Graywater treatment on recreational crafts in sensitive watercourses

Boating is becoming an increasingly popular leisure activity, attracting ever growing numbers of enthusiasts to watercourses such as lakes, bays, lagoons and costal waters. In order to protect the natural resources in these environmentally sensitive areas, while at the same time allowing intensive touristic usage, wastewater produced on board watercrafts must be treated before discharge into a watercourse. Up till now, technical solutions have only been available for large vessels.

The aim of the project described below is to design a graywater filter system for installation on boats and yachts under 24m in length. The graywater treated with this system has to be of such quality that it can be safely discharged into the watercourse without fear of pollution. Our partner from industry on this project is Wave International Ltd.


Graywater is defined as wastewater produced on board from showers, washbasins, washing machines, kitchen sinks and dishwashers. Technical solutions already exist for the treatment of wastewater from toilets and urinals on board.

The International Maritime Organization (IMO) is a specialized agency of the United Nations that has been regulating the prevention of marine pollution from ships since 1973 through the MARPOL Convention. In order to ensure the implementation of fixed regulations for disposal, MARPOL annexes also prescribe technical equipment on board. These globally recognized regulations do not, so far, include small boats and small yachts. However, increasingly, national legislation is addressing this issue, and regulations at the European and international level are expected in the near future for coastal waters. Therefore, our initial focus is on the development of a graywater filter system for marine applications.

The challenge is to develop a marine graywater filter system that

  • meets future disposal standards,
  • ensures high water flow,
  • features very compact construction,
  • has low energy requirements and
  • is robust.

Concept and results

Tourism has greatly increased in sensitive watercourses.
Tourism has greatly increased in sensitive watercourses.

The proposed marine graywater filter system is a multi-stage process. The first stage involves elimination of coarse pollution and large particles, while at the same time subsequent filtration stages are protected from blockage. In the second stage, grease is eliminated by a special filter. This filter was designed for the elimination of oil and separation of grease from bilge water (water collected in the bilge of the boat and often contaminated with engine oil) and has been used effectively for many years. The third, and, where necessary, a fourth, stage involves the elimination of soluble nutrients. The filter media are configured into cartridges or filter bags and can be exchanged easily after use and disposed of on shore.

First, we searched for filtration medium suitable for eliminating soluble nutrients (carbon, nitrogen and phosphorous compounds) from graywater. For this purpose a pilot plant was operated with synthetic graywater and different filter media were screened.

In the screening process, the aim is to make findings quickly while under realistic circumstances. Therefore, filter media were tested configured into cartridges and high filtration velocity was chosen. Comparable and significant results could be derived within 60 minutes. As an example, the chart shows the results of a screening for the removal of carbon compounds, measured as chemical oxygen demand (COD). We were able to demonstrate that with the best filter material configuration loading – and thus COD elimination - could be achieved two to three times higher than with conventional filter cartridges within the same time and at the same capacity (see GWF04 in the chart).

The removal of particulate and colloidal material by the filtration system was tested with real graywater from showers and washing machines. For this purpose, the best-performing filter media from the screening were used in order to identify in turn the best filter media and filtration system configuration. Preference was given to a specially configured activated carbon filter that can be combined with an ion exchange resin.


Using the results from particulate material and nutrient removal, work is now under way on constructing a prototype system which will be tested under real conditions on a leisure boat.