Scientific and technological trajectory in the recovery of value-added products from wastewater: A general approach
Graphical abstract
Source: Elaborated by the authors.
Introduction
In any economy, sustainability and its associated problems require a multidisciplinary approach for adequate management [1]. Many of the problems posed by wastewater production can be solved by implementing resource recovery systems and through them implement actions aimed at improving sustainability [2]. Therefore, the efficient management of these wastes and the optimization of water use must be incentivized [3]. Additionally, any evaluation of wastewater resource recovery systems should consider not only the evaluation method but also the contexts and opinions of all stakeholders [4]. This concern has been addressed by the European Union (EU), where the criteria behind sustainable policies and the use of resources were based on guidelines provided the circular economy, seeking tangible economic, social, and environmental benefits [5].
Therefore, different countries have embraced circular economy because it offers an environmentally-friendly alternative to the current production model [6]. Circular economy represents an interesting framework to integrate relevant elements such as reuse, recycling, and reduction, although it is crucial to continue constructing the concept and increasing its accuracy and promoting its association with the notion of sustainable development [7,8]. Therefore, circular economy can be seen as an alternative to optimize the use of resources and, simultaneously, decrease waste [9], it is considered to have a positive relationship with sustainability, and it represents a good starting point for sustainable production [10]. The expected results of circular economy and its methods will vary depending on the approach, leaning either toward waste mitigation and saving resources or toward economic development [11].
The current approach to wastewater treatment has incorporated the recovery of value-added products [12], increasing its relevance and potential. For instance, wastewater from fruit and vegetable processing industries contain large amounts of bioactive compounds that can be recovered not only to mitigate environmental impact but also to promote the development of other value-added products [13]. Similarly, wastewater resulting from the nixtamalization process (alkaline treatment) of maize allows for the recovery of gelling arabinoxylan (prebiotic), which is useful as a probiotic vehicle (for example, Bifidobacterium) [14]. In the case of wastewater from shrimp cooking, the recovery of astaxanthin and bioactive peptides can be used in important applications [15]. Additionally, antioxidants such as melanoidins and polyphenols with potential applications in pharmacy, cosmetics, and food production can be obtained from wastewater generated during the distillation process of sugarcane molasses [16].
Concerning energy, the bio–electrochemical system (BES) provides an attractive mechanism to recover bioelectricity from wastewater, as well as other components such as methane, industrial chemicals, hydrogen, and heavy metals, among others [17]. Similarly, Microbial electrosynthesis systems (MESs) have shown their potential to provide solutions not only in terms of resource recovery but also to obtain energy sustainably through wastewater treatment [18]. On the other hand, microbial electrometallurgical processes are useful for the recovery of metals (Sn, Fe, and Cu) and organic products (for example, acetates and polymers) from wastewater produced by the manufacture of printed circuit boards (PrCB), which contributes to compliance with disposal requirements for this type of effluent [19]. Electrodialysis anti-fouling configurations used to treat tannery wastewater represent an option for the recovery of proteins, amino acids, and low molecular weight charged species, while at the same time, water is desalinated and treated for reuse [20]. Similarly, the recovery of gold (ions) contained in gold-plating wastewater can be achieved by using low-cost activated carbon from agricultural waste via the absorption process [21].
Biorefineries can be used to transform waste from the dairy industry into useful products ranging from biofuels and bioplastics to food additives and different chemicals [22]. Another potentially attractive case for a biorefinery is the use of microalgae to produce biomass and energy from urban wastewater and as the primary treatment of such type of effluent [23]. In fact, due to their versatility, microalgae have many applications in sustainable biorefineries that help to meet two critical objectives: product recovery and biofuel recovery [24]. A series of interesting biorefinement processes using industrial wastewater have to do with biohydrogen production [25]. The biorefinery is also being used to produce cellulases and decrease pollutants in domestic wastewater [26]. Microbiological methods (e.g., microremediation) are being used to decrease contaminant load in wastewater from refineries, for example, phenols, and at the same time, to produce useful enzymes [27].
Actors in the food industry have developed applications to recover products of interest, for example, olive oil mills, where a spray-drying system can recover the product and decrease contaminant loads [28]; in this type of mills, phenolic compounds can also be recovered and eliminated using physical (extraction) and chemical and biological methods (reduction of organic load) [29]. Another example is the production of potato chips, in which separation methods can mitigate the organic load of wastewater, decrease water consumption, and help to recover by-products such as starch, oil, and fat, which have different uses, for instance, in paper and soap manufacturing processes [30]. The textile industry uses ultrafiltration processes to recover and reuse polyvinyl alcohol (PVA) with the purpose of mitigating environmental impact [31].
Photosynthetic bacteria (PSB) are an exciting group of microorganisms that have different functions: they are used in wastewater treatment processes, they produce proteins, carotenoids, and coenzyme Q10, among others, and they help to eliminate residual sludge [32,33]. Anaerobic processes to recover energy in the form of methane are used in the treatment of low-resistance domestic wastewater [34]. Methanotrophs represent an option to reduce methane production during wastewater treatment; methanol and biopolymers can be synergistically obtained in the process, and these microorganisms also participate in biological nitrogen mitigation [35]. In wastewater from egg-processing plants, electrocoagulation provides an alternative for the efficient recovery of protein and fat [36].
