Risk Evaluation and Control for treated Wastewater Reuse in Green Spaces

The presence of human enteric pathogens and microorganisms has been detected in raw or treated wastewater, as well as differences between removal rates for various elements or organisms, such as Giardia duodenalis cysts or Cryptosporidium oocysts (Gomes et al., 2019).

There is also evidence which shows the chemical and biological risk in water reuse, potentially exposing humans – as end users – via micro-sprinklers or breathing in biological and virological pollution (Sidhu et al., 2019). However, it is already known that safely managed, treated, and monitored wastewater reuse in agricultural irrigation systems can reduce exposure risks for the general population and contaminants.

This principle has been demonstrated in a small number of cases, with recent scientific studies on agriculture and aquaculture in the urban and peri-urban environment leading the way.

The combination of population growth, pollution, and climate change has exacerbated water scarcity problems. Treated wastewater reuse represents a sustainable water resource alternative for urban areas (Warsinger et al., 2019a) . The expansion of urban areas, municipal water consumption, and pollution place heightened pressure on at-risk ecosystems (Tilley et al., 2014) (Kusumawardhana et al., 2021).

Wastewater treatment plants release considerable amounts of treated water to surface water; this treated effluent has the potential to replenish water ecosystems and restore environmental function through integrated water management (Sheer & Lundie, 2018) (E. Schoen et al., 2018).

2.1. Microbial Contamination

In the case of the irrigated wastewater, the microbial quality also needs to be taken care of in terms of coliforms, Escherichia coli and Salmonella, total microbial content in the form of Bacteria (TVCC-Total Viable Cell Count) at least on a regular time interval or at least annually for Escherichia coli and Salmonella.

Microbial count over 100 cfu/100 ml of Escherichia coli and 1000 cfu/100 ml of TVC at the point of compliance is being considered as the permissible limit according to the relevant standard. According to the recommendation of the WHO, 2004, effluent for unrestricted irrigation should have no more than 1 E. coli per 100 mL in at least 80% of the samples collected over the previous 4-6 weeks, and no sample should have more than 10 E. coli per 100 mL.

On the other hand, if the public is not allowed in the irrigation area, up to 5 E. coli per 100 mL are recommended and not every sample has more than 10 E. coli per 100 mL in effluent for unrestricted irrigation (ODwyer et al., 2020). The risk associated with direct oral ingestion of untreated primary effluent and secondary effluent are calculated to be 100 to 1000 times more than that of treated effluent per year at the scenario evaluated in this study. Streams receiving untreated sewage are found to be disproportionately contaminated.

Given the occurrence of multiple drought periods and the water shortage in Oman, the application of treated wastewater in the soil-plant system has received much attention (Gatica & Cytryn, 2013). However, with such reuse concerns related to human health and environmental risks need to be appraised for minimizing the potential threats.

Among others, microbial quality, the presence of heavy metals, organic micro-pollutants, and antibiotic resistance, emerge as potential issues that need to be addressed. According to the standard guidelines, a microbial quality assessment of E. coli has always been the preferred bacterium for the assessment of microbial load in treated wastewater and reclaimed water (E. Schoen et al., 2018).

2.2. Chemical Contaminant

Assessment of risks to prevent WW from reaching natural water systems is fundamental for public health and ecosystem preservation. Authors should consider proteomic CPU for the multipurpose use and convey its significance regarding WW management.

However, the environmental risks of the WW use can be catagorised into three major concerns: microbial hazards, chemicalcontaminants, and physical hazards during direct contact between the individuals and WW; irrigation of crops with untreated or partly treated WW; and leaching of pollutants from untreated or partly treated WW that is used for crop irrigation into the surrounding underground and/or surface water resources (E. Akpan et al., 2020).

Some recent examples from WW management should also be included. Moreover, many governments have not adopted uniform international standards for the WW management using the multi-step adaptive measures through its release, uses for domestic and commercial purposes, and its other safe disposal methods.

This risk is compounded by the fact that surface water is routinely mixed with inadequately treated domestic and industrial wastewater and subjected to polluted downstream groundwater or subsequent water sources (Abd-Elaty et al., 2021). These facts underscore the significance of preventing anthropogenic chemicals from reaching natural water bodies (Grozavu & MIHAI, 2018).

A significant amount of regulated chemicals and unregulated commercial chemicals commonly used in the manufacturing, agricultural and aquaculture industries are discharged into domestic and industrial wastewater. The presence of these anthropogenic chemicals raises concern over the potential impact of WW on human health, aquatic ecosystem, and animals through repeated WW exposure.

