Anyone who lives in Okinawa, a subtropical island in Japan, has an idea of the intensity of its pork industry. Farms have a significant effect on the economy and culture of the island. According to the Cabinet Office of Japan, in 2018, there were more than 225,000 pigs in Okinawa. Pork is a staple in the local diet and is found in many dishes in traditional restaurants. But the presence of pig farms has another, less welcome impact, that of odors. Drive through particularly agricultural areas with the car windows down and you are sure to be filled with regrets.
This odor is, at least in part, caused by a by-product of pig farming. Across Okinawa, large amounts of wastewater are produced by farms. Today, researchers from the Biological Systems Unit of the Okinawa Institute of Science and Technology Graduate University (OIST) have created a new system for treating this wastewater, which they have successfully tested on a pig farm. local in Okinawa.
“Our new system uses two different chambers,” explained Dr Anna Prokhorova, lead author of a recently published article in Bioresources Technology. “In one chamber, full-power porcine wastewater is treated to remove odors, pathogens and organic matter, while in the other chamber excess nitrate and phosphate is removed from the wastewater that has already been processed by the traditional ventilation system. To our knowledge, this is the first system to successfully treat two different types of wastewater at the same time. “
This is in stark contrast to the traditional aeration system currently used by farmers which primarily treats organic matter in wastewater and also converts ammonium present to nitrate but does not further treat nitrate. In Japan, the nitrate discharge limit for the livestock industry will soon be lowered to one-fifth of the current level (which now stands at 500 milligrams of nitrate nitrogen per liter) to be in line with other industries. More than 35% of Okinawa’s farms are at risk of overtaking this impending change.
“This is of great concern as contamination with nitrates can have disastrous effects on human health and the environment,” said Dr Mami Kainuma, group leader in the Biological Systems Unit. “When nitrate is ingested by humans, it is converted to nitrite, which impacts the blood’s ability to carry oxygen and can lead to methemoglobinemia or blue baby syndrome.”
This new system relied on the existence of a rich community of bacteria to start the process. In the first chamber – the anode chamber – bacteria reacted with the organic molecules present, releasing electrons in the process. These electrons were then transferred to the second chamber – the cathode chamber – via the electrodes. The cathode chamber contained sewage that had already undergone the aeration process and therefore had high levels of nitrate. Bacteria on the surface of the cathode chamber accept these electrons and use them to fuel the conversion of nitrate to nitrogen gas. The advantage of this system is that nitrate removal can occur in wastewater with a low organic content, such as already aerated water.
After successfully testing this system in the laboratory, the researchers set up a first pilot experiment at one of the pig farms of the Okinawa Prefecture Livestock and Grassland Research Center by working with the Okinawa Prefecture. Okinawa Prefecture Environmental Sciences and Okidoyaku. There they had access to both the aeration basin and raw sewage. The project was funded by the Okinawa prefectural government and monitored for over a year. Because of the essential role played by bacterial communities, the researchers also analyzed which species were present, how the composition of the community changed over time, and which species were responsible for each step.
Long-term experience has shown that the dominant nitrate-eliminating bacteria are the ones that can receive electrons to grow. During the treatment, their activity was stimulated by the potential applied to the electrode in a range of -0.4 V to -0.6 V, which led to more efficient treatment of wastewater. These bacterial communities increased by more than 60% in total in the cathode chamber and continued to exhibit high activity, leading to a high rate of nitrate reduction. Another great advantage was that, as organic matter and, in particular, volatile fatty acids were degraded in the raw wastewater, the odor was diminished and the number of pathogens reduced.
“We are very satisfied with the results so far. It is much more effective than expected,” said Dr Prokhorova. “This system is scalable, inexpensive, easy to assemble and requires little maintenance. We hope that over the next few years it will be used by farmers in Okinawa and other places facing similar problems, such as than the rural communities of mainland Japan and Southeast Asia. “
The work will be continued as a POC program in OIST.