Recent developments in the anaerobic digestion of olive mill effluents

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Process Biochemistry xxx (2015) xxx–xxx

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Review

Recent developments in the anaerobic digestion of olive mill effluents Ahmet Gunay a , Dogan Karadag b,∗ a The Scientific and Technological Research Council of Turkey (TUBITAK), Marmara Research Center, Environment and Cleaner Production Institute, Kocaeli, Turkey b Yildiz Technical University, Department of Environmental Engineering, Davutpasa, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 13 March 2015 Received in revised form 2 July 2015 Accepted 9 July 2015 Available online xxx Keywords: Oil mill effluent Methane Bioreactor Suspended Granular Biofilm

a b s t r a c t Liquid and solid olive mill effluents contain considerable quantities of organics, phenols and lipids. Comprehensive papers have been published on the treatment of olive mill effluents. Methane production and organic removal have been studied under varying environmental conditions. This paper reviews the recent reports on anaerobic reactors which have been published during the last 15 years. Olive mill effluents have high amounts of hardly biodegradable substances, with most of them being toxic to microorganisms. It has been proven that pretreatment with aerobic, advanced oxidation and heat methods are an efficient way of removing toxic materials and improving anaerobic treatment efficiency. The effects that organic loading, hydraulic retention time, and temperature have on suspended, biofilm, and granular reactors are discussed. Anaerobic treatment has been performed by feeding only olive mill effluents or co-digestion with other waste streams. Co-digestion enhances methane productivity by balancing nutrient and alkalinity levels. Furthermore, a comprehensive discussion of studies regarding pretreatment is carried out by comparing their performances. © 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

4. 5.

6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Characteristics of olive mill effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Pretreatment of olive mill effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1. Physical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2. Chemical pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3. Biological pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 The anaerobic treatment of olive mill effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Anaerobic reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.1. Suspended bioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.1.1. Completely stirred tank reactor (CSTR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.1.2. Other suspended reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.2. Biofilm reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.2.1. Up-flow anaerobic filter (UAF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.3. Granular reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.3.1. Up-flow sludge blanket (UASB) reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 5.3.2. Hybrid reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Evaluation of bioreactors and management of treated effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Abbreviations: OM, olive mill; OMWW, olive mill waste water; OMSR, olive mill solid residue; COD, chemical oxygen demand; BOD5 , 5-day biochemical oxygen demand; LCFA, long chain fatty acids; SS, suspended solid; TKN, total Kjhedahl-nitrogen; OLR, organic loading rate; HRT, hydraulic retention time; VFA, volatile fatty acids; CSTR, continuously stirred tank reactor; ASBR, anaerobic sequencing batch reactor; PABR, the periodic anaerobic baffled reactor; UAF, up-flow anaerobic filter; UASB, upflow anaerobic sludge blanket bioreactor. ∗ Corresponding author. E-mail address: [email protected] (D. Karadag). http://dx.doi.org/10.1016/j.procbio.2015.07.008 1359-5113/© 2015 Elsevier Ltd. All rights reserved.

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1. Introduction Olive oil production is one of the most important agro-industrial activities in the economics of Mediterranean countries. Worldwide olive oil production has been gradually increasing, with around 3 million tonnes produced in 2010 alone [1]. An excess amount of water is consumed during olive oil extraction, with annual wastewater generation being estimated as being around 30 million m3 [2,3]. Olive oil is produced with either a two- or three-phase extraction method; olive mills (OM), however, are mostly operated under a two-phase method due its low water consumption and less generation of waste streams [4,5]. In addition to olive mill wastewater (OMWW), olive mill effluents contain a highly polluted solid residue as well. Olive mill solid residue (OMSR) – also known as pomace – contains a considerable amount of humidity. Indeed, one tonne of olive oil processing generates around 800 kg of OMSR under a two-phase extraction system [4]. Researchers reported that the amount and characteristics of OM waste streams are affected by a variety of olive fruits, the cultivation conditions of the trees, the degree of their ripeness, climatic conditions, harvesting time and extraction methods [5,6]. OMWW is high strength wastewater with a considerable amount of organic content in terms of COD (25–220 g/l) and BOD5 (9–100 g/l) [7–9]. Furthermore, OMWW is also rich in phenolic compounds which change the colour of discharging bodies, induce toxicity in living organisms, and decreases the biodegradability in treatment plants [10]. OMSR consists of olive pulp, stones, water, and leftover oil and is rich in lignin, cellulose and hemicelluloses. Moreover, it is also characterised by a remarkable concentration of organic matters, phenols, and volatile fatty acids (VFA) and a low pH [4]. Pomace is generally stored in open ponds which results in the formation of leachate with a dense colour and an organic content of up to 30 g/l along with 3.5 g/l of phenols [11]. The uncontrolled discharge of large quantities of OM waste streams into receiving bodies causes severe environmental problems. Nevertheless, treatment of OM waste streams is complicated due to the inherent characteristics of olive harvesting and OM operation. Olive production varies considerably from year to year and olive mills are operated for only a short amount of time in a given year. In order to remove the pollutants from OM effluents, researchers have proposed various individual treatment methods including advanced chemical [12,13] and membrane [14,15]. The combination of different singular treatment systems, however, is more appropriate since single-step treatments are insufficient for meeting discharge limits. Among other alternatives, biological treatment systems have provided promising successes for the removal of organics and other pollutants from OMWW, and other food production industries as well [16,17]. Aerobic systems have limited application on the treatment of high strength wastewaters since continuous aeration considerably increases operational cost and excess sludge is generated which needs additional treatment. On the other hand, anaerobic technologies provide less sludge generation, overall cost, and nutrient requirements [16,18,19].

