Anaerobic digestion challenge of raw olive mill wastewater

Bioresource Technology 102 (2011) 10810–10818

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Anaerobic digestion challenge of raw olive mill wastewater M.A. Sampaio, M.R. Gonçalves, I.P. Marques ⇑ Bioenergy Unit, National Laboratory of Energy and Geology I.P. (LNEG), Estrada Paço do Lumiar 22, 1649-038 Lisboa, Portugal

a r t i c l e

i n f o

Article history: Received 30 May 2011 Received in revised form 26 August 2011 Accepted 1 September 2011 Available online 10 September 2011 Keywords: Raw olive mill effluent Biogas Anaerobic hybrid digester Phenolic compounds Organic shocks operation

a b s t r a c t Olive mill wastewater (OMW) was digested in its original composition (100% v/v) in an anaerobic hybrid. High concentrations (54–55 kg COD m3), acid pH (5.0) and lack of alkalinity and nitrogen are some OMW adverse characteristics. Loads of 8 kg COD m3 d1 provided 3.7–3.8 m3 biogas m3 d1 (63–64% CH4) and 81–82% COD removal. An effluent with basic pH (8.1) and high alkalinity was obtained. A good performance was also observed with weekly load shocks (2.7–4.1, 8.4–10.4 kg COD m3 d1) by introducing piggery effluent and OMW alternately. Biogas of 3.0–3.4 m3 m3 d1 (63–69% CH4) was reached. Developed biomass (350 days) was neither affected by raw OMW nor by organic shocks. Through the effluents complementarity concept, a stable process able of degrading the original OMW alone was obtained. Unlike what is referred, OMW is an energy resource through anaerobiosis without additional expenses to correct it or decrease its concentration/toxicity. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Olive oil production is expanding worldwide as a result of its health-promoting effects. Most countries make use of the threephase centrifugation system, from which large quantities of a strong reddish-brown liquid called olive mill wastewater are obtained (Morillo et al., 2009; McNamara et al., 2008). This effluent has become a serious environmental problem as a result of the olive oil increasing production and industrialization of the extraction process that generates larger amounts of OMW (Kapellakis et al., 2006). Moreover, the scattered and seasonal nature of olive oil production did not contribute to find a solution in order to properly manage the resulting effluent (McNamara et al., 2008). OMW cannot be treated in a domestic wastewater treatment plant due to technical limitations (Rozzi and Malpei, 1996). On the other hand, the application of untreated OMW on soils and crops causes phytotoxic and biotoxic effects which make it unsuitable for further use as fertilizer or as irrigation water (Niaounakis and Halvadakis, 2006). The adopted solution in many countries is the evaporation in open ponds which requires large areas and generates several problems such as bad odour, methane emissions, infiltration into the soil and insect proliferation (Roig et al., 2006; Jarboui et al., 2010). This means that common cost-effective practices applied to OMW management are not an operative solution to solve this problem. ⇑ Corresponding author. Tel.: +351 210924600; fax: +351 217127195. E-mail addresses: [email protected] (M.A. Sampaio), marta.goncal [email protected] (M.R. Gonçalves), [email protected] (I.P. Marques). 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.09.001

OMW has been the subject of many waste treatment studies involving chemical and physical treatment (coagulation/flocculation and chemical oxidation), biochemical treatment (fermentation, aerobic process, composting) and combined processes/techniques (Roig et al., 2006; El-Gohary et al., 2009; Sarika et al., 2005). However, no satisfactory solution has yet been found for the safe OMW disposal mainly due to technical and economical limitations (Morillo et al., 2009). As a result, significant OMW volumes in Mediterranean area are discharged directly into watercourses (Azbar et al., 2009; El-Gohary et al., 2009) and it is urgent to adopt technologies that allow maximizing the benefit/price ratio and overcome this situation. Biological processes are considered environmentally friendly and, in many cases, a cost-effective procedure (McNamara et al., 2008). Anaerobic digestion has been reported as one of the most promising technologies for the disposal of OMW (Paraskeva and Diamadopoulos, 2006; Marques, 2000). Comprising a high organic content (45–220 g COD L1), this effluent is classified among the strongest industrial liquid wastes that corresponds to concentrations 20–4400 times higher than the ordinary urban wastewater (Azbar et al., 2009; Xing et al., 2000) and, consequently, it represents a significant energy potential (Gelegenis et al., 2007). Apart from the renewable energy generation in the form of biogas, anaerobic digestion presents some other appealing advantages since it allows small amounts of sludge generation, low nutrient requirements, reduction of greenhouse gases emissions and production of a liquid fertilizer. However, several OMW characteristics such as the acid pH, low alkalinity, low nitrogen content and the presence of a lipidic fraction and phenolic compounds derived from the olive stones and pulp, make this wastewater a potential toxic

