Wastewater: An Energy Pile

Water management has historically been a serious problem for societies, both from a health point of view and from an environmental point of view. But in addition, water management represents an important economic problem, because to solve the previous problem, societies have had to implement expensive purification and wastewater treatment processes that require the consumption of chemicals and, above all energy, enormous amounts of energy.

An energy that, depending on the country we are talking about, can account for an energy consumption of around 3% or 5% of the total energy consumption of an entire country, accounting for between 30% and 40% of the energy consumption carried out by a municipality, and even 40% of the operating costs of said treatment plant

Although there is currently a lot of scope for improving treatments and their energy consumption, simply by proposing short-term measures or even integrating energy efficiency into treatment plans in the long term, some pioneers are beginning to see wastewater as more than a cost that needs to be optimized, radically changing their concept, and starting to truly consider that wastewater is a usable energy resource.

An alternative to those proposed in this novel line is anaerobic co-digestion, applied to the treatment of wastewater and its sludge.
Many of the wastewater treatment plants that currently exist have a system for treating and stabilizing sludge from their sludge line (the solid fraction that remains after all water treatment), and in many cases this treatment includes anaerobic digestion, a biological process for the decomposition of organic matter in the absence of oxygen that produces biogas that can be used as fuel, thanks to its high methane content (CH4 between 50% and 70%) with an average calorific value of 6.4 Kw/m3 (PCI for 60% biogas).
The idea of co-digestion involves carrying out a joint treatment of sewage sludge and other substrates, which can be of multiple origins, which compensate for nutrients and moisture, bringing the substrate to the closest point to the optimum for anaerobic digestion, which in the end will result in a substantial increase in biogas generation, in addition to being able to integrate the treatment of other organic waste into existing facilities, which would reduce its management costs.

Laboratory tests have even recorded increases in biogas generation through anaerobic co-digestion with sludge of up to 200%, and the experiences carried out on a field scale are relevant enough that we can think of a new sludge management model, with increases that lead to a generation of biogas of more than double that generated under normal conditions.

There are currently also two other technological alternatives that contemplate wastewater purification as a way of generating a useful fuel for subsequent use in combustion or energy generation processes.

The first of these is purification with microalgae, also called photobiotreatment. A technology that has been emerging in recent years as an alternative to the expensive tertiary nitrogen and phosphorus removal treatments that are ultimately carried out in our urban wastewater treatment plants and whose objective is to eliminate the excess nutrients carried by the water, which cause the eutrophication problems experienced in rivers and lakes, and which can also provide the added value of consuming CO2 for the generation of biomass.

The principle is simple and is inspired by the normal functioning of nature. It is a matter of promoting and even accelerating the growth of microscopic algae, specifically selected for their properties and for the potential to generate various useful compounds, by introducing them into wastewater with a high nutrient content, aerated and exposed to sunlight.

In this way, microalgae find the perfect medium to grow and spread, consuming the nutrients that pollute the water and converting them into biomass.

In addition, photobiotreatment has the possibility of integrating the treatment of pollutant emissions from certain processes, such as combustion, by having the capacity to assimilate CO2.

In fact, if CO2 is used to dissolve it in water, carbon is increased in the basic nutrient equation C:N:P (Carbon/Nitrogen/Phosphorus) just for the nutrient where urban wastewater may be most deficient. Thus, by bringing them to a greater balance of nutrients, in addition to eliminating a pollutant such as CO2 in the same purification process, the reduction of the rest of the nutrients present is substantially increased, by balancing their composition in the water, and promotes greater growth and development of biomass.

The second technology seeks to give an energy value to sludge, the waste from urban wastewater treatment par excellence, and an important problem due to its constantly increasing volume and an increasingly limited recovery output.

Although they may have an interesting calorific value for their energy recovery, these sludges present the problem of high humidity, which is why this alternative is often discarded, given the costs for prior drying. This is where supercritical water gasification technology comes in, a technology currently under development that would allow biomass and carbonaceous waste with a high degree of humidity to generate a synthesis gas, also called syngas, that could be used for energy generation. Here, the degree of humidity of the sludge ceases to be a disadvantage, since water becomes the sinus in which the gasification reaction takes place.
Applied to a gasification process, supercritical water allows the solid fraction of the sludge to be closely mixed with water, favoring decomposition reactions, transfers of matter and, ultimately, the ultimate decomposition of the organic matter in the sludge into a synthesis gas composed of H2, CO, CH4 and CO2, which will be of interest for energy generation.

The use of supercritical water is already a reality and has been successfully applied on a small scale for waste co-oxidation, presenting itself as a promising technology in projects such as Life Lo2x — Supercritical Water Co-oxidation of Urban Sewage Sludge and Water, with the Agrifood Technological Institute (AINIA) in the lead, where it is proposed as a means for the cost-effective treatment of sewage sludge.

The next step will soon reach the industrial or semi-industrial scale, in the form of pilot plants, and soon there will be a valid alternative for the treatment of biological sludge and even wastewater through its gasification into supercritical water, so that its purification can be achieved at the same time as its energy use in situ, allowing for an alternative source of supply to treatment facilities, even beyond self-sustainability.

But the alternatives do not end the technologies seen so far, since in addition to what has been seen, in the last five years, an interesting line of work has also been taking on a special strength in the treatment of wastewater, which proposes a method for converting the chemical energy they contain (in the form of dissolved organic pollutants) into electrical energy, using Bioelectrogenesis for this purpose.

This novel technology, based on the discovery and development of bacteria capable of producing electricity, proposes bacterial electrochemistry (MET) as a means of purifying wastewater while generating electricity.

Microbial fuel cells, bioreactors or electrogenic lagoons are the three lines currently being proposed to accommodate this technology in wastewater treatment. The three lines have numerous studies in this regard, which have been growing exponentially in recent years, and in fact, there are even developments already on an industrial or semi-industrial scale brought to the market by some startups.
Bioelectrogenesis thus becomes a very interesting alternative for anaerobic secondary treatment of wastewater, which can even replace conventional wastewater with low polluting loads or even complement them, allowing for energy self-consumption (or even a surplus for other processes) and a low production of sludge, achieving yields similar to conventional treatments.

Soon, and following the trend of these new technologies, wastewater generation will be a competitive advantage for all those who know how to see “the energy they have inside”.

If you want to know more about this topic read: Environmental Quality