Comparison Study of Urban Wastewater Treatment Using Conventional Biologic Treatment and Submerged Membrane Bioreactor Processes

Abstract

Today, the demand for clean water resources causes the rapid consumption of water and the finding of alternative water resources. The recovery and reuse of wastewater after treatment is important for water sustainability, and in recent years, the use of wastewater by completely or partially treating it has gained importance due to the water shortage that has emerged as a result of global climate change. It can be used in agricultural areas where water is frequently used, especially if the water content is suitable after treatment. In this study, the use of water from the treatment plant as irrigation water in agricultural areas was investigated. The effluent of the Mezitli and Kızkalesi Wastewater Treatment Plants in Mersin was used for this purpose. In the investigation of the usability of the treated water in agricultural irrigation, analyses were made for many pollutants. In order to examine the usability of wastewater after treatment in irrigation water, parameters such as total phosphorus (TP), total nitrogen (TN), biological oxygen demand (BOD) and heavy metals were examined in order to meet the nutrient needs of plants. The analysis results were evaluated according to the agricultural irrigation water criteria specified in the Wastewater Treatment Plant Technical Procedures Communiqué. As a result, the analysis results of the treated water were compared with the limit values in the regulations, and it was evaluated that the treated water in the Mezitli Wastewater Treatment Plant did not meet the limit values of irrigation water usage criteria. However, it has been evaluated that the effluent from the Kızkalesi Wastewater Treatment Plant, which was treated with the MBR process, meets the limit values and therefore can be used for agricultural irrigation purposes. As a result, when the analysis results performed on treated water were compared with the Wastewater Treatment Plant Technical Procedures Communiqué irrigation water limit values, it was evaluated that the treated water of the Mezitli Wastewater Treatment Plant did not meet irrigation water limit values, but the treated water of the Kızkalesi Wastewater Treatment Plant with the MBR process met the irrigation water limit values and therefore could be used for agricultural irrigation purposes. The study results showed that the treated water in the Mezitli Wastewater Treatment Plant could not be used for agricultural irrigation, but the treated water in the Kızkalesi Wastewater Treatment Plant could be used for agricultural irrigation.

1. Introduction

The usability of surface and groundwater of different qualities varies according to their intended use. Such water resources can be polluted as a result of both natural events and human-added activities, and their physical and chemical structure can change. This situation is increasing in time with the increasing population of the problems related to water resources and protection of water quality in developing countries [1]. In order to prevent these problems that may cause water scarcity, it has been aimed to obtain different water resources without changing the amount of existing water resources. It is rainwater as an alternative water source. However, it may be more advantageous to treat polluted waters in order to obtain a larger amount of water [2].

Water is of great importance in agriculture. In general, the area that has a share of 70% in water consumption is irrigation water used in agricultural activities. This percentage may vary according to the changing climate and changes in agricultural intensity. Improvements are made in irrigation infrastructure and techniques in order to reduce the use of water needed in the agricultural field without changing the agricultural yield. However, despite these measures, the gradual depletion of water resources has not completely disappeared. For this reason, polluted water classified as waste may contain nutrients, organic matter and minerals that will support plant growth. It is dangerous to use wastewater generated as a result of industrial and human activities directly in agricultural areas, but after improving the quality of the purified water, this water can be used as an agricultural water source [3,4]. Advanced treatment methods of wastewater have been examined, and the main advantages of reusing wastewater in agricultural and aquaculture irrigation by treating it with appropriate methods have been revealed. These include increased income, employment and food security [5]. Miller-Robbie et al. (2017) conducted a study on the agricultural reuse of effluent from a wastewater treatment plant [6]. In their study, they used the effluent of the central wastewater treatment plant for irrigation purposes instead of groundwater. They irrigated an agricultural field in the city of Hyderabad (India) for one year with the effluent of the treatment plant, which has an efficiency of 99% in coliform removal and 81% in BOD removal. Thus, they stated that the harvest yield in the agricultural field increased 10 times. Thus, it has been stated that the amount of unrefined nutrients in the treated water has a positive effect, such as preventing the environmental effects of chemical fertilizers and recycling nutrients. It is thought that the research conducted on the fact that the use of treated wastewater against water scarcity in agricultural areas may be appropriate and can be preferred as alternative water sources may be positive [7,8]. As a result of experimental studies conducted on treated water taken from 289 wastewater treatment plants in Chile, the average nitrogen concentration in treated water was determined to be 25 mg/L and the phosphorus concentration to be 8 mg/L. Therefore, it has been reported that it can be used in agricultural irrigation [8].