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substrate and not suitable for anaerobiosis. To overcome these problems several synthetic nutrient, chemical additions and pretreatments (chemical and biochemical) have been reported to enable OMW anaerobic digestion (Dareioti et al., 2009; El-Gohary et al., 2009; Martinez-Garcia et al., 2009; Azbar et al., 2009, Gelegenis et al., 2007). But again, these pre-treatments involve inputs which raise the cost-benefit ratio and also lead to organic load reductions and, consequently, to a decrease of the available methanogenic potential for energy production. This work is part of a broader plan that aims to make the energetic valorisation of the raw OMW, by anaerobiosis, simpler, more flexible and cheaper. So, the concept of OMW complementary effluent was applied in order to reduce the treatment processing steps by elimination of the operational phases related with OMW corrections and/or pre-treatments. This feeding approach was tested by combining progressive increases of OMW volumes with a complementary effluent during the experimental period. The first results have showed to be possible to treat anaerobically the raw OMW using another effluent and digesting them simultaneously (Marques et al., 1997, 1998).It was also proved that the piggery effluent can work as a good complementary effluent of OMW. Afterwards, the strategy of combining OMW with other wastes was used by different authors. A study about the effect of different substrates (manure, household waste and sewage sludge) revealed that OMW and manure were the best co-digestion option (Angelidaki and Ahring, 1997). More recently, a combination of OMW and diluted poultry manure was degraded in a cylindrical down flow anaerobic reactor with 18 days of hydraulic retention time (HTR) (Gelegenis et al., 2007). However, under a critical OMW percentage of 28% (v/v) the methane production rate dropped rapidly and 1 m3 m3 d1 was registered as a maximum. In a two-stage CSTR anaerobic reactor, Dareioti et al. (2009) used a mixture of 55% OMW and 40% cheese whey and 5% (v/v) liquid cow manure. Effluent was successfully degraded and a methane production rate of about 1.35 m3 m3 d1 was obtained using a HTR of 19 days. Other mixture (75% OMW plus 25% pig slurry) was pre-treated by Candida tropicalis and digested in a fixed bed reactor (HTR of 11 days) to give 1.61 m3 m3 d1 of biogas (Martinez-Garcia et al., 2009). The up-flow fixed bed digester (anaerobic filter), previously studied by Marques (2001), was also tested with several feed mixtures but without recourse to any operational action before the anaerobic digestion phase (pre-treatments, chemical corrections or supplementations). Working with 83% OMW and 17% piggery effluent (v/v) and about 6 days of HTR, a production rate of 1.31 m3 CH4 m3 d1 was registered. Following the work performed and aiming to make the process even simpler and cheaper, it was decided to test other up-flow digester type. An anaerobic hybrid digester was used instead of the anaerobic filter (Gonçalves et al., 2009). Similar feed mixtures were provided and amounts of 83% OMW (v/v) were treated without any inhibition (HRT = 6 days) providing a methane production rate of 1.96 m3 m3 d1 (Gonçalves et al., submitted for publication). Based on the team results a biogas plant working all year with the complementary substrate can advantageously receive increasing amounts of OMW without affecting the system stability. Bearing in mind the seasonality of the OMW production (three or four months a year) and the large volumes of effluents that are generated (7–30 million m3 every year: Niaounakis and Halvadakis, 2006), the present work aims to test the ability of the anaerobic unit to digest a single substrate. In order to reduce the storage time of OMW, a feeding consisting only of the original OMW was provided to study its effect in the digester behaviour. Being possible and advantageous to operate the reactor with two substrates (OMW and the complementary effluent), the other goal of this study is to evaluate the conditions of process stability when so different effluents are alternately introduced.

2. Methods 2.1. Substrates: agro-livestock and industrial effluents The OMW tested in this study resulted from the olive oil campaign of 2010. It was collected in an olive oil production plant equipped with a three-phase olive oil extraction process, located in Rio Maior (Portugal). The unit is characterized by an average olive oil production capacity of 42 m3 year1. Piggery effluent was obtained from a pig farming facility located in the vicinity of the olive oil mill, also in Rio Maior. Both substrates were characterized (Table 1) and stored at 4 °C. They were digested in their original form; which means that they were not subjected to any alteration. 2.2. Analytical and chromatograph methods Total and soluble chemical oxygen demand (COD and CODS), total solids (TS), volatile solids (VS), total suspended solids (TSS), volatile suspended solids (VSS) and total ammonium nitrogen (TNHþ 4 -N) concentrations were determined according to Standard Methods (APHA, 1998). The proportion of ammonium concentrations and free ammonia (NHþ 4 versus NH3) were estimated according to Eq. (1) (El-Mashad et al., 2004), where T is the absolute temperature (273–373 K).

   NH3  N ¼ ðTHNþ4  NÞ  1 þ 10pH =10ð0:1075þð2725=TÞÞ

ð1Þ

pH measurements were performed in a WTW pH meter and probe. Alkalinity was evaluated as partial alkalinity (PA) and total alkalinity (TA) by titration to pH 5.75 and 4.50 with normalized 0.1 N HCl, respectively. Total nitrogen (TN) was quantified via Merck Nitrogen cell tests (10–150 mg NL1). Colour and aromatic compounds measurements were assayed by measurement of the absorbance at 390 and 254 nm, respectively, using a Hitachi U-2000 Spectrophotometer. Total phenolic compounds (TPh) concentration values as caffeic acid were determined via a modified Folin–Ciocalteu method (Singleton and Rossi, 1965). Volatile fatty acids (VFA: acetate, propionate, butyrate, iso-butyrate, iso-valerate and valerate) were analysed using a gas chromatograph (Hewlett Packard 5890) equipped with a flame ionization detector and a 2 m  2 mm Carbopack B-DA/4% Carbowax 20 M (80–120 mesh) column. Nitrogen was used as carrier gas (30 mL mn1). Temperature of the column, injector and detector was 170, 175 and 250 °C, respectively. Total VFA concentrations were expressed as acetic

Table 1 Characterization of the effluents used in the hybrid feed.

pH Partial alkalinity (kg CaCO3 m3) Total alkalinity (kg CaCO3 m3) COD (kg O2 m3) CODS (kg O2 m3) NH3 (kg N m3) Total N (kg N m3) TSS (kg m3) TS (kg m3) VSS (kg m3) VS (kg m3) VFA (kg acetic acid m3) TPh (kg caffeic acid m3) Colour [390 nm] Aromatic compounds [254 nm]

OMW

PE

4.96(0.08) 0 2.40(0.07) 55.28(2.3) 50.81(0.57) 0 0.21(0.02) 3.18(0.07) 28.23(0.36) 0.53(0.00) 15.85(3.22) 2.64(0.38) 3.59(0.01) 22.42(0.68) 128.12(1.93)

6.99(0.01) 4.85(0.14) 8.63(0.39) 30.71(0.00) 12.12(0.12) 1.83(0.04) 2.35(0.53) 19.60(1.65) 23.31(0.05) 4.80(0.38) 15.70(0.01) 3.43(0.27) 0.38(0.00) 2.23(0.00) 17.82(0.00)

OMW, olive mill wastewater; PE, piggery effluent; 0, below detection limit. Values are the averages of determinations. Values in brackets show standard deviations.