Furthermore, methane is a valuable renewable energy source and digestate could be used as fertiliser in for agricultural purposes. The removal of pollutants and methane production from the OM effluents by anaerobic bioreactors has also been extensively documented in the literature. The present study contains considerable evaluation of recent developments in the anaerobic treatment of OM effluents. The literature review provided focuses mainly on papers which were published during the last 15 years. In addition, anaerobic bioreactors have been evaluated based on treatment performance, operational schemes, pretreatment technologies. 2. Characteristics of olive mill effluents A summary of composition values for OM waste streams is presented in Table 1. In comparison to other food industry wastewaters, OM effluents generally contain more organic pollutants and phenols [20–23]. Both liquid and solid waste streams have acidic characteristics, with pH values ranging from 4.0 and 6.5; in addition, they contain great amount of solids. In OMSR streams, concentrations of total and suspended solids rise up to 206.7 and 143 g/l, respectively. Moreover, El-Gohary et al. [24] have shown that suspended materials in OM effluents are mostly comprised of colloidal solids with low settleability. Although OM waste streams have high COD amount up to 178 g/l, the biodegradability ratio of BOD5 /COD is very low due to the presence of excessive toxic phenolic compounds [25–27]. In contrast, wash water from olive mills has the least amount of pollutants when compared to other wastewater sources. Additionally to having a great number of organic acids, OM waste streams include more than 30 different phenolic compounds, while the type and concentration of individuals varies significantly with respect to region, type of process, local operational procedures, fruit maturity, storage time and oil extraction method [3,28–30]. OM wastewater is also characterised by a dense colour which varies from brown to black depending on the degradation stage, olive origin, and the amount of solid matter and phenolic compounds [31]. Although some OM waste streams have been reported with sufficient nutrient balances, most studies revealed the deficiency of appropriate nitrogen and phosphorus levels for efficient anaerobic treatment [32]. Solid waste streams generally have a low nitrogen content of less than 0.2% of COD, while ammonium ion ranges from 5.5% to 45% in OM effluents [33–36]. Mineral content comprises 0.5% to 2% of OM effluents, while individual amounts of K+ , Ca2+ , Na+ , Mg2+ and Fe2+ change due to the oil extraction method, the nature of the soils, and the fertiliser and quality of water used in extraction [4,5,37]. 3. Pretreatment of olive mill effluents The anaerobic degradation of OM waste streams has its own difficulties due to the high content of hardly degradable cellulosic materials and toxic substances that they consist of, such as phenols,

Table 1 Reported composition values of olive mill effluents. Effluent

pH

COD (g/l)

BOD5 (g/l)

Solids (g/l)

VS (g/l)

Nitrogen (mg/l)

Phenols (g/l)

Lipid (g/l)

Ref.

Two phase OMWW Three phase OMWW Three phase OMWW Two-phase OMSR Two phase OMSR Settled OMWW OM wash water Pomace leachate

4.89 5.14 5.0 5.3 4.9 5.20 6.0 6.0–6.5

21.5 68.78 131 162 187.9 95 2.735 25–30

NA 17.12 41 NA NA 19 NA NA

16.7 (TS) 49.14 (TS) 83.3 (TS) 143 (TS) 206.7 (SS) 15 (SS) 0.456 (TS) 1.5–2.0 (SS)

14.0 NA 54.9 126 158.2 NA NA 0.3–0.4

210 (TKN) 220 (TN) 0.7 NA NA NA NA NA

0.06 5.06 6.8 14.9 NA 11.5 0.291 3–3.5

NA NA NA NA NA 9.8 NA NA

[84] [26] [37] [77] [80] [88] [39] [11]

NA: not available; VS: volatile solid, TS: total solid, SS: suspended solid; TN: total nitrogen; TKN: total Kjhedahl nitrogen.

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Table 2 Comparison of pretreatment methods applied on olive mill effluents. Effluent

Pretreatment conditions

Effects of pretreatment

Ref.

OMWW

Sand filtration + AC adsorption Heat treatment at 120, 180 ◦ C for 180 min

BOD5 /COD increased from 0.6 to 0.88, COD removal in UASB increased from 40% to 68% Soluble COD increased 42%, Contents of hemicellulose, cellulose and lignin decreased CH4 productivity increased 1.9% to 5.0% 72% increase in soluble organics, CH4 productivity decreased 14% due to the release of inhibitory substances Soluble COD increased from 59% to 79%, CH4 productivity improved 20%, Turbidity increased from 1200 to 1800 NTU Kinetic constant increase 48%, CH4 production rate increased 12%, CH4 yield increased 5.6% 52% and 42% COD removals by Al and Fe anodes, Colour removal up to 97%, Suspended solid removal increased from 48% to 68%

[46]

Over 70% phenol removal, BOD5 /COD ratio increased from 0.33 to 0.58, Colour reduced 90%, inhibition decreased 66.4%, CH4 yield increased 90% BOD5 /COD ratio increased from 0.14 to 0.53

[51]

% 6.2 COD reduction, 94.3% phenol removal, Improvement in CH4 productivity Polyphenol removal up to 90%, Organic degradation increased 31.65%, CH4 yield increased 2.4 times COD reduction of 19%

[38]

OMSR

Olive husk + OMWW OMWW

OMSR

OMWW

OMWW

OMWW OMWW

OMWW

OMWW

Heat treatment at 134 ◦ C and 3.12 bar for 20 min Ultrasound at 20 kHz and 0.4 W/ml for 10 min Ultrasound at 24 kHz and 200 W for 90 min Electrochemical treatment at pH 6.2, 12 V and 20 mA cm−2 for 30 min Electrocoagulation with iron electrode + sedimentation Fenton at pH 2–4 for 24 h Ozonation at 20 ◦ C, pH 4.85 and 40 l/h for 8 h Aeration with indigenous bacteria for 5d Aeration with fungus P. chrysosporium

long-chain fatty acids (LCFA), ethanol, tannin, etc. [5,38,39]. The slow-growing anaerobic microbial community is very sensitive to inhibitory conditions, with ethanol being more toxic than phenol [30]. Phenol concentration is around 15 g/l; in addition, low molecular phenols have been reported with more toxic properties as well [40,41]. Polyphenols are phytotoxic and resistant to biological degradation, while a 65.8% reduction of polyphenolic compounds subsequently decreased the OMWW toxicity to around 33% [42,43]. Fiorentino et al. [44] concluded that phenolic compounds have a strong toxic potential on aquatic systems, whereas Justino et al. [45] indicated that OMWW with phenolic content is highly toxic, not only to microorganisms but also to plants and algae as well. Pretreatment enhances methane productivity by removing toxic materials, increasing biodegradability, and reducing cellulosic content. Various individual and combined physical, chemical, and biological methods have been applied prior to anaerobic degradation. Dilution; heating; ultrasound; treating with acidic, basic, and salt chemicals; advanced chemical oxidation; and biological treatments are all common pretreatment methods applied to OM waste streams. 3.1. Physical pretreatment A visual comparison of the pretreatment methods applied to olive mill effluents are provided in Table 2. Sabbah et al. [46] compared the pretreatment impacts of sand filtration and adsorption onto activated carbon and filtration through filter paper of 1.6 ␮m pore size. In comparison to other materials, activated carbon was superior in respect to removing toxic substances and improving the BOD5 /COD ratio from 0.60 to 0.88. Heat treatment, on the other hand, is mainly conducted in order to degrade the cellulosic content of OMSR effluents, with the process’s efficiency being dependent on temperature and time. In their study, Rincón et al. [47] found that 31–42% of COD solubilised into the liquid phase and that methane

[47]

[33]

[26]

[48]

[50]

[42]

[53]

