Sludge dewatering and stabilization in drying reed beds: Characterization of three full-scale systems in Catalonia, Spain

Optimization of sludge management can help reducing sludge handling costs in wastewater treatment plants. Sludge drying reed beds appear as a new and alternative technology which has low energy requirements, reduced operating and maintenance costs, and causes little environmental impact. The objective of this work was to evaluate the efficiency of three full-scale drying reed beds in terms of sludge dewatering, stabilization and hygienisation. Samples of influent sludge and sludge accumulated in the reed beds were analysed for pH, Electrical Conductivity, Total Solids (TS), Volatile Solids (VS), Chemical Oxygen Demand, Biochemical Oxygen Demand, nutrients (Total Kjeldahl Nitrogen (TKN) and Total Phosphorus (TP)), heavy metals and faecal bacteria indicators (Escherichiacoli and Salmonella spp.). Lixiviate samples were also collected. There was a systematic increase in the TS concentration from 1–3% in the influent to 20–30% in the beds, which fits in the range obtained with conventional dewatering technologies. Progressive organic matter removal and sludge stabilization in the beds was also observed (VS concentration decreased from 52–67% TS in the influent to 31–49% TS in the beds). Concentration of nutrients of the sludge accumulated in the beds was quite low (TKN 2–7% TS and TP 0.04–0.7% TS), and heavy metals remained below law threshold concentrations. Salmonella spp. was not detected in any of the samples, while E. coli concentration was generally lower than 460 MPN/g in the sludge accumulated in the beds. The studied systems demonstrated a good efficiency for sludge dewatering and stabilization in the context of small remote wastewater treatment plants.