Sewage Treatment Plants
As a project with the goal to use light-switchable proteins to bind and later reuse phosphate from wastewater, we need to understand the current practices of phosphate removal in sewage treatment plants. The basic setup of a sewage treatment plant is depicted in figure 1.
Figure 1: Basic structure of a sewage treatment plant
Getting a better understanding of the system we are trying to implement our project in, we met with Prof. Schäfer. He studied sanitary environmental engineering and currently works at the Erftverband, an association for water management, as deputy of sewage treatment plant. As such, he answered some of our more specific questions concerning conditions, facts, and processes in a sewage treatment plant (figure 2).
Figure 2: Meeting with Prof. Schäfer from the Erftverband
He told us that on average, every person is responsible for 120 L of sewage every day. Seeing that there are sewage treatment plants with an urban catchment of a few thousand to a few hundred thousand people, one can imagine the immense load of sewage that needs to be handled every day. The sewage treatment plant in Bergheim in Germany e.g treats $12000 m^{3}$ of sewage on a daily basis.
The average concentration of phosphorous in sewage is around 10 mg/L, assuming dry weather. The sewage treatment plants precipitate around 95% of the phosphorous from the sewage, using ferric chloride. Per precipitated kilogram of phosphorous, 2.7 kg ferric chloride is used.
Possible Implementation Points
After talking to farmers and experts (see Human Practices), we concluded that MEtaPhos could be used in multiple locations in sewage treatment plants. One possibility is using MEtaPhos after the final sedimentation tank or using the ash from the mono-incinerator.
1. Final Sedimentation Tank
This is the point in the process where precipitation with ferric chloride usually occurs. To completely replace this procedure, a biological filtration using our switchable immobilized proteins would have to take place here. This would make the precipitation with metals completely obsolete, however, there are some challenges to consider.
Firstly, the proteins would need to be immobilized on a membrane, since immobilization on beads or via encapsulation is not suitable for such an approach. The functionality of light-switchable proteins on a membrane would have to be established. Furthermore, the membrane would have to be able to handle huge volumes of water, which brings up other problems such as membrane fouling and leaching that need to be fixed before the application.
Another aspect to consider is the temperature and pH of the medium in which the proteins work. In the final sedimentation tank, the water has a pH of 6 to 7.5 and is 10 °C to 20 °C warm, depending on the season. The protein would need to be stable and ideally have a high activity within these operating conditions.
Finally, the economic aspect must also be considered when thinking about implementing our project in practice. This would be a concern because replacing precipitation with our method would mean that in a decentralized approach every wastewater treatment plant would have to use our biological filtration method. This would make it a rather expensive project.
As you can see, there are quite a few hurdles to overcome before light switchable phosphate binding proteins can actually be implemented into sewage treatment plants this way, but we believe that in the long term this approach represents the best alternative to precipitation with heavy metals.
2. Mono-Incinerator
The incineration of sewage sludge among other things has the purpose of destroying potentially harmful organic substances. After the incineration, the ashes are left over to be disposed.
This poses another possible entry point, as the ashes can be resuspended in water, thus providing a much lower volume that needs to be handled. Therefore, the exact mechanism of how the proteins can be immobilized and used is much more flexible. Possible methods include immobilization on beads in a continuously stirred basket reactor, on a column or on a membrane where the resuspended ash could pass through.
Next to a lower volume, this bears some other advantages, such as a much more defined reaction system. Temperature, pH and as well as the medium itself could be controlled very easily, providing optimal conditions for the proteins to do their work.
Another beneficial aspect of this approach lies in the fact that it could happen centralized, as the incinerator plants are not necessarily coupled to the sewage treatment plants. This could mean a much more economical solution since our project would only have to be implemented in a few incinerator plants instead of every sewage treatment plant.
One problem that would need to be addressed though is the pollution of the ashes with everything from heavy metals to antibiotic residues.
This approach would, however, not replace heavy metal precipitation, as the sewage still would need to be purified before introducing it to the natural water cycle again. Nevertheless, it could be a start to recover and reuse it after converting it to polyphosphate instead of throwing it away and mining new phosphorous.