[56]

productivity rose to 5% at 120 ◦ C with the lowest lignin content. On the one hand, heat treatment reported 14% less CH4 production from olive husks due to their releasing inhibitory substances into the liquid stream, although organic matter solubilisation was improved at 72% [33]. Extensive ultrasound applications have been conducted for the pretreatment of various OM streams. The efficiency of ultrasound pretreatment mainly depends on frequency and exposure time. In comparison to high frequency, low frequency significantly increases the solubilisation and eliminates the more toxic substances. Oz and Uzun [26] found that the amount of soluble COD rose from 59% to 79%, thereby improving the methane productivity of subsequent anaerobic treatments. Similar enhancements have been obtained by Rincón et al. [48] for 20% diluted OMSR. 90-min exposure time solubilised 57% of the COD into liquid phase and promoted a methane yield of 5.6%. 3.2. Chemical pretreatment Researchers reported that acidification decreased the phenolic content of OMSRs to around 40.7%, whereas treatment with basic chemicals degraded lignin and cellulosic materials and improved CH4 production [4,49]. Battista et al. [49] stated that the highest improvement was contributed by adding the salt chemicals FeCl3 and CaCO3 . The improvement of CaCO3 was interpreted as being due to the enhancement of the buffering capacity in the reactor and the positive influence of calcium on methanogens. Advanced oxidation processes offer high removal of organics and toxic chemicals and improve anaerobic biodegradability. The pretreatment efficiency of the electrochemical process varies based on electrode type and pH. Inan et al. [50] applied electrochemical pretreatment at 6.0 pH and obtained 52% and 42% COD removals with the use of aluminium and iron anodes, respectively. On the

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other hand, Neffa et al. [25] reported that 57% of phenol removal was achieved by aluminium electrodes with the increase of the BOD5 /COD ratio from 0.33 to 0.58. Khoufi et al. [51] found that, by using an iron electrode, they were able to remove over 70 phenolic compounds and improved CH4 yield to around 90%. Moreover, reduction in COD and inhibition were 43% and 66.4%, respectively, with CH4 yields increasing to around 60%. The Fenton process has been assessed as being one of the best pretreatment methods for removing toxic pollutants from OM waste streams and improving biodegradability. It also provides comprehensive COD removal, with the reported efficiencies varying from 80% to 95% [23,52]. Kallel et al. [42] found that the Fenton treatment at pH (2–4) provides 92% of COD removal and improves the BOD5 /COD ratio to 53% by oxidising phenolic compounds. Organic matter removal from OM wastewater increases with ozonation time while 6.2% of COD and 94.3% of phenol removals were achieved at 8 h [40]. The CH4 yields in 50% diluted OM wastewater increased from 194 ml CH4 /g COD to 266 ml CH4 /g COD by decreasing the amount of toxic compounds in the mixture through ozonation. On the other hand, anaerobic biodegradation might also be inhibited by using ozonation by-products while inhibition is much higher at lower COD concentrations.

3.3. Biological pretreatment Aerobic pretreatment has been widely used to selectively remove phenolic compounds from OM effluents; however, a decrease in COD concentration reduces CH4 production potential. Aerobic methods can be easily applied at ambient temperatures (20–35 ◦ C) but pH adjustment is critical for successful operation. In a study of Hafidi et al. [28], they found that pH adjustment by phosphate provided a good stabilisation for organic matter a considerable amount of nitrogen being found in humic form. González-González and Cuadros (2014) aerated OMWW by using indigenous microorganisms. As a result, they obtained 56% polyphenol removal after the first day of aeration, with it increasing to 90% by day 7; further aeration, however, did not contribute any remarkable advancement [53]. Aeration enhanced CH4 yields per COD removed by 2.4 times. Economic analysis revealed that aerobic pretreatment plus anaerobic digestion have 5.82 years for the return of the investment costs. Researchers were able to remove a considerable amount of COD and phenols when OMWW was aerated with the fungi Geotrichum candidum and Aspergillus terreus [54]. G. candidum removed 75% of COD from the OMWW while aeration with P. chrysosporium was effective on the degradation of low molecular polyphenols and reduced COD by 20–50% and toxicity by 5% [53,55,56]. In contrast, the main disadvantage of aerobic pretreatment is that it consumes a considerable amount of energy in order to continuously aerate for long periods of time [57,58].

4. The anaerobic treatment of olive mill effluents The stability of an anaerobic reactor could be maintained by adequate alkalinity levels in the reactor. In most cases, OM waste streams lack sufficient alkalinity, so external alkaline chemicals, including Ca(OH)2 [5], NaHCO3 [56] and Ca(HCO3 )2 [36], have been added into anaerobic reactors. Mixing OM waste streams with alkaline-rich wastewaters is cheap and also adds ammonium and other essential nutrients to the reactors for microorganisms [59,60]. The anaerobic treatment of OM effluents has commonly been conducted in mesophilic temperature ranges of 32–40 ◦ C [3,6,11,61]. Few thermophilic studies, however, have been performed at 55 ◦ C [60,62]. At thermophilic conditions, a 17.3% to 35% improvement in methane yields has been reported during the

co-digestion of OM waste streams, with the net energy gain being higher than that of mesophilic systems [62,63]. OM waste streams generally do not have a sufficient carbonto-nitrogen–to-phosphorus (C/N/P) ratio for anaerobic treatment. Supplementing external nitrogen and phosphorus sources or mixing OM waste effluents with nutrient-rich streams significantly improve reactor performance and counteract the operational problems related to phenolic compounds and lipids [59,64]. As external nitrogen sources, urea [5,36,65], NH4 Cl [64] and aqueous ammonia [66] can be added, while K2 HPO4 [38] and (NH4 )2 HPO4 [5] are supplementary phosphorus chemicals which can be added. Mixing OM effluents with other waste streams promotes a C/N/P balance and decreases operational costs. For this purpose, various waste streams, including slaughterhouse, whey, municipal, manures, treatment plant sludge and microalgae waste streams, have been mixed at ratios [60,67–71]. Co-digestion is highly recommended for improving organic removal and biogas production from wastewaters having high lipid content, such as is the case for the meat and dairy industries [22,72]. Furthermore, co-digestion also contributes to the dilution of toxic substances and reduces inhibition and operational problems [70,73]. For efficient co-digestion, waste streams should be mixed at optimum ratios. In a study made by Kougias et al. [74], it was reported that the optimum mixing ratio for the co-digestion of OMWW and manure was that of 0.4:0.6 and that ratio produced 277 ml CH4 /g COD – 79% of the theoretical yield. However, further increasing the OMW ratio led to an accumulation of LCFA, thereby inhibiting methane production. In another study, Agdag [35] found that the co-digestion of OM pomace with municipal solid waste at a ratio of 0.7:0.3 had the highest treatment efficiency. The recirculation of leachate generated during the digestion into the reactor increased treatment performance and CH4 productivity. Single-stage anaerobic treatment is easily destabilised by the accumulation of VFAs, especially when sufficient buffering chemicals are not present in the reactor. Researchers have found that the two-stage treatment of OM waste streams contributes high stability over elevated OLR and promotes bioreactor performance with respect to methane productivity and removal efficiency [60]. Acclimating inoculum into waste streams improves the successful start-up and operation of anaerobic reactors. In comparison to pretreatment, acclimation provides more tolerance against inhibitory substances [61]. Azaizeh [75] reported that inoculum from plants treating agricultural wastes are more compatible for the treatment of OM wastes since microorganisms are more tolerant to toxic compounds. It has been documented that microbial diversity is dynamic and that several members of the bacterial community during the start-up process were lost with the development of new bacterial species at subsequent operation times. Rizzi et al. [76], for instance, found that Methanobacteriaceae were the sole dominant archea species during the anaerobic digestion of OM wastewater; nevertheless, the presence and relative abundance of bacteria changed with OLR. In another study, researchers concluded that Clostridium were the predominant bacteria at low OLR but that other bacterial communities, such as Gammaproteobacteria, Actinobacteria, Bacteroidetes and Deferribacteres, were relatively higher at elevated OLR [77]. In addition, Kougias et al. [74] indicated that changes in substrate composition impacts microbial diversity. Furthermore, Azaizeh’s [75] investigation indicated that sulfatereducing bacteria within the anaerobic community have the ability to remove sulfates and convert phenol into methane.

