Het Tigray gebied in Ethiopië wordt gekenmerkt door langdurige periodes van droogte. Daarom heeft de lokale bevolking er waterreservoirs aangelegd om regenwater te verzamelen en een constante toegang te hebben tot zuiver drinkwater. Het water in deze reservoirs is echter sterk vervuild met pesticiden, die plaatselijk in de landbouw nog veel worden gebruikt. In deze masterproef zal de cactus Opuntia Ficus-Indica onderzocht worden als natuurlijk adsorbens voor de organochloorpesticiden Aldrin, Dieldrin en DDT in het water. Deze cactussoort groeit talrijk in de hooggebergten van het Tigray gebied. Vooreerst zal er kleinschalig worden getest aan de hand van experimenten in het labo, om daarna een ‘echte’ opstelling te ontwerpen, die kan worden toegepast in Ethiopië. Er zal door middel van batch- en kolomexperimenten getracht worden zuiver drinkwater te verkrijgen. De adsorptiecapaciteit van de Opuntia Ficus-Indica wordt vergeleken met die van actieve kool en Moringa Oleifera zaden, beide eveneens afkomstig uit het Tigray gebied. Als conclusie zal nader worden onderzocht in hoeverre deze waterbehandelingsmethode kan bijdragen tot het bereiken van de Millenniumdoelstellingen, die moeten worden behaald tegen 2015.

Abstract:

Ethiopia is socio-economically one of the poorest countries in the world. It has to deal with long periods of drought. Rainfall occurs very irregularly. Because of this limited, irregular and unreliable access to water, Ethiopia, especially the northern highlands of Tigray, has suffered from poignant periods of famine. The urgent necessity for storage of water led to the installation of reservoirs filled with rainwater. These reservoirs face problems of pollution. There is a lot of algae growth in the reservoirs and a strong presence of phytoplankton flowers, which produce extremely toxic cyanobacteria. Because of the location of the reservoirs, in the middle of farming area, pesticides are a real threat. They are the cause of serious health issues among the local population. This masterthesis emphasises on the removal of pesticides from these reservoirs. The pesticides that will be investigated are Aldrin, Dieldrin and DDT. The reason of this specific choice of organochlorine pesticides lies in the ease of analysis with an Electron Capture Detector, and all three components are reported to be used in significant quantities in the Tigray region.
The challenge is to come up with a low-cost, easy and efficient method to purify the water in the reservoirs. There are different treatments to get rid of pesticides in water, but their use is limited because they often entail high operational costs. Adsorption is a versatile and widely used method, thanks to the ease of use at a large scale. Solutes as well as micro-organisms and pathogens can be removed in this manner. Cost-efficient technologies, like natural and synthetic plant-based adsorbents, are preferred over chemicals. Cactus, which is considered as a ‘magic plant’ in Ethiopia (because of the many possibilities), is ideal for the removal of organochlorine pesticides, including Aldrin, Dieldrin and DDT. The Opuntia Ficus-Indica cactus grows abundantly in Ethiopia. In ideal circumstances, the yearly production of the Opuntia Ficus-Indica reaches as much as 50 ton/hectare (Bhaskar et al., 2005).
Investigators at the University of South-Florida demonstrated that the Opuntia cactus adsorbs solutes through a coagulation mechanism that works via adsorption (Miller, 2008). There are four different mechanisms of coagulation: double layer compression, sweep flocculation, adsorption combined with charge neutralisation and adsorption combined with interparticle bridge formation. The main mechanism for the Opuntia is adsorption and interparticle bridge formation. The adsorption capacity of the Opuntia is largely due to the presence of a viscous mucilage, that is stored in certain external parts and the internal of the cactus (Young et al., 2005). This mucilage, called Nopal Dribble, contains 23.4 wt% galacturon acid (Matsuhiro et al., 2006), which is the active component that is responsible for the strong adsorption capacity of the Opuntia cactus. This galacturon acid only takes care of the removal of 50% of the turbidity in the water. This indicates that there are other components in the cactus that contribute to the adsorption capacity. However, little research has been done on this matter (Miller, 2008).

