Recreation : Potential effects : Sewage

The potential effects of sewage

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 section concentrates.

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 available include:

  • 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 Discharge

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 bacteria’s 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 craft.


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 minimal.


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