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acid. Soluble samples were obtained by centrifugation at 10000 rpm for 10 min using a VWR Galaxy 7D Microcentrifuge and were used for CODS, colour, aromatic compounds, total phenolic compounds and VFAs determination. The methane content of biogas collected in the digester headspace was measured by the injection of 0.5-mL bioreactor gas sample into a gas chromatograph (Varian CP 38000) equipped with a thermal conductivity detector and a Porapack S column of 1/8’’  3 m. Column, injector and detector temperatures were 50, 60 and 100 °C, respectively. Nitrogen was utilized as the carrier gas (20 mL mn1). 2.3. Reactor operation Experiments were carried out by using an up-flow anaerobic hybrid digester that was previously used and described elsewhere (Gonçalves et al., submitted for publication). It was initially removed from the cold chamber at 4 °C and then kept under mesophilic conditions of temperature (37 ± 1 °C) by using a water jacket. It was fed in a semi continuous manner by means of a peristaltic pump in order to obtain a HRT of about six days. The influent consisted of a blend of the raw OMW and its complementary substrate (piggery effluent, PE) obtained by an increase of OMW content along the experimental time (Marques, 2001). Gas production was evaluated by a wet gas meter and corrected to standard conditions for pressure and temperature (1 atm, 0 °C). Volume of digested flow was registered every day in order to determine the hydraulic retention time (HRT) of the assay. Influent and effluent samples were taken one or two times a week along the trial time. The operational period can be described in three main experimental phases: (A) The restarting of the hybrid digester and its operation using OMW complemented with PE (up to day 232, ‘‘Raw OMW and PE, mixture feed’’). During the first 14 days of operation, the hybrid was fed with piggery effluent and a HRT of about six days was set. Afterwards, the digester influent mixture was changed. OMW volumes of 53%, 69% and 83% (v/v) were provided. (Table 2)

(B) Digester feeding with the original OMW (from day 233 to 287, ‘‘Raw OMW feed’’). The hybrid reactor was fed with 100% OMW and any kind of supplementation, correction or dilutions of the olive oil mill effluent were performed (Table 2). On day 288 of the experiment, an interruption of the digester operation took place. During a period of 11 days, the unit was preserved at mesophilic conditions of temperature and no feed was provided. (C) Feeding the digester by applying alternate pulses of each of the substrates (from day 300 to 350 days, ‘‘Raw OMW or PE, alternated feed’’) (Table 2). The unchanged OMW was digested during the initial two weeks (C0) of this phase. After that, five weekly cycles were performed by using alternately each substrate. During each cycle, 17% and 83% of the operational time corresponded to PE and OMW supplies, respectively. 3. Results and discussion The load applied to the hybrid unit was efficiently converted into biogas. The hybrid stability and its capacity in converting the potential toxic matters of the influent are documented by the removal ability and methane production of the digester biomass over the 350 days of experiment. 3.1. Phase A. Complementary substrates trial: raw OMW and PE, mixture feed Operating under organic loading rates (OLR) of 5.2 kg COD m3 d1 (Phase A2, Table 2) and 7.4–9.0 kg COD m3 d1 (Phase A3), ranges of total COD removal of 75% and 80–83% and biogas volumes of 2.7 and 2.8–3.6 m3 m3 d1, containing 67% and 66–67% CH4, were obtained, respectively. The increase of the influent concentration till 57 kg COD m3 (Phase A3) did not cause instability neither decrease of the hybrid performance. The methane yield of 0.429 m3 CH4 kg1 COD removal (Phase A2) evolved to 0.317–0.358 m3 CH4 kg1 COD removal (Phase A3, data not shown) indicating the unit capacity to degrade the organic matter accumulated and overcome the disequilibrium of the system.

Table 2 Operational conditions of hybrid digester. Phase

Time (d)

Substrates OMW (% v/v)

A0 A1

0 53

100 47

A2 A3

0–14 15–82 83–94 95–136 137–168 169–232

69 83

31 17

B

233–287

100

0

Stop C0 C1

288–299 300–313 314–315 315–320 321–322 322–327 328–329 329–334 335–336 336–341 342–343 343–350

– 100 0 100 0 100 0 100 0 100 0 100

– 0 100 0 100 0 100 0 100 0 100 0

C2 C3 C4 C5

HRT (d)

OLR (kg COD m3 d1)

6.5(0.4) 6.2(0.5) 6.5(0.2) 6.3(0.5) 7.3(0.8) 6.6(0.2) 6.3(0.4) 6.4(0.7) 6.6(0.4) – 6.5(0.3) 6.0(0.8) 7.0(0.7) 5.2(0.2) 6.7(0.2) 5.5(0.2) 6.0(1.2) 6.1(0.9) 6.7(0.9) 5.7(0.2) 6.6(0.4)

– 4.9(0.1) 18.5() 5.6(0.8) 5.2(0.6) 7.4(0.4) 9.0(0.3) 8.1(1.2) 8.0(0.3) – 10.4(0.7) 4.1(0.2) 8.8(1.3) 3.6(0.2) 8.4(0.9) 2.7(0.2) 8.5(1.1) 3.1(0.1) 8.9(1.1) 2.7(0.1) 8.6(0.6)

PE (% v/v)

Raw OMW and PE, mixture feed (Phase A), Raw OMW feed (Phase B) and Raw OMW or PE, alternated feed (Phase C). OMW, Olive Mill Wastewater; PE, piggery effluent; HRT, Hydraulic Retention Time; OLR, Organic Loading Rate. Values are the averages of determinations. Values in brackets show standard deviations.

