\[Image: Image21\]

\[Image: Image21\]
Performance Evaluation of Oke-Afa Wastewater Treatment Plant in Isolo,
Lagos State, Nigeria

\*Ijebor O.L and Ihimekpen N. I.
Department of Civil Engineering, University of Benin, Benin City, Nigeria

Department of Civil Engineering, University of Benin, Benin City, Nigeria
\*Corresponding Author Email: [okwudili2lucky@gmail.com](mailto:okwudili2lucky@gmail.com)

Article Info

Keywords:Performance Evaluation,
wastewater,wastewater characteristics,
treatment plant, wastewater treatment,
Oke-Afa.

Received 21 September 2023
Revised 25 October 2023
Accepted 26 October 2023
Available online 22 Dec. 23

[https://doi.org/10.5281/zenodo.10426207](https://doi.org/10.5281/zenodo.10426207)

Abstract

For three months, the Oke-Afa wastewater treatment plant (O-AWWTP)
located in Oke-Afa was studied. Data was collected from both the raw
influent, which enters the WWTP, and the treated wastewater (effluent)
from the plant. The parameters indicators, including pH, dissolved oxygen
(DO), biochemical oxygen demand (BOD5), chemical oxygen demand
(COD), total suspended solids (TSS), Ammonia (NH3), electrical
conductivity (EC), fecal coliform (FC), and Phosphate concentration
(PO4) were tested for both samples to evaluate the performance of O-
AWWTP. The removal efficiency of the plant in treating these parameter
indicators was estimated, and the treatment plant reliability factor (RF) of
each parameter indicator was calculated based on the NESREA-approved
acceptable limit for discharge. Based on the study conducted, the effluent's
average concentration of parameter indicators was determined. BOD5 had
a concentration of 18.242mg/l, pH had a concentration of 7.186, DO had
a concentration of 7.499mg/l, COD had a concentration of 44.458mg/l,
TSS had a concentration of 10.586mg/l, NH3 had a concentration of
4.392mg/l, EC had a concentration of 237.917µS/cm, FC had a
concentration of 31.250 MPN/100ml, and PO4 had a concentration of
1.903mg/l. The plant's removal efficiency for these pollutants: BOD5,
COD, TSS, NH3, FC, and PO4 was 85.5%, 78%, 93%, 67.5%, 97%, and
71.2%, respectively. However, the plant's performance was deemed
unsatisfactory in terms of COD, TSS, and FC removal as their average
pollutant concentration in the effluent still exceeded the maximum
permissible discharge limit set by the NESREA. Specifically, the limits for
COD, TSS, and FC are ≤20mg/l, ≤0.75mg/L, and ≤1.0MPN/100ml,
respectively. Based on the indicators tested, most of the estimated RF
values were less than 1.0. However, the RF values for COD, TSS, and FC
were above 1.0, with values of 2.20, 14.11, and 31.25, respectively.

1. Introduction
   Wastewater refers to water that is contaminated to a degree where it cannot be used without

Wastewater refers to water that is contaminated to a degree where it cannot be used without
treatment for most purposes. Water that contains various impurities can lead to diseases that may
cause a pandemic. Hence, it is essential to use available water in the cleanest form, and this is where
the concept of a water treatment plant, WWTP, comes in. A WWTP is used to convert wastewater
with various impurities, such as garbage, chemicals, and biological matter, into a form that is fit for
discharge. Untreated or inadequately treated wastewater poses a direct threat to the environment.
Discharging untreated sewage into the water body can lead to severe contamination, resulting in
eutrophication and intoxication of aquatic organisms, as well as chemical and biochemical
transformations of pollutants that release harmful gases and disrupt the functioning of ecosystems.

* * *

5(4) 2023 pp. 87-96

All these factors can cause changes in the biotic conditions and the physicochemical composition
of wastewater receivers \[1, 2, 3, 4\]. Therefore, it is crucial to ensure the proper functioning of the
WWTP to effectively protect water resources from pollution from sewers \[3, 4,5, 6, 9\].
In a typical wastewater treatment plant, water is drawn from the nearest source and undergoes