2.3. Nutrient Imbalance

Nutrient imbalances, represented by factors such as tank depletion and drying periods, are observed in treated wastewater (TWW) usage across different geographical and climatic conditions.

These imbalances affect green spaces, leading to changes in vegetation patterns and crop productivity. The supplementation of potable water with TWW for irrigation purposes has shown mixed results, impacting both vegetable production and plant growth characteristics.

 Concerns about urban environmental management, including water scarcity and agricultural risks, have prompted a shift towards reducing water-intensive horticulture, particularly in urban areas where greywater is commonly used. However, reliance on effluent-derived channels for irrigation exacerbates water scarcity issues.

 Nutrient imbalances, exacerbated by chemical fertilizers in TWW, have become characteristic of urbanization, particularly in low-income countries. The role of nutrient dispersal and floc development in urban agro-ecosystems has been discussed, highlighting the need for balanced nutrient loops. TWW usage in green spaces often results in theoretical deficiencies of nitrogen, carbon, and sulfur, due to the absence of local soluble nutrients, necessitating careful management to address these shortcomings.

3. Strategies to Mitigate Risks

This study starts by discussing very broadly the risk burden and the risk management strategies that can be developed for treated wastewater use. It extrapolates treated wastewater use into the production of forages and describes likely health and crop processing variables that determine consumer risks.

It then describes the Epi Model technique which, even though it is very sophisticated, can be quickly implemented and is built on general principles concerning concessional risks and limits. In this article, by understanding a model for acute illness risks and analyzing the scientific literature it describes choices for a total pathogen concentration strategy suitable to the cultivated forage plant and uses these options to support a case study of human health risk analysis.

 It indicates the limitations of advanced practical knowledge and research that are required in this period of use of agricultural treated wastewater supply.

All over the world, reused water is inseparably linked to detailed and continually evolving water guidelines. In the absence of an intricate understanding of the dangers, handling and management of reused water would have been quite unrealistic and open to mismanagement.

Besides confusing water quality goals, these standards face various problems, including capacity and complexity, broad range factors for hazard analysis, health hazard exposure evaluation and outcomes, and the lawful compliance of honesty and equivalence of scales.

Poorly judged and possibly costly soil and groundwater investigations can exist with reused water risk analyses. Furthermore, there are circumstances when levels of substance or distinct persistent distributions of various pollutants cause Euclidean calculations not to improve the understanding of risks. It is at such times that classes logic (CL) is line-oriented and enables locations with specific hazards.

This research focuses on the potential risks of treated wastewater use and the management strategies employed to achieve paratrophic conditions (Gatica & Cytryn, 2013). While there are numerous risks associated with the use of treated wastewater, this review has primarily brought together various strategies that have been useful in reducing the risks associated with its use.

It is worth noting that city planning should consider these potential strategies for the management of treated wastewater at the outset in order to lower the risk of accidents and disasters (E. Akpan et al., 2020).

3.1. Water Treatment Technologies

To provide irrigation quality water, a secondary traditional wastewater treatment approach integrates a coagulation process and rapid sand filtration. This technology usually combines the application of coagulants/flocculants and an HRT of 1 h in a clarifloatation process to separate organics and suspended solids from water.

Rapid gravity sand filtration removes impurities mixed with water from many physical and chemical processes. Rapid sand filters are the tanks that can be created from any bed material, but have to exhibit a noticeable variety of grain sizes for effective bed formation (B. L. Neto et al., 2024). If the treated water contains sediment, it can be directed to an underground storage pond or a sedimentation tank. The physical separation is followed by a multi-stage mechanical and biological purification process in the artificial wetland. The constructed wetland promotes water purification from natural and artificial wetlands.

The Water Treatment Technologies Since the mid-20th century, chlorination is widely accepted as a water treatment technology for public use for producing water suitable for drinking, bathing, and swimming.

The rapid disinfection action of chlorine (Cl2) has also been found to remove many other pathogens, including amoebas, tuberculosis, and the microsporidia of public health significance.

The coagulation process is normally dissolved air flotation (DAF) for physical separation and producing effluents directly reusable as irrigation water. However, DAF is restricted by complicated preparedness and adjustment, low solid concentration, and low load capacities.

