Sustainable Fuel Practices in Roman North Africa and the Contemporary Mediterranean Basin

Abstract

As a readily available and renewable resource, olive pomace has been used as a fuel throughout the Mediterranean for centuries. This article will first discuss the extensive use of pomace fuel in Roman North Africa, introducing and adding the once coastal city of Utica to our growing list of sites with archaeobotanical evidence for pomace residue. The paper will then focus on the ways in which the Romans linked olive oil and pottery production. While environmental sustainability was unlikely to have been one of the Romans’ conscious objectives, the use of this fuel was vital to the continued production of North African ceramics, particularly in more arid areas. Today, in the face of increasing energy demands, pomace is once again being recognized as an important and sustainable resource. More work, however, still needs to be done to improve the efficiency of pomace use. The article will conclude by highlighting the valuable lessons that can be learned from ancient practices, especially the efficient pairing of olive cultivation and pottery production. IANSA 2018 ● IX/2 ● 147–156 Erica Rowan: Sustainable Fuel Practices in Roman North Africa and the Contemporary Mediterranean Basin 148 pomace fuel assemblages from olives burnt for ritual purposes or as table waste. In all cases, the burning, or carbonization process, turns the olive flesh and skin to ash and as a result we are often only left with burnt olives stones (endocarps) and occasionally the seeds. Usually a pomace assemblage will appear as hundreds or thousands of fragmented olive stones in a concentrated deposit (see, for example, Smith, 1998; Margaritis and Jones, 2008; Rowan, 2015). The high degree of fragmentation is the result of crushing the olives prior to the pressing stage. Many of the olive stones will not survive combustion, especially when the pomace is subject to high temperatures such as those found inside a kiln. Consequently, a high concentration suggests largescale and/or repeat burning events and thus pomace fuel (Mason, 2007, p. 333; Warnock, 2007, p. 47). In the case of ritual or table waste, the assemblage is usually smaller and contains a greater quantity of intact stones despite lower burning temperatures increasing the chances of preservation. Reflectance measurements can also be used to confirm the use of olive pomace as a fuel and distinguish between the use of air-dried pomace and pomace that has been converted into charcoal (Braadbaart, Marinova and Sarpaki, 2016). 1.1 Current uses of pomace Today, 97% of the world’s olive oil is still made in the Mediterranean and in particular in Spain, Greece, Italy, Turkey, Morocco, and Tunisia (Christoforou and Fokaides, 2016; IOOC EU Olive Oil Figures 2018). Since 1990, EU production of olive oil has increased from 994,000 tonnes to 2.17 million tonnes or 2.36 million litres per annum (IOOC World Olive Oil Figures, 2018).1 As olive oil output increases so too does the volume of pomace. Although the modern twoand three-phase press extraction processes make pomace output more difficult to calculate, generating 2.36 million litres of oil results in the creation of approximately 4.13–4.72 million kg of pomace. Modern press methods differ from traditional methods in that they create a more mixed and chemically toxic pulp, especially the two-phase method where all the pomace and olive waste water are mixed together. Consequently, different treatment methods must be applied to the pomace prior to its utilization as a fuel (for a good overview of the different outcomes using traditional and modern presses, see Caputo et al., 2003 or Azbar et al., 2004). However, since the ratio of modern continuous presses to traditional presses varies by country, for simplicity, in this article, all pressing waste with a solid component will be called pomace regardless of water content. It is beyond the scope of this article to discuss the various uses and challenges associated with olive waste water, which does not contain the flesh or stones (Niaounakis, 2011). In the light of higher energy demands and a decreasing fossil fuel supply, in addition to the challenges associated with global warming, renewable and sustainable biomass fuels such as pomace are becoming ever more important. Unlike the combustion of fossil fuels, burning pomace 1 1 litre of olive oil weighs circa 0.92 kg (Marzano, 2013, p. 99). will not increase levels of atmospheric carbon dioxide and therefore not contribute to rising levels of greenhouse gasses. Any CO2 generated during combustion is offset by the continued presence of olive trees and other plant matter that photosynthesizes CO2 (Ali Rajaeifar et al., 2016, p. 87). Experiments have shown olive pomace to be a viable alternative to fossil fuels and unlike other biomass sources such as wheat or corn, the use of pomace does not act as competition for the food supply (Intini et al., 2011, p. 165). Throughout the Mediterranean and the