'Sewer assets': Use of sludge as fertilizer leads to rich-tasting foods

Tomatoes growing alongside the Aichi Prefectural Toyo River Sanitation Center are seen being harvested in the city of Toyohashi, Aichi Prefecture. (Mainichi)
Tomatoes growing alongside the Aichi Prefectural Toyo River Sanitation Center are seen being harvested in the city of Toyohashi, Aichi Prefecture. (Mainichi)

There's a new taste sensation cropping up nationwide, and it might not be what you expect: foods grown using sewage sludge and treated wastewater.

New technologies are springing up for increasing the production of tomatoes, as well as heightening the deliciousness of nori (edible seaweed). And the Japanese government, aiming to export waste treatment technology and its application to agriculture, is dubbing the previously incongruous combination of sewage and gastronomy "sewer bistro."

The Aichi Prefectural Toyo River Sanitation Center -- which is responsible for sewage treatment serving four cities in the eastern part of the prefecture, including Toyohashi -- has on its premises something that appears to be quite out of place: a 500 square meter agricultural greenhouse, filled with a crop of plump tomatoes.

At first, the operation appears to be that of regular hydroponic cultivation. In fact, however, carbon dioxide resulting from an electricity generation process involving gas that has originated from sewage sludge is being blown onto the tomato seedlings through greenhouse ducts -- with increased concentrations of carbon dioxide around the seedlings stimulating the process of photosynthesis.

In addition, the seedlings are introduced to treated sewage that includes elements such as phosphorous. The first experiment of its kind anywhere in the world, this process has resulted in crop yields that are around 30 percent higher than normal.

"They were very rich in flavor. They were delicious," commented Aichi Gov. Hideaki Omura after sampling the tomatoes.

Toyohashi University of Technology professor Hiroyuki Daimon, 48, who has been conducting substantiative experiments on the process since last year, indicated confidence in the project. "The complex technology increases the existential value of the wastewater treatment plants," he noted.

There are, additionally, numerous entrepreneurial examples already existing in this regard.

In the city of Saga, which is famous for its production of well-known nori brands, a tie-up was begun in 2007 between a sewage treatment plant and a local fisheries cooperative association. Through the collaboration, the plant releases treated sewage into the ocean with high concentrations of nitrogen during the winter months -- when nori is traditionally cultivated -- in order to help increase the level of amino acids, which are responsible for creating nori's rich flavor. Additionally, the city of Saga contracted a company to produce around 1,400 tons of sewage sludge fertilizer each year, which is in turn sold to farms.

The treatment plant uses microorganisms to break down contaminants in dirty sewage water, which is then allowed to settle at the bottom, and sluices the water that remains at the top. Although the precipitated sewage sludge had long been regarded as a nuisance -- with the majority of it sent to landfills until the 1990s -- the substance in fact contains abundant amounts of the three major components of fertilizer: nitrogen, phosphorous and potassium. And with the prices of commercial fertilizer on the rise, the once-shunned substance is consequently now getting a second, more favorable look.

According to the Ministry of Land, Infrastructure, Transport and Tourism (MLIT), which oversees sewage systems, the amount of sewage sludge being shipped off to landfills is continuing to decrease almost every year, with a total of 78 percent being recycled in 2010. Some 60 percent of this was utilized for cement and other construction-related materials, however, with biomass usage standing at a mere 15 percent for agricultural fertilizer, and 1 percent for electricity.

And with regard to the image of the substance as being unhygienic, an Aichi Prefecture project member insists, "Numerous types of heat treatments are utilized, and testing for heavy metals such as arsenic and cadmium also reveal that official standards are being met. If the process is properly managed, there will be no harm from using (the sludge)."

The MLIT reports more than 20 examples of effective utilization and shipping in this regard, such as the use of treated sewage for breeding soft-shelled turtles in the city of Saga, and the application of sewage sludge as fertilizer for rice cultivation in the city of Iwamizawa, Hokkaido Prefecture. Additional examples include pumpkins grown using sewage sludge as fertilizer in Iwamizawa; garlic grown using sewage sludge as fertilizer in Hachinohe, Aomori Prefecture; strawberries grown using sewage sludge as fertilizer in Kochi Prefecture; rice grown using treated sewage in the city of Kumamoto; and sugar cane grown using sewage sludge as fertilizer in Amami, Kagoshima Prefecture.

MLIT set up a strategy team for such "sewer assets" last summer, and this April, a public relations campaign was spearheaded with the publication of the first-ever cookbook using recipes of food grown from this method.

The strategy team has identified its next challenge as that of overseas launches. Japan has already exported treatment technology to countries including the United States -- but continues to face stiff competition from European nations. An MLIT representative commented, "The added value of utilization for food production will be a helpful advantage in terms of targeting developing countries with increasing populations. It is now the time to begin showing to the world our existing successes in this area."

"The total amount of phosphorous found in sewage sludge is around one-fifth of the amount that Japan imports yearly," explained Dr. Masaki Takaoka, an environmental engineering professor at Kyoto University who is an expert in sewage sludge treatment issues.

"Phosphorous is regarded globally as a valuable strategic resource, and imports in this area are likely to become restricted," he continued. "We must begin thinking about the efficient utilization of sewage sludge and treated water, which are in fact examples of domestic resources."

It bears asking, then: Have we neared the time when sewage sludge will no longer be regarded as the dirty substance it has long thought to be?

May 05, 2014(Mainichi Japan)