Wastewater Pollution Introduction

Sewage pipe. Photo © Joe Miller

Wastewater 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 wastewater – which includes human sewage – is discharged into the environment without treatment, releasing an array of harmful contaminants into the ocean and causing direct harm to people and coral reefs. ref Research shows that wastewater pollution often occurs in proximity to coral reefs around the world due to nonexistent or inadequate wastewater management. ref

Terminology: Sewage vs Wastewater

Sewage and wastewater are terms that are often used interchangeably, but there are important differences between the two. Sewage (human waste transported through sewers) is a major component of wastewater, which is a collective term for the used water of a community or industry. Wastewater contains dissolved and suspended matter from a variety of domestic, commercial, or industrial sources including chemicals, soaps, heavy metals, nutrients, and effluent from sewered and non-sewered systems (like septic treatment tanks).

We recognize that reef managers and practitioners are likely more familiar with the term sewage when considering the major impacts to coral reefs, however, we will be using the term wastewater throughout the toolkit since it accurately describes various sources of pollution impacting coral reefs. And because collaboration is key to addressing the threats of ocean pollution from human waste and the use of consistent terminology helps to facilitate partnership with other sectors like the sanitation sector.

Major coastal cities and urban environments are usually significant sources of pollution, especially in low-income countries. Globally, there are approximately 4.5 billion people who do not have access to safe sanitation. Included in this group are 2.5 billion people who have no toilets at all and an estimated 1 million people who practice open defecation. ref However, even in high-income countries this challenge is present. For instance, more than 1.2 trillion gallons of effluent (including untreated wastewater, stormwater, and industrial waste) are discharged into waterways in the United States every year. ref 

Map sewage pollution

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

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

The impacts of wastewater pollution vary and are influenced by population, geography, and infrastructure. Wastewater 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 (e.g., sargassum seaweed) 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. These impacts are due to both direct effects of wastewater pollution on individual fish as well as decreased dissolved oxygen (due to increases in nutrient levels) and algal toxins. ref

When wastewater 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 wastewater 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 wastewater 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 wastewater pollution.

Climate change (specifically rising temperatures, sea levels, and ocean acidification) intensifies the impacts of wastewater 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 wastewater pollution in the ocean is becoming more critical. Reef managers address 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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