Sewage sludge contains every imaginable substance being flushed down toilets, sinks and sewers from homes, businesses, laboratories, industrial applications, and landfills. It is a mess of pathogens, heavy metals, dioxins, and other contaminants that gets treated to become a biosolid.

So what does that have to do with Ontario farms? In what some would call "recycling gone wrong" Toronto's biosolids ended up on 15,000 hectares of farmland last year.

While coverage of this food safety issue has been picking up in recent months, it's is an area of waste management that many Torontonians know little or nothing about.

While the practice of spreading sewage sludge on farmland (aka land application) has been happening at a smaller scale for over 30 years, the widespread land application of Toronto's biosolids has been practiced since 1996. It was driven forward by the stiffened sewage treatment guidelines in the Great Lakes Water Quality Agreement, which created an increased volume of sludge and consequently the problem of how to dispose of it.

As a solution, the Ontario Ministry of Health encouraged the uptake of land application, which is governed by the provincial Nutrients Management Act (NMA). It is completely legal to spread treated sewage onto farmlands from which we harvest crops (mostly corn, wheat, soy and pasture for livestock).

Why are we using sewage on our farmlands? How is this practice considered "recycling" or even acceptable?

In the NMA, biosolids are identified as containing beneficial soil nutrients such as nitrogen, phosphorous and potassium. They can help improve the pH levels in soil and increase its water retention capacity. And they provide farmers with an affordable (often free) way to fertilize their crops.

To make sewage sludge safe for land application, it is treated by anaerobic digestion for about two weeks in an environment heated to 37째C, which is supposed to "significantly reduce" the pathogen content.

However, heavy metals (arsenic, cadmium, chromium, cobalt, copper, lead, mercury, molybdenum, nickel, selenium and zinc), dioxins, and pathogens that were not killed during "digestion" can remain in the sludge.

So what does this mean for our food?

The NMA sets forth waiting periods that must be observed before harvesting or pasturing animals on sludge-covered fields in order to "prevent possible transmission of disease." Assuming these guidelines are followed, the Ministry believes that land application of sewage sludge is safe for humans and the environment.

But it has been documented that substances such as PCBs, dioxins, and brominated flame-retardants are being expressed in the milk and meat of cattle. And plants can absorb heavy metals, or they can be left behind as residue on the surface of fruits and vegetables like tomatoes and potatoes.

Big names in the food business like Campbell Soup, who operates a plant right here in Toronto, have taken the safe route and refused to use produce from fields being sludged. They claim there just isn't enough information to support the safety.

Right now the city is mapping out how biosolids will be managed for the next 45 years. A public consultation process is in place, and the next set of public meetings will take place in June 2009. From there the final Master Plan will be developed and is slated for completion in the fall of this year. Because our solid waste is for the most part out of sight and out of mind, public discourse has been limited.

Something to think about next time you flush the toilet... or eat a salad.

For current news and updates on the sludge situation in Toronto, you can join the SludgeWatch listserv, which is open to anyone to subscribe and post to.

Photo of Ashbridges Bay Water Treatment Plant by Lauren Wilson