The amount increases in direct proportion to the amount of houses, education areas, hospitals and industries that increase with urbanization. In addition to the collection of these waters in the sewer system, urban wastewater is formed by the inclusion of rainwater in the sewer system. Different treatment methods are used in order to treat such waters according to the content of each urban area or wastewater [9]. The most common method used in the treatment of urban wastewater is wastewater treatment systems using biological treatment systems. Most of the organic and inorganic pollutants in urban wastewater can be removed in activated sludge treatment systems. However, such treatment systems may be insufficient according to the pollutants in the water [10]. For this reason, advanced treatment systems are applied to treat the non-biodegradable part, such as ozonation, advanced oxidation and membrane systems. Thus, with other treatment systems used together with biological treatment, water is purified with higher efficiency [11].

In this study, the utilization potential of the treated wastewater from wastewater treatment plants as irrigation water and the usability of wastewater treatment plant effluent in two different districts of Mersin province as irrigation water were investigated. In order to evaluate the usability of the treated water in the agricultural field, it has been evaluated together with the classifications of the quality of irrigation water in the “Wastewater Treatment Plants Technical Procedures Communiqué” in the national legislation. The original aspect of this study is that alternative water sources are being investigated due to the need for agricultural irrigation water in the regions where treatment plants are located. In the region, due to the pressure of climate change, especially the pressure of agricultural water demand, farmers and their social environment, agricultural irrigation unions, seriously demand irrigation water sources. The region has extremely suitable climate conditions for agricultural production, but water scarcity prevents the use of these suitable conditions. Therefore, providing a water source is a study that will enable both agricultural production and the sustainability of natural resources.

2. Materials and Methods

2.1. Information About Wastewater Treatment Plants

Mezitli was founded as Soli by Greek settlers coming from Rhodes in the 7th century BC and later renamed Pompeiopolis by Roman general Pompey the Great after his campaign against Mediterranean pirates in 67 BC. It hosted an important port city under Roman and Byzantine rule. In the early 20th century, the region became a living space for people as a small village named after the Oghuz tribe, “Mezitoğlu”. In 1968, a municipality was established, and in 2008, a district was established. With the rapid migration, its population increased, and as of 2022, its population is approximately 222,436. The geographic coordinates of the Mezitli Wastewater Treatment Plant are approximately latitude 36.7301° N and longitude 34.5048° E.

The Mezitli Wastewater Treatment Plant carries out the carbon, nitrogen and phosphorus removal of the region with a population of approximately 384,000 and a capacity of approximately 55,000 m³/day by biological treatment. The treated water is discharged to the Mediterranean by deep-sea discharge. The wastewater coming to this wastewater treatment plant passes through the screen unit, sand-oil trap, and pre-settlement tank in order to separate the solid parts (paper, wood, plastic material, etc.). Then, bio-phosphorus and an aeration pool are used to remove the nutrients in the water. Then, it is transferred to the final settling tank for both the return of the settled sludge and the retention of the water. The excess sludge that is not recycled is disposed of using a mechanical sludge thickening unit (Figure 1).

The population of Kızkalesi Neighborhood, Erdemli District of Mersin Province, as of 2022, is 1781 people, and this figure represents the winter population. In the summer months, especially in July and August, the population reaches approximately 50,000–60,000 due to the influx of local and foreign tourists. The settlement, which attracts visitors with its historical sites, serves as an important tourist center, especially hosting the ancient Korykos Castle and the Maiden’s Castle, located on a small island just off the coast. The Kızkalesi Wastewater Treatment Plant serving the region has geographic coordinates latitude 36.463821 N and longitude 34.137694 E.

