Salma Rayan, (Water Engineering Services, Downer Group), Mark Newland (reviewing author, Downer Group), Logan Water Infrastructure Alliance
Biosolids generated from wastewater treatment plants can be beneficially reused as an effective soil conditioner for agricultural, forestry and soil rehabilitation purposes, as well as providing a source of renewable energy. Despite this, New Zealand disposes the majority of biosolids to landfill, at rates much higher than Australia. The reason for this is a mixture of negative perceptions, low landfill levy’s and limited national experience with the technologies and practises for biosolid treatment and reuse.
Several biosolid management processes exist which could be applied successfully in New Zealand. This paper describes one such process - biosolid gasification, and reveals the results of Australia’s first full-scale 350,000 EP demonstration (now permanent) biosolids gasification plant which was constructed in 2020 at Loganholm in Queensland by The Logan Water Infrastructure Alliance (LWIA), a partnership between Downer Group and Logan City Council.
The project sought to prove the performance of this previously unproven process again biosolid and air emission environmental standards and characterise various elements of the gasification process such as the characteristics of the resultant biochar product, biosolid volume reduction, destruction rates of persistent organic pollutants like perfluoroalkyl substance, heat balance and operating costs
The key outcomes and benefits shown from this demonstration plant which is of relevance to New Zealand wastewater treatment plant operators include >90% reduction in biosolid volume (and related transport and disposal costs), sterilisation of pathogens, destruction of persistent organic pollutants and micro-plastics and retention of the plant-available nutrients. Biochar does not generate offensive odours, is not subject to a restricted storage time before transport/application and is easier to handle than standard dewatered sludge cakes. Furthermore, Logan City Council has found that valuable markets exist for biochar as soil conditioner for the agricultural sector, fuel source and associated carbon credits, such that the biochar is a significant revenue source and contribution to a circular economy for Council.
Given the required capital investment and operating capability, this paper explores the feasibility of implementing biosolid gasification plants New Zealand. Early indications show that this process favours intensified, and landfill limited New Zealand cities such as Auckland, New Plymouth, Wellington and Otago. Additionally, these plants have the potential to be effective in regions where populations are scattered where they can act as decentralised points that receive sludge streams from several smaller surrounding townships.