Effect of anaerobic pre-treatment on environmental and physicochemical characteristics of duckweed stabilization ponds.

Duckweed stabilization ponds, as an alternative for wastewater treatment, are attracting a growing interest from researchers because they are low cost, easy to built and operate and produce tertiary quality effluents. Besides, this technology offers the possibility of resource recovery by producing high quality duckweed protein, which can be of further use. The presence of a layer of duckweed on the surface of the ponds is expected to produce different environmental and physicochemical conditions in the water compared to those in conventional stabilization ponds. The environmental and physicochemical conditions affect both plant growth and microbiological treatment processes in the system. Two series of continuous flow pilot plants, composed of seven ponds in series each, were operated side by side. One system received artificial sewage with anaerobic pretreatment, while the other system received the same wastewater without anaerobic pretreatment. The flow was kept constant during the operation. pH, temperature, dissolved oxygen, alkalinity, conductivity, biochemical oxygen demand, total and ammonium nitrogen, nitrites and nitrates, phosphorus were monitored under steady state conditions. The main conclusions from this study include the following: pH levels are very stable in both systems with and without anaerobic pretreatment. Temperature gradients are present during daytime but not as high as they may be in conventional stabilization ponds. Oxygen levels are significantly higher in the duckweed system with anaerobic pretreatment, especially in the top layer. Nevertheless, re-aeration rates are low in both systems. Both systems were efficient in removing organic matter. The system without pretreatment obtained 98% of BOD 5 removal in pond 4, so 12 days of retention time will be enough to reach high organic matter removal. The system with pretreatment obtained also 98% BOD 5 removal (92% of BOD 5 in UASB reactor). In this case with a very efficient UASB reactor, the duckweed ponds will serve as a polishing step for remaining organic matter. Nutrient removals were 37-48% for nitrogen and 45-50 % for phosphorus in the lines with and without pretreatment respectively. It is important to establish the nitrogen balance for the systems in order to generate a better understanding of the nitrogen transformations in the duckweed system.