Anyone who lives in Okinawa, a subtropical island in Japan, appreciates the intensity of its pork industry. Farms have a significant effect on the economy and culture of the island. According to the Japanese Cabinet Office, in 2018 there were more than 225,000 pigs in Okinawa. Pork is a staple of 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 areas particularly packed with farms with the car windows closed and you are sure to be filled with regret.
This odor is, at least in part, caused by a by-product of pig farming. All over Okinawa, large amounts of wastewater are produced by farms. Now, researchers at the Biological Systems Unit of the University of 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 farm. local pig 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 pig wastewater is treated to remove odors, pathogens and organic matter, while in the other chamber excess nitrate and phosphate is removed from the wastewater which have already been treated by the traditional aeration system. To the best of 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 mainly treats organic matter in wastewater and also converts the 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 keep pace with others industries. Over 35% of Okinawa farms are likely to overtake this impending change.
“This is of great concern as nitrate contamination can have disastrous effects on human health and the environment,” said Dr Mami Kainuma, group leader of the Biological Systems Unit. “When nitrate is ingested by humans, it is converted to nitrite, which affects 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 wastewater which 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 in one of the pig farms of the Okinawa Prefecture Cattle and Grassland Research Center in collaboration with the Science Center. environment of Okinawa Prefecture and Okidoyaku. There they had access to both the aeration tank and raw sewage. The project was funded by the Okinawa prefectural government and followed for over a year. Due to the critical 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 a potential applied to the electrode in a range of -0.4V to -0.6V, 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 in the next few years it will be used by farmers in Okinawa and other areas with similar problems, such as rural communities on the mainland. Japan and Southeast Asia. “
Work will continue as a POC program in OIST.