The effects of wastewater loading rates and two macrophyte species on treatment of sugar factory stabilization pond effluent were investigated in a pilot-scale free water surface constructed wetland (FWS CW) system in western Kenya. For 12 months, four CWs were operated at a hydraulic loading rate of 75 mm day⁻¹ and four at 225 mm day⁻¹. Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Water samples were taken at the inlets and outlets and analyzed for TP, TDP, NH₄-N, and TSS. Mass removal rates of the selected water quality parameters were compared during three periods designated the short rain (period 1), dry (period 2), and long rain (period 3) seasons. There was a significant linear relationship between the mass removal rate of TP, NH₄-N, and TSS and the mass load, and season had a significant effect on the mass removal rate of TSS, NH₄-N, and TDP. Mass loading rates for TDP were about 78% of those for TP, whereas TDP comprised 78–99% of TP mass outflow rates, indicating a release of dissolved P within the CWs. The only significant difference between the two macrophyte species was associated with mass removal of NH₄-N, with more efficient removal in CWs planted with C. papyrus than those with E. pyramidalis. TP mass removal rates were 50–80% higher when a mean water loss for CWs 6–8 during periods 1 and 2 was assumed to represent evapotranspiration for all CWs in period 3 instead of pan evaporation data. This illustrated the importance of accurate estimations of evapotranspiration for pollutant mass removal rates in CWs in tropical climates.

Wastewater treatment and nitrogen and phosphorus (P) recovery in harvested biomass of two macrophyte species receiving two wastewater loading rates was studied in a free water surface constructed wetland (FWS CW) in Kenya. Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Inlets and outlets water samples were analysed for selected water quality parameters. Macrophytes were harvested at around 7 month intervals on three occasions for determination of biomass, P and N content. Area specific removals of TP, TSS and Nh₄⁺-N were higher in the high-load CWs and in the low-load ones, but the relative removal was lower. For Nh₄⁺-N, there was a significantly higher removal in C. papyrus CWs- Each macrophyte species had similar tissue P content independent of mass load suggesting excess available phosphorus in all CWs, as supported by the low N:P ratios. During a 7 month period, the amount of P stored daily in the green biomass of the macrophytes represented 18-29% and 25-100% of the daily removal of TP and TDP, respectively.

This study investigated, using a lithium salt tracer, how macrophyte species and hydraulic loading rate (HLR) of wastewater influenced hydraulics in constructed wetlands (CWs). Four pilot-scale CWs received 45 mm day(-1) of pre-treated sugar factory effluent and another four received 110 mm day(-1). Half the CWs were planted with Cyperus papyrus and half with Echinochloa pyramidalis. Results showed a significant negative connection between tracer mass recovery and wetland water leakages. Also, a significant negative relationship between active wetland water volume and macrophyte density was detected. Further, a significant effect of HLR on mass removal rates of NH4+-N was observed. However, no significant effect of either HLR or macrophyte species on wetland hydraulic parameters was found.

Effects of inlet design and vegetation type on tracer dynamics and hydraulic performance were investigated using lithium chloride in 18 experimental free water surface wetlands. The wetlands received similar water flow but had different vegetation types: 6 emergent vegetation wetlands (EVWs), 6 submerged vegetation wetlands (SVWs) and 6 free development wetlands (FDWs). Two types of inlet designs were applied: half of each wetland vegetation type had a barrier near the inlet to help distribute incoming tracer solution, while the rest had no barrier. Residence time distribution (RTD) functions were calculated from tracer data using two techniques: method of moments and a novel Gauss modelling approach. RTD functions were used to quantify hydraulic parameters: active wetland volume (e-value), water dispersion (N-value) and hydraulic efficiency (λ-value).

For wetlands without barrier, significantly lower tracer mass recoveries were found from EVWs compared to FDWs and SVWs, signifying a risk of tracer methodological problems in small densely vegetated wetlands. These problems were minimized in wetlands with an inflow construction promoting distribution of incoming tracer solution.

Compared to the method of moments, Gauss modelling seemed to produce more reliable λ-values but less reliable N-values. Data for precise hydraulic quantification were lost by Gauss modelling, as indicated by overall lower variance in these data sets and lower mass recoveries. However, Gauss modelling may minimize uncertainties associated with lithium immobilization/mobilization. Parameters were significantly affected by the RTD data analysis method, showing that the choice of method could affect evaluation of wetland hydraulics.

The experimental wetlands in this study exhibited relatively high e-values and low N-values. This was probably caused by the small size of the wetlands and low water flow velocities, emphasizing that hydraulic parameter values obtained in small experimental wetlands may not be applicable to hydraulics in larger wetlands.

The method of moments revealed lower e-values from EVWs compared to SVWs and FDWs. It was indicated that lower e-values were mainly caused by vegetation volumes. This highlighted a need for regular maintenance to secure efficient treatment volume in wetlands with dense vegetation.

Hydraulic tracer studies are frequently used to estimate wetland residence time distributions (RTDs) and ultimately pollutant removal. However, there is no consensus on how to analyse these data. We set out to (i) review the different methods used and (ii) use simulations to explore how the data analysis method influences the quantification of wetland hydraulics and pollutant removal. The results showed that the method influences the water dispersion (N) most strongly and the removal least strongly. The influence increased with decreasing effective volume ratio (e) and N, indicating a greater effect of the method in wetlands with low effective volume and high dispersion. The method of moments with RTD truncation at 3 times the theoretical residence time (t_n) and tracer background concentration  produced the most dissimilar parameters. The most similar parameters values were those for gamma modelling and the method of moments with RTD truncation at tracer background concentration. For correct removal estimates, e was more important than N. However, the results from the literature review and simulations indicated that previously published articles may contain overestimated e and underestimated N values as a result of frequent RTD truncations at 3t_n when using the method of moments. As a result, the removal rates may also be overestimated by as much as 14% compared to other truncation methods or modelling. Thus, it is recommended that wetland hydraulic tracer studies should use the same method, specifically, RTD truncation. We conclude that the choice of tracer data analysis method can greatly influence the quantifications of wetland hydraulics and removal rate.