The Physico-Chemical Status of Impacted Environment
One of the most important indicators of the chemical disposition of wastewater and its potential to cause environmental damage is the pH. The pH of cassava wastewater, cassava sludge and impacted soils ranged from 6.6 – 6.9, 6.6 – 6.7 and 8.0 – 11.9 respectively (Table 2). The determined pH values are not within the FEPA permissible limit of effluent discharge ((FEPA), 1991). The pH of cassava of freshly collected cassava wastewater from the study area ranged from 3.8 to 4.3 which indicate a very acidic condition induced by the presence of hydrogen cyanide a conjugate acid released by the decomposition of cyanogenic glycosides - linamarin and lotaustralin (Priya, et al., 2011). The low pH of cassava wastewater will cause damage to plants and other flora and fauna when disposed on land without appropriate treatment to conform to effluent standards. This does not agree with the optimal range of soil pH stated between 6.5 and 7.5 (Anon, 2004b). Acidic condition can also interfere with treatment processes hence the need for adjustment of pH using strong alkaline such as NaOH in order to render it amenable to biochemical treatment. It has been reported that optimal adsorption of cyanide from cassava wastewater occurs at a pH of 10 (Akpan, et al., 2019). It has been reported that hydrogen cyanide inhibits the activities of methanogens and that three days of acclimatization might be required for cyanide concentration up to 10mg/L (Glapracha, et al., 2018). When left to stand for one month, the pH of cassava wastewater obtained from two plants increased steadily from 3.79 and 4.29 to 9.24 and 9.72 respectively. Samples from the other two plants increased only slightly from 3.8 and 4.0 to 4.0 and 4.36 respectively. The increase in pH over time can partly be attributed to the escape of hydrogen cyanide from cassava wastewater when exposed to the atmosphere. Microorganisms such as Fusarium Solani and Pseudomonas Fluoresens that use ferrocyanide as a source of nitrogen and carbon can also decompose hydrogen cyanide. Therefore, maintaining proper pH of the soil is essential since it tends to determine the nutrient uptake in the soil. The acidic properties of the soil can also be linked to the presence of high hydrocarbon cyanide content. In forest and savannah region, the pH range that favours the growth of plant and microorganisms is 5.70-6.50 (Banjoko, et al., 1994). However, low soil pH less than 6.0 enables and raises the exchangeability of aluminum which is highly toxic to crops.
Figure 1.pH of wastewater at four different locations.
The hydrogen cyanide content in the collected cassava wastewater sample from different locations ranged from 8.6 to 42.8mg/L. Report has been made that 85% to 90% of cyanide are lost during the process of oven drying at 65oC and there is about 56% of cyanide loss from sun drying (Cooke, et al., 1978). Therefore, the amount of cyanide in cassava is unusually high when compared with the concentrations of total cyanoglucosides in cassava wastewater which ranged between 10.4 to 274mg/L (Balagopalan, et al., 1998).
Table 3. Descriptive statistics of the monitored physicochemical parameters and heavy metals
The electrical conductivity of cassava wastewater in the sample areas ranged from 4006μS/cm to 5240μS/cm. This is within the range reported in literature. The total suspended solid (TSS) ranged from 11900 mg/l to 40800 mg/l. This is above the FEPA permissible limit for TSS. According to Ehiagbonare et al. (Ehiagbonare, et al., 2009), TSS increases the absorption of heat from the sun in the wastewater thereby increasing its temperature. It also influences the turbidity of wastewater which prevents the penetration of sunlight when discharged in a given water body, thus impacting negatively on the aquatic lives in the water.
The concentration of heavy metals in samples generally exceeded the effluent discharge guidelines for the heavy metals investigated except copper. The concentration of copper in the different samples (sludge, wastewater and soil) ranged from 0 to 0.194 ppm across the various locations which are less than the permissible limit for effluent discharge. Among the samples, sludge has the least concentration of copper. In this study, the concentration of copper in the contaminated soil displayed slight deviation from the values reported by other authors. This study shows that the concentration of copper in the soil ranged from 0.1293 to 0.194 ppm. Concentrations of copper reported by other researchers ranged from 0.146 to 2.90 ppm (Nwakaudu, 2012; Osakwe, 2012; Igbinosa, et al., 2015). Cadmium concentration in the different samples of this study ranged from 0 to 0.1429 ppm. The concentration of cadmium in the sludge was found to be higher than the concentration in wastewater and soil. The low level of contamination of cadmium in the soil has been found to be of similar range with previous studies. Cadmium values of 0.012 ppm and 0.006-0.04 ppm have been reported (Osakwe, 2012; Nwaugo VO et al., 2008). Notably, cadmium as well as other heavy metals is one of the additives in lubricating oil used for lubricating processing machine parts. The level of concentration of chromium in the samples ranged from 0 to 0.233 ppm. Cassava wastewater has higher chromium concentration than sludge and Chromium concentration in the soil exhibits some level of disparity with previous works (Osakwe, 2012; Nwaugo VO et al., 2008).
