Sewage Pollution Introduction

Sewage pipe. Photo © Joe Miller

Ocean sewage pollution is a growing threat to people and marine life and makes up the largest percentage of coastal pollution worldwide. ref Globally, an estimated 80 percent of sewage is discharged into rivers, lakes, streams and the ocean untreated. ref Research shows that sewage pollution often occurs in proximity to coral reefs around the world due to nonexistent or inadequate sewage management. ref

Map sewage pollution

Documented coastal sewage pollution in 104 of 112 areas with coral reefs. Source: Wear and Vega Thurber 2015

Major coastal cities and urban environments are usually significant sources of pollution, especially in low-income countries. However, even in high-income countries this challenge is present. For instance, more than 1.3 trillion gallons of wastewater (including untreated sewage, stormwater, and industrial waste) are discharged into rivers in the United States every year. ref Over half of the world’s population, 4.2 billion people, use sanitation services that leave human waste untreated and an estimated 673 million people have no toilets at all and practice open defecation, often in tropical areas. ref

Typical components of sewage include water, nutrients, organic matter, bacteria, viruses, parasites, endocrine disruptors, suspended solids, micro and macro plastics, microfibers, industrial chemicals, slaughterhouse wastes, sediments and heavy metals—each of which individually and together work to harm coastal and marine ecosystems. ref The high volume of sewage humans discharge into the ocean degrades critical habitats, kills marine life, harms ecosystems upon which humans depend, and threatens human health.

The impacts of sewage pollution vary and is influenced by population, geography, and infrastructure. Sewage pollution can be linked to the following impacts ( opens in a new windowOur Shared Seas):

  • Physical and biological damage to coral reefs, seagrasses, and salt marshes where it can smother habitat, lead to high levels of local acidification, and increase risk of disease. ref
  • Eutrophication due to nutrient overload that depletes oxygen, kills marine flora and fauna, and disrupts ecological processes. ref
  • Loss of coastal ecosystem services, like erosion control, buffers from storms, and nurseries for juvenile fish. ref
  • Harmful algal blooms that can produce toxins or physical material (i.e., sargassum) that kill marine life, close beaches, and can cause human disease through direct exposure and indirectly through consumption of contaminated seafood. ref
  • Animal and human diseases resulting from pathogens, heavy metals, and toxic chemicals that can cause acute disease, as well as long-term disruptions to biological processes. ref
  • Contamination of fisheries and fish mortality as well as reductions in species diversity in polluted areas because of decreased dissolved oxygen (due to increases in nutrient levels) and algal toxins. ref

When sewage enters the ocean and mixes with seawater, pollutants are dispersed and diluted. This has led to a persistent assumption that “the solution to pollution is dilution”. However, ongoing sewage contamination limits the ocean’s capacity to dilute those contaminates, particularly in areas with limited tidal flux, or vulnerable biodiversity. Strategies to collect and treat sewage can effectively avoid further pollution of the ocean, although treated effluent can also be harmful due to contaminants left behind by outdated or inadequate treatment. See Impacts on Marine Life and Impacts on Human Health for additional information on the effects of sewage pollution.

Climate change (specifically rising temperatures, sea levels, and ocean acidification) intensifies the impacts of sewage pollution. Oxygen depletion, resulting from nutrient loading and corresponding algal blooms, leads to increased production of nitrous oxide, a greenhouse gas further contributing to climate change. As global populations and climate change threats increase, the need to mitigate sewage pollution in the ocean is becoming more critical. Reef managers can engage in addressing this threat and help ensure that sanitation interventions consider natural ecosystems.

Watch the webinar on Addressing the Threat of Ocean Sewage Pollution:

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