DANISH GUIDELINES FOR SMALL CONSTRUCTED WETLAND SYSTEMS Hans Brix

Transcription

1 DANISH GUIDELINES FOR SMALL CONSTRUCTED WETLAND SYSTEMS Hans Brix Introduction Discharge of domestic sewage from single households to streams and lakes in the countryside is resulting in poor freshwater quality in many areas of Denmark. Therefore new legislation from 1997 requires improved treatment of sewage from single households in the countryside. Following the new legislation in 1997, the Danish EPA developed official guidelines for various treatment options for systems up to 30 person equivalents. The official guidelines include Environmental Guidelines for Root-Zone Systems up to 30 PE ( Environmetal Guidelines for Soil Infiltration Systems up to 30 PE ( Environmetal Guidelines for Biological Sandfiltration Systems ( Soil infiltration is the preferred solution but at many locations this is not possible because of clayish soil conditions or high ground water tables. The degree of treatment required in rural areas is determined by regulations (Table 1) and depends on the desired quality of the receiving water body. Table 1: Treatment classes that has to be meet in rural areas Treatment class BOD 5 Total-P Nitrification SOP SO OP O 95% 95%

2 Environmental guidelines for root-zone systems In the following the EPA guidelines for root-zone systems from 1999 will be briefly summarised. The root-zone systems will only meet the less stringent treatment class (O), i.e. removal of BOD 5. The sewage must be pre-treated in a sedimentation tank (minimum size 2 m 3 for a single household The necessary surface area of the root-zone system is 5 m 2 per PE (minimum size for a single household is 25 m 2 ) Minimum length of the root-zone system is 10 m The root-zone system is enclosed by a tight membrane (minimum 0.5 mm thickness) The membrane must be protected by a geotextile or sand The filtermedium should be sand with a d 10 between 0.3 and 2 mm, d 60 between 0.5 and 8 mm, and the uniformitycoefficient should be <4. The bed is planted with common reed (Phragmites australis) Construction guidelines for the inlet and outlet arrangement is also included (se Figs. 1-4) Figure 1: Overview of a root-zone system according to the Danish Guidelines 110

3 Figure 2: Sketch of a single household root-zone system according to the guidelines. Surface area is 25 m 2, length is 10 m, bed depth is 0.6 m at the inlet side, and bottom slope is 10 o/oo Figure 3: Cross section through the inlet arrangement of a roo-zone system 111

4 Figure 4: Cross section through the outlet arrangement of a root-zone system Proposed guidelines for a compact vertical flow constructed wetland system During the past few years investigations have been carried in order to develop a constructed wetland system that will meet the most stringent treatment class, i.e. 95% removal of BOD, removal of total-p and nitrification. Previous studies have shown that compact subsurface flow constructed wetland systems with vertical flow will be able to fulfil the treatment demands in rural areas. The studies have produced the necessary background documentation for the development of official guidelines for the design and construction of vertical flow constructed wetland system for use in the rural areas. In order to clarify the area demand under Danish conditions, an experimental vertical flow constructed wetland system was constructed at a traditional municipal wastewater treatment plant so that the loading rate can be manipulated as desired. The system consists of a 10-m 2 and a 5-m 2 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- m 3 three-chamber sedimentation tank before application to the beds. Part of the effluent can be recycled to the sedimentation tank in order to study the possibility of enhancing denitrification. The experimental system also comprise three filter-units in series 112

5 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 experiences from the experimental system, a full-scale system for a single house with four persons was constructed. The system consists of a 2-m 3 threechamber sedimentation tank, a level-controlled pump, a 15-m 2 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. Figure 5: Sketch of a compact vertical flow constructed wetland system. In future constructions the splitter well will be placed before the P-filter. 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 (<2 m 2 /person). Recycling 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 has to be sufficient for the binding 113