The concentration of Nickel in the samples ranged from 0 to 0.8205 ppm at various locations. Sludge has the maximum concentration of nickel followed by soil with 0.8205 ppm and 0.5128 ppm respectively. Mercury is a naturally occurring metal which has a silver white, odourless liquid but becomes colourless and odourless gas when heated. Mercury concentration of the different samples in this study ranges from 0 to 6.0 ppm which is associated with the degree of anthropogenic activities. This range is way high above the permissible limit of 0.001 ppm for effluent discharge into the environment. The Arsenic concentration in the different samples of this study ranged from 0.9032 to 8.5806 ppm which is above the 0.1 permissible limit of effluent discharge. This implies that the effluent and soil in the various locations are highly polluted with arsenic. The concentration of arsenic was found to be the highest in the sludge sample and lowest in the wastewater with 8.5806 ppm and 0.9032 ppm respectively. This implies that the samples are all above the permissible limit of effluent discharge for arsenic concentration. At low concentration, arsenic has a high adverse effect on the environment and that of public health. The fermentation period and human anthropogenic activities tend to affect the arsenic concentration in the various locations.
Impact Assessment Of Cassava Processing Into Garri In The Environment
The processing of cassava into garri involves the following operations; peeling, washing, grating, dewatering /pressing, fermentation, sieving and roasting/frying. The process material sequence is shown in Figure 2. The first stage in the processing of cassava into garri after harvesting involves peeling to remove the outer brown skin and inner thick cream layer. This process generates large amounts of solid waste whose management constitute a problem to the environment (Figure 3). Considering that about 20 – 35% of cassava tube consists of peels (Oghenejoboh, et al., 2021), the dimensions of the problem it creates can better be envisaged. This situation introduces a complicating dynamics to the already overwhelming burden of solid waste management in Nigeria. On an annual basis, between 11 and 20 million metric tons of cassava peels are generated from about 59.5 million metric tons of cassava produced annually. Added to this is the potential of leaching of toxic substances such as heavy metals and cyanide from the cassava peels into surrounding soils and water bodies. After peeling, the cassava tubers are then washed to remove stains and dirt, and then crushed/grated to reduce the particle size to smaller particles for ease of dewatering and further processing. This process can be achieved manually or mechanically. The crushing of the cassava tubers causes rupture of cells and releases some of cyanogenic glucosides into the liquid phase. The moisture-laden cassava pulp is then subjected to dewatering in a filter press. Cassava roots are high in moisture content (60 – 70% on wet basis) most of which (40 – 50%) are lost during dewatering (Dahdou, et al., 2020). It has been reported that 90% of the total cyanogens present in cassava tubers were removed during fermentation and that one-third of the original linamarin were found in the water (Montagnac, et al., 2009). In most cases, the wastewater effluent from the dewatering process is freely discharged into the environment without any form of treatment, thus negatively impacting on soil and water resources. An estimated 20 million tons of cassava wastewater is produced annually in Nigeria, which translates into about 20,000 m3 of cassava wastewater per annum or a daily stream of 57m3/day.
Subsequently, the mashed cassava is then packed, tied and secured in a jute bags. The mashed cassava in the bags are then pressed/dewatered using a metallic presser or heavy stone in order to dewater and remove the starchy juice. The fermentation process is the final process of cyanide removal for it to be good for consumption. At this point, they are left for at least two days for fermentation based on the consumer tastes and preferences. The fermented pressed cassava is then sun-dried and rubbed over a sieve tray to take out the roughages. The sieved grains are then roasted over fire using a cast iron made frying pan which can be sieved further to remove lumps where necessary. Lumps found after roasting can be due to water not properly removed during pressing. While roasting, red palm oil can be added to prevent the garri from getting burnt. Finally, the roasted cassava is air dried and allowed to cool before storing or packaging for sale.
