Title: Optimization of the Duplex constructed wetland design for total microbial activity removal of faecal indicator organisms and organic matter from domestic wastewater
Author: Emma Immaculate Kyomukama
Supervising Institution: IHE Delft - Institute for Water Education
Constructed wetlands (CWs) are low cost, simple to operate yet effective alternatives for secondary treatment of different types of wastewater. In developing countries, they have been used to treat domestic wastewater (DWW) for small communities in a bid to curb the rampart spread of water-borne diseases, which result from poor wastewater disposal. This study was carried out on a novel CW, known as the "duplex CW," arranged in a stack design. The stack design comprises of a vertical flow CW (VFCW) on top of a horizontal flow filter (HFF). In a treatment wetland, the major contributor to organic matter breakdown and transformation is attributed to microbial activity. In view of this, the study sought to optimise the performance of the duplex CW for total microbial activity. The removal of organic matter, pathogenic and faecal indicator organisms was investigated.
The performance of three set-ups of the duplex CW design was compared. Two of the set-ups were intensified; one with artificial aeration, the second with recirculation of the effluent. The third set-up had neither aeration nor recirculation (control).The performance of the HFFs of two of the set-ups (recirculation and control) was also compared with and without an extra source of carbon (wastewater bypass).
Experiments were conducted for four months from mid November 2013 to March 2014. During the experimental period, different parameters on water, sand, and roots were analysed; Physical chemical parameters were measured insitu. Total microbial activity of the viable cells in the sand and root biofilm was determined by the use of Fluorescein Diacetate Assay (FDA hydrolysis). Removal efficiency of organic matter (COD, BOD, and DOC) pathogenic and faecal indicator organisms (Salmonella, E .coli, and faecal coliforms) was assessed. Enumeration of the micro-invertebrate population (protozoa and metazoa) in the wetland was done and their grazing effect on faecal indicator bacteria (E. coli) was investigated by use of a batch experiment.
Total microbial activity correlated positively (P=0.07, R2 =0.059) with organic matter (BOD5) reduction but with marginal significance. Removal efficiencies of the organics (COD, BOD5 and DOC) were in the range of 63-87% while that of pathogenic and feacal indicator organisms ranged between 3-6 log removals by the set-ups. Notable reductions in the concentration of E. coli were observed in aliquots of the Duplex CW sand treated with the protozoan inhibitor, cycloheximide. In conclusion, all the set-ups of the Duplex CW performed equally well in the removal of organic matter and feacal indicator organisms from DWW. Therefore, due to cost implications, the control set-up would be suitable for the Duplex design in a developing country.
Key words: Artificial aeration, Constructed wetlands, Domestic wastewater, Duplex CW, Faecal indicators, Pathogens, Protozoa, Metazoa, Microbial activity, Recirculation