Vertical-flow constructed wetlands as sustainable sanitation approach for faecal sludge dewatering in developing countries

Researches aiming at assessing the potentials of vertical-flow constructed wetlands vegetated with Echinochloa pyramidalis as technically feasible approach for sludge dewatering were conducted at yard-scale experimental plant in Yaounde (Cameroon). The performances of the system and the quality of the by-products generated were assessed for more than 6 months of constant solid loading rates with raw faecal sludge. Results revealed that at loading rate of 100–200 kg TS/m2/yr, the system performed well for solid–liquid separation, with an average dry matter content of biosolids !30% and pollutant removal efficiencies higher than 77%, 86%, 90%, 90% and 95% for NH4+, TSS, TS, NTK and COD, respectively. Besides this primary goal, the system could generate 100–150 dry tons/ha/yr of forage plant and biosolids rich in nutrients that could be valorised to sustain the technology if safely managed, since helminth egg concentrations in biosolids remained relatively high (79 eggs/g TS).

This study revealed that the indigenous macrophytes E. pyramidalis could be used to sustain the solid liquid separation of FS produced in most African countries. The removal efficiencies obtained for most of the parameters considered was >90%. Loading the beds with FS at 100–200 kg TS/m2/yr permitted to limit the occurrence of clogging. Meanwhile, 100–150 dry tons/ha of plant biomass could be generated by the system if properly managed. The biosolids exhibited high nutrients contents with total N and P2O5 accounting for up to 2% and 2.3% DM respectively. However, helminth eggs remained relatively higher (79 eggs/g TS on an average basis) to the WHO guidelines for direct reuse in unrestrictive agriculture, therefore requiring a longer storage time or co-composting to accelerate its safety. The high biomass production on dewatering beds and its use as forage crop in many African countries as well as the good quality of biosolids generated show that this system has the potential to generate substantial revenue to lower the treatment plant operational and maintenance costs if safely managed, thus linking sanitation to business opportunity [23].