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VTT: Ship wastewater discharges cause minor though not insignificant nutrient input in the Baltic Sea



According to a survey carried out by VTT, ship wastewater discharges have a minor impact on the state of the Baltic Sea. However, the impact is not insignificant as the nutrients entering the sea with unprocessed wastewater from ships accelerate algae growth. A much more prominent source of nitrogen is ship exhaust gases. The largest nutrient inputs into the Baltic Sea are caused by agriculture in the watershed and by municipal wastewater.

The aim of the survey conducted by VTT was to investigate the scope of ship wastewater nutrient inputs in the Baltic Sea. The results indicated that ship wastewater only makes up a small part of the total loading, being 0.04 % for nitrogen and 0.3 % for phosphorus. Compared with wastewater discharges, the amount of eutrophicating nitrogen entering the Baltic Sea with ship exhaust gases each year is substantially higher. At the turn of the millennium, 6 % of the nitrogen fallout in the Baltic Sea was caused by exhaust emissions from ships.

However, the environmental impact of ship wastewater is much more severe than suggested by mere percentages in that most of its nutrients are in a form biologically exploitable by algae. In addition, most wastewater nutrients enter the Baltic Sea in summer, at which point algae have already depleted most of the nitrogen and phosphorus having dissolved into run-off water in spring. Wastewater discharges also have local harmful effects on heavily trafficked shipping routes. The wastewater released into the sea also contains pathogens, heavy metals and organic compounds harmful to aquatic organisms.

The largest sources of nitrogen and phosphorus in the Baltic Sea are agriculture in the watershed and insufficiently disinfected municipal wastewater. Approximately one-quarter of the nitrogen load takes the form of aerial fallout. Based on point-type sources in rivers and the coast, a total of 744,900 tons of nitrogen (approximately 74 % of the total amount) entered the Baltic Sea each year at the beginning of the millennium. The nitrogen fallout was 264,100 tons (approximately 26 %). Almost all (99 %) of the 34,500 tons of phosphorus in the Baltic Sea is caused by run-off water. It has been estimated that the nutrient loading caused by ship wastewater each year is 356 tons for nitrogen (0.04 %) and 119 tons for phosphorus (0.3 %).

Current situation with processing ship wastewater

In Helsinki, Stockholm or Tallinn, most of the wastewater from vessels in regular traffic is pumped into the sewerage system in ports and from there to wastewater purification plants. According to the Port of Helsinki, all passenger ships in regular traffic pumped wastewater into the sewerage system in the port in 2008. The environmental awareness of shipping companies has increased as the amount of wastewater pumped out of ships in ports is much higher than 10 years ago. Furthermore, port reception facilities have been improved.

According to international maritime regulations, the discharge of sewage into the sea is prohibited, except when the ship is discharging comminuted and disinfected sewage using a approved system at a distance of more than 3 nautical miles from the nearest land, or sewage which is not comminuted or disinfected at a distance of more than 12 nautical miles from the nearest land, provided that in any case, the sewage that has been stored in holding tanks shall not be discharged instantaneously but at a moderate rate when the ship is en route and proceeding at not less than 4 knots.

If restrictions are placed on the nutrient content of ship wastewater in future, this will also impose requirements on the manufacturers of waste water purification equipment, shipping companies and port reception facilities. Purifying wastewater on board or transporting it to the port may be a competitive advantage to shipping companies as passengers are concerned about the poor condition of the Baltic Sea.

Considering the impaired state of the Baltic Sea, it is absolutely essential that the amount of all types of nutrients is reduced. It would be easier to control ship wastewater than the nutrient inputs caused by agriculture. It is already technically possible to purify wastewater on board. However, this calls for investments in expensive purification technology, so on-board purification is not in widespread use. The possibility to leave wastewater in the port should also be provided as an alternative to on-board purification. Shipping companies currently have a reserved stand towards ship wastewater intake services as their amount, quality and reliability vary considerably according to the port.

Collecting wastewater in ship tanks and pumping it into port reception facilities is also problematic as large, heavy tanks limit the space available to other functions and affect ship ballast and thereby safety. According to VTT’s survey, it seems that there could be need for further development in onboard wastewater purification technology and thus also business potential.

The results are being utilised in the work of the Maritime Group of the Helsinki Commission, HELCOM Maritime ( HELCOM Maritime works to prevent any pollution from ships including deliberate operational discharges as well as accidental pollution. The working group also aims to assign the status of a special area to the Baltic Sea through the IMO’s agreement. The status involves the requirement that the nutrient loading caused by ship wastewater discharges must be limited in the area.

VTT’s survey is based on information obtained from port facilities, shipping companies and reference literature. It was concerned with passenger ships, cruise ships and cargo vessels, but not recreational vessels. The share of ship traffic wastewater discharges causing eutrophication in the Baltic Sea was investigated by VTT for the first time in 2007 and an update was published this year. The investigation was financed by the Finnish Maritime Administration.


Baltic Sea Portal


Research publication on the Internet: /Documents/2009_VTT_R_07396_08.pdf