5. Anaerobic reactors Different anaerobic reactors have been used for treating various olive mill effluents during the last 15 years. Anaerobic studies

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5

Table 3 Operational conditions and performances of bioreactors treating olive mill effluents. Reactor

Feed

Temp. (◦ C)

pH

HRT (d)

OLR (g COD/l d)

Influent COD (g L−1 )

Removal efficiency (%)

CH4 production rate (l/l d)

Ref.

CSTR

OMSR

35

>7.0

15–215

0.8–11

162

0.2–1.7

[78]

CSTR

OMSR

35

7.3–7.7

NA

0.75–3.0

162

0.164–0.659

[81]

CSTR

OMWW + manure OMWW + cheese wastewater+ manure OMWW OMWW

35

>7.0

16

3.63

162

0.91

[37]

35

6.5–7.9

19

4.5, 5.5

85.5–104.5

44.5–97 (COD) 36.5–88 (TVS) 95.6–97.0 (COD) 50 (COD) 34.2(VS) 75.5–85.2 (COD) 30–41 (TS)

1.35

[36]

30 35

7.0 NA

0.5–3 3.75–17.5

5.3–31.8 1.46–6.0

16.0 19.5–25.2

NA 0.346–0.580

[87] [29]

OMWW + Cheese wastewater OMWW + Piggery slurry OMWW Pretreated OMWW OMWW Pomace leachate OMWW + manure OMWW

35

7–7.8

NA

0.2–3

90

53–83 (COD) 58.2–81.6 (COD) 83 (COD)

1.25

[90]

37

6.9–7.8

11–45

0.2–5.0

NA

62–89 (COD)

NA

[91]

37 35

7.4 7.0

14.5, 26 5

1.5, 2.75 8

36, 45 40

40–90 (COD) 80–85 (COD)

0.32 NA

[55] [46]

35 32

6.7–8.4 6.8–8.0

2.5–10 3

0.45–32 0.33–1.67

4.56–109.8 1–5

34–79 (SS) 35–70 (COD)

NA 0.12

[97] [11]

36

7.0

1.4–7.6

38–110

5–28

85–95 (COD)

NA

[75]

35

6.6–8.9

2.5–10

4.55–109.8

0.45–32

50–95 (COD) 19–37 (SS)

NA

[5]

CSTR

ASBR PABR UAF

UAF UAF UASB UASB UASB UASB Hybrid UASB

NA: not available, OMWW: olive mill waste water, OMSR: olive mill solid residue, CSTR: continuously stirred tank reactor, ASBR: anaerobic sequencing batch reactor, PABR: the periodic anaerobic baffled reactor, UAF: up-flow anaerobic filter, UASB: upflow anaerobic sludge blanket bioreactor, ASBR: anaerobic sequencing batch reactor.

have been commonly conducted at laboratory scale and mostly performed utilising three reactor types of suspended, biofilm and granular. A comparison of anaerobic reactors based on operational conditions and performances is shown in Table 3. 5.1. Suspended bioreactors 5.1.1. Completely stirred tank reactor (CSTR) CSTRs are well-known suspended-biomass reactors and have been widely used in anaerobic treatments of OM waste streams and other wastewaters as well. CSTRs provided comparable COD removal and CH4 production (Table 3). In comparison to other reactors, CSTRs have a lower biomass concentration. In addition, its methanogens are easily washed out due to the equality of hydraulics and solid retention times. HRT and OLR are crucial parameters in both designing and operating anaerobic digesters. In a study by Rincon et al. [78], the researchers found that CSTR performance deteriorates when the HRT was less than 17 days during the single-stage treatment of OMSR and considerable biomass washout was reported when a CSTR was operated at an HRT of 15 days. On the other hand, COD removal efficiency is virtually independent of HRT when the reactor is operated above an HRT of 28 days in single-stage treatment [86]. During the treatment of OM effluents, a CSTR was successfully operated at a higher HRT from biofilm and granular reactors but at a lower HRT than tubular digesters [34]. Like other high-rate reactors, the performance of CSTRs is maximised by operating the reactor at elevated OLRs. Researchers reported different optimum OLR values for the treatment of OM waste streams in CSTR systems. For instance, Rincon et al. (2007) reported that COD removal efficiency declined from 97% to 82.6% when OLR was increased from 0.8 g COD/l d to 8.3 g COD/l d during a single-stage treatment of OMSR [78]. When the reactor was operated at over 9.2 g COD/l d, pH suddenly dropped and the reactor failed because of its low COD removal and CH4 production. Borja et al. [79] reported that single-stage CSTRs could tolerate high OLRs

