Dewatering sludge originating in water treatment works in reed bed systems

The dewatering process of the liquid water works sludge was examined in a trial with a series of six trial beds, each 20 m2. These were monitored from April 2008 to June 2010. It is possible to get the vegetation to grow in ferric sludge (∼300,000 mg Fe/kg dry solid, pH 7,7). It has not been necessary to use fertilizer. The influence of the loading programs (15–50 kg dry solid/m2/year) was tested with 1–5 days of loading and 35–55 days of rest. It is possible to drain and treat ferric sludge. Generally the dewatering profile is a peak with a maximum over 0.015–0.025 L/s/m2. The times for dewatering of 6–12 m3 are ∼15 h and over 90% of the load is dewatered in that period. The dry solid (0.16–0.20%) in the sludge has been concentrated ∼200 times. The dewatering phase results in ferric sludge with 30–40% dry solid which cracks up very quickly. The volume reduction is over 99%. The trend shows that the main volume of reject water has a turbidity level below 5 NTU even in the loading periods.