Making the most of our biosolids - What are the options?

Excess sludge generated from wastewater treatment plants has been a headache for many utilities and operators in New Zealand and around the world. Traditionally, the excess sludge has long been viewed as a waste, requiring waste management (i.e. to be disposed of). The disposal routes often include ocean disposal, landfilling of dewatered sludge (with or without stabilisation), incineration and agricultural application.

Stringent legislation, perceived risks of emission and public opinion has made the option of incinerating undigested sludge very unpopular. Similarly, land application for agricultural use has been a subject of negative public perception over odour and pathogens. Landfilling excess sludge has been a common practice for past decades but it is difficult to build new landfills in large metropolises and the cost of transporting dewatered sludge to landfill has been progressively increased. Thus, new management practices need to be identified.

During the last decade, the fossil fuel crisis has resulted in developing new initiatives and technologies to minimise/replace fossil fuel uses. As a result, biosolids are increasingly being recognised a fuel to generate electricity after being dried or via combustion of biogas which originates from anaerobic digestion, as well as its nutrient value as an attractive alternative fertiliser source.

Moreover, a range of process technologies have since been developed and commercialised to improve the current biosolids management practice with an objective of “greener future”.

This paper presents an overview of these new technologies and how they can be applied in the New Zealand context to deliver the desired outcomes.