez et al. (2005) and better than thoseobtained by Callaghan et al. (1999) and Dinsdale et al. (2000). It isvery likely that the high degradation efficiency in the co-fermen-tation was due to an improved ratio of nutrients and better avail-ability of the organic substances, which facilitate their assimilationby anaerobic flora and increases the degree of degradation (Kruppand Schubert, 2005). Furthermore, the AW addition improves theproteins availability that were used by anaerobic bacteria toproduce new cells and enzymes.
3.1.3. Alkalinity, total VFAs and pH variationIn a well balanced anaerobic digestion process, total VFAs levels
are low (Fernandez et al., 2005; Chen et al., 2007). In this study allthe combinations examined, except FVW–FW showed lower levelsof total VFAs in their digested effluent at steady-state (Fig. 4).During the period days 1–25 high levels of total VFAs of up to
0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
20 22 24 26 28 30 32 34C/N ratio
Bio
gas yield
(l g
-1
rem
oval V
S)
60
65
70
75
80
85
90
VS
rem
oval (%
)
Fig. 3. Effect of C/N ratio variation on the VS removal efficiency (,) and biogasyield (-).
Table 3Digesters performances.
R1 R2 R3 R4 p
OLR (g/l.d) 2.48� 0.05 2.46� 0.1 2.51� 0.1 2.5� 0.04 –VS inlet (g/l) 24.8� 0.5 24.6� 1 25.1� 1 25.06� 0.4 –VS outlet (g/l) 5.85� 0.2 3.92� 0.1 3.66� 0.2 6.74� 0.3 –VS removal (%) 76.4� 0.98 84.06� 1.2 85.4� 1.51 73.1� 1.1 0.000
p1¼0.000 p1¼0.000 p1¼0.006Biogas production
rate (l/d)1.53� 0.1 2.53� 0.2 2.49� 0.1 1.6� 0.09 0.000
p1¼0.000 p1¼0.000 p1¼0.569Biogas yield
(l/g removal VS)0.4� 0.05 0.61� 0.03 0.58� 0.01 0.44� 0.03 0.000
p1¼0.002 p1¼0.003 p1¼0.415Biogas yield
(l/g added VS)0.31� 0.02 0.51� 0.03 0.49� 0.01 0.32� 0.02 0.000
p1¼0.001 p1¼0.001 p1¼0.712VFAs (mg/l) 750� 20 300� 20 520� 20 1900� 100 –Alkalinity (mg/l) 1300� 50 4400� 100 4800� 150 7000� 300 –tVFAs/Alkalinity 0.57� 0.01 0.07� 0.005 0.11� 0.01 0.27� 0.015 0.000pH 6.9� 0.3 7.33� 0.1 7.17� 0.15 7.57� 0.2 –Ammonia (mg/l) 120� 10 900� 30 700� 30 2200� 100 –
p: Indicated the statistical difference between all digesters performances (R1–R4).p1: Indicated the statistical difference between digesters R1 and one of otherdigesters (R2–R4).
6,8
6,9
7
7,1
7,2
7,3
7,4
7,5
7,6
7,7
7,8
7,9
Time (day)
pH
0
500
1000
1500
2000
2500
3000
3500
4000
0 5 10 15 20 25 30 35 40 45
0 5 10 15 20 25 30 35 40 45
0 5 10 15 20 25 30 35 40 45
Time (day)
VF
As (m
g l-1)
0
1000
2000
3000
4000
5000
6000
7000
8000
Time (day)
Alkalin
ity (m
g l-1)
Fig. 4. VFAs, alkalinity and pH variation during anaerobic digestion of W1: 30%FVW/70%Water (-), W2: 30%FVW/70%AW (:), W3: 30%FVW/70%WAS (,) and W4:30%FVW/1.4%FW/68.6Water (�), under mesophilic condition and an HRT of 10 days.