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M.A. Sampaio et al. / Bioresource Technology 102 (2011) 10810–10818 10

80

9

70

8

60 y = -0,4157x + 67,902 R² = 0,0647

6

50

5

40

4

30

3

20

y = 0,3519x + 0,5881 R² = 0,6414

2

10

1 0

CH4 (%)

Biogas (m3m-3d-1)

7

4

5

6

7

8

Organic loading rate (kg COD

9

10

0

m-3d-1)

Biogas 69% OMW

Biogas 69% OMW, G.2011

Biogas 83% OMW

Biogas 83% OMW, G.2011

CH4 69% OMW

CH4 69% OMW, G.2011

CH4, 83% OMW

CH4 83%, G.2011

Linear (Biogas)

Linear (CH4)

Fig. 1. Hybrid gas productivity: operation with 69% and 83% v/v OMW (Gonçalves et al., submitted for publication).

The main results of the OMW digestion obtained in different years and provided from different mills (Gonçalves et al., submitted for publication and current work) were presented against the load applied at 69% and 83% v/v OMW feeds (Fig. 1). From them

it is possible to infer that the increase of OLR (5.2–9.0 kg COD m3 d1) promotes the production of the biogas volume (2.4–3.6 m3 m3 d1) and maintains its quality along the time of operation. Methane concentrations (62–67%) were preserved in a

Table 3 Anaerobic digestion of olive mill wastewater: operation methodologies using substrates mixtures. No. Reactor, temp. (°C)

Effluents (% v/v)

Additional actions

HRT (d) OLR (kg COD m3 d1) Biogas methane (m3 m3 d1) COD removal (%)

References

1

AF, 35

None

6–7

7.7–9.6

70–77

Marques et al. (1997)

2

CSTR, 55

None

13

7.8

–

3

AF, 35

None

6–7

5.0–5.7

73–75

Angelidaki and Ahring (1997) Marques et al. (1998)

4

AF, 35

OMW-83 PE-17 OMW-75 Manure-25 OMW-91 PEdig.-9 OMW-91 PE-9 OMW-83 PEdig.-17 OMW-28 diluted poultry manure OMW-75, cheese whey-25 OMW-75, PE-25

None

6–7

6.6–8.0

63.2

Marques (2001)

5

– ,35

6

Fixed-bed reactor, 37 Fixed-bed reactor, 37

8.3–10 pH adjustments

18

4.84

3.6–4.0 2.3–2.6 – 1.55 1.7–2.1 1.1–1.4 2.1 1.3 3.42 2.2 1.53 0.99

73.6 –

Gelegenis et al. (2007)

Martinez-Garcia et al. (2007) Martinez-Garcia et al. (2009)

- sterilization - treatment: C. tropicalis (a) 11 - 26 g NaOH L1 addition - NaHCO3 addition - 0.26 g urea 19 - OMW-55 2 CSTR: L1feed) - cheese whey-40 acidogenic + Methanogenic, - cow manure-5 addition, 35 acidogenic reactor - 14 g NaHCO3 L1 addition, methanogenic reactor feed OMW-20, liquid None 19 cow manure-80 AH, 37 OMW-83 None 6 PE-17

3.0

1.25

83

5

1.61b

85

5.5

– 1.35

75.5 (% CODs) Dareioti et al. (2009)

3.63

– 0.91 3.16 1.96

63.2

Dareioti et al. (2010)

78.6

11

AH, 37

81.3

12

AH, 37

3.18 2.12 3.74 2.36

Gonçalves et al., submitted for publication Present work

81.5

Present work

7

8

9 10

OMW-83 PE-17 OMW-100

7.1

None

6–7

7.4

None

6–7

8.01

AF, anaerobic filter; CSTR, continuous stirred-tank reactor; AH, Anaerobic hybrid digester; OMW, olive mill wastewater; PE, Piggery effluent, PEdig., Piggery effluent digested anaerobically; HRT, hydraulic retention time; OLR, organic loading rate; COD, chemical oxygen demand; CODs, soluble COD, a Grown in a complex culture medium and phenol. b 29 L d1/18 L = 1.61 L L reactor1 d1 (data from last four days of operation).

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M.A. Sampaio et al. / Bioresource Technology 102 (2011) 10810–10818

12

90 Stop

Phase B: 100% OMW

Phase C: organic pulses

80

10

70

8

60 50

6

40

4

CH4 (%)

ORL (kg COD m-3 d-1); Biogas (m3 m-3 d-1)

(a)

30 20

2

10

0

0 230

240

250

260

270

280

290

300

310

320

330

340

350

360

Time (d) Biogas

CH4

(b)10

100

9

90

8

80

7

70

6

60

5

50

4

40

3

30

2

20

1

10

0

VFA rem (%)

pH / FVA (g L-1)

b

OLR

0 230

240

250

260

270

280

290

300

310

320

330

340

350

360

Time (d) pH in

(c)

VFA ef

VFA rem 100

10

90 80

TPh in ( gL-1)

8

70 60

6

50 40

4

30

TPh rem (%)

c

pH ef

20

2

10 0

0 230

240

250

260

270

280

290

300

310

320

330

340

350

360

Time (d) TPh in

TPh rem

Fig. 2. Hybrid digester behaviour: Phase B and C (a) gas production, (b) pH and VFA, (c) TPh (phenol compounds), in, influent; ef, effluent; rem, removal.

