Fifty years of sewage sludge management research: Mapping researchers' motivations and concerns.

Sewage sludge management is torn between a desire for pollution prevention and reuse of a valuable resource. Reconciling these interests in sustainable management is a challenge for researchers. This study focuses on how research on sewage sludge management practices has evolved and scrutinizes how this research is interlinked with concerns and societal issues such as contaminants, economic efficiency, and legislation. Based on published academic papers on sewage sludge management between 1971 and 2019, this study found four trends in research focused on sewage sludge management: a decreasing interest in disposal (landfilling and sea dumping), a dominant interest in land application, a growing interest in sewage sludge as product, and a stable interest in energy recovery. Research on disposal focuses on increasing sludge volumes, legislative changes, and economic challenges with an interest in waste co-treatment. Research on land application concerns nutrient use and contaminants, mainly heavy metals. Research on sewage sludge as a product focuses on the extraction of certain resources and less on use of sewage sludge specifically. Research on energy recovery of sewage sludge focuses on volume reduction rather than contaminants. Two-thirds of the papers are detailed studies aiming to improve single technologies and assessing single risks or benefits. As management of sewage sludge is multifaceted, the narrow focus resulting from detailed studies promotes some concerns while excluding others. Therefore, this study highlights potential gaps such as the combination of nutrient use and disposal and energy recovery and nutrient use.

Co-digestion of sewage sludge and wood fly ash.

This study uses a new approach for the recycling of plant nutrients by co-digesting sewage sludge with fly ash from a wood combustion. Sewage sludge and fly ash both are enriched with nutrients of the wastewater resp. wood, which makes these products an enhanced source for recycled fertilizers. The effects of the ash addition to the anaerobic digestion are studied in several lab scale experiments including effects on the gas production and microbial activity. Following that, the fertilizing qualities of the digestate are evaluated by plant growth experiments. The results show that the fertilizing qualities of the digested sludge were improved by the ash addition. Next to this, gas production results show that the methane production was not affected by the ash addition, while the total gas release was reduced. The sulphur addition by the ash stimulated sulphate reducing bacteria. The sulphate reducing bacteria did not markedly inhibit the methanogens.