The wastewater from a fertilizer company was sampled in Epe, Lagos State, in southwest Nigeria. Two closed basins containing fertilizer wastewater at various concentrations were chosen for the collection of effluent samples. In addition to the outfall basin (Sample 2) because of the potential effects on aquatic life once it entered the lagoon, samples were also taken from the final discharge basin (Sample 1) because it comprises effluent from both the equalization basin and cooling tower blowdown. There is a fishing settlement along its banks.

The analytical procedures used to determine these parameters have been modified from Theoretical Aspects of Laboratory Analysis in 1992, Guidelines and Criteria for Water Quality Management in Ontario in 1967, the Laboratory Manual on Soil and Plant Analysis in 1995, and A.O.A.C. (Association of Analytical Chemists, Official Methods of Analysis, 1990). The wastewater quality characteristics of these effluents were measured, including temperature, total suspended solids (TSS), urea, total nitrogen, NH₃, electric conductivity, color, nitrate, total dissolved solids (TDS), turbidity, residual chlorine, total phosphorus, sulphates, pH, and heavy metals.

The biological treatment of fertilizer effluent made it possible to install and use GPS*, version 8.0, a modelling and simulation program developed by Hydromantis Environmental Software Solutions Inc., the most advanced tool currently accessible for the mathematical optimization, modelling, and management of wastewater treatment plants [17] .

The software was started, and the process water treatment library (proc water lib) option was selected. After finding the process table, a plant layout was made utilizing its icons. On the drawing board, every element of the process model was moved, arranged, and labelled. The process table was especially used to build all of the flow connections between the components of the process model. After adjusting the flow connections, stream labels were chosen using the labels button on the main toolbar. Following that, all process model object flow lines were given new names and labels.

The collected fertilizer effluent samples were evaluated in reference to temperature,

Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), Total Dissolved Solids (TDS), Electric Conductivity (EC), Sulphates, Turbidity, Total Phosphorus (T/ PO 4 − ), Residual Chlorine, Nitrate (NO₃), Total Nitrogen (TN), Total Suspended Solids (TSS), Colour, pH, Urea, NH₃ and heavy metals (Cu, Cd, Cr, Pb, Ni and Fe). Results are shown in figures that compare the outfall effluent of the urea fertilizer business to Federal Environmental Protection Agency (FEPA) criteria from 1991.

Sample 1’s color was a pale shade of blackish white, while sample 2 was found to be colorless. The fertilizer factory outfall effluent (sample 2) recorded high concentrations for all the water quality physicochemical parameters, and these concentrations are higher than the FEPA (1991) standard, with the exception of TSS, PO 4 − , SO 4 − , and NO 3 − . Temperature and pH were determined from sample 2 analysis to be 41˚C and 9.7, respectively. Because it has an impact on aquatic life, the outfall effluent temperature is a crucial variable. An abrupt change in temperature may lead to a high mortality rate for aquatic species [1] . In rivers, streams, and canals, high pH wastewater has the potential to reduce the solubility and toxicity of pollutants, which could have an impact on aquatic life (Umer et al., 2017) [7] . High pH wastewater can also promote the solubility of many important elements, such as Selenium (Se), in place of Mn, Cd, Al, B, Hg, and Cd. Additionally, high urea and NH₃ concentrations of the process condensates from the urea plant, ammonia plants, and from the sanitary sewage system following organic matter decomposition are connected with high pH values of the

outfall effluent. The pH of natural waters is a crucial quality indicator [1] . Salts are present in the samples, as determined by the measurements of EC and TDS. The EC and TDS values of the outfall effluent were measured to be 5848 S/cm and 2658.18 mg/l, respectively. These values are higher than the FEPA standard of less than 1000 μS/cm and 2000 mg/l in reported in the [7] , indicating their propensity to increase the salinity level of water bodies, which can have detrimental ecological effects on aquatic biota. High solubility compounds in aquatic environments may also slow down the rate of sunlight penetration into aquatic microsystems, which would otherwise have been a key factor in the emergence of photosynthetic organisms [7] . However, a high concentration of conducting particles in the effluent from production processes and chemical spills may be to blame for the small rise in EC and TDS [1] . The outfall effluent’s turbidity was estimated to be 267 NTU, exceeding the FEPA requirement of 100 NTU. High turbidity can affect aquatic life, raise the expense of treating drinking water, and have a negative effect on tourism and recreation, greater turbidity of wastewater correlated with greater conductivity, and vice versa.

Figure 6(a) and Figure 6(b) shows that the outlet effluent’s concentrations of urea, ammonia, phosphates, sulfates, nitrates, and total nitrogen were 219 mg/l, 20 mg/l, 0.358 mg/l, 26.80 mg/l, 1.70 mg/l, and 2.80 mg/l, respectively. The results of the current analysis show that the effluent quality of fertilizers is poor and that inorganic elements are present in considerable concentrations. The levels of urea, ammonia, and T/N are over the FEPA’s acceptable limits of 100 mg/l, 0 - 5 mg/l, and 0.6 mg/l, respectively. The lagoon’s water quality has greatly decreased as a result of this. High quantities of (NH₂)₂CO and NH₃ in process condensates from the urea plant and the ammonia plant, which were improperly hydrolyzed or stripped, are linked to the high values. Excretory materials from the sanitary sewage system and seal leakage from ammonia pumps may both cause a high level of NH₃ to appear in the outfall effluent. (Obire, Ogan, & Okigbo, 2008) stated that high concentrations of (NH₂)₂CO and NH₃may have an effect on available trace metals that brings about detrimental or beneficial effect and also cause eutrophication issues on water. These activities include urea synthesis, housekeeping activity, and granulation section cleaning that dispose/discharge urea granules into open drains. When nitrogen is released into the environment above the necessary level, it may have undesirable consequences on the environment’s ecology and human health [18] . In order to maintain oxidation (degradation) of the available nutrients and support the typical spectrum of aquatic life, the available dissolved oxygen (DO) must be used [1] .

