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CHAPTER ONE: INTRODUCTION
1.1 Background of Study
Untreated sewage effluent refers to wastewater generated from domestic, municipal, and industrial sources that is discharged into the environment without undergoing any treatment processes (primary, secondary, or tertiary treatment) to remove contaminants (Metcalf and Eddy, 2019). This wastewater contains a complex mixture of organic matter, inorganic nutrients (nitrogen, phosphorus, potassium), pathogenic microorganisms (bacteria, viruses, protozoa, helminths), heavy metals (lead, cadmium, chromium, copper, zinc, nickel, mercury), emerging contaminants (pharmaceuticals, personal care products, microplastics), and other toxic substances (WHO, 2022). In many developing countries, including Nigeria, untreated sewage effluent is frequently discharged directly into water bodies (rivers, streams, lakes) or onto land, including agricultural farmlands, due to inadequate sewage treatment infrastructure (UNEP, 2021).
The use of untreated sewage effluent for irrigation of farmlands is a common practice in peri-urban and urban areas of Nigeria, particularly in cities with high population density and limited access to clean water for agriculture (Adebayo and Ogunyemi, 2020). Farmers are often driven to use sewage effluent because it is readily available, inexpensive (or free), and contains plant nutrients (nitrogen, phosphorus, potassium) that can act as a fertilizer, potentially increasing crop yields (Eze and Nweze, 2019). However, the simultaneous presence of toxic heavy metals, pathogens, and other contaminants poses significant risks to soil health, crop quality, food safety, environmental quality, and human health (Okafor and Nwosu, 2020).
The composition of untreated sewage effluent varies depending on the source (domestic, industrial, stormwater), but typical characteristics include (Metcalf and Eddy, 2019):
| Parameter | Typical Range (mg/L) | Significance for Farmlands |
| Biochemical Oxygen Demand (BOD) | 200-600 | High organic load; depletes soil oxygen |
| Chemical Oxygen Demand (COD) | 400-1,200 | Indicates organic pollution potential |
| Total Suspended Solids (TSS) | 200-500 | Clogs soil pores; reduces infiltration |
| Total Nitrogen (N) | 30-100 | Plant nutrient (excess causes eutrophication) |
| Total Phosphorus (P) | 5-20 | Plant nutrient (excess causes eutrophication) |
| Potassium (K) | 10-30 | Plant nutrient |
| Heavy metals (Pb, Cd, Cr, Cu, Zn, Ni) | 0.1-5.0 (varies) | Toxic to plants, soil microbes, humans |
| Pathogens (faecal coliforms) | 10⁶-10⁹ CFU/100mL | Disease transmission (cholera, typhoid, dysentery) |
| Helminth eggs (Ascaris, Trichuris) | 100-1,000 eggs/L | Parasitic infections |
| pH | 6.5-8.5 | Affects nutrient availability |
| Electrical Conductivity (EC) | 1,000-3,000 µS/cm | Salinity risk |
The impact of untreated sewage effluent on farmlands can be categorized into several interrelated dimensions (Okafor and Ugwu, 2021):
Impact on Soil Physical Properties:
| Effect | Mechanism | Consequence |
| Soil clogging | Suspended solids block soil pores | Reduced infiltration, increased runoff, waterlogging |
| Reduced porosity | Organic matter and solids fill pore spaces | Reduced aeration, poor root growth |
| Surface crusting | Fine particles deposit on surface | Reduced seedling emergence |
| Increased bulk density | Compaction from solids accumulation | Restricted root penetration |
Impact on Soil Chemical Properties:
| Effect | Mechanism | Consequence |
| Heavy metal accumulation | Pb, Cd, Cr, Cu, Zn, Ni accumulate in topsoil | Plant uptake → food chain contamination |
| Nutrient enrichment | Excess N, P, K | Eutrophication of water bodies; crop toxicity |
| Soil acidification | Nitrification of ammonium produces H⁺ | Reduced pH; aluminum toxicity; nutrient imbalance |
| Salinization | High electrical conductivity (salts) | Osmotic stress on plants; reduced yields |
| Organic matter increase | Organic input | Can improve soil structure (positive) but may cause oxygen depletion |
Impact on Soil Biological Properties:
| Effect | Mechanism | Consequence |
| Pathogen contamination | Faecal coliforms, Salmonella, Vibrio cholerae, helminth eggs | Crop contamination → human illness |
| Microbial community disruption | Heavy metals toxic to beneficial microbes | Reduced nutrient cycling, nitrogen fixation |
| Antibiotic resistance | Antibiotics and resistant bacteria in sewage | Spread of antimicrobial resistance |
Impact on Crop Growth and Yield:
| Effect | Mechanism | Consequence |
| Reduced germination | Toxic substances inhibit seed germination | Poor stand establishment |
| Stunted growth | Heavy metal toxicity, salinity stress | Reduced plant height, biomass |
| Chlorosis (yellowing) | Heavy metal interference with chlorophyll | Reduced photosynthesis |
| Reduced yield | Combined stress factors | Lower crop yields, economic losses |
| Crop contamination | Uptake of heavy metals and pathogens | Food safety risk |
Impact on Human Health (via consumption of contaminated crops):
| Contaminant | Health Effect |
| Heavy metals (Pb) | Neurological damage, kidney damage, developmental delays in children |
| Heavy metals (Cd) | Kidney damage, bone demineralization (Itai-Itai disease) |
| Heavy metals (Cr) | Carcinogenic (Cr VI) |
| Heavy metals (Ni) | Allergic dermatitis, respiratory effects |
| Pathogens (faecal coliforms, Salmonella, Vibrio cholerae) | Gastroenteritis, typhoid fever, cholera, dysentery |
| Helminth eggs (Ascaris, Trichuris, hookworm) | Intestinal parasitic infections |
| Antibiotic-resistant bacteria | Treatment failure for infections |
The sources of untreated sewage effluent contamination of farmlands include (WHO, 2022):
| Source | Description |
| Direct discharge | Untreated sewage pumped or drained onto farmlands |
| River/stream irrigation | Farmers pump water from rivers receiving untreated sewage |
| Flood irrigation | Flood events spread sewage-contaminated water onto fields |
| Overflow from sewage systems | Broken pipes, overflow from treatment plants |
| Open defecation near farms | Direct contamination from human waste |
The regulatory framework for sewage disposal and agricultural use of wastewater in Nigeria includes (FMEnv, 2019; NESREA, 2020):
| Regulation | Year | Relevance |
| National Environmental Standards and Regulations Enforcement Agency (NESREA) Act | 2007 | Enforcement of environmental standards |
| National Guidelines for Wastewater Reuse | 2019 | Standards for treated wastewater use in agriculture |
| National Environmental (Surface and Groundwater Quality Control) Regulations | 2018 | Limits for discharge into water bodies |
Despite these regulations, enforcement is weak, and untreated sewage continues to be used on farmlands in many parts of Nigeria (Okonkwo, 2020).
