| A full-scale wetland system was established in 1994 to improve
the quality of tertiary effluent from a sewage treatment plant
(STP) on the island of Texel, The Netherlands. The effects
of this system on STP effluent quality have been monitored
and studied between 1995 and 1998 in a research project funded
by the Dutch government.
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Sewage treatment plant Everstekoog
- oxidation ditch
- phosphorus removal with FeSO4
- up to to 45,000 population equivalents during summer
- 3,000-4,000 m3/day at dry weather, maximum flow 10,000 m3/day
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Aerial photo of the constructed wetland and the oxidation ditch
(© Simon Smit Aerial Photography, Texel)
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Wetland system
- surface water through flow system
- 3.5 ha: 13,130 m2 water surface
- mean hydraulic retention time (HRT):
2.4 days
- in 1995/96: same flow through nine ditches
- in 1997/98: four different flows through the ditches resulting in HRTs of 1.7 up to 10.7 days
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Rationale for construction
of the wetland system
STP effluent quality not sufficient:
- STP effluent is drained off to the small surface waters
of Texel, resulting in a considerable influence of this
effluent on surface water quality and quantity
- high natural values on the island
solution is a full-scale wetland system to change the
STP effluent into "living" surface water
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Monitoring and research
- fortnightly routine water analyses
- continuous measurements including water levels, oxygen
concentration, light intensity and precipitation
- study of separate processes involved in nutrient removal
(denitrification, storage in macrophytes and periphyton,
accumulation in the soil)
- compilation of water and mass budgets
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Results
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| Conclusions
A diurnal pattern of the O2 concentration as
long as sufficient light can reach the submerged aquatic plants
Clear HRT and seasonal effects:
- Nutrient removal: ammonium and nitrate removal at longer
HRTs even in wintertime. Annual phosphate removal was small
- Turbidity: increases due to conversion of nutrients into biomass
- Faecal coliforms: a consistent effective removal, HRT
of 5.5 days often leads to < 1/ml
The combination of open water, helophytes and submerged
aquatic plants in a constructed wetland is an effective
way to change "dead" effluent from a sewage treatment plant
into usable "living" surface water
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| All details can be found in the |
THESIS |
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