The Kızkalesi Wastewater Treatment Plant is located in a region where summer tourism is intense. In general, biological treatment can be performed in the treatment plant with a capacity of 1000 m³/day. However, with the opening of summer tourism in the region in the summer months, the capacity has been increased to 7940 m³/day with the increasing population. Therefore, although the nominal operating capacity of the plant is 1000 m³/day according to the winter population, since the population of the region is expected to reach approximately 50,000–60,000 people with tourism in the summer season, the design capacity of the plant has been determined as 7940 m³/day to meet this increase. Therefore, both values are valid: 1000 m³/day represents the continuous operating capacity, and 7940 m³/day represents the maximum loading capacity in the summer season. Modern membrane systems can easily adapt to seasonal fluctuations, especially thanks to their high efficiency and flexible operating capacity modular structures. The high-flow scenario in the summer period can be successfully managed with strategies such as activating additional modules when necessary, optimizing cleaning frequencies and dynamically adjusting operating parameters. This shows that the system can be operated safely, sustainably and effectively all year round, including during the summer period. Therefore, the available membrane area is suitable and sufficient for all design flows, including summer high flow loads.

The operating flux values calculated for different flow rates on the 5927 m² membrane area installed in our system were evaluated. The flux values for the measured minimum and maximum flow rates are 3.5 L/m²/h and 21.0 L/m²/h, respectively, and these values are fully compatible with the 15–25 L/m²/h design range accepted in the literature, especially at maximum flow. The flux value under the 1000 m³/day flow rate for the winter period is 7.0 L/m²/h, allowing the system to operate energy efficiently under low load.

For an expected high flow rate of 7940 m³/day in summer, the calculated theoretical flux value is approximately 55.8 L/m²/hour. Although this value is above the conventional design range, it has been confirmed with field data that the system can tolerate this load thanks to its high-performance membrane modules, modular operating structure and adaptable operating strategies. The high adaptability of the system in meeting short-term peak loads, especially in the summer months, supports the stability of the operation and the sustainability of the process.

As a result, despite the flow rate changes in different seasons, the available membrane area and system configuration are sufficient for all operating scenarios. By optimizing the operating parameters, safe and efficient operation of the membranes is ensured under both low- and high-load conditions.

In the Kızkalesi Wastewater Treatment Plant, where the MBR system is installed, all of the treated water is currently discharged to the Mintan stream close to the Erdemli district of Mersin. After the water coming to MBR passes through coarse and fine screens, it is transmitted to the sand-oil trap unit and passes into the balancing pool. Here, after stabilization of the water, water is transferred to the de-oxidation pool and the anoxic pool, respectively. When the water passing through the pre-aeration pool passes through the last membrane system, the water is purified. The sludge formed in the membrane system is returned to the system. (Figure 2).

2.2. Real Wastewater Characteristics of Wastewater Treatment Plants

In this study, the characterizations of wastewater coming to two different wastewater treatment plants in Mersin province were made. The parameters examined in wastewater are pH, conductivity with sodium (Na), sodium adsorption rate (SAR), biological oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), nutrients (total phosphorus (TP), total nitrogen (TN) and nitrate nitrogen (NO₃-N)), total dissolved solids (TDS), chloride (Cl), active chlorine, coliform and heavy metals.

For the characterization of raw wastewater and treated wastewater, the following parameters were monitored using standard analytical methods. Two replicates were performed for all analyses.

The facilities are publicly inspected. For the study, the inlet and outlet water parameters of the facility were analyzed in a nationally accredited laboratory. Since periodic validation studies are carried out in the laboratory where the analyses of this study were carried out, the analysis results using standard methods are reliable. The analysis methods used for the analyses conducted in the study are presented in the Supplementary File S1.

3. Result and Discussion

3.1. Characterization Results of Initial Wastewater and Treated Water

The Mezitli Wastewater Treatment Plant and the Kızkalesi Wastewater Treatment Plant use the activated sludge process and the membrane process to treat, respectively. Therefore, in this study, the Mezitli Wastewater Treatment Plant is named ASP and the Kızkalesi Wastewater Treatment Plant is named MBR. In this study, initial concentrations of wastewater belonging to ASP and MBR and decreasing concentrations of treated water from each treatment plant were analyzed. Since MBR is intense in the summer months, the changes in the starting wastewater depending on the season are also indicated. As a result of the analyses made, the initial concentration values of the wastewater in the treatment plants are given in

Table 1. Initial wastewater concentration values of wastewater treatment plants.

