Abstract
Due to high waste sludge disposal at wastewater treatment plants and increasing phosphorus scarcity, there is a need to combine waste removal and nutrient recovery. One way to achieve this is by incorporating ecological processes into wastewater treatment. Positive species interactions, such as facilitation, are critical to increase productivity of biomass and nutrient recovery. In this study we showed the potential of using ecological principles including interspecific facilitation processes of aquatic plants (Azolla filiculoides) and worms (Oligochaeta, Tubificidae) in waste recovery and biomass production. This was investigated by developing a biocascade with monocultures of plants and aquatic worms that was fed on activated sludge. Tubificidae had an average relative growth rate of 0.02 g g−1 DW d−1 whereby sludge predation resulted in 45% sludge reduction. When Tubificidae were present in the biocascade, A. filiculoides biomass production significantly increased to a relative growth rate of 0.15 g g−1 DW d−1. The activity of Tubificidae mostly affected total suspended solids, chemical oxygen demand and ammonium concentration in the first compartment of the biocascade. Additionally, nitrification rates increased and the water acidified, leading to increased carbon dioxide concentrations and dissolved phosphorus-binding metals (zinc, iron, aluminium and manganese) that stimulated A. filiculoides growth. The high sludge reduction (45%) and phosphorus sequestration (133 mmol m−3 d−1) show a strong potential of the biocascade for combined sludge waste reduction and phosphorus recovery from wastewater.
Highlights
Wastewater treatment plants (WWTPs) are a point source of pollution and produce large volumes of sludge with a limited application potential that needs to be disposed against high cost
In this study we showed the potential of using ecological principles including interspecific facilitation processes of aquatic plants (Azolla filiculoides) and worms (Oligochaeta, Tubificidae) in waste recovery and biomass production
We found an increase in cumulative biomass of A. filiculoides grown downstream the control aquaria too, this was mainly caused by floating algae (Fig. 4)
Summary
Wastewater treatment plants (WWTPs) are a point source of pollution and produce large volumes of sludge with a limited application potential that needs to be disposed against high cost. Production of N via the Haber-Bosch process is a costly process that requires extensive use of non-renewable resources (> 1% of the global energy consumption) and P is non-renewable as it can only be obtained from a limited number of P mines (Chowdhury et al, 2017; Ullmann et al, 1985). There are several methods to recover P from sludge (e.g. direct use of sewage sludge, precipitation in form of struvite, recovery from ashes through thermochemical treatment), they often have limited application potential (Cieślik and Konieczka, 2017) and there is not one solution to combat the scarcity of P in the future. There is an urge to implement the new concept of circular economy in wastewater management, in order to reuse sewage sludge as a valuable resource of matter and energy (Kacprzak et al, 2017)
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