Technical, economic and environmental feasibility of resource recovery technologies from wastewater
Graphical abstract
Introduction
Traditionally, the goal of wastewater treatment technologies has been to keep the treated water within permissible nutrient levels, ensuring that it poses no risks to humans, animals, or the environment (Mo and Zhang, 2013; US EPA, 2009). However, due to the growing freshwater demand, population explosion, and depleting non-renewable natural reserves, sustainable and circular use of resources are encouraged. Consequently, there has been a paradigm shift in the research community to develop and implement innovative strategies that not only facilitate waste treatment but also create a system in which generated waste is converted back into raw matter, closing the loop of circularity and making systems truly self-sustaining (Svardal and Kroiss, 2011). As a result, there will be less dependence on fossil fuel reserves, resulting in lower GHG emissions. Of all the waste that is dumped, about 50–100% are contained in the wastewater streams (Puyol et al., 2017). Currently, a very less proportion of recycled goods are being reintroduced into the economy and our production system is still largely reliant on raw material mining and manufacturing transformation into products (Lovins, 2008). This necessitates a shift from the current linear model to a more circular model, which emphasizes on zero-waste concept and wastewater as a renewable resource (Diaz-Elsayed et al., 2019). The implementation of resource recovery technologies, however, is a challenge. This is because, in the past, wastewater treatment plants were designed and constructed in a centralized manner with the sole purpose of reducing pollutant load. To transition from pollution removal to recovering resources, existing treatment facilities must be redesigned and modified with the aid of governments, scientific bodies, private stakeholders, and end-users (Kehrein et al., 2020). Additionally, socio-economic challenges due to the past and future political and socio-economic situation of the country also pose a major roadblock (Montwedi et al., 2021). Developing countries do not have reliable data regarding wastewater flows, the performance of treatment systems, and resources recovery, which adds further complications (Chrispim et al., 2020).
The deployment of resource recovery facilities must be phased in over time and include government benefits (subsidies) and financing. In addition, there needs to be a cultural shift in society to embrace recovered products. A staged approach is most beneficial for getting the most nutrients or energy products out of waste streams, where various primary, secondary, and tertiary treatment procedures are used at each step, depending on the purity need and contaminant characteristics. As indicated in Table 1, different types of wastewaters require specific attention in order to effectively apply an appropriate treatment technique. More than one technology or a hybrid method may be most useful in some cases (Yenkie, 2019). Depending on the type and source of wastewater, it can be divided into several types viz. domestic or municipal, industrial, agricultural, medical, or nuclear. Thus, in this review article, we highlight key resources and the innovative approaches for its full economic potential, which is currently not exercised in traditional treatment facilities.
The objective of this review is to explore the emerging and innovative resource recovery technologies that have the ability to change the focus of current treatment systems towards resource recovery in a systematic way. A comprehensive and critical review of traditional wastewater treatment systems was carried out to identify their drawbacks, challenges, and scope of improvement in the state-of-art. Several advanced RRT's were highlighted such as advanced oxidation processes with hydrodynamic cavitation. Finally, the feasibility of these technologies with respect to sustainable extraction of the nutrients is evaluated, covering the technical, economic, and environmental performance. It is proposed that the overall water and carbon footprint will be lowered when water is recovered along with other resources from both wastewater and sewage sludge, thereby offsetting the local operational and maintenance cost of local treatment facilities.
Section snippets
Biogas
Biogas is the most common source of energy from the WWTPs produced by anaerobic digestion. Anaerobic digestion (AD) is one of the most popular techniques applied to industrial and municipal wastewater treatment to recover biogas. Biogas comprises mainly methane (50–70%), carbon dioxide (30–50%), and some traces of nitrogen, hydrogen, hydrogen sulfide, and water vapor (Manyuchi et al., 2018). The integration of AD processes into existing CHP plants has the potential to address global climate
Conventional
There are several phases in the wastewater treatment process as highlighted in Fig. 2. Firstly, the raw wastewater is passed through mechanical screens or grits where coarse solids and floating debris are removed. This step helps prevents the downstream treatment unit operations. In the next stage, the water is treated to remove the 55% of suspended solids by chemical precipitation (coagulation and flocculation), gravitational settling, dissolved air flotation, and/or filtration techniques. At
Technical feasibility of resource recovery technologies
Despite diverse scientific output on solutions for effective wastewater treatment, the success stories in the wastewater field of large-scale resource recycling technologies are still in their infancy stage. This is due to the design and operation of existing wastewater treatment technologies which limits the smooth transition to resource recovery technologies outlook. Indeed, the proposed transition will not only incur costs but create operational distraction, and requires technical know-how,
Economic feasibility of resource recovery technologies
Water is often available for free or at a lower cost, so the cost is not really important to us for its use in process or manufacturing industries. However, when considering wastewater effluent on a larger scale, resource recovery and water reclamation can be a win-win scenario. For the implementation of a successful full-scale RRT, it is important that it competes economically well with the conventional methods. In order to be truly successful, RRTs must be taken as first step to re-organize
Environmental feasibility of resource recovery technologies
Environmentally, all RRTs must adhere to the guidelines and regulations set forth by government agencies for the permissible limit of nutrients in the solid, liquid or gaseous effluents. It is important to first know the end-users of the treated systems and products. For example, if the reclaimed water after wastewater treatment is being used for potable uses, it must strictly adhere to the stricter limits set by the US Environmental Protection Agency (USEPA, 2009). Otherwise, for non-potable
Future directions
Fresh water is a limited resource so do energy reserves. Gradually, we are planning towards moving to a circular economy from linear economy. While linear economy emphasis on take-make and dispose, circular economy (CE) is based on make-use and recycle. In the current scenario when world population is growing at an unprecedented rate, to ensure there is enough food, water and energy for our future generations, sustainable growth based on circular economy model is required. CE ensures generation
Conclusions
While the original purpose of wastewater treatment technologies was to preserve water quality, resource depletion and sustainability concerns are driving big global changes today. Wastewater is increasingly being regarded as a reliable source of renewable energy, pure water, nutrients, and other high-value resources. The emphasis of this review paper has been on the identification of wastewater sources and a comprehensive list of different types of services that can be recovered. In addition,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
AM and PG would like to acknowledge IIT Kharagpur for providing all of the essential internet access and scientific literature and records. The authors would like to express their gratitude to IGPRED (www.igpred.com) for giving valuable insight and knowledge on the study issue, as well as for assisting with the article.
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