Characterisation of NZ digestate for contamination risk and safe use

[Y2M7 final report]

Introduction:  Anaerobic digestion is gaining importance in New Zealand as a means of managing organic waste while recovering value through biogas and digestate. This study evaluated digestate quality across four key feedstocks—food waste, dairy waste, manure waste, and sewage sludge—and compared results against soil, compost, and synthetic fertiliser references. Using literature review, laboratory testing, and international screening benchmarks, the assessment examined nutrients, metals, pathogens, PFAS, and plastics/microplastics.

Results showed that digestate quality is strongly feedstock-dependent, with significant variation in nutrient value and contaminant profiles. Elevated zinc and copper were observed in manure digestate, sewage-derived digestate showed higher overall metal burden, and PFAS analysis highlighted limitations of targeted testing approaches. Regulatory frameworks were found to address traditional contaminants more effectively than emerging contaminants such as PFAS and microplastics.

The findings demonstrate that digestate should not be treated as a uniform product and support the need for feedstock-specific assessment, targeted monitoring, improved source control, and development of New Zealand-specific digestate guidance for safe and beneficial land application.

Useful links and reports

• Application of biofertiliser to pasture - risk management for disease exposure