Treatment Evaluation of Aluminium Can Manufacturing Wastewater

316 Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 p ISSN: 2635-3342; e ISSN: 2635-3350 Original Research Article Treatment Evaluation of Aluminium Can Manufacturing Wastewater *1Ekebafe, O.L., 1Uduma, A., 2Mahmud, H. and 2Nworie, E.C. 1Department of Chemistry, Faculty of Science, University of Lagos, Lagos, Nigeria. 2Department of Chemical Engineering Technology, Auchi Polytechnic, Auchi, Nigeria. *[email protected] http://doi.org/10.5281/zenodo.18061001 ARTICLE INFORMATION ABSTRACT Article history: Received 27 Jul. 2025 Revised 15 Sep. 2025 Accepted 02 Oct. 2025 Available online 30 Dec. 2025 Untreated aluminium can manufacturing effluent can threaten both the environment and public health. This study assessed the physicochemical and biological characteristics of wastewater from the rinse–mobility sections of two aluminium packaging plants, referred to as alpha and beta, and evaluated the effectiveness of their treatment processes. Representative samples were collected before and after treatment and analysed for potential of hydrogen (pH), total suspended solids (TSS), chemical oxygen demand (COD), biochemical oxygen demand over five days (BOD₅), nutrients, heavy metals, and microbial presence using standard procedures. Posttreatment, pH values approached neutrality: 6.25 at alpha and 6.62 at beta, both within the discharge limits of the National Environmental Standards and Regulations Enforcement Agency (NESREA) (6.0–9.0) and the Ogun State Environmental Protection Agency (OGEPA) (6.5–8.5). BOD₅ values were below the permissible 30 mg/L, at 18 mg/L for alpha and 22 mg/L for beta. COD varied: alpha recorded 80 mg/L, exceeding OGEPA’s limit (50 mg/L) but within NESREA’s allowance (250 mg/L), while beta achieved 70 mg/L. TSS reductions were notable, at 12 mg/L (alpha) and 6 mg/L (beta). Both plants achieved compliance for iron, zinc, copper, chromium (VI), nickel, and aluminium, with final aluminium levels of 0.05 mg/L (alpha) and 0.06 mg/L (beta). Microbial loads also fell significantly, with bacteria reduced from 10⁶ to 10³ colony-forming units per millilitre (CFU/mL) at alpha and from 10³ to 10² CFU/mL at beta, alongside similar yeast reductions. Overall, treatment substantially improved effluent quality, though further optimisation is required to achieve full compliance and ensure environmental safety. © 2025 RJEES. All rights reserved. Keywords: Aluminium Can manufacturing Pollutants Treatment Wastewater 1. INTRODUCTION The aluminium can-making business is very vital to the world packaging market and to the beverage market in particular. Aluminium is used largely in foil form for packaging at a thickness of just over 6 to about 150 microns, and one of the most important properties of aluminium as a packaging material 317 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 is its inertness compared to most metals (Kamalesh et al., 2024). The aluminium industry is one of the largest sectors, and wastewater generated from this industry could cause crucial environmental problems due to high heavy metal concentration and conductivity (Kaya et al., 2022). The aluminium can manufacturing industry faces multifaceted challenges regarding wastewater management that are increasingly critical in today's environmentally conscious climate. Aluminium is one of the most widely used metals due to its high strength, corrosion resistance, heat and electrical conductivity (Kaya et al., 2022). One of the foremost issues is the generation of wastewater that is often laden with various contaminants, including oils, heavy metals, surfactants, and residual chemicals from the surface treatment processes. These contaminants are not only hazardous to aquatic ecosystems; still, they can also pose significant risks to human health if they enter the water supply or are released into the environment without proper treatment (Hu et al., 2019; Silva, 2023). Many current wastewater treatment practices in the aluminium manufacturing sector are outdated and insufficient to handle the complexities of modern production processes (Kaya et al., 2022). Traditional methods such as sedimentation and basic biological treatment may be ineffective in meeting regulatory standards due to their inability to adequately remove specific contaminants like heavy metals and oils (Oladimeji et al., 2024). As a result, numerous manufacturers are