Dewatering model for optimal operation of sludge treatment wetlands

Sludge treatment wetlands (STW) are used as a dewatering technology in some European
countries since the 80’s. Although the efficiency of this technology in terms of sludge
dewatering and mineralisation is well known, design and operation parameters are yet to
be standardised. The aim of this study is to develop a mathematical model capable of
predicting the water loss with time, in order to optimise the feeding frequency enhancing
sludge dewatering and expanding the lifespan of the system. The proposed model is
validated with experimental data from one pilot and two full-scale STW. The scenarios
considered indicate that the optimum feeding frequency decreases with the sludge layer
height. In this way, systems with a sludge layer of 20 cm, 40 cm and 80 cm (corresponding
to 2, 4 and 8 years of operation), should be fed every 2.5, 10 and 30e40 days, respectively.
On the other hand, evapotranspiration (ET) has no effect on the feeding frequency,
although it does increase the sludge dryness from 25% to 45% (for ET of 2.5 and 14.5 mm/
d in the case of 20 cm of sludge height). According to the model output, the sludge loading
rate is determined as a function of evapotranspiration, feeding frequency and sludge
height.