Research Article pubs.acs.org/journal/ascecg
Olive Mill Wastewater: From a Pollutant to Green Fuels, Agricultural Water Source and Biofertilizer Khouloud Haddad,†,‡ Mejdi Jeguirim,*,† Boutheina Jerbi,‡ Ajmia Chouchene,§ Patrick Dutournié,† Nicolas Thevenin,∥ Lionel Ruidavets,∥ Salah Jellali,‡ and Lionel Limousy† †
Institut de Science des Matériaux de Mulhouse, 3 bis rue Alfred Werner, 68093 Mulhouse, France Wastewaters and Environment Laboratory, Water Research and Technologies Centre, BP 273, Soliman 8020, Tunisia § Higher Institute of Sciences and Technology for Environment, B.P. 1003, Hammam chat, 2050 Borj Cedria, Tunisia ∥ Rittmo Agroenvironment, ZA Biopôle, 37 rue de Herrlisheim, CS 80023, F-68025 Colmar Cedex, France ‡
ABSTRACT: This investigation has established a complete environmentally friendly strategy for the valorization of olive mill wastewater (OMW). This valorization process includes different steps, namely, OMW impregnation on sawdust, drying, biofertilizer production, and soil amendment. The OMW impregnation on raw cypress sawdust (RCS) was performed using batch procedure mode. During this impregnation, 59% and 71% of the chemical oxygen demand and total dissolved salts of OMW were adsorbed on RCS. The drying of the impregnated sawdust (IS) and OMW was realized in a convective dryer at temperature ranging between 40 and 60 °C and air velocity ranging between 0.7 and 1.3 m/s. Comparison between both samples demonstrated clearly that the impregnation procedure accelerated the drying process and consequently allowed an ecologic recovery of water from OMW that could be reused. The IS sample was pyrolyzed at 500 °C for green fuel (bio-oil, gas) and char production. This residual char (IS-Char) exhibited higher mass fraction of 34.5%. The IS char characterization showed the presence of important nutrients (potassium, nitrogen, and phosphorus) contents. The application of the IS char as a biofertilizer for rye-grass growth studies under controlled conditions showed promising results in terms of leaf dimensions and mass yields of the plant. These preliminary results indicated the validity of the established strategy to convert OMW from a pollutant to green fuels, agricultural water source, and biofertilizer. KEYWORDS: Wood sawdust impregnation, Olive mill wastewater, Biochar, Green fuel, Soil fertilization
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INTRODUCTION In the Mediterranean countries, olive oil production is considered as one among the oldest agri-food industries. About 1.8 × 106 t of olive oil are produced world wide every year. The major production is located in the Mediterranean region.1,2 The process of oil extraction engenders huge byproduct amounts, such as olive solid wastes (OSW) and olive mill wastewater (OMW). The typical quantity of OMW generated during the extraction process is 1.2−1.8 m3 t−1 of olives. In the Mediterranean basin, the generated quantity of OMW exceeds 30 million m3 per year. Because of their high pollution potential, the good and integrated management of these byproducts has been pointed out as a real challenge by the olive professional sector, as well as the actors in charge of environment protection and preservation.1,2 © 2017 American Chemical Society
The OMW management is currently considered as an imperative challenge for the olive oil industry. In this context, several technologies have been developed and tested. They mainly deal with elemental operations, such as flocculation,3 ultrafiltration,4 chemical treatments,5 or combined operations, such as centrifugation-ultrafiltration,6 composting, and direct watering on lands.7 However, these techniques were mainly tested in a laboratory or a pilot plant. Several management and recovery systems have some degree of relevance such as land spreading, composting and natural evaporation. However, these Received: June 5, 2017 Revised: August 15, 2017 Published: September 5, 2017 8988
DOI: 10.1021/acssuschemeng.7b01786 ACS Sustainable Chem. Eng. 2017, 5, 8988−8996
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Photograph of the drying setup.
