1. Introduction
One of the major constraints to the socioeconomic sustainability of human livelihoods is water scarcity, which is the disparity between the water supply and demand. The increase in water demand is due to urbanization, population growth, changing patterns of consumption, and supply-side limiting factors, such as water pollution [
1]. A reduction in precipitation by 10–20% has been predicted due to climate change [
2]. Globally, the largest water demand is from the agricultural sector, accounting for around 70% of all freshwater abstractions. Reusing treated wastewater has become a main low-cost and reliable alternative. It has been considered a sustainable unconventional water resource that will increase in the future due to the continuous population increase especially in urban areas [
3]. Macronutrients (nitrogen, phosphorous, and potassium) and several micronutrients (iron, zinc, manganese, and copper) are found more in wastewater than in any synthetic fertilizer, and these components are needed for optimal plant growth [
4,
5]. As a result, reusing these components may enhance soil structure and physical characteristics and may reduce the chemical fertilizers’ requirements [
6].
On the other hand, wastewater reuse has negative impacts that include risks to human health and adjacent ecosystems due to its microbial and chemical components [
7]. Excreta-related pathogens and disease agents such as bacteria, nematode eggs, viruses, and protozoa are found in high amounts in wastewater causing health risks for exposed groups. Many excreta-related pathogens have shown resistance to biological wastewater treatment processes [
8]. The most affecting pathogens on public health in Egypt are Rotavirus, Salmonella, Giardia duodenalis, and Ascaris. They are considered the pathogenic indicators representing each of the major groups of organisms (i.e., bacteria, viruses, protozoa, and helminths) associated with acute and chronic public health consequences in Egypt. Wastewater irrigation can negatively impact soil properties and fertility, crop productivity, quality of groundwater, and aquatic ecosystems. The magnitude of potential effects depends on several factors such as chemical concentrations in the wastewater, solubility, and inherent toxicity [
9].
To reduce the negative impacts, standards and guidelines have been adopted governing wastewater reuse in agriculture. The tolerable health risk of 1 × 10
−6 Disability-adjusted life year (DALY) loss per person per year is recommended to protect public health from different exposures [
8,
10,
11]. Quantitative microbial risk assessment (QMRA) is a probability method that integrates data on pathogen abundance, human exposure, and infection to ensure the safety of the management strategies of wastewater reuse. Quantitative health-based risk assessments incorporate hazard concentrations, dose responses, exposure assessments, and risk characterizations [
12]. This framework has been used for the examination of different reuse scenarios. Farmers can become infected by accidentally swallowing soil particles that are saturated with wastewater. Moreover, consumers of crops irrigated with wastewater, especially vegetables eaten uncooked, are at risk of infection since these crops might be exposed to high and unacceptable levels of pathogens [
13]. Based on the quantitative microbial risk assessment Monte Carlo program, Mara and Sleigh [
14] studied the infection risks of the Rotavirus and bacterial infection while using reclaimed water in restricted and unrestricted irrigation. Hamilton [
15], Mara and Sleigh [
14,
16], and Sant’Ana [
17] used QMRA models to estimate infection risks with
E. coli, Salmonella, helminths egg, and enteric viruses from irrigation with wastewater for food crops.
Globally, the evaluation and design optimization of wastewater treatment alternatives based on dynamic modeling and simulation is a common practice. Software programs such as GPS-X 8.0 (Hydromantis, Ontario, Canada), BioWin 6.2 (EnviroSim Associates Ltd., Ontario, Canada), STOAT (Sieker, Hoppegarten, Germany), SIMBA (inCTRL Solutions, Ontario, Canada), and WEST (MIKE Powered by DHI) are important tools that can be used for the quick evaluation of alternatives. They give accurate evaluations, improve design, and estimate costs by taking into account all wastewater characteristics [
18]. The total cost of wastewater treatment plants includes capital costs (CAPEX) and operation and maintenance costs (OPEX). Capital costs are the expenses associated with long-term investments in assets. The operation cost of a wastewater treatment plant consists of labor cost (operational and maintenance), supervision and administration, power requirements, chemical requirements, cost of all repairs, and miscellaneous supplies and services [
19].
