Building a system that consists of a combination of geothermal component (water well (pit)) and heat recovery from aerobic biological fermentation of olive cake for hot water production at an olive mill is examined in this work. Hot water is essential for mill operation and constitutes a main operational cost, and in many countries, including Jordan, it is normally produced using diesel fuel. In this process, treated and untreated olive cake was characterized. Results show that olive cake is rich in crude fiber and NFE, contains moderate amounts of crude protein and fat, and a good amount of ash. The as-received moisture content ranged from 33.3 to 35.6%, while water activity was between 0.93 and 0.96. The total counts, thermophilic bacteria, and the total mold count of fermentation ranged, respectively, from 2.1 × 108 to 2.4 × 108, 1.7 × 104 to 1.9 × 104, and 1.5 × 102 to 1.7 × 102. The temperature results showed that the well and the covered tank led to a rise in water temperature before entering the boiler in the range of 7 to 13°C. The system effected significant raises in water temperature entering the boiler ranging from 19°C up to 25°C, which holds a promising potential for the system to satisfy much of the mills needs at this range of temperature before entering the boiler provided a large enough pile (pile scale up) is used to handle larger flow rates. The exhausted cake may well be utilized as a soil organic fertilizer.
In most Mediterranean countries including Jordan, olive tree culture goes back to many centuries, and olive trees in the region enjoy a special cultural and economic significance. Due to various social and economic transformations, the region has witnessed substantial expansion in olive trees in the last several decades. In fact, in Jordan alone, there are more than 110 olive mills that process olive fruits for olive oil production. The latter enjoys several proven health advantages [
In addition to olive oil, two other materials are produced by olive processing mills, namely, the solid residue and wastewater as by-products, both of which constitute a serious environmental risk if not adequately disposed of or properly utilized [
Recently, a strong correlation was observed between olive oil production and environmental pollution, and researchers became concerned with the basic issue of the wastes produced after extracting oil from the fruits. Aqueous sludge, which makes about 45% to 54% of the total waste, is treated by using different methods such as chemical, biochemical, and electrochemical treatments, supercritical extraction, and separation processes based on membrane technology [
However, these application areas require drying of the olive cake from a moisture content of 20% to 45% to approximately 5% to 6%; this process is regarded as energy intensive [
During olive oil production, mills need large amounts of hot water that are normally obtained from boilers that operate on diesel fuel. In Jordan, that imports more than 97% of its energy, the annual energy bill averages about JOD 10,000 per mill (equivalent to US$15,000) and is projected to be doubled with the continuous increase in diesel fuel prices. Mills use large amounts of hot water at about 60 to 70°C for about four to five months, which is the length of the annual pressing season. In addition, some mills have on-site olive oil storage facilities for filling and exporting purposes. These storage facilities also require additional large volumes of hot water at 30 to 40°C for oil processing.
Olive processing yields enormous quantities of solid waste after the separation of the aqueous phase. This residue contains 4% to 9% of olive oil, depending on the extraction system used: continuous or discontinuous pressure [
The current practice regarding hot water production for the milling process at the mill under consideration, as well as in most other mills, involves transporting water from nearby springs and storing them in bear (uninsulated) metal tanks on the rooftop of the mill and then feeding this water to the diesel-fired boiler. Given that most of the milling season coincides with the cold winter season when ambient temperatures in most regions including the mill under study are very low and even get close to zero during part of the milling season, large sensible heat is needed to raise the water temperature up to 60 to 70°C needed for olive processing. This fact is reflected in substantial diesel fuel requirements that under current price trends present a heavy toll and a major operational expense of the mill. Based on the above, it may be obvious that the energy bill especially that pertains to hot water makes a major operational cost, and any reductions in energy expenses will effect significant savings in the running costs of operating the mill.
In light of the facts that many families depend on olive oil as an important source of living, the tighter competition among oil producers in available markets, and that Jordan imports almost all its energy needs, this study was set out to explore the possibility of building an alternative novel the on-site system for hot water production. The proposed system which was erected on the mill premises consisted of a combination of geothermal component (water well or pit) and heat recovery from the process heat of the biological fermentation of olive cake that is produced in large quantities at the mill in an attempt to reduce/supplement the diesel fuel consumption at the mill. In fact, heat recovery from the aerobic treatment of organic wastes was investigated although in very limited instances [
Proximate analysis was carried out according to the procedures outlined by the AOAC [
The regular pH meter was used to determine the pH of fermented and unfermented olive cake. Six measurements were taken for each sample, and the final reported reading was the arithmetic average of the readings.
The regular water activity instrument was used to determine the water activity of fermented and unfermented olive cake. Three determinations (about 25 grams each) were taken for each measurement, and the final reported reading was the arithmetic average of all readings.
Twenty-five-gram samples were taken from fermented and unfermented olive cake. Each sample was then added to 250 ml of peptone water to a dilution of 1 : 10.
From the previous solution (Section
From the previous solution, 0.1 and 1 ml were put in Petri dishes, and potato dextrose agar that was prepared by standard methods was poured on them. After that, the dishes were put in an incubator for 3 to 5 days at 25°C. The colony was counted after the growth.