6 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 fulfils the most stringent standards in rural areas, i.e. 95% removal of BOD, nitrification and removal of phosphorus, when effluent is recirculated in the system. 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 to avoid the effects of recirculation. The results also indicates that the size of the 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 are 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 is at the same level as the costs of a soak-away system. The Danish Environmental Protection Agency is presently producing official guidelines for the construction and establishment of vertical flow constructed wetland systems in rural areas. Proposed guidelines for willow systems A novel constructed wetland system based on willows has been developed to treat sewage, evaporate water and recycle nutrients from single households and villages at sites where effluent standards are stringent and soil infiltration is not possible. Main attributes of the willow wastewater cleaning facilities are that the systems have zero discharge of water (because of evapotranspiration) and nutrients can be recycled via the willow biomass. Furthermore, the harvested biomass may be used as a source of bioenergy. The willow wastewater cleaning facilities generally consist of c. 1.5 m deep highdensity polyethylene-lined basins filled with soil and planted with clones of willow (Salix viminalis L.). The surface area of the systems depends on the amount and quality of the sewage to be treated and the local annual rainfall. For a single household in Denmark the area needed typically is between 120 and 300 m 2. Settled sewage is dispersed underground into the bed under pressure. The stems of the willows are harvested on a regular basis to remove nutrients and heavy metals and to stimulate the growth of the willows. 114

7 Removal of water from the systems occurs by evaporation from the soil and plant surface and transpiration. The following factors are important for maximising evaporative loss of water: High energy input (solar radiation), high air-temperatures, low relative humidity in the air, exchange of air (wind), canopy resistence, stomata resistance, and leaf area index. Factors like the oasis effect, which is the phenomenon where warmer and dry air in equilibrium with dry areas flows across a vegetation of plants with a high water availability. The vegetation experiences enhanced evaporation using sensible heat from the air as well as radiant energy, and air is cooled by this process. In addition, the so-called clothesline effect, where the vegetation height is greater than that of the surroundings (different roughness conditions), may increase evaporative water loss. This occurs where turbulent transport of sensible heat into the canopy and transport of vapour away from the canopy is increased by the 'broadsiding' of wind horizontally into the taller vegetation. In addition, the internal boundary layer above the vegetation may not be in equilibrium with the new surface. Therefore, evapotranspiration from the isolated expanses, on a per unit area basis, may be significantly greater than the calculated potential evapotranspiration. Examples of the clothesline or oasis effects would be evapotranspiration from a single row of trees surrounded by short vegetation or surrounded by a dry non-cropped field, or evapotranspiration from a narrow strip of cattails (a hydrophytic vegetation) along a stream channel. The EPA is currently producing guidelines for two types of willow systems: Closed systems without any outlet, and systems with infiltration (i.e. not enclosed by a water tight membrane). The systems with infiltration is intended to be used on clayish soils, whre infiltration is low. Characteristics of the systems are: Willow beds are generally constructed with a width of 8 m, a depth of minimum 1.5 m, and with 45 degree slopes on the sides The evapotranspiration from the systems is assumed to 2.5 times the potential evapotranspiration at the location as determined by climatic parameters The necessary area of the systems is determined by the amount of wastewater, the normal precipitation, and the potential evapotranspiration at the location of the system Details on dimensioning and construction will be included in the guidelines. The willow systems will meet the most stringent treatment class, i.e. 95% removal of BOD, removal of phosphorus and nitrification. 115

8 BUNDFÆLDNING PUMPEBRØND OPFØRINGSRØR FORDELERRØR FORDELERLAG OPSAMLINGS- OG TØMMEDRÆN MEMBRAN INSPEKTIONS- OG TØMMEBRØND Constructed wetlands: applications and prospects (Edited by Angiolucci, S.) PILETRÆER Figure 6: Sketch of a willow system with no outflow (evaporative system) 8 m 1,1 m 1,1 m 1,1 m 1,1 m 0,3 m FORDELERRØR TØMMEDRÆN MEMBRAN 1,5 m 5 m Figure 7: Cross section of a closed willow based system 116

9 BUNDFÆLDNING PUMPEBRØND FORDELERRØR FORDELERLAG JORDDÆKNING Constructed wetlands: applications and prospects (Edited by Angiolucci, S.) PILETRÆER Figure 8: Sketch of a willow system with infiltration 1,2 m 1,2 m 1,1 m 1,1 m 1,1 m 0,3 m 8 m Figure 9: Cross section through a willow system with infiltration 117