A sewage treatment plant in Canada has figured out a way to reduce the levels of chemicals that stick around in the treated wastewater, according to a new study. These include chemicals that have caused fish to develop both male and female sex organs. Eliminating them from our water system is important because they are devastating fish populations, and they can end up back in our bodies, too.

Hormones from drugs like birth control, as well chemicals that mimic hormones, get into the water through normal human usage, because most drugs aren’t completely broken down and used up by the body. Medications can also get into the sewage system when people and organizations like hospitals, and nursing homes flush unused drugs.

These pharmaceuticals aren’t completely removed by the wastewater treatment process. Trace amounts of these compounds in drinking water probably aren’t that dangerous to people in the short term, according to the WHO. But we don’t know what they could do over the long term, and in combination with other chemicals.

What we do know is that they can severely alter fish development, causing males to grow ovarian tissue in their testes, and females to stop producing eggs normally. That can have a major impact on local ecosystems. In one study, adding a synthetic estrogen found in birth control pills to an experimental lake feminized the male fish, screwed up egg production in the female fish, and caused the population to collapse in just a few years.

A decade ago, researchers from the University of Waterloo in Canada began studying a fish called the rainbow darter at sites downstream of a sewage treatment plant in a watershed that flows into Lake Erie. At some of the downstream research sites, every single one of the male rainbow darters the scientists examined were intersex. For many of these intersex males, their gonads were more like female ovaries than testes.

But in mid-2012, the sewage treatment plant got an upgrade to improve the plant’s efficiency and reduce the amount of pollution it released back into the environment. These upgrades included strategies to decrease the ammonia in the wastewater, since ammonia can lead to algal blooms and overabundant plant growth that can choke water systems. To do this, the plant operators increased aeration of the solid parts of the waste and extended the amount of time those solids are allowed to stay in the system. These are pretty typical upgrades that gave the bacteria more time to feast on the waste — breaking down ammonia and, it turned out, gobbling up the natural and synthetic female hormones and hormone mimics.

When the research team measured the hormone levels in the treated wastewater after the upgrades, they’d dropped significantly. What’s more, the number of intersex fish, and the severity of the intersex characteristics, declined as well. At one site, the percentage of intersex fish dropped from 100 percent to 29 percent in just one year. At another, over two years, it plummeted to 9 percent, according to results recently published in the journal Environmental Science and Technology.

The researchers’ findings means that upgrading water treatment systems with existing technology can make big differences for ecosystem health. If these were implemented everywhere, it could also be a big help to humans; after all, we’re exposed to these chemicals through our food and water, too.