Saturday, February 24, 2024

Converting the sewage sludge into energy and mineral salts

Wastewater Treatment Plants convert wastewater into bilge water that can be discharged back into the environment. During this process, it produces liquid waste or sewage generally known as Sludge. Earlier, the operators of the plant used to dispose of it directly as fertilizer. But around ten years ago, Switzerland made this illegal due to the increased proportion of the polluted substances found in that effluents.

Therefore, now this sludge is mostly dried into the cake and burned, taking thousands of tons of phosphorus with it every year. It matters a lot as in most of the biological processes like photosynthesis, phosphorous is an important compound. But so far there was no viable method of recycling phosphorus in effluent streams.

Now, EPFL’s Laboratory of Sustainable and Catalytic Processing Engineers have designed a mechanism capable of recovering phosphorus, whose market CHF is 33 billion. The system was developed by TreaTech, a company spun off from the lab. Plus, all thanks to the thermal gasification technology developed by the Paul Scherrer Institute, TreaTech’s system can also produce biogas from the effluent.

In order to reduce the transportation costs, the sludge which is 95% water is first dehydrated by wastewater plants, requiring in-turn a great deal of energy that has a cost of its own. The residue is then incinerated elsewhere.

Our system can recover sludge directly from wastewater treatment plants without any drying or other preliminary processing needed,” says Frédéric Juillard, CEO of TreaTech. The team noted that in their system, the effluent stream is fed into a high-pressure, high-temperature separator (> 22.1 MPa and 400°C) where the fluid enters a supercritical state (i.e., between liquid and gas). That sharply lowers the solubility of the phosphorus and mineral salts in the fluid so it can be easily crystallized into a recoverable solid. “Over 90% of phosphorous can be recuperated,” underlines the CEO.

The team aims to convert almost 100% of organic matter into biogas

Gaël Peng, co-founder, and CTO of TreaTech explained that the biodigesters that are currently used in some of the water-waste treatment plants are able to convert only 40–50% of the organic matter. While the remaining waste is then dried and transported to incineration facilities that require a huge cost and a lot of energy. “Sludge processing and disposal account for around 40% of a wastewater treatment plant’s total operating costs,” added Peng.

To help the plant operator reduce the cost and increase the conversion rate, Juillard, therefore, wanted to include technology to create biogas in his system.

After a few months of studies and researches, he got a solution at the Paul Scherrer Institute, where the scientists were working on a new kind of reactor. The reactor uses ruthenium as the catalyst and that can achieve a nearly 100% conversion rate into biogas. The biogas then can be used to produce heat or electricity, or even as biofuel. The resulting water you get is free of toxicity and can be pumped directly back into municipal water systems.

What’s more

This new technology also saves a considerable amount of time as the reactors can turn sludge into biogas in just 20 minutes, unlike the existing biodigesters which need around 30 days. That can also save space at wastewater treatment plants and leaves no waste.

The company has already tested the prototype successfully and started building a large-scale version. The larger version is 100 times bigger than the prototype and can treat 100 kg/h of sludge. The team plans to install its system at a wastewater treatment plant by 2022.

The team is further planning to adapt the system for use in other applications, such as for treating industrial wastewater, wastewater from desalination plants or biomass residue.