The potential effects of
The impact of boat sewage discharge
Grey water discharges
Pollution from a wide variety of human activities
impacts upon water quality. Industrial discharges, agricultural
run-off, municipal waste and oil spills all contribute to declines
in water quality. One important source of aquatic pollution
is human sewage discharge which enters the UK's coastal waters
from several sources. Water company waste treatment plants are
by far and away the largest single source of sewage, discharging
millions of gallons of raw and treated sewage into rivers, lakes,
estuaries and oceans daily. Another source of human waste discharge
is boat-generated sewage and it is on this issue that the following
Craft that spend a large proportion of time
in use and away from land require some type of toilet system
for dealing with sewage. There is a large variety of different
toilet systems available to the boat owner ranging from portable
designs to fully installed systems. The choice of system depends
on a number of parameters including system cost, the size of
the boat, the availability of handling facilities, the type
of use to which the boat is subjected and local and national
regulations governing the system specification. The systems
- portable toilet with integral waste tanks
- chemical re-circulating toilet
- sea toilet
- installed toilets with holding tank systems
It should be stressed that smaller craft with
living accommodation are unlikely to have the room to retro-fit
a holding tank system.
The Impact of Boat Sewage
Installed toilets with holding tank systems
tend to have the least potential impact on the environment,
providing it is possible to pump out the tank at suitable shore-side
pump-out facilities. There are, however, very few of these facilities
in coastal areas. As a result, those craft with such systems
tend to pump out their tanks at sea. This is likely to have
minimal impact where the operation is performed in open sea
areas, well away from land, as the waste will be quickly diluted
and dispersed by wave actions and currents.
Boats which have holding tanks and use land-side
pump-out facilities do not contribute directly to marine pollution.
This is, however, outside the control of the boat owner. Toilets
which discharge raw or treated sewage overboard may contribute
a number of potentially harmful pollutants to the marine environment.
The resultant impact depends upon the flow characteristics of
the water body and the proximity to sensitive marine features.
The effect of raw and treated sewage discharged
from boats in fast flushing coastal areas is negligible in the
context of its dilute nature and in comparison to sewage discharge
from water companies treatment plants. Boat sewage discharge
in poor flushing estuarine areas, for example, inlets and bays,
however, can have a significant localised impact on the environment.
It is difficult to quantify this impact but it is likely to
be greatest in areas which already suffer from environmental
stresses from other sources such as agricultural run off. Whilst
there has been research into the potential impacts of sewage
on human health and aesthetic issues in the vicinity of popular
anchorage sites and bathing beaches, little research has been
carried out into its effect on the natural environment.
Biochemical Oxygen Demand - when human waste
is discharged into the water, bacteria feed on the organic matter
within the sewage. As the organic substances are decomposed
by the bacteria, dissolved oxygen in the water is consumed.
If large quantities of waste are discharged into the water the
bacterias biochemical oxygen demand (BOD) can seriously
deplete dissolved oxygen levels in the water.
The reduction in dissolved oxygen levels can
have serious consequences for fish and other flora and fauna,
which depend upon oxygen for survival. High levels of BOD resulting
from waste discharge are a particular problem in low flushing
areas where water circulates only slowly. However, it should
be noted that there are many other significant sources of high
BOD including decaying plants and animal wastes.
Nutrient Enrichment - human waste discharges
contain phosphorus and nitrogen in varying quantities. Phosphorous
and nitrogen are nutrients which are essential for plant growth.
However, when present in the water in excessive quantities these
nutrients can trigger algal blooms which reduce
light penetration through the water column. Populations of submerged
aquatic vegetation or macrophytes which rely on light transmission
for survival can be seriously affected by such blooms. As the
algae die, the process of decomposition also increases BOD in
a similar way to that of sewage decomposition.
Toxic Chemicals - portable toilet effluent
and some holding tank systems contain chemical additives used
to disinfect, breakdown and deodorise waste. The most commonly
used substances are chlorine, formaldehyde, ammonium and zinc
compounds. All these chemicals, if discharged into the water
in sufficient concentrations, are toxic to marine life and,
therefore, have the potential to affect marine flora and fauna.
Grey Water Discharges
Grey water discharges originate from onboard
sinks, showers and washing machines. These discharges may bring
with them potential pollutants in the forms of soaps and detergents,
food wastes and dyes. Of these pollutants, detergents are the
most significant. Detergents often contain phosphates, which
can contribute to nutrient enrichment as described above. Additionally
they may contain chlorine which can be toxic to flora and fauna.
However, grey water discharges from boats are in a very dilute
form and the impacts are likely to be negligible from small
Litter, whether on the land or in the water,
can cause significant visual impact. In addition, litter which
takes a long time to degrade can be harmful to wildlife through
ingestion and entanglement. Whilst a very small proportion of
the litter found in the marine environment may come from small
craft, the vast majority enters coastal waters either from land
or commercial vessels further out to sea. The majority of boat
owners are unlikely to dispose of rubbish over the side of their
craft as it would detract from their own enjoyment of the activity.
Provided there are sufficient land-side disposal and recycling
facilities, litter resulting from small craft is likely to be