struggling to comply with stringent environmental regulations that govern effluent discharge. This non-compliance can lead to serious repercussions, including hefty fines, legal challenges, and negative public perception. Furthermore, the industry is under pressure to adopt more sustainable practices in light of global efforts to reduce waste and promote recycling. Wastewater management is a crucial component of sustainability, yet many facilities lack the necessary infrastructure or technologies to effectively treat wastewater, resulting in significant environmental impacts. The existing treatment systems often produce secondary waste, which requires additional processing and can complicate overall waste management strategies (Rahimi et al., 2022). This study sets its sights on analysing the wastewater generated by aluminium can plants in southern Nigeria and assessing the effectiveness of their wastewater treatment practices in compliance with relevant environmental regulations. 2. MATERIALS AND METHODS 2.1. Materials and Sample Collection Wastewater samples were collected from two aluminium can manufacturing plants in Ogun State, designated Alpha and Beta. The plant's facility generates wastewater from distinct stages of the can washing process, namely the pre-wash to drag-out section and the rinse to mobility section. However, in line with internal operational protocols and for this study, only the rinse–mobility wastewater stream and the final treated effluent were selected for laboratory analysis. Two representative samples were collected: one untreated sample from the rinse–mobility section before treatment, and one treated effluent sample from the final discharge point after treatment. Samples were collected in pre-washed polyethylene containers, properly labelled to indicate sampling points, and transported under ice-cooled conditions to the laboratory for immediate analysis. Parameters were assessed in accordance with APHA Standard Methods (2017). The analysis of the wastewater parameters was performed using instruments and test kits such as a UV Spectrophotometer (HACH: DR3900), pH Meter (OHAUS; Starter 3000), Conductivity/TDS Tester (HACH: Pocket Pro™+ Tester); HACH Hardness Test Kit, HACH COD Test Kit, and HACH BOD Test Kit. 2.2. Analysis of the Wastewater Samples Standard procedures according to the American Society for Testing and Materials (ASTM) and American Public Health Association (APHA) methods of analysis were adopted in the analysis of the wastewater samples. The appearance and colour of wastewater provide preliminary information about its quality, level of contamination, and potential treatment needs. Visual evaluation and industrial wastewater Method 2120 C (Spectrophotometric Method) of the American Public Health Association (APHA) et al., (2017) was used. Odour determination is a critical sensory test used to assess water and wastewater for the presence of volatile pollutants such as hydrogen sulphide, phenols, or industrial effluents. ASTM D1292-15 (2021) was used to determine the threshold at which odour is perceptible (ASTM International, 2021). Temperature is a critical 318 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 parameter in water and wastewater analysis. The procedure followed the APHA Standard Methods for the Examination of Water and Wastewater (20th ed.), Method 2550 B, for field and laboratory temperature determination (APHA, 2017). The electrical conductivity (EC) determination method follows ASTM D1125, which outlines procedures for both static (non-flowing) and continuous (in-line) measurement of water conductivity (ASTM International, 1999). The pH of water and wastewater was determined using ASTM D1293-12, Test Method A, which uses electrometric measurement with a glass electrode under controlled laboratory conditions (ASTM International, 2012). Total Suspended Solids (TSS), also referred to as non-filterable matter, was determined using ASTM D5907, Test Method A, which provides standardised procedures for the gravimetric determination of TSS in aqueous samples (ASTM International, 2018). TSS (mg/L) was calculated using Equation 1: TSS (mg/L) = 𝑀𝑎𝑠𝑠 𝑜𝑓𝑓𝑖𝑙𝑡𝑒𝑟 + 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑓𝑖𝑙𝑡𝑒𝑟 𝑆𝑎𝑚𝑝𝑙𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 (𝑚𝐿) × 1000 (1) Total Dissolved Solids (TDS), also referred to as filterable matter, was determined using ASTM D5907, Test Method A, which