Performance of duckweed-covered sewage lagoons in Sana'a, Yemen, depending on sewage strength

The performance of a duckweed (Lemna gibba) sewage lagoon (DSL) was investigated in non-continuous batch system reactors using high strength sewage under natural environmental conditions in Sana'a. Wastewater effluent from the anaerobic ponds of the Sana'a waste stabilization ponds (WSPs) was used with dilution factors (DF) of 0, 2, 3 and 4. The initial COD concentration range applied was 254–600 mg COD l⁻¹ (150–250 mg BOD l⁻¹) and NH₄⁺ of 25–100 mg N l⁻¹, while the duckweed stock density used was 500 g wet weight m⁻². The duration of the experiments was 10 days with a harvesting frequency of 5 days. NH₄⁺ in this very concentrated Sana'a sewage was possibly the most important limiting factor for growth of L. gibba. High pH near the end of the reaction time and lower temperatures at night-time probably also contributed to slower growth. Relative growth rate (RGR) decreased from 0.17±0.04 d⁻¹ at an NH₄⁺ concentration of 23–40 mg N l⁻¹ to around 0.00 d⁻¹ at a concentration of 100 mg N l⁻¹. Fresh wastewater helped to grow duckweed, especially at NH₄⁺ <50 mg N l⁻¹, while after 5 days, algae proliferation and probably the exhaustion of other essential nutrients started to inhibit duckweed growth. COD removal correlated strongly with the applied initial surface loading. At a higher initial COD loading (λ_s) of 869 kg COD ha⁻¹, the removal loading (λ_r) was 710 kg COD ha⁻¹ 10 day⁻¹, while at a lower initial COD loading of 344 kg ha⁻¹, the removal loading was 210 kg COD ha⁻¹ 10 day⁻¹. Total Nitrogen removal (λ_r,N) increased with initial NH₄⁺ concentration and with initial surface loading (λ_N). At initial nitrogen loading (λ_N) of 28 and 164 kgN/ha, the removal loading (λ_r,N) was 25 kgN/ha.10 d and 148 kgN/ha.10 d, respectively. At the same time, the first order COD kinetic removal rate constant increased from 0.10 to 0.16 d⁻¹ at initial COD concentration of 254 and 621 mg/l, respectively. The total nitrogen kinetic removal constant increased from 0.16 day⁻¹ at NH₄⁺ concentration 93 mg N l⁻¹ to 0.26 day⁻¹ at NH₄⁺ 34 mg N l⁻¹. The high DO and pH encountered under outdoor environmental conditions are probably the main cause of the high N removal compared with removal under laboratory conditions. Therefore, total nitrogen removal was taking place through nitrification/denitrification and probably NH₃ stripping.