Clogging patterns in vertical flow constructed wetlands treating combined sewer flows under different feeding modes | |
| Author | Rajapakse, Eranda Pradeep |
| Call Number | AIT Thesis no.UWEM -11-01 |
| Subject(s) | Constructed wetlands Combined sewer overflows |
| Note | A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Urban Water Engineering and Management |
| Publisher | Asian Institute of Technology |
| Abstract | Treating combined sewer flows (CSFs) is extremely difficult due to its stochastic nature of the flow and concentration fluctuations. A common treatment system for CSFs is vertical flow constructed wetlands (VF CW5) as a promising technology in treating high fluctuating wastewater. However, clogging is one of the major operational problems of VFCWs which could result in deteriorating treatment performance and shortening the effective operational period. Hence this study aimed to investigate the clogging patterns of VFCWs treating CSFs under different feeding modes with the objectives of studying the clogging patterns, relate clogging patterns to various parameters and model the clogging process. Two pilot-scale and six lab-scale VFCW units were Operated in continuous feeding (CF) and intermittent feeding (IF) modes under three hydraulic loading rates (HLRs) varied from 0.3 to 0.9. m3/m2d. Each VFCW unit consisted three layers of sand (0.1 m), small gravel (0.15 m) and large gravel (0.35 m). CSF characteristics were simulated by using synthetic waste-water prepared in two mixing tanks, operated separately for dry and wet weather conditions. Tracer test was conducted in the initial stages and after two month of operation to determine the changes of wetland hydraulics. Influent and effluent samples were analyzed for chemical oxygen demand (COD), biochemical oxygen demand (BOD), dissolved oxygen (DO) and total suspended solids (TSSs) while changes of bed properties were observed by measuring-residence time distribution, hydraulic conductivity, effective porosity, solid deposition and pending height. Tracer study showed considerable changes of hydraulic properties by increased average volumetric efficiency from 49% to 251% and broaden residence tine distribution (number of tank in series increased from 1.5 to 2.9). Gradual increase of TSS removal was observed with increasing clogging while COD and BOD removals also increased particularly for intermittent fed units. However the effluent DO concentrations decreased with increasing clogging. Exponential reduction of hydraulic conductivity was observed every lab—scale VFCW units and a critical hydraulic conductivity of l m/d was determined. Effective porosity of every lab-scale VFCW units dropped gradually but the higher reduction of above 80% was observed in the top sand layer. Solid deposition in the top sand layer was considerably high comparing to the other layers. Noticeable amount of deposition was found on top of the sand layer which formed into 3 to 9 mm thick layer at the end of operation of each unit. The units operated under higher HLRs reached clogging faster than the units operated under lower HLRs. CF units showed early clogging than the IF units demonstrating capability of longer Operation of IF VFCWs. The clogging processes were described by using simple mathematical model which considered the filling of the pore Spaces by accumulation of fixed and volatile solids and the effect of top deposition layer. This model was used to calculate remaining pore spaces and total clogging time. The clogging time calculated by the model was always higher than the observed clogging time but it was below 140% of the observed clogging time for most of the VFCW units. However this model predicted reasonably accurate clogging time for VFCWS treating CSFs compared to the predictions of other available clogging time models. |
| Year | 2011 |
| Type | Thesis |
| School | School of Engineering and Technology (SET) |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Urban Water Engineering and Management (UWEM) |
| Chairperson(s) | Thammarat Koottatep |
| Examination Committee(s) | Visvanathan, Chettiyappan;Babel, Mukand Singh;Hooijmans, Tineke |
| Scholarship Donor(s) | UNESCO-IHE;AIT Fellowship |
| Degree | Thesis (M. Eng.) - Asian Institute of Technology, 2011 |