prescribes a gravimetric approach for TDS determination (ASTM International, 2018). TDS (mg/L) was calculated using Equation 2: TDS (mg/L) = 𝑀𝑎𝑠𝑠 𝑜𝑓𝑑𝑖𝑠ℎ + 𝑟𝑒𝑠𝑖𝑑𝑢𝑒 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑖𝑠ℎ 𝑆𝑎𝑚𝑝𝑙𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 (𝑚𝐿) ×1000 (2) Chemical oxygen demand (COD) was determined as the amount of oxygen required to chemically oxidise organic and inorganic matter in water using a strong oxidant under acidic conditions, following ASTM D1252-06 (2006), Test Method A. This method employs a macro reflux digestion step followed by titration to quantify COD levels. COD (mg/L) was calculated using Equation 3: COD (mg/L) = (𝐴−𝐵) × 𝑁 − 8000 𝑆𝑎𝑚𝑝𝑙𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 (𝑚𝐿) (3) Where A represents the volume of titrant for blank (mL), B represents the volume of titrant for sample (mL), while N represents the normality of ferrous ammonium sulphate Biochemical Oxygen Demand (BOD5) is a measure of the amount of oxygen consumed by microorganisms in the biological degradation of organic matter over a five-day incubation period. The procedure follows APHA Standard Methods 5210 B (APHA, 2017), which standardises the dilution and seeding techniques necessary for reproducible BOD results. When nitrification is inhibited using specific chemicals, the result is reported as CBOD5 (Carbonaceous BOD₅). Calculate BOD₅ using Equation 4: BOD5 (mg/L) = (𝐷𝑂𝑖− 𝐷𝑂𝑓)−𝐵 (4) Where 𝐷𝑂𝑖 represents the DO of the diluted sample immediately after preparation, mg/L, 𝐷𝑂𝑓 represents the DO of the diluted sample after 5 days of incubation at 20 °C, mg/L, and B represents the DO depletion in seeded dilution water (blank). If a dilution factor (D) is applied, then BOD5 (mg/L) is calculated using Equation 5: BOD5 (mg/L) = ((𝐷𝑂𝑖− 𝐷𝑂𝑓)−𝐵)×𝐷 (5) Nitrate (NO3⁻) is a naturally occurring form of nitrogen that originates from the oxidation of nitrite or ammonium compounds. The method follows ASTM D3867-16, which outlines the determination of nitrate and nitrite nitrogen using cadmium reduction and colorimetric detection (ASTM International, 2016). 319 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 Nitrate (NO3-N) = Total (NO3 + NO2-N) – Nitrite (NO2-N) (6) Nitrite (NO2⁻) is an intermediate compound in the nitrogen cycle and is often indicative of partial nitrification or contamination from industrial or agricultural sources. The procedure follows ASTM D3867-16, which prescribes a colorimetric method involving diazotization without cadmium reduction (ASTM International, 2016). Phosphate concentration in the wastewater samples was determined using APHA Standard Method 4500P.E – Ascorbic Acid Method (APHA et al., 1999). Total phosphorus (mg/L) was calculated using Equation 7: Total phosphorus (mg/L) = 𝐴𝑏𝑠𝑜𝑟𝑏𝑎𝑛𝑐𝑒 × 𝐶𝑎𝑙𝑖𝑏𝑟𝑎𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 (7) Aluminium (Al) content was determined using ASTM D857-12, which utilises direct flame atomic absorption spectrophotometry to determine aluminium concentrations in the range of 0.5–5.0 mg/L without dilution (ASTM International, 2012). The Aluminium (Al) content in the wastewater samples was calculated using Equation 8: Al (mg/L) = 𝐶 × 100 𝑉 ⁄ (8) Where C represents the concentration from the calibration curve (mg/L) and V represents the volume of the aliquot taken (mL). Iron is commonly present in natural and treated water systems, either in dissolved or particulate form. The procedure described in ASTM D1068, Test Method A, was used for the determination of total and dissolved iron in the wastewater samples using Atomic Absorption Spectrophotometry (AAS). For diluted samples, iron concentration (mg/L) is calculated using Equation 9: Iron (mg/L) = 𝐼𝑛𝑠𝑡𝑟𝑢𝑚𝑒𝑛𝑡 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 × 𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 (9) Metals, including zinc (Zn), copper (Cu), chromium (Cr), and lead (Pb) were determined using flame atomic absorption spectrophotometry (AAS). The procedures followed the respective ASTM standards: zinc in accordance with ASTM D1691-02 (ASTM International, 2007), copper following ASTM D1688-02, Test Method A (ASTM International, 2002), chromium according to ASTM D1687, Test Method B (ASTM International, 1992), and lead in accordance with ASTM D3559, Test Method A (ASTM International, 2008). For diluted samples in each case, concentrations were calculated using Equation 10: Metal (mg/L) = 