H. Bouallagui et al. / Journal of Environmental Management 90 (2009) 1844–1849 1847
2800 mg l�l, 2000 mg l�1, 1500 mg l�1 and 2300 mg l�1 experi-enced in digesters treating W1, W2, W3 and W4, respectively,indicating that the reactors were not operating at their optimum.An average of 750 mg l�1, 300 mg l�1, 520 mg l�1 and 1900 mg l�1
total VFAs were found, respectively for the different digesters atsteady-state. Total VFAs concentration remained at high level forthe digester treating FVW–FW indicating a digestion limitationaccompanied by the lowest biogas yield and volatile solid removal.Although, the pH and the partial alkalinity of this reactor were high,indicating good process stability.
An average value of total VFAs between 1330 and 1800 mg l�l
was also found in the effluent of a successful methanogenic reactortreating FVW with WAS as a co-substrate (Dinsdale et al., 2000). Incontrast, levels of 55–505 mg l�l total VFAs were found in thereactors treating multi-component agro-wastes (Misi and Forster,2001) indicating that higher and lower levels of total VFA arepossible for organic wastes co-digestion.
The initial partial alkalinity ranged between 600 mg l�1 and2400 mg l�1 while the final range was 1300 mg l�1 and 7000 mg l�1
(Fig. 4). The latter, demonstrated an increased partial alkalinity inthe digesters compared to the initial values before anaerobicdigestion stability. This provided further evidence that the co-digestions of FVW and co-substrates studied were successful.Previously, laboratory studies on mesophilic and thermophilicanaerobic organic wastes digestion reported a range of 2000–4000 mg l�1 partial alkalinity as being typical for properly oper-ating digesters (Chen et al., 2007; Sharma et al., 2000). The initialvalues reported in this study fall within this range. However, thefinal values are higher than the reported values, especially forFVW–FW co-digestion. This increase could be due to generation ofNH4þ during the digestion of protein in fish waste which resulted in
an increased digester buffering capacity and hence stability of thedigesters. This is an interesting cost effective approach since noexternal buffer sources were added.
The pH was monitored continuously in the digesters. Theevolution of the pH values obtained under different conditions ispresented in Fig. 4. Despite the low pH of the feed substrate (4.3–5.4),the pH increased to its neutral value (between 6.9 and 7.57) dueto the process stability and the activity of methanogenic bacteria.The outlet pH value increased with the addition of high nitrogencontent co-substrate on the FVW. The highest values were obtainedfor the digester treating FVW–FW co-substrates due to a highpartial alkalinity of 7000 mg l�1 and ammonia concentration of2200 mg l�1.
One of the criteria for judging digester stability is the VFA-s:Alkalinity ratio. There are three critical values for this (Switzen-baum et al., 1990; Callaghan et al., 2002). If this ratio was lower than0.4, the digester should be stable. While, when the ratio ranged0.4–0.8, some instability will occur on the digester performances.However, the ratio higher than 0.8, indicates a significantly insta-bility. When FVW being digested alone (Fig. 5), the VFAs:Alkalinityratio (0.57) did not rise above the criteria value of 0.4, implying thatdespite the results for the biogas yield and VS reduction, there wasthe potential for instability. Generally, FVW is thought of as being
0
1
2
3
4
5
6
7
8
9
0 5 10 15 20 25 30 35 40 45Time (day)
AG
Vs/A
lkalin
ity ratio
Fig. 5. VFAs/Alkalinity ratio variation during anaerobic digestion of W1: 30%FVW/70%Water (-), W2: 30%FVW/70%AW (:), W3: 30%FVW/70%WAS (,) and W4:30%FVW/1.4%FW/68.6Water (�), under mesophilic condition and an HRT of 10 days.
H. Bouallagui et al. / Journal of Environmental Management 90 (2009) 1844–18491848
highly degradable, but it is essential that there is an adequatealkalinity (Gunaseelan, 1997). Lane (1984) suggested that, fora balanced digestion of FVW, the alkalinity should not be less than1500 mg l�1 and that the VFAs:Alkalinity ratio should be less than0.7. The addition of AW, WAS and FW in the feedstock of FVWproduced a decrease of VFAs:Alkalinity ratio in the digesters to bearound 0.07, 0.11 and 0.27, respectively, showing better processesstabilities and buffer capacities. Consequently, the occurrence of animbalance between the different groups of anaerobic bacteriawhich may take place in an unstable anaerobic digestion of FVWprocess could be prevented.