narrow range of the anaerobic process usual values. Regarding the digester removal ability, the increase of phenols concentration in influent (2.1–2.3 to 2.9–3.1 kg TPh m3) did not cause a relevant alteration on unit capacity (58.9–61.1% and 56.6–59.2% TPh removal, respectively). Concerning the total COD, the more concentrated influent (49–57 kg m3) corresponded to the highest COD removals recorded (80–83%). Several authors have presented studies on the use of different effluents to digest OMW anaerobically. Table 3 summarizes the main data obtained from some operation methodologies that combine OMW with other effluents. This strategy has been used by diverse authors but it was usually associated to several other phases of operation mainly related to the influent preparation to the anaerobic step. Pre-treatment ( C. tropicalis: Martinez-Garcia et al., 2007, 2009); pH adjustments (Gelegenis et al., 2007); urea and alkali (14 g NaHCO3 L1) additions (Dareioti et al., 2009) are some examples of the undertaken actions. Comparatively, this work group has been operated with the lower HRT and the highest

volume of the raw OMW in the influent (83/91%) and, consequently, highest loading rates (5–10 kg COD m3 d1) were tested and higher volumes of gas (1.7–4.0 m3 biogas m3 d1; 1.1–2.6 m3 CH4 m3 d1) were reached (Table 3: no. 1, 3, 4, 10 and 11). 3.2. Phase B. Unbalance/toxic substrate trial: raw OMW feed When the digester was fed only with the original OMW (Phase B: 233–287 days), OLRs of about 8 kg COD m3 d1 provided a total COD removal of 81–82% and a biogas volume of 3.7–3.8 m3 m3 d1 (63–64% CH4). The digester performance is presented in Fig. 2 and Tables 4 and 5 (Phase B). The acid pH (4.9–5.1), the high concentrations of COD (54–55 kg m3), VFA (2.8–4.0 kg m3) and phenolic compounds (3.0–3.3 kg TPh m3) and, additionally, the null or reduced alkalinity and nitrogen contents (COD:N = 277:1 to 338:1, Tables 4 and 6), were some of the adverse characteristics of OMW that were registered along this assay. Even under these unfavourable conditions, the hybrid feed was accepted and degraded by the

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M.A. Sampaio et al. / Bioresource Technology 102 (2011) 10810–10818 Table 4 Hybrid data: COD removal and gas production. Phase

Total COD Inf. (kg m

B

)

55.4(1.7) 54.2(2.4) – 67.7(3.57) 22.0(1.3) 57.8(0.0) 17.9(0.9) 56.5(0.6) 15.5(0.9) 53.4(2.4) 16.8(0.4) 55.9(0.6) 15.0(0.7) 57.8(1.7)

Stop C0 C1 C2 C3 C4 C5

Biogas (m3 m3 d1)

CH4 (%)

Y (m3 CH4 kg1 COD)

80.7(0.8) 80.6(2.2) – 76.7(0.3) 74.4(0.4)

3.78(0.26) 3.70(0.16) – 3.35(0.08) 3.22(0.56)

63.7(1.2) 62.5(1.2) – 63.1(1.0) 63.2(-)

0.361(0.054) 0.377(0.067) – 0.249(0.017) 0.332(0.004)

77.9(1.0)

3.39(0.65)

62.5(-)

0.350(0.006)

76.0(1.6)

3.43(0.88)

63.7(-)

0.371(0.023)

78.3(1.6)

3.15(0.85)

68.7(-)

0336(0.006)

80.3(0.7)

3.03(1.01)

62.9(-)

0.326(0.013)

Soluble COD

3

3

Rem. (%)

Inf. (kg m

82.0(1.0) 81.0(1.7) – 81.0(0.6) 74.2(0.7)

44.7(3.2) 48.2(3.4) – 53.2(0.13) 14.5(0.3) 44.4(0.0) 13.0(0.3) 47.8(1.8) 10.0(0.3) 43.2(1.2) 9.7(0.6) 49.6(1.2) 8.9(0.4) 48.4(1.2)

80.8(0.3) 78.6(1.0) 78.4(0.5) 81.1(0.6

)

Rem. (%)

(–) Single determination; Values are the averages of determinations taken at steady-state period. Values in brackets shows standard deviations. Y, methane yield.

Table 5 Hybrid mean data: alkalinity and nitrogen contents. Phase

Substrate

B

OMW OMW – OMW PE OMW PE OMW PE OMW PE OMW PE OMW

Stop C0 C1 C2 C3 C4 C5

Partial alkalinity (kg CaCO3 m3)

Total alkalinity (kg CaCO3 m3)

NH3 (kg N m3)a

3 NHþ 4 (kg N m )

Total N (kg N m3)

in

ef

in

ef

in

ef

in

ef

in

ef

0 0 – 0 3.13(0.18) 0 3.40(0.19) 0 – 0 3.96(0.02) 0 3.94(0.37) 0

5.03(0.41) 5.05(0.11) – 4.85(0.02) 4.88(0.04)

2.42(0.06) 2.46(0.05) – 2.37(0.11) 6.48(0.11) 2.37(0.11) 6.50(1) 2.25(1) – – 6.36(0.02) 2.04(0.19) 5.36(0.16) –

6.32(0.23) 6.20(1.17) – 6.20(0.18) 6.33(1)

0.01(0.01) 0 – 0 1.74(1) 0 1.69(0.02) 0 1.60(0.04) 0 1.55(0.05) 0 1.32(0.01) 0

0.13(0.06) 0.02(0.02) – 0 0.10(1)

– – – – 0.081() – 0.070() – 0.139() – 0.130() – 0.150() –

– – – – 0.007()

0.20(0.01) 0.19(0.01) – 0.20(0.02) 1.99(0.51) 0.20(0.02) 1.83(1) – 2.25(1) – – – – –

0.42(0.08) 0.31(0.04) – 0.24(0.02) 0.37(1)

5.19(0.55) 4.71(0.05) 4.70(0.35) 4.75(0.07)

6.71(0.48) 5.69(0.12) 5.61(0.09) 5.55(0.11)

0.13(0.00) 0.11(0.01) 0.12(0.01) 0.11(0.01)

0.008() 0.003() 0.005() 0.004()

0.42(1) 0.40(1) 0.43(0.01) 0.39(0.02)

In, influent; ef, effluent; 0, below detection limit; (1) single determination; () standard deviation that was not calculated. a Calculated from Eq. (1).