Treated wastewater reuse

Treated wastewater reuse can provide a sustainable and low-energy solution to water scarcity in urban green spaces (UGS). This work includes five processes, including coagulation, precipitation, sedimentation, filtration, and chlorination, for treating secondary effluents. For every 100 m3 of water treated, 10 m3 of gypsum will be generated as the byproduct, as well as 10 m3 of waste activated sludge from sedimentation (A. C. Castellar et al., 2021). The treated water (O Dwyer et al., 2020) will then be ready to reuse in urban landscapes without causing substantial damage.

3.2. Monitoring and Testing Protocols

On the other hand, the approach proposed in this work is focused on the rationalization of the monitoring and testing investment, keeping a balance between the cost of data acquisition and the benefits obtainable from a safer operation of the system.

Data must be acquired for the trend of variables driving the risks such as failure in system, process stress, the composition of the effluent, of the sludge and of the overflow; process reliability in case of cyber-physical attacks and composition of the wastewater during this case.

There is a need to focus on the combined action of inorganic and organic solids in wastewater, including nanoparticles and microplastics and to account for the presence of nitrogenous and phosphorus compounds of anthropogenic origin between the composition of the wastewater driving the environmental risk.

The number of species coexisting in the wastewater fractional matrix representing a unique hazard index for the complexity of beasts can be used, in addition to other microbiological indicators (Jesus Garcia-Galan et al., 2020).

Moreover, when separatite storm-water and wastewater, both should be monitored and tested regularly. As precautionary measures, they propose to analyze the overall quantity of heavy metals in the municipal and industrial wastewater based on the importance of hazards and their synergistic effect.

Monitoring and testing protocols are the cornerstone of the risk analysis (Dunton, 2023). A large empirical data set is necessary to improve the credibility of the risk model. As part of urban subsurface system infrastructures, wastewater networks should work with predictable and acceptable margins of residual risk.

The problem of allocating the maintenance and renewal budget should therefore be tackled by means of optimizing decision models to be used in the framework of risk-based strategies. Nevertheless, such models need the support of monitoring and testing protocols given in terms of hazard identification data. The usual approach of exploiting the experience feedback from the analysis of accidents records and the chemical composition of sewage overflow is limited by the very low frequency of accidents and may not correspond to the real pollution scenario for the waters due to a selection bias (Alevizos et al., 2023).

3.3. Proper Irrigation Practices

Some of the wastewaterborn pathogens that are associated in the soil and human heath risks in the field crops, green spaces, vegetable gardens are generenllly the Salmonella spp., Citrobacter, shigella spp. etc. and also some species belong on Aspergillus, penicillium and cladosporium that may cause allergies and respiratory disorders to the exposed.

Some parasites from humans and animals, like certain types of worms and protozoa, can live in the soil or sewage for less than a year. There are also parasites like roundworms, tapeworms, and others, along with larvae from Guinea worms and around 60 other types of parasitic worms, that can be found in soil contaminated with feces from infected hosts. To safely use reclaimed water for irrigation, it's important to assess and manage the risks to human health and the environment. Preventing human illness is the main concern when dealing with these hazards.

Introduction of irrigation involves practices that shall be followed for the plants to grow and mature properly (Gatica & Cytryn, 2013). Over-irrigated sites may display deep water penetration that will enable leaching of pollutants, and the over-leached site at an evaporation phase may cause up-welling from the surface.

 On the contrary, under-irrigation will allow the plants not to get the water it needs for its life. Both over-irrigation and under-irrigation may save the risk for harmful contamination within the soil and plant root boundary in the respective sites of irrigation (Alevizos et al., 2023). It is however essential never to water log or under irrigate under the proper management practices for irrigation even when using treated wastewater for irrigation.

Soil properties have high impact on the treatment of soil as an irrigation method, of soils are well drained, tolerant to salinity, maintained good amounts of soil organic matters thus decreasing the reflected and blocked photons which are key for pathogen inactivation as a UV rays attenuates with an increased amount of pathogens in the water.

If there are more Nickel, Calcium, Chromium, Iron, Magnesium, Manganese, Selenium, Sodium, Lithium, Cadmium, Copper, Lead, and Zinc in the Wastewaters that are required to be used for irrigation, it might be risky for carve minerals upon absorption and corrupts the soil structure for some situations on those subject (Taheri, 2021).

4. Quality Standards for Water Intended for Reuse

Inappropriate irrigation practices and reclaimed water with high quality pose risks. The standards for treated wastewater reuse in irrigation were observed to lack these. water quality standards for urban green spaces (TWW-I-UGS) defining irrigation with treated wastewater.

