Introduction
The ever-increasing human population and industrial growth have posed a huge burden on existing resources and have led to an increase in environmental pollution and climate change. The imprudent use of water resources and overall wastes released from domestic, agricultural and industrial sectors in natural water bodies has exacerbated the challenges relating to availability, quality, and purity of water resources. Pollutants such as oxygen-demanding substances, pathogens, nutrients, inorganic, and synthetic organic chemicals have been listed as potential contaminants in municipal wastewater. Oxygen demanding substances such as ammonia pose potential dangers to aquatic life. Pathogens are carried into the groundwater through sewage sourcing from industrial waste, storm runoff, and municipal sources. Nutrients such as carbon, nitrogen, and phosphorus are found in large quantities in agricultural wastewater. If not treated properly, the large amounts of nutrients, primarily phosphorus, and nitrogen cause nutrient enrichment resulting in algae growth and eutrophication. Heat reduces the capacity of water to retain oxygen, industrial water utilized for cooling is often too hot to be released back to the ecosystem. Thus, wastewater needs to be checked and treated before it is released in the ecosystem, or else it will have detrimental effects on marine life and natural water bodies. Furthermore, freshwater resources are limited and the only way to satisfy the increased demand for water is to rely on wastewater treatment(WWT) systems that can provide reusable water or reduce contaminant concentrations to acceptable levels which can be handled by natural remediation systems (biogeochemical cycles).
Current wastewater treatment plants (WWTPs) comprises multiple treatment technologies to obtain necessary purity standards set by the regulatory agencies such as the US EPA (United States Environmental Protection Agency), European Union’s Water Policy, United Nations Environmental Programme. Technologies are based on physical, biological, chemical processes or their combination [1]. Physical processes are applied for the removal of solids from wastewater usually using screens and filters. Biological processes use small organisms to remove and break down harmful sewage. Chemical processes are often combined with physical processes to remove complex pollutants. Thus, appropriate characterization of wastewater streams is essential to identify candidate technologies which will reduce the contaminants to acceptable levels.
Judicious water use and minimization in wastewater release are equally important due to the scarcity of water resources and WWTPs energy costs [2]. Considerations may include overall volume reduction, pollutant strength reduction, or a combination. Wastewater volume reduction can have a significant impact on technology capacity, flow/loadings of WWTPs, operation/maintenance costs, energy requirements, and ecological impacts. Approximately, 20–30% water savings can be achieved with flow reduction devices such as sensors for fault detections and concentration measurements, and efficient controllers [3,4]. Industrial plants can achieve wastewater volume reductions by utilizing multi-point waste collections, reuse, and elimination of sludge discharges. Finally, WWTPs need efficient design and evaluation metrics to minimize costs, energy requirements and subsequent environmental impacts while meeting the regulatory guidelines. To this end, this review summarizes the most recent developments in the areas of wastewater characterization, treatment technologies, modeling, and optimization framework for designing efficient WWTNs, economic analysis, and sustainability assessment (see Figure 1).
Figure 1. Schematic of an integrated approach for designing wastewater treatment networks. The complexity lies in the selection of appropriate technologies based on contaminants present in different sources of wastewater for treatment network synthesis followed by cost evaluations and sustainability assessment.Wastewater characterization
The wastewater characterization includes total suspended solids (TSS), total dissolved solids (TDS), pH, organic loadings, chemical oxygen demand (COD), biochemical oxygen demand (BOD), toxic ions, active pharmaceutical ingredients (APIs), endocrine disrupting chemicals (EDCs), and others [1,5,6]. Their typical range can vary significantly based on the source of the contaminant stream. Municipal wastewater from residential sources have BOD values in the range of 100–400 mg/l, nitrogen as 20–85 mg/l and phosphorous as 6–23 mg/l [7••]. Food, drinks, and milk (FDM) sector can have effluents with 10–100 times higher BOD and COD values as compared to the municipal sector [8]. Pharmaceutical effluents contain a high concentration of organics, and APIs [9]. Thus, source of the effluent stream, its contaminant properties, and relative amounts, as well as information about limits for safe discharge or reuse of treated water enables the connection of appropriate WWT technologies to design a case-specific process flow diagram.
Treatment technologies
WWT is most effective when accomplished in stages and usually comprises preliminary, primary, secondary, and tertiary stages along with sludge treatment options [10•]. An overview of the treatment stages and technologies involved are described in Figure 2. Generally, WWTPs utilize one technology from each stage; however, depending on the purity requirements, contaminant properties and their amounts in the inlet waste stream, more than one technology might be needed in a stage or some stages might be bypassed.