Figure 2. Material cycle in garri processing
Figure 3. Waste generation from Garri processing unit operation
Each of these processing stages generates some level of environmental impacts starting from cassava harvesting to the closure of the process as demonstrated in Figure 4. The harvesting of cassava involves pulling the roots and tubers out of the soil, thus dislodging some soil particles. Sumithra et al (2013) estimated that about 80.7 g/root of soil is lost during harvesting of cassava; and the crop-specific soil loss is 7.64 kg/ha (Sumithra, et al., 2013). The environment is also impacted due to generation of litters during peeling, sieving and frying. The litters can be in the form of large particles such as those generated during peeling or particulate matter such as those generated during sieving and frying which contribute to air pollution. The major contributor to air pollution in garri processing hinge on burning of fossil fuel due to transportation, grinding and frying. The severity of this comes to the fore considering the fact that most of the vehicles used for transportation of agricultural products are very old and spit out excessive amounts of combustion by-products, including carbon sooth. The diesel generators used to drive the grinding machines are no better. Ai et al (2012) reported that NOX and PM from agricultural machineries are equivalent to 17% and 22% of the total on-road traffic emissions in Anhui, China (Ai, et al., 2012). Aged vehicles emit significantly higher amounts and more toxic pollutants than newer ones due to changes in the properties of emissions such as hygroscopicity, particle size distribution, phase partitioning, morphology and chemical constitution (Liu, et al., 2019). The issue is further worsened by the overloading of these worn out trucks that transport agricultural produce from farm clusters in rural areas to the cities. Garri processing is a semi-mechanized process which is energy intensive. Almost every stage of the process exerts significant amount of energy demand. Energy is required at the following stages: transportation, peeling, grinding, pressing, sieving and frying. Jekayinfa and Olajide (2007) estimated a total energy demand of 327.17 MJ per tonne of fresh cassava tuber processed (Jekayinfa, et al., 2007).
Figure 4. Environmental impact matrix for Garri processing
Water demand kicks in when washing peeled cassava tubers to remove soil particles and stains smeared during peeling. Water is also required for cooling the diesel engines used for driving the grinding machines. The wash water forms the first phase of liquid effluent from the process. Cassava Large amount of effluent are generated from cassava processing mills which contains highly toxic substances that affect biodiversity, water and soil physicochemical characteristics (Kolawole, 2014; Izah, et al., 2017; Omomowo, et al., 2015). Generally, cassava effluents are discharged directly into the soil and surface water which tends to pollute the receiving environment due to high cyanogenic contents, total suspended solid, total colour, heavy metals, chemical demand oxygen (COD) etc. This alters the suitability of the receiving surface water from drinking and washing. Ehiagbonare et al. (Ehiagbonare, et al., 2009) reported that cassava effluent has negative impacts on plants, air, domestic animals, soil and water. If cassava wastewater is not properly treated before disposal, it causes serious pollution by percolating into the soil or contaminating the runoff that drains inside the stream or nearby water bodies. Ogundola and Liasu (Ogundola, et al., 2007) reported that cyanide stays in the soil at a negligible level when it is added to soil but becomes toxic to soil microorganism at high concentrations.
None of the processing plants investigated in this study has a treatment unit for any of the effluent categories before discharge into the environment. In all cases, the effluents are discharged directly into the immediate environment of the setup. This continuous input of pollutants into the environment through unregulated effluent discharge leads to a build-up of contaminants in the environment. Besides, the processing of cassava into garri involves partial fermentation of the crushed tubers which usually generates foul smell due to the production of volatile acids. This odour can be perceived as far as 90.3m to 102.3m from the source (Ehiagbonare, et al., 2009). The unpleasant sensation of odour from cassava effluent may induce adverse physiological and olfactory reaction. This effluent could actually breed insects that may later cause disease infection in humans. Some side-effects caused by cassava odour pollution include loss of appetite, coughing, breathing difficulty and nose irritation. It is quite unfortunate that environmental agencies in developing countries like Nigeria are yet to adopt practical measures to mitigate the adverse impacts of cassava processing in the country. Apart from odour from fermentation, other forms of air pollutants, notably greenhouse gases are emitted during transportation of the raw or processed product, grinding and frying. Most of the grinding machines examined in the area were diesel powered engine with either automated starter or manual starter (Table 3). Diesel engines are considered as one of the largest source of environmental pollution in the form of carbon monoxide (CO), hydrocarbons (HC), particulate matter and nitrogen oxides – NOX (Reşitoğlu, 2015). Diesel emissions contribute to the development of cancer; cardiovascular and respiratory health effects; pollution of air, water, and soil; soiling; reductions in visibility; and global climate change (Lloyd, 1995). These processing plants are located in close proximity to people’s houses, thus making them direct recipients of the adverse impacts.