WWTP to effectively protect water resources from pollution from sewers \[3, 4,5, 6, 9\].
In a typical wastewater treatment plant, water is drawn from the nearest source and undergoes
several stages of treatment. The first stage is screening, which removes larger floating particles.
This is followed by aeration, which further treats the water. Next, coagulation is done in a flash
mixer, and the water enters a tube settling tank to remove suspended impurities. After settling, the
water goes through a filtration unit to remove fine particles. Then, chlorination is done using liquid
chlorine to kill pathogenic bacteria and other harmful microorganisms. Finally, the treated
wastewater is discharged. However, over time, the performance of the wastewater treatment plant
may decrease, resulting in lower discharge standards. Therefore, it is necessary to periodically assess
the plant's performance to ensure it meets regulatory standards. A survey showed that some
wastewater treatment plants are not producing water that is safe for discharge. The current situation
may result in numerous hygiene issues related to water. Each unit in the water treatment facility
plays a crucial role in purifying wastewater. Hence, it is vital to examine the functioning
performance of each unit. Over time, treatment plants may be required to upgrade their wastewater
treatment units using the latest technologies. Evaluating the performance of a WWTP is essential in
monitoring and determining its efficiency, providing better insight into design and operational
problems in water treatment facilities.
As modern societies continue to develop, their growing populations increase the demand for water

As modern societies continue to develop, their growing populations increase the demand for water
supply and wastewater treatment. Unfortunately, there is concern in the water sector due to the
inadequate treatment of sewage and the resulting lack of clean water. According to the World Health
Organization (WHO), up to 80% of illnesses and infections worldwide are caused by poor sewage
treatment. Additionally, more than 3.4 million people die each year due to pathogens in the aquatic
environment. This is why the environment and public health have been affected by current trends
and practices of wastewater disposal and treatment, which often result in poor effluent quality. The
Oke-Afa plant was originally designed and constructed in 1982 to serve around 40,000 inhabitants
of Jakande estate. However, it is now serving over 50,000 people, which exceeds its intended
capacity. A study conducted by (Yahaya et al. 2016) found that the Oke-Afa canal, which is the
discharge point for treated water from the plant, is heavily polluted when compared to the National
Environmental Standard and Regulation Enforcement Agency (NESREA)\[8\] standards. This
situation calls for an evaluation of the system's performance to ensure the protection of water
resources and management. Although this approach to wastewater treatment plants is common in
more developed areas, similar studies in Nigeria and the African context have not been widely
reported. Therefore, an attempt has been made to evaluate the performance of the wastewater
treatment plant (WWTP) at Oke-Afa in Jakande estate to determine its removal efficiency and
treatment capacity.

2 Materials and Method
2.1 Study Area

The research was carried out in Jakande estate, located at Isolo in the Oshodi-Isolo Local
Government Area of Lagos State where the Oke Afa wastewater treatment plant is constructed.
Jakande Estate has 600 buildings with six flats in each building. The estate was housing about
18,000 inhabitants at the time the treatment plant was built. Over time, the population figures for
the estate increased as the estate is observed to be housing other public users who now rent
apartments in the estate. The estate is purely residential with different commercial and other
economic activities taking place. The estate has primary and secondary schools, churches, mosques,
and markets in different locations within the estate. These led to the population growth to over
50,000 in the estate according to National Population Commission. The Oke-Afa Wastewater

2.1 Study Area
The research was carried out in Jakande estate, located at Isolo in the Oshodi-Isolo Local

* * *

5(4) 2023 pp. 87-96

o
Treatment Plant, (O-AWWTP) lies within longitude 6.532 E of the Greenwich meridian and latitude
o
3.304 N of the equator. It was built to serve about 40,000 people through the activated sludge
process, with the capacity of doing primary and secondary treatment. The Oke-Afa plant treats
domestic wastewater within the estate. Treated water from the plant is discharged into the Oke-Afa
canal. It was observed that each of the residents uses about 135 litres of water daily, about six million
litres of water is generated daily of which 80 percent or over four million litres become wastewater.
The processes involved in the wastewater treatment at the Oke-Afa plant consist of Primary and
secondary treatment with activated sludge process, and Tertiary treatment processes. Figure 1 shows
the Google Earth imagery of the plant.