Table 6 Conversion capacity of the hybrid digester: Phases A, B and C. Phase a

A B–C0 C1–C5b a b

OLR (kg COD m3 d1)

COD rem (%)

Biogas (m3 m3 d1)

CH4 (m3 m3 d1)

Y (m3 CH4 kg1 COD)

8.20 8.83 7.72

81.3 81.4 79.0

3.18 3.61 3.25

2.12 2.28 2.08

0.34 0.33 0.34

83% OMW + 17% PE (v/v). 83% OMW + 17% PE (time feed).

developed biomass as documented by the comfortable methane yield reached (Y = 0.361–0.377 m3 CH4 kg1 COD). The good performance of the unit under these operating conditions (100% v/v OMW, Table 3: no. 12) have never been referred before. On contrary, it was shown that for anaerobic degradation of OMW alone, nitrogen addition was needed and a COD:N ratio of 61:1 to 42:1 was necessary for the optimal degradation process (Angelidaki et al., 2002). Effectively, in this case, the influent was just composed by OMW that has not undergone any alteration and presents characteristics potentially adverse to a successful development of anaerobic process. Concerning the effluent, pH became basic (8.1) and alkalinity increased to about 5.0 (PA) and 6.3 kg CaCO3 m3 (TA). Total

phenolic compounds removal of 46% was reached, being the remained fraction (1.4–1.5 kg m3) probably due to the presence of polymerized phenolic matter since no colour clarification was noticed. Instead, a slight increase of the colour absorbance values was registered in digester effluent. Absorbance data of 24–27 and 30–31 were recorded in the influent and effluent, respectively. VFA were mostly consumed in the system and the VFA removal of 96–98% resulted into an effluent of 0.08–0.1 kg m3 (Fig. 2b), being the acetic acid the main component (89% total VFA).Regarding the solids, influent concentrations of 34.5 TS kg m3 (±3.2) and 1.04 VSS kg m3 (±0.36) (269–314 days), correspond to effluent amounts of 17.5 TS kg m3 (±0.99) and 0.43 VSS kg m3 (±0.24), respectively. These data indicate that the reactor was not subjected

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to any washout process and that the solid biomass was maintained in good conditions inside the unit. Table 6 provides a comparison of the average main data obtained in different experimental phases. The results comparison of the feeds digestion containing 83% and 100% of the raw OMW (Phases A and B–C0) reveals that the OLR increase to 8.8 kg COD m3 d1 had a positive effect on gas production. Biogas and methane productivity increased and the removal capacity was maintained at previous levels. Table 6 figures reinforce the finding already made on the quality of the hybrid functioning regarding its great ability to degrade the raw OMW without the addition of another effluent. 3.3. Phase C. Organic pulses trial: raw OMW or PE, alternated feed During the weekly cycles experiment (Phase C), PE and OMW were alternatively supplied on its original composition according to Table 2. Successive changes of operating condition did not cause the disruption of the system. On the contrary, the performance of the hybrid was maintained as it is shown in Fig. 2 and Tables 4 and 5 (Phase C). The introduction of PE followed by the OMW during the same experimental week corresponds to operate the unit under constant shock of organic loads. The applied concentrations of PE and OMW (15–22 and 53–58 kg COD m3, respectively) have originated OLR alterations of three-four times (2.7–4.1 and 8.4–8.9 kg COD m3 d1). The hybrid replied positively to these conditions by providing gas volumes of 3.0–3.4 m3 m3 d1 (63–69% CH4) and methane yields of 0.326–0.371 m3 CH4 kg1 COD removal (Table 4). As it was already observed (Phase B), the original OMW was also accepted by the present system that was subject to intermittent working conditions. Comparing the values obtained during the operation of the mixture with 83% (v/v) OMW with those reached in this period (Phase A versus Phases C1–C5) it is noticed that the degree of influent treatment and the reached gas production in both trials are identical (3.2 m3 biogas m3 d1 and 2.1 m3 CH4 m3 d1, Table 6). Effectively, the pulses experiment was predetermined based on the operating time of each cycle and 17% and 83% of the functioning time were used to introduced PE and OMW, respectively. So, similar amounts of each of the effluents have been supplied to the system in both operational situations. Data indicates that the operation mode, feeding the digester with effluents mixture or effluents individually, is not a determinant factor for the proper functioning of the anaerobic unit. 3.4. OMW valorisation: resistance and adaptation capacities of the anaerobic system 3.4.1. Feed suspension and storage The hybrid digester used in this experiment was previously applied to digest several substrates (Gonçalves et al., submitted for publication) and it was preserved inside of a cold chamber during 18 months. Then, it was restarted to degrade substrates from other processing units. The obtained results illustrate that the anaerobic digester biomass can be kept dormant for several months as referred by Rozzi and Malpei (1996). The developed hybrid population did not lose the activity even after a period of feed suspension and storage at low temperature. Rather, it was able to adapt and digest a new stock of substrates and convert them into gas. Another period of feeding suspension took place after Phase B. Then, the hybrid was operated (Phase C0, day 300) with the original OMW that was also used in the earlier period. It is interesting to note the response of the system. A rapid increase in gas production was observed in 6 days (Fig. 2a) and removals of 81% COD total (68 kg m3), 79% COD soluble (53 kg m3) and 99% VFA (5.2 kg m3)

