3th International Faecal Sludge Management, Hanoi/Vietnam, Jan.2015
Application of High Organic Loading Rate in Sludge Treatment Wetlands as an Option for Developing Countries with Tropical and Sub-Tropical Climate S C Kafer*, M E Magri**, J G Zaguini*, L S Philippi* * Universidade Federal de Santa Catarina, Departamento de Engenharia Sanitária e Ambiental, Campus Universitário – Trindade, Florianópolis/SC. Brazil 88040-970. ** (corresponding author) Universidade Federal de Lavras, Departamento de Engenharia, Lavras/MG. Brazil 37200-000.
[email protected] Key words: Faecal Sludge; Wetlands; Dewatering.
1. Introduction Since 1990 almost 2 billion people gained access to improved sanitation according to the Joint Monitoring Program (WHO, 2014). An improved sanitation facility is one that hygienically separates excreta from human contact, which includes ventilated improved latrines, pit latrines with slab, composting toilets and flush toilets connected to piped sewer systems or on-site septic tanks. Although there was an improvement on sanitation in the last decade, much remains to be done. One of the main concerns about septic tanks and latrines is the treatment and disposal of faecal sludge. Constructed wetlands have been studied aiming sludge dewatering in order to attend constraints such as construction and energetic costs and simplified operation. Sludge treatment wetlands can promote dewatering, mineralization and depending on the treatment duration, even pathogens can be reduced to safe levels (Kengne et al. 2008). There are no standard project criteria for sludge treatment wetlands. The most used is to choose a loading rate in terms of total solids (kg TS.m-².year-¹), and based on that the required area of the treatment wetland is calculated. Commonly used rates vary from 50 to 125 kg TS.m-².year-¹ (UGGETTI et al., 2010). Higher rates are not common with some exceptions (i.e. 500 kg TS.m².year-¹ studied by Koottatep et al., 2001), but we believe that rates can be optimized when the system is constructed in areas were tropical and sub-tropical climates are predominant, since temperature influences directly the dewatering process. With optimized rates the required area can be optimized which reflects in the decrease of costs, a main constrain for making the technology available to all. In that context the aim of the present study was to evaluate the efficiency of a treatment wetland in dewatering sludge from septic tank, under a high organic loading rate.
2. Materials and Methods The research was conducted in Southern Brazil – city of Florianópolis, Santa Catarina State. The region has sub-tropical climate with high air moisture (80%) and average temperatures of 15° C during winter and 25° C during summer. The experimental unit was composed by a pilot vertical sludge treatment wetland planted with Zizaniopsis bonarienses. The wetland had 1.6m² of superficial area, and its bed was composed of 0.10 m of sand, 0.15 m of gravel and 0.35 m of large gravel. Additionally the wetland had a ventilation pipe placed vertically over the drainage pipe. Before the present experiment, the pilot wetland was operated for over 2 years treating anaerobic sludge under lower rates (125 – 200 Kg TS.m-².year-¹), what made a sludge layer of 0.20 m to accumulate over the wetland surface. After that period the wetland started to be fed with sludge removed from a septic tank on a weekly base, under a rate of 700 Kg TS.m-².year-¹ (data presented here for 3 months of monitoring). The period between each sludge application determined the hydraulic retention time which was 6 days. The pilot wetland was monitored to evaluate the efficiency in dewatering the sludge. With that purpose some laboratory analysis were conducted in samples taken from the influent sludge
and from the effluent produced during the dewatering process (called Liquid Percolated). The analysis were: pH, chemical oxygen demand (COD), total ammonium nitrogen (TAN), nitrate (NO3), orthophosphate (PO4), total solids (TS), total fixed solids (TFS), total volatile solids (TVS), suspended solids (SS), Total coliforms, Escherichia coli and Enterococus faecalis.
3. Results and Discussion Results show that even when fed with high loading rates in terms of Total Solids, the studied wetland planted with Zizaniopsis bonariensis had a good performance in dewatering the sludge. The plants were also resistant to the high loads, which was possible since they were well adapted to the systems by previous feeding with smaller organic loads. The temperature was considered to be a key issue in the process. The average temperature during the monitoring period presented here was 23°C (with maximum of 37ºC), which allowed good evaporation. From Table 1 it can be seen the good removal of Total solids and COD, which produced a liquid percolated with 290 mg.L-1 of COD. The TAN removal was 84% in average, and the balance of measured forms of nitrogen indicates that both processes might have taken place, nitrification and denitrification. Orthophosphate was removed from effluent by 88%. The sludge layer reached 20 cm in 3 months of experiment. Another important result was the removal/retention of microorganisms in the filter, which were 2.5 to 2.7 log units for the tested bacteria. Table 1 Results from monitoring the dewatering process efficiency. Data removal efficiency. Influent Sludge pH 7,1 ± 0,2 -1 COD (mg.L ) 35518 ± 27360 -1 TS (mg.L ) 29977 ± 13872 -1 TVS (mg.L ) 13987 ± 7501 -1 TFS (mg.L ) 17602 ± 8531 -1 SS (mg.L ) -1 TAN (mg.L ) 72 ± 28 -1 NO3 (mg.L ) 7,2 ± 3,5 -1 PO4 (mg.L ) 151 ± 104 -1 Total coliforms (MPN.100mL ) 1,8E+07 n.a. -1 Escherichia coli (MPN.100mL ) 2,4E+06 n.a. -1 Enterococcus faecalis (MPN.100mL ) 8,5E+05 n.a. n.a. not applicable.
is shown for average, standard deviation and Liquid Percolated 6,0 ± 0,4 290 ± 209 599 ± 148 236 ± 91 364 ± 133 37 ± 27 11 ± 11 10 ± 11 18 ± 19 6,0E+04 n.a. 8,0E+03 n.a. 1,6E+03 n.a.
Removal efficiency n.a. 99% 98% 98% 98% n.a. 84% n.a. 88% 2,7 logs 2,5 logs 2,5 logs
4. Conclusion We could see with our preliminary results that the sludge treatment wetlands can achieve good dewatering efficiencies even when submitted to high organic loading rates, since temperature is favorable. When applying a sludge rate of 700 kgTS.m-².year-¹ the area required constructing a plant could be reduced by 64%, in comparison with a rate of 125 kgTS.m-².year-¹, commonly used. This would directly reflect in the reduction of costs, what makes the technology more accessible to all. However more studies are needed to confirm the sustainability of the system in a long term.
5. References Kengne I. M., Akoa A., Soh E. K., Tsama V., Ngoutane M. M., Dodane P. H., Koné D. Effects of faecal sludge application on growth characteristics and chemical composition of Echinochloa pyramidalis (Lam.) Hitchc and Chase and Cyperus papyrus L. (2008). Ecol. Eng. 34, 233-242. Koottatep, T.; Polprasert, C.; Oanh, N. T. K.; Montangero, A.; Strauss, M. Sludges from on-site sanitation systems – lowcost treatment alternatives. In: IWA Conference on Water & Wastewater Management for Developing Countries, Kuala Lumpur, Malaysia. (2001). 10p. Uggetti E., Ferrer I., Lorens E., Garcia J. (2010). Sludge treatment wetlands: A review on the state of the art. Biores. Tech. 101, 2905-2912. Progress on sustainable and drinking water – 2014 Update. (2014). WHO and UNICEF. Switzerland, 78p. Available on line at: http://www.wssinfo.org/