The calculated biological and chemical oxygen demands were 91.7 mg/l and 116.7 mg/l, respectively. Figure 6(c) demonstrates that despite the normally low amounts, both samples have BOD and COD levels that are greater than the FEPA’s recommended limits of 30 mg/l for each.

According to Umer et al. (2017), high BOD and COD of effluent samples indicate that organic and inorganic matter is present in the fertilizer effluent

sample in high concentration, suggesting the possibility that the sample could enhance algal blooms and destabilize aquatic systems due to the high amount of nutrients present. The rate at which oxygen is used up in the stream increases with BOD. Similar to low dissolved oxygen levels, too much BOD stresses aquatic life, suffocates it, and ultimately kills it.

According to Figure 6(d), the amounts of Cd, Cr, Cu, Fe, Pb, and Ni in the fertilizer outfall effluent sample were 1.7 mg/l, 5.1 mg/l, 4.6 mg/l, 2.0 mg/l, 9.2 mg/l, and 4.7 mg/l, respectively. Except for the Fe concentration, which was not as high as observed in other detected heavy metals, the results of all heavy metals detected show concentrations exceeding the FEPA recommended values of the release of heavy metals into the environment. This suggests that the fertilizer plant’s piping system is sufficiently protected from corroding by chemical and biological actions. Metal and non-metal ion concentrations in aquatic environments have been shown to impact the metabolism of higher creatures and bacteria in seawater. These ions, particularly cations, are hazardous to aquatic life at varying levels. This can be caused by the impact of soluble elements in the effluent generated by the overall functioning of the fertilizer plant. Lead is a toxic metal that can harm the liver and kidneys. Chromium in its hexavalent form is extremely toxic because it has the ability to cross cell membranes and interact with genetic materials after being reduced to trivalent form, which is what causes its mutagenic and toxic effects. As a result, the effluent from the fertilizer outfall can be deemed inappropriate for environmental release (Umer et al., 2017). When applied to the soil, high iron concentrations in wastewater can cause soil acidity and lower the amount of molybdenum and phosphorus that is readily available. When copper concentrations rise slightly over those needed as a micronutrient, they become hazardous, especially to marine invertebrate larvae [19] .

The model layout was built due to what is obtainable from Figure 1, the process model objects were calibrated as per the data extracted from the Biological Treatment and Sludge Handling Package design manual and data sheet used in the fertilizer plant, and the influent was characterized using the physicochemical parameters got from the analyzed influent sample. From the process simulation of the wastewater treatment, the analysis of results revealing the chemical analysis of the effluents is shown in Table 4. Hence, the treated water simulation results clearly showed high reduction in BOD and COD concentration by 36% and 54.5% respectively, total nitrogen (TN), Nitrate-N and total phosphorus (TP) were also seen to be within the permissible FEPA standard as can be seen in Table 4. Acetic acid and sodium hydroxide flow rates were used as the output variables to investigate the effects of changes in the influent flow rates on the plant effluent qualities. On increasing the acetic acid dosage at 45 L/hr, 60 L/hr, 75 L/hr and 90 L/hr into anoxic tank-1 and keeping sodium hydroxide flow at 6 m³/hr, some changes in the treated water simulation results were observed.

As the acid flow rate increases and more simulations are run, a continuous concentration decrease was seen in the TSS, BOD, COD, Volatile Suspended

Solids (VSS), Ammonia-N, Total Kjeldahl Nitrogen (TKN), Nitrate-N and Total Phosphorus in the treated water simulation results as shown in Table 5 below. Hence, this indicates a proportional relationship with acid flow rate and the waste-water treatment performance.

Similarly, on increasing the sodium hydroxide dosage at 9 m³/hr, 12 m³/hr, 15 m³/hr and 18 m³/hr into the aeration tank-1 and keeping acetic acid flow at 31 L/hr, some changes on the treated water simulation results were observed. As the sodium hydroxide flow rate increases and more simulations are run, even though, continuous concentration decrease was observed in total alkalinity but, a continuous concentration decrease was sighted in the TSS, BOD, COD, Volatile Suspended Solids (VSS), Ammonia-N, Total Kjeldahl Nitrogen (TKN), Nitrate-N and Total Phosphorus in the treated water simulation results as shown in Table 6.

Also, on increasing both sodium hydroxide dosage at 9 m³/hr, 12 m³/hr, 15 m³/hr and 18 m³/hr into the aeration tank-1 and acetic acid flow at 45 l/hr, 60 l/hr, 75 l/hr and 90 l/hr into anoxic tank-1, some changes on the treated water simulation results were observed. As the flow rate of both chemicals increases with corresponding values and more simulations are run, even though, continuous concentration decrease was sighted in total alkalinity and Nitrate-N but, a continuous concentration decrease was observed in the TSS, BOD, COD, Volatile Suspended Solids (VSS), Ammonia-N, Total Kjeldahl Nitrogen (TKN) and Total Phosphorus in the treated water simulation results as shown in Table 7.

However, no significant change was observed in all the three scenarios as

regard Dissolve oxygen (DO), soluble PO₄-P and P^H, this could be owing to the fact that limited parameters were used in characterizing the influent causing most required parameters to run on default. Another factor may be that the acid dosage flow rate is not sufficient enough to bring down the P^H and alkalinity concentration of the treated water.