Figure 2. Stage-wise wastewater treatment design and technologies involved in each stage.The treatment results in water-rich and contaminant-rich outlet streams. The contaminant-rich stream consists of sludge which can be treated to recover nutrients via technologies such as hydrothermal liquefaction [11], thermophilic digestion [12], and fermentation, or incinerated to recover energy [13••]. The details of some existing and novel WWT technologies is provided in Table 1. This information can be used when selecting candidate technology alternatives for performing a designated task and technology network connections [7••,14••,15•].
Table 1. Wastewater treatment technologies, their advantages, and limitations
| Technology | Features | Advantages | Limitations |
|---|---|---|---|
| Screening [1,5] |
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| Flocculation/ Coagulation [10•,16,17] |
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Sedimentation/ Clarifier [5,18] |
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Filtration (granular) [5] |
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| Rotating biological contactors [5,19] |
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| Adsorption/ Ion exchange [10•] |
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| Disinfection [1] |
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| Activated sludge [5,20] |
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| Microbial fuel cells [21,22] |
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| Constructed wetlands [23•,24] |
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Advanced oxidation process [25,26] |
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| Membrane processes [10•,27] |
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Modeling approaches for efficient design and economic viability
WWT network (WWTN) design is a complex problem due to a high number of technology alternatives, multi-criteria design considerations of purity, costs, operational safety, and environmental impact, and lack of knowledge integration from experts. Some modeling approaches implemented in WWTN design are discussed here.
Data-driven, heuristics and performance index models
Ruiz-Rosa et al. [28•] proposed a data-driven Activity Based Cost (ABC) management model for WWT and reuse processes in four phases: (i) identification of final products and their measurement units, (ii) definition of product transformations and activities, (iii) relation and classification of resource groups consumed in WWT, and (iv) development of logical associations between resources, activities and products. Here, activities denote technologies, resource groups denote fixed assets, labor, energy, maintenance, social and other services, and logical associations denote cost distributions in each technology for achieving desired purity.
The work by Liu et al. [29•] proposed an enhancement of the numerical indicator of total mixing influence potential (TMIP) [30], based on WWT systems with minimum treatment flowrate and pinch analysis, by including heuristic rules for situations when one contaminant can be removed in multiple technologies or when multiple contaminants can be removed in a single technology. The improved model could design more complex treatment networks. Viciano et al. [31] introduced a performance index (Z) in the cost function to incorporate the fact that equipment does not always run on their optimal capacity due to seasonal changes, population shifts, thus impacting the energy costs. This methodology was applied to empirical data from 156 WWTPs in Valencia to represent economies of scale in efficient design and cost estimates. Fuzzy logic models, intuitional fuzzy sets and multi-criteria decision making were integrated to represent numerical and verbal information and subsequently applied to optimal WWT technology selection [15•,32].
Superstructure synthesis approaches
Garibay-Rodrigues et al. [33••] proposed a constraint-based MINLP (mixed-integer nonlinear programming) for integrating optimal resource management to the synthesis of distributed WWTNs. The approach is demonstrated via a typical river system that serves as a source and natural drainage to domestic, industrial and agricultural sectors with constraints of maximum allowable concentration of pollutants discharged and limit on water consumption from fresh sources and solving the total annual costs minimization problem. Alnouri et al. [34••] presented an approach for interplant water network synthesis in industries by combining the central and decentral treatment options and merging the common pipe segments carrying water and wastewater with similar properties, which allowed for a reduction in network complexity and overall costs.
Lu et al. [35••] addressed the problem of optimal synthesis and operation of WWTNs with multi-scenario influent streams under different discharge standards and penalty ratios of non-compliant emissions. They employed solution methods involving disjunctive programming, multi-period MINLPs to minimize Total Annual Costs (TAC) to provide management insights and assist policymakers. Some other studies from the group [36,37] also highlighted the importance of superstructure synthesis and technology modeling for WWT.
Process network synthesis approaches
Kollmann et al. [38••] applied the Process Network Synthesis (PNS) approach to optimize the economics of a WWTP while recovering energy in the form of heat and supplying the surplus to the public energy distribution grids. They utilized the Geographical Information System (GIS) based Energy Zone Mapping and established a feasible WWTN structure in P-graph [39] based PNS studio software. The varied applications of P-graph based PNS approach [40,41,42] indicates its potential in the field of WWTN synthesis.
Methods for sustainability assessment
Life-cycle assessment (LCA) has been the methodology of interest for many research groups [13••,43] when evaluating the sustainability of a WWTP and integrating its economics. LCA usually includes four steps: (i) goal and scope definition, (ii) life cycle inventory (LCI), (iii) life cycle impact assessment (LCIA), and (iv) life cycle interpretation. Piao et al. [13••] demonstrated an integrated LCA and economic efficiency analysis for WWTPs and sludge management systems by subdividing them into plant operation, electricity and chemical consumption, and transport to landfills, which could prove valuable in managing urban water systems.