A pilot plant experiment was carried out to assess differences in environmental conditions and treatment performance in two systems for wastewater treatment: algae-based ponds (ABP) and duckweed-based (Lemna gibba) ponds (DBP). Each system consisted of a sequence of 4 equal ponds in series and was fed with a constant flow rate of partially treated wastewater from Birzeit University. Physico-chemical parameters and the removal of organic matter, nutrients and faecal coliforms were monitored within each treatment system over a period of 12 months. The results show clear differences in the environmental conditions. In ABP significantly (P>0.05) higher pH and DO values were observed than in DBP. DBP were more efficient in removal of organic matter (BOD and TSS) than ABP. The faecal coliform reduction was higher in ABP. However, the quality of the effluent from the third and fourth duckweed pond (total retention time of 21 and 28 days) did not exceed the WHO-criteria for unrestricted irrigation during both the summer and winter period, respectively. During the summer period, the average total nitrogen was reduced more in ABP (80%) than in DBP (55%). Lower values were measured during the winter period. Seasonal nitrogen reductions of the two systems were significantly different (P>0.05). In DBP, 33% and 15% of the total nitrogen was recovered into plant biomass and removed from the system via duckweed harvesting during the summer and winter period, respectively. This study showed that there were differences in the environmental conditions and treatment efficiencies between the two systems.

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