The system in this work utilized a combination of renewable energy sources for hot water production. The sources included geothermal energy by digging a ground pit (well) with a total capacity of 20 m3 of water. The well (first station) receives water from tankers that bring water from nearby springs. The water is then pumped to the partially buried tank (second station) that is covered with olive cake for merely thermal insulation purposes and utilizing the possible uncontrolled compost process heat to further heat the water. Water then leaves the tank to a heat exchanger that is completely inserted into a controlled olive cake compost pile (third station) where temperature reaches 60°C to 70°C due to the biological activity. A schematic of the system is shown in Figure
A schematic diagram of the combined system used in this work for hot water production at the olive mill.
A 100-meter long heat exchanger made of aluminum tubes with an internal diameter of 18 mm was used. The tubes were wound at the Engineering Workshops of Jordan University of Science and Technology to fit within a 13-meter long pile of olive cake. The water flow rate into and out of the heat exchanger at steady-state conditions was measured by a calibrated (graduated) beaker with a stop watch from which the hot water volumetric flow rate was calculated.
As shown in Figure
A schematic of the compost pile of olive cake used in this work [
A sketch of the perforated pipes used for aerating the pipe [
A valve was fitted at the exit point of each of the three stations, and numerous measurements of water temperatures were made at these points in addition to the ambient temperature using thermocouples along with a digital multimeter. The water temperature data were averaged for every two weeks (15 days) and reported as a single value. Water temperature measurements were started at the end of January and extended over the following three consecutive months (till the end of April).
Data are presented as means of three determinations and analyzed using the general linear model procedure with SAS Version 8.2 software package [
The chemical composition of fermentation periods of olive cake during 90 days of fermentation is shown in Table
Chemical composition of olive cake during 90 days of fermentation.
Fermentation period (day) | Dry matter | % of dry matter | ||||
---|---|---|---|---|---|---|
Ash | Crude protein | Crude fiber | Ether extract | NFE |
||
0 | 65.3 ± 4.4 | 4.4 ± 0.2 | 7.2 ± 0.5 | 43.6 ± 3.3 | 11.4 ± 0.8 | 32.3 ± 2.5 |
30 | 66.7 ± 4.5 | 4.6 ± 0.3 | 7.1 ± 0.4 | 44.0 ± 3.8 | 11.3 ± 0.7 | 32.3 ± 2.7 |
60 | 65.9 ± 4.1 | 4.2 ± 0.2 | 7.4 ± 0.5 | 44.5 ± 3.2 | 11.5 ± 0.8 | 32.0 ± 2.4 |
90 | 65.3 ± 4.4 | 4.3 ± 0.3 | 7.1 ± 0.4 | 45.5 ± 3.9 | 11.4 ± 0.7 | 32.7 ± 2.3 |
Moisture content and water activity values of olive cake over 90 days of fermentation are shown in Table
Moisture contents and water activity of olive cake during 90 days of fermentation.
Fermentation period (day) | Moisture contents (%) | Water activity |
---|---|---|
0 | 34.7 ± 2.2 | 0.95 ± 0.03 |
30 | 33.3 ± 2.1 | 0.93 ± 0.04 |
60 | 34.1 ± 2.5 | 0.94 ± 0.05 |
90 | 35.6 ± 2.1 | 0.96 ± 0.04 |
Figure
The total counts, the thermophilic bacteria, and the total mold count of olive cake during 90 days of fermentation.
The data that pertain to temperature over the course of the study are reported in Figure
Water temperature data for three months.
It should be emphasized here that the well can provide this boost in water temperature at a steady basis with no regard to flow rate. That is, the well can provide the total water requirements of the mill while maintaining this temperature boost. Also, the data indicate that the combination of the well and the covered tank lead to a rise in water temperature before entering the boiler in the range of 7 to 13°C. This rise in water temperature is apparently better (higher) than that by the well alone and could lead to significant fuel savings. It should be emphasized here that, given ample time, the tank, like the well, can provide this boost in water temperature at a steady basis with no regard to flow rate. That is, the tank can provide the total water requirements of the mill while maintaining this temperature boost.
As for the whole system (the three stations), the rise in water temperature relative to the ambient is reported in the last column of Figure
Water temperature entering the boiler with and without the system.
Given that 1.0 ton of olive fruit requires 200 kg of hot water (a ratio of 5 : 1) and should a scale up system be considered, the current system is sufficient. The 20 m3 well can process 100 tons of olive fruits, and the 0.4 m3/h flow rate from the pile is sufficient for 2 tons of olives/h, which is more than the capacity of almost all existing mills. The scenario may involve filling the well in the mill’s off time each night which is equivalent to a 4-hour residence time during which water temperature rises by 2 to 8°C above the ambient as was demonstrated during our work.
Based on the findings reported herein and the fact that olive milling season in the Mediterranean region coincides with cold winter times, it may be concluded that combining geothermal (well) and on-site aerobic biological treatment of the milling solid by-product (olive cake) holds a promising potential as a simple, low-cost energy source for the needed hot water. This is evidenced by the fact that the system effected significant raises in water temperature entering the boiler to satisfy much of the mill needs at this range of temperature before entering the boiler provided a large enough pile (pile scale up) is used to handle larger flow rates. Consequently, significant savings are possible by implementing the system.
The authors declare that they have no conflicts of interest.
The authors express their gratitude to the Higher Council of Science and Technology, Jordan, for funding this study. Thanks are extended to the mill owners, as well as Engineer Omar Alzoubi for his assistance in the figures presented herein.