Maintenance wastes and marina/harbour runoff

Biocides and bleach

Detergents

Antifouling paints

Maintenance wastes can enter a harbour as a result of a number of activities including scraping old paint from vessels, cleaning pontoons, cleaning jetties and wharves or cleaning vessels. Within marinas and boatyards there are often commercial maintenance areas that are usually housed in large hangers, and there are also general boat areas where boat owners can carry out their own maintenance of craft, whether in dry berthing areas, against walls or on scrubbing grids. Within these areas pressure washing, scraping and painting operations take place (UK CEED 1993). As a result of spillages, debris and wastes produced during these activities, the waters used to wash down maintenance areas may contain a mixture of contaminants including oils, oil emulsifiers, paints, solvents, detergents, bleach, antifouling paint scrapings or sandblasting wastes.

Contaminated cleaning waters can be washed down into the harbour or marina basin directly or via the drainage system. Even when no chemicals are used in the cleaning of harbour structures and just water and elbow grease is applied, the runoff that enters the harbour may be contaminated with oil, debris, heavy metals or sediments from the surfaces of the jetties, pontoons and wharves. On occasion dirt, debris and spillages have been known to be swept directly over the side and into the water.

On entering the marine environment these pollutants can have harmful or toxic effects on the animals and plants. In many cases the effects may be temporary and minimal, however risks of possible adverse effects increase where cleaning agents and other chemicals are used incorrectly or in large quantities far in excess of needs. The dilution of wastes in the harbour waters means that in most cases any possible adverse effects will be only localised and temporary. However, there may be a problem where wastes are washed into enclosed waters, such as docks, or areas with low tidal flushing. The use of biocides, bleach and detergent in the maintenance of harbour structures and vessels are discussed further below.

Fouling of harbour structures, such as slipways, steps, jetties, pontoons, can result in surfaces becoming covered in layers of bacterial and algal slime that must be removed. Obviously ensuring that walkways are safe for staff and the public is an important consideration in harbour management. A number of methods have been used to overcome the effects of fouling of both harbour structures and boats with variable success. These range from manual washing and scraping to the application of chemicals to kill and remove the fouling organisms (biocides). Unfortunately for the environment, in most cases the use of biocides is the simplest and most effective means of maintaining safe harbours.

Bleach is a popular solution used by a number of small ports and harbours to remove algae from slipways, ladders and steps. However severe damage can occur to the local marine environment where chlorine-containing agents, such as bleach, are used in large quantities at any one time. The impact of chlorine on the marine environment has been monitored for many years and has been shown to be toxic to shellfish and fish as well as causing the localised lowering of species diversity. The relatively widespread use of bleach is encouraged by the fact that it works very well as an inexpensive, easily applied biocidal agent, and there are few non-polluting alternatives which easily remove algae and prevent its occurrence for sometime. Practices such as pressure hosing or scraping are very labour intensive and often do not achieve a level of removal that is safe for the public (Rennis 1995). Where biocides and bleach must be used, dilution is the key to minimising the potential affects. One must also have regard to the possibility of the storage of contaminants in sediments and their accumulation in marine animals.

Soaps and detergents are often used within harbours for general cleaning operations, particularly for cleaning vessels. When detergents enter harbour waters they can cause the formation of ‘grey water’ which contains phosphate nutrients that encourage algal growth. Under certain conditions, which depend on a number of variables (including background water quality and season), when present in low concentrations this can have the effect of enhancing plant productivity. When detergents are present in high concentrations the formation of algal blooms may occur. The breakdown of these blooms causes the removal of oxygen from the surrounding waters, which can disturb or suffocate sensitive marine animals within the area. The tendency for algal bloom formation is highest during the warmer spring and summer months.

Boat and ship hulls spend a large proportion of the time submerged in water and as a result they become prone to colonisation by marine micro-organisms, weed, barnacles and so forth. Fouling of craft increases the drag on the hull, which can lead to increased fuel consumption. The most effective means of protecting boats from the fouling is to apply a coating of antifouling paint which contains a biocide that is designed to leach into the almost static layer of water next to the hull preventing organisms adhering to the paint by poisoning the settling organisms. World-wide the use of these paints has made a significant contribution to the reduction of costs to maritime industry, through savings in fuel consumption, dry docking and maintenance costs (ICS 1997). However, their release into the marine environment has also been found to have harmful effects on non-target organisms, such as shellfish.

Much of the pollution problem associated with the use of antifouling paints derives from the traditional widespread use of the old types of paint. When these were first applied the leaching rate was high and more biocide was released into the environment than was necessary to control fouling, and when the paint aged, the rate of leaching was much lower, releasing too little biocide to be effective. Modern antifouling paints are carefully formulated to release biocide at a constant rate and in concentrations just above that needed to kill the juvenile organisms that cause the problems (BMIF 1997).

There are numerous types of antifouling paints, containing a range of different active ingredients, and varying in their longevity (and therefore their application rates), compatibility with certain substrates and leaching behaviour (Boxall, Conrad & Reed 1998). Antifouling biocides enter the environment during application of antifouling paint, leaching from paint into the surrounding water and during the removal of paint and discard of contaminated remnants.

Over the past two decades the most commonly used biocides in antifouling paints for recreational vessels and larger commercial vessels have been tributyl tin (TBT) and copper compounds, which are discussed below. There are non-toxic alternatives to the use of biocides in antifouling paints available, such as silicon-based paints which make the surface of the ship slippery so that fouling organisms have difficulty attaching themselves to the hull and are washed off as the ship moves through the water. At present these alternatives are less effective, but are undergoing further development.

TBT- & Copper-based antifouling paints

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