6.532^{\\circ{\\mathrm{E}}}

3.304^{\\mathrm{0}}\\mathrm{N}

Figure 1: Map of the study area (Source: Google Earth Maps, 2019)

2.2 Sample Collection
For the study, wastewater samples were collected weekly for three (3) months, from July to

For the study, wastewater samples were collected weekly for three (3) months, from July to
September 2021 from the Oke-Afa Wastewater Treatment Plant (O-AWWTP). A total of twelve
(12) samples of untreated wastewater (influents) and twelve (12) samples of treated wastewater
(effluents) were collected for characterization and evaluation. Two sampling points were identified
which include the wastewater treatment plant influent and the wastewater treatment plant effluent.
The samples were collected with great care to avoid any disturbance or exposure to air. The
collection time was chosen to be between 10:00 am and 3:30 pm, which is the period when most
residents of the estate would have left their homes for their daily activities. This was to ensure that
the sample was not fresh. To maintain hygiene, one-litre plastic containers that were previously
cleaned were rinsed three times with the wastewater and labelled appropriately. The samples were
then transported to the laboratory for analysis. Before analysis, the samples were preserved in a
refrigerator and stored according to the recommended procedures in the standard methods, as stated
in \[7\].
Pollutant parameters tested for these samples were the biochemical oxygen demand (BOD5),

in \[7\].
Pollutant parameters tested for these samples were the biochemical oxygen demand (BOD5),
chemical oxygen demand (COD), total suspended solids (TSS), Ammonia (NH3), fecal coliform

\\mathrm{(N H\_{3})}

* * *

Ijebor O.L & Ihimekpen N. I./Journal of Science and Technology Research

5(4) 2023 pp. 87-96

(FC), and Phosphate concentration (PO4). Other parameters include pH, dissolved oxygen (DO), and electrical conductivity (EC). The values of the tested parameters were analyzed by using Excel 2010 software. Concentrations of effluent pollutants indicators after treatment were examined and compared with the permissible discharge limits of the Nigeria Environmental Standard Regulatory and Enforcement Agency (NESREA)\[8\]. The removal efficiency of the wastewater treatment plant was assessed on a pollutant-by-pollutant basis using equation 1 as follows: 100 ( _Influent-Effluent_) Re moval Efficiency = (1) _Influent_ Where; _Influent_ is the concentration of untreated wastewater pollutant indicator and Effluent is the concentration of treated wastewater pollutant indicator. In addition, the value of the wastewater treatment plant reliability factor (RF) was estimated by the ratio of the averaged concentration of an evaluated pollutant indicator in the effluent wastewater and its permissible value in the wastewater discarded to the receiving river, as provided in the work of \[4\] _X_ _RF =_ (2) _X_ _Per_ Where; _X is the average concentration of a pollutant indicator in the effluent sewage (mg/L) and_ 𝑋𝑝𝑒𝑟 is the permissible concentration of a pollutant indicator in the treated sewage (mg/L) When the value of RF is greater than 1.0, it indicates efficiency of the wastewater treatment plant is low. Also, when the reliability factor (RF) is less than 1.0, it means the wastewater treatment plant is functioning optimally \[9\].

## 3\. Results and Discussion

## 3.1 Wastewater Sample Assessment

The pollutant parameter indicators for wastewater samples were analyzed and the results were compared with the NESREA permissible discharge limits. The results of the analysis are presented in Tables 1, 2, and 3.

**3.1.1 Total Suspended Solid (TSS): According to Table 1, the influent TSS concentrations varied** between 102.300- 227.300mg/L, with a weekly average concentration of 155.183mg/L. Table 2 shows that the corresponding effluent ranged from 6.800-17.500mg/L, with an average weekly concentration of 10.586. The plant's TSS removal efficiency during the study period ranged was
93.0455%. However, the plant's performance was poor for TSS, as the effluent concentration exceeded the permissible discharge limit (<0.75 mg/L) set by NESREA \[8\], and the RF value was greater than 1.0, as shown in Table 3.

* * *

Table 1: Descriptive statistics of influent wastewater

| Wastewater Parameter Indicator | Statistics |  |  |  |
| --- | --- | --- | --- | --- |
| Range | Median | Mean | Standard dev. |  |
| BOD(mg/L) | 97.900-169.20 | 124.000 | 127.508 | 22.155 |
| COD(mg/L) | 154.00-253.00 | 202.000 | 206.250 | 32.550 |
| DO(mg/L) | 2.760-4.230 | 3.760 | 3.662 | 0.501 |
| EC(μS/cm) | 217.300-321.000 | 263.000 | 267.500 | 31.567 |
| TSS(mg/L) | 102.300-227.300 | 152.850 | 155.183 | 40.276 |
| pH(Nil) | 7.120-7.760 | 7.410 | 7.406 | 0.197 |
| PO4(mg/L) | 5.430-7.440 | 6.985 | 6.648 | 0.729 |
| NH3(mg/L) | 10.800-17.300 | 14.100 | 13.917 | 1.700 |
| FC(MPN/100ml) | 840.000-1440.000 | 1125.000 | 1133.583 | 230.780 |