Parameters	Unit	ASP	MBR Summer and Winter Average
pH	-	7.11 ± 0.25	7.50 ± 0.5
Conductivity	µS/cm	6.920 ± 1.130	1.438 ± 142
BOD	mg/L	386.25 ± 23.97	149.5 ± 10.75
COD	mg/L	496.4 ± 19.15	192.5 ± 10.58
SS	mg/L	247.5 ± 24.62	106 ± 5.44
TP	mg/L	7.28 ± 1.43	3.18 ± 0.30
TN	mg/L	31.5 ± 3.38	50.51 ± 1.80
Cl	mg/L	6848 ± 463.11	2244 ± 73
Arsenic	mg/L	0.055 ± 0.012	0.053 ± 0.011
Cadmium	mg/L	0.038 ± 0.018	0.056 ± 0.008
Chromium (Cr6+)	mg/L	0.05 ± 0.018	0.058 ± 0.023
Cobalt	mg/L	<0.005	<0.005
Copper	mg/L	0.059 ± 0.036	0.0695 ± 0.038
Lead	mg/L	0.25 ± 0.032	0.3 ± 0.024
Nickel	mg/L	0.26 ± 0.036	0.21 ± 0.033
Zinc	mg/L	0.11 ± 0.026	0.18 ± 0.025

, and characterization values of treated water after treatment for each plant are given in

Table 2. Treated water characterization values of wastewater treatment plants.

Parameters	Unit	ASP	MBR
pH	-	7.39 ± 0.12	7.47 ± 0.2
Conductivity	µS/cm	6430 ± 1490	1056 ± 110.3
BOD	mg/L	18.45 ± 6.55	5.96 ± 2.6
COD	mg/L	23.21 ± 5.19	11.17 ± 4.12
Coliform *	CFU/100 mL	0	0
SS	mg/L	21.79 ± 13.21	7.2 ± 2.3
TP	mg/L	5.25 ± 0.75	1.43 ± 1.1
TN	mg/L	19.66 ± 4.34	10.2 ± 6.1
NO3-N	mg/L	17.8 ± 5.64	17.20 ± 4.89
Cl	mg/L	457.79 ± 32.25	71.1 ± 18.4
Active chlorine	mg/L	0.79 ± 0.12	0.35 ± 0.05
TDS	mg/L	26.7 ± 6.12	9.44 ± 6.5
Arsenic	mg/L	0.049 ± 0.029	0.023 ± 0.02
Cadmium	mg/L	0.025 ± 0.025	0.0415 ± 0.008
Chromium (Cr6+)	mg/L	0.029 ± 0.021	0.05 ± 0.023
Cobalt	mg/L	<0.005	<0.005
Copper	mg/L	0.054 ± 0.046	0.0625 ± 0.04
Lead	mg/L	0.179 ± 0.121	0.15 ± 0.121
Nickel	mg/L	0.15 ± 0.05	0.15 ± 0.05
Zinc	mg/L	0.25 ± 0.025	0.08 ± 0.02

Note: *: The analysis was performed in an accredited laboratory and the measurement uncertainty for the parameter was reported as 13.86%.

.

3.2. Evaluation According to the Classification of Treated Wastewater to Be Used in Irrigation

The Wastewater Treatment Plants Technical Procedures Communiqué, published in the Official Gazette of the Republic of Turkey dated 20 March 2010 and numbered 27527, is prepared with the aim of “regulating the basic technical procedures and applications to be used for the selection of technology, design criteria, disinfection, reuse and deep-sea discharge of treated wastewater and disposal of sludge generated during treatment activities of wastewater treatment plants related to the treatment of wastewater originating from residential areas” and is being implemented in Turkey. In ANNEX 7 of this communiqué, Areas and Criteria for Reuse of Treated Wastewater (Table E7.1), a classification of treated wastewater to be reused in irrigation was made as follows: “Class A: a—agricultural irrigation: food products not commercially processed, b—irrigation of urban areas, and Class B: a—agricultural irrigation: food products commercially processed, b—irrigation areas with restricted access, c—agricultural irrigation: non-food plants”. Again, in the same legislation (Table E7.2) an evaluation of the chemical quality of irrigation water was made.

