Untreated grey water contains many pathogens and, in addition to those listed above, can also include, Salmonella, shigella, hepatitis A and E, and gastro intestinal viruses (national Research Council, 1993). Pathogens can pose danger to human health by contact or ingestion of contaminated water, or by consuming shell fish which feed by filtering water.
The 1986 EPA Quality Criteria for Water, commonly known as the ‘Gold Book’, references pathogen indicators and has defined water quality standards based on two activities of import to yachting: marine water bathing and, shellfish harvesting.
From the samples of untreated grey water analysed in the study, the average fecal coliform contamination exceeded the standard of 43 MPN/100mL for shellfish harvesting and, the average enterococci contamination exceeded the standard of 35 MPN/100mL for marine water bathing.
Although not a major indicator in this study, chlorine is used on yachts in a variety of applications including sanitising fresh water tanks, the discharge of which is generally pumped overboard, often via the grey water tanks, so levels must be carefully controlled so they do not exceed recommended limits.
Studies have found that chronic affects to the marine biota such as poor reproduction and health can be observed at concentrations above 230mg/L with acute effects, severe illness or death, at concentrations exceeding 860 mg/L.
Organic matter from the grey water acts as food for water borne bacteria. The more food available, the greater the number of bacteria decomposing the waste and using oxygen in the process. Nitrates and phosphorous also contribute to high oxygen demand by providing nutrients for plants and algae to grow quickly, contributing to organic waste when plants die and decompose.
Reductions in oxygen level can be harmful to aquatic species and can, in extreme cases, create ‘dead zones’ where no fish or other organisms can live.
Suspended solids can affect the clarity of the water and in turn adversely affect the photosynthetic activity of marine biome.
This is a more recent and topical concern relating to plastics entering the food chain, consumption by humans, and the longer term environmental and health impacts.
A number of studies have shown that during machine washing of man-made clothing significant numbers of tiny fibres are released into the waste water; and, in the case of yachts, into the grey water tank and eventually into the sea.
One such study is ‘The release of microplastic micro fibres from domestic washing machines: Effects of fabric type and washing conditions’ (Elsevier: Imogen E. Napper, Richard C. Thompson).
This study examined the release of textile fibres during machine washing of clothes from three commonly used fabrics; polyester, polyester-cotton mix and acrylic. The results showed that laundering 6kg of synthetic material could release between 137,951 – 728,789 fibres per wash, ranging in size between 20 𝜇m and <5mm. Given the load on most laundries and, composition of the fabrics washed, one can see the potential for a significant number micro plastics being discharged into the sea on a daily basis.
It is important to note that when grey water is discharged into the surrounding sea it mixes with the sea water and dilution takes place. This effect can be affected by factors such as discharge rate, salinity, water temperature, wind, currents and, of course, a yachts movement.
The Alaska Department of Environmental Conservation (ADEC) concluded that dilution factor would range from approx. 5 to 60 and occur between 1 and 7 meters from the ship (ADEC, 2004). The EPA report suggests that the initial dilution estimated by ACSI and ADEC for a vessel at rest would not likely be great enough for untreated grey water to meet the ‘Gold Book’ standards for fecal coliform and enterococci
From tests conducted by the EPA, it was shown that the dilution effect, due to the movement of a vessel and the mixing by their propellers, for a ship underway between 9.1 and 17.4 knots was a factor of between 200,000:1 and 640,000:1 immediately behind the vessel and, based on those results all ‘Gold Book’ standards for water quality, apart from fecal coliform, would be met.
From this it can be seen there are significant benefits in only discharging when underway and away from the near shore.