\\mathbf{P O}\_{4}\\left(\\mathbf{m g}/\\mathbf{L}\\right)

Table 2: Descriptive statistics of effluent wastewater, Removal Efficiency, and Standard
Limit.

| Wastewater Parameter Indicator | Statistics |  |  |  | Removal Efficiency(%) | Standard Limit(NESREA,2011) |
| --- | --- | --- | --- | --- | --- | --- |
| Range | Median | Mean | Standard dev. |  |  |  |
| BOD(mg/L) | 11.400-27.800 | 18.450 | 18.242 | 5.115 | 85.49719 | 30.000 |
| COD(mg/L) | 27.600-62.100 | 43.850 | 44.458 | 9.578 | 78.08495 | 20.000 |
| DO(mg/L) | 6.980-8.230 | 7.460 | 7.499 | 0.357 |  | - |
| EC(μS/cm) | 200.000-301.000 | 227.000 | 237.917 | 32.397 | 11.07719% | 400.000 |
| TSS(mg/L) | 6.800-17.400 | 9.800 | 10.586 | 3.284 | 93.0455 | 0.750 |
| pH(Nil) | 7.020-7.520 | 7.135 | 7.186 | 0.162 |  | - |
| PO4(mg/L) | 1.030-3.220 | 1.825 | 1.903 | 0.648 | 71.21919 | 3.50 |
| NH3(mg/L) | 2.660-6.880 | 4.265 | 4.392 | 1.237 | 67.46351 | 10.00 |
| FC(MPN/100ml) | 10.000-80.000 | 20.000 | 31.250 | 26.894 | 97.00082 | 1.000 |

* * *

Table 3: Treatment plant reliability factor (RF) for the evaluated wastewater indicators

| Wastewater Parameter Indicator | Average Effluent Concentration | Maximum Allowable Concentration | Estimated Plant Reliability Factor (RF) |
| --- | --- | --- | --- |
| BOD | 18.242mg/L | 30.000mg/L | 0.608 |
| COD | 44.458mg/L | 20.000mg/L | 2.223 |
| EC | 237.917μS/cm | 400.000μS/cm | 0.595 |
| TSS | 10.586mg/L | 0.750mg/L | 14.11 |
| PO4 | 1.903mg/L | 3.500mg/L | 0.544 |
| NH3 | 4.392mg/L | 10.000mg/L | 0.439 |
| FC | 31.25MPN/100ml | 1.0MPN/100ml | 31.25 |

\ {bf P0}\_{4}

Figure 1: Weekly variation of TSS in the influent and effluent wastewater

3.1.2 Electrical Conductivity (EC): This measures the wastewater's ability to conduct an electrical
current. The electrical conductivity levels of the influent wastewater, as shown in Table 1, ranged
from 217.0-321.0 µS/cm with an average weekly value of 267.50 µS/cm. The corresponding
effluent value ranged from 200.0-301.0 µS/cm with an average value of 237.92 µS/cm, as presented
in Table 2. The average enhancement efficiency of the plant for EC, as shown in Table 3, was
11.07719%. Although the plant's performance in handling this pollutant was poor, the observed
(EC) values of both influent and effluent concentrations were below the permissible limit (<400

3.1.2 Electrical Conductivity (EC): This measures the wastewater's ability to conduct an electrical
current. The electrical conductivity levels of the influent wastewater, as shown in Table 1, ranged
from 217.0-321.0 µS/cm with an average weekly value of 267.50 µS/cm. The corresponding
effluent value ranged from 200.0-301.0 µS/cm with an average value of 237.92 µS/cm, as presented
in Table 2. The average enhancement efficiency of the plant for EC, as shown in Table 3, was
11.07719%. Although the plant's performance in handling this pollutant was poor, the observed
(EC) values of both influent and effluent concentrations were below the permissible limit (<400

(EC) values of both influent and effluent concentrations were below the permissible limit (<400
µS/cm) set by NESREA \[19\]. Additionally, the RF value remained below 1.0 during the study
period.

* * *

Ijebor O.L & Ihimekpen N. I./Journal of Science and Technology Research

5(4) 2023 pp. 87-96

**Figure 2: Weekly variation of EC in the influent and effluent wastewater**

Figure 2 shows that the greatest reduction of EC was observed during the first and eleventh weeks,

whereas the lowest concentration of effluent EC was recorded during the first, eleventh, and twelfth weeks as presented in Figure 2. Overall, the treatment plant exhibited a limited ability to handle the presence of EC in the influent wastewater, with minimal variation between the influent and effluent concentrations.

