Phosphorus (P) is a popular fertiliser product that is widely used across Western Europe, despite it not being mined on the continent. In fact, 70% of Earth’s Phosphate reserves can be found in Morocco, with the other 30% mainly spread across China, Russia and the United States. However, these reserves are not inexhaustible. Wastewater is one source of P that is readily available to us, and with the current tightening of Ofwat’s regulations as water companies move through AMP7, alternative sources for P removal are being investigated. Here, we discuss one such P removal method - with the use of algae.
Algae is a chemical free solution that captures carbon dioxide (CO2) through photosynthesis and releases oxygen into the atmosphere. While its production using wastewater as a nutrient source can limit its valorisation, algae can still be harvested and sold as a biomass for a range of applications.
However, creating the optimum conditions for the use of algae in P removal can be difficult, requiring Nitrogen, Phosphorus, light for photosynthesis and a controlled level of CO2.
There are two current solutions for P removal processes using algae: open ponds and compact bioreactors. Open ponds, or raceways, are cost-effective to build and maintain, but require a very large footprint and have a high weather and daylight dependence. Therefore, they have a low overall productivity. On the other hand, compact suspended-solid phase photobioreactors (ssPBR) have a significantly lower footprint and higher productivity, but also have a far higher OPEX cost to build and operate.
In addition to the process, a wastewater treatment plant would also need to implement a source or method of CO2 production, to complement atmospheric CO2 to enable the algae to flourish. As Nitrogen and Phosphorus promote algae growth, these nutrients can be absorbed from the wastewater to do so.
In terms of optimal lighting for photosynthesis to occur, artificial lighting can be considered to provide 24-hour productivity but this can be costly. Alternatively, natural light can be used. However, this would require large open plants which would be cheaper to operate but would only be productive during natural daylight hours. Seasonal variability must also be considered, although this can be solved through the installation of the reactor within a greenhouse structure.
It is important to remember that while algae absorbs CO2 as part of the photosynthesis process, opting for an artificial light source can have a negative effect on the CO2 footprint of the plant itself. The power consumption of the plant needs to be considered for carbon neutrality. With this in mind, there is ongoing research into how it may be possible to provide a 24-hour light source to an algae reactor with an energy efficient method.
Harvesting Techniques
Currently, there are a number of known techniques for harvesting algae. These include settling, flotation, centrifugation and filtration. As well as each individual method’s pros and cons, the selected harvesting technique will also be impacted by the species of algae and intended application of the biomass. Furthermore, it is critical that the method does not cause contamination or influence the resultant biomass’ quality to affect its value.
Outcomes
There are a number of ongoing studies investigating how to adapt these techniques, as well as the development of new technologies, to simplify the algae harvesting process. This focus includes the use of spiral-plate centrifugation, electro-assisted flotation and bio-flocculated settling as possible solutions.
While there are examples of functioning algae plants globally, the key to its widespread implementation relies on the development of a method that is energy efficient and can be easily and cost-effectively implemented.
Veolia Water Technologies UK is pioneering P removal with a number of technologies, in addition to our ongoing research into alternative sources such as algae, that are currently used by thousands of wastewater treatment sites worldwide. As the water industry enters AMP7, VWT UK offers economical and reliable methods to continue this process while adhering to the tightening regulations.
For more information on P removal and the current technologies offered by VWT UK, click here.