as much as 15.03 g COD/l d with COD and volatile solid removal efficiencies of around 85% during the treatment of 20% dilutedOMSR. The maximum CH4 production rate was 2.12 l CH4 /l d at an OLR of 12 kg COD/m3 d, with VFA accumulating quickly when OLR increased even further. Borja et al. [80] stated that OM solid residue needs to be diluted and that OLR should be lower than 18.81 g COD/l d for the stable operation of single-stage CSTRs. Different reported optimal OLR values are associated with variations in the composition of waste streams, microbial diversity, and environmental factors. Overall CSTR performance could easily be destabilised during single-stage operations due to the progressive accumulation of VFAs within the reactor [6]. Inhibition could be eliminated and performance could be enhanced by using a two-phase digestion, co-digesting with other waste streams, or developing biofilms on materials added inside the reactors. A two-stage treatment allows the stable operation of a CSTR at elevated OLRs by separating acid and methane production phases. In acidogenic reactors, organic materials are hydrolysed and converted into VFAs whilst acetic acid is most abundant within the reactor. Acidogenic reactor performance increases with HRT; however, controversial results have been reported for optimum HRT and treatment efficiencies. Fezzani and Cheikh [34] indicated that the optimal HRT is 24 days for a semi-continuous acidogenic CSTR. Both Fezzani and Cheikh [34] and Dareioti et al. [36] reported no dissolved COD reduction; however, Dareioti et al. [37] obtained a 2.4% COD removal with low biogas production. On the other hand, Dareioti et al. [36] reported 10.3% and 20% removal efficiencies of suspended solids and phenol, respectively. During the two-stage treatment process, 40% phenol removal in an acidogenic reactor resulted in a 90.5% improvement of CH4 production rate and a 10% CH4 yield during anaerobic treatment [4,77]. Fezzani and Cheikh [41] reported that CH4 productivity was two times higher with increments in the removals of COD, colour, and phenolic compounds. An increase in both alkalinity and TKN

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in acidification effluents made the methane reactor more resistant to pH changes and prevented the risk of a nitrogen shortage. Rincón et al. [81] treated OMSR in two stages at 35 ◦ C and found an inverse relationship between the increase in OLR and reactor performance. COD removal slightly decreased from 97.1% to 95.6% with the increase of OLR from 0.75 g COD/l d to 3 g COD/l d. CH4 yields were highest at 0.225 l CH4 /g COD and methane production was inhibited when OLR was over 3.0. Co-digestion of OM streams advances CSTR performance and enables the reactor to be operated at a higher OLR and a shorter HRT. Co-digestion also increases microbial activity and reactor stability by supplying nitrogen and essential micro-nutrients. Fountoulakis et al. [63] stated that OM wastewater could be successfully treated when mixed with agricultural waste streams from slaughterhouse wastewater and wine-grape residues. Mixing OMWW with agricultural wastes at a ratio of 1:1 increased CH4 yields from around 3% to 36% while co-digestion with wine-grape residues provided a rather methane yield (214 l CH4 /kg COD) compared with slaughterhouse residues (184 l CH4 /kg COD). Angelidaki and Ahring [59] obtained a 75% COD removal when treating OM wastewater with manure at a ratio of 1:1; however, co-digestion with household waste or sewage sludge resulted in lower efficiencies due to there being a deficient amount of ammonia in the mixture. Manure has been widely used as co-substrate since it is rich in nitrogen, has complementary nutrients, and a helpful buffering capacity. The mixing of OMWW with cattle manure at a ratio of 1:3 contributed to an adequate buffering capacity along with quite stable CH4 production. Changes in the mixing ratios affected microbial diversity and reactor performance during the co-digestion of OMWW and swine manure [74]. The microbial community, however, easily adapted to a new substrate composition. Indeed, and the stepwise increase of OLR did not have a detrimental effect on reactor performance, even though it increased inhibitory substances. Maximum CH4 production and organic matter removal were obtained when OM waste was mixed with swine manure at a ratio of 0.4:0.6. The optimum OLR was 4.4 g VS/l d for co-digestion whereas increasing OLR caused a foam layer to form on the surface of the reactor and slightly reduced its treatment efficiency. Similarly, other researchers reported foam formation during the co-digestion of OMWW with poultry manure [82]. A two-stage methane reactor provided 50% of COD and 26% total solid removals during the co-digestion of OMWW with liquid manure at a ratio of 0.2:0.8 at 35 ◦ C [39]. The CH4 yield was 250.9 l CH4 /kg COD, which is higher than the 170 l CH4 /kg COD reported by Fountoulakis et al. [63]. Dareioti et al. [36] reported that mixing OMWW with cheese wastewater and cow manure at a ratio of 0.55:0.40:0.05 provided a better nutrient balance and microbial community in the methane reactor at a relatively short period of 10 days. In the methane reactor, COD and SS removals were 64% and 41%, respectively. It was recorded that co-digestion was very effective for phenol removal (61%) while the CH4 yield was 243 l CH4 /kg COD. The main problem in CSTRs is the washout of slow-growing methanogens when operated at short HRTs. The immobilisation of methanogens by adding support materials into CSTRs avoids the loss of microorganisms and enables the operation of the CSTR at a lower HRT or at a higher OLR. Researchers indicated that the addition of bentonite and sepiolite into CSTRs enabled the operation of CSTRs at shorter HRTs and enhanced the reactor stability at lower VFA/alkalinity levels [83–86]. Researchers improved CH4 production at 53% for the treatment of OMSR without diluting at an OLR of 20 g COD/l d by incorporating saponite into the CSTR [4]. Although support material enables massive biomass development inside the reactor, excess support media could increase the apparent viscosity of the medium and slow down biodegradation by hindering mass transfer [84]. It has been reported that half of low molecular phenols can be easily adsorbed onto bentonite at the

pretreatment stage [42]. The addition of a phenol-saturated bentonite slurry into a CSTR contact reactor enabled organics to be the slowly released into liquid its phase and provided an average of 70% phenol removal. Post-treatment with activated sludge improved the removal of phenols while a combination of adsorption, anaerobic digestion, and activated sludge treatment has an economic advantage over treating it with activated sludge alone. 5.1.2. Other suspended reactors An anaerobic sequencing batch reactor (ASBR) was subjected to an increment in HRT from 0.5 to 3 days during the treatment of OMWW [87]. COD removal increased from 53% to 83% with the increase in HRT, whereas reactor performance deteriorated following the increase of influent organic concentrations from 16 g COD/l to 32 g COD/l. SS removal was 91% at an HRT of 3 days with the sludge in the reactor settling easily. In addition, lipid removal was always higher than SS and rose up to 99%. The periodic anaerobic baffled reactor (PABR) is a novel bioreactor having compartments configured circularly which was developed in order to overcome the operational problems of conventional baffle reactors. The circular design enables the moving of wastewater sequentially through all compartments so that the organic strength of the wastewater is exposed equally to all biomass retained in the bioreactor [29]. The periodic switching of the feeding point along the compartments imposes a periodic disturbance on the dynamics of the bioreactor. A PABR was operated at HRTs ranging from 3.75 to 17.5 days and an OLR from 0.94 to 6 g COD/l d. Inhibition was linked to a rapid increase in the acetic acid concentration, with a constant increase in the propionic acid stopping biogas production. The process’s failure was attributed to the increase in phenol concentration over 3 g/l and the solids accumulated in the bioreactor. Researchers indicated that the increase in OLR by COD increment failed to produce biogas by means of VFA accumulation. In contrast, increasing OLR by HRT reduction led to stable operation with a COD removal of 72% at an HRT of 3.75 days. On the other hand, COD removal increased up to 80% when the reactor was operated at an OLR of less than 3.5 g COD/l d. The post membrane treatment of PABR effluents reduced COD concentration to less than 0.1 g/l, increased water quality, and made it suitable for irrigation. 5.2. Biofilm reactors Biofilm reactors have a concentrated biomass and could be operated at elevated organic loads with high treatment efficiency. It has been assessed that the CH4 produced in anaerobic filtration could cover the overall energy consumption during the treatment of OM waste streams [88]. Researchers reported that biofilm systems are highly adaptable to changes in organic loadings caused by fluctuations in season [89]. Anaerobic microorganisms could be inhibited by excess ammonia resulting from the degradation of protein-rich organics; nevertheless, biofilm reactors recover performance rapidly following the removal of inhibitory substances or detrimental environmental conditions [9,90]. 5.2.1. Up-flow anaerobic filter (UAF) UAFs have been comprehensively operated for the treatment of different OM effluents. Various packing materials, including activated carbon, silica, foam, PVC rings and wood chips have been used, though activated carbon is superior to all other methods because of its tremendous surface area for biofilm development coupled with the considerable adsorption capacity of the phenolic compounds [91–95]. Besides, granular activated carbon has been reported to have a tenfold higher biomass concentration than silica beads, along with 62% and 78% higher COD and phenol removal efficiencies [89]. UAF reactors have a great quantity of

