Assessment of wastewater treatment efficiency of a constructed wetland at finlays flower farm, Kericho, Kenya

Abstract

An assessment of wastewater treatment efficiency of Chemirei constructed wetland (CW) at James Finlay's farm in Kericho was carried out from November 2014 to February 2015. Water samples were collected twice per month from seven sampling points (S1-S7) using acid cleaned bottles for analysis. In situ measurements of Dissolved Oxygen (DO), pH, EC and temperature were done using calibrated meters and probes. Wastewater inflow and outflow rates for each purification cell were obtained using the volumetric method. Macrophyte biomass was determined using harvest method. Hydraulic retention time (t) and loading rate (q) were determined using mean flow rate (Q), system volume (V) and wetted surface area (A). In the laboratory; SRP, TP, NH4-N, NO3-N, NO2-N, TN, TSS, BOD and COD were determined using Standard Methods for Analysis of Water and Wastewater (APHA, 2004). Data were checked for normality and homogeneity of variance prior to parametric test. Analysis was done using IBM SPSS statistics 21 (USA) and comparison of means of different wastewater variables were performed using Analysis of Variance (ANOVA). Tukey HSD post hoc test was applied to separate means between the sampling sites where all statistical tests were considered significant at p<0.05 (95% confidence interval). The mean inflow rate was 37.91 ± 9.96 m3 and outflow 12.31 ± 4.67 m3 per day with HRT of 14 days and HLR of 0.23 m per day. The results showed mean removal efficiency of NH4-N (98%), TP (93.6%), SRP (61.6%), NO3-N (88.6%), TN (88.6%), TSS (98.1%), BOD (69.5%) and COD (57.2%). Macrophyte nitrogen accumulation was highest in Fimbristylis complanata with 57.70 gm-2 and biomass of 3085 ± 99.31 gm-2 while phosphorus accumulation was highest in Cyperus alternifolius at 7.29 gm-2 with biomass of 8896 ± 195.61 gm per m2. Pistia stratiotes had the lowest nitrogen accumulation at 3.73 gm-2 with biomass of 333 ± 18.59 gm-2 while Cyperus rotundus had the lowest phosphorus at 0.58 gm-2 with biomass of 503 ± 23.99 gm-2. There was significant removal of nutrients and TSS (p<0.05) between the wetland inlet and outlet. This study found that there was no significant impact (p>0.05) on the receiving stream water at the point of effluent discharge with respect to nutrients and TSS. The constructed wetland was efficient in removing nutrients and TSS. However, it was not able to remove the COD to the required Kenyan effluent standard. The low removal rate is an indication of the presence of non-biodegradable compounds in the wastewater.