The review aimed to identify differences and similarities in the adsorption process of five azo dyes [congo red (CR), reactive black 5 (RB5), methyl orange (MO), orange II (OII), and methyl red (MR)] on natural materials, biosorbents, industrial and agricultural waste, or biomass, which are alternatives of costly activated carbon and are locally available. The azo dyes were characterized and compared based on their molecular structure and weight, water solubility, acid dissociation constant, n-octanol-water partition coefficient, and maximum absorbance. RB5 and CR are diazo dyes, whereas MO, OII, and MR are mono-azo dyes. MO, OII, and MR are anionic acid dyes, RB5 is an anionic reactive dye, and CR is an anionic direct dye. CR, RB5, MR, and OII molecules contain one or more sulfonate functional group(s), but MR does not. We performed a literature review based on the following parameters: initial dye concentration, adsorbent dosage, pH, temperature, isotherm, kinetic models, thermodynamic parameters, and synergetic or competitive interactions. The azo dyes tended to adsorb best in an acidic medium and at higher temperatures. The initial dye concentration and adsorbent dosage studies indicated the importance of using an appropriate amount of adsorbent dosage for an effective removal. The studies tended to follow the Langmuir isotherm and kinetic pseudo-second-order model. Most adsorption processes were endothermic and spontaneous, leading to an increase in randomness at the solid-liquid interface. These results indicate similarities between the adsorption process of the five azo dyes. Relevant adsorption mechanisms in azo dye adsorption processes were assumed to be electrostatic forces, hydrogen bonding, and π–π interactions, among others. Nevertheless, the focus of the studies lies more on the development and characterization of adsorbent materials, not on the study of influences from the matrix “industrial wastewater”. Therefore, more research is needed to develop adsorption units for application in textile industries.

The use of decentralised, sustainable urban drainage systems (SUDS) for the treatment of stormwater runoff is becoming increasingly prevalent in Germany. Decentralised SUDS can offer a viable and attractive alternative to end of pipe treatment systems for stormwater runoff from urban areas. However, there is still some uncertainty regarding the long-term performance of SUDS, and the general legislative requirements for SUDS approval and testing. Whilst the allowable pollution levels in stormwater runoff that infiltrate into ground and/or water table are regulated across Germany by the Federal Soil Protection Law, there is presently no federal law addressing the discharge requirements for surface water runoff. The lack of clear guidance can make it difficult for planners and designers to implement these innovative and sustainable stormwater treatment systems. This study clarifies the current understanding of urban stormwater treatment requirements and new technical approval guidelines for decentralised SUDS devices in Germany. The study findings should assist researchers, designers and asset managers to better anticipate and understand the performance, effective life-spans, and the planning and maintenance requirements for decentralised SUDS systems. This should help promote even greater use of these systems in the future.

Over the decades, the rise in nitrate levels in the ecosystem has posed a serious threat to the continuous existence of humans, fauna, and flora. The deleterious effects of increasing levels of nitrates in the ecosystem have led to adverse health and environmental implications in the form of methemoglobinemia and eutrophication, respectively. Different pathways/routes for the syntheses of perovskites and their oxides were presented in this review. In recent times, electrocatalytic reduction has emerged as the most utilized technique for the conversion of nitrates into ammonia, an industrial feedstock. According to published papers, the efficiency of various perovskites and their oxides used for the electrocatalytic reduction of nitrate achieved a high Faradaic efficiency of 98%. Furthermore, studies published have shown that there is a need to improve the chemical stability of perovskites and their oxides during scale-up applications, as well as their scalability for industrial applications.