age is its low biomass yield and incomplete oxidation to acetate which lead to effluent with high content of COD  .
The ethanol production from cassava peel has different steps and the first one is to prepare the cassava peel for the process. In order to increase the availability of sugar that can be hydrolyzed biologically for ethanol production using cassava peel, it is necessary to make a pretreatment in cassava peel biomass to reduce re- calcitrance biomass  . The polysaccharides (carbohydrates) in cassava peel bio- mass must be broken down into monosaccharide to increase availability of hydrolysis. Catalyzed steam explosion (CSE) is the preferred method to perform this pretreatment for cassava peel biomass because addition of catalyst in this treatment can increase significantly sugar yields  . Steam (T = 433 K and p = 1.5 MPa) is fed into a reactor where it gets in contact with cassava peel for about 5 min and then the pressure is drastically dropped to cause expansion of lignocellulosic matrix in order to cause separation of individual fibers  . Since diluted sulfuric acid is inexpensive, it is used as a catalyst which reduces the content of xylose up to 90% contributing for increase in ethanol yield  . In the hydrolysis phase, cellulose is broken down into glucose and reducing sugar. Simultaneous sacharification and fermentation (SSF) seem to be the process with best efficiency when enzymatic hydrolysis is carried out during bioethanol production  .
The procedures for ethanol production and subsequent treatment of mine water are described in the coming section. Cassava in Mozambique can be bought in local markets and from local farmers. The cassava must be washed with clean water and the peel must be removed. Cassava peel contain cyanide and thus, it is necessary to soak it in water for 5 hours to remove cyanide  and then dry it in the sun for 7 days to remove moisture content (Figure 3). The dried cassava peel must be chopped and milled to increase surface area for chemical reactions, to increase the digestibility of cellulose and reduce dry matter losses  . The grounded cassava peel can be stored in silos for ethanol production or for other uses.
Chemical precipitation, ion exchange, adsorption, ultrafiltration, reverse osmosis, nan-filtration, electro-dialysis, coagulation, flocculation, flotation and electrochemical process are quite used to remove metals and metalloid from wastewater. According to,  adsorption has advantages comparatively with the above mentioned methods because it is easy to operate, it is economically feasible, and the design is simple. Adsorption has high metal removal efficiency and adsorbent can be reused  .
To date, activated carbon is the adsorbent that is most used in the world due to its high efficiency in removing metals and nonmetal pollutants from polluted water. Activated carbon has high surface area, good microporous structure and
Figure 3. Cassava peel preparation process for silage (Mozambique).
porosity. For that reason, it can be used in batch and continuous operation  . There are other adsorbents that are used for metal sequestration: zeolites, clay, chitosan, red mud, dairy sludge and metal oxides  . Due to high costs of com- mercial activated carbon, researchers are now focused on low cost adsorbents. Cellulosic low cost bio-adsorbents (e.g: agricultural waste) are used to remove metal due to its low economic value, high availability and its potential to treat large scale wastewater. The cellulosic adsorbents that have been used for metal removal are rice husk  , banana peel  , wheat straw  , grape bagasse  , corn stalk  , coir pith  , bel fruit shells  , corn cob  , and hemp fibers  . The majority of cellulosic waste is constituted by cellulose, hemicelluloses and lignin. Metals from wastewater adhere on the surface. The adsorption capacity of these materials can be improved by chemical and physical treatment  . Cassava peel can also be used as adsorbent in the treatment of mine water from coal mine in Moatize. According to  , cassava peel chemically modified is an excellent adsorbent to remove Cd, Pb and Cr. From economic point of view the cassava peel used as bio-sorbent is a sustainable alternative compared with conventional treatment methods, because it can remove effectively metals to very low concentrations at low cost  . Research done by Simate & Ndlovu (2014) about “removal of heavy metals in a packed bed column using immobilized cassava peel waste biomass” concluded that metals can be removed efficiently by cassava peel.
Cassava peel can be used as filter medium in passive treatment of mine water. In this low cost treatment process, flow of mine water goes through filter medium (e.g cassava peel) and pollutants are removed. Organic filter material such as lignite  and rice husk  were used successfully in the removal of metal and sulfate from mine drainage. According to  , there are some disadvantages in using filter material in large scale, namely: high cost, variable composition of mine water, instability, and risk of leaching hazard substances from filter material. Cassava peel contain cyanide and if appropriate treatment it not done, cyanide can end up in the environment. A pre-treatment of cassava peel would be necessary to remove the cyanide and then use it as filter medium for treatment of mine water.