From a theoretical perspective, this study is supported by three theories: Environmental Pollution Theory (Mackay, 2019), which explains the transport, fate, and effects of pollutants in the environment; Soil-Plant-Contaminant Interaction Theory (Alloway, 2018), which explains how contaminants in soil are taken up by plants and enter the food chain; and Risk Assessment Theory (USEPA, 2020), which provides a framework for evaluating the probability and magnitude of adverse health effects from exposure to contaminants.
In summary, the use of untreated sewage effluent on farmlands is a common practice in Nigeria due to water scarcity and the perceived benefits of nutrient addition. However, this practice poses significant risks to soil health (physical, chemical, biological degradation), crop quality (heavy metal contamination, pathogen contamination), and human health (disease transmission, heavy metal toxicity). This study aims to evaluate the impact of untreated sewage effluent on farm lands by comparing soil properties (physical, chemical, biological), crop growth parameters, and heavy metal accumulation in crops between sites irrigated with untreated sewage effluent and control sites irrigated with clean water (borehole/well water).
1.2 Statement of Problems
The use of untreated sewage effluent for irrigation of farmlands is a widespread but highly problematic practice in many parts of Nigeria, particularly in peri-urban and urban areas where access to clean water for agriculture is limited. While farmers may benefit from the nutrient content (nitrogen, phosphorus, potassium) of sewage effluent, which can temporarily enhance crop growth and reduce fertilizer costs, the practice simultaneously introduces a complex mixture of toxic heavy metals (lead, cadmium, chromium, copper, zinc, nickel, mercury), pathogenic microorganisms (faecal coliforms, Salmonella, Vibrio cholerae, helminth eggs), and other hazardous contaminants into the agricultural environment. The cumulative impact of these contaminants on soil health, crop quality, food safety, environmental quality, and human health is not adequately understood or quantified in many affected areas.
Specific problems addressed by this study include:
Soil degradation (physical, chemical, biological): Untreated sewage effluent contains high levels of total suspended solids (200-500 mg/L), which clog soil pores, reduce infiltration rates, increase surface runoff, and cause waterlogging. The accumulation of heavy metals in soil (lead, cadmium, chromium, copper, zinc, nickel) alters soil chemistry, reduces pH (acidification), increases salinity (electrical conductivity), and disrupts soil microbial communities (beneficial bacteria, fungi, nitrogen-fixing organisms). The long-term effects include reduced soil fertility, loss of soil structure, and potential abandonment of farmland.
Heavy metal contamination of crops: Crops grown on soils irrigated with untreated sewage effluent absorb and accumulate heavy metals (lead, cadmium, chromium, copper, zinc, nickel) from the soil. These metals are translocated to edible plant parts (leaves, fruits, tubers, grains). When consumed by humans, these heavy metals cause serious health effects: lead causes neurological damage (especially in children), kidney damage, and developmental delays; cadmium causes kidney damage and bone demineralization (Itai-Itai disease); chromium (VI) is carcinogenic; and nickel causes allergic dermatitis and respiratory effects. The concentration of heavy metals in crops often exceeds permissible limits set by WHO/FAO and NAFDAC.
Pathogen contamination of crops: Untreated sewage effluent contains high levels of faecal coliform bacteria (10⁶-10⁹ CFU/100mL), pathogens (Salmonella, Shigella, Vibrio cholerae), and helminth eggs (Ascaris, Trichuris, hookworm). Crops irrigated with untreated sewage become contaminated with these pathogens, especially leafy vegetables (lettuce, spinach, cabbage) and root crops that come into direct contact with contaminated soil or water. Consumption of these crops causes waterborne and foodborne diseases including gastroenteritis, typhoid fever, cholera, dysentery, and intestinal parasitic infections. Outbreaks of such diseases have been linked to consumption of sewage-irrigated vegetables in various Nigerian cities.
Antimicrobial resistance spread: Sewage effluent contains antibiotics (from human excretion and pharmaceutical discharge) and antibiotic-resistant bacteria. When these are introduced into agricultural soils, they contribute to the development and spread of antimicrobial resistance (AMR) in environmental bacteria, which can then be transferred to human pathogens through the food chain. AMR is a global health crisis, and agricultural use of untreated sewage is a contributing factor.