In Egypt, the availability of water resources has become a limiting factor for the country’s development. To reduce the effects of water shortage in Egypt, sustainable water resource management practices are required. The main practice of sustainable management is to develop integrated strategies that efficiently allocate the available water to meet all demands such as wastewater reuse [
20]. Many studies have looked at the estimation of cubic meters cost from treatment alternatives, but there is no indicative price per cubic meter for reclaimed water used for agriculture in Egypt. There are also several studies evaluating health risks depending on the exposure route for selected pathogens, but there has been little attention given to the most common pathogens affecting public health in Egypt and the effectiveness of low-cost treatment alternatives to achieve health risk management.
This paper attempts to provide an evaluation of six wastewater treatment alternatives for the Alexandria Western Wastewater Treatment Plant (case study). The evaluation approach has three levels: (1) evaluate the treatment efficiency and effluent quality of proposed wastewater treatment alternatives using the software GPS-X 8.0; (2) use QMRA- Monte Carlo simulations model to evaluate health risks from four common waterborne pathogens in Egypt, namely Rotavirus, Salmonella, Giardia duodenalis, and Ascaris, which analyzes the safety of wastewater reuse for farmers during irrigation practices and consumers of wastewater irrigated crops; and (3) estimate the costs of treatment alternatives, and calculate the cost per cubic meter.
4. Conclusions
This paper aimed to evaluate the restricted and the unrestricted treatment alternatives’ efficiencies, estimating the cubic meter cost of reclaimed water that can be used in irrigation and managing health risks to achieve the target annual infection risk. The CAS, CAS-N, MBR, MBR-N, CAS-Sand, and CAS-N-Sand treatment alternatives were evaluated using simulation programs to estimate effluent quality, cost, and health risks. The GPS-X 8.0 simulation program was used to verify the designed processes. CapdetWorks 4.0 program was used to estimate the treatment alternatives costs. Log unit reductions from treatment and post-treatment were checked to be sufficient in reducing infection risks estimated using the QMRA model to tolerable risks.
Based on the value of pathogen concentrations in wastewater in Egypt, it can be noticed that both Salmonella and Giardia duodenalis, compared with Rotavirus and Ascaris, seem to have limited significance when reclaimed water is utilized for irrigation. Therefore, under the investigation conditions used in this study, only Rotavirus and Ascaris can be used for the evaluation and comparison of the public health protection effectiveness of different treatment alternatives. This study indicated that irrigation with tertiary treated wastewater can achieve the target microbial risk reduction for unrestricted irrigation (one DALY loss per million people per year) without the need to add advanced treatment facilities. Although activated sludge alternative gave higher health risks than other alternatives, it produced an effluent with appropriate qualities that can be used in restricted irrigation.
Based on the results of this study, it can be concluded that the cubic meter cost of wastewater treatment for agricultural purposes in Egypt was estimated and ranges from 0.082 USD to 0.133 USD. Using activated sludge with sand filtration provides effluent with suitable characteristics for unrestricted irrigation with a small cost per cubic meter of 0.088 USD. Although CAS provides the lowest capital and O&M costs, it gives lower efficiencies and higher health risks than other treatment alternatives. The usage of membrane filtration provides higher protection from health risks, especially those caused by Giardia duodenalis and Ascaris. Membrane filtration also gave higher removal efficiencies and a lower footprint, but it requires higher construction and running costs.
Usage of the GPS-X 8.0 and CapdetWorks 4.0 programs was very helpful in verifying the process performance for treatment alternatives and in estimating the costs of the treatment alternatives. It is necessary to investigate pathogen concentration in further research and to test the effectiveness of different treatments in pathogen removal for different case studies. The GPS-X 8.0 and CapdetWorks 4.0 programs should be tested in other case studies with different conditions; the wastewater flow, wastewater characterizations, and results should be compared to results obtained by other simulation programs.