3. Integrating Bioethanol Production and Subsequent Treatment of Mine Water
Figure 4 illustrates mine water treatment scheme. The cassava peel that was previously treated and stored in silos would be fed simultaneously with steam in a reactor to increase ethanol yield as explained before. The fuel that can be used for the boiler could be thermal coal that is produced in Moatize or the bioethanol to avoid usage of fossil fuel. A flash separator could be used to remove by-products (eg. Xylose). The next step would be a pre-hydrolysis of cellulose during 24 hours where cellulose would be reduced into glucose and reduced sugar.
Figure 4. Production of bioethanol from cassava peel to feed SRB in the treatment of mine water. Adapted from (Adekunle, et al. 2016).
SSF would be used in the enzymatic hydrolysis to obtain high efficiency in ethanol production. After fermentation, the mixture should go to two distillation co- lumns organized in series. In the first column, carbon dioxide that remained would be released and unreacted substrate (including unfermented sugar) would be collected and sent back to SSF. The ethanol produced with an estimated concentration of 37 wt%  would be collected and introduced in the second distillation column to increase the concentration up to 95 wt%  . Part of the produced ethanol would be collected and stored in proper reservoir and others would be used as carbon source for SRB. The carbon source (bioethanol) and mine water could be mixed together in a mixing tank. The mixture would be introduced in a bioreactor that was inoculated before with SRB. The treated water must be sent to a clarifier to allow precipitation of metal sulfide (sludge). Treated mine water which is rich in alkalinity () that comes from the clarifier would be sent to adsorption column to remove calcium, magnesium and other metals that cannot form metal sulfide during bioremediation. Part of the treated mine water coming from the adsorption column would be released to the environment and other would be sent to a boiler to produce steam. Part of the produced sludge should be sent back to SRB reactor and the other part could be used to recover metals in the sludge.
The coal mines in Moatize are inside of one of the most important river basins in Southern Africa, the Zambezi river basin. Due to climate change, the temperature is expected to rise by 0.3˚C to 0.6˚C per decade. This rise in temperature will have impact in the rainfall which is expected to decrease by 10% to 15% and the evaporation also is expected to decrease by 10% to 25% by 2050. It means that Zambezi river basin will become more dry and less water will be available for mining activities, agriculture and drinking water supply for the local people  . Mining industry is one of the most water consuming industries in the world, and thus water must be used in sustainable way in order to avoid compromising the future generation. One way of doing that is to treat the mine water that is generated in the mines and use it for different activities inside Zambezi river basin.
According to FAO, Mozambique produced 10 million tonnes of cassava in 2012, which means that 1.5 million tonnes of cassava peel were generated. This agro-waste was just discarded. According to Vale, coal mine in Mozambique, 5 billion liters of mine water were generated inside Vale coal mine in Moatize in 2012. Only 47% of the generated mine water was recycled and the other 53% was discharged to the local environment  . Instead of discarding 1.5 million tonnes of cassava peel and release to the environment 2.65 billion liters of polluted mine water, it could be good to use the agro-waste to treat the polluted mine water and use the treated mine water for agriculture.
Mine water that is treated using SRB eliminates toxic metals that are able to form metal sulfide as shown in Equation (2). The metal sulfides have low solubility and thus, they can be removed through sedimentation. The disadvantage of SRB is that it cannot remove some metals (Ca2+, Mg2+) because they are not able to form metal sulfide. To overcome this problem, adsorption can be coupled with SRB bioreactor to increase the treatment of mine water. Cassava peel would be used as carbon source for SRB and as adsorbent for adsorption process. The treated mine water could be used for the irrigation of native plants and crops that are produced in the Zambezi river basin. Some of the coal mines in Moatize have plans to restore the environment impacted by mining through planting native species. For that, mine water treated with combination of SRB and adsorption can be used as irrigation water.
Ethanol produced from cassava peel can be used as fuel and as a carbon source for SRB. The use of ethanol as a carbon source for SRB in the treatment of mine water appears to be a good solution for polluted mine water in Moatize. There is a need to couple bioremediation with an adsorption column where the cassava peel would be used as adsorbent to remove metals that do not form metal sulfide in the bioremediation process. The treated mine water could be used for various purposes inside of the Zambezi Basin. In conclusion, applying agro-waste generated in developing countries in the treatment of mine water could be a good solution to protect the environment against pollution and also a source of water for local farmers to do agriculture.
The authors would like to thanks SIDA for providing financial support for this research and also the Lund University for providing technical support.