Friday, May 20, 2022

Turning toxic sewage sludge into renewable energy

Every year, a huge amount of sewage sludge, a byproduct of wastewater treatment, is produced all around the world that creates greenhouse gases and water pollution when dumped into landfills. This raises the question of whether this material can be used as a new type of bioenergy.

Well, a team of researchers at Worcester Polytechnic Institute (WPI) seems to think so. The team recently received a 3-year, $2 million grant from the U.S. Department of Energy (DOE) to develop a viable process to convert the sewage sludge into renewable fuel that can be used to power the wastewater treatment process and potentially supplement municipalities’ power grids.

The researchers are developing a system that uses hydrothermal processes, high temperatures and pressure, and inexpensive catalysts to turn sewage sludge into natural gas. The advanced hydrothermal gasification process heats the water in the toxic sludge to a supercritical stage between water and gas, turning the raw material into biofuel. From there, it takes several additional steps to convert the sludge into renewable energy.

Michael Timko, the team leader, is partnering with three other WPI professors on different technologies and methods important to create this new process. He is collaborating with Geoffrey Tompsett, WPI assistant research professor in Chemical Engineering, to study hydrothermal liquefaction, which is another thermal process used to convert the sludge into crude oil-like liquid – the first step in converting the sludge into usable gas.

Timko is also working with Andrew Teixeira, who is focused on designing a supercritical salt precipitation process that uses high temperatures and high-pressure steam to recover nitrates, sulfates, and phosphates from the sewage sludge – a part of the process of turning the raw material into biofuel. And Nick Kazantzis is simulating the economic and environmental performance of the process, a critical step to proving the commercial viability of the technology and for guiding research and development efforts.

Another collaborator on the research project, Harold Walker, will focus on ensuring the technology being developed works within conventional wastewater treatment facilities.

According to DOE, the energy in wastewater entering a typical wastewater treatment plant is five times greater than the energy needed to treat it. So the project’s overall objective is to recover and convert this wasted energy so it can be utilized to replace or supplement purchased energy sources. In addition, nitrates and phosphates extracted during the new process can be used in agriculture.

DOE estimates that the U.S. municipal wastewater treatment plants consume more than 30 terawatt-hours per year of electricity, which equals about $2 billion in annual electric costs. Electricity alone can constitute 25% to 40% of a wastewater treatment plant’s annual operating budget, so there is a lot of opportunities to reduce costs.

Turning a waste material like sludge into a valuable resource has so many environmental benefits, from reducing greenhouse gas emissions to reducing water pollution and soil contamination,” said Walker. “If successful on a large scale, this new approach could dramatically increase the recovery of energy at wastewater treatment facilities all over the world and bring us a lot closer to net energy neutral wastewater treatment.”

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