PHOSPHORUS REMOVAL IN CONSTRUCTED WETLANDS OF SUBSURFACE AND SURFACE FLOW

Transcription

1 WATER MANAGEMENT PHOSPHORUS REMOVAL IN CONSTRUCTED WETLANDS OF SUBSURFACE AND SURFACE FLOW Water Management Institute of Lithuanian University of Agriculture Abstract Constructed wetland (CW) treatment systems are engineered systems designed to treat wastewater with the usage of the same processes that occur in natural wetlands. The full-scale investigations of free water flow surface filters (FWF), subsurface flow filters, including filters of vertical flow (SVF) and horizontal flow (SHF) were carried out in Lithuania. The investigations database collected within the study period of was used for the estimation of phosphorus removal efficiency of different constructed wetlands. It was established that phosphorus removal efficiency depends on construction of filters and the load according to total phosphorus for the area unit of filter surface. Subsurface flow filters are distinguished by better phosphorus removal. At the same load of all filters, i.e., total phosphorus of 0.3 g m -2 d -1, the removal efficiency of subsurface flow horizontal, vertical, and free water flow filters reaches 53.0, 46.5 and 28.0%, respectively. The wastewater contains phosphorus in mineral and organic forms. Due to the aerobic conditions in filters of vertical flow, the amount of organic phosphorus removed by SVF is 3 times greater than in horizontal filters. Organic phosphorus in wastewater before the treatment by vertical filters constituted 27% of total P, and after the treatment 21%. In wastewater treated by horizontal filters this ratio was 21% and 31%, respectively. Key words: constructed wetlands, phosphorus removal. Introduction Constructed wetland (CW) treatment systems are engineered systems designed to treat wastewater with the usage of the same processes that occur in natural wetlands. However, it is done within a more controlled environment. Constructed wetlands may be divided into two types: subsurface and surface flow. These two types of CW may be of different constructions. They may be designed as free-water surface wetlands, further in the text called as free water flow filters (FWF). Another type of constructed wetlands is subsurface flow filters, including filters of vertical flow (SVF) and horizontal flow (SHF). All those systems have one common element plants. Principal filter schemes of common construction are presented in Figure 1. horizontal filter (SHF) vertical filter (SVF) free water flow filter (FWF) Fig. 1. Principal filters schemes of common construction. sand chippings direction of water flow 207