techniques have several drawbacks such as groundwater contamination, bad smell, evaporation duration (limited by the formation of residual oil layer).1,2,8,9 The thermal treatment seems to be a promising technique to benefit from the energy content of the OMW. In particular, the combustion of OMW mixed with OSW has been examined.10−12 However, the high moisture content of OMW makes its direct thermochemical conversion not economically viable. Recently, a combined strategy for the treatment of OMW has been developed. This strategy consisted in the impregnation of lignocellulosic biomass by OMW, followed by its pelletization for green pellets production.13,14 The combustion of the green pellets shows some limitations due to the ash content and particulate emissions increase, which was attributed to the relatively high K, Cl, and Na contents in OMW.14 Although combustion is the mainly applied thermochemical process for biomass conversion at industrial scale, gasification, and pyrolysis processes are promising techniques for biomass valorization.15,16 In particular, pyrolysis has the advantage to produce solid, liquid, and gaseous valuable products that could be recovered in different manners.16 Gaseous product has a high calorific value and could be converted into heat/electricity and therefore used in the pyrolysis plant.16 The liquid product, named bio-oil, could be used as a fuel or added to petroleum refinery feedstocks.17 The solid product, named char, can be used as biofuel,18 gasified for syngas production,19 activated to prepare efficient adsorbents,20,21 or used for soil amendment (called biochar for this specific application).22 During the last decades, the recovery of biochar obtained from the biomass pyrolysis for agronomic applications has received particular attention.22 In fact, carbon balance investigations indicate that biochar production from biomass allows sequestration of 50% from the initial carbon in soil. Furthermore, several investigations showed that biochar application to soil improves the quality of soils through the acidity decrease, water retention, and cationic exchange capacities enhancement and microorganism activity improvement.22 In addition, the relatively high contents of nutrients contained in biochar especially in terms of nitrogen (N), phosphorus (P), and potassium (K) could enhance the amended soils fertility and therefore improve crops growth yields.22,23
The pyrolysis process benefits and biochar advantages motivate the implementation of the previous developed strategy for OMW valorization24 and to replace the combustion step by a pyrolysis step. Hence, this work aims to validate anew environmental friendly strategy based on the impregnation of low cost sawdust by OMW, generated by the 3-phase extraction system, followed by a drying step for water recovery and later by slow pyrolysis for biofuels (gas and oil fractions) and biofertilizer (solid) production.
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EXPERIMENTAL SECTION
Raw Biomass and Olive Mill Wastewater. Raw cypress sawdust (RCS) was collected from a carpentry manufactory located in the region of Menzel Bouzelfa (North East of Tunisia). It was initially airdried for about 1 week until a constant weight. Then, it was sieved by using mechanical sieve shakers (Retsch, Haan, Germany). Only the fraction with particle diameters lower than 2 mm was selected and dried in an oven at 60 °C for 24 h and stored in bags for further use. It is worth mentioning that the use of cypress sawdust was justified by the fact that it is an abundant local waste and also low in cost. Furthermore, the use of sawdust as a filter material for industrial effluents has already showed significant removal yields of both organic25 and inorganic26 compounds. OMW were collected from an olive oil mill plant (three phases centrifugal steps) located in the city of Touta (North East of Tunisia). It was stored in a refrigerator at a temperature lower than 4 °C for all the experiments carried out throughout this study. Biomass Impregnation with Olive Mill Wastewater. The RCS was impregnated with the OMW by using a batch mode procedure. During these assays, 20 g of RCS were stirred in 400 mL of OMW by using a magnetic stirrer “Agimafic-S, I.P Selecta compagny”. The kinetic retention of the organic matter and salts on RCS was monitored through the measure of the OMW chemical oxygen demand (COD) and total dissolved salts (TDS) progress in time (between 0 and 120 min), respectively. The resulting biomass (mixed with retained organic matter and salts) at equilibrium (IS) was used as feedstock for the pyrolysis process. Drying Investigation. To examine the possible water recovery, experimental drying of OMW and IS was performed using a forced convective dryer (Figure 1). The pilot dryer used is equipped with a balance to monitor the sample mass and consequently the moisture content progress with the drying time. The biomass weight, the drying chamber temperature and air velocity evolution in time (each 2 s) were followed through specific sensors that are connected to an acquisition system. 8989
DOI: 10.1021/acssuschemeng.7b01786 ACS Sustainable Chem. Eng. 2017, 5, 8988−8996
