Can additional air supply enhance decomposition processes in sludge treatment reed beds?

Abstract

This work was designed to investigate the influence of artificial aeration on the sludge decomposition process in mesocosm sludge treatment reed beds (STRBs). In addition to the typical STRB design, where ventilation is mainly provided by a drainage pipe, passive aeration via a "chimney" and active aeration via a blower were introduced. During the entire observation period of 1.5 years, O₂ concentrations in the upper part of the filter were significantly higher in the artificially aerated beds. To determine decomposition rates, a study with decomposition bags, measurements of CO₂ emissions from the STRB and isotopic partitioning of CO₂ emissions were performed. The results indicate an accelerated sludge degradation process in both active and passive beds. However, this effect was limited to part of the season and could not be demonstrated by episodic measurements of CO₂ efflux. The CO₂ efflux showed a seasonal pattern. Average CO₂ efflux was below 3.0 μmol m^-2 s^-1 in the winter months and reached 43 μmol m^-2 s^-1 in the warmer months. The low sludge load and drought period in summer 2018 resulted in an extremely low CO₂ efflux in August 2018. Isotopic analyses revealed changes in decomposition dynamics for certain parts of the season, differences in contributions of sludge and plant derived CO₂ to total CO₂ emissions from differently aerated beds. Overall, passive aeration proved to be similarly efficient as active aeration and could therefore be considered for application in a full-scale system.

Keywords: Active aeration; CO2 efflux; Isotopic partitioning of CO2 emissions; Passive aeration; Sludge decomposition; Sludge treatment wetland.