## 3.1.3 Phosphate (PO4) and Ammonia (NH3): The levels of phosphate and ammonia in wastewater

are important indicators of water quality and pollution. High levels of these substances can lead to nutrient pollution, which can harm aquatic ecosystems and human health. During the study period, the minimum and maximum weekly influent concentrations of phosphate were 5.430mg/L and

7.44mg/L, respectively, with an average weekly concentration of 6.648mg/L. The minimum and maximum weekly concentrations of ammonia were 10.80mg/L and 17.30mg/L, respectively, with an average concentration of 13.92mg/L as presented in Table 1. The effluent concentration of phosphate ranged from 1.03-3.20mg/L, with an average concentration of 1.90mg/L. The effluent concentration of ammonia ranged from 2.66-6.88mg/L, with an average concentration of 4.39mg/L. These concentrations are below the standard limit as reported in \[8\]. The removal efficiency of phosphate was 71.21919%, while the removal efficiency for ammonia was
67.46351%. These efficiencies are moderate in value and may be related to the variability of the effluent wastewater in terms of its source and composition. Nevertheless, the treatment plant showed good capacity to treat these parameters since their RF is less than 1.0, which indicates optimal plant performance \[9\]. The weekly variations of PO4 and NH3 in the influent and effluent of the treatment plant are presented in Figures 3 and 4 respectively.
**Figure 3: Weekly variation of PO4 in the Figure 4: Weekly variation of NH3 in the influent** **and effluent wastewater influent and effluent wastewater**

* * *

Based on the findings in Figure 3, the greatest reduction in PO4 was observed in weeks 6 and 10,
with the lowest concentration of PO4 found in the effluent during the same period. Similarly,
Figure 4 shows that the highest percentage decrease in NH3 occurred in weeks 6 and 10, with the
lowest NH3 concentration in the effluent recorded during weeks 4 and 10.

\\mathrm{P O}\_{4}

\\mathrm{P O}\_{4}

\\mathrm{N H}\_{3}

3.1.4 Assessment of Oxygen Demand (BOD, COD, DO): The amount of dissolved oxygen (DO)
consumed by biological organisms when they decompose organic matter in water is known as the
biochemical oxygen demand (BOD). On the other hand, the chemical oxygen demand (COD) is the
amount of oxygen consumed when the wastewater sample is chemically oxidized. Both BOD and
COD can have negative effects on the oxygen levels of lakes and rivers, causing eutrophication and
harm to aquatic life. In Tables 1, 2, and 3, the oxygen demand of pollutant indicators during the
investigation period was presented. The plant demonstrated optimal performance for BOD, with an
effluent concentration that was less than the permissible discharge limit (≤ 30mg/L) set by
NESREA, an average weekly removal efficiency of 85.5%, and an estimated RF of 0.608. However,
the average weekly effluent concentration for COD (44.46mg/L) was still higher than the
recommended permissible limit of 20mg/L \[8\]. This may be due to the presence of nonbiodegradable organic matter in the wastewater or the plant operating below full capacity. The RF
for COD was estimated to be 2.223. Nevertheless, the effluent DO concentration during the study
period was observed to be higher than the NESREA standard, indicating that the receiving water
body is safe for aquatic life as there is enough oxygen in the effluent of the wastewater. The weekly
variations of BOD and COD in the influent and effluent of the treatment plant are presented in
Figures 5 and 6 respectively.

(\\leq,30\\mathrm{m g/L)}

20\\mathrm{m g/L}

Figure 5: Weekly variation of BOD5 in the
the influent and effluent wastewater influent and effluent wastewater

According to the findings in Figure 5, the greatest reduction in BOD levels occurred during the ninth
week, while the lowest effluent BOD concentration was measured during the fourth and ninth
weeks. The wastewater treatment plant demonstrated effective management of BOD in the influent
wastewater. In Figure 6, the highest percentage removal of COD was reported during the second
and tenth weeks, with the lowest effluent COD concentration being recorded during the ninth and
twelfth weeks. The treatment plant demonstrated moderate effectiveness in the COD management
of influent wastewater. The effluent concentration of both BOD and COD showed minimal variation
compared to their influent concentration. The weekly variations of DO in the influent and effluent
of the treatment plant are presented in Figure 7.