This study was designed according to national irrigation water standards, and the evaluations were made according to the national standards that are the field of application.

In order to evaluate the output results of the wastewater treatment plants, the values obtained according to the classification of the treated wastewater to be used for irrigation in Table E7.1 of the Wastewater Treatment Plant Technical Procedures Communique were compared. Although MBR provides the communiqué limit values regarding the results of the effluent treated wastewater, it is in the category of treated wastewater in Class A. When the treated wastewater results of ASP are examined, it is seen that it is in the Class A category except for the BOD parameter, and it is in the treated wastewater of the Class B category with the BOD parameter (

Table 3. Classification of treated wastewater to be used in irrigation according to the Wastewater Treatment Plant Technical Procedures Communiqué.

Parameters	Unit	Results		Classification of Treated Wastewater to Be Used in İrrigation (Table E7.1)
		ASP	MBR	A	B
pH	-	7.39 ± 0.12	7.47 ± 0.2	6–9	6–9
BOD	mg/L	18.45 ± 6.55	5.96 ± 2.6	<20	<30
Active chlorine	mg/L	0.79 ± 0.12	0.35 ± 0.05	>1	>1
SS	mg/L	21.79 ± 6.21	7.2 ± 2.3	-	<30
Coliform *	CFU/100 mL	0	0	0	<200

Note: *: The analysis was performed in an accredited laboratory and the measurement uncertainty for the parameter was reported as 13.86%.

).

Treated wastewater in Class A can be used in food products that are not processed commercially, in food products that are irrigated with surface and sprinkler irrigation and directly eaten raw, and in all kinds of green field irrigation. However, these irrigations should be performed at a distance of at least 50 m from the wells from which drinking water is supplied. Treated wastewater belonging to class B should be used in commercially processed food products, in irrigation areas with restricted access, in agricultural irrigations without food products, in places where public access is restricted such as grass production and aquaculture, and in pasture irrigation of grazing animals [12].

3.3. Evaluation of the Chemical Quality of Treated Water for Use in Irrigation Water

In order to evaluate the chemical quality of the irrigation water, the values of the treated water were compared with the values in Table E7.2 of the Wastewater Treatment Plant Technical Procedures Communiqué. It is seen that the output values of ASP do not comply with the limit values in terms of conductivity. It has been concluded that the reason for this is that Na and Cl parameters are high, and this increases salinity and therefore conductivity. It has first-quality water quality in the TDS parameter. It is in the third-quality water category in terms of chloride, boron and sodium parameters. It has been determined that the treated water of MBR has first-quality water features in terms of TDS and chloride parameters, second-quality water in terms of conductivity and boron parameters, and third-quality water according to sodium results (

Table 4. The quality of the treated wastewater to be used in irrigation according to the Wastewater Treatment Plant Technical Procedures Communiqué.

Parameters	Unit	Results		Chemical Quality of Irrigation Water (Table E7.2)
		ASP	MBR	(I)	(II)	(III)
Conductivity	µS/cm	6430 ± 1490	1056 ± 110.3	<700	700–3000	>3000
TDS	mg/L	78 ± 15	73.3 ± 9.5	<500	500–2000	>2000
Cl	mg/L	457.7 ± 32.2	71.1 ± 18.4	<140	140–350	>350
Na	mg/L	321.1 ± 13.1	59.98 ± 14.1	<3	3–9	>9

).

Since the conductivity parameter of the treated water in ASP does not meet the standard values, it is thought that if it is used as irrigation water, it may cause the plant to dry out and die because it gives the cell water to the soil to dilute the soil water due to the osmotic pressure difference between the cell water in the plants and the plant cells [13,14]. In addition, it creates problems such as deterioration of soil structure due to salinity. Salinity in water is measured by conductivity and TDS parameters. The increase in salinity in irrigation water is the decrease in the osmotic gradient between the water and the plant cell of the soil. The plant releases the water in its cell back to the soil to dilute the salty water in the soil, and as a result, plant growth is inhibited. No adverse results were observed in plants when TDS was less than 500 mg/L. Sensitive plants may be affected if it is in the range of 500–1000 mg/L. In the range of 1000–2000, many plants are affected by this, and care should be taken. Generally, irrigation waters with a TDS value above 2000 mg/L can be used on permeable soils for salinity-tolerant plants [15,16].