 The proposed TWW-I-UGS was found to be more conservative than other standards. The findings of the study will improve the guidelines and quality standards for treated wastewater reuse in urban green spaces (Al-Sa’ed, 2007).

Water quality standards are rules that make sure water is safe for people and the environment. They focus on two things: chemicals and living things in the water. The World Health Organization sets these standards.

The California standards did not contain pathogen related criteria in its standard. The microbial quality of treated wastewater substantially meets WHO guidelines. The treated wastewater does not comply with the other standards (Chen & Franklin, 2023).

Documents related to treated wastewater reuse for the green spaces were considered for the comparison analysis. The California state regulations and WHO guidelines were observed to be inadequate for treated wastewater reuse in urban green spaces.

Reuse of treated wastewater for irrigation in urban green spaces is an integral part of a sustainable urban water cycle (UWC) due to the rapidly expanding urbanization and water scarcity (Gatica & Cytryn, 2013). Risk assessment and management (RAM) of wastewater reuse for irrigation substantially depends on the regulations that define the water quality standards.

 The safe and reliable reuse in urban green spaces depend on the quality and degree of treatment and methods of safe application. The review critically assesses the guidelines and quality standards for irrigation water reuse.

Comparison of various regulations across the world for different standards necessitates comprehensive discussion. Furthermore, the present knowledge of the quality of treated wastewater reuse is posed to reclaim as considered against the recommended standards. Reclaimed wastewater has been used for irrigation without considering the health hazards associated with contaminated food and soil.

4.1. Environmental Considerations for Water Reuse

The underlying values to the water reuse technologies are critical at grasping the public significance and environmental and infrastructural challenges. They include securing the quality of water and security; protecting the dignity of fresh water; and the integration and cultivation of green and sustainable techniques for water in structural innovations.

In view of the harsh climate conditions and water scarcity, the adoption of water reuse techniques has been the highest in Kuwait, United Arab Emirates, Israel, Singapore and Bahrain. They are regularly strapped with limited resources, so it has often required money, operating funding and inputs.

This process has been found to be competitive across all countries, but consumers primarily use wastewater for irrigation, particularly in Mediterranean and western Asia but are not incentivized to use wastewater on a larger scale. Water reuse is, indeed, predicted to rise by 50 percent globally (Kulionis et al., 2024).

Water scarcity is a growing threat that endangers food security and the growth of communities, primarily in the developing world. Water reuse, also known as water recycling, reclaims water from sewage, greywater and black water for reuse (Yang et al., 2020).

 The aim is to mitigate the drinking water crisis by obtaining water that meets only quality requirements and improving environmental problems related to water, energy and ecosystem conservation through urban and residential activities. In some of the global scenarios for climate change, the form and quantity of available water resources will vary across the northern, sub-tropical, and tropical latitudes. Techniques for risk control include proper and effective handling of the wastewater before use, and regular evaluation of procedures for water quality (Alevizos et al., 2023).

4.2. Text of standards for water intended for Reuse

The standards that exist worldwide for treated wastewater reusewastewater reuse for irrigation are agnostic concerning the proposed method of irrigation, either drip, sprinkler irrigation, or spread but advise users when the treated wastewater reuse risk presented a subcategory of potential public exposure. The risk is calculated using the likelihood and the potential public offset, not only the total amount of water that is reused but also the microbial water quality of the treated wastewater and the actual paths to the people that are reusing it.

If irrigation presents no potential for aerosol inhalation, not all faecal paths are planned for reuse in the irrigation project, such as urinal flows, and the hydraulic loading is high, all the faecal bacterial indicators maybe at a low predefined concentration for the potential public exposure, otherwise the potential public exposure is much higher than in this case. All these parameters could be incorporated in a biological green project risk adjusted microbiological standard (R. Srivastava & K. Singh, 2021).

Water reuse presents an innovative approach to managing urban water and closing the urban water cycle (Reynaert et al., 2021). Water intended for reuse could be either stormwater or treated wastewater, as they all could be used for various reasons, such as, for the irrigation of green spaces in the cities. Some countries have developed standards based on the risk management of potentially pathogenic and toxicants to protect public health.

 The risk management approach uses microbiological standardsmicrobiological standards as entry-level barriers for facial bacterial indicators, but these results may not be sufficient to assess the public health risk related to the other potential contaminants in the treated wastewater or stormwater. There are currently no microbiological standards, specifically designed for the water intended for reuse in a biological green project.