Is Poo a Problem?

The following article appeared on the ABC Web Site yesterday

Researchers map the impact of human sewage on coastlines around the world

By environment reporter Nick Kilvert

Researchers have mapped the discharge of global wastewater — treated, septic, and raw sewage — into coastal ecosystems in what they say is the finest detail yet.

And the numbers are intimidating.
Key points:

  • Researchers found just 25 watersheds contribute half of the total wastewater nitrogen to the world’s oceans
    Meat-heavy diets are a significant factor in nitrogen levels
    High nitrogen levels can lead to algal blooms
  • Scientists from the University of California and Columbia University calculated the volume of nitrogen and faecal indicator organisms entering the ocean from about 135,000 watersheds around the world.
  • A watershed is an area of land, often bounded by hills or mountains, that drains all the water from that land to a common outlet such as a river.

They found that just 25 watersheds contribute nearly half of all wastewater nitrogen, with the Yangtze River in China contributing 11 per cent of the world’s total.

The researchers calculated that, in 2015, around 6.2 million tonnes of nitrogen entered coastal waters from human wastewater. Polluted water from a stream flowing into the Yangtze River in China. China’s Yangtze River contains 11 per cent of the world’s total wastewater nitrogen.(Getty Images: China Photos). This was about 40 per cent of the nitrogen that agricultural runoff would normally feed into coastal waters.

The watersheds that released the most nitrogen from wastewater were located in Korea, India and China. However, in terms of the overall contribution of nitrogen to coastal ecosystems, the United States was responsible for the third-highest level behind China and India. Just under a third of the wastewater-generated nitrogen reaching our oceans globally was from untreated sewage, with treated and septic making up the rest. While treating sewage removes solid and some organic matter, nitrogen is still present in treated wastewater.

The research is published today in the scientific journal PLOS ONE.

High meat diets equal high nitrogen

Organic crops and organically-reared livestock typically produce lower greenhouse gas emissions than non-organic. But lower yields mean more land must be farmed, resulting in higher emissions overall.  To get these figures, the researchers combined data on population density, wastewater treatment, and diet, down to a resolution of 1-square-kilometre grids, for watersheds worldwide.

The most surprising variable that changed the volume of nitrogen discharge was the amount of meat in local diets, lead author and postdoctoral researcher Cascade Tuholske from Columbia University said.   “As someone who studies food systems, the most surprising result to me was the contrast between the Yangtze [China] and Brahmaputra River [Tibet, India, Bangladesh] in terms of nitrogen inputs,” Dr Tuholske said. “The Yangtze added far more nitrogen to coastal waters than what would be expected.”

One possible solution to reduce nitrogen loads in wastewater would be to cut down on our meat consumption, he said. “The more burgers we eat, the more nitrogen we poop, the worse outcomes for coastal habitats. “This suggests to me that diets in China have shifted far faster to animal-based protein than diets in India.”

Why ocean nitrogen levels matter

Elevated nitrogen levels can lead to algal blooms, which can in turn deplete oxygen levels in water as that algae decomposes, according to Megan Huggett, a marine and coastal ecosystem ecologist from the University of Newcastle. “It can lead to decomposition of algal cells, which can deplete oxygen and lead to fish kills,” said Dr Huggett, who wasn’t involved in the study.“[Wastewater] also brings in things like herbicides and pesticides into the system, and plastics of course.” The algae Caulerpa sertularioides engulfs a reef in Culebra Bay on the Costa Rica Pacific coast

The researchers also estimated the potential exposure of coral reefs and seagrass beds to elevated nitrogen due to wastewater. They found that more than half of the world’s coral reefs and nearly 90 per cent of seagrass beds experience some elevated nitrogen from human sources. According to Dr Huggett, some research has actually shown that seagrass beds can help mitigate the impacts of sewage and pathogen loads on nearby reefs.

All of the above is a verbatim extract from the ABC article. While it basically correct, it is not the whole story.

Algae or phytoplankton play a major role in the carbon cycle.  They remove more carbon dioxide from the atmosphere by photosynthesis than do plants on land. Seventy percent of the world is ocean and ninety percent of the ocean is almost sterile. It is a nutrient limited “desert”. These are the blue parts in satellite images. The green bits near coastlines are coloured by phytoplankton.

In 1934, Alfred Redfield discovered that the ratios of carbon to nitrogen to phosphorus are a nearly constant 106:16:1 throughout the world’s oceans, in both phytoplankton biomass and in dissolved nutrient pools. These are the Redfield Ratios. We can use them to compute how much carbon will be removed from the atmosphere due to the 6.2 million tonnes of nitrogen quoted in the article, viz.:

6.2 X 106 / 16 = 41 MtC/yr= 150 MtCO2/yr

(Australia emits 414 MtCO2/yr)

Algal blooms are only a problem when they occur in coastal or estuarine waters near sea grass beds or corals because they absorb sunlight before it can reach the sea grass beds underneath.

What is needed is a means to transport our sewage offshore to the deep blue sea where it can generate carbon credits.



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