Oxidation Ditches for Recycling and Reusing Wastewater Are Critical for Long-Term Sustainability—A Case Study

Abstract

Because the cost of operation and maintenance is lower than those of chemical treatments, the aerobic biological oxidation method used to treat wastewater is very effective. An oxidation ditch can be classified as progressive aeration-activated sludge capable of removing organic pollutants and also nitrogen and phosphorus. The overall goal of this research is to analyse influent, effluent, and operating data over a seven-year period (2014–2020) to better understand process performance, identify knowledge gaps, and suggest potential improvements for the operating efficiency of the wastewater treatment plant (WWTP) in Bishah Governorate, which works with oxidation ditch technology. An examination of historical influent, effluent, and operational data shows that the Bishah WWTP has consistently met the national and international guidelines for wastewater reuse in restricted and unrestricted irrigation. The effluent ratio of the biodegradable organic material (BOD₅)/chemical oxygen demand (COD) values ranged from 0.3 to 0.51 with an average of 0.41. Significant Pearson correlation coefficients (bivariate) between physico-chemicals merit, especially in total coliforms form, BOD₅ and ammonia. It could be concluded that the operational performance of a wastewater treatment plant with an oxidation ditch in Bishah is working well according to national and international standards.

1. Introduction

The availability of clean water resources is increasingly capturing the global community’s attention and emerged as a key issue at the Sustainable Development Goals (SDGs) Summit on 25 September 2019 in New York. Recycling and reuse are important for sustainability for the following reasons [1,2,3,4]: Population growth in the second half of the twentieth century put a strain on water resources. Furthermore, as a result of urbanisation, weather patterns and the agrarian nature of many areas have been altered [1,5].

Arab countries receive about 2% of the world’s average annual revenue rainwater and only account for 0.3 percent of the world’s renewable water resources on an annual basis, despite accounting for roughly 10% of the world’s land area. Despite the fact that much of the Arab region is extremely dry, the average annual rainfall is around 250 mm, with a yearly precipitation rate of no more than 5 mm. A variety of approaches will be used to determine the reuse of treated wastewater, including improving the precision of treated water as an alternative source of irrigation, improving public awareness and perception of reused water, establishing resident environmental and public health criteria for reuse, and investigating efficient utilisation policy in crop expansion production as well as groundwater protection [6,7,8].

Aerobic biological oxidation wastewater treatment technology includes the activated sludge process, aeration tank, aerated lagoon, sequence batch reactor, trickling filter, rotating bio contactor, and the combination and further development of these process systems [4,9]. The process of treating wastewater with aerobic biological oxidation is highly efficient because the operating and maintenance costs are lower than those of chemical treatment. Performance indicators are a useful tool for communicating process conditions and energy efficiency to facility plant operators. Flow rate, BOD5, TSS, PO4, and power consumed/m³ are the operational indicators of a wastewater treatment plant [10,11,12]. Due to the various components involved in each industry, there has been no safe way to treat waste until now. In practise, a combination of methods is frequently used to achieve the desired water quality at the lowest possible cost [13].

Oxidation ditch technology was developed in the Netherlands in the 1950s [14,15]. Oxidation ditches are advanced aeration-activated sludge capable of removing organic parameters as well as nitrogen and phosphorus [16,17]. Oxidation ditches are biological treatment methods that achieve high efficiency while requiring only basic operational knowledge. The first stage is an intermittent oxidation ditch, aeration, and precipitation evolution. The second is an oxidation ditch with a vertical aerator and nitrification and denitrification reactions. The third is oxidation ditch development, which has high phosphorus and nitrogen removal efficiency. The circulation oxidation ditch is the fourth generation of oxidation ditch, and it effectively treats wastewater while reducing plant human labour and increasing sewage treatment plant efficiency [18].

The following are the advantages of oxidation ditches: the impact of a shock load is reduced by a long hydraulic retention time and complete mixing; sludge production is lower than in other biological treatment methods; and, when compared to other biological treatments, the operations use less energy [19]. The cost of constructing oxidation ditches varies depending on wastewater treatment plant capacity, effluent standards, land price, local construction prices, and local-specific design effluent limitations [19]. To achieve sustainable community development, wastewater treatment plants must achieve two main goals: energy saving and funds saved for WWTP improvements. Aeration energy typically accounts for half of the total energy consumption in WWTPs, making it imperative to identify the most critical optimization conditions in order to reduce the process’s carbon footprint [18].

