Performance of subsurface flow constructed wetland in domestic wastewater treatment

The performance of Constructed Wetland (CW) at the University of Dar es Salaam main campus to treat domestic wastewater and prevent environmental pollution was investigated. The aim of this research was to assess efficiency of CW in the removal of fecal indicator bacteria, organic Bio-chemical Oxygen Demand (BOD5) and inorganic nutrients from domestic wastewater using Subsurface Flow Constructed Wetland (SSF), after a long time usage (>1O years). Concentrations of microbial indicators of faecal pollution, nutrients and BOD5 were assessed weekly over a three month's period (November 2009 to January 2010) using standard methods and the results were compared with the World Health Organization (WHO) water quality guidelines. To implement this, cells planted with Phragmites and bare (control) were used. Higher counts (CFU per 100 ml) were observed at influents than effluents in both old and new wetland systems. Influent counts in planted cell of old system ranged from 2.1 x 104 to 8.5 x 104 Escherichia coli (EC) and 1 .8 x 104 to 4.0 x 104 Fecal coliforms (FC); The effluent counts in planted cells ranged from 1.1 x 103 to 6.0 x103 (EC) and 3.0 x102to 6.0 x 103 (FC). Counts in effluent of unplanted cell, ranged from 4.0 x 103 to 1 .8 x 104 (EC), and 4.0 x103 to 2.0 x 104 (FC) CFU/100 ml. In the new wetland, influent ranges were: 4.0 x 103 lo 1.0 x 104 (EC); 3.5 x.103 to 9.0 x.103 (FC). Effluent counts were 2.0 x102 to 1.3 x 103 (EC) and 5.0 x102 to 2.6 x103 (FC). Percentage fecalindicator bacteria removal in planted cells were significantly higher than in unplanted cell (P < 0.001) both in old and new CW. Bio-chemical Oxygen Demand values in influent of old CW ranged from 60 to 70 mg/I, while in effluent of planted cells, the range was 13 to 27 mg/l; and in effluent of unplanned cell the range was 3 I to 38 mg/I. Bio -chemical Oxygen Demand in influent of new system ranged from 34 to 47 mg/l; and in effluent the range was 12 to 22 mg/l. Bio-chemical Oxygen Demand removalwas significantly higher in planted cells than in unplanted one (P < 0.001) in both old new CW. Similarly, nutrients were significantly higher (P < 0.01) in the influents than in the effluents. In influent of old system, the concentration (µmol L -1) ranged from O.18 to 0.984 (NO3) and 0.0081 to 0.013 (PO4); whereas in effluent, nutrient ranges were 0.1 1 1 to 0.156 (NO3) and 0.0038 to 0.0071(PO4). Fecal indicator bacteria numbers were low in effluent than in influent indicating that CW has a buffering capacity to assimilate higher influent contents of wastewater. This study demonstrated that Phragmites plants in constructed wetland systems enhance fecal indicator bacteria, BOD and nutrients removal. It is recommended that CW be promoted for sewage treatment in a strategy to reduce wastewater pollution in Tanzania. The system continues to function for long time (>10 years) which signifies its cost effectiveness.