For its part, municipal wastewater treatment can be approached by using microalgae to obtain proteins and carbohydrates originating from the produced biomass [37]. The recovery of phosphorus from sludge from urban effluents for fertilizer production is unviable due to the scarce presence of this element and the high costs associated with its production (reactants and energy) [38]. Despite the potential of phosphorus recovery from municipal waters, it is important to note the technical and environmental challenges associated with replacing phosphorus contained in phosphate rocks for agricultural use [39].
Hydroxyapatite precipitation is one method to obtain phosphorus from (acid) wastewater [40]; crystallization processes are viable in high-phosphorus content wastewater [41], as well as in urban and livestock wastewater, via struvite crystallization technologies [42]. The reduction of total ammonia-nitrogen (TAN) and the recovery of total orthophosphate in wastewater from swine production is a focal point in the industry; in this regard, a proposed technology uses bittern as a less expensive alternative magnesium source that yields positive results in terms of performance and profitability [43].
Legislative aspects, socioeconomic impact, safety, and hygiene issues, and the evaluation of heavy metal and organic micropollutant contents associated with products recovered from wastewater, for example, from domestic wastewater, must be addressed by research as a sine qua non condition toward effective solutions [44]. The following are regulatory obstacles and milestones around the world: In the EU, carrying out recycling and phosphorus recovery activities using wastewater requires complying with regulations established by the market and environmental and health laws [45]; value-added products generated from sewage sludge must be carefully evaluated for endocrine-disrupting compound (EDC) content and the presence of toxic intermediaries and to avoid further restrictions to their use, storage, and commercialization [46]; in the EU, sewage sludge storage criteria are being replaced by initiatives aimed at stabilization and safe recycling in favor of the environment and enhancing product recovery. Therefore, it is relevant to evaluate the safety of each application for an adequate classification [47].
At the legislative level, several economies are prioritizing research on two key elements: alternative fuels and sustainable biorefineries using waste products, favoring a model centered on circular economy [48]. In China, industrial wastewater is marginally controlled, which aggravates the environmental impact of the country's increasing economic activity [49]. In response to its large demand for water resources, however, the country has advanced a wastewater reuse strategy, but the lack of public awareness, cooperation from other actors, and the unhurried adoption of wastewater-related programs have been obstacles to its implementation. The guidelines and integral management of these resources need to be further developed, in addition to technical requirements for water quality, and aspects related to commercialization must be defined [50].
In developing countries, the irrigation of crops using treated water also requires the development of policies incorporating financial mechanisms and institutional work to address this problem [51]. In general, developing economies frequently fail to provide comprehensive information to formulate adequate policies to address problems associated with the recovery of resources from this type of effluent [52]. Although wastewater reuse is certainly promoted, these countries often lack regulatory frameworks dealing with environmental protection and health [53]. Consequently, addressing these problems should be a priority in developing countries, as well as the incorporation of endogenous technologies providing effective and efficient solutions, although developing countries must also be capable of creating their own technological solutions and compete at the global scale.
On the other hand, knowledge about wastewater treatment and sanitation technologies will allow for a better understanding of how to comply with existing regulations and norms, which will result in efficient solutions for water disposal or reuse [54]; therefore, progress in political, normative, technical, and environmental matters is a relevant area of focus in this field. Wastewater treatment indicators reveal the degree of sustainability associated with a technology [55,56], and the use of these indicators is also necessary for activities involving the recovery of value-added products. It is also essential to outline the scientific and technological trajectory of this field of knowledge to identify its current status and provide information for the design of strategies to develop original basic and applied research projects aimed at advancing technological solutions that have a positive impact at the economic, social, and environmental levels. Therefore, the main goal of the present study was to provide a first approach to the scientific and technological trajectory around the recovery of value-added products from wastewater.
Section snippets
Method
The nature of technological progress is evolutionary, shaped by multiple dynamics and uncertainty during its development [57]. The technological innovation of products and processes is increasingly complex, and innovation is a key factor for industrialization and economic growth [58,59]. Thus, the definition of a technological trajectory (technological development) is quite useful to outline the state of a technology over a period of time, although identifying such trajectory is of course not
Discussion and conclusions
The development and use of technologies and bioprocesses related to the treatment and recovery of value-added products in wastewater is a relevant issue that needs to be further developed by promoting scientific research and technological development to achieve effective and efficient solutions to pressing issues. Thus, the wide range of wastewater compositions represents a potential that can be channeled via two types of applications: wastewater treatment and recovery of high value-added
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
We wish to acknowledge the support provided by the National Polytechnic Institute (Instituto Politécnico Nacional) and the Secretariat for Research and Postgraduate Studies (Secretaría de Investigación y Posgrado), grant numbers 20195587 and 20200773.
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