BIOSOLIDS

Optimization of sludge dewatering and drying through a better understanding of its rheological characteristics and evolution within the whole treatment chain

Financing

Project FNRS T.0159.20-PDR.

Context

Wastewater sludge (also known as biosolids) treatment and management is a growing challenge for municipalities around the world. If all of the wastewater from the world’s urban population was collected and treated, 83 Mt of dry sewage sludge would be generated by 2020. Sludge management is an environmental issue with economic and political overtones as well, due to the cost and pollution concerns associated with their disposal and due to the potential, they constitute for energy production. It is the next decade major issue.

Until recently, biosolids were considered as basic waste with no interest and no economic value for which final disposal routes were landfilling or spreading on agricultural fields (often miles away from its production location). But with the rapid growth of sustainable management, sludge is now considered as a valuable and permanent resource of organic matter and nutrients which can feed the circular economy pathway.

Considering that (i) 60% of the world population will live in urban areas by 2030, (ii) the energy demand will continue to increase and (iii) our fossil-oil dependence remains high, local and renewable energy production is a big challenge that need to be met. Sludge pyrolysis represents a big opportunity as it allows the conversion of sludge into biofuel products, namely bio-oil, syngas and biochar (Pokorna et al., 2009).

Fast pyrolysis in fluidized bed reactors is to be the most efficient process but it requires specific operating conditions. Indeed, Bridle and Pritchard (2004) showed that according to sludge characteristics (such as size distribution, flow rate, etc.), energy consumption for pyrolysis may vary from 1 to 3, moving from environmentally friendly to energy-guzzling technology. Time requirements, yields, flow rate are impacted by particle size and initial water content. Thus, a drying step is mandatory prior to sludge pyrolysis with the objective of producing a dry material with less than 15% of water.

However, drying efficiency is hardly predictable as it was clearly established that structural and mechanical properties, among which rheological ones, have a major impact on the process. Indeed, Leonard et al. (2010) established that mechanical history, through pumping and conveying, negatively impacts drying rate: the more intense the mixing, the lower the drying rate. A similar observation was made by comparing the drying rate of pre-limed and post-lime sludge. Moreover, it was also observed that both the conditioning (coagulant/flocculant nature and dosage) and dewatering steps (filtration or centrifugation) had also an impact on sludge structure and subsequent drying (Peeters et al. (2011), Pambou et al. (2016)). From our best knowledge, there was no attempt to correlate drying efficiency and sludge rheological characteristics, considered the whole treatment chain. As partially shown in the PhD thesis of Pambou (2016), a reduction of the drying time resulting from an optimised choice of conditioning/dewatering conditions can lead to large energy and consequently costs savings.

Objectives

• to understand the main role of conditioning and dewatering on sludge rheological
• characteristics
• to understand how rheological characteristics and sludge drying rate are related, focusing
• on convective drying
• to model drying kinetics by taking rheological properties into account
• to give industries usable tools and support to manage drying efficiency.

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