Forsøg med rensning af spildevand i det åbne land i beplantede filteranlæg

Experiements with Filter-plant Purification of Wastewater in the Open Country

Summary and conclusions

Project background and objectives

New legislation requires that sewage produced in rural areas be treated adequately in order to improve the environmental conditions in streams and

lakes. The preferred disposal solution in rural areas in Denmark is soak-aways (soil infiltration). But at many sites it is not possible to use soak-aways because of clayish soil conditions or high water tables. There is therefore a need to develop new and effective on-site treatment technologies.

Previous studies have shown that compact subsurface flow constructed wetland systems with vertical flow will be able to meet the treatment requirements in rural areas. However, knowledge concerning the required size of the systems is still lacking, and further work is needed to improve the capacity of the systems to remove phosphorus.

The overall aim of the present study is to provide the necessary background documentation for the development of official guidelines for the design and construction of vertical flow constructed wetland system for use in rural areas. It is expected that the systems will be able to meet the most stringent treatment class, i.e. 95% removal of BOD, 90% nitrification and 90% removal of phosphorus.

Methods

In order to clarify the area requirement under Danish conditions, an experimental vertical flow constructed wetland system was established at a traditional municipal wastewater treatment plant, ensuring that the loading rate can be manipulated as desired. The system consists of a 10-m² and a 5-m² vertical flow constructed wetland. The two vertical beds can be loaded in series (in any order) or in parallel. The wastewater is pre-treated in a 2-m³ three-chamber sedimentation tank before application to the beds. Part of the effluent can be recirculated to the sedimentation tank in order to enhance denitrification. The experimental system also comprises three filter-units in series containing calcite and with vertical upflow to study the removal of phosphorus. Different loadings and operation regimes have been tested in the experimental system.

Based on the initial experience from the experimental system, a full-scale system for a single house with four persons was set up. The system consists of a 2-m³ three-chamber sedimentation tank, a level-controlled pump, a 15-m² vertical flow constructed wetland followed by a filter-unit containing calcite for the removal of phosphorus. Effluent from the system can be recirculated to the sedimentation tank to enhance removal of total-nitrogen by denitrification. The performance of the single-household system has been monitored under conditions with recirculation as well as without recirculation.

Main conclusions

The studies in the experimental system showed that vertical flow constructed wetland systems have a high capacity to remove BOD and to nitrify the wastewater using a relatively small area of land (<2 m²/person). Recirculation of effluent to the sedimentation tank improves and stabilises the performance of the system and enhances the removal of nitrogen by denitrification. Phosphorus can be removed in a separate filter unit with calcite. The residence time in the calcite filter must be sufficient for the binding processes to occur. At high hydraulic loading rates, the filter showed decreased performance and symptoms of clogging.

The removal performance of the full-scale single household system did not fulfil the outlet concentration requirements applying in the most stringent treatment class, i.e. BOD(mod)< 10 mg/L, NH4-N < 5 mg/L and total-P < 1,5 mg/L. The reason for the lack of compliance with the standard is probably that the sewage loaded onto the system was very concentrated, that the filter layer was too shallow, and that the filtersand in the bed was too coarse. Recycling of effluent to the sedimentation tank resulted in lower outlet concentrations. The position of the phosphorus filter in the system was not optimal, because recirculation in the system increased the water flow through the filter and hence decreased the residence time. In future constructions, the P-filter should be placed at the outlet, in order to avoid the effects of recirculation. The results also indicate that the size of the P-filter unit should be extended to achieve sufficient capacity for removal of phosphorus for a period of one year.

The project documents that constructed wetland systems with vertical flow is an attractive treatment option in rural areas, and that the systems, if correctly designed and constructed, are probably capable of meeting the most stringent treatment standards. The vertical flow system is small and compact, and the removal performance is robust. The costs of construction for a single-house system are at the same level as the costs of a soak-away system.

Results from this projects have been presented at the 8^th International Conference on Wetland Systems for Water Pollution Control, September 2002, Arusha, Tanzania /1-4/