Summary and conclusions, Miljøstyrelsen

Miljøkonsekvenser ved nedsivning af spildevand renset i økologiske renseanlæg sammenlignet med traditionel nedsivning

Summary and conclusions

The main objective of the project was to assess environmental impacts from percolation of domestic wastewater using traditional percolation methods, including mound systems. Further, the objective was to assess the effect of a possible ecological treatment of the wastewater or separation of the black (toilet) wastewater before percolation.

The focus in the project was on organic micropollutants. However, assessments have also been made on the impact from general organic matter, ammonia, nitrate, phosphorus, heavy metals and pathogens.

The study describes the actual conditions in Denmark. By means of a literature survey, these conditions were compared with studies in Denmark and abroad at locations with similar climate, soil and groundwater conditions, primarily in Scandinavia, Northern Germany and the Netherlands.

Literature Survey:

Limited literature available

Few articles and reports were identified that described occurrence and behaviour of organic micropollutants in connection with percolation of wastewater into soil. Only one Danish reference was identified dealing with this subject, a thesis work from Roskilde University (Højenvang et al., 1999). Therefore, other relevant field investigations were examined dealing with percolation under different climatic conditions, artificial groundwater formation at infiltration etc.

On the basis of the examined literature, the following was concluded on performance of organic micropollutants in soil and groundwater:

At present no data are available that can enable an actual quantification of the removal/retention of organic micropollutants etc. in connection with percolation of wastewater (possibly for very few substances).

Percolation of (treated) wastewater to unsaturated or saturated soil will cause a considerable reduction in the concentration of most substances.

Percolation of domestic wastewater will for more substances not result in complete removal of the substance before the wastewater reaches groundwater level.

Generally, decomposition seems to be more important than sorption when it comes to removal of organic pollutants, however, examples of the opposite were also found.

Most substances are more easily decomposed under aerobic than under anaerobic conditions. A reduction can, however, be detected under both conditions.

Proper conditions for reduction of substances are primarily related to processes in the unsaturated zones and locations with great microbial activity.

Investigation Programme:

Six plants investigated

Six percolation plants for domestic wastewater were investigated. These plants were all constructed in accordance with the Danish EPA's guidelines on percolation plants. All these plants had been in operation for between 3 and 12 years.

Eight drillings per plant

Six test drillings were conducted underneath/around each plant and 2 reference drillings 10-20 metres upstream.

Tests

The following samples were taken for analysis:

Reference drilling at all plants

Most polluted groundwater underneath each plant (highest chloride content)

Inlet samples at two plants (outlet from septic tank).

To examine the effect of possible pre-treatment in an "ecological" plant, inlet and outlet samples were taken at two reed beds.

Analysis programme

All samples were analysed for:

organic micropollutants (approx. 20 inclusive groups)

traditional parameters (COD, BOD₅, NH₃-N, NO₃-N, Tot-N, Tot-P)

heavy metals (7)

hygienic parameters (selected bacteria and Collony Forming Units (CFU))

Results

High treatment efficiency

Most substances and pathogens examined were considerably reduced at the passage of the unsaturated zone. The results showed that most substances were decomposed/transformed or retained up to 100% in the unsaturated zone. The concentrations in the most polluted groundwater underneath the plants were generally close to or below the detection limit for the relevant substances. Where substances occurred in concentrations above the detection limit, the values were generally below the requirements for drinking water.

Conclusion

Environmental impacts in connection with percolation of domestic wastewater from single households are assessed to be connected to very few parameters in a relatively limited area underneath and immediately downstream the plant. Plants established in accordance with the percolation guideline have a high treatment efficiency for most parameters irrespective of the soil composition. Furthermore, the results indicate that the clay soil, found underneath several of the examined plants has fissures, root holes, worm holes etc. increasing the drainage possibilities compared to the theoretically obtainable. At the same time these fissures, holes etc. contribute to the treatment, perhaps due to an increased oxygen admission.

There is a limited knowledge on efficiency and methods for retention/ demolition of organic micropollutants passing through the unsaturated zone underneath a percolation plant. Similarly limited literature is available documenting the processes in the saturated zone.

The organic micropollutants examined were treated nearly 100% underneath the examined percolation plants or the concentrations were below the detection limit, thus a treatment degree could not be calculated.

Compared to drinking water requirements, most LAS values underneath most of the plants were too high. However, 20-50 meter downstream the plant, it was estimated that drinking water requirements could be fulfilled merely by dilution.

For other chemical substances and for the bacteria examined, the treatment efficiency is similarly high irrespective of soil composition.

Compared to drinking water requirements too high values of nitrogen, phosphorus, Collony Forming Units (CFU) and E.coli were found underneath more plants. The nitrogen and phosphorus values were, however, already exceeded in the reference drillings.

High treatment efficiency is seen in both clay soil and sandy soil. The sandy soil with better oxygen conditions results in better nitrification and biological degradation, whereas clay soil is better for adsorption.

Ecological pre-treatment is estimated to have a limited effect on the impact on groundwater. The reduced LAS load is not sufficient to fulfil drinking water requirements.

Percolation of gray wastewater (kitchen, wash) only gives a reduced nitrogen and pathogen load, however a minor reduction in the amount of detergents (LAS) and phosphorus. This could be reduced treatment efficiency due to the high BOD/nitrogen relation.