Figure 6: Weekly variation of COD in
influent and effluent wastewater influent and effluent wastewater

* * *

5(4) 2023 pp. 87-96

Figure 7: Weekly variation of DO in the influent and effluent wastewater

Based on the findings presented in Figure 7, the highest percentage increase in dissolved oxygen
(DO) was observed during week 1, week 10, week 11, and week 12. Conversely, the concentration
of effluent DO remains consistent throughout the entire sampling period. Generally, the treatment
plant exhibited effective capabilities in managing low DO levels in the influent wastewater, with
only low fluctuations observed in the effluent and influent concentrations.

3.1.5 Fecal Coliform (FC): Fecal pollution refers to the presence of disease-causing
microorganisms in bodies of water, which typically results from human sewage or excreta from
warm-blooded animals. Some coliform bacteria can make people ill, leading to symptoms such as
vomiting, fever, diarrhea, or an upset stomach. In the treatment plant under investigation, both the
influent and effluent concentrations exceeded the allowable discharge limit of 1.0MPN/100ml per
week, as shown in Tables 1 and 2. Despite achieving a removal efficiency of over 97%, the average
weekly concentration (31.250MPN/100ml) still exceeded the permissible discharge limit and had
an RF value greater than 1.0, as indicated in Table 3. The treatment plant currently cannot effectively
handle the presence of fecal coliform in the influent wastewater. Therefore, it is necessary to
improve the plant's capacity to treat this presence before discharge. This will ensure that harmful
microbes are eliminated and the receiving canal remains safe.

4.0. Conclusion
The performance of the Oke Afa wastewater treatment plant was evaluated, and the results indicated
that the plant is not operating at its optimum level. The analysis of the effluent from the plant showed
that the concentration of effluent parameters such as fecal coliform (FC), total suspended solids
(TSS), and chemical oxygen demand (COD) exceeded the permissible discharge limit set by
NESREA for effluent wastewater before it is discharged into the environment. The discharge limit
set by NESREA for TSS and COD is 0.75mg/L and 20mg/L, respectively, and that for FC is 1.0
MPN/100ml. The analysis revealed that the average weekly concentration of effluent TSS was
95

* * *

5(4) 2023 pp. 87-96

10.586mg/l, that of COD was 44.458mg/l, and FC was 31.250 MPN/100ml. The corresponding
average removal efficiencies for COD, TSS, and FC are 78.08%, 93.05%, and 97%, respectively.
The reliability factor (RF) of the treatment plant estimated was 2.2 for COD, 14.11 for TSS, and
31.250 for FC. Since the estimated reliability factor was greater than 1.0, it was concluded that the
plant was not functioning optimally.

\[1\] Masłoń A., Tomaszek J.A. (2013), "Evaluation of the Effectiveness of Wastewater Treatment Plant in Lubaczów.
Czasopismo Inżynierii Lądowej, Środowiska i Architektury,," vol. 60, no. 3/13, pp. 209-222.
\[2\] Al-Zahiri A. (2015), " Assessment of Performance of Wastewater Treatment Plants in Jordan," International
Journal of Civil and Environmental Engineering, pp. 1-6, 2015.
\[3\] Młyński D., Chmielowski K., Młyńska A., Miernik W. (2016a), "Evaluation of Efficiency of Sewage Treatment
Plant in Jasło. Infrastruktura i Ekologia Terenów Wiejskich," vol. 1, pp. 147-162.
\[4\] Młyński D., Chmielowski K., Młyńska A. (2016b), "The Assesment of the Efficiency and Stability of Work
Sewage Treatment Plant in Zabajka. Inżynieria Ekologiczna," vol. 47, pp. 123-130.
\[5\] Mielenz B. (2008), "The Assessment of Working Sewage Treatment Plant at Wadowice Before the
Modernization. Infrastruktura i ekologia terenów wiejskich," vol. 2, pp. 129-138.
\[6\] Łagożny P., Maj K., Masłoń A. (2015), "Technological efficiency of the wastewater treatment plant in Krosno,"
Technological efficiency of the wastewater treatment plant in Krosno, vol. 17, no. 2, pp. 113-122.
\[7\] NESREA, (2011) "National Environmental Standards and Regulations Enforcement Agency," in National
Environmental (Surface, and Groundwater Quality Control) Regulations.
\[8\] APHA (2015), "Standard Methods for the Examination of Water and Wastewater, (21st Ed)," in Environmental
Management, Washington, D. C, pp. 79-83.
\[9\] Garbowski T. Wiśniewski J. & Bawiec A. (2018) "Analysis and Assessment of the Wastewater Treatment Plant
Operation in the City of Kłodzko," Journal of Ecological Engineering, vol. 19, no. 2, pp. 66-72.

References