𝐼𝑛𝑠𝑡𝑟𝑢𝑚𝑒𝑛𝑡 𝑟𝑒𝑎𝑑𝑖𝑛𝑔 × 𝐷𝑖𝑙𝑢𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 (10) Oil and grease content represents a diverse group of substances, including hydrocarbons, fats, waxes, and oils, which are extractable in organic solvents. The procedure followed the APHA Standard Methods for the Examination of Water and Wastewater (20th ed.), Method 5520 B, using a partition-gravimetric approach for quantifying oil and grease content in water and wastewater (APHA et al., 1999). The oil and grease content in wastewater is determined gravimetrically using Equation 11: Oil and Grease (mg/L) = [(𝐴−𝐵) ×1000] 𝑉 (11) Where A represents the weight of residue in the sample extract (mg), B represents the weight of residue in the solvent blank (mg), and V represents the volume of sample extracted (mL). Microbial Diversity (Fungi and Bacteria Presence) determination procedure follows manufacturer guidance from ChemAqua Technical Bulletin 2-001 (2018) and Palintest TTC/M Dip slides Method BACT.3. 2.3. Plant Alpha Influent Collection and Storage 320 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 The process begins with the collection of wastewaters from the production floor (Figure 1). Wastewater generated at the plant is first collected in 2 different kinds of holding tanks, each kind corresponding to a different section of the production line. • Tank 1 receives water from the Pre-wash to Drag-out stages of the washer, which is typically acidic and contains emulsified oil. To moderate the acidity, fresh water is added to raise the pH. An acidic compound is introduced to help with demulsification. This helps in breaking the oil-water emulsion, causing the oil to separate and float to the surface while the water settles at the bottom. The oil layer is skimmed off and treated as waste. This stream is sent to the local regulatory authority for further handling. • Tanks 2 contains effluent from the Rinse 1 to Mobility stages of the production line. This wastewater typically has a higher pH and is more chemically stable, thus requiring no initial adjustment. These tanks serve as holding vessels where the wastewater is allowed to stabilise before further treatment begins. Figure 1: An overview of Plant Alpha wastewater treatment process 2.3.1. Plant treatment procedure Oil-water separation To separate oil from the water, a chemical agent (demulsifier) is added. This demulsifier breaks the oil-water emulsion, enabling the oil to separate and float to the surface. The separated oil is then removed to protect subsequent treatment processes from oil contamination. It also helps to improve the efficiency of subsequent chemical treatments. Stage 1: pH adjustment and precipitation with hydrated lime After oil removal, the wastewater enters the first chemical treatment stage where hydrated lime (calcium hydroxide (Ca(OH)₂)) is dosed into the system. This alkaline compound performs dual functions. It adjusts the pH of the water to an optimal neutral-to-slightly-alkaline range (around pH 7 to 9) and facilitates the precipitation of dissolved metals and impurities. Many metal ions form insoluble hydroxides at elevated pH, which then settle out as solids. These solids are suspended in the water temporarily and will be removed in later stages. Stage 2: Coagulation and flocculation in a multi-compartment reaction tank In this stage, the partially treated water flows through a three-compartment reaction tank, each section facilitating a different part of the coagulation-flocculation process, which is essential for removing fine 321 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 particles and colloids. In the first compartment, aluminium sulphate (alum) is added to the wastewater as a primary coagulant. The alum destabilizes negatively charged particles in the water by neutralizing their surface charges, causing them to clump together and form tiny aggregates (micro-flocs). In the second compartment, a high molecular weight polymeric flocculant is introduced. This flocculant acts like a molecular net, bridging the micro-flocs together into larger, heavier flocs that are easier to settle out. In the third compartment, an additional coagulant polymer is dosed to stabilize the flocs and enhance their strength, ensuring they don’t disintegrate as they pass into the clarification unit. This final mixing phase also helps in capturing any residual particles or emulsions left over from earlier