Wastewater sludge treatment, energy recovery and sustainability: the Waternet case

Abstract

Sludge production and handling have always been the most challenging components of wastewater treatment. Sludge disposal is the most expensive and energy intensive treatment, and now that the Netherland’s focus is on CO2 reduction (with the goal to be carbon neutral by 2020), research on this topic is needed more than ever. The present research is two-fold. The first part focuses on primary sedimentation; from previous research, it emerged that for a wastewater treatment plant serving 46,000 PE it is much cheaper to not implement it and provide for the extra energy demand by means of solar panels. This study enquired on whether this is true also for much bigger scales (i.e. 500,000 PE). The second part focuses on sludge treatment, and whether it is more sustainable to digest and dry the sludge or to dry it directly, in both cases with the production of a biofuel destined to the cement industry. Primary energy and CO2 reduction potentials were calculated. This was enquired on both with an upstream situation including primary sedimentation and one not including it. What emerged is that indeed, primary sedimentation is not convenient even for scales as big as 1 million PE; even then, solar panels are a cheaper measure to reduce the plant’s carbon footprint. In regard to final sludge handling, it emerged that if only secondary sludge is present, it is better to not digest it and directly dry it; when the sludge is mixed, it is better to digest both primary and secondary sludge prior to drying. However, when a future situation as close as 2020 is analysed (with the use of waste heat and updated conversion factors for CO2 and primary energy), it turned out that when both primary and secondary sludge are present, it is better to digest primary sludge only. Further research is needed to determine whether the most sustainable alternative is also financially convenient when compared to other sustainability measures like solar panels.