Lack of quantitative data: There is limited empirical data quantifying the concentrations of heavy metals, pathogens, and other contaminants in soils, crops, and water sources in areas where untreated sewage is used for irrigation. There is also limited data on the health status of farmers and consumers exposed to these contaminants. Without such data, risk assessment and policy development are impossible.
Weak regulatory enforcement: Although Nigeria has environmental regulations (NESREA Act, National Guidelines for Wastewater Reuse, Surface and Groundwater Quality Control Regulations) that set standards for wastewater discharge and reuse, enforcement is weak. Many farmers are unaware of the risks, or continue the practice because of economic necessity (lack of alternative water sources). There is no systematic monitoring of sewage effluent quality, soil quality, or crop quality in affected areas.
Conflicting farmer perceptions: Many farmers believe that sewage effluent is beneficial because it contains plant nutrients (free fertilizer) and is readily available. They are often unaware of the hidden dangers of heavy metals, pathogens, and other contaminants. There is a gap between farmer perception (positive) and scientific evidence (negative).
Lack of comparative data: Few studies have systematically compared soil properties (physical, chemical, biological), crop growth parameters, heavy metal concentrations, and pathogen loads between sites irrigated with untreated sewage effluent and control sites irrigated with clean water (borehole/well water). Such comparative data are essential for quantifying the magnitude of the impact.
Health impact assessment gap: There is limited epidemiological data linking consumption of crops grown on sewage-irrigated soils to specific health outcomes (gastrointestinal diseases, heavy metal toxicity, parasitic infections). Such data are needed to inform public health interventions and policy.
Remediation and mitigation options: There is limited research on cost-effective remediation options (phytoremediation, soil amendments) for contaminated soils, or on post-harvest treatments (washing, cooking) to reduce pathogen and heavy metal loads in contaminated crops.
In summary, the use of untreated sewage effluent on farmlands poses serious risks to soil health, crop quality, food safety, and human health. However, there is a lack of comprehensive empirical data quantifying these risks in the Nigerian context. This study aims to evaluate the impact of untreated sewage effluent on farm lands by comparing soil properties, crop parameters, heavy metal concentrations, and pathogen loads between sewage-irrigated sites and control sites, and to assess the potential health risks to consumers.
1.3 Aim of the Study
The specific aim of this research work is to evaluate the impact of untreated sewage effluent on farm lands by comparing soil properties (physical, chemical, biological), crop growth parameters, heavy metal accumulation in crops, and pathogen contamination between sites irrigated with untreated sewage effluent and control sites irrigated with clean water (borehole/well water), and to assess the potential health risks to consumers.
1.4 Objectives of the Study
- To determine the physical (texture, bulk density, porosity, infiltration rate), chemical (pH, electrical conductivity, organic matter, total N, available P, exchangeable K, heavy metal concentrations: Pb, Cd, Cr, Cu, Zn, Ni), and biological (total coliform, faecal coliform, helminth eggs) properties of soils from sewage-irrigated sites and control sites.
- To determine the heavy metal concentrations (Pb, Cd, Cr, Cu, Zn, Ni) in edible parts of crops (vegetables, tubers, grains) grown on sewage-irrigated sites and control sites.
- To determine the pathogen contamination (total coliform, faecal coliform, Salmonella, helminth eggs) of crops grown on sewage-irrigated sites and control sites.
- To compare crop growth parameters (plant height, biomass, yield) between sewage-irrigated sites and control sites.
- To assess the potential human health risks (hazard quotient, hazard index, cancer risk) from consumption of crops grown on sewage-irrigated soils.
1.5 Research Questions
- What are the physical, chemical, and biological properties of soils from sewage-irrigated sites compared to control sites?
- What are the concentrations of heavy metals (Pb, Cd, Cr, Cu, Zn, Ni) in edible parts of crops grown on sewage-irrigated sites compared to control sites?
- What are the levels of pathogen contamination (total coliform, faecal coliform, Salmonella, helminth eggs) in crops grown on sewage-irrigated sites compared to control sites?
- What is the difference in crop growth parameters (plant height, biomass, yield) between sewage-irrigated sites and control sites?
- What are the potential human health risks (hazard quotient, hazard index, cancer risk) from consumption of crops grown on sewage-irrigated soils?
1.6 Research Hypotheses
Hypothesis One
- H₀ (Null): There is no significant difference in soil properties (physical, chemical, biological) between sewage-irrigated sites and control sites.
- H₁ (Alternative): There is a significant difference in soil properties between sewage-irrigated sites and control sites.
Hypothesis Two
- H₀ (Null): Heavy metal concentrations in edible parts of crops from sewage-irrigated sites do not exceed permissible limits set by WHO/FAO and NAFDAC.
- H₁ (Alternative): Heavy metal concentrations in edible parts of crops from sewage-irrigated sites exceed permissible limits.
Hypothesis Three
- H₀ (Null): There is no significant difference in pathogen contamination levels (total coliform, faecal coliform, Salmonella, helminth eggs) between crops from sewage-irrigated sites and control sites.
- H₁ (Alternative): There is a significant difference in pathogen contamination levels between crops from sewage-irrigated sites and control sites.
Hypothesis Four
- H₀ (Null): There is no significant difference in crop growth parameters (plant height, biomass, yield) between sewage-irrigated sites and control sites.
- H₁ (Alternative): There is a significant difference in crop growth parameters between sewage-irrigated sites and control sites.
Hypothesis Five
- H₀ (Null): Consumption of crops grown on sewage-irrigated soils does not pose significant health risks (hazard quotient >1, hazard index >1, cancer risk > acceptable level).
- H₁ (Alternative): Consumption of crops grown on sewage-irrigated soils poses significant health risks.
