Sludge degradation, nutrient removal and reduction of greenhouse gas emission by a Chironomus-Azolla wastewater treatment cascade.

Wastewater treatment plants (WWTPs) are a point source of nutrients, emit greenhouse gases (GHGs), and produce large volumes of excess sludge. The use of aquatic organisms may be an alternative to the technical post-treatment of WWTP effluent, as they play an important role in nutrient dynamics and carbon balance in natural ecosystems. The aim of this study was therefore to assess the performance of an experimental wastewater-treatment cascade of bioturbating macroinvertebrates and floating plants in terms of sludge degradation, nutrient removal and lowering GHG emission. To this end, a full-factorial experiment was designed, using a recirculating cascade with a WWTP sludge compartment with or without bioturbating Chironomus riparius larvae, and an effluent container with or without the floating plant Azolla filiculoides, resulting in four treatments. To calculate the nitrogen (N), phosphorus (P) and carbon (C) mass balance of this system, the N, P and C concentrations in the effluent, biomass production, and sludge degradation, as well as the N, P and C content of all compartments in the cascade were measured during the 26-day experiment. The presence of Chironomus led to an increased sludge degradation of 44% compared to 25% in the control, a 1.4 times decreased transport of P from the sludge and a 2.4 times increased transport of N out of the sludge, either into Chironomus biomass or into the water column. Furthermore, Chironomus activity decreased methane emissions by 92%. The presence of Azolla resulted in a 15% lower P concentration in the effluent than in the control treatment, and a CO2 uptake of 1.13 kg ha-1 day-1. These additive effects of Chironomus and Azolla resulted in an almost two times higher sludge degradation, and an almost two times lower P concentration in the effluent. This is the first study that shows that a bio-based cascade can strongly reduce GHG and P emissions simultaneously during the combined polishing of wastewater sludge and effluent, benefitting from the additive effects of the presence of both macrophytes and invertebrates. In addition to the microbial based treatment steps already employed on WWTPs, the integration of higher organisms in the treatment process expands the WWTP based ecosystem and allows for the inclusion of macroinvertebrate and macrophyte mediated processes. Applying macroinvertebrate-plant cascades may therefore be a promising tool to tackle the present and future challenges of WWTPs.

Wastewater treatment plant contaminant profiles affect macroinvertebrate sludge degradation.

Disposal of the overwhelming amounts of excess wastewater treatment plant (WWTP) sludge is an increasing financial and environmental problem, and new methods to reduce the amount of excess sludge are therefore required. In the natural environment, interactions between multiple macroinvertebrate detritivores mediate the degradation of organic matter. Macroinvertebrates may thus also be able to degrade WWTP sludge, but may meanwhile be impacted by the associated contaminants. Therefore, the aim of the present study was to examine if WWTPs contaminant concentrations and profiles affect the biotic interactions and macroinvertebrate mediated degradation of sludge. Assessing degradation of sludge from three WWTPs differing in contaminant profile by (combinations of) three macroinvertebrate detritovore taxa, revealed that macroinvertebrate enhanced sludge degradation was WWTP and taxa combination specific. Yet, taxa combinations only had an additional positive effect on sludge degradation when compared to single taxa in sludge with a higher contaminant load. This was confirmed by the results of a Cu-spiked sludge degradation experiment, indicating a possible effect of biotic interactions. It was concluded that macroinvertebrates are a potential tool for the reduction of excess WWTP sludge, and that using multispecies assemblages of detritivorous macroinvertebrates may increase the resilience of this additional treatment step.

Organic matter degradation and redistribution of sediment associated contaminants by benthic invertebrate activities.

The fate of sediment associated compounds is the combined result of chemical properties and biological activities. Yet, studies simultaneously addressing the effects of biota on the redistribution and bioaccumulation of contaminants are scarce. Our aim was therefore to assess the effect of benthic invertebrate activities on organic matter degradation and the redistribution of metals and Polycyclic Aromatic Hydrocarbons (PAHs) in contaminated sediment. To this end, we introduced egg ropes of the non-biting midge Chironomus riparius into wastewater treatment plant sludge and allowed these to either develop until fourth instar larvae or to fully complete their life cycle into terrestrial flying adults. Chironomid larvae enhanced sludge degradation, resulting in increased metal concentrations in the sludge and in a flux of metals into the overlying water. Moreover, they hampered PAH degradation in the sludge. Contaminant transport from aquatic to terrestrial ecosystems with emerging invertebrates as a vector is widely acknowledged, but here we showed that biomanipulation prevailed over bioaccumulation, since due to chironomid activity, the flux of metals from the sludge into the overlying water was larger than into chironomid biomass. It is therefore concluded that contaminant-macroinvertebrate interactions are bilateral relationships driven by the interplay between macroinvertebrate traits and contaminant properties.