Removal of bacteria and chemical species from sewage applied to forest soil

Abstract

In a society faced with the problems of eutrophication and pollution of natural waterways, caused in part by the disposal of inadequately treated sewage effluents, land application has considerable potential as a means of treatment and disposal of wastes. Considerable quantities of both nutrients and water may also be provided to crops grown on such a system.

In view of projected future population increases in the Rolleston area, an investigation of the viability of Pinus radiata forest soil for such a land application system was commenced. Wastewater was applied to the forest soil at three different application rates. Both the percolating wastewater during the applications and soil after the applications were sampled and analysed for changes in nitrogen, coliform bacteria and pH and compared to changes on control sites. The soil was also analysed for changes in the concentrations of four cations. The relevant literature was reviewed in order to compare results gained with those reported by other workers.

Results showed that NH₄-N, although retained by the soil during effluent applications, was subsequently nitrified (to N0₃-N) by aromonium-N oxidisers and nitrite-N oxidisers (which increased in numbers in response to wastewater applications) and leached from the upper soil intervals.

The soil did not form an effective barrier to the passage of coliform bacteria in the percolating wastewater and survival times of those retained by the humus and 0-50 mm soil layer were high.

The adsorption and leaching of various soil cations required for tree growth was such that no net advantages for the cation balance in the soil were reported.

It is recommended, therefore, that application of treated wastewater to the Burnham P. radiata forest soil should not proceed as the risks of groundwater contamination are high. Alternative cropping systems which can reduce the risk of nitrate-N contamination of groundwater were discussed and it is also recommended that these be further investigated.