Summary and conclusions, Miljøstyrelsen

Danske pileanlæg

Summary and conclusions

In principle, willow evaporation beds are earth filled containers with willow planted on the surface. In the container the annual production of wastewater is accumulated. The eminent ability of the willow to evaporate water and lignify secures resorption of the wastewater. Thus there is no discharge from these treatment plants, only nutrients in the form of wood as well as perhaps evaporation of nitrogen compounds. These plants are particularly interesting, as not only they eliminate the load on the water courses, but also they recycle water as well as the nutrients from the wastewater.

The first Danish willow evaporation bed is from 1992. In 1993 one more plant was established, whereas the third was not established until 1997. It is estimated that after this another 30-60 willow evaporation beds have been established in Denmark.

The interest for these plants is high, especially due to new demands to treatment of wastewater from the scattered houses. There is thus a large demand for a clarification, information and directions for establishment and operation. The construction of the plants and the influencing physical-chemical factors have, however, never been examined and compared. There are no general directions for construction of the plants.

Consequently, a number of willow evaporation beds are described in this project. The aim is to explain administrative regulations, construction, function and the destiny of admitted substances and groups of substances and the importance of the growth of the willows.

The results are summarised below.

Establishment and construction

The wastewater, which is discharged into the examined willow evaporation beds, has previously been lead through a sedimentation tank. Afterwards it is pumped from pumping well or lead to the willow evaporation bed via gravitation. The wastewater is dispersed on top, in the middle or below the earth layer. All evaporation beds are established with minimum 2 inspection tubes, which are lead to the bottom. An overflow drain has been established in one of 9 examined plants, 8 of them have no discharge. Drain has been established under 2 plants for registration of any leakage of wastewater containing fluid. The membrane at the bottom and along the sides is made of bentonite or 0,5-0,75 mm low density polyethylene.

There has been no traffic in the plants. Traffic causes risk for pressure damages.

Plants with surface admission, where the earth has been put back irregularly so that the surface is uneven, shows irregular growth potential for the willows creating water-logged hollows, where the willow grows poorly.

On the surface 1 to 4 willow cuttings per square meter are planted. The willow evaporation beds established in Autumn have accumulated considerable amounts of water through the winter that the relatively young willow cuttings have not had sufficient roots and evaporation capacity to evaporate the accumulated fluid as well as the fluid, which has been admitted through the subsequent growth season.

There has not been any problems with root penetration in the discharge and water distribution systems connected to the willow evaporation beds. The root development is weak in the willow evaporation beds, which is due to plentiful admission to fluid and nutrients.

As the detention time of the water in the plant can be long, the demand for infiltration velocity is so low, that earth types with up to 20% clay and 20% silt have appeared to be useful. It has not been examined whether more clay or silt containing earth types are suitable.

Shelter, shadow, slopes above the plant as well as tilts on the sides of the plant must be avoided, as it can be crucial to the water balance in the plant.

Working stability and operating demands

Willow plants are, as far as periods with hydraulic peak loads and lacking wastewater production is concerned, relatively robust. Furthermore willow is a tolerant plant as far as potential phytotoxic compounds are concerned.

The operating demand is weeding of the willow evaporation bed through the first season and approx. 2 hours per person for harvest every winter. The sedimentation tank must also be emptied according to the instructions of the authorities.

In chapter 14 and Annex B the recommended demands for permission to establishment of willow evaporation beds at scattered houses are described. The recommended demands in connection with establishment as well as operation are included.

Mass balance for water

Seven factors are essential to the size of the plant: The wastewater production, rainfall in the local area, the inclination of the sides of the plant, the depth of the plant, the sun influx of light, the influence of the wind and the pore volume of the earth. Removal of the admitted water quantities to the plants have been attempted explained by

Registrations of admitted rain (data from DMI) and wastewater.

Regular registrations of the water level in the inspection tubes of the plants.

Regular analyses of water contents in depth profiles of the earth.

Analyses of pore volume in depth profiles of the earth.

Measurements of length, depth of the willow evaporation beds and levelling of the surface.

