Performance evaluation of a vertical flow constructed wetland for treatment of sludge from septic tanks.

This study investigated the performance of vertical flow constructed wetland (VFCW) units at full and pilot scale that are used to treat septic tank sludge. The final disposition and the treatment of sludge is considered a major problem in the field of sanitation in most Latin America and developing countries. The research was carried out at the Centre for Research and Training in Sanitation (CePTS) of the Federal University of Minas Gerais, located in Belo Horizonte, capital of Minas Gerais state, Brazil. This research was done within the framework of the double degree master Programme in Sanitary and Environmental Engineering between Universidad del Valle (Cali-Colombia) and UNESCO IHE Institute for Water Education (Delft, The Netherlands), and was funded by the Bill and Melinda Gates Foundation (BMG), as part of an ongoing doctoral project focusing on the evaluation of the behaviour of vertical flow constructed wetlands for combined treatment of domestic wastewater and septic tank sludge at the Federal University of Minas Gerais. At the same time, this research is part of a broader international project entitled `Stimulating Local Innovation on Sanitation for the Urban Poor in Sub Saharan Africa and South East Asia¿ SaniUP, sponsored by BMG Foundation and coordinated by UNESCO-IHE.The study was divided into two Phases. Phase 1 considered the application of real septic tank sludge, or septage, into a VFCW unit (FSW1) which belongs to a Full Scale System (FSS). Additionally this phase involved the construction and start-up of six VFCW units in a Pilot Scale System (PSS) that was built with the aim of comparing different operational conditions. In Phase 2, application of septic tank sludge was run in parallel in both, the FSS and the PSS. The applied septage presented great variability in its composition. Moreover, recirculation of the percolated effluent from FSW1 was executed at full-scale in other VFCW units (FSW+R). The operational conditions tested at pilot scale included the same Hydraulic Loading Rate (HLR) as the one evaluated at full scale (average); half and the double of that condition; an unplanted control, a unit with the presence of worms; and last, a unit with recirculation of the sludge into itself. Substantial variability was observed in terms of Solids Loading Rates (SLR) from week to week due to the different origins of the incoming septic tank sludge, which is the reason why median values were adopted as a measure of central tendency for data analysis. The vegetation planted in the VFCW units in FSS and PSS was Cynodon spp., which in previous studies in Brazil showed tolerance to the raw wastewater feeding strategies and to the tropical environment. The FSS was also integrated with two units for raw sewage treatment according to the first stage of the French System of vertical flow wetlands. All full-scale units have been in operation since 2009 treating wastewater, but one out of the three cells stop this operation and started treating solely septic tank sludge in September 2013. The sludge was applied once a week under batch conditions and the rest of the week was used as a resting period. The sludge was collected by trucks, and the full volume from each truck (typically 10m 3) was discharged into the wetland unit without strict control of the hydraulic (HLR) and solids loading rate (SLR), thus reproducing real conditions found in most treatment plants in Brazil. Recirculation of the percolate effluent was performed to one of the two vertical wetlands for domestic sewage. The surface area of the full-scale unit is 29.1m2 (3.1 m in width, 9.4 m in length), with a filter bed of 0.70 m thick with fine, medium and coarse gravel. For Phase 2, a detailed analysis was carried out in terms of solids, organic matter and nutrients (TS, TVS, TCOD, TBOD5 and TKN), with the resulting median HLR applied in the unit FSW1 as 10 m 3 week-1 , which is equivalent to 0.34 m 3 m -2 per weekly batch. The resulting median SLR was 12 kg TS m-2 year -1). The median influent total solids (TS) concentrations in the raw septage, FSW1 and FSW1+R drained liquid were 706 mg L -1 , 612 mg L-1 and 676 mg L-1 respectively, with median removal efficiencies of 22% and 27% for FSW1 and FSW1+R correspondingly. In terms of chemical oxygen demand (TCOD), median concentration values attained were 487 mg L -1, 240 mg L -1 and 246 mg L -1, respectively, leading to median removal efficiencies of 41% and 71% in FSW1 and FSW1+R. In the pilot-scale columns, the different operational conditions and loading rates were compared to Performance Evaluation of a Vertical Flow Constructed Wetland for Treatment of Sludge From Septic Tanks ii performance of a full scale units, However, efficiencies achieved for all the parameters in all of units evaluated were lower than those attained for full-scale units, and through statistical analysis based on mixed models, it was observed that at a confidence level of 95% there were no significant differences among the efficiencies obtained in each of the units evaluated excepting for the removal of total Kjeldahl Nitrogen (TKN), for which the size of the wetland is an aspect of influence. In general, the plants showed good resistance to the septic tank sludge characteristics and to the weekly feeding strategies provided in the research, which was reflected in healthy growth with high yield and productivity. In contrast; the sludge layer formed on the top of the wetlands was still thin and did not seem to have a key role in the filtration process, which is expected to happen in long term operation according to the literature reviewe