Our modern cities are resource sinks designed on the current linear economic model which recovers very little of the original input. As the current model is not sustainable, a viable solution is to recover and reuse parts of the input. In this context, resource recovery using nature-based solutions (NBS) is gaining popularity worldwide. In this specific review, we focus on NBS as technologies that bring nature into cities and those that are derived from nature, using (micro)organisms as principal agents, provided they enable resource recovery. The findings presented in this work are based on an extensive literature review, as well as on original results of recent innovation projects across Europe. The case studies were collected by participants of the COST Action Circular City, which includes a portfolio of more than 92 projects. The present review article focuses on urban wastewater, industrial wastewater, municipal solid waste and gaseous effluents, the recoverable products (e.g., nutrients, nanoparticles, energy), as well as the implications of source-separation and circularity by design. The analysis also includes assessment of the maturity of different technologies (technology readiness level) and the barriers that need to be overcome to accelerate the transition to resilient, self-sustainable cities of the future.
The issue of ever-increasing amounts of waste activated sludge (WAS) produced from biological wastewater treatment plants (WWTPs) is pointed out. WAS can be effectively reduced in the anaerobic digestion (AD) process, where methanogens break down organic matter and simultaneously produce biogas in the absence of oxygen, mainly methane and CO2. Biomethane can then be effectively used in gas turbines to produce electricity and power a part of WWTPs. Hydrodynamic cavitation (HC) has been identified as a potential technique that can improve the AD process and enhance biogas yield. Rotary generators of hydrodynamic cavitation (RGHCs) that have gained considerable popularity due to their promising results and scalability are presented. Operation, their underlying mechanisms, parameters for performance evaluation, and their division based on geometry of cavitation generation units (CGUs) are presented. Their current use in the field of wastewater treatment is presented, with the focus on WAS pre/treatment. In addition, comparison of achieved results with RGHCs relevant to the enhancement of AD process is presented.
Biogas plants receive inputs of different sources of carbon, nutrients, metals and other pollutants from large areas that result in a digestate that is a very complex and concentrated matrix. How to redistribute all these components without causing imbalances in the receiving environments is one of the main questions that arises regarding the reuse of digestate. The main end destinations of digestate within the EU are agriculture, landfill and incineration, in addition to open-mine land reclamation. There are European and country specific end destinations of digestate that have been recently reviewed and made publicly available in an EU commission report. In terms of agricultural application, digestate is seen as a valuable source of carbon and nutrients, but its application is conditioned by disposal limits for nitrogen, phosphorous and metals. Here, we discuss the need for redesign of the process of digestate manipulation in order to increase its value as fertiliser, through addition of compounds, different solid/liquid phases separation or additional treatments. Potential recovery techniques are also discussed. Phytoremediation, the use of plants to uptake metals from different substrates, can be used not only to remove trace metals from the digestate but also for the recovery of metals from plant biomass or their reintroduction into the biodigester. In addition, a combination of landfill with phytoremediation can be a good alternative for the recovery of degraded soils, or for the reclamation of polluted soil for landscape recovery. Another option can be the use of digestate to produce biochar to be applied in agriculture, a technique that increases carbon content in soils while decreasing trace metal bioavailability. Finally, we discuss the new opportunities that are arising for the use of digestate, including microalgae biomass production and bioenergy.
Anaerobic digestion is a process by which energy from organically degradable waste can be recovered in the form of biogas. The uncontrolled disposal of such organic waste is very burdensome to the environment. In Slovenia, large quantities of waste edible oils are produced every year, which could be used for biogas production. In this study, we investigated the methane potential of waste edible oil in a batch reactor. Further in the pilot scale in a semi-continuous experiment we evaluated the use of waste edible oil in anaerobic digesters of wastewater treatment plant. The results show that the quantity of oil per day that is fed into the anaerobic reactor is limited due to the process's instability and lower biogas production. The optimal daily organic loading of waste edible oil was between 1.6 and 2.4 g VS/L inoculum. During these conditions, the process of anaerobic digestion was stable and methane production was highest. At higher organic loading, the process became unstable, pH was reduced, volatile fatty acids increased, COD at the outflow of reactor increased, and biogas production was significantly reduced.
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