Data from these examinations have been used for, in different ways, to calculate approximated mass balances for the water currents to, in and from the willow evaporation beds. The conclusions are as follows:

Registrations of the water level in inspection tubes can be used as targets for the actual water contents of the earth at a given time. Registrations in inspection tubes can, however, be used as indicator for leaks, if any. A leakage in a plant will appear the following winter as a considerably lower water level than the previous winters.

As it is assumed that the annual admission of wastewater and rainfall evaporates in the course of the year, the evaporation is between 2 an 3 times higher than calculated values for the normal potential evaporation from close-cropped grass.

Regular analyses of the water contents of the earth compared to known values for admitted rainfall and wastewater also indicates that there is a considerable evaporation from willow evaporation beds in the growing season, as well as it exceeds the annual potential evaporation considerably. Direct measurements of the evaporation can confirm this.

The considerable difference of calculated evaporation from close-cropped grass and apparent evaporation from the willow evaporation beds can hardly cause leaks in the membranes of the plants. After the end of the growing season the water contents increases accordingly to the admitted quantities of rain and wastewater.

The difference of the evaporation from the willow evaporation beds and from close-cropped grass can be due to several factors:

The willow has a longer growing season, rapid growth, a high maximum leaf-area index and generally easy access to water. As the willow evaporation beds are generally placed with the purpose of maximising of the wind influence, it is probable that a so-called "oasis-effect" occurs: The conditions that a well-watered smaller area placed in a more dry (or with slow growth) large area will have a much larger evaporation than estimated on the basis of influx of light and wind velocity, as the difference in humidity causes advection, where drier air blows over the willow evaporation bed and increases the humidity gradient from leaf to the surroundings, so that the evaporation increases. It is probable that the oasis-effect is so considerable that the annual evaporation does not correspond with the calculations of the potential evaporation.

Furthermore the wind velocity and the humidity in the height of leafs can be considerably more effecting to the evaporation than the wind velocity and the humidity in the height of close-cropped grass.

The last, plausible explanation to why the evaporation apparently is relatively rapid could be that the willow evaporation beds can be regarded as a cube, where both the total surface of the tree crowns upwards and the four 1-4 meter high sides of a plant (fringes) are exposed to influx of light and wind with dry air, whereas the calculated potential evaporation is only based on one surface.

In the project the maximum evaporation in willow evaporation beds from wastewater and rain it is not clarified, how much wastewater and rain at the most can evaporate from willow evaporation beds at different locations in Denmark, and which factors are essential for the maximising of the evaporation. A clarification of these conditions will have considerable influence on the dimensioning of the willow evaporation beds and consequently on the price of these plants.

The evaporation ability is increasing over the first 3 to 5 years. Thus it must be determined whether overload during the initial phase is acceptable and how this overload is handled. In this connection the local conditions must be crucial for the decision whether the risk of a temporary overload is acceptable. If the willow evaporation bed e.g. receives wastewater from a property, from which the wastewater has been lead to drain or receiving waters for 20 years, it is possible that a temporary overload, if any, can be lead to the drains/receiving waters, as a possible overload will only occur in extremely rainy periods, where the rainfall may dilute the wastewater.

The evaporation ability from the willow evaporation beds has influence on the annual water capacity of the willow evaporation beds. The storage capacity in willow evaporation beds is essential for sizing of the plant in order to store the rainfall and wastewater accumulated in the off season for growth. To estimate the storage capacity the plant volume and the pore volume of the earth must be measured. Furthermore rainfall, amounts of wastewater and the period, during which a net accumulation of rainfall and wastewater in plants, must be known.

In this project it is not clarified, when a plant changes between showing net evaporation to net accumulation and vice versa. In stead scenarios are set upwhere the duration of the period with net accumulation can be varied. In the shown scenarios only in one of 6 of the examined plants it can be clarified that the rainfall and the wastewater can be accumulated in the plant in the off season. The explanation can be that none of the scenarios are in accordance with the actual length of the period without growth. The explanation can also be that all plants have leaked. That is, however, unlikely as all plants are relatively young in relation to the normal lifetime of the membranes.