1.7 Justification of the Study
This study is justified on several grounds. First, the use of untreated sewage effluent for irrigation is a widespread practice in many Nigerian cities, but the environmental and health impacts are not adequately quantified. Second, there is limited empirical data on heavy metal accumulation in soils and crops, pathogen contamination, and human health risks in the Nigerian context. Third, the findings will inform policy development (enforcement of wastewater reuse standards) and public health interventions. Fourth, the study will provide baseline data for risk assessment and for monitoring the effectiveness of remediation efforts. Fifth, the study will contribute to the limited literature on sewage irrigation impacts in sub-Saharan Africa.
1.8 Significance of the Study
The findings of this research will be significant to several stakeholders. To farmers, the study will provide evidence of the hidden dangers of using untreated sewage effluent (heavy metals, pathogens) and inform safer alternatives (treated wastewater, clean water). To consumers, the study will inform awareness of potential health risks from consuming crops grown with untreated sewage. To government agencies (Federal Ministry of Environment, NESREA, NAFDAC, State Ministries of Agriculture, State Environmental Protection Agencies) , the findings will inform policy (enforcement of wastewater discharge standards, regulation of irrigation water quality, monitoring of soil and crop quality), and public health interventions (health education, disease surveillance). To water and sanitation utilities, the study will inform investments in sewage treatment infrastructure. To academic researchers, the study will contribute empirical data on environmental contamination and health risk assessment, testing and extending environmental pollution theory, soil-plant-contaminant interaction theory, and risk assessment theory.
1.9 Scope of the Study
The scope of this study is delimited to the evaluation of the impact of untreated sewage effluent on farm lands. The study compares two types of sites: (1) sewage-irrigated sites (farmlands that have been irrigated with untreated sewage effluent for at least 5 years); and (2) control sites (farmlands irrigated with clean water (borehole/well water) located at least 1 km away from sewage-irrigated sites, with similar soil type and crop type). Soil sampling: surface soil (0-20 cm) and subsoil (20-40 cm) collected from 5-10 points per site (composite sample). Crop sampling: edible parts (leaves, fruits, tubers, grains) of commonly grown crops (vegetables: amaranth, spinach, lettuce, cabbage; tubers: cassava, sweet potato; grains: maize, rice). Laboratory analyses: soil physical properties (texture by hydrometer method, bulk density by core method, porosity by calculation, infiltration rate by double-ring infiltrometer), soil chemical properties (pH by pH meter, electrical conductivity by conductivity meter, organic matter by Walkley-Black method, total N by Kjeldahl method, available P by Bray-1 method, exchangeable K by flame photometry, heavy metals (Pb, Cd, Cr, Cu, Zn, Ni) by atomic absorption spectrophotometry (AAS)), soil and crop biological properties (total coliform and faecal coliform by membrane filtration or multiple tube fermentation, Salmonella by culture method, helminth eggs by sedimentation or flotation method), crop growth parameters (plant height by ruler, biomass by drying and weighing, yield by weighing). Health risk assessment: estimated daily intake (EDI) of heavy metals, hazard quotient (HQ = EDI / reference dose (RfD)), hazard index (HI = sum of HQs), cancer risk (CR = EDI × slope factor). The study covers selected sites in a specified state/local government area. The study does not extend to other contaminants (pharmaceuticals, personal care products, microplastics), other exposure pathways (drinking water, soil ingestion, dermal contact), or epidemiological studies (cohort, case-control).
1.10 Definition of Terms
Untreated Sewage Effluent: Wastewater generated from domestic, municipal, and industrial sources that is discharged into the environment without undergoing any treatment processes (primary, secondary, or tertiary treatment) to remove contaminants.
Irrigation: The artificial application of water to farm lands to supplement rainfall and support crop growth.
Farm Land: Land used for agricultural production (crop cultivation).
Soil Physical Properties: Characteristics of soil related to its physical condition, including texture (sand, silt, clay percentages), bulk density (mass per unit volume), porosity (pore space volume fraction), and infiltration rate (rate at which water enters the soil).
Soil Chemical Properties: Characteristics of soil related to its chemical composition and fertility, including pH (acidity/alkalinity), electrical conductivity (salinity), organic matter, total nitrogen, available phosphorus, exchangeable potassium, and heavy metal concentrations (lead, cadmium, chromium, copper, zinc, nickel).
Heavy Metals: Metallic elements with high atomic weight and density that are toxic to living organisms at low concentrations. In this study: lead (Pb), cadmium (Cd), chromium (Cr), copper (Cu), zinc (Zn), nickel (Ni).
Pathogens: Microorganisms that cause disease, including bacteria (faecal coliforms, Salmonella, Vibrio cholerae), viruses, protozoa, and helminths (parasitic worms).
Total Coliform: A group of bacteria used as indicators of faecal contamination; includes faecal coliforms and other coliforms.
Faecal Coliform: A subgroup of coliform bacteria that originate from the intestines of warm-blooded animals; used as an indicator of recent faecal contamination and potential presence of pathogens.
Helminth Eggs: Eggs of parasitic worms (nematodes: Ascaris lumbricoides, Trichuris trichiura, hookworms) that are transmitted through the faecal-oral route; can survive in soil for months.
Crop Growth Parameters: Measures of crop development and yield, including plant height (cm), biomass (dry weight, g/plant), and yield (kg/ha or tons/ha).
Estimated Daily Intake (EDI): The amount of a contaminant (e.g., heavy metal) ingested per day through consumption of contaminated crops; calculated as EDI = (crop heavy metal concentration × daily crop consumption rate) / body weight.
Hazard Quotient (HQ): The ratio of estimated daily intake (EDI) to the reference dose (RfD). HQ < 1 indicates no significant health risk; HQ > 1 indicates potential health risk.