stages. This stepwise approach maximizes particle aggregation and prepares the wastewater for efficient separation in the next stage. Stage 3: Water clarification The chemically treated effluent is now directed into a clarifier, where gravity separation occurs. The heavy flocs formed during coagulation and flocculation settle to the bottom of the tank, forming a sludge layer, while the clarified water rises to the top. This upper layer of water is collected and subjected to quality testing, checking for key indicators like pH, chemical oxygen demand (COD), and total suspended solids (TSS) to ensure it meets environmental discharge standards. Stage 4: Sludge dewatering The sludge settled at the bottom of the clarifier is transferred to a filter bed press, where pressure is applied to extract as much water as possible. This dewatering process results in two streams: The removed water (filtrate) which may be recycled back into the treatment system or treated further if necessary. A solid sludge cake, which is typically reused in non-hazardous applications such as road base materials or sent to landfill under controlled conditions. This stage reduces waste volume significantly and ensures that solid waste handling is both efficient and environmentally safe. 2.4. Plant Beta Wastewater Treatment Process The treatment process is designed to manage oil-laden acidic wastewater and rinse water with varying pH levels, progressing through oil-water separation, pH neutralisation, flocculation, and solid-liquid separation via filtration and sludge dewatering. 2.4.1. Plant Beta influent collection and segregation Wastewater generated at the plant is first collected in two separate holding tanks, each corresponding to a different section of the production line. • Tank 1 receives water from the Pre-wash to Drag-out stages, which is typically acidic and contains emulsified oil (Figure 2). To moderate the acidity, fresh water is added to raise the pH. An acidic compound is introduced to help with demulsification. This process facilitates the breakdown of the oil–water emulsion, allowing the oil to separate and float to the surface while the water settles below. The oil layer is skimmed off and treated as waste. • Tank 2 contains effluent from the Rinse 1 to Mobility stages of the production line. This wastewater typically has a higher pH and is more chemically stable, thus requiring no initial adjustment 2.4.2. Combined lime treatment After the initial separation and pH balancing in the individual tanks, wastewater from both tanks is pumped into the same treatment tank for chemical conditioning. Here, hydrated lime (Ca(OH)₂) is added. The lime serves multiple functions: i. It adjusts the overall pH of the mixture to a neutral or slightly alkaline range (typically pH 7–9), and ii. It promotes the precipitation of dissolved metals and other impurities. These contaminants convert into insoluble hydroxides that can be separated from the water in later stages. 322 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 Figure 2: Overview of Plant Beta wastewater treatment process 2.4.3. Flocculation and secondary lime dosing The lime-treated effluent flows into a flocculation chamber, where a polyacrylamide-based flocculant is added along with additional lime. The flocculant helps bind together fine suspended particles, colloids, and precipitated contaminants, forming larger and more settleable aggregates known as flocs. The secondary dose of lime continues to support pH control and aids in further destabilising emulsified materials. This combined chemical conditioning maximises particle aggregation, making it easier to filter the contaminants in the subsequent stage. 2.4.4. Filtration through filter beds The flocculated water is passed through a set of filter beds, which serve as a physical barrier to trap and remove remaining suspended solids. These beds consist of layers of sand, gravel, or other porous media designed to improve water clarity and reduce the load on the final dewatering step. The solid waste (sludge) collected from this process during flocculation and filtration is directed into a filter press (bed press). This mechanical device applies pressure to the sludge, squeezing out the liquid and leaving behind a compact sludge cake. The dewatered sludge is usually collected by a government agency and repurposed for pot-hole filling during road construction and maintenance operations. The separated filtrate (water) is usually subjected to final quality testing before safe discharge. 