As stated in the Kingdom of Saudi Arabia (KSA) strategy for water resources management, one of the important goals is to ensure safe and sustainable water of high quality. Furthermore, the oxidation ditch process is a well-proven secondary wastewater treatment technology that can be used in any situation where activated sludge treatment (conventional or extended aeration) is appropriate. Because it requires more land than conventional treatment plants, the process is very effective in small installations, small communities, and isolated institutions. As a result, the goal of this study was to evaluate influent, effluent, and operational data over a seven-year period (from 2014 to 2020) in order to better understand process performance, identify knowledge gaps, evaluate process efficacy, and suggest possible improvements for efficiently operating the Bishah Plant for Wastewater Treatment, which uses oxidation ditch technology.

2. Materials and Methods

The wastewater treatment plant (WWTP) located in Elgemaie District, Bishah Governorate, Aseer Region, KSA is an oxidation ditch activated sludge constructed in 2005 and designed to handle a typical daily flow of 14.400 m³/day of municipal wastewater. The flow diagram of the plant operation (Figure 1) reveals it consists of:

A review of historical influent, effluent, and operational data are as follows:

(Profile of influent wastewater treatment plant):

• Aeration tank volume: 4200 m³
• Detention time: 1.45 day
• Sludge age: 20 day
• MLVSS: 2868 mg/L
• MLSS: 3824 mg/L
• F/M ratio: 0.13 mg BOD₅/mg MLVSS
• Waste sludge flow: 0.248 mg/d
• TSS in waste sludge: 10.920 mg/L
• Waste sludge: 3920 mg/L (TSS)
• Return sludge flow: 50.47% of raw wastewater
• Aeration horsepower: 73.76 hp
• The oxidation ditch was designed for the following influent loadings:

  o

  Design influent flow: 14.400 m³/day

  o

  Influent BOD₅ concentration: 383 mg/L

  o

  Influent TSS concentration: 242 mg/L

  o

  Influent ammonia concentration: 72 mg/L

2.1. Inlet Works

• Preparation: Oxidize the gases resulting from activity of the anaerobic bacteria inside the pipes and helps to float oils, fats, and greases.
• Oil, grease, and floating materials separation unit: The oil collection well is cleaned regularly, and the quantities that are collected are emptied into its own container and discharged continuously in the places designated for it.
• Mechanical strainers: Ensure the cleanliness of the mechanical filters and the non-accumulation of solid and large-sized wastes on the rods of the strainers.
• Sand filter: The filter medium is usually a mix of coarse and soft sand. The recommended effective media size (ES) range is 0.75 to 2.0 mm, and the uniformity coefficient (UC) should be less than 4.
• Flow meter: Should be regularly calibrated for accurate and proper operation.

2.2. Biological or Secondary Treatment, which Consists of

• Aeration ponds: Controlled dissolved oxygen to activate the aerobic bacteria in order to set a parameter (F/M) which is the percentage of bacteria with the amount of organic load in the wastewater.
• Sedimentation ponds: In which bacterial cells are deposited at the bottom of the sedimentation pond and the clear waters regularly overflow through V-shaped holes. On the surface of the water in the sedimentation basin, there is an upper scraper that removes the floating suspended solids collected on the surface of the pond.

2.3. Tertiary Treatment, Which Carried Out Through

• Sand filters: Receive the clear water from the sedimentation pond. The efficiency of the filter in terms of removing solid particles must be measured by measuring TSS of the water entering and leaving the filter.
• Chlorination: The appropriate chlorine dose (Kg/hour) is determined based on the amount of filtered water and the percentage of free chlorine remaining in the water as specified in the regulations.

An examination of historical influent, effluent, and operational data of the Bishah WWTP from 2014 to 2020 was conducted. Wastewater grab samples were collected monthly for a period of seven years to determine which of their quality indicators complied or did not comply with Saudi Arabia specifications [20] and to determine the appropriate wastewater discharge methods. The samples were collected, preserved, and analysed physically and chemically as well as bacteriologically and parasitologically, using the Standard Methods for the Examination of Water and Wastewater [21].

2.4. Physico-Chemical Characteristics

The physico-chemical properties include the following parameters: pH-value, total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD₅), nitrate (NO₃-N), total Khjeldahl nitrogen (TKN), which includes organic nitrogen and ammonia (NH₃-N), and residual chlorine. Unless otherwise specified, all analyses were performed in accordance with APHA [21]. Water samples were also digested using the method described in [21] to measure heavy metals (Cr, Zn, Cd, Pb, Co, Mn, Cu, Fe, and Ni) using the atomic absorption spectrophotometric method.

