The factory I visited, called Tengu Natto, was located just 9 minutes from Mito station. The factory also has a museum and store, both of which are free to enter. Unlike the traditional method, which takes several days, the factory first sanitizes the wara and uses commercial natto-kin. This special bacterium, which is three times stronger than that found on unsterilized wara, is only made in Sendai, Tokyo, and Yamagata. This factory uses natto-kin from Sendai.
The factory takes into account the weather forecast of that day and adjusts the time the natto is kept in the heating units. The heating units are kept at only 40 celcius, which is roughly the same temperature as a warm bath. If it is a hot day, then the natto stays in the units for around 16-18 hours. If it's a cold day it will stay longer. The average fermentation time is twenty hours. Although the tools used today are different, the methodology is still the same as ancient times.
If you like natto or traditional Japanese dishes, I highly recommend checking out the factory and museum to learn more about the delicious natto.