The IWA Anaerobic Digestion Modelling Task Group was established in 1997 at the 8th World Congress on Anaerobic Digestion (Sendai, Japan) with the goal of developing a generalised anaerobic digestion model. The structured model includes multiple steps describing biochemical as well as physico-chemical processes. The biochemical steps include disintegration from homogeneous particulates to carbohydrates, proteins and lipids; extracellular hydrolysis of these particulate substrates to sugars, amino acids, and long chain fatty acids (LCFA), respectively; acidogenesis from sugars and amino acids to volatile fatty acids (VFAs) and hydrogen; acetogenesis of LCFA and VFAs to acetate; and separate methanogenesis steps from acetate and hydrogen/CO₂. The physico-chemical equations describe ion association and dissociation, and gas-liquid transfer. Implemented as a differential and algebraic equation (DAE) set, there are 26 dynamic state concentration variables, and 8 implicit algebraic variables per reactor vessel or element. Implemented as differential equations (DE) only, there are 32 dynamic concentration state variables.

Limits in resource availability are driving a change in current societal production systems, changing the focus from residues treatment, such as wastewater treatment, towards resource recovery. Biotechnological processes offer an economic and versatile way to concentrate and transform resources from waste/wastewater into valuable products, which is a prerequisite for the technological development of a cradle-to-cradle bio-based economy. This review identifies emerging technologies that enable resource recovery across the wastewater treatment cycle. As such, bioenergy in the form of biohydrogen (by photo and dark fermentation processes) and biogas (during anaerobic digestion processes) have been classic targets, whereby, direct transformation of lipidic biomass into biodiesel also gained attention. This concept is similar to previous biofuel concepts, but more sustainable, as 3rd generation biofuels and other resources can be produced from waste biomass. The production of high value biopolymers (e.g. for bioplastics manufacturing) from organic acids, hydrogen, and methane is another option for carbon recovery. The recovery of carbon and nutrients can be achieved by organic fertilizer production, or single cell protein generation (depending on the source) which may be utilized as feed, feed additives, next generation fertilizers, or even as probiotics. Additionlly, chemical oxidation-reduction and bioelectrochemical systems can recover inorganics or synthesize organic products beyond the natural microbial metabolism. Anticipating the next generation of wastewater treatment plants driven by biological recovery technologies, this review is focused on the generation and re-synthesis of energetic resources and key resources to be recycled as raw materials in a cradle-to-cradle economy concept.

Bio-electrochemical systems (BESs) have recently emerged as an exciting technology. In a BES, bacteria interact with electrodes using electrons, which are either removed or supplied through an electrical circuit. The most-described type of BES is microbial fuel cells (MFCs), in which useful power is generated from electron donors as, for example, present in wastewater. This form of charge transport, known as extracellular electron transfer, was previously extensively described with respect to metals such as iron and manganese. The importance of these interactions in global biogeochemical cycles is essentially undisputed. A wide variety of bacteria can participate in extracellular electron transfer, and this phenomenon is far more widespread than previously thought. The use of BESs in diverse research projects is helping elucidate the mechanism by which bacteria shuttle electrons externally. New forms of interactions between bacteria have been discovered demonstrating how multiple populations within microbial communities can co-operate to achieve energy generation. New environmental processes that were difficult to observe or study previously can now be simulated and improved via BESs. Whereas pure culture studies make up the majority of the studies performed thus far, even greater contributions of BESs are expected to occur in natural environments and with mixed microbial communities. Owing to their versatility, unmatched level of control and capacity to sustain novel processes, BESs might well serve as the foundation of a new environmental biotechnology. While highlighting some of the major breakthroughs and addressing only recently obtained data, this review points out that despite rapid progress, many questions remain unanswered.

Technologies to recover nitrogen, phosphorus, and potassium from waste streams have undergone accelerated development in the past decade, predominantly due to a surge in fertilizer prices and stringent discharge limits on these nutrients. This review provides a critical state of art review of appropriate technologies which identifies research gaps, evaluates current and future potential for application of the respective technologies, and outlines paths and barriers for adoption of the nutrient recovery technologies. The different technologies can be broadly divided into the sequential categories of nutrient accumulation, followed by nutrient release, followed by nutrient extraction. Nutrient accumulation can be achieved via plants, microorganisms (algae and prokaryotic), and physicochemical mechanisms including chemical precipitation, membrane separation, sorption, and binding with magnetic particles. Nutrient release can occur by biochemical (anaerobic digestion and bioleaching) and thermochemical treatment. Nutrient extraction can occur via crystallization, gas-permeable membranes, liquid–gas stripping, and electrodialysis. These technologies were analyzed with respect to waste stream type, the product being recovered, and relative maturity. Recovery of nutrients in a concentrated form (e.g., the inorganic precipitate struvite) is seen as desirable because it would allow a wider range of options for eventual reuse with reduced pathogen risk and improved ease of transportation. Overall, there is a need to further develop technologies for nitrogen and potassium recovery and to integrate accumulation–release–extraction technologies to improve nutrient recovery efficiency. There is a need to apply, demonstrate, and prove the more recent and innovative technologies to move these beyond their current infancy. Lastly, there is a need to investigate and develop agriculture application of the recovered nutrient products. These advancements will reduce waterway and air pollution by redirecting nutrients from waste into recovered nutrient products that provides a long-term sustainable supply of nutrients and helps buffer nutrient price rises in the future.