All materials associated with trace metal sampling and analyses were thoroughly acid cleaned before use to avoid contamination. Glassware and Teflon vessels were immersed in a 10% v/v nitric acid solution for 24 h before being washed with distilled and deionized water.

2.5. Bacteriological Examinations

Bacteriological sampling, dilutions of inlet and final effluent samples, and transferring samples were carried out according to APHA [21]. Determination of total coliforms (Part: 9000–9215) and E. coli (Part: 9000–9221) were carried out with the multiple tube fermentation technique, most probable number (MPN/100 mL) in accordance with the standard methods, APHA [21] as a follows:

• Enumeration of total coliform by multiple tube fermentation (MTF) technique: Total coliform (TC) was determined using MTF technique according to APHA (2017).
• Presumptive test: Lauryl tryptose broth medium was used for presumptive test of TC. Appropriate three-decimal dilutions from each sample in three replicate tubes (ten tubes containing double strength of lauryl tryptose broth) were employed. The inoculated tubes were incubated at 37 ± 0.5 °C for 48 h, after which acid and gas production was recorded as positive presumptive test.
• Confirmed test: Brilliant green lactose bile broth (BGB) was used in the confirmatory test. Tubes containing BGB were inoculated with the positive presumptive tubes and incubated at 37 ± 0.5 °C for 48 h. The production of acid and gas was recorded as a positive confirmative test for TC. Referring to MPN tables for three rows of tube dilutions, the results were reported as MPN-index of confirmed TC per 100 mL of sample.
• Enumeration of E. coli: E. coli was detected by adding a few drops of kovacs reagent on positive EC broth tubes. The results were calculated according to three tubes of MPN table and expressed as MPN-index/100 mL.

2.6. Helminths Eggs (Parasitology)

The wastewater samples (volume 5 L for each sample) were collected from influent as well as effluent of Bishah Wastewater Treatment Plant and processed according to APHA [21]. After centrifugation, the deposit was then transferred to one or more microscope slides, covered with a cover slip, and examined under the microscope to differentiate and enumerate parasitic helminth ova using the 10× objective lens and the 40× objective lens to confirm any uncertainties.

Bacteriological examination and Helminth egg count were performed on samples collected from the final effluent after chlorination in sterile plastic containers containing sodium thiosulfate pellets for dechlorination of the samples, which were then transferred to the laboratory and tested immediately.

2.7. Statistical Analysis

The statistical analyses were conducted using correlation analysis “ggplot2” package and “corrplot” for box-plot histograms and correlation matrix in R 4.2.0. in R 4.1.1 [22,23].

3. Results and Discussion

In treatment plant administration and engineering, key parameters are a type of metric used to express process conditions and energy efficiency. It should be kept to a minimum, clearly defined, easily measurable, verifiable, and understandable [12]. The WWTP’s Kay Performance Indicators (KPIs) are flow rate, BOD₅, TSS, PO₄, kWh/m³, kWh/TSS, and solid generated/TSS [13]. The primary indicators of activated sludge process operations are dissolved oxygen, return activated sludge rate, and waste-activated sludge rate [10]. In addition to controlling these critical operational indicators, efficient activated sludge operation necessitates regular inspection of operational control parameters, such as microscopic examination of activated sludge, control of mixed liquor respiration rate or nitrification rate, measurements of sludge volume index SVI, and control of clarifier sludge blanket depth. Because there is no clear boundary zone between anoxic and aerobic conditions, aeration intensity and resulting DO are the primary parameters of the nitrification and denitrification processes [24].

According to the operational profile (Figure 2), the Bishah WWTP has consistently met NPDES (National Pollutant Discharge Elimination System) permit limits, but the performance of WWTPs is ultimately dependent on operating practices and decisions made by decision-makers. Many WWTPs are focusing on reducing carbon footprints and avoiding energy consumption in addition to complying with NPDES permits [25,26,27,28]. Monitoring and controlling the process is critical for efficient operation, which saves energy and lowers operating costs, while effluent characteristics demonstrate the need for effluent reuse and reclamation. The Bishah Wastewater Treatment Plant has three major treatment steps: I-Inlet works, II-Biological (secondary) treatment, and III-Tertiary treatment. For BOD₅, COD, NH₃-N, and TSS, respectively, the inlet works resulted in removal efficiencies of 94.75%, 93.07%, 89.47%, and 96.72%. Furthermore, biological treatment was less efficient, with removal efficiencies of 90.5%, 89.3%, 83.3%, and 94.2% for BOD₅, COD, NH₃-N, and TSS, respectively. Because there is no clear boundary zone between anoxic and aerobic zones, aeration intensity and resulting DO are the primary parameters of the nitrification and denitrification processes. DO concentrations in the range of 0.4–0.8 mg/L have a significant impact on the process [24]. As a result, tertiary treatment and disinfection are required for the Bishah Wastewater Treatment Plant. As a result, the final effluent produced after the third step meets national and international standards.

