Impacts of Cassava Mill Effluents in Nigeria

Cassava is the fourth largest staple food after rice, wheat and maize. Cassava is produced in the tropical and sub-tropical countries. Currently, the global production of cassava is about 215,436,496 tons. Out of these, Nigeria accounts for 20.3%, being the largest producing nation. During processing of cassava flour (Gari), three main wastes are generated including cassava mill effluents (CME), solid and gaseous emission. This paper reviews the impacts of CME in Nigeria. The study found that CME’s physicochemical quality often exceeds the limit for effluents discharge onto land and surface water as recommended by Federal Environmental Protection Agency (FEPA), Nigeria. CME alters the quality of soil and water with regard to physicochemical, heavy metal and microbial characteristics. CME can induce toxicological effects on the environments and its biota including humans, fisheries, flora and fauna. The impacts are mostly associated with physicochemical (viz: odour, cyanide, acidic, dissolved oxygen, biological and chemical oxygen demand, conductivity) and heavy metals characteristics. Therefore, there is the need for treatment and sustainable management strategies of CME through biotechnological advancement. DOI : 10.14302/issn.2637-6075.jpae-17-1890 Corresponding Author: Sylvester Chibueze Izah, Department of Biological Sciences, Faculty of Science, Niger Delta University, Wilberforce Island, Bayelsa State, Nigeria, Tel: +234 703 0192 466, E-mail: chivestizah@gmail.com


Introduction
Like rice and maize, cassava (Manihot esculenta Crantz) which belongs to the Euphorbiaceae family is a major staple food in Africa especially Nigeria.Cassava is a typical food security crop [1].According to Nwokoro et al. [2], Afuye and Mogaji [3], cassava is one of the most vital food crops consumed in developing countries especially in tropical areas.Its cultivation and processing into useful products such as gari and fufu [4] Cassava is cultivated in over 80 countries of humid tropical region of the world [5].Cassava products are rich in carbohydrates [6], vitamins (mostly vitamins B and C), essential minerals and low in protein.The nutrient composition depends on the variety, age, and prevailing environmental condition including soil characteristics.Ukwuru and Egbonu [7] reported that cassava is a major source of energy for more than 2 billion people in the world especially in the tropical region.Cassava is consumed by more than 500 million people in developing nations [8] and about 300 million in the tropical countries [9].
Moreover, smallholder processors have dominated the enterprises before the presidential cassava initiative of 2002-2003.According to Knipscheer et al. [24], smallholder cassava processors account over 80% of cassava production and processing into useful products for Nigeria.After petroleum, cassava is a major contributor of Nigeria's gross domestic product (GDP) [25].
Basically, cassava tuber contains about 70% water [24,26,27].During cassava processing into gari, several by-products are derived including cassava peelings (21.8%), cassava mill effluents (CME) (16.2%), sieviates (7.5 %), air emission (19.8%), high quality cassava flour (25.0%) [28,29].In Nigeria, these byproducts (mainly solid and liquid wastes) are discharged into the ecosystem without treatment.Elijah et al. [30] opined that wastewater of cassava processing units could pose more intense problem in near future probably due to lack of effluent treatment facility, as effort of Nigerian Government is ongoing to boost cassava based products.These wastes stream could lead to environmental impacts especially on soil fertility, water and air quality.The solid wastes are consumed by domestic animals such as goat in some part of Nigeria.
The liquid wastes are also consumed by domestic animals such as goat, but instances of toxicity leading to death of flora and fauna have been reported in literatures.Furthermore, CME contaminates agricultural farmland, surface water (creek, river, stream, pond etc) and percolates into sub-soil and groundwater resource [10].The discharge of effluents, sludge, and biosolid from food processing such as cassava on the land has been an age long practice [31].Sackey and Bani [32] have reported instances of CME flowing into vegetation, abandoned into living communities.
In Nigeria several varieties of cassava abound, but the two major cultivars cultivated are sweet and bitter variety.Bitter cassava is known to contain glucoside which forms hydrocacyanic acid during processing [9].Adeyemo [33], Abiona et al. [34], Kolawole [1], Eze and Onyilide [9], Arimoro et al. [35] reported that cassava contains cyanogenic glucoside viz: linamarin and lotaustralin which is stored in the vacuole of plant cell and are converted into hydrogen cyanide, and when it comes in contact with cell wall hydrolysis of linamarin and lotaustralin takes place.During processing (cooking, frying, boiling), the linamarin is reduced because it is hydrolyzed in the digestive system of humans and animals by indigenous microbial flora and in the process hydrogen cyanide (HCN) is released [8].
Cyanide enters the human body through inhalation, ingestion and/ or skin contact and distribute round the body through the blood stream [36].
Typically, a life cycle assessment (LCA) framework of any processing outfit is used to evaluate the impacts associated with the life cycle of a product.Some of the major impacts that can be assessed include social, health, economic and environmental components.Among these impacts, environment components is frequently assessed and some of the   Ohimain [49] also reported that less emphasis on bioethanol led to decline in bioethanol projects.Physicochemical CME is a colloidal suspension of fine particles of cassava i.e. starch in water.CME is highly acidic in nature containing high organic matter, suspended solid, sulphur dioxide and cyanide [32].The cyanide is reduced during cassava processing.Izah et al. [17] reported that heating, fermentation and addition of palm aid in the reductions of cassava cyanide content.[21,[66][67][68], manganese [21,66,68], zinc [21,67,68], iron, chromium, silver and mercury [68].

