Wastewater treatment by soil infiltration for long-term phosphorus removal

Phosphorus leaching from onsite wastewater treatment systems may contribute to eutrophication, especially in freshwater systems.

In developed countries, the most common onsite treatment technique is septic systems with soil infiltration. However, current knowledge about long-term phosphorus removal in soil treatment systems is not well developed, and the data used for estimation of phosphorus losses from such systems are unreliable.

In this study Swedish researchers sampled four filter beds from community-scale soil treatment systems with an age of between 14 and 22 years, to determine the long-term phosphorus removal and investigate the chemical mechanisms behind the observed removal.

For one site the long-term phosphorus removal was calculated using a mass balance approach.

After analysis of the accumulated phosphorus, it was estimated that on average 12% of the long-term phosphorus load had been removed by the bed material. This indicates a low overall capacity of soil treatment systems to remove phosphorus.

Batch experiments and chemical speciation modelling indicated that calcium phosphate precipitation was not an important long-term phosphorus removal mechanism, with the possible exception of one of the sites.

More likely, the phosphorus removal was induced by AlPO4 precipitation and/or sorption to poorly ordered aluminium compounds. This was evidenced by strong relationships between oxalate-extractable aluminium and phosphorus.

Journal of Contaminant Hydrology, Volumes 140–141, October 2012, Pages 24–33.