Building a Circular Economy Option Through Wastewater Treatment and a Resource Recovery Approach ^†

Abstract

This work studies and analyzes the transition from a linear to a circular economy through wastewater treatment and resource recovery. As wastewater volumes grow, sustainable management becomes critical. This study highlights the reuse of treated effluent, beneficial sludge utilization, and energy generation via anaerobic digestion. Wastewater treatment plants should be envisioned as hubs for recovering water, materials, and energy, rather than disposal facilities. Emphasizing resource efficiency, the circular economy approach offers viable solutions to challenges related to resource scarcity, climate change, and ecological impact.

1. Introduction

The global economy is experiencing significant pressures due to the depletion of finite resources. Since the 1970s, global resource extraction has increased more than threefold and is projected to double again by 2060, according to the United Nations [1].

The escalating demand for critical minerals, driven by advancements in emerging technologies and the energy transition from fossil fuels to renewable energy sources, heightens the vulnerabilities of supply chains. This trend underscores the unsustainable nature of the prevailing traditional linear economy model that follows a straightforward path: extract resources, manufacture goods, distribute them, and dispose following post-consumption. This “take, make, and dispose” system approach does not leverage the full potential of our resources, hastening their depletion. It is a path that leads to less productive land, polluted oceans, lost biodiversity, scarce freshwater, and fewer forests.

Regarding wastewater, as the global population and urbanization experience rapid growth rates, the respective wastewater volumes are rising at a parallel rate. This increase is further propelled by improvements in water supply, elevated living standards, and economic development. The 2017 UN World Water Development report [2] corroborates: “On average, high-income countries treat about 70% of the municipal and industrial wastewater they generate. However, this ratio drops to 38% in upper-middle-income countries and to 28% in lower-middle-income countries. In even lower-income countries, only 8% undergoes treatment of any kind”.

In recent years, the concept of circular economy for wastewater management has garnered increasing attention among policymakers, engineers, and environmental scientists. This model advocates for a fundamental shift in perspective, redefining wastewater from a waste by-product requiring treatment and disposal towards a valuable resource. When managed effectively, wastewater can serve as a critical component in advancing sustainable development, aligning economic progress with environmental sustainability.

In this sense, wastewater is no longer seen as a problem needing a solution but rather as part of the challenging solutions that societies are facing today. Wastewater can also be a cost-efficient and sustainable source of energy, nutrients, and other potentially useful by-products. The expected benefits of extracting such resources from wastewaters go well beyond human and environmental health, with further implications for food and energy security issues, as well as climate change mitigation. In the context of a circular economy, whereby economic development is balanced with the protection of natural resources and environmental sustainability, wastewater represents a widely available and valuable resource.

2. Linear to Circular Economy Transformation for Sustainable Wastewater Treatment

The concept of a circular economy (CE) emphasizes the transformation of waste into valuable resources that can be re-integrated into production cycles, fostering sustainability and efficiency (“prosumer” approach instead of “simply “consumer”). The most important driver behind the CE is ever-increasing resource scarcity. This approach has significant implications for various sectors, including wastewater treatment, where its principles align closely with relevant specific goals, such as resource conservation, energy efficiency, and environmental protection [3]. In particular, the application of CE practices in municipal wastewater treatment plants (WWTPs) enables the recovery of essential nutrients, such as nitrogen, phosphorus, and organic carbon, along with the generation of renewable energy and the reuse of clean/treated water. This shifts the perception of wastewater from a disposal problem to a resource-rich opportunity [4]. Furthermore, these practices support the broader objectives of the bio/economy by promoting the circular use of materials and minimizing the waste production [5].

2.1. Treated Effluent Reuse

Wastewater is an effective alternative water supply, which lowers the pressure on natural water bodies and enhances the EU’s ability to adapt better to climate change challenges. Reusing water after appropriate treatment extends its life cycle, thereby preserving the use of virgin water resources. Treated wastewater is predominantly utilized for non-potable applications, such as agricultural irrigation, land enhancement, groundwater recharge, golf courses irrigation, vehicle washing, toilet flushing, firefighting, and construction activities. It also finds applications in industrial cooling processes, particularly in thermal power plants [6]. More than 58,000 wastewater treatment plants are included in the global database HydroWASTE [7].The reuse of treated wastewater has grown rapidly in recent years, with annual increases in usage ranging from ~10% to 29% in regions such as Europe, the USA, and China, and up to 41% in Australia [6]. In Asia, China leads the wastewater reuse efforts, irrigating approximately 1.3 million hectares of agricultural land. In Greece, there are currently 240 WWTPs registered in the National Monitoring Database, but only 11% follow the practices of reusing the treated wastewater (Figure 1).

2.2. Treated Sludge Utilization

Sludge is the main solid by-product generated during the wastewater treatment process, consisting of a heterogeneous mixture of water, organic matter, inorganic substances, nutrients, metals, and pathogens. The specific composition of sludge varies significantly, depending on the characteristics of the influent wastewater and the specific treatment processes employed. Sludge (or “biosolids”) can generally be classified in two types: primary and secondary sludge. Primary sludge is derived from the preliminary treatment stages, such as sedimentation, is characterized by a higher concentration of organic matter and typically produced in larger quantities. In contrast, secondary sludge is generated during the biological treatment phase and is composed mainly of microbial biomass and inorganic materials, resulting in comparatively smaller production volumes.

Beneficial reuse involves the re-purposing of treated sludge as a resource, thereby reducing its classification as waste. Treated sludge can be utilized in several applications, including as soil amendment [8], as a source of nutrients in agriculture (biosolids) [9,10], or as feedstock in anaerobic digestion processes for biogas (and energy) production [11,12]. These applications support the circular economy principles by promoting resource recovery options and reducing the environmental burdens associated with sludge disposal. However, the implementation of beneficial reuse strategies requires rigorous treatment processes to ensure that sludge meets stringent quality standards. These standards are designed to minimize the risks associated with pathogens, heavy metals, and other organic contaminants, thus safeguarding human health and the environment during its application.

2.3. WTP Energy Efficiency

The challenges posed by rapid population growth have escalated the energy demands and increased waste generation, hence placing the aspects of (proper) waste management, material recycling, climate change mitigation, and greenhouse gas emission reduction at the forefront of political and environmental agendas. In this context, the waste-to-energy (WTE) supply chain emerges as a viable and practical strategy within the framework of circular economy.

Anaerobic digestion (AD) is a well-established and efficient technology for the treatment of sewage sludge, providing the added benefit of renewable energy generation. During the AD process, microorganisms metabolize the organic matter present in the sludge under anaerobic conditions, resulting in the production of biogas. This biogas primarily consists of methane (CH₄) and carbon dioxide (CO₂) and serves as an energy source. The utilization of energy generated through AD is contingent on local regulations and incentives. In some cases, the biogas produced can be directly harnessed onsite through a combined heat and power (CHP) system, providing both electricity and thermal energy (mainly for local use). Alternatively, the biogas can be purified to produce biomethane, which can be injected directly into a natural gas grid if the respective infrastructure permits. By enabling both waste treatment and energy recovery, AD aligns with the circular economy principles and contributes to sustainable waste management and energy systems.

3. Conclusions

WWTPs can play a crucial role in advancing sustainability by facilitating the circular flow of water, waste, materials, and energy, thereby contributing to the optimization of resource re/use and a reduction in environmental impacts. Traditionally viewed as facilities for wastewater and sewage sludge treatment, WWTPs are now being redefined for their potential in resource and energy recovery. In the near future, WWTPs are expected to serve as essential hubs for sustainable resource management, contributing to the reduction in environmental impacts and fostering the transition to circular economies.