The average daily flow rate is approximately 14.4 m³/day with a TSS influent concentration of approximately 242 mg/L. The average daily organic loading (BOD₅) is approximately 383 mg/L, and influent ammonia levels have been approximately 72 mg/L. Furthermore, the BOD₅/COD ratio is frequently constant (0.57–0.67) over the course of the study. The results showed that the raw domestic wastewater was of medium strength, and its concentrations remained relatively constant throughout the operating years (Figure 3).

The key performance study of the integrated Bishah oxidation ditch wastewater treatment plant (the study years extend from 2014–2020) showed the effluent ratio of BOD₅/COD values ranged between 0.3 to 0.51 with an average of 0.41, as demonstrated in Figure 4.

BOD₅/COD ratios less than 0.5, according to Tchobanoglous et al. [29], indicate that chemical substances with low biodegradability may slow or delay the biological process, consuming easily biodegradable organic matter (BOD₅). As a result, the biodegradability of domestic wastewater has been deemed to be somewhat low. Furthermore, Liu et al. [30] found that different operational parameters (sludge load, influent component, temperature, hydraulic retention time (HRT), dissolved oxygen, diversity in the dominant microbial communities, and operational mode) had a significant impact on wastewater biodegradability. Furthermore, because it requires more land than conventional treatment plants, this technology is very effective in small installations, small communities, and isolated institutions [31]. Bishah Governorate, for example, is distinguished by vast land areas with low population density [6,7].

In addition, the integrated system presented a high removal efficiency of carbon as well as nutrients. The average removal among the years of monitoring is shown in Figure 5 and Figure 6.

The removal of BOD₅, nitrate, and coliforms in 2015 is appropriate and indicates that the WWTP is operating properly. However, these parameters indicate that there was a drop in operation in 2017. Furthermore, significant removal of pollutants such as BOD₅, COD, TSS, total Kjeldahl nitrogen (TKN) (ammonia (NH₃-N), and nitrate (NO₃-N) is observed. Because the influent BOD₅ concentration is 383 mgO₂/L, the final effluent annual mean concentrations range from 5 mgO₂/L to 11.8 mgO₂/L with a maximum percentage removal of 98.7% (SD 0.6). Furthermore, a significant removal was detected in Figure 5 when comparing the valuation to TSS influent and effluent. The maximum TSS removal percentage is 99% (SD = 0.38).

Moreover, the annual concentration of ammonia fluctuates from 1.8 to 2.2 mg NH₃/L; simultaneously, its removal percentage ranged between 96.9% and 98.3% (SD ± 0.47). Furthermore, the ratio of TKN/COD was ranging between (0.1 to 0.22) with an average value of 0.16, as shown in Figure 7. This is also appropriate with the data specified by [32].

Each sample was collected monthly and measured in triplicate. Error bars indicate the minimum and maximum values. BOD₅ = biological oxygen demand (BOD₅); Total Kjeldahl Nitrogen (TKN) (Ammonia = NH₃-N; Nitrate = NO₃-N); TSS = Total Suspended Solids; COD = Chemical Oxygen Demand.

The NH₃ removal occurs theoretically via three mechanisms: gaseous NH₃ stripping to the atmosphere, NH₃ assimilation in microbe biomass, and biological nitrification. From the results, it can be noticed that there is limited fluctuation in nitrate concentration, as recorded in Figure 5 in the final effluent; this indicates that nitrification has limited presentation [33]. The nitrification process is accomplished by two groups of nitrifying bacteria Nitrosomonas and Nitrobacter [33].

Furthermore, according to [34,35,36], the average daily effluent ammonia-N concentration was 0.15 mg/L, while the average daily effluent BOD₅ concentration was 5 mg/L, resulting in effluent ammonia-N and BOD₅ reductions of 89 percent and 50%, respectively. Every gram of nitrate-N converted to nitrogen gas saves 2.86 g of oxygen in terms of CBOD₅ removal. Providing an anoxic zone within the ditch can thus result in energy savings ranging from 10 to 20%. Denitrification is a zero-order reaction that reduces nitrate concentrations to extremely low levels, indicating that it is dependent on dissolved oxygen concentration and carbon source rather than NO₃-N concentrations.