Research Article
ACS Sustainable Chemistry & Engineering During drying test, a fan (1) introduces air in the dryer via a pipe (2) for low flows. Air is supplied in the drying chamber (6) via a cylindrical vein (3) and heated by electric resistances (4). The drying chamber is seated on a balance (7). Several thermocouples measured temperatures in real time in the drying chamber, at the inlet and outlet. The air velocity is adjusted by controlling the fan rotation speed and measured by two Pitot tubes. The power input for heating is adjusted via a proportional−integral−derivative(PID) control loop and the measured temperatures at the chamber inlet. All the sensors are connected to the control unit (5) for monitoring. Less than 5 min are required to reach steady operating conditions. Wet samples of around 0.3−0.5 cm thick are placed on a plate in a rectangular slice. All the constituent parts of the dryer are in stainless steel. The studied operating drying conditions were 40, 50, and 60 °C for temperature and 0.7, 1.0, and 1.3 m·s−1 for air velocity, respectively. Pyrolysis Tests. Thermogravimetric analyses were performed using a Mettler-Toledo TGA/DSC3+ apparatus to assess the different phases of the IS sample thermal decomposition and consequently to optimize the biochar preparation. Before each experiment, about 10 mg of the biomass sample was put in an alumina crucible. TGA experiments were performed under nitrogen gas flow of 100 mL min−1 at heating rates of 5 °C min−1 from room temperature to 800 °C. The biochar production was performed in a homemade pyrolyzer pilot. This pyrolyzer (see axial plane in Figure 2) consists in vertical tubular furnace. During pyrolysis experiments, 2 kg of the raw biomass was placed in the different drawers of the pyrolyzer (Figure 2). Then, nitrogen flow was supplied continuously at room temperature during
30 min in order to remove residual oxygen. Afterward, the temperature increased to the setting value (500 °C) at a range of 5 °C min−1 under nitrogen at a flow rate of 5 N L h−1. The sample was maintained during 1 h at 500 °C then cooled naturally under nitrogen flow. During the pyrolysis test, the emitted gases reached a condensing system formed by refrigerant connected to the liquid collector where they were cooled by circulating cold water to collect the recovered biooil. Characterization Techniques. Different analytical techniques were used to determine the main properties of the OMW, RCS, IS, as well as the produced chars. OMW was characterized through the assessment of its main physicochemical properties, such as the pH, the electrical conductivity, the biological oxygen demand in 5 days (BOD5), chemical oxygen demand (COD), total dissolved salts (TDS) according to standards methods.27 Furthermore, the contents of ammonium, potassium, magnesium sodium, and calcium were determined by using an ionic chromatograph (Metrohm). The elemental compositions of RCS, IS, and the elaborated chars were determined by means of a CHNS-Flash smart apparatus. Proximate analyses were realized in triplicate using thermogravimetric analyzer (Mettler TGA/DSC 1) according to ASTM method. The mineral elements contents of the different chars were measured by an XRF (X-ray fluorescence) spectrophotometer (Philips PW2540) equipped with a rhodium target X-ray tube and a 4 kW generator. During the analysis, 100 mg of the produced chars were ground and mixed with 200 mg of boric acid, and then pressed into a pellet under a 9.00 × 108 Pa pressure for 15 min. The use of boric acid is required to pelletize the char powder since the char has a hydrophobic character and could not be densified without a binder. The boric acid signal is easily eliminated during the XRF analysis. The specific area and porosity of the biochars generated from RCS and IS were evaluated based on CO2 adsorption method. These measurements were carried out by using a Micromeritics ASAP2020. Prior to analysis, all samples were outgassed at 150 °C for 24 h under vacuum to remove surface water and volatile organic species. Analysis of the samples was carried out using CO2 adsorption at 0 °C, with temperature control being achieved with an ice-water bath. CO2 adsorption was chosen for several reasons, CO2 has greater accessibility to the ultramicroporosity (pore diameter lower than 1 nm) compared to N2, and as such any surface and porosity measurements will be more representatives in its overall application, also CO2 is a noncondensing gas under these conditions, which means that the microporosity of the biochar can be readily examined. Biofertilizer Tests. To evaluate the application feasibility of the two produced biochars as fertilizers, indoor plant growth experiments were conducted for rye-grass (Lolium perenne).The experiments were conducted in 1.5 L flowerpots (Ø 11 cm, height 19 cm) with four holes (Ø 0.5 cm) at the bottom. Loamy soil (sand 76.2%, silt 14.4%, and clay 9.1%) was collected from the upper layer (horizon: 0−30 cm) of an agriculture soil in the research field plot of “Rittmo agro-environment institute, Colmar, France”. The used soil was subsequently air-dried and then sieved (