Hazard Index (HI): The sum of hazard quotients for multiple contaminants or multiple exposure pathways. HI < 1 indicates no significant health risk; HI > 1 indicates potential health risk.
Cancer Risk (CR): The probability of an individual developing cancer over a lifetime from exposure to a carcinogenic contaminant; calculated as EDI × slope factor. Acceptable risk is typically <10⁻⁶ (1 in 1,000,000) to 10⁻⁴ (1 in 10,000).
Environmental Pollution Theory: A theory (Mackay, 2019) explaining the transport, fate, and effects of pollutants in the environment (air, water, soil, biota) and the factors affecting their distribution and persistence.
Soil-Plant-Contaminant Interaction Theory: A theory (Alloway, 2018) explaining how contaminants (heavy metals) in soil are taken up by plant roots, translocated to shoots and edible parts, and enter the food chain; bioavailability depends on soil properties (pH, organic matter, clay content).
Risk Assessment Theory: A framework (USEPA, 2020) for evaluating the probability and magnitude of adverse health effects from exposure to environmental contaminants, including hazard identification, dose-response assessment, exposure assessment, and risk characterization.
CHAPTER TWO: LITERATURE REVIEW
2.1 Conceptual Framework
The conceptual framework for this study is organized around the key concepts of untreated sewage effluent, farm land contamination, the mechanisms of impact on soil and crops, and the pathways to human health risk. These concepts are defined, operationalized, and related to one another below.
2.1.1 Concept of Untreated Sewage Effluent
Untreated sewage effluent is wastewater generated from domestic, municipal, and industrial sources that has not undergone any treatment (primary, secondary, or tertiary) to remove contaminants (Metcalf and Eddy, 2019).
Characteristics of Untreated Sewage Effluent:
| Parameter | Typical Range | Significance for Farmlands |
| Biochemical Oxygen Demand (BOD) | 200-600 mg/L | High organic load; depletes soil oxygen |
| Chemical Oxygen Demand (COD) | 400-1,200 mg/L | Indicates organic pollution potential |
| Total Suspended Solids (TSS) | 200-500 mg/L | Clogs soil pores; reduces infiltration |
| Total Nitrogen (N) | 30-100 mg/L | Plant nutrient (excess causes eutrophication) |
| Total Phosphorus (P) | 5-20 mg/L | Plant nutrient (excess causes eutrophication) |
| Potassium (K) | 10-30 mg/L | Plant nutrient |
| Lead (Pb) | 0.1-1.0 mg/L | Toxic to plants, soil microbes, humans |
| Cadmium (Cd) | 0.01-0.1 mg/L | Toxic; accumulates in kidneys |
| Chromium (Cr) | 0.05-0.5 mg/L | Toxic; Cr(VI) is carcinogenic |
| Copper (Cu) | 0.1-2.0 mg/L | Toxic at high levels; essential at low levels |
| Zinc (Zn) | 0.5-5.0 mg/L | Toxic at high levels; essential at low levels |
| Nickel (Ni) | 0.05-0.5 mg/L | Toxic; causes dermatitis |
| Faecal coliforms | 10⁶-10⁹ CFU/100mL | Indicator of faecal contamination; pathogen presence |
| Helminth eggs | 100-1,000 eggs/L | Parasitic infections (Ascaris, Trichuris, hookworm) |
| pH | 6.5-8.5 | Affects nutrient availability and metal solubility |
| Electrical Conductivity (EC) | 1,000-3,000 µS/cm | Salinity risk |
(Source: Metcalf and Eddy, 2019; WHO, 2022)
2.1.2 Impact on Soil Properties
| Property | Effect of Sewage Effluent | Mechanism |
| Texture | Suspended solids add fine particles | Increased silt/clay content |
| Bulk density | Solids fill pore spaces; compaction | Increased bulk density |
| Porosity | Pore spaces blocked by solids | Decreased porosity |
| Infiltration rate | Surface sealing; pore clogging | Decreased infiltration |
| Aggregate stability | Organic matter may improve; salts may disperse | Variable effect |
| Property | Effect of Sewage Effluent | Mechanism |
| pH | Often decreases (acidification) | Nitrification of NH₄⁺ produces H⁺ |
| Electrical Conductivity (EC) | Increases (salinity) | Accumulation of salts (Na⁺, Cl⁻, SO₄²⁻) |
| Organic matter | Increases | Addition of organic waste |
| Total Nitrogen | Increases | Addition of N from sewage |
| Available Phosphorus | Increases | Addition of P from sewage |
| Exchangeable Potassium | Increases | Addition of K from sewage |
| Heavy metals (Pb, Cd, Cr, Cu, Zn, Ni) | Increases | Accumulation from sewage |
| Property | Effect of Sewage Effluent | Mechanism |
| Microbial biomass | May increase (organic matter) or decrease (heavy metals) | Variable effect |
| Total coliform | High levels | Direct contamination from sewage |
| Faecal coliform | High levels | Direct contamination from sewage |
| Helminth eggs | Present | Direct contamination from sewage |
| Beneficial microbes (nitrifiers, decomposers) | May be inhibited by heavy metals | Toxicity |
(Source: Alloway, 2018; Okafor and Nwosu, 2020)
2.1.3 Impact on Crop Growth and Quality
Crop Growth Parameters:
| Parameter | Effect of Sewage Effluent | Mechanism |
| Germination rate | May be reduced | Toxicity from salts, heavy metals |
| Plant height | May be reduced (toxic) or increased (nutrients) | Variable effect |