The adsorption capacities of furnace-dried and fresh Opuntia cactus, activated carbon en Moringa Oleifera seeds will be investigated. First, small-scale experiments are performed. From this, adsorption isotherms can be composed. Adsorption is a process that is determined by the diffusion of the solutes from the solution to the internal structure of the adsorbent. The relationship between the amount adsorbed per gram adsorbent at equilibrium and the concentration in the solution at equilibrium is shown in an adsorption isotherm. Freundlich isotherms give the best agreement with the results of adsorption in this masterthesis. Then the results are scaled up in order to construct a large-scale installation which can be used in the highlands of Ethiopia.
The furnace-dried Opuntia cactus particles give good results for the adsorption of Aldrin, Dieldrin and DDT, but they can only be used in batch experiments. The small dried cactus particles absorb a lot of water. As a consequence, the porosity of the dry particles drops and they swell up so that the column gets clogged. The size of the particles is of course determinative for the amount of pesticides that they can adsorb. The smallest particles have the largest surface area-to-volume ratio, so they have the largest adsorption capacity. Fresh Opuntia cactus can be used in both batch and column experiments. The adsorption isotherms are derived from the batch and column experiments. Breakthrough curves are determined from column experiments. These curves are used for the scale-up of the water purification installation. From breakthrough curves it is possible to deduct after how many liters of water the adsorption capacity of a certain amount of cactus is exhausted. The fresh cactus gives even better results than the dry cactus. But because of the mucilage, it has a very high turbidity. This can be eliminated by flushing the cactus with a large amount of distilled water (For 500 grams of cactus one needs 10 liters of distilled water to get an acceptable degree of turbidity.). The consequence of this flushing is that the cactus loses 25%-30% of its adsorption capacity because the mucilage is gone. But when scaling up the amount of cactus, less water is needed for the flushing and the adsorption capacity will still be sufficient to purify the water of the Ethiopian reservoirs.
Activated carbon or charcoal as an adsorbent gives the best results of adsorption of the three organochlorine pesticides. A small amount of this adsorbent is capable of removing about 99% pesticides, which indicates a large efficiency. Both batch and column experiments are done to obtain the adsorption isotherms and the breakthrough curve. Activated carbon also needs flushing to keep the turbidity of the purified water low enough, but less distilled water is needed as opposed to fresh Opuntia cactus (For 500 grams of activated carbon one needs a maximum of 8.3 liters of distilled water.). On the downside, activated carbon is a very expensive material. This is compensated by the fact that activated carbon can be regenerated by a thermal treatment in a furnace. So the investment cost of activated charcoal may be high, but the operational costs are relatively low. If the investment costs can be reduced in the future and if the preparation can be simplified, it is possible to apply this adsorption material in the purification of the water from the reservoirs.
Moringa Oleifera is a tree that grows abundantly in Africa. This tree contains seeds that are already used as an adsorption material for heavy metals. This study investigates if these seeds can be used as an adsorption material for organochlorine pesticides. The column experiments show that 15 grams of peeled Moringa Oleifera seeds adsorb 80%-95% of the organochlorine pesticides in 100 ml of water. The flushed Moringa Oleifera seeds show an even larger adsorption capacity than the washed fresh Opuntia cactus slices, which loose a lot of their mucilage during this pretreatment of the adsorbent.

The laboratory tests are terminated with a design on full-scale on paper of the water purification installation as it can be build in the highlands of Tigray. For the large-scale installation, one first takes a look at the possible materials. The costs must remain as low as possible. Moreover, the system is going to be used by the local population, so it has to be of a technological simplicity so that the local people can build, maintain and repair it, if any defects occur, without difficulties. It turns out that polypropylene has the most advantages. It is the material pipelines are made of. They have the perfect shape for a column application. It is resistant to bacterial growth, which is essential for the production of drinking water.
Fresh Opuntia cactus has the disadvantage that it quickly grows moldy, even quicker in the warm atmosphere in Ethiopia. The challenge is to keep the cactus cool as long as possible. Polypropylene has another advantage in this respect: it has a low thermal conductivity (0.12 W/m K) (M. Lucisano, 2009) so that the heat slowly penetrates into the cactus. But this alone is not enough to keep the atmosphere cool enough. An extra aid is to put the pipes in vessels filled with cold sand. When sand is dry, it is a strong insulator. As a surplus, the sand provides stability for the pipes. The problems concerning flushing are already discussed. Activated carbon is better resistant to the high temperatures in Ethiopia. It does not grow moldy so no measures need to be taken in this area. But because of its high price, it’s not considered for use on a larger scale. Since the Moringa Oleifera trees are not as abundantly available in Tigray compared with the Opuntia Ficus-Indica cactuses, further investigations to upscale this process have not been done.
The United Nations Millennium Developments Goals (MDGs) are eight international goals that need to be reached by 2015. One of them is the protection of a sustainable environment, including increasing the access to pure drinking water. Ethiopia achieved this goal in 2010. Nevertheless there are still 15 million people who suffer from a lack of drinking water. The water purification installation, which uses the fresh Opuntia Ficus-Indica cactus as a natural adsorbent, helps realizing this goal. It also helps the poor local community to anticipate in their own drinking water needs. The installation is also an aid in achieving the sixth MDG: the combat against HIV/AIDS, malaria and other diseases, caused by pollution of drinking water because of the removal of pesticides and toxic cyanobacteria in the water of the reservoirs.