3. RESULTS AND DISCUSSION 3.1. Analysis of Water Samples from Plant Alpha 3.1.1. Analysis of untreated wastewater samples The results of the physicochemical and biological analysis conducted on the untreated wastewater sample collected from the Rinse–Mobility section of the can washer are presented in Table 1. This sample represents wastewater prior to entry into the plant’s internal treatment system. The untreated wastewater sample collected from the Rinse–Mobility section of the can washer at Alpha displayed characteristics typical of a moderately polluted industrial effluent. Although this stream is expected to carry less organic and metal load than the prewash–drag-out stage, the results clearly indicate the presence of acidic, metal-laden, and biologically active wastewater, making in-house treatment essential. One of the most critical findings was the highly acidic nature of the sample, with a pH of 2.79. This falls 323 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 significantly outside the acceptable regulatory ranges of 6.0–9.0 (NESREA, 2009) and 6.5–8.5 (OGEPA, 2023). Such acidity can pose environmental hazards if released untreated, including altering the natural pH of receiving waters, harming aquatic life, and corroding sewer infrastructure. Table 1: Untreated wastewater characteristics – Plant Alpha compared with the NESREA and OGEPA regulatory limits Parameter Unit Untreated (Rinse - Mobility) NESREA Limit OGEPA Limit Appearance - Hazy, colourless-white — — Odour - Characteristic — — Turbidity NTU 52 - - Temperature (°C) ℃ 26 Ambient +3 Ambient ±3 Electrical Conductivity 𝜇𝑆/𝑐𝑚 1,600 - 1,200 pH - 2.79 6.0–9.0 6.5–8.5 Total Suspended Solids (TSS) (mg/L) 232 50 25 Total Dissolved Solids (TDS) (mg/L) 960 - 1,200 Chemical Oxygen Demand (COD) (mg/L) 720 250 50 Biological Oxygen Demand (BOD5) (mg/L) 35 30 30 Ammonia (mg/L) 3.0 20.0 - Nitrate (NO3-) (mg/L) 3.8 5.0 (Total N) 100 (Total N) Nitrite (NO2-) (mg/L) 33 5.0 (Total N) 100 (Total N) Phosphate (PO43-) (mg/L) 0.04 1.0 30 (to sewer) Aluminium (Al) (mg/L) 6.2 0.1 - Iron (Fe) (mg/L) 2.86 0.2 — Zinc (Zn) (mg/L) 0.06 1.0 0.5 Copper (Cu (mg/L) 0.98 3.0 1.0 Chromium (Cr6+) (mg/L) 0.131 0.1 0.05 Nickel (Ni) (mg/L) 0.01 0.2 0.3 Lead (Pb2+) (mg/L) 0 0.01 0.01 Cadmium (mg/L) 0 0.01 0.01 Mercury (mg/L) 0.002 0.5 0.005 Oil and Grease (mg/L) (mg/L) 2 10 10 Surfactant Conc. (mg/L) Not Determined - 15 (to sewers) Phenolic Compounds (mg/L) Not Determined - 0.001 Cyanide (mg/L) Not Determined 0.2 0.2 Bacteria CFU/mL 106 - - Yeast CFU/mL 105 - - Note: “–” indicates not regulated or data not available. Source: NESREA (2009); OGEPA (2023) The organic pollution load was also substantial. The chemical oxygen demand (COD) was 720 mg/L, while the biochemical oxygen demand (BOD₅) was 35 mg/L. Both values exceed the discharge limits of 250 mg/L (COD) and 30 mg/L (BOD₅) stipulated by NESREA, and far exceed the 50 mg/L COD limit imposed by the OGEPA. These results suggest the presence of non-biodegradable and oxygen-depleting substances, likely residual cleaning agents or organic process chemicals, which pose serious threats to receiving water bodies by promoting oxygen depletion and secondary pollution (Kaya et al., 2022). 324 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 The electrical conductivity of the sample was measured at 1,600 µS/cm, which is above OGEPA’s permissible limit of 1,200 µS/cm, indicating a significant concentration of dissolved ionic substances. This value remains within OGEPA’s TDS limit of 1,200 mg/L, but it does suggest a high dissolved ion load that could influence conductivity, corrosion potential, and the overall salinity profile of the effluent. Elevated conductivity is typical of aluminium finishing operations due to the presence of sulphates, nitrates, and metal ions in rinse water (APHA, 2017; Chaubey, 2021). On the other hand, the total suspended solids (TSS) concentration was 232 mg/L, which is nearly five times above the NESREA limit (50 mg/L) and almost ten times the OGEPA threshold (25 mg/L), pointing to a significant load of particulate matter in the rinse water. The sample also showed contamination by toxic metals, most notably aluminium, iron, and chromium (VI). The concentration of aluminium was 6.00 mg/L, which is 60 times higher than the NESREA limit of 0.1 mg/L. This