Furthermore, heavy metals of Ni, Pb, Mn, Cr, Zn, Cu, Cd, Co, and Fe measured have mean values of 0.0145, 0.02, 0.004, 0.002, 0.0101, 0.0032, 0.0003, 0.0004 and 0.02 mg/L, respectively. Moreover, the maximum and minimum values (

Table 1. Minimum, maximum, and mean annual values of heavy metals in treated wastewater produced from Bishah oxidation ditch wastewater treatment plant.

Heavy Metals	Unit	Final Effluent Produced from Bishah Oxidation Ditch Treatment Plant				MWE (2006) for		WHO (2006) in Irrigation Water	EU (2020)
		Min.	Max.	Mean	SD	RI	URI
Nickel	mg/L	0.0085	0.0200	0.0145	0.00349	0.20	0.20	0.20	0.020
Lead	mg/L	0.0014	0.0400	0.0200	0.00151	0.10	0.10	5.00	---
Copper	mg/L	0.0003	0.0060	0.0032	0.00208	0.40	0.40	0.20	2.000
Manganese	mg/L	0.0013	0.0070	0.0040	0.00226	0.20	0.20	0.20	---
Chromium	mg/L	0.0010	0.0030	0.0020	0.00092	0.10	0.10	0.10	0.025
Cadmium	mg/L	0.0001	0.0006	0.0003	0.00019	0.01	0.01	0.10	0.005
Zinc	mg/L	0.0001	0.0200	0.0101	0.00584	4.00	2.00	2.00	---
Iron	mg/L	0.0100	0.0400	0.0200	0.01221	5.00	2.00	5.00	---
Cobalt	mg/L	0.0001	0.0007	0.0004	0.00020	0.05	0.05	0.05	---

Means are average of 84 collected monthly random samples for each parameter. Min. = Minimum; Max. = Maximum; SD = Standard Deviation; MWE = Standard Limits of treated wastewater according to Ministry of Water and Electricity in 2006; RI = Restricted Irrigation; URI = Unrestricted Irrigation; WHO (2006) = FAO guidelines for trace metals in irrigation water, Compendium of standards for wastewater reuse in the Eastern Mediterranean Region, World Health Organization Regional Office for the Eastern Mediterranean Regional Centre for Environmental Health Activities (https://applications.emro.who.int/dsaf/dsa1184.pdf, accessed on 8 January 2022); EU (2020) = Guidelines to support the application of Regulation 2020/741 on minimum requirements for water reuse (2022/C 298/01) for class (A) of reclaimed water according to the regulation for irrigation methods (https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:52022XC0805(01)&from=EN, accessed on 8 January 2022).

) of these samples met the Saudi and WHO standards [20,37].

Bacteriological counts recorded during the study years revealed that E. coli was removed by 2 log with an average residual count of 4102 MPN/100 mL, as shown in Figure 5 and Figure 6. The annual average E-coli removal percentage was 99.9%. The disinfection efficiency of chlorination to final effluent is expected to affect the final effluent’s viability for reuse. The significant correlation (Figure 6) found between E. coli removal and chlorine dose is consistent with previous research, which found that as chlorine dose increased, so did E. coli inactivation [38,39,40,41]. Aside from that, the average annual residual chlorine value for municipalities at the Bishah oxidation ditch sewage treatment plant ranged between 0.3 and 0.53 mg/L. More than half (57%) of the samples examined contained less than 0.5 mg/L of free residual chlorine, which is less than the maximum allowable mean value set by the Saudi Water and Electricity Ministry [20,42], which states that residual chlorine should be 0.5 mg/L.

The results, on the other hand, show the efficiency of the chlorine dose used, which lead to the complete removal of parasites (helminthes eggs) in the final effluent of the Bishsah oxidation ditch municipal wastewater treatment plant, as shown in Figure 3.

A Risk Management of Wastewater Reuse assessment approach was developed by conducting a risk analysis of the various recommended wastewater irrigation microbial health guidelines for unrestricted irrigation. The World Health Organization and the US Environmental Protection Agency have proposed international guidelines for the safe reuse of domestic wastewater [43]. Treatment and microbial standards with varying microbial water quality criteria have been proposed to improve the local state of affairs and health risks.

Guidelines for wastewater recycling and reuse have been developed based on the presence of bacterial indicators and nematode eggs. Furthermore, it is linked to the pathogen’s last point of contact with the host as well as chemical ingestion and contact with the human body [44,45].