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biomass, with biofilm concentrations increasing up to 37 g/l [93]. Most microorganisms are fixed on a packing medium while the remaining bacteria are suspended in the voids of the packing materials. In an OMWW-treating UAF, identified biofilms were mainly rod-shaped bacterial cells and distributed randomly throughout the packing materials. Distinctive types of microorganisms function at different parts of the anaerobic filters during the treatment of OMWW. The filter medium which was closer to the feeding point was rich in hydrolytic and acidogenic bacteria, along with a high production of acids, whereas reactor mediums near the outlet was quite buffered and dominated with acetogenic and methanogenic bacteria [94]. Despite having a dense community of microorganisms, UAFs reported detrimental treatment performances due to a great amount of biomass being washedout when the reactor was operated at an OLR of over 15 g COD/l d [76]. On the other hand, variations in organic loadings did not adversely affect methanogenic activity but altered the relative amount of bacterial species. Furthermore, the biofilm community also contains sulfate reducers which have been reported to reduce SO4 −2 by 90% to 95% in UAF reactors [89,93]. In a study by Sayadi et al. [55], the researchers concluded that the treatment performance of biofilm reactors is dependent on the COD fractions of OMWW. Low molecular-phenols are degraded well by an adapted anaerobic consortium without toxicity; however, higher molecular weight organics resulted with very low CH4 yields and bioconversion efficiencies of less than 40%. So far, biofilm reactors have been mostly fed by OM waste streams which have been mixed with other types of wastes. Gannoun et al. [95] applied UAFs in order to co-digest OMWW and slaughterhouse wastewater, obtaining the greatest performance with the mixing ratio of 0.4:0.6. In all conditions, COD removal efficiencies were quite high as around 80% with the reactor performance being stable even at an OLR as high as 12 g of COD/l d. Variations in operational temperature may fluctuate the reactor performance and even fail in some circumstances. Mechichi and Sayadi [94] indicated that decreasing the temperature from 37 ◦ C to 30 ◦ C and then increasing it to 45 ◦ C in one step caused VFA accumulation with a sharp rise of acetates and a gradual increase in propionate concentration. The CH4 yield and production rate were at their highest at 45 ◦ C, while they were at their lowest at 30 ◦ C. In one study by Gannon et al. [95], the researchers stated that thermophilic conditions (55 ◦ C) enhanced the biodegradability of organics and resulted in higher colour and COD removal (80%) with better biogas yields compared to mesophilic treatments. Thermophilic temperatures enabled the operation of UAFs at elevated OLRs (12 g COD/l d) and at a short HRT of 3.33 days, while the maximum OLR was 9 g COD/l d in mesophilic reactors and was already overloaded at an HRT of 4.5 days. Martinez-Garcia et al. [91] operated an UAF reactor at an HRT between 11 and 45 days. They obtained high COD removal with methane-rich biogases during the co-digestion of aerobically treated OMSRs and piggery slurry. On the other hand, Mechichi and Sayadi [94] reported that substrate degradation was not completed and resulted in lower biogas production when UAFs were operated at a HRT of 5 days. Increasing the OLR from 0.75 g COD/l d to 1.87 g COD/l d considerably affected the reactor performance, with CH4 productivity decreasing continuously. In contrast, MartinezGarcia et al. [90] operated the UAF reactor satisfactorily up to an OLR of 3 g COD/l d that resulted in 83% of COD being removed. Similarly, Marques [96] reported closer COD removals during the co-digestion of OMWW and piggery manure at an OLR of 3–4 g COD/l d, while their efficiency declined to 63% when the OLR increased to 8 g COD/l d. Martinez-Garcia et al. [91], however, obtained 85% COD removal at 5 g COD/l d during the co-digestion of OMWW after the acclimation of biomass at an OLR range of 0.33 and 1.11 g COD/l d.