Quality of Cassava Mill Effluents
This suggests that contamination caused by CME in the environment is mostly from acidity, cyanide, heavy metals and odour.

Microbiology of CME
Microbes are often described as ubiquitous organisms due to the ability to thrive in nearly all environments under different conditions.Some of the environments could be stressful to the microbes.For instance, CME is acidic in nature and contain high cyanide content.This makes it toxic to some certain group of life.However, some still survive under this environment.These microbes are also transferred to the environment (soil or surface water) receiving the CME via discharge.The microbial diversity associated with CME in its environment and populations from cassava mills are presented in Table 2  processing including water used in washing, knife/ cutlass used peeling, bag used in storing prior to pressing and hygienic status of the processors.
Microorganisms present in soil could be affected by the toxicity of the CME effluents.As such, the density of the microbes typically reduces as compared to noneffluent soil and surface water.The reduction in population of microbial parameters including total heterotrophic bacteria, total coliform, E.coli counts, Staphylococci counts, fecal coliform counts, lipolytic bacteria, cellulolytic bacteria, phosphate solubilizing bacteria, nitrifying bacteria and total fungi (Table 3).
The decline in microbial population due to the effect of CME could also affect the environment receiving the effluents (soil and water).

Impacts of Cassava Mill Effluents
Generally, cassava processing units generate large volumes of effluent [1,16,17,21,28,70], which contain highly lethal substances, mobile in soil, affect biodiversity including marine lives, benthic macro-invertebrates, fisheries, microbes, plants [70], human, domestic animals (goat and sheep), fauna and flora, and affect water and soil physicochemical parameters [64].The current trend is direct discharge of CME onto soils and nearby surface water including rivers and streams [78].Probably due to high cyanogenic contents, chemical oxygen demand (COD), BOD, total suspended solid, total dissolved solid, colour, the receiving water bodies could get polluted and its suitability is hindered for downstream utilization such as drinking and washing.CME can also cause alteration in aquatic ecology, plant and animal composition and distribution, and human health.
Air CME is typically known to cause bad odour.
Ehiagbonare et al. [64] reported that foul odour of CME can be perceived as far as 90.3-102.3m of its source.In a developing country like Nigeria, air quality studies is still at infant stage and government agencies have not seen food processing sectors like cassava processing as a major area for which limits need to be established.
During cassava processing, odour emanates from the decomposition of nutrients and this could be highly offensive [79].

Soil
The soil plays several functions to human and their abiotic components including social, ecological and economical [18].The soil is a platform through which the several economic activities take place including construction works [18].The soil harbors several economic microbes (including aerobic and anaerobic) and they play essential roles in transformation, biodegradation and mineralization.The soil also contains several minerals and organic matter needed by plant growth.There are different strata found in the soil.
Every strata supports plant growth in addition to several other functions.Soil is the top layer of the lithosphere formed during weathering [77,80] and combination of weathering, geologic materials and microbial interactions [18].The soil is a major recipient of agricultural and industrial wastes [81].These wastes stream often alter the physicochemical and microbial community of soil [9].
Typically soil contamination affects the alteration of soil's fauna and flora, thereby leading to low productivity due to reduced fertility.Instances of CME leading to reduction in height and leaves of Telfairia occidentalis Hook F [69] and root of Allium cepa L [68] have been reported.Cyanide is a metabolic poison and has the tendency to reduce the biomass of microorganisms within the impacted soil.Due to acidic and high cyanide content (i.e.cynogenic glucoside such as linmarin) of CME, i could reduce the activities of most microbes involve in biogeochemical nutrient cycling [77].solubilizing and total heterotrophic bacteria [76,77].
CME typically reduces the concentration of acid and alkaline phosphate, dehydrogenase, lipase, cellulose and lipase, and elevates urease concentration [77,82].
The alteration could also affect soil geotechnics.
Instance of heavy metal contamination of soil due to CME has been documented by Aiyegoro et al.
[89], Izah et al. [18].High concentration of heavy metal especially non-essential metals has the tendency to affect soil native micro-biota [89].Recently, the role of essential heavy metals has been comprehensively documented by Izah et al. [90].Some of these heavy metals have the tendency to persist in the environment for a long time [91] as they have low degradation potentials.

