The science behind a little poo poo- A Ecotoxicologist explains

By TWS resident Marine Ecotoxicologist Sam Gaylard

Picture this, a hot, still afternoon in a beautiful anchorage. There are plenty of boats anchored and it’s a small bay. You jump into the water to cool off and have a quick dip and then out of the corner of your eye.......a blind mullet floats on past! Other than the gross nature of this, there are some real potential health risks, and unfortunately, it’s not that uncommon.

Boats anchored in a small bay

Photo credit @thecruisingkiwis whilst on anchor with a few of their friends at Rottnest Island,

Western Australia

Abiding by the rules

The majority of countries have strict regulations with regard to the discharge of sewage from vessels. In both the United States and Australia, you must not discharge sewage within 3 nautical miles of the coast. The enforcement of these regulations varies from place to place but typically vessels need some kind of sewage holding tank or other methods to prevent discharge of sewage while at an anchorage. In offshore waters (ie: > 3nm) it is assumed that there is adequate flushing and dilution to reduce the risk of health and environmental problems. But what about in nearshore waters and particularly a crowded anchorage?

What is sewage and how many germs are we really talking about here?

The definition of sewage is vague and often does not differentiate between urine and solid matter. I personally consider that urine discharged overboard within coastal waters is low risk to the environmental or human health impacts. It is sterile and poses very little health risk. On the other hand, it is estimated that there are nearly 100 billion bacteria per gram of wet poop [1].

Substantial loads of bacteria and pathogens are discharged when a toilet flushes or macerates, these will disperse into waters around the vessel and move with the tide and currents. Bacteria can survive for more than 2 and a half days in salt waters [2] and the load in surrounding waters can accumulate depending on how many people on board the vessel, how many vessels are within the anchorage, and the flushing of the water in the anchorage. So, the load of bacteria and pathogens in the water after the morning coffee can be considerable.

So, if you do ingest some human sewage, what can be the effect on our bodies?

This thought alone certainly evokes an instant dry retching effect but the potential health implications for ingesting human sewage can be considerable. This can range from mild gastroenteritis through to some particularly nasty viruses and parasitic protozoa which can cause serious illnesses such as dysentery and hepatitis [3]. These illnesses can be debilitating and in extreme cases, life-threatening. Certainly not something that is favorable for boat life in any capacity.

What happens in the anchorage?

The tidal flow will partially exchange waters within an anchorage roughly every six hours. The amount of tidal exchange will depend on the size and depth of the anchorage and the tidal amplitude (a measure of how much water comes in). As tides move through the anchorage it will bring new cleaner water in and push the bacteria out of the anchorage. Larger anchorages with more open waters will flush better than tightly enclosed bays, but these are also not as protected from wind and waves. Spring (larger) tides will bring more water into an anchorage providing higher exchange than during neap (smaller) tides. So it’s not hard to imagine that you could get considerable amounts of residual bacteria in an anchorage, particularly if the anchorage is small and enclosed with numerous large vessels and during neap tides.

Think about what could happen if you are down current of another large anchorage!

If you chose to swim around your anchorage, then consider whether you and surrounding boats are doing the right thing with their sewage. Think about the size of the anchorage and the water flowing through and make an educated assessment.

Another risk that is not often considered, is the risk from harvesting oysters from rocks at low tide. These shellfish are a delicacy but they filter vast volumes of water taking out particles including algae for food. If bacteria and pathogens are in the water, these will be filtered into the tissue of the oyster and can concentrate to very high concentrations. Eating contaminated oysters can result in a very nasty bout of gastro. Harvesting oysters is a fantastic sustainable practice, but consider where you do this.

Sewage and its effects on the environment

It's not just bacteria and pathogens that can be a concern from the mismanagement of sewage. From an environmental point of view, discharges contain nitrogen and phosphorus, which are nutrients. These nutrients promote algal blooms which deplete oxygen in the water and can lead to smothering crucial habitats including seagrass. This process is called eutrophication, and it is one of the biggest causes of habitat loss throughout the world.

Sewage discharge from boats is a very small proportion of the overall load of nutrients coming from agricultural runoff, sewage treatment plants, and aquaculture industry but every little bit adds to the problem.

What can we do as environmentally conscious sailors?

The best thing to do is to not discharge sewage into the waters. We all suffer from the attitude that one discharge will make very little difference, but if we all have this attitude, then each discharge can add up. In some locations inspection of vessels occur but these are generally focused on sensitive areas. It's up to each of us to ensure we are not contributing to a potentially dangerous situation for ourselves, our neighbor's and also the environment.

On our sailing vessel, SV Allusive, we use a composting toilet and we see the benefit;

  • That it has no through-hull connections making it safer for the vessel and no pipes where bacteria grow and create foul odors.

  • It separates the liquids and solids and maintains a very dry and well-aerated compost with no smell.

  • This solids compartment will last the 4 of us (2 adults and 2 kids) approximately 2-3 weeks. Friends on other vessels have a 110L sewage holding tank with the ability to discard liquids overboard and retain solids. They suggest that for the two of them, they can last about 1.5 weeks between discharges. So in comparison, we also get a longer period between disposals.

  • Sewage holding tanks can be expensive to retrofit into a yacht but if your vessel has one of adequate size installed then this is also a great option but please consider where and when you pump it out.

To learn more about the conversion process of making the switch to a composting head, check out Michael's article Economy, Ecology & Excrement: Why we installed a Composting Toilet.

Sam Gaylard

Sam has been sailing for over 30 years in both social and high performance racing all-size boats from dinghies, high powered sports boats through to carbon fiber race machines. For the past 2 years Sam and his family have been living aboard their Lyons 47 in Adelaide and cruising up the east coast of Australia diving the vast kelp forests of southern Australia and the warm tropical coral in the north. Sam trained as an environmental toxicologist specializing in how pollution impacts on marine systems and has been working as a leading marine scientist for the South Australian government for over 20 years. If that is not enough, Sam is also working on a PhD focusing on how we can improve environmental decision making.

Follow along with Sam and his family adventures



  1. Ho, V. Your poo is (mostly) alive. Here’s what’s in it. The Conversation, 2018 5 December 2022.]; Available from:

  2. Dufour, A.P., Bacterial indicators of recreational water quality. Canadian Journal of Public Health/Revue Canadienne de Sante'e Publique, 1984. 75(1): p. 49-56.

  3. NHMRC, Guidelines for Managing Risks in Recreational Waters, National Health and Medical Research Council, Editor. 2008, Australian Government: Canberra. p. 216 pp.

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