The Mezitli region’s sewerage system is collected at zero/minus sea level on the seashore. The collected wastewater reaches the treatment plant from here by pumping. Therefore, it has been concluded that there is seawater intrusion in the collection pit at the pumping center or in the sewerage line at the minus level. This situation has been reported to the infrastructure operator, and it has been reported that it has been included in the investment program.

The sodium adsorption rate (SAR) changes to the water content after the water comes into contact with sodium. It both swells and hardens the soil during irrigation by adsorption of sodium by water. In this case, irrigation with water with a high SAR value in the agricultural area slows down the infiltration rate of the soil, and the water that cannot leak from the solidified soil prevents the cultivation of agricultural products after a certain period of time [17]. In the Wastewater Treatment Plant Technical Procedures Communiqué, the values were compared according to the sodium tolerance of different plants in the irrigation water. As a result of this evaluation, it was determined for which kinds of plants the treated water should be used according to the sensitivity of the plants to sodium (

Table 5. Evaluation of sodium tolerance in irrigation of plants classified according to their sensitivity.

Parameter	Unit	Results		Tolerance of Plants to Sodium (Table E7.4)
		ASP	MBR	Very Sensitive	Sensitive	Medium Tolerant	Tolerant
SAR	mg/L	26.7 ± 6.12	9.44 ± 6.5	2–8	8–18	18–46	46–102

). According to the SAR results, considering the results of ASP, it has been observed in the table to have medium tolerance. Considering the mean and standard deviation results of the MBR, it is seen that it is in the very sensitive or sensitive classes.

In the very sensitive class, it can cause leaf burn on deciduous fruit trees, citrus and avocado plants. In the precision class, the growth of beans can be inhibited. In the medium tolerance class, in plants such as alfalfa, oats and rice, growth may be inhibited due to the nutrient and soil structure, and the plant may be stunted. In the tolerant class, for wheat, alfalfa, barley, tomato, sugar beet and some grass species, there may be a possibility of stunting and stunting due to poor soil structure [18,19].

3.4. Evaluation of Heavy Metal Content of Treated Water for Use in Irrigation Water

Heavy metals are found in very low concentrations in the aquatic environment. The effect of heavy metals on plants varies according to their concentration. If these elements are in high concentrations in irrigation water, effects such as plant and soil incorporation of such elements, damage to leaves or regression in growth can be seen. The concentration of heavy metals in domestic wastewater is generally low. However, when industrial wastewater is discharged into domestic wastewater, the concentrations may rise [20]. Therefore, in this study, values were compared according to the maximum allowable heavy metal and toxic element concentrations in Table E7.7 of the Wastewater Treatment Plant Technical Procedures Communiqué (

Table 6. Maximum permissible concentrations of heavy metals and toxic elements in irrigation water.

Parameters	Unit	Results		Maximum Permissible Concentrations of Heavy Metals and Toxic Elements in İrrigation Water (Table E7.7)
		ASP	MBR	Limit Values for Continuous İrrigation on All Kinds of Solids	Limit Values for İrrigation Less than 24 Years in Clay Soils with a pH Between 6.0 and 8.5
Al	mg/L	<0.7	<0.7	5.0	20.0
As	mg/L	0.049 ± 0.029	0.023 ± 0.02	0.1	2.0
Be	mg/L	<0.0005	<0.005	0.1	0.5
Cd	mg/L	0.025 ± 0.025	0.0415 ± 0.008	0.01	0.05
Cr	mg/L	0.029 ± 0.021	0.05 ± 0.021	0.1	1.0
Co	mg/L	<0.005	<0.005	0.05	5.0
Cu	mg/L	0.054 ± 0.046	0.0625 ± 0.04	0.2	5.0
F	mg/L	<0.2	<0.2	1.0	15.0
Fe	mg/L	<0.2	<0.2	5.0	20.0
Pb	mg/L	0.179 ± 0.121	0.15 ± 0.121	5.0	10.0
Li	mg/L	<0.015	0.001	2.5	2.5
Mn	mg/L	<0.05	<0.05	0.2	10.0
Mo	mg/L	<0.005	<0.005	0.01	0.05
Ni	mg/L	0.15 ± 0.05	0.15 ± 0.05	0.2	2.0
Se	mg/L	<0.005	<0.005	0.02	0.02
V	mg/L	<0.001	0.003	0.1	1.0
Zn	mg/L	0.025 ± 0.025	0.08 ± 0.02	2.0	10.0