Water Quality
Like food, water is also a prime resources required by living things for the sustenance of life [93].
Water resources are majorly contaminated by anthropogenic activities [96][97][98].CME could lead to reduction of oxygen demand in water [28].CME has the tendency to turn water brownish/milkish thereby impacting on the colour and turbidity level.Omotioma et al. [72] reported that colour of water sample at the source of CME has 9 Hazen unit being higher than the control samples (4 -6 Hazen unit) respectively.
Acidification have widely been reported in CME due to low pH (<5) [16, 27, 99 -101].As the water becomes acidic due to the presence of CME, the turbidity also elevated.Table 9 present variations associated in the physicochemical characteristics of surface water receiving CME.Microbial quality of the water receiving the effluent are also altered (Table 2 and 3).
High concentration of total suspended solid, BOD, COD and complex polymers and minerals in CME suggests that it could contaminate water sources causing eutrophication in the process without treatment.
Also, runoff associated with CME discharge may result to percolation into the ground water thereby causing contamination.

Toxicological Impacts Associated with Cassava
Mill Effluents CME elicit several toxicological impacts which are caused by the presence of cyanogenic glucoside; lionamann (synthesized from valine, amino acid) and lotaustralin (synthesized from isoleucine, also an amino acid) [9,79].This section of the paper is organized into various impacts associated with CME toxicity.

Food Resources (Flora and Fauna)
Due to acidic nature of CME, it is toxic to household animals, fisheries and other organisms [1].organosomatic and mortality rate [112][113][114][115][116][117][118][119][120][121][122][123][124].Several fish species have been studied depending on its availability and region.Among the family that had been widely studied is Clariidae.Of all the species, Clarias gariepinus and Heterobranchus bidosalis are commonly used for toxicology study.This could be due to the ability to withstand stress and unfavorable conditions.However, Clarias gariepinus is a common fish found in most surface water in Nigeria.This fish species have been widely described as a common Niger Delta fish [125,126].
Studies on effect of CME on some indices (mortality, behavioral, enzyme, histopathology and haematology) (Table 10) causes toxicological abnormality in fish [33,[127][128][129].The toxicity in fisheries exposed to CME is most likely to be caused by cyanide content and acidity.According to Adeyemo [33] cyanide is a potent respiratory poison and could kill life in aquatic ecosystem especially fisheries.Small concentration of cyanide could elicit physiological and pathological effects in fisheries [33].However, alteration in the various indices could lead to stress, disease and even death.This is because the toxic component could impede the metabolic and physiological responses.For instance, CME could decrease mean cell volume in the blood and haemoglobin content which is an indication of shrinkage of red blood cell caused by microcytic or hypoxia [33].
Furthermore, due to eutrophication caused by CME, oxygen content could be reduced.This could eventually affect rate of respiration and photosynthesis, thereby inducing behavioural responses leading to mortality [127].Typically, eutrophication could increase growth of aquatic plants and marsh transformation [79].
The nutrient level in CME could also intensify the rate of eutrophication.Typically various nitrogenous compounds are found in CME including nitrate, nitrite etc. and these compound could be toxic to fisheries especially at high concentration over a prolonged period of time.

Microorganisms
Acidification in environment (soil and water) water could impact microscopic organisms including bacteria and fungi.In acidic environment, non-acid tolerant microbes do not thrive well.Presence of cyanide resulting from the discharge of untreated CME into soil could prevent oxidation/reduction process in non-resistant microbes, thereby leading to decline in productivity probably due to the effect on soil microorganisms [82].Similarly, Ezeigbo et al. [10] have reported that high cyanogenic glycoside limit the growth of microbes.This is a typical observation in CME contaminated soil and non-polluted soil (Table 3).Vol-  The presence of coliforms in CME suggests fecal contamination and is mostly of the genus Enterobacter and Escherichia.The contamination could also be from the environment i.e. soil or water used for washing prior to processing.