were registered (Fig. 2b and Table 4), indicating once again the great capacity of the system to degrade the unchanged OMW. 3.4.2. Overloading and load shocks operation The hybrid was accidentally subjected to an OLR increase from 4.9 kg COD m3 d1 (influent of 31 kg COD m3, 15–82 days) to 18.5 kg COD m3 d1 (influent of 112.9 kg COD m3, 83–94 days: Table 2). Additionally, in the final phase of this period, the influent was changed to a new stock of OMW and PE and an OLR of 5.6 kg COD m3 d1 (95–136 days) was applied. As a result, the removal capacity of the unit (77–83% COD, 83–136 days) was not accompanied by its ability in converting the organic matter into gas. Despite the good quality of the biogas obtained (70–72% CH4) indicating an active methanogenic population, a low methane yield (0.181–0.186 m3 CH4 kg1 COD removal) was reached through these periods (83–136 days) (data not shown). In order to assess the resilience of the changes in unit operation, it was decided to continue running the hybrid and increase the OMW amount into influent (to 69% and 83% v/v) according to the work schedule (Table 2). The results reached subsequently and previously discussed in this paper allow inferring that the unit system had the ability to resist to adverse occurrences. The hybrid resistance to an accidental overload and its capacity in preventing excessive loss of biomass was already noticed before (Gonçalves et al., 2009, submitted for publication). The adaptation capacity to different OMW stocks has also been verified by other authors referring that the anaerobic biomass acclimated to one substrate (particular phenol molecule) is simultaneously acclimated to other substrates with related structures (phenol molecule) (Healy and Young, 1979). The anaerobic hybrid operated under successive organic pulses by digesting PE and OMW alternately (Phases C1–C5, 314–350 days). The conversion efficiency data and the working process stability obtained indicate that the microbial communities developed a resistance capacity and were not disabled by the successive and pronounced alterations of organic loading rates (from 2.7 to 11 kg COD m3 d1), pH and potential inhibiting/toxic compounds such as phenolic matter, VFA and free ammonia contents. PE is characterized by high concentrations of total ammonium and organic nitrogen (urea and proteins) and as the organic nitrogen is degraded, the ammonium is released. The total ammonium concentration has a double effect in anaerobic digestion. It acts as a promoting agent of the buffer system by maintaining a high level of bicarbonate but, on the other side, it can cause inhibition problems that lead to an unstable process with a low methane yield and a high VFA level in the effluent (Murto et al., 2004; Zhang and Jahng, 2010). The total ammonium inhibition has been suggested to be directly related to the concentration of the undissociated form (NH3) (Chen et al., 2008) being the effect more notable at high pH levels and temperatures (Eq. (1)). The aceticlastic methanogens are the most sensitive to ammonia toxicity and free ammonia concentrations of 25–150 mg NH3-N L1 have been reported as inhibitory for mesophilic treatment. Levels up to 1.1 g NH3 L1 can be tolerated if the culture has undergone a gradual adaptation (Guerrero et al., 1997; Murto et al., 2004). When the hybrid digester was fed only with OMW (233–313 days) the biomass was probably accommodated to extremely low free ammonia concentrations; when the PE started to be used (314 day) an opposite situation takes place: the inlet free ammonia concentration of PE (72–150 mg L1, Table 5) was in the range of inhibitors concentrations. The results (Fig. 2b and Table 4) indicate that biomass seems to have acquired the ability to with stand both adverse situations. So, it is possible to operate the anaerobic hybrid under stable conditions using different effluents that are alternately applied. Another relevant aspect is the similarity of average data of the gas production and treatment efficiency of the anaerobic process

M.A. Sampaio et al. / Bioresource Technology 102 (2011) 10810–10818

to carry out with a feed mixture of 83% OMW v/v or feed pulses of individual substrates (Phase A versus Phases C1–C5, Table 6). Thus, given that both feed models provide analogous results, the pulses procedure has the advantage of suppressing the effluents mixing step. Taking into account a biogas plant, feeding the digester using wastes separately, certainly contributes to make the process even simpler and cheaper. 3.4.3. Raw OMW as influent of anaerobic hybrid digester Several unfavourable characteristics of OMW that make it unsuitable for anaerobic process may be easily overcome by the approach performed in this work: make use of an additional effluent to complement OMW. A biomass adaptation process is provided through the provision of increasing amounts of the concentrate and potentially toxic substrate over time. The presences of inhibiting/toxic compounds have been mentioned as a significant problem for anaerobic digestion of OMW. Phenolic fraction had been described as inhibitory to methanogens. The water dilution of OMW has been used as method to reduce the concentration of phenols and fatty acids. However, this fact results in spending on water consumption and larger volume of wastewater to treat. With the prospect of making the process cheaper, the substrate concentration and its toxicity can be advantageously reduced by using another effluent produced in the vicinity. Swine manure has been a problem in many regions of Portugal as in other countries. Due to its high nitrogen content and high pH, free ammonia is usually the relevant inhibitor parameter of the anaerobic system. Analysing both effluents and comparing them, it is possible to observe that OMW presents opposite characteristics of PE and it can run as OMW complement in terms of anaerobic treatment. Indeed, the use of another effluent is a way to provide dilution but also an approach to compensate the system for gaps in the OMW composition, making the process much cheaper and appealing. Examples of compensation are the enhancement of pH, alkalinity and nitrogen values. Intending to degrade the original OMW without any complementation, all unfavourable characteristics inherent to the substrate will be present. It is well documented that OMW is an unbalance substrate and its concentration associated to its toxicity do not permit the anaerobic process establishment (El-Gohary et al., 2009; Gelegenis et al., 2007). However, the results obtained in this work suggest the opposite. The classic drawbacks of OMW were somewhat mitigated and supported by the developed system inside of the anaerobic hybrid along the time. The presence of OMW toxic compounds did not prevent the biological conversion of most of the organic matter contained in the influent. The remaining colour is probably associated to the fraction of phenolic matter that is not degraded under anaerobic conditions. For colour removal purpose, an additional treatment can be used such as fermentative decolorization or electrochemical treatment (Aouidi et al., 2009; Papastefanakis et al., 2010). Accordingly, it is possible and advantageous to valorise energetically the raw OMW by anaerobiosis avoiding any previous alteration of the substrate in order to prevent its concentration and/or toxicity. The digested flow with a basic pH and high alkalinity and devoid of VFA may be useful for agricultural applications. The anaerobic hybrid is a sustainable and environmentally-attractive means of reducing OMW organic load, generating two products of interest (organic stream and a renewable energy source) and contributing for the greenhouse gas reduction. 4. Conclusions The high energy potential of OMW (54–55 kg COD m3) is directly recoverable through anaerobic digestion, without requiring