| Biomass | May be reduced (toxic) or increased (nutrients) | Variable effect |
| Yield | May be reduced (toxic) or increased (nutrients) | Variable effect |
Crop Quality (Contamination):
| Contaminant | Effect | Health Risk |
| Heavy metals (Pb, Cd, Cr, Cu, Zn, Ni) | Accumulate in edible parts | Toxicity; carcinogenicity (Cr) |
| Pathogens (faecal coliforms, Salmonella, helminth eggs) | Contaminate edible parts | Gastroenteritis, typhoid, cholera, parasitic infections |
(Source: Okafor and Ugwu, 2021)
2.1.4 Permissible Limits for Heavy Metals in Soils and Crops
| Heavy Metal | Permissible Limit in Soil (mg/kg) | Permissible Limit in Crops (mg/kg) | Reference |
| Lead (Pb) | 50-100 | 0.1-0.5 | WHO/FAO, 2019; NAFDAC, 2020 |
| Cadmium (Cd) | 1-3 | 0.05-0.2 | WHO/FAO, 2019; NAFDAC, 2020 |
| Chromium (Cr) | 100 | 1.0-2.3 | WHO/FAO, 2019; NAFDAC, 2020 |
| Copper (Cu) | 50-100 | 10-20 | WHO/FAO, 2019; NAFDAC, 2020 |
| Zinc (Zn) | 100-200 | 20-50 | WHO/FAO, 2019; NAFDAC, 2020 |
| Nickel (Ni) | 20-50 | 0.5-1.0 | WHO/FAO, 2019; NAFDAC, 2020 |
(Source: WHO/FAO, 2019; NAFDAC, 2020)
2.1.5 Permissible Limits for Pathogens in Irrigation Water and Crops
| Pathogen Indicator | Permissible Limit for Irrigation (CFU/100mL) | Permissible Limit for Crops (CFU/g) | Reference |
| Faecal coliforms | <1,000 (restricted irrigation); <200 (unrestricted) | <100 | WHO, 2022 |
| Helminth eggs | <1 egg/L | 0 eggs/g | WHO, 2022 |
(Source: WHO, 2022)
2.1.6 Human Health Risk Assessment Parameters
| Parameter | Formula | Interpretation |
| Estimated Daily Intake (EDI) | (C × IR) / BW | C = crop concentration (mg/kg), IR = ingestion rate (kg/day), BW = body weight (kg) |
| Hazard Quotient (HQ) | EDI / RfD | HQ < 1 = no significant risk; HQ > 1 = potential risk |
| Hazard Index (HI) | Σ HQ | HI < 1 = no significant risk; HI > 1 = potential risk |
| Cancer Risk (CR) | EDI × SF | Acceptable risk: <10⁻⁶ to 10⁻⁴ |
(Source: USEPA, 2020)
2.1.7 Conceptual Framework Diagram (Described in Text)
The conceptual framework can be visualized as follows:
Untreated Sewage Effluent (Source) → Environmental Pathway → Impact → Health Risk
Source:
- Untreated sewage effluent (domestic, municipal, industrial)
↓ Environmental Pathway (Farm Land):
- Soil contamination (physical, chemical, biological)
- Crop uptake and contamination
- Pathogen transfer to crops
↓ Impact:
- Soil degradation (reduced fertility, heavy metal accumulation, pathogen contamination)
- Crop contamination (heavy metals, pathogens)
- Reduced crop growth and yield (may be positive from nutrients, negative from toxicity)
↓ Human Exposure:
- Consumption of contaminated crops
↓ Health Risk:
- Heavy metal toxicity (Pb: neurological, Cd: kidney, Cr: cancer, Ni: dermatitis)
- Pathogen infection (gastroenteritis, typhoid, cholera, parasitic infections)
Moderating Variables:
- Duration of sewage irrigation (years)
- Type of crop (leafy vegetables have higher uptake and contamination)
- Soil type (clay soils retain more metals; sandy soils allow more leaching)
- Post-harvest practices (washing, cooking reduce contamination)
The framework posits that untreated sewage effluent is the source of contaminants. These contaminants enter farm land through irrigation, affecting soil properties and crop growth. Crops grown on contaminated soils accumulate heavy metals and pathogens, which are then consumed by humans, leading to health risks.
2.2 Theoretical Framework
This study is anchored on three supporting theories that provide a comprehensive theoretical foundation for understanding the impact of untreated sewage effluent on farm lands. These theories are Environmental Pollution Theory, Soil-Plant-Contaminant Interaction Theory, and Risk Assessment Theory.
2.2.1 Environmental Pollution Theory
Environmental Pollution Theory, developed by Mackay (2019), explains the transport, fate, and effects of pollutants in the environment (air, water, soil, biota) (Mackay, 2019).
- Sources: Pollutants originate from point sources (sewage outfalls, industrial discharges) or non-point sources (agricultural runoff, urban runoff).
- Transport: Pollutants move through environmental media (water, air, soil) via advection (flow), diffusion, and dispersion.
- Fate: Pollutants undergo transformation (biodegradation, hydrolysis, photolysis) or partitioning (adsorption to soil, volatilization to air, uptake by biota).
- Persistence: Some pollutants (heavy metals) are persistent (do not degrade); others (pathogens) have limited persistence outside the host.
- Bioaccumulation: Pollutants accumulate in organisms (uptake from water, soil, food). Heavy metals bioaccumulate in plants and animals.
- Effects: Pollutants cause adverse effects on ecosystems (soil health, plant growth, aquatic life) and human health (toxicity, disease).
Application to Sewage Effluent Impact
Environmental Pollution Theory predicts:
- Heavy metals from sewage effluent are persistent in soil (do not degrade) and accumulate over time.
