Short-term water content analysis for the optimization of sludge dewatering in dedicated constructed wetlands (reed bed systems)

The performance of reed bed systems is usually presented following a long-term evaluation of case studies. However, short-term water content analyses may be useful to better understand the dynamics of the dewatering process and hence to rationally describe the system, with the ultimate aim of improving the system overall. In this study we performed daily water content analyses in the interval between subsequent sludge loadings in a realscale reed bed system situated in Pisa (central Italy). Water content analyses were performed at different periods of the year: in winter, drainage quickly led to a maximum of about 8% dry matter while in summer the dewatering process significantly continued after the first week thanks to evapotranspiration. A comparative analysis of planted and unplanted beds confirmed that the mechanical function of plants is essential in the winter, when the unplanted bed did not present any dewatering capability. A semi-empirical equation describing the dynamics of the dewatering process is then proposed and calibrated. This equation forms the basis for the technical and economical optimization of the resting time in terms of required surface area and drying efficiency. An optimum of 11 days resting time was finally evaluated in the presented case study.

Short-term water content analysis is useful for optimizing a reed bed system from both a technical and a financial point of view. Our analysis highlighted the following points: • The dewatering process in summer conditions is significant for the entire three weeks of resting time leading to about 40% d.w. sludge, thanks to ET. In winter conditions the drainage • • • mainly occurs in the first week leading to about 8% d.w. sludge; thereafter the dewatering process is limited, since ET is negligible. For the first year the presence of young reed plants could even reduce the overall evapotranspiration loss from the bed; the positive effects of the reeds on evapotranspiration should only be expected after the second and third years. Moreover, the maximum efficiency of the dewatering process may be limited by the wilting of reeds. On the other hand, the reeds are essential to ensure drainage in winter periods: traditional sand drying beds cannot be operated with subsequent sludge loading for the whole year. Reeds are therefore useful to maintain the long-term efficiency of the beds. The semi-empirical equation that we proposed to describe the dynamics of the dewatering process fits the experimental WC data between subsequent loadings very well. The dewatering equation provides the basis for evaluating the influence of resting time on the life expectancy of the bed(s) and therefore for defining the minimum main financial and maintenance costs. In our case we estimated an optimum of 11 days resting time. Further studies are required to generalize the dewatering equation. Future experiments should investigate the explicit influence of evapotranspiration and medium design on the empirical coefficients we introduced. Moreover, a study of the hydraulic aspects of the process should be coupled with a more thorough analysis of sludge mineralization.