7

Khoufi et al. [51] reported that CH4 production was inhibited at a loading rate of 2–4 g COD/l d when the reactor was fed raw OMWW. When OMWW was pretreated with elecrocoagulation plus sedimentation, however, it counteracted the toxic impact and increased methane yield two fold. Sedimentation after electrofenton eliminated COD, SS, and polyphenols at 52.6%, 83.8%, and 78% with more than 93% lipid removal while effluents were treated in a UAF reactor without dilution and pH regulation [88]. The reactor started with an organic loading of 1 g COD/l d while the OLR was gradually increased up to 10 g COD/l d. The mean COD reduction was 75%, though, at an OLR of 8–10 g COD/l d, the removal efficiency progressively declined due to a lower retention time. Researchers indicated that ultrafiltration (UF) of post-treatment AF effluents eliminated all suspended solids, half of COD, and 96% of the colour. Dhouib et al. [57] worked a UAF reactor at OLR ranges of 2–8 g COD/l d, with the reactor being operated in stable conditions without any inhibition from an even higher influence of COD 80 g/l. When biogas production became more than 900 l/d, cross-mixing increased the pressure inside the reactor and the biogas escaped in the form of liquid effluent. This problem was solved by modifying the anaerobic filter and by feeding the wastewater into the reactor through parallel entrances for the purpose of preventing it from clogging. UF post treatment completely removed polyphenols and toxins from the effluents of the UAF reactor that they used. Rizzi et al. [76] obtained around 90% COD removals under all operational conditions, while OLR was progressively increased from 1 to 15 g COD/l d. Increasing the OLR to 20 g COD/l d deteriorated the reactor’s performance due to biomass washout and the accumulation of tannins, lipids and phenols [90,94]. 5.3. Granular reactors 5.3.1. Up-flow sludge blanket (UASB) reactor The UASB is the most popular granular bioreactor since it has been proven effective and economical treatment for waste streams from both food industry and olive mills. Moreover, undiluted wastewater can be satisfactorily treated by UASB reactors under elevated organic loads. During the treatment of OM effluents, UASB reactors can generally be operated under higher organic loadings than other reactors (Table 3). Especially, during the treatment of wastewater with high lipid content, UASB reactors are also subject to typical operational problems, such as foam formation and biomass washout. In order to overcome these types of problem, researchers have suggested converting UASB reactors to hybrid reactors. Inserting packing materials at the top of UASBs allow the development of a biofilm which prevents biomass washout and enhances the tolerance of detrimental effects [61]. UASBs contain quite a large amount of biomass in the form of granules; however, a long start-up time for the development of granules is an important obstacle. The use of acclimated biomass contributes to more stable start-ups, an advanced tolerance to toxicity, and better organic removal [64]. Additionally, the supplementation of external nitrogen sources or co-digestion with nitrogen-rich waste streams promotes the growth of new bacterial cells and accelerates granule formation. It has been reported that granules within OMWW-treating UASBs had a diameter ranging from 3 mm to 8 mm with excellent settleability [97]. Following the formation of mature granules, UASBs can be safely operated at high organic loadings and at short HRTs. In a study conducted by Azbar et al. [5], the researchers suggest that keeping the HRT between 5 and 10 days allows for the efficient treatment of OMWW. Short HRT values declined COD removal efficiency by around 30%, whereas HRT values of over 8 days did not affect organic removal [98]. In contrast, researchers reported a higher treatment performance by operating the UASB at very short retention times. For instance, Ubay and Ozturk [97] operated a UASB at

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HRTs ranging from 0.83 to 2 days and obtained about 70–75% COD removal efficiency with a CH4 yield of 0.35 m3 per kg CODremoved . In another study undertaken by El-Gohary et al. [23], the researchers operated two sequential UASBs for the purpose of treating Fentonpretreated OMWW. Both reactors had equal retention times, with a total HRT of 2 days. Phenol removal was 57% along with a COD removal of 77% while SS and lignin removals were at 86% and 90%, respectively. The greater performance of the second stage UASB was attributed to the removal of some phenols in the first reactor. Researchers reported that the treatment efficiency of UASBs was independent of influent COD concentrations during the treatment of pomace leachate [11]. However, Boari et al. [99] indicated that the organic matter concentration should be less than 18 g COD/l d for the efficient treatment of OMWW. In another study, Azbar et al. [98] stated that UASB reactors could tolerate high organic loadings and that COD removal efficiency was over 90% as long as OLR was kept less than 10 g COD/l d. Further increasing OLR contributed to the accumulation of polyphenols and inhibited biogas production. Overall phenol removal efficiency varied between 34 and 72%, while the removal efficiencies of phenols and suspended solids declined at an OLR of 10 g COD/l d. Sabbah et al. [46] operated a UASB reactor for treating pre-treated OMWW and obtained a 90% COD removal efficiency at OLRs between 7 and 8 g COD/l d at a fixed HRT of 5 days. In comparison to tubular digesters and CSTRs, UASB reactors have been operated at higher OLR values with satisfactory performances [60]. UASBs are quite reliable and have provided higher treatment performance for the co-digestion of OM waste streams. Mixing OMWW and swine manure at a ratio of 1:1 inhibited anaerobic treatment; co-digestion at a ratio of 1:2, however, resulted in a remarkable COD removal efficiency of between 85% and 95% and a biogas production of 550 l CH4 /kg COD. UASBs also have promising phenol removal potential since sulphate reducers within the structure of the granules have the ability to convert phenol to CH4 [75]. Even though the researchers observed a decline in the performance of the reactor, inhibition was reversible and microbial activity was easily recovered when toxic compound levels were reduced [45,75]. In some cases, the quality of effluents from UASBs does not meet the discharge limits and the researchers recommended post-treatment. Katsoni et al. [11] applied a post electrochemical treatment on UASB effluents with complete COD removal being achieved within 7 h of operation. 5.3.2. Hybrid reactors Foam formation and biomass washout are the main detrimental influences of UASB operation caused by lipids in OMWW. Additionally, the physical entrapment of solids within sludge beds also deteriorates reactor performance and reduces COD removal efficiency [11]. To overcome operational problems, researchers adopted several approaches. Gonc¸alves et al. [64] worked with an inverted UASB reactor by connecting a solid–gas liquid separator to the middle of the reactor in order to avoid washout and in order to promote the degradation of the accumulated substrate. The acclimation of biomass also contributes to the improvement of toxicity tolerance and organic matter removal. Reducing the COD/N ratio from 270/1 to 100/1 by nitrogen supplementation increased the CH4 yield from 20% to 93%. Nitrogen supplementation also increased the biomass/LCFA ratio and counteracted LCFA accumulation at elevated OLRs. Alternatively, intermittent feeding leads to the advanced mineralisation of LCFA and the degradation of phenolic compounds, with phenol removal efficiencies up to 81%. Similar improvements to intermittent feeding have been reported for the anaerobic treatment of lipid-rich wastewaters [22,100]. Azbar et al. [5] have overcome these operational problems by applying a hybrid reactor on OM effluents. Biofilm development on water-resistant cardboard in the upper parts of the reactor prevented biomass

washout and enhanced operational stability. The researchers operated the reactor at OLRs between 0.45 and 32 g COD/l d by changing the influent COD concentrations or HRTs. COD removal increased up to 94% while phenol removal increased between 39% and 80% when the reactor was operated at OLRs less than 10 g COD/l d. OLRs over 20 g COD/l d inhibited reactor performance, with COD removal decreasing to 51% due to the high concentrations of polyphenols. Although the removal of colour and suspended matters increased up to 54% and 87%, respectively, the researchers suggested that it should be treated with polish in order to comply with discharge standards. UASBs and UAFs were sequentially operated in order to treat OMWW. The OLR was between 2.8 and 12.7 g COD/l d and the C/N ratio was adjusted to 20/1 by adding aqueous ammonia. As organic loads were increased gradually, the treatment efficiency of UASB increased up to 75–85%, with the UAF contributing a further 20% of COD removal. In the case of phenol treatment, the UASB and UAF provided 73% and 35% removal efficiencies, with the effluents from the combined systems being free of toxicity [66]. Gonc¸alves et al. [6] do not recommend the construction of a separate treatment plant for the treatment of OMWW. They concluded that seasonallygenerated OMWW could be stored in a tank and co-digested in an anaerobic treatment plant running all year long. The gradual increase of the OMWW ratio allows the biomass to adapt and prevents operational problems. Researchers reported that OMWW was successfully treated by increasing the ratio from 8% to 83% and the OLR from 3.3 to 8.0 g COD/l d. No inhibition occurred even though the reactor was fed with high phenol and lipid concentrations. Accidental overload disturbance moved the biologic solids blanket upwards and penetration into the fixed bed section caused washout of some of the biomass, along with a relatively poor effluent quality. Nevertheless, reactor performance recovered fast after deterioration.