2 In free water flow filters, the wastewater is related to atmosphere. Such kind of filter contains a filtration body (plant roots + soil) with an isolated bottom for the protection of subsurface water from pollution. Free water filters often include a pre-settling basin and several of compartments with a shallow water layer ( m) planted up with helophytes such as Phragmites, Typha or Scirpus spp. (Verhoeven and Meuleman, 1999). In filters of subsurface flow, the wastewater is flowing under the ground surface. The filters consist of excavated beds filled with soil in which marshy plants are growing; water level is below the ground surface in such filters. In filters of horizontal flow, the wastewater flows horizontally from the inflow zone through the body of the filter (i.e., roots of marshy plants, rhizomes and soil), where it is purified. The functioning principle of vertical filters is similar, only here the wastewater is spread on the surface of the filter; then it flows vertically through a m deep sand layer into the collection pipes. Phosphorus (P) within constructed wetlands is removed from wastewater via physical, chemical and biological processes occurring between the wetland substratum, vegetation and wastewater stream (Kim and Geary, 2000). Biological oxidation of phosphorus within CW converts most phosphorus species to an orthophosphate (soluble) form (Cooper et al., 1996; Kadlec and Knight, 1996). Phosphate can also precipitate with iron and aluminium oxides to form new mineral compounds (Fe- and Al-phosphates). These compounds allow the removal of phosphorus from a wastewater via sedimentation and filtration (Richardson and Craft, 1993; DeBusk and Dierberg, 1999). Based on experience in the USA, it has been established that the purification efficiency of surface-flow wetlands is high for chemical oxygen demand (COD) and biochemical oxygen demand (BOD) - (90%), but substantially lower for nitrogen (N) and P (10 15%) (Kadlec and Knight, 1996). In South Sweden, surface-flow wetlands have been built to treat wastewater from municipal wastewater treatment plants. Commonly, nitrogen removal has been the prime aim, though a significant removal of total P and BOD 7 has been observed. P removal varied between 10 and 41 kg ha -1 yr -1, and was related to differences in loads (Andersson et al., 2005). In subsurface flow wetlands, P removal is greatly influenced by physico-chemical characteristics of used sand (Brix et al., 2001). However, other researchers indicate that only some physicochemical characteristics are influential. The P- adsorption capacities of 13 Danish sands were studied by short-term isotherm batch experiments and related to the physico-chemical characteristics of the sands. If the most efficient sand for P- adsorption was used, the adsorption capacity would be used up after about 1 year, while, for the less efficient sands, the P-retention would go on for about 2 months. P-binding energy constants were not significantly related to the physico-chemical properties of the sands, except the Ca content, which showed, however, a low correlation coefficient (Del Bubba et al., 2003). Arias et al. (2001) data show that the content of calcium is particularly important, whereas iron and aluminium contents are of less importance. According to the data of Verhoeven and Meuleman (1999), in most filters phosphorus removal efficiency does not exceed 50%. The objectives of the article are to summarize the data collected within the period of and evaluate the phosphorus removal efficiency in constructed wetlands of different construction. Materials and Methods The full-scale investigations of free water flow surface filters (FWF), subsurface flow filters, including filters of vertical flow (SVF) and horizontal flow (SHF), were carried out in Lithuania. Due to construction differences, wastewater treatment conditions in filters are not identical, which has the influence upon the pollutant removal efficiency. The investigations database of different constructed wetlands collected within the study period of was used for the estimation of phosphorus removal efficiency. For horizontal filters, the studies were carried out in 6 wastewater treatment facilities where wastewater of different chemical composition was treated. All objects under investigation contain filters of common construction. After the primary treatment, wastewater is distributed into the filter via a distribution manhole. Chippings prisms arranged in filters contain distribution pipes. Here water is distributed evenly within the whole filter. Then, wastewater is filtered horizontally through a semicoarse sand medium with the filtration coefficient of 5-8 m d -1. Size of sand particles d 10 is fluctuating between 0.15 and 0.17 mm, the ratio d 60 /d 10 is 2-5. The depth of filters is 0.8 m, filtration distance

3 5.5 m. Filters are planted up with common reed (Phragmites australis). The investigations of vertical filters systems were carried out in wastewater treatment facilities arranged in motels Nikola, Pastoge, and village Aristava, Kėdainiai district within the period of After the purification in septic tanks, wastewater is directed into vertical filters for secondary (biological) treatment. Surface areas of filters are 300, 250, and 950 m 2 in motels Nikola, Pastoge, and Aristava, respectively. With the help of a pump, wastewater from septic tanks is distributed into a distribution manhole. Further wastewater is directed into distribution pipes, and finally it is sprayed into a layer of chippings arranged on the surface of the filter. Vertical filters contain a 20-cm thick layer of fine chippings with distribution pipes arranged at the spacing of 1 m. With the help of a pump, wastewater is distributed from the pump shaft into the distribution pipes. Further wastewater is spread throughout the chippings layer and is filtered downwards in a vertical direction via a 0.8-m deep sand layer into the collecting pipes arranged on the bottom of the filter. Sand filtration coefficient is 35.5 ± 5.9 m d -1, the ratio of sand particles d 60 /d 10 is 5-6. Filters are planted up with common reed (Phragmites australis). For free water filter, the investigation object included the Babenai wastewater treatment facilities (Kedainiai town) that have been re-constructed from an aeration wastewater treatment plant into a natural one. A free water filter has been arranged for additional wastewater treatment. The filter was set up in Currently it is completely overgrown with reed and cat s tail (Typhaceae). The area of the filter is 675 m 2. Water depth in the filter may be regulated from 0.1 to 0.8 m. Water depth is maintained to be at the depth of cm in the warm period of the year and at the depth of 50 cm in winter. Due to the entrance of surplus water into the wastewater network, its discharge is fluctuating from 10 to 90 m 3 d -1. The data of measurements taken in different study periods ( ) was used for the analysis. Another examined free water filter set in village Aristava was used for the secondary wastewater treatment after the wastewater treatment in the vertical filter. The filter was set up in The area of the filter comprises 130 m 2. The amount of wastewater varies from 10 to 30 m 3 d -1. Water levels are the same as in the Babenai examined object. The free water filter is completely overgrown with cat s tail (Typhaceae). Further analysis of wastewater treatment efficiency is performed only from the beginning of influent into the filters. To evaluate phosphorus removal efficiency, for statistical data analysis 182 measurement data of different periods for SHF, 90 for SVF and 55 for FWF were taken. Regression and analysis of variance were used for the mathematical processing of data. Results and discussion To estimate total phosphorus (TP) removal efficiency of filters with different construction, the load for filter surface area and removed TP amount for one area unit were calculated. The calculation results are presented in Fig. 2. The average load of studied SHF according to TP was 0.29±0.27, of SVF 0.32±0.25, and of FWF 0.25±0.23 g m -2 d -1. To compare the treatment efficiency, the calculations of pollutants removal efficiency have been made at the same load of all filters, i.e., TP 0.3 g m -2 d -1. To make the calculations, the results of relationships presented in Fig. 2 have been used. In this case the treatment efficiency of horizontal, vertical, and free water flow filters reached 53.0, 46.5, and 28.0%, respectively. 209