As it is clear from

Table 2. Standard limits of treated wastewater reuse of Saudi Arabia.

Parameters	Unit	MWE (2006) for		WHO (2006)	European Commission
		URI	RI		2017	2020
pH		6–8.5	6–8.4	6–8.5	6–8.5	6–8.5
Turbidity	NTU	5	5	5	≤5	≤5
Total suspended solids	mg/L	10	40	40	≤10	≤10
Biological oxygen demand	mgO2/L	10	40	40	≤10	≤10
Chemical oxygen demand	mgO2/L	50	---	50	---	---
Ammonia	mgNH3/L	5	5	5	---	---
Nitrate	mgNO3/L	10	10	10	---	---
Residual chlorine	mgCl2−/L	0.5	0.5	0.5	---	---
E. coli	%positive MPN/100 mL Log10 reduction Cfu/100 mL	8.3 1000 --- ---	8.3 1000 --- ---	8.3 ≤100 --- ≤1	--- --- ≥5 ---	--- --- ≥5 ≤1
Helminths eggs	Ova/L	1 viable egg/L	1 viable egg/L	1 viable egg/L	---	---

MWE = Standard limits of treated wastewater according to Ministry of Water and Electricity in 2006 for Unrestricted Irrigation (URI) and Restricted Irrigation (RI) in accordance with the reclaimed water standards for URI and RI in Saudi Arabia (https://applications.emro.who.int/dsaf/dsa1184.pdf, accessed on 8 January 2022); WHO (2006) = Compendium of standards for wastewater reuse in the Eastern Mediterranean Region, World Health Organization Regional Office for the Eastern Mediterranean Regional Centre for Environmental Health Activities; European Commission (2017) = (EC) Minimum quality requirements for water reuse in agricultural irrigation and aquifer recharge (reclaimed water quality class A); EC (2020) = Guidelines to support the application of Regulation 2020/741 on minimum requirements for water reuse (2022/C 298/01) for class (A) of reclaimed water according to the regulation for irrigation methods.

, the effluent produced from the oxidation ditch wastewater treatment plant in Bishah Governorate conforms to the Saudi standard specifications for unrestricted irrigation, but so far, it is only used in restricted irrigation and the European Commission (EC) minimum quality requirements for water reuse in agricultural irrigation and aquifer recharge (reclaimed water quality class A) [20,46,47].

As a result, future wastewater recycling and reuse laws, rules, and regulations must include: (1) more detailed biological and chemical analyses of recycled water and the environment, (2) the development of tools for determining the environmental impact of recycled water and recycling byproducts, (3) the establishment of tools to ensure a reduction in host-pathogen contact, and (4) risk assessment and management mechanisms [48,49]. This is to compensate for the severe water shortage that most countries around the world, particularly the Arab countries, are expected to face. Furthermore, these are consistent with KSA’s water resource management strategy to ensure safe and sustainable water of high quality in the southern region [50].

Because the risks are lower than expected, health risk assessment and data will be useful for lowering reclamation and reuse costs where the treatment technology used must be inexpensive. The most important considerations are economic and technical. Water and wastewater treatment costs, maintenance costs, labour employment, irrigation network structure, and agricultural patterns are among these factors [51]. Risk analysis is a comprehensive framework for wastewater treatment and reuse that includes risk assessment (physical system, loads, uncertainties, and risk quantification), risk management, and risk communication [52,53].

4. Conclusions

Sufficient treatment of municipal wastewater is critical for environmental and public health preservation. Traditional wastewater treatment processes are significantly less expensive and more sustainable; however, they are unfriendly to the environment. For unrestricted irrigation, reusing wastewater could be a useful provision. The outcomes of the risk assessment are relevant to the needs of local management. The surveyed Bishah oxidation ditch municipal wastewater treatment plant met the Saudi standards specified by the Ministry of Water and Electricity (2006) for the reuse of unrestricted irrigation according to the study’s findings. However, it is still used in limited irrigation (irrigation of agricultural land or landscaping). As a result, adequate indicators for wastewater reuse and risk management are required, and data on maximum pollutant concentrations play an important role. More microbial and chemical risk assessment is required before setting guideline limits for reclaimed wastewater reuse. The microbiological parameters that are typically determined are insufficient or incorrect to perform a thorough risk analysis. More information on parasitic parameters and viruses is required. Moreover, values of chemical parameters should be considered when evaluating various wastewater reclamation and reuse alternatives.