Mankind demographic and social growth, and the development of sanitation and water treatment infrastructures are closely linked. Thanks to them, we live in healthy environments and we minimize the impact to the environment around us and from which we get our supplies.
The growing importance of the circular economy is driving the water treatment sector towards a paradigm change. Many of our water pollutants – organic load, nitrogen, metals, paper pulp, fats and oils, etc. – are nothing more than possible resources for obtaining other high-value products, such as energy, bioplastics or fertilisers. Water treatment plants, both urban and industrial, are thus beginning to evolve from the sites where we treat our water to resource recovery and obtention centres; to this end, the following technologies are being researched:
Using membrane technologies such as direct osmosis or membrane contactors, or using combined technologies – such as anaerobic membrane bioreactors-, research is carried out on the concentration and extraction of nutrients (mainly nitrogen and phosphorus) from the wastewater, while generating quality water for reuse and biogas.
By using this type of technology it is possible to retain the resources present in water currents. Biotechnological processes, based on microorganisms, fungi and enzymes, are used to carry out the selective biosorption of metals with high economic value present in these flows.
Furthermore, these adsorption technologies can also be applied, based on novel materials, such as biochar, for the recovery of other metals of interest.
Electrochemical technologies have a high potential for recovering ions present in different types of aqueous waste flows. They are also environmentally friendly as they avoid the addition of large quantities of reagents to the medium and they allow metal recovery without a subsequent separation phase, as the precipitate of the metal of interest will be found on the surface and in the area around the electrode. Electroprecipitation processes have been developed for recovery, in form of hydroxides, of metals of economic interest such as the Magnesium present in landfill leachates.
All of the above applications have one thing in common: the recovery of existing resources in wastewater. But it is also possible to obtain resources that, even if they are not present in the water, can be generated from it. Sewage sludge is mainly made up of bacterial biomass, which is rich in organic biomolecules such as proteins, carbohydrates, lipids, etc. These biomolecules can be transformed through a process of enzymatic hydrolysis into peptics and amino acids, which are a source of organic nitrogen, which can be used in agricultural fertilisation.
Similarly, by means of specific bacteria that accumulate PHAs as carbon and energy reserves, streams with a high content of biodegradable organic matter can be transformed into bioplastics.
The most common example of the application of circular economy strategies in sewage treatment plants is urban sewage sludge, a by-product often revalorised in agriculture. The vision of the WWTP as a resource centre goes one step further. The aim is not only to revalue the waste generated in a conventional WWTP, but also to design and operate the waste water treatment processes in order to purify the incoming water and to recover and obtain secondary raw materials.
The secondary raw materials to be obtained vary greatly from the type of water that the WWTP treats – water from a dairy industry, for example, will be rich in fats, proteins or lactic acids, among others, while the water from a municipality will be rich in nitrogen in the form of ammonia and ammonium, phosphorus, or materials such as cellulose from toilet paper. However, whatever their type, all treatment plants have resources in great quantity and variety, waiting for the implementation of the technologies and processes necessary for their renewed inclusion in industrial value chains.