- Pathogens from sewage effluent have limited persistence (days to weeks) but are continuously replenished by repeated irrigation.
- Transport of contaminants from sewage-irrigated soil to crops occurs via root uptake (heavy metals) and surface contact (pathogens).
2.2.2 Soil-Plant-Contaminant Interaction Theory
Soil-Plant-Contaminant Interaction Theory, developed by Alloway (2018), explains how contaminants (heavy metals) in soil are taken up by plant roots, translocated to shoots and edible parts, and enter the food chain (Alloway, 2018).
- Bioavailability: Not all soil heavy metals are available for plant uptake. Bioavailability depends on soil properties: pH (lower pH increases metal solubility), organic matter (binds metals, reduces availability), clay content (binds metals), and redox potential.
- Uptake mechanisms: Plants take up heavy metals through root cation channels (non-selective) or specific transporters. Essential metals (Cu, Zn) are actively taken up; non-essential metals (Pb, Cd) are passively taken up.
- Translocation: Heavy metals are translocated from roots to shoots via the xylem. Translocation efficiency varies by metal: Cd and Zn are highly mobile; Pb and Cr are less mobile.
- Accumulation: Some plants are accumulators (high uptake, high tolerance); others are excluders (low uptake). Leafy vegetables accumulate more heavy metals than fruit/ grain crops.
- Food chain transfer: Heavy metals in edible plant parts are transferred to humans through consumption, leading to health risks.
Application to Sewage Effluent Impact
Soil-Plant-Contaminant Interaction Theory predicts:
- Soils irrigated with untreated sewage will have elevated heavy metal concentrations.
- Heavy metal uptake by crops will be higher on sewage-irrigated soils than on control soils.
- Leafy vegetables (spinach, amaranth, lettuce) will accumulate higher heavy metal concentrations than fruit/ grain crops.
- Cadmium (Cd) and zinc (Zn) are more mobile and will be found in higher concentrations in edible parts than lead (Pb) and chromium (Cr).
2.2.3 Risk Assessment Theory
Risk Assessment Theory, developed by the USEPA (2020), provides a framework for evaluating the probability and magnitude of adverse health effects from exposure to environmental contaminants (USEPA, 2020).
- Hazard identification: Identify the contaminants of concern (heavy metals, pathogens) and their adverse health effects.
- Dose-response assessment: Determine the relationship between dose (amount of contaminant ingested) and adverse health effects. Reference dose (RfD) is the daily exposure without appreciable risk; slope factor (SF) relates dose to cancer risk.
- Exposure assessment: Estimate the amount of contaminant ingested by humans. Estimated Daily Intake (EDI) = (C × IR) / BW, where C = concentration, IR = ingestion rate, BW = body weight.
- Risk characterization: Calculate Hazard Quotient (HQ = EDI / RfD), Hazard Index (HI = Σ HQ), and Cancer Risk (CR = EDI × SF). Compare to acceptable levels.
Application to Sewage Effluent Impact
Risk Assessment Theory predicts:
- If HQ > 1 or HI > 1 for heavy metals, there is potential non-cancer health risk.
- If CR > 10⁻⁴ (1 in 10,000) for carcinogenic heavy metals (Pb, Cd, Cr), there is potential cancer risk.
- The presence of pathogens (faecal coliforms, Salmonella, helminth eggs) in crops indicates risk of infectious disease.
Integration of the Three Theories
The three theories are complementary and collectively provide a robust theoretical framework for this study:
| Theory | Focus | Contribution to Study |
| Environmental Pollution Theory | Transport, fate, and effects of pollutants in environment | Explains how sewage contaminants (heavy metals, pathogens) move from source to farm land |
| Soil-Plant-Contaminant Interaction Theory | Uptake of heavy metals by plants | Explains how heavy metals in soil are taken up by crops and accumulate in edible parts |
| Risk Assessment Theory | Human health risk from contaminant exposure | Provides framework for estimating health risk (HQ, HI, CR) from consumption of contaminated crops |
Together, these theories support the study’s evaluation of the impact of untreated sewage effluent on farm lands, recognizing that: (1) contaminants from sewage enter the environment and persist (Environmental Pollution); (2) heavy metals are taken up by crops and accumulate in edible parts (Soil-Plant-Contaminant Interaction); and (3) consumption of contaminated crops poses health risks to humans (Risk Assessment).
2.3 Review of Related Empirical Studies
This section reviews empirical studies relevant to the impact of untreated sewage effluent on farm lands.
2.3.1 Studies on Soil Contamination from Sewage Effluent (Nigeria)
Adebayo and Ogunyemi (2020) studied the impact of sewage effluent irrigation on soil properties in Oyo State. Using a comparative design (sewage-irrigated vs. control sites), they found: sewage-irrigated soils had higher pH (7.2 vs. 6.5), higher electrical conductivity (2,500 vs. 500 µS/cm), higher organic matter (3.5% vs. 1.2%), higher total N (0.25% vs. 0.08%), and elevated heavy metals (Pb: 45 mg/kg vs. 12 mg/kg; Cd: 2.5 mg/kg vs. 0.3 mg/kg; Cr: 30 mg/kg vs. 8 mg/kg; Cu: 50 mg/kg vs. 15 mg/kg; Zn: 120 mg/kg vs. 30 mg/kg; Ni: 25 mg/kg vs. 5 mg/kg). The study concluded that sewage irrigation degrades soil quality.
Eze and Nweze (2019) studied heavy metal accumulation in soils irrigated with sewage effluent in Enugu State. Soil samples (0-20 cm) were collected from three sewage-irrigated sites and one control site. Heavy metal concentrations (mg/kg): Pb (52 vs. 10), Cd (3.2 vs. 0.2), Cr (35 vs. 6), Cu (55 vs. 12), Zn (130 vs. 25), Ni (28 vs. 4). All heavy metals exceeded permissible limits for agricultural soils. The study recommended that farmers stop using untreated sewage.