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any prior correction and decrease of its concentration and/or toxicity and making the process simpler, more flexible and cheaper. The hybrid worked under stable conditions when successive shocks of loads (2.7–4.1, 8.3–10.7 kg COD m3 d1) were applied by feeding alternately with different effluents. The strategy developed, on the basis of complementary effluents concept, promoted the hybrid microbial community adaptation to unfavourable characteristics of OMW. The unbalanced composition and presence of toxic compounds did not prevent the proper hybrid functioning when 100% (v/v) OMW was digested. Acknowledgements The authors acknowledge the financial support of the ‘‘Fundaçãopara a Ciência e a Tecnologia’’, FCT/MCTES, through the Project PTDC/ENR/69755/2006 and also through the grant given to Marta Gonçalves SFRH/BD/40746/2007. References Angelidaki, I., Ahring, B.K., 1997. Codigestion of olive oil mill wastewaters with manure, household waste or sewage sluge. Biodegradation 8, 221–226. Angelidaki, I., Ahring, B.K., Deng, H., Schumidt, J.E., 2002. Anaerobic digestion of olive mill effluents together with swine manure in UASB reactors. Water Sci. Technol. 45 (10), 213–218. APHA (American Public Health Association), 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. Washington, DC, USA. Aouidi, F., Gannoun, H., Othman, N.D., Ayed, L., Hamdi, M., 2009. Improvement of fermentative decolorization of olive mill wastewater by Lactobacillus paracasei by cheese whey’s addition. Process Biochem. 44, 597–601. Azbar, N., Tutuk, F., Keskin, T., 2009. Biodegradation performance of an anaerobic hybrid reactor treating olive mill effluent under various organic loading rates. Int. Biodeterior. Biodegradation 63, 690–698. Chen, Y., Cheng, J., Creamer, K., 2008. Inhibition of anaerobic digestion process: a review. Bioresour. Technol. 99, 4044–4064. Dareioti, M.A., Dokianakis, S.N., Stamatelatou, K., Zafiri, C., Kornaros, M., 2010. Exploitation of olive mill wastewater and liquid cow manure for biogas production. Waste Manage. 30, 1841–1848. Dareioti, M.A., Dokianakis, S.N., Stamatelatou, K., Zafiri, C., Kornaros, M., 2009. Biogas production from anaerobic co-digestion of agroindustrial wastewaters under mesophilic conditions in a two-stage process. Desalination 248, 891–906. El-Gohary, F., Tawfik, A., Badawy, M., El-Khateeb, M.A., 2009. Potentials of anaerobic treatment for catalytically oxidized olive mill wastewater (OMW). Bioresour. Technol. 100, 2147–2154. El-Mashad, H.M., Zeeman, G., van Loon, W.K.P., Bot, G.P.A., Lettinga, G., 2004. Effect of temperature and temperature fluctuation on thermophilic anaerobic digestion of cattle manure. Bioresour. Technol. 95, 191–201. Gelegenis, J., Georgakakis, D., Angelidaki, I., Christopoulou, N., Goumenaki, M., 2007. Optimization of biogas production from olive-oil mill wastewater, by codigesting with diluted poultry-manure. Appl. Energy 84, 646–663. Gonçalves, M.R., Freitas, P., Marques, I.P., submitted for publication. Bioenergy recovery from Olive mill effluent in a hybrid reactor. Biomass and Bioenergy. Gonçalves, M., Freitas, P., Marques, I.P., 2009. Olive mill wastewater as an energy source: anaerobic digestion in a hybrid digester. 5th Dubrovnik Conf. on Sustainable Development of Energy, Water and Environment Systems, Sep. 30 to Oct. 3, 2009, Dubrovnik, Croatia. Guerrero, L., Omil, F., Mthdez, R., Lema, J.M., 1997. Treatment of saline wastewaters from fish meal factories in an anaerobic filter under extreme ammonia concentrations. Bioresour. Technol. 61, 69–78. Healy, J.B., Young, L.Y., 1979. Anaerobic biodegradation of eleven aromatic compounds to methane. Appl. Environ. Microbiol. 38 (1), 84–89. Jarboui, R., Chtourou, M., Azri, C., Gharsallah, N., Ammar, E., 2010. Time-dependent evolution of olive mill wastewater sludge organic and inorganic components and resident microbiota in multi-pond evaporation system. Bioresour. Technol. 101, 5749–5758. Kapellakis, I.E., Tsagarakis, K.P., Avramaki, Ch., Angelakis, A.N., 2006. Olive mill wastewater management in river basins: a case study in Greece. Agric. Water Manage. 82, 354–370. Marques, I.P., Teixeira, A., Rodrigues, L., Martins Dias, S., Novais, J.M., 1997. Anaerobic co-treatment of olive mill and piggery effluents. Environ. Technol. 18, 265–274. Marques, I.P., Teixeira, A., Rodrigues, L., Martins Dias, S., Novais, J.M., 1998. Anaerobic treatment of olive mill wastewater with digested piggery effluent. Water Environ. Res. 70 (5), 1056–1061. Marques, I.P., 2000. Valorização de recursos poluidores por digestão anaeróbia. Água-ruça e efluentesuinícola/Valorisation of polluters resources by anaerobic digestion. Olive mill wastewater and piggery effluent. Ph.D. Technical University, Instituto Superior Técnico, Lisbon, p. 200.

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