A significant share of sewage treatment plants (STPs) worldwide rely on iron coagulants to bind and remove phosphorus. Their use is expected to increase further as phosphorus effluent limits tighten. These coagulants are typically soluble iron salts and represent an important cost and environmental factor for STP operators. Especially the acid dissolution step that is typically used to produce soluble iron coagulants impacts costs and environmental impact, and the supply can be affected by fluctuating energy prices (e.g. 2021-2022). The counterions of the iron in these salts (Cl–, SO43-) will also increase the salinity of sewage effluents, and this can become increasingly problematic as fresh water becomes scarcer due to climate change.
Higher iron dosages are also required to maximize phosphorus recovery via vivianite formation. Fortunately, magnetic separation allows the recovery of a significant share of this iron (and phosphate) as vivianite. This mineral can potentially be split into iron and phosphorus for reuse. The sustainability of this route can be significantly improved if recovered iron from split vivianite can be reused in STPs. So far, developments indicate that these recovered iron products are likely insoluble iron hydroxides. The impact of iron use for phosphorus removal and recovery in STPs can therefore be significantly reduced if insoluble iron oxides (e.g. magnetite, iron hydroxides) could be used to replace soluble iron salts to bind and recover phosphorus.
Research challenges
Although iron coagulants for phosphate removal are widely used, little research has been conducted on this topic in recent years. Three main challenges have been identified for this topic:
- The mechanisms of Fe binding to P are largely unexplored. It is now established that vivianite will be the final FeP sink after anaerobic retention time, but the formation route remains understudied.
- Current Fe salts are soluble and react fast with P, which is one of the main reasons for their use. Using less soluble salts is therefore challenging, especially in the water line, where phosphate must be captured quickly. However, evidence showed that vivianite can already form in the waterline under anaerobic conditions, suggesting that insoluble iron could be used there. A good understanding of the Fe oxidation/reduction in the different stages of the STP will be essential.
- S and Fe interactions play an important role and impact Fe consumption, especially considering that Fe will preferentially bind S before P. The dosing point for Fe, for example, may influence the species to which Fe binds.
Objectives and methodology
The main objective of this project is to understand the Fe speciation along the STPs and the interactions with P and S in the different units. A study of existing installations using different Fe dosing strategies appears to be an interesting initial step. Advanced analytical techniques, such as Mössbauer spectroscopy and SEM-EDX, will be used to better understand the formation and interactions of Fe and P.
With a good understanding of the current situation, it will be possible to investigate mechanisms underlying the reduction and dissolution of insoluble Fe species. The results will be connected to practical use, mainly regarding P binding in anaerobic tanks, anoxic tanks and/or sludge return systems.
A final output of the project would be a strategy to use insoluble iron sources for P removal in the waterline, vivianite formation, without losing the COD coagulation properties. If the proof of principle is promising, this strategy will be tested at full-scale.
Your profile
We are looking for a highly motivated recent MSc graduate with a background in either chemical engineering, process engineering, chemistry, biotechnology or a related field. You need to be able to bridge fundamental knowledge, such as crystallization science, with applied process engineering concepts. An affinity for and understanding of biological processes are assets.
Our ideal candidate thrives in an international, multidisciplinary team and is interested in collaborating with industrial stakeholders. In addition, we are looking for a curious person with initiative, who critically analyses and discusses the project results.
During your PhD, you will have the opportunity to collaborate with the stakeholders who are currently bringing Vivimag® technology to the market and with those responsible for developing new value chains for vivianite. It will be a very exciting project with a concrete impact on the field of resource recovery!
Keywords
Iron dosing, phosphate recovery, crystallization, solids characterization, wastewater
Supervisory Team:
University promotor and co-promotor: dr. Mario Pronk, Applied Science Department, TUDelft
Wetsus supervisors: ir. Leon Korving, Scientific Program Manager, dr. Thomas Prot, Scientific Project Manager,
Project partners: Phosphate Recovery
Only applications that are complete, in English, and submitted via the application webpage before the deadline will be considered eligible.