4 0.8 Removed P total, g m -2 d y (SHF)= 0.46x R 2 = 0.81 (0<x<1.5) y (SVF)= 0.43x R 2 = 0.72 (0<x<1.5) y FWF= 0.43x R 2 = 0.56 (0<x<1.0) Load P total, g m -2 d -1 Fig. 2. Removal rate of total phosphorus (TP) in different filters with respect to the load SHF SVF FWF SVF SHF FWF Under aerobic conditions with the variation from neutral to acid reaction, trivalent iron joins phosphates into stable complexes. Due to the change of conditions into the anaerobic ones when sand is flooded, trivalent iron is reduced into bivalent thus causing less adsorbtion and release of phosphates (Faulkner and Richardsson, 1989). This may explain rather little phosphorus removal efficiency in free water flow filters, because wastewater treatment processes take place under anaerobic conditions. Filters of horizontal flow are more efficient for phosphorus removal than the vertical ones because of the longer residence time of wastewater in the filter and because of the greater volume of sand used for the same amount of wastewater treatment. Ciupa (1996) has found that, at first, with the operation of plant filters phosphorus removal efficiency is quite high, then it decreases to the minimum. According to the data of our research, phosphorus treatment efficiency in vertical filters during the first two months exceeded 90.0%. Then it started decreasing and in 4-6 months set at the level of 40-60%. According to the Lithuanian regulations, when wastewater treatment level according to P is not regulate, the concentration of total P in wastewater released into surface water should not exceed 4.0 mg l -1 (Nuotekų ). In order to estimate the possible filter load not exceeding the allowable concentration, a regressive analysis on the dependence of total P concentration in filters of effluent wastewater with respect to its load was carried out. The results of the analysis are presented in Figure 3. According to the dependencies presented in the figure, it is possible to calculate the possible filter load not exceeding 4.0 mg l -1 of total P concentration in the filters after the treatment. In this case, it comes out that filter load according to P total in filters of horizontal flow (SHF) cannot exceed 0.38 g m -2 d -1, and in filters of free water flow (FWF) 0.26 g m -2 d -1. Whereas filters of vertical flow (SVF) failed to reach the wastewater treatment level according to total P lower than concentration of 4.0 mg l -1, because the initial wastewater pollution with phosphorus was twice bigger than in other filters. 210

5 20 18 Effluent P total, mg l y(fwf) = 7.82x R 2 = 0.39 (0<x<1.0) y(svf) = 4.72x R 2 = 0.32 (0<x<1.5) y(shf) = 7.53x R 2 = 0.49 (0<x<1.5) Load P total g m -2 d -1 Total P in wastewater occur in mineral and organic forms. Phosphorus of the organic form before treatment in filters constituted 21-27% from total P. Organic phosphorus in filters of horizontal flow constituted 21% before and 31% after the treatment from total P. It reveals that filters withheld more phosphorus of mineral form. Organic phosphorus in vertical filters constituted 27% before and 21% after the treatment. In free water flow filters, it constituted respectively 24 and 19%. Water treatment processes in filters of different construction take place under unequal conditions thus influencing phosphorus removal efficiency according the given phosphorus forms. Average Fig. 3. Concentration of total P in effluent with respect to the load of filters. SHF SVF FWF SHF SVF FWF values of total phosphorus and its forms in influent and effluent wastewater are presented in Table 1. The analysis of the research data shows that phosphorus removal in subsurface flow filters is influenced by conditions caused by filter construction for wastewater treatment. Removal efficiency of phosphