An automated medium scale prototype for anaerobic co-digestion of olive mill wastewater

An automated medium scale prototype for

anaerobic co-digestion of olive mill wastewater

B. Bernardi, S. Benalia

_{, D.A. Zema, V. Tamburino, G. Zimbalatti}

Department of AGRARIA, University Mediterranea of Reggio Calabria, Localita` Feo di Vito, 89122 Reggio Calabria (RC), Italy

A R T I C L E I N F O Article history:

Received 19 January 2017 Received in revised form 29 June 2017

Accepted 30 June 2017 Available online 8 July 2017 Keywords:

Medium scale prototype Olive mill wastewater (OMWW) Anaerobic co-digestion (AcoD) Automatic process

A B S T R A C T

Olive oil production constitutes one of the most important agro-industrial business for Mediterranean countries, where 97% of the international production is focused. Such an activity, mainly carried out through three phase olive oil mill plants, generates huge amounts of solid and liquid by-products further than olive oil. Physico-chemical features of these by-products depend on various factors such as soil and climatic conditions, agri-cultural practices and processing. As currently carried out, the disposal of these by-products may lead to numerous problems taking into account management, economic and particularly environmental aspects. Indeed, olive mill wastewater is not easily biodegradable due to its high chemical and biochemical oxygen demand, its high content in phenolic compounds, high ratio C/N and low pH, leading consequently to soil and water source pollution. Considering, the above-mentioned statements, olive mill waste disposal constitutes nowadays a challenge for oil industry stakeholders. It becomes necessary to look for alternative solutions in order to overcome environmental problems and ensure the sustainability of oil industry. Anaerobic co-digestion of olive mill wastewater with other agro-industrial matrices could be one of these solutions; since it offers the possibility to produce green energy and break down toxicological compounds contained in these wastewater for a better disposal of the digested matrices as soil conditioner. In this contest, this note reports the functioning principle of an automated medium scale plant for anaer-obic co-digestion of olive mill wastewater.

Ó 2017 China Agricultural University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Olive oil industry plays an important role for all Mediterranean countries[1], where 97% of the worldwide oil production is focused [2]. Calabria region counts over 182.133 ha of olive orchards[3], and is able to produce more than 100.000 tons of olive oil[4], mainly extracted by three phase processing system. Such plants produce, further than olive oil, solid wastes (or

olive pomace) and aqueous component engendered from olive constitution[5]as well as from washing water and eventually added water during extraction process. Nature, quantities and physico-chemical features of these by-products depend on numerous factors such as olive varieties, soil and climatic conditions, agricultural practices and extraction processes

[6]. Two phase system generates just one kind of by-product that is olive pomace, with a high content of moisture, whereas three phase system, which requires additional water use dur-ing processdur-ing, generates huge quantities of olive wastewater moreover than olive pomace.

http://dx.doi.org/10.1016/j.inpa.2017.06.004

2214-3173Ó 2017 China Agricultural University. Publishing services by Elsevier B.V.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). * Corresponding author.

E-mail address:soraya.benalia@unirc.it(S. Benalia).

Peer review under responsibility of China Agricultural University.

INFORMATION PROCESSING IN AGRICULTURE 4 (2017) 316–320

These by-products, especially olive mill wastewater (OMWW), may represent a serious environmental threat if they are not managed in a suitable and eco-friendly way

[2,6]. Indeed, they are characterized by high chemical oxygen

demand and biochemical oxygen demand, a high concentra-tion of suspended solids and contain diverse complex sub-stances that are not easily degradable because of their high contents in organic carbon such as lipids, tannins and pheno-lic compounds. This composition confers OMWW toxicologi-cal properties and compromises biologitoxicologi-cal degradation processes, especially when associated to a high ratio Car-bon/Nitrogen and low pH values[7]. Furthermore, they may contaminate watercourses and groundwater due to washout and run-off phenomena, and alter soil pH and salinity engen-dering possible asphyxia conditions.

A further problem is linked to the seasoning of olive oil production that generates improvised and huge quantities of olive mill wastes in a short period of time, making the adoption of sustainable approaches more difficult[2,6,8].

Up to date, the management of OMWW, exclusively gener-ated from mechanical processes, still consist in their storage and subsequent controlled disposal on agricultural terrains. Although this practice is the most employed one, it presents numerous inconvenient, from economic and environmental points of view[9], making OMWW management an effective challenge for olive sector stakeholders[7]. It is, therefore, nec-essary, to look for more sustainable solutions in order to avoid these problems.

In this context, numerous treatments and valorisation processes were studied in view to recover OMWW into green and sustainable energetic source. Among these, one promis-ing process lies in the anaerobic digestion for biogas produc-tion. However, due to the formerly mentioned features, the application of the anaerobic digestion to OMWW does not enable to reach satisfactory results, in both mesophilic and thermophilic conditions[10]; unless previous physical, chem-ical and/or biologchem-ical treatments are achieved[2,7,11–16].

The scientific literature reports several studies aiming to improve biogas production from OMWW and decrease pollut-ing substance content in the digestate. This was mainly real-ized through biochemical methane potential (BMP) assays in batch experiments using lab reactors with low working

vol-umes[5,10,16]and the subsequent scaling up of the results,

using especially developed pilot plants, having working vol-umes that can reach up to 220 L[17,18], in order to optimise operational parameters and stabilize the process [19,20]. According to their functioning, various types of anaerobic digester can be distinguished; namely: the Continuously Stir-red Tank Reactor, the Up-Flow Anaerobic Filter, the Up-flow Anaerobic Sludge Blanket reactor, the Expanded Granular Sludge Bed reactor, the Internal Circulation reactor as well as the Jet Loop Reactors reactor[21–23].

In this context, a medium scale plant to study continuous processes of anaerobic co-digestion of olive mill wastewater with other matrices, such as sewage, olive pomace and citrus pomace, was developed.

2.

Medium-scale

prototype

for

OMWW

recovery

2.1. Composition

This plant (Fig. 1) is composed of a stainless steel tank having a capacity of about 100 L surrounded with a water heater with a thermostat that enable to operate in either mesophilic or thermophilic conditions. Heating is ensured by a couple of armoured heater elements of 1500 W, while cooling, can occurs thanks to a water main directly connected to the reac-tor, whenever temperature exceeds the set thresholds.

Digestion chamber feeding is carried out, either manually or automatically, through two upload lines respectively for acid feed and for alkaline feed that could be filtered sewage for example, according to the variation of the substarte pH value, thanks to a self-priming pump. Presently, and taking into account the nature of biomass to be used, these compo-nents make this reactor suitable for both wet and semi-dry conditions, it is therefore suitable also for two phase extraction system pomace.

The substrate is continuously stirred in the digestion chamber thanks to a mixer auger acted by a motor of 1.5 kW at a nominal rotation speed. The mixer auger rotation is manually adjustable through a mechanical gearmotor from 8 to 50 rpm.

The digester is also provided by a temperature sensor (Pt100) that controls water heater functioning and a suitable pH probe (InPro4501VP electrode, Mettler Toledo) for wastewater and high flow-through rate applications, which controls feeding process. It consists in a rugged pH-electrode with polyvinylidene fluoride shaft, built-in temperature sensor, which provides automatic compensa-tion of the electrode slope to ensure accurate readings and Solution Ground that assures accurate measurements for diagnostic purposes.

The whole is connected with a cable to the M300 Multipa-rameter transmitter, which is a single channel online process instrument for measuring fluid properties. It is equipped with a USB communication interface that provides real-time data output and complete instrument configuration capabilities for central monitoring via Personal Computer (PC).

Both probes are connected to a switchboard provided by an IP65 protection degree, an emergency button according to IMQ CEI 23-48/23-49 and IMQ EN 62208 certifications and conform to IECEE CB SCHEME IEC 60670-24 norms. Inside the switchboard, there is a CPU memory (ABB, series Ac500, model PM564-T-ETH B0) for automation management.

Pressure is checkable thanks to a manometer placed at the top of the digestion chamber. Moreover, the plant is provided by a series of safety valves and vacuum breakers in order to prevent critical overpressure or under pressure.

At the end of a process, tank drain can be effectuated by means of a manual valve present in the bottom.

2.2. Functionning

The prototype is able to operate both manually or automatically.

As previously mentioned, pH and temperature are the main parameters that control the prototype automatism (Fig. 2). Particularly, feeding runs if the pH value goes beyond the set thresholds, loading so, acid feed or alkaline one according to the increase or decrease of the registered pH.

Analogously, water heating is controlled by the tempera-ture probe, so, if substrate temperatempera-ture goes bellow the set value the thermos-resistances run, otherwise, the water hea-ter is fed through the wahea-ter main for cooling.

The produced biogas is measured with a Ritter Drum type Gas Meter, while methane quantity included in the biogas is measured subsequently through a qualitative analysis with a gas chromatograph. Biogas sampling is effectuated thanks to a specific gasbag.

Finally, the plant can be surveyed and managed, even remotely, by an ad-hoc software through which it is possible to insert the various functioning parameters (working days, pH range, temperature range, pump run and stop), as well as the whole automation process.

3.

Conclusion

The present prototype has been developed to scale-up batch experiment outcomes related to olive mill waste recovery through anaerobic digestion as well as to deepen the knowl-edge about the continuous processes. It is mainly character-ized by flexibility and operating easiness and thus could be employed in small and medium olive oil mills, contributing therefore, to their sustainability according to a multifunc-tional approach considering, both of economic and environ-mental aspects[24,25].

Acknowledgment

The medium scale plant was realized and funded in the framework of the National Operative Project PON Ricerca e Competitivita` 2007–2013, PON01_01545 OLIOPIU` ‘‘Sistemi tec-nologici avanzati e processi integrati nella filiera olivicola per la valorizzazione dei prodotti e dei sottoprodotti, lo sviluppo di nuovi settori e la creazione di sistemi produttivi ecocompatibili”.

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