Impact on Human
Like fisheries, cyanide content could affect human.But humans are not sensitive to cyanide as compared to fisheries [33].Odour pollution may trigger unpleasant sensation which could have adverse physiological reactions and olfactory functions [79].
Some of the adverse response associated with odour pollution include breathing and sleeping difficulty, coughing, stomach and loss of appetite, eye, nose and throat irritation, disturbance from external environment, annoyance etc [79,130].According to Ero and Okponmwense [79], hydrogen cyanide, a type of cyanide found in CME contains toxic materials that could cause partial blindness in human exposed to environment containing decomposing CME.
In addition, Uhegbu et al. [111] reported CME dumpsite could significantly influence cyanide concentration in some common root crops of Nigeria including Dioscorea dumetorum (domestic yam), Dioscorea dumetorum (wild yeam), Dioscorea rotundata (white yam), Dioscorea alata (water yam), Xanthosoma sagittifolium (red cocoyam), Colocasia esculenta (white cocoyam), Ipomeabatatas (red sweet potato), Ipomeabatatas (white sweet potato).Consumption of food material containing high concentration of cyanide has some health implications.When food containing high cyanide ion concentration is ingested, they are absorbed by the gastro intestinal tracts and could lead to nutritional neuropathies such as tropical ataxic neuropathy and epidemic spastic paraparesis [111,131].This disease affects the spinal cord.Oluwole et al. [132] reported that ataxic polyneuropathy occurrence is endemic area in south west Nigeria, which is associated to cyanide exposure from cassava foods.Famuyiwa et al.
[131] also reported that tropical ataxic neuropathy is associated to chronic cyanide intoxication.
Over time, oral ingestion of cyanide can lead to neurological health issues.In man, cyanide toxicity is characterized by hyperventilation, headache, collapse and coma, nausea and vomiting, generalized weakness, perhaps with convulsion and then respiratory depression [111].When cyanide combines with noxious gases such as carbon monoxide, hydrogen sulphide and azide they inhibit cytochrome oxidase activities, thereby

Socioeconomic Impacts
Odour from cassava processing mill has socioeconomic influence in the society.Ero and Okponmwense [79] opined that odour from CME could worsen or down grade community pride especially in communities with high rate of cassava processing, interfere with human relation leading to unhealthy annoyance, discourage capital investment leading to slow growth in such community outside cassava processing.Sackey and Bani [32] also reported that sanitation and environmental challenges associated with cassava wastes and CME could have adversely affected processors and the larger community of the processing mills.

Conclusion and Future Direction
Nigeria is the largest cassava producing nation.
Cassava cultivation and processing is a major source of livelihood to several families especially in rural area.This study reviews the impacts of CME in Nigeria and found that it causes air, soil and water pollution, and toxicological responses in human, fisheries, flora and fauna.Despite these impacts, management and treatment strategies are poor in developing countries like Nigeria.Therefore, there is the need for treatment of CME prior to discharge and/ or utilization through biotechnological advancement.

Figure 2
Figure 2 and 3 present production cassava output and hectares of land between 1961 to 2014 respectively.At as 2014, Nigeria cassava cultivated area was 7,102,300 hectares while the rest of the world production area is 40,632,527 hectares.As such Nigeria occupies 29.3% of global cassava plantation.Congo Democratic Republic, Brazil, Thailand and Indonesia occupy 2 nd , 3 rd , 4 th and 5 th position respectively of largest cassava area.Due to the different intervention programmes of the Federal government of Nigeria, private sector investment was included in the cassava sector [61] for the adoption of improved cassava yield variety [62].According to Aniedu and Omodamiro [63], about 6 new varieties of cassava processing pro-vitamin A (β Carotene) were released specifically for bread-making through research and development.Unfortunately, some of the intervention strategies have been abandoned probably due to change in regime.
phototrophic bacteria that play essential role in sulphur cycle could be impacted upon.The groups of microbes that can be affected by cynogenic glucoside found in CME include nitrifying, lipolytic, cellulolytic, phosphate

Table 10 :
Toxicological response of fisheries (Clarias gariepinus) to cassava mill www.openaccesspub.orgJPAE CC-license DOI : 10.14302/issn.2637-6075.jpae-17-1890Vol-1 Issue 1 Pg.no.-34Due to availability of microbial isolates in CME, it's a medium through which pathogenic (bacteria and fungi) diseases can be transmitted[9].This typically occurs when microbial contaminated CME come in contact with human skin for some microbes or incidentally ingested.Some of the microbes found in CME could induce health impacts when unintentionally ingested especially in immune-compromised patients.

Table 9 :
Physicochemical characteristics of surface water receiving cassava mill effluents CME-Cassava mill effluents; NS-Note stated