likely originates from contact with aluminium substrates during the rinsing stage. Iron was recorded at 2.86 mg/L, also significantly exceeding the NESREA limit of 0.2 mg/L, and indicative of metal surface oxidation or corrosion within the process line. The level of chromium (VI) was 0.131 mg/L, surpassing both the NESREA threshold of 0.1 mg/L and the more stringent OGEPA limit of 0.05 mg/L. This is a serious concern due to the high toxicity and carcinogenic nature of hexavalent chromium in aquatic environments. Although lead and cadmium were both reported as 0.00 mg/L, the method detection limits were not specified. In the absence of that information, it cannot be ruled out that trace levels are present, especially since these metals are commonly associated with industrial and metal-finishing processes and are highly toxic even at very low concentrations (Zhang et al., 2023). While concentrations of zinc (0.06 mg/L), copper (0.98 mg/L), and nickel (0.01 mg/L) remained within permissible limits, the overall presence of heavy metals remains significant, particularly due to cumulative environmental effects. The biological quality of the wastewater was equally concerning. High microbial loads were observed, with bacteria recorded at 1.0 × 10⁶ CFU/mL and yeast at 1.0 × 10⁵ CFU/mL. These figures indicate a biologically active effluent, likely supported by the high BOD₅ and nutrient availability, which presents a risk of pathogen transmission and further biological pollution if discharged untreated (Ali et al., 2017). The untreated Rinse–Mobility wastewater displays significant deviations from discharge limits for key parameters such as pH, COD, BOD₅, TSS, electrical conductivity, aluminium, iron, chromium (VI), and microbial counts. This confirms that the effluent is not safe for direct discharge and strongly supports the need for a comprehensive and targeted treatment process to ensure regulatory compliance and protect the receiving environment 3.1.2. Analysis of treated wastewater samples from Plant Alpha The results of the physicochemical and biological analysis conducted on the treated wastewater sample are presented in Table 2. This sample was collected at the final discharge point of the in-house treatment facility, after undergoing pH adjustment, oil-water separation, chemical coagulation, and sludge removal. The pH of the treated effluent was 6.25, which falls within the discharge limits set by both NESREA (6.0– 9.0) and OGEPA (6.5–8.5). Total suspended solids (TSS) were reduced to 12 mg/L, representing a substantial reduction from the untreated value of 232 mg/L, and meeting both NESREA (50 mg/L) and OOGEPA (25 mg/L) limits. The biological oxygen demand (BOD₅) was brought down to 18 mg/L, well below the 30 mg/L limit prescribed by both regulatory agencies. Importantly, the chemical oxygen demand (COD) was reduced to 80 mg/L, which is well within the NESREA limit of 250 mg/L, though still above the OGEPA threshold of 50 mg/L. This indicates a significant reduction in the overall organic load, with most oxidisable substances effectively removed. The remaining COD may reflect low levels of refractory compounds, but the overall organic pollution burden has been considerably diminished (Chaubey, 2021). Regarding dissolved substances, the electrical conductivity (EC) was recorded at 50.2 μS/cm, and the total dissolved solids (TDS) at 30.1 mg/L was comfortably below OGEPA’s TDS limit of 1,200 mg/L. These figures suggest a relatively low dissolved ion content in the effluent and support the effectiveness of the treatment process in removing ionic and inorganic constituents. The removal of nutrients was also highly effective. Nitrate and phosphate were reduced to 2.7 mg/L and 0.04 mg/L, respectively, well below their regulatory thresholds. However, nitrite remained at 8 mg/L, which exceeds NESREA’s total nitrogen threshold of 5 mg/L. This suggests incomplete nitrification, possibly due to inhibited microbial conversion 331 O.L. Ekebafe et al. / Nigerian Research Journal of Engineering and Environmental Sciences 10(2) 2025 pp. 316-331 ASTM International. (2002). ASTM D3223-02: Standard test method for total mercury in water. West Conshohocken, PA: ASTM International. ASTM International. (2002). ASTM D1688-02: Standard test methods for copper in water. West Conshohocken, PA: ASTM International. ASTM International. (2006). 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