Geotechnical and chemical characteristics of ETP and WTP biosolids

Aruna Ukwatta and Abbas Mohajerani


Stricter regulations on the quality of wastewater treatment by-products are giving rise to an increasing volume of stockpiled biosolids. The annual production of biosolids in Australia is approximately 300,000 dry tonnes, which involves a biosolids management cost of about $A90 million. Biosolids are the end product and the main solid component collected from the wastewater treatment process. This paper presents some of the geotechnical and chemical properties of two samples of biosolids collected from Melbourne Water’s Eastern Wastewater Treatment Plant (ETP) stockpile No. 22 and the Western Wastewater Treatment Plant (WTP) stockpile No. 10. Various geotechnical tests – liquid limit, plastic limit, particle density, particle size distribution, organic content, and linear shrinkage – were undertaken. In addition, chemical tests comprising leachate analysis for heavy metals and chemical composition were conducted on the samples of biosolids. From an environmental perspective, all the samples of biosolids were found to be safe in terms of leaching for use as a landfill application material. The experimental results showed that the ETP biosolids have about 7% of organic content with some of the geotechnical and chemical properties similar to a conventional soil with similar particle size distribution. In addition, empirical relationships were obtained for the compaction behaviour of the ETP biosolids and a comparison soil used in this study. The results obtained in this study can be used as a guide for the use of ETP and WTP biosolids in different civil engineering applications.