). In order to determine heavy metals and elements that may cause toxicity, analyzes for Aluminum (Al), Arsenic (As), Beryllium (Be), Cadmium (Cd), Chromium (Cr⁶⁺), Cobalt (Co), Copper (Cu), Fluoride (F), Iron (Fe), Lead (Pb), Lithium (Li), Manganese (Mn), Molybdenum (Mo), Nickel (Ni), Selenium (Se), Vanadium (V) and Zinc (Zn) were made in treated waters.

As a result of heavy metal parameters made in both plants, all results except Cadmium are in the “limit values in case of continuous irrigation in all kinds of soil”. Cadmium, it is in the category of “irrigation less than 24 years on clayey soils with a pH between 6.0 and 8.5”.

3.5. Evaluation of the Nutrient Content of Treated Water for Use in Irrigation Water

The presence of organic and inorganic compounds in various amounts in wastewater increases the diversity of sources. In addition to the purification of compounds that can harm the environment and plants in treated water, the presence of untreated nutrients in irrigation water reduces the use of industrial fertilizers and increases the efficiency of plant growth [21]. For this reason, the values according to the nutrient levels that may be in the recycled wastewater were compared according to the Wastewater Treatment Plant Technical Procedures Communiqué (

Table 7. Comparison of nutrient values of treated water with AST and MBR.

Parameters	Unit	Results		Nutrient Levels that May Be Present in the Recovered Wastewater (Table E7.9)
		ASP	MBR	ASP	MBR
TN	mg/L	19.66 ± 4.34	10.2 ± 6.1	15–35	7–18
NO3-N	mg/L	17.8 ± 5.64	7.20 ± 2.82	10–30	5–11
TP	mg/L	5.25 ± 0.75	1.43 ± 1.1	4–10	0.3–5

). In the study, the characterization of the treated water of two different treatment plants was compared with the values specified in the Wastewater Treatment Plant Technical Procedures Communiqué. In order to evaluate nutrient treatments, some reference values in the literature were taken. For ASP, references to conventional activated sludge treatment systems were taken (Ref. ASP); for MBR, references to membrane bioreactors (Ref. MBR) were taken [22,23,24,25].

ASP meets all of the classical activated sludge process limit values. When the MBR limit values of MBR are considered, TN and TP values provide limit values. In the effluent of MBR, the NO₃-N parameter cannot provide the mean value limit value, but it can be seen that it can reach the desired value with the standard deviation value.

4. Conclusions

In this study, the evaluation of the use of treated wastewater from wastewater treatment plants as irrigation water and the usability of wastewater treatment plant effluent in two different districts in Mersin as irrigation water in agricultural areas were investigated. For this purpose, main pollution parameters were analyzed in the treated waters of two different wastewater treatment plants. The results of the analysis were evaluated according to the Wastewater Treatment Plant Technical Procedures Communiqué in the national legislation.

As a result, when the analysis results of the treated water and the Wastewater Treatment Plant Technical Procedures Communiqué irrigation water limit values are examined, it has been evaluated that the treated wastewater in ASP does not meet the limit values to be used as irrigation water and therefore cannot be used in agricultural irrigation in its current form. However, it has been evaluated that the treated wastewater in MBR meets the limit values for irrigation water and can be used for agricultural irrigation purposes.

This study has been shared with the Mersin Water and Sewerage Administration. Due to the intensive demands of agricultural irrigation unions and the critical levels of agricultural irrigation water demand, the Mersin Water and Sewerage Administration Kızkalesi Wastewater Treatment Plant has made the necessary application to the Ministry of Agriculture and Forestry for the discharge water to be used for agricultural water purposes.