6. Evaluation of bioreactors and management of treated effluents Among the bioreactors, the CSTR was very effective for treating OM effluents with high solid concentrations. The CSTR also provides higher organic removal efficiency and CH4 production for the co-digestion of OM effluents. Overall treatment could be significantly improved if a CSTR is installed as an acidogenic reactor in a two-stage system. A CSTR’s performance deteriorates, however, when it is operated under high OLRs or low HRTs. UASBs, on the other hand, could be effectively operated at elevated organic loadings. Although UASBs are superior for treating liquid wastes at short HRTs, operational problems related with high lipid content could be overcome by applying hybrid UASB reactors. Biofilm development in both hybrid UASBs and anaerobic filters improve the resistance to changes in environmental conditions and wastewater composition. Biofilm systems yield a higher treatment efficiency with better contact between substrate and microorganisms. Irrigation by treated OMWW could offer a cost-effective and environmentally-friendly solution for agricultural fields in arid regions. OMWW is rich in organics, macro-nutrients, and microelements which could be useful for plant growth; however, phenolic and other toxic compounds, along with low pH and salts, may possess phytotoxicity and, in turn, negatively affect agricultural crops [101–103]. Aggelis et al. [104], for instance, reported that treated OMWW had negative effects on agriculture crops; this is probably due to its high electrical conductivity which causes difficulty in water and nutrient uptake by plants. The long-term application of OMWW on agricultural areas may increase electrical conductivity, the sodium adsorption ratio, and the content of phosphorus, nitrogen and potassium. The post-treating anaerobically-treated

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OMWW with membrane systems considerably reduces all parameters along with electrical conductivity and produces a high quality effluent suitable for irrigation [29,88]. Therefore, membrane filtration effluents following the anaerobic digestion of OMWW could be safely spread in agricultural fields but the changes in soil characteristics should be monitored closely in order to avoid future negative effects. 7. Conclusions and recommendations Olive mills consume vast amounts of water, while liquid and solid effluents are rich in organic pollutants phenols and lipids. The anaerobic treatment of olive mill waste streams is a complicated process due to the inherent characteristics of olive harvesting and oil extraction methods. Due to the high amounts of toxic and hardly biodegradable substances, pretreatment is required for the efficient operation of anaerobic reactors. Among other alternatives, advanced oxidation and heat pretreatment technologies significantly reduce toxicity and increase biodegradability. Aerobic pretreatment also enhances subsequent anaerobic treatment performance, but the consumption of a considerable amount of energy for aeration is an obstacle for its widespread application. OM effluents generally lack an appropriate nutrient and alkalinity balance; thus, co-digestion with agricultural waste streams reduces the start-up time and provides a cost-effective removal of organics and methane production. Studies regarding anaerobic treatments have primarily been performed by suspended, biofilm, and granular reactors. CSTR and UAF reactors were the ones mostly applied; however, UASB reactors are highly effective for high strength OM effluents and tolerant against changes in operational conditions. The co-digestion of agricultural waste streams in two-stage anaerobic systems resulted in satisfactory treatment efficiency and energy production. Finally, the reports have shown that all of the bioreactors’ performances can be maximised by simply optimising the organic loading rate and carbon to nitrogen ratio. References [1] F. La Cara, E. Ionata, G. Del, M.R. Gonc¸alves, I. Paula, Olive mill wastewater anaerobically digested: phenolic compounds with antiradical activity, Chem. Eng. Trans. 27 (2012) 325–330. [2] R. Casa, A. D’Annibale, F. Pieruccetti, S. Stazi, S.G. Giovannozzi, C.B. Lo, Reduction of the phenolic components in olive-mill wastewater by an enzymatic treatment and its impact on durum wheat (Triticum durum Desf.) germinability, Chemosphere 50 (2003) 959–966. [3] J. Morillo, B. Antizar-Ladislao, M. Monteoliva-Sánchez, A. Ramos-Cormenzana, N.J. Russell, Bioremediation and biovalorisation of olive-mill wastes, Appl. Microbiol. Biotechnol. 82 (2009) 25–39. [4] B. Rincón, R. Borja, M. Martín, A. Martín, Evaluation of the methanogenic step of a two-stage anaerobic digestion process of acidified olive mill solid residue from a previous hydrolytic-acidogenic step, Waste Manag. 29 (2009) 2566–2573. [5] N. Azbar, F. Tutuk, T. Keskin, Biodegradation performance of an anaerobic hybrid reactor treating olive mill effluent under various organic loading rates, Int. Biodeterior. Biodegrad. 63 (2009) 690–698. [6] E. Eroglu, I. Eroglu, U. Gunduz, L. Turker, M. Yucel, Biological hydrogen production from olive mill wastewater with two-stage processes, Int. J. Hydrogen Energy 31 (2006) 1527–1535. [7] A. Günay, M. C¸etin, Determination of aerobic biodegradation kinetics of olive oil mill wastewater, Int. Biodeterior. Biodegrad. 85 (2013) 237–242. [8] I. Ntaikou, C. Kourmentza, E.C. Koutrouli, K. Stamatelatou, A. Zampraka, M. Kornaros, G. Lyberatos, Exploitation of olive oil mill wastewater for combined biohydrogen and biopolymers production, Bioresour. Technol. 100 (2009) 3724–3730. [9] M.A. Dareioti, M. Kornaros, Effect of hydraulic retention time (HRT) on the anaerobic co-digestion of agro-industrial wastes in a two-stage CSTR system, Bioresour. Technol. 167 (2014) 407–415. [10] G. Padovani, C. Pintucci, P. Carlozzi, Dephenolization of stored olive-mill wastewater, using four different adsorbing matrices to attain a low-cost feedstock for hydrogen photo-production, Bioresour. Technol. 138 (2013) 172–179. [11] A. Katsoni, D. Mantzavinos, E. Diamadopoulos, Sequential treatment of diluted olive pomace leachate by digestion in a pilot scale UASB reactor and BDD electrochemical oxidation, Water Res. 57 (2014) 76–86.

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