32-51% of the volume of the plants are pores. The pore volume is relatively high in plants, or zones of plants, water-filled during most of the year. Besides the pore volume is generally higher in plants with sandy soil than in plants with clay soil. It is recommended that pore volume is determined as being 35%, as pore volume is included in calculations of the specified plant volume, whereas several factors preclude examination of pore volume prior to construction of the plant.

Between half and 4/5 of the fluid to be stored and evaporated is rainfall in the examined plants. The local rainfall conditions have thus great importance to the necessary depth of a plant. The tilt on all the sides of the examined plants is low, 45^o. Consequently, the sides function as a funnel for rainfall and cause that the demand for volume for accumulation becomes considerably higher than if the sides were upright. At the same time the depth has influence on how much wastewater can be stored under each square meter of surface apart from the rainfall. The deeper a plant is, the deeper an earth pile the willows can evaporate fluid from, the less rain accumulating area it will demand, which is of great importance to the price of the plant.

As it is not possible to construct a model based directly on the known processes in the willow evaporation beds an empirically based model for dimensioning of the willow evaporation plants is constructed in stead. The model is based on conservative estimates and includes calculations of the reduction percentage of the plant size at a steeper slope than 45^o and/or deeper beds than 1,5 m. It is recommended that this conservative, empirically based model is used, until the basic knowledge is sufficient for elaboration of a specific instruction.

Nutrients, organic compounds, salinity and matters hazardous to the environment

There are large variations in admitted amounts and compound of wastewater from one willow evaporation bed to another. Thus there are also large variations in the absorption of nutrients in the willows. There is a risk, that willow evaporation plants to which only grey wastewater is admitted, must be fertilised the first year.

It is estimated that, on the basis of experiences with sand filters, root zone systems and percolation plants, there will be no problems with constipation of the distribution systems in willow evaporation plants due to accumulation of easily degradable compounds.

Salinity will be accumulated in the willow evaporation beds. It is still uncertain, whether the concentration of salinity will influence the evaporation ability of the willows during the lifetime of the plants.

The heavy metals, which are admitted to the willow evaporation plant, will partly be accumulated in the earth, and partly it will be absorbed by the willow and thus removed. Two scenarios for the heavy metal absorption of the willows are shown. There are several dimensions of difference on the result. As the oldest willow evaporation beds in Denmark are only 8 years old, and as the heavy metals may be absorbed as a function of the concentration in the earth, the destiny of the heavy metals in willow evaporation beds cannot yet be determined.

The destiny of the organic matters hazardous to the environment has not been measured. It is likely that the detention time and large annual fluctuations in the water contents, and consequently the reduction potential, will ensure decomposition of the organic matters hazardous to the environment.

Microbiological examinations

The contents of faecal streptococci on the surface of the earth has been analysed. This goes for both plants with surface and plants with subterranean distribution of wastewater. To accomplish an evaluation the earth has been analysed in the immediate surroundings of each willow evaporation bed, the reference. Here several examples are shown of collected samples, where the contents of faecal streptococci is up to one dimension higher that in the willow evaporation plants. Thus analyses of faecal streptococci can not substantiate a recommendation of one distribution system or another. However, smell from wastewater at the willow evaporation beds with surface distribution of the wastewater has not been registered. Furthermore there is a risk for direct contact with wastewater in the plants with surface distribution. Subterranean distribution should thus be preferred.

Administrative regulations

In the administration willow evaporation plants come under the designation "no-outlet water plants" and must be treated as such, administratively, as a container buried in the ground. It is demanded that sides and bottom must be waterproof, that there is no surface run-off, that there is no risk for nuisance or health hazard to humans or beasts, and that the distance to nearest water catchment plant must be 15 to 50 meters.

As willow evaporation beds are wastewater treatment plants, the general rules of environmental protection, terms of payment and wastewater permissions for wastewater treatment plants will also be valid for willow evaporation beds. As there is no discharge from willow evaporation beds, they comply with all the treatment classes mentioned for the scattered houses in the wastewater regulations. Willow evaporation beds can thus in principle be relevant to all the properties of the scattered houses, on which wastewater treatment is imposed.