Abstract

Large quantities of wastewater are generated from municipal and industrial sources in New Zealand. More stringent discharge standards and recent improvements in wastewater treatment technology have contributed to the production of large quantities of solid residuals. Increasing public, cultural, and regulatory pressures to protect water quality have resulted in significant interest in applying residuals to plantation forests.

In a well-managed land application site, the forest ecosystem can efficiently handle organic contaminants and recycle nutrients provided through the residuals, resulting in significant increase in plant growth. Due to large variations in characteristics of different residuals, land application system design must be tailored to match a particular residual with the assimilation capacities of a particular soil–plant system. In New Zealand, many wastewater irrigation systems are employed in forested land as an alternative treatment of liquid residuals. In recent years, large-scale forested land application of municipal and pulp and paper industrial solid residuals have also increased as a preferred alternative to landfilling. It has been suggested that natural abundance of ¹⁵N can be used to trace the fate of residual-derived N in the ecosystems. This technique can resolve one of the main environmental concerns associated with application of residuals in forested land, namely nitrate leaching. Intensive research indicates that significant economic and social benefits can be achieved with minimal adverse impact on the receiving environment.

This paper provides an overview of the research issues associated with application of residuals on forested soils in New Zealand, such as sustainability, beneficial effects, and potential adverse impacts on soil and environmental quality. Performances of 4 case studies, representing application of 2 liquid residuals and 2 solid residuals to forested soils in New Zealand, are discussed.