2.3.2 Studies on Crop Contamination from Sewage Effluent (Nigeria)
Okafor and Nwosu (2020) studied heavy metal accumulation in vegetables irrigated with sewage effluent in Edo State. Vegetable samples (amaranth, spinach, lettuce) were collected from sewage-irrigated farms and control farms. Heavy metal concentrations in vegetables (mg/kg fresh weight): Pb (0.45-0.85 vs. 0.05-0.10), Cd (0.12-0.25 vs. 0.01-0.03), Cr (0.35-0.65 vs. 0.05-0.10), Cu (5.5-12.5 vs. 2.0-3.5), Zn (25-45 vs. 8-15), Ni (0.25-0.55 vs. 0.05-0.10). Pb, Cd, and Cr exceeded permissible limits (0.1-0.5, 0.05-0.2, 1.0-2.3 mg/kg respectively). The study concluded that vegetables from sewage-irrigated farms are unsafe for consumption.
Okafor and Ugwu (2021) studied pathogen contamination of vegetables irrigated with sewage effluent in Anambra State. Vegetable samples (lettuce, cabbage, amaranth) were analyzed for faecal coliforms and helminth eggs. Faecal coliform counts (CFU/g): 10⁴-10⁶ (sewage-irrigated) vs. 10²-10³ (control). Helminth eggs: present in 60% of sewage-irrigated samples (Ascaris, Trichuris). The study concluded that vegetables from sewage-irrigated farms pose a high risk of pathogen infection.
2.3.3 Studies on Health Risk Assessment (Nigeria)
Okonkwo (2020) conducted a health risk assessment for heavy metals in vegetables from sewage-irrigated farms in Cross River State. Estimated Daily Intake (EDI) values for Pb, Cd, and Cr exceeded reference doses (RfD). Hazard Quotients (HQ) were >1 for Pb (2.5), Cd (3.2), and Cr (1.8). Hazard Index (HI) was 7.5 (>1). Cancer Risk (CR) for Cr (carcinogenic) was 2.5 × 10⁻⁴ (>10⁻⁴ acceptable limit). The study concluded that consumption of vegetables from sewage-irrigated farms poses significant non-cancer and cancer health risks.
2.3.4 Studies on Remediation Options (International)
| Study | Country | Remediation Method | Effectiveness |
| Sharma et al. (2019) | India | Phytoremediation (water hyacinth, duckweed) | 70-90% heavy metal removal |
| Khan et al. (2020) | Pakistan | Soil amendments (lime, biochar, compost) | 30-60% reduction in bioavailability |
| Liu et al. (2021) | China | Washing and cooking vegetables | 50-80% reduction in pathogens and heavy metals |
2.3.5 Summary of Empirical Findings
The empirical literature reveals consistent findings: (1) sewage irrigation significantly increases heavy metal concentrations in soil (Pb, Cd, Cr, Cu, Zn, Ni); (2) crops grown on sewage-irrigated soils accumulate heavy metals, often exceeding permissible limits; (3) pathogens (faecal coliforms, helminth eggs) contaminate crops; (4) health risk assessment indicates significant non-cancer and cancer risks from consumption; (5) most Nigeria studies are limited to single states. This study addresses these gaps.
2.4 Summary of Literature Review
The table below summarizes key theoretical and empirical literature relevant to the impact of untreated sewage effluent on farm lands.
| Author(s) and Year | Focus of Study | Strength | Weakness | Limitation | Gap Identified |
| Mackay (2019) | Environmental Pollution Theory | Explains transport, fate, effects | Complex models | General theory | Application to sewage needed |
| Alloway (2018) | Soil-Plant-Contaminant Interaction Theory | Explains uptake, translocation, accumulation | Requires many parameters | General theory | Application to sewage needed |
| USEPA (2020) | Risk Assessment Theory | Framework for health risk assessment | Requires RfD, SF values | General theory | Application to sewage needed |
| Adebayo and Ogunyemi (2020) | Soil contamination (Oyo State) | Heavy metals elevated in sewage-irrigated soils | Single state | Geographic gap | Multi-state study needed |
| Eze and Nweze (2019) | Heavy metals in soils (Enugu State) | Pb, Cd, Cr, Cu, Zn, Ni exceeded limits | Single state | Geographic gap | Multi-state study needed |
| Okafor and Nwosu (2020) | Heavy metals in vegetables (Edo State) | Pb, Cd, Cr exceeded limits | Single state | Geographic gap | Multi-state study needed |
| Okafor and Ugwu (2021) | Pathogens in vegetables (Anambra State) | Faecal coliforms, helminth eggs present | Single state | Geographic gap | Multi-state study needed |
| Okonkwo (2020) | Health risk assessment (Cross River State) | HQ >1, HI >1, CR >10⁻⁴ | Single state | Geographic gap | Multi-state study needed |
| Sharma et al. (2019) | Phytoremediation (India) | 70-90% heavy metal removal | India, not Nigeria | Geographic gap | Nigeria study needed |
| Khan et al. (2020) | Soil amendments (Pakistan) | 30-60% bioavailability reduction | Pakistan, not Nigeria | Geographic gap | Nigeria study needed |
| WHO/FAO (2019) | Permissible limits | International standards | Not Nigeria-specific | Geographic gap | Nigeria compliance needed |
| NAFDAC (2020) | Permissible limits (Nigeria) | National standards | Not enforced | Enforcement gap | Monitoring needed |
