Polishing wastewater in the Netherlands

On the island of Texel the effluent of the oxidation ditch Everstekoog is being treated in a full scale wetland system (nearly 4000 m3/day). The project shows that it is possible to change the effluent from the Sewage Treatment Plant into a more natural water with a normal diurnal oxygen pattern with very high oxygen levels at daytime. The water becomes more clear and the typical scent of treated sewage disappears. Moreover the change of sludge particles into inactive organic matter and algae contributes to a very satisfying desinfection. The removal of nitrogen and phosphorus is limited at the short retention time of just over two days.

Short project description/ project function

The constructed wetland consists of:

• Presettling basin: settling of material in suspension in the effluent, turned out to be a good opportunity for Daphnia, which live on activated sludge particles from the effluent
• 9 parallell ditches with a length of 150 m and a width of 6,3 – 7 m
• first part with reed, with attached algae mats which works as a biological filter;
• second part with submerged waterplants leading to a more natural oxygen regime.
• Area: 1,3 ha
• Hydrological residence time: 2,2 days
• Hydrological load: 0,24 m3/m2/day

Summary of experiences

• N-removal is relatively high in this system, taken into account the low concentration of N in the water flowing in the system.
• P-removal is very low which is normal in these situations; the concentration of the P in the water flowing into the system is also very low.
• The water after passing the constructed wetland is more natural than the water flowing out of the waste water treatment plant. The material in suspension is changing from flocks with bacteria till flocks consisting of plantrests and algae.
• Birds which are breeding in the constructed wetland are: Reed Warbler, Tufted Duck, Shoveler, Redshank.
• Interesting is the use by Spoonbills, which feeds on the Sticklebacks, which eat Daphnia in the constructed wetland.

Project benefits

The Everstekoog constructed wetland has been monitored and researched extensively. During the period 1995 – 1999:
A report in Dutch can be downloaded through www.rekel.nl/water/

The thesis: A treatment wetland used for polishing tertiary effluent from a sewage treatment plant: performance and processes” can be downloaded from www.compris.nl/eco/thesis.php

The mean result of the project was that the benefits of effluent polishing in a constructed wetland has been demonstrated on a practical scale, leading to a renewed interest for the method in the Netherlands:

• the Waterharmonica project (www.waterharmonica.nl ) , see also the IEES Newsletter No. 10, December 2004
• the constructed wetlands Land van Cuijk, Waterpark Groote Beerze in Hapert, etc.