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Thermophilic and hyper-thermophilic anaerobic digestion as novel treatment technologies for safe nutrient recovery from concentrated black water

25 October 2021

Eline Anvik

Domestic wastewater, specifically its toilet fraction (black water; BW), contains nutrients (mainly nitrogen, phosphorous and potassium) that can be used as alternative fertilizer sources in the agricultural sector. During source separated sanitation different household waste streams are collected separately. The main fraction of nutrients and organic matter is present in BW. However also the main pathogen fraction ends up in BW, which restricts nutrient reuse. Within the Horizon2020 project Run4Life the challenge is to safely recover these nutrients through high rate thermophilic (55 °C) and hyper-thermophilic (70 °C) anaerobic digestion (TAD/HTAD) in Upflow Anaerobic Sludge Blanket (UASB) reactors. These innovative treatment technologies have the potential for simultaneous biogas production and pathogen elimination due to high temperatures to ensure safe reuse of recovered nutrients.

Thermophilic and hyper-thermophilic anaerobic digestion (AD) are promising techniques for the treatment of concentrated black water. It was shown that thermophilic AD of concentrated BW reaches the same methanisation and COD removal as mesophilic anaerobic treatment of BW (conventional vacuum toilets) and kitchen waste while applying a higher loading rate (OLR) (2.5–4.0 kgCOD/m³/day) [1]. The retention time was 8.7 days with an organic loading rate of >3 kgCOD/m³/day. This resulted in a COD removal of 70% and a methanisation of 62% (based on CODt) during thermophilic AD. Hyper-thermophilic (70 ◦C) reached lower levels of methanisation (38%).

During both TAD and HTAD pathogen indicator organisms were removed to a high extent. Compared to mesophilic anaerobic digestion there is a significant increase in the removal of Escherichia coli and extended-spectrum β-lactamases producing E. coli. Both were almost fully eliminated during TAD and HTAD. Since there was no significant difference between TAD and HTAD in terms of pathogen removal, there is no additional benefit for hyper-thermophilic AD over thermophilic AD. Additionally, TAD has better COD removal and methanisation performance than HTAD. Therefore, TAD is suggested as novel treatment technology for concentrated BW with simultaneous pathogen removal. Next challenge is the development of efficient nutrient recovery strategies during TAD, preferably with separate product streams for nitrogen, phosphorous and potassium.

Source: Moerland, M. J. 2021. Thermophilic and hyper-thermophilic anaerobic digestion as novel treatment technologies for safe nutrient recovery from concentrated black water. Resource recovery and wastewater treatment technologies, Clean Water & Ecosystem Restoration. AIWW 2021