Middle East pomace is still used in traditional ways. In Jordan and Syria, olive pomace is used to heat homes and cook food, while in Turkey it is used in bakeries and olive mills (Doymaz et al., 2004, p. 214; Azbar et al., 2004, p. 238; Warnock, 2007, p. 47–57; Rowan, 2015, p. 466). Other small-scale uses of pomace in Spain, Italy, Greece, Croatia and Slovenia include the heating of factories, private homes, and hotels, all of which make use of local resources (M.O.R.E., 2008). While these traditional small-scale uses of pomace remain important, a greater number of factories and hotels, for example, could take advantage of this resource. Unless local demand increases, pomace will continue to be generated in quantities that far outstrip local consumption. Governments, universities and research institutions have begun to dedicate considerable resources to developing more efficient ways to exploit this clean energy resource (Demicheli and Bontoux, 1996, p. 49–53; Arvanitoyannis, 2007; Vera et al., 2014; Christoforou and Fokaides, 2016; European Commission, 2017; M.O.R.E, 2018). Some of the major olive oil producing countries in the Mediterranean have started to make use of olive pomace fuel for various industrial activities and most commonly the generation of electric and thermal energy (Demicheli and Bontoux, 1996; García-Maraver et al., 2012). While today’s motivations are both financial and environmental, the drive to link industrial-scale, olive oil production with industrial-scale, energy generation is remarkably similar to the events that took place during the Roman period. 1.2 Olive oil production in Roman North Africa Roman conquest of the Mediterranean began in earnest in the 3rd century BC. By the late first century BC, Rome controlled all the land around the Mediterranean Sea and, in effect, all olive oil producing regions. Although olive oil was made in many parts of the Mediterranean prior to Roman hegemony, Roman territorial expansion brought about a significant expansion of olive groves, resulting in an increase in olive oil and pomace production (Mattingly, 1988a; 1988b). This expansion is no more readily apparent than in North Africa, which underwent an “olive boom” starting roughly in the 2nd century AD, and reaching its peak in the 3rd to 5th centuries AD (Mattingly, 1988a, p. 56; 1996, pp. 235–237; Hobson, 2015a, p. 148; 2015b, p. 219). The Romans invested significant capital in the planting of olive groves and the construction of presses along the Tunisian and Libyan coasts, as well as in the Tunisian Sahel (Figure 1). An even more dedicated investment can be seen in the planting IANSA 2018 ● IX/2 ● 147–156 Erica Rowan: Sustainable Fuel Practices in Roman North Africa and the Contemporary Mediterranean Basin 149 of enormous olive groves and the construction of hundreds of multi-presses sites in the Tunisian High Steppe and Libyan Djebel, semi-arid regions that only receive 200–300 mm of rainfall per annum (Mattingly, 1988a, pp. 44–45; 1996, pp. 236–237; Hobson, 2015a, p. 99). The successful planting and cultivation of these olive trees resulted in the output of millions of litres of oil. The territory around the three cities of Lepcis Magna, Sabratha and Oea in modern Libya, for example, may have been producing up to 30 million litres of oil per year (Mattingly, 1988a, p. 37). If olive oil was being produced on an industrial scale, so too was pomace (Mattingly, 1988a; 1988b; Hitchner, 2002). 2. Pomace use at Utica The site of Utica is located on the western side of the Mejerda estuary in northern Tunisia, 10 km from the coast (Hay et al., 2010, p. 325). Originally a Punic settlement, the earliest structures date to the 8th century BC. After the Roman defeat of Carthage in 146 BC, Utica was made the capital of the newly founded province of Africa. Although the city lost its capital status to Carthage after the Roman civil wars of the 1st century BC, it nevertheless continued to prosper as an important port centre and many public buildings associated with large Roman cities, such as baths, basilicas and theatres Figure 1. Coastal Tunisia and Libya with major sites and regions mentioned in the text (author). Figure 2. Area IV. The large lime kiln is visible at the top of the photo while smaller circular kiln 4022 is visible on the right-hand side. A rectangular cistern with rounded edges is located near the smaller kiln. Note the dark ash scatter near the lime kiln (photo courtesy of Andrew Wilson). IANSA 2018 ● IX/2 ● 147–156 Erica Rowan: Sustainable Fuel Practices in Roman North Africa and the Contemporary Mediterranean Basin 150 were constructed. During the imperial period it had the status of a municipium before achieving the higher rank of colonia under Hadrian. By the mid-3rd century it had become an important Christian centre. The city then declined during the Late Roman period and was captured by the Vandals in AD 439 and then the Byzantines in AD 534. Sometime between the early 5th and mid-6th centuries the alluvial fans in the es