Table 4. Common types of power engines for cassava processing (grinding)
The average pollution load index of the recipient soils ranged from moderately polluted (2.48) to highly polluted (4.56) with an average value of 3.05. This signifies a significant adverse shift from the background concentration. The values of pollution index were highest for chromium in two of the locations and for arsenic at another location with values greater than 5 (very highly polluted). Arsenic exhibited high pollution index (PI>3) in two locations and moderate pollution index (2<PI<3) in two locations. Chromium exhibited high pollution index in two locations and moderate pollution index in one. Copper exhibited high pollution index at one location and moderate pollution index at three locations, while nickel and lead generally exhibited only slight level of pollution. Based on the average values of pollution indices, two locations were highly polluted, while the other two were moderately polluted. Nemerow’s pollution index (NPI) indicated that soil samples from three of the locations were severely polluted (>3.0), while one was moderately polluted (2<PI<3). Based on the modified degree of contamination assessment, soil samples from three of the locations were moderately contaminated with heavy metals (2<mCD<4), while one location had a high degree of contamination. The geoaccumulation indices ranged as follows: 0.58 (slight contamination) to 2.59 (heavy contamination) for arsenic; 1.0 (slight contamination) to 2.59 (heavy contamination) for chromium; 0.58 (slight contamination) to 1.58 (moderate contamination) for copper; -0.03 (uncontaminated) to 0.32 (slight contamination) for nickel; 0.03 (slight contamination) to 1.03 (moderate contamination) for cadmium. It is obvious that the soils in the immediate vicinity of garri processing plants are being adversely impacted due to poor waste management and uncontrolled effluent discharge. Arsenic and chromium are ranked the highest with respect to pollution indices. Arsenic and chromium have a lot in common: they are both known carcinogens with high level of toxicity; widespread environmental contaminants; commonly present in air, soil and water; and readily intrude into the human body via the major routes of inhalation, ingestion and skin absorption (Vimercati, et al.). Once in contact with the soil, heavy metals are quickly absorbed and re-distributed in the soil by mineral precipitation, ion exchange, adsorption, aqueous complexation, biological mobilization/immobilization and plant uptake (Wuana, et al., 2011). This combination and sequence of processes make the contaminants readily available to plant uptake, leading to bioaccumulation and bio-magnification up the food chain. It is difficult to attribute the presence of these heavy metals in the soil within the vicinity of garri processing plants to any specific source. It would, however, appear that the major source are the cassava tubers themselves which had absorbed the contaminants from their native soils during the growing season. Atafar et al (2010) reported an overall increase in the concentrations of Cd, Pb and As in soil due to fertilizer application (Atafar, et al., 2010). It has also been reported that the concentrations of Cd, Cr and Pb in cassava leaves are above the WHO limits of 0.20mg/kg,0,05mg/kg and 0.30mg/kg respectively (Ajiwe, et al.). Hence, the cassava tubers serve as a medium of heavy metal, recycling, transport and redistribution. While this is not desirable, it has an intrinsic advantage in that the heavy metals which would have been directly ingested by humans while eating the cassava tubers have been screened out through cassava processing. Other sources of heavy metals input to the environment include: wearing of the metallic moving parts of the grinding machine and filter press, spillage of diesel and condemned engine oil as well as exhaust carbon soot from diesel power engines.
The environmental risks associated with the input and build up of heavy metals in the soil within the immediate environment of the gari processing plants were assessed using the ecological risk factors (Er) and the potential ecological risk index (RI). The values of Er ranged from 5.63 to 91.8 for all metals investigated. The order of ecological risk of various metals based on their average values was: As>Cd>Cu>Cr>Pb>Ni with corresponding ecological risk values of 50.63 (moderate risk), 34.43 (low risk), 15.93 (low risk), 10.51 (low risk), 8.93 (low risk) and 5.63 (low risk). Based on individual metal assessment per location, cadmium and arsenic posed the highest risk with Er values of 91.8 and 90.0 respectively. Overall, low ecological risk was recorded 83.3% of the times, which indicates that despite the gradual build up of heavy metals in the immediate environment of the processing plants, the ecological risk posed remains low. However, a red flag exists with respect to certain heavy metals whose concentrations at some spots present a reasonable probability of high ecological risk. A preponderance of heavy metal concentration in the soil has the potential to affect soil native microorganisms and most heavy metals have low tendency to be degraded with high tenacity to exist in the environment for a long period of time (Aiyegoro, et al., 2007; Izah, et al., 2016). The presence of these heavy metals in the soil has immediate and remote adverse consequences which include phyto-toxicity, reduces soil productivity, water pollution, illnesses associated with intake of contaminants either through ingestion of impacted water or plant. For instance, exposure to arsenic even at low concentrations can lead to vomiting and reduction in the production of red and white blood cell while ingestion of high concentration can cause death. Nickel and Copper can be associated with the following side effect on human like kidney failure, diarrhea, headaches, allergy, cardiovascular, lung and nasal cancer. Cadmium, Chromium, Mercury and Lead are potential human carcinogens that affect human organs such as lungs, kidney etc.