3.2. Digesters performances
The results of the digesters performances are shown in Table 3.Compared to methane yield for the pure FVW, co-digestions of theFVW–AW, FVW–WAS and FVW–FW enhanced the biogas yield by51.5%, 43.8% and 8.1%, respectively. The better biogas yield(0.61 l g�1 removal VS) and VS removal (85.4%) were obtained byW2 and W3 co-digestions, respectively. This could be due to posi-tive synergism in the digestion medium, especially for FVW–AW,FVW–WAS combinations, supplying missing nutrients andreducing of inhibitory materials in feedstock by the co-substrates(Mshandete et al., 2004). The average CH4 content of the biogasproduced from different treated wastes range between 64% and66%. This range of methane content is closer to the range of 55–65%which is normally obtained from conventional anaerobic digestionof organic wastes conducted in single stage digesters.
The results of FVW–FW digestion showed a decrease of biogasproduction rate due to the high amount of ammonia (2200 mg l�1)and total VFAs (1900 mg l�1). In fact, the total ammonia nitrogenand VFAs both are important intermediates and potential inhibitorsin the anaerobic digestion process. High concentration of ammoniaand VFAs in the digester would decrease the methanogens activityand further accumulation could inhibit the anaerobic digestion(Chen et al., 2007).
The estimated free ammonia (FA) concentrations based on pHand total ammonia for the four digestion processes (R1–R4) were3.2, 63.4, 34.4 and 264.5 mg l�1. It is generally believed thatammonia concentrations below 200 l�1 are beneficial to anaerobicprocess since nitrogen is an essential nutrient for anaerobicmicroorganisms (Liu and Sung, 2002). Gallert and Winter (1997)studied the anaerobic digestion of organic wastes and reported thatmethane production was inhibited 50% by 220 l�1 FA at 37 �C andby 690 l�1 FA at 55 �C, indicating that thermophilic flora tolerated
at least twice as much FA as compared to mesophilic flora. Severalmechanisms for ammonia inhibition have been proposed, such asa change in the intracellular pH and the inhibition of specificmethane synthesising enzyme reaction (Calli et al., 2005). FA hasbeen suggested to be the main cause of inhibition since it is freelymembrane-permeable. The hydrophobic ammonia molecule maydiffuse passively into the cell, causing proton imbalance, and/orpotassium deficiency (Gallert et al., 1998).
4. Conclusion
An interesting option for improving yields of anaerobic diges-tion of solid wastes is co-digestion. Its benefits include improvedbalance of nutrients, synergistic effect of microorganisms,increased load of biodegradable organic matter and better biogasyield. Combination of FVW with other substrate like AW and WAScan significantly improve the waste treatment efficiency. Thisresulted in a highly buffered system as the high nitrogen contentco-substrate contributed to high amount of ammonia. Fish wastewas not as successful as a co-substrate for FVW digestion. Asa consequence to the FW addition, the VS reduction deterioratedand the methane yield increased slightly. This appeared to be due tothe concentration of ammonia. Results indicate that the ratio of C/Nis a determining parameter which influenced the methaneproduction and the organic matter bio-degradation. The biogasproduction yield was enhanced by 51.5% and 43.8% by the additionof AW and WAS, respectively to FVW feedstock. It was verified thatthese combinations could be a promising and practical alternativefor the simultaneous recycling of different types of organic wasteswith high stability. It seemed that carbohydrate rich substrates aregood producers of VFAs and that protein rich substrate are yieldinggood buffering capacity. The high values for the methane yield andthe VS reduction were indicatives for a high content of biode-gradable organic matter in the co-substrate due to an improvedratio of nutrients and better availability of the organic substances.
Acknowledgements
The authors wish to acknowledge the Ministry of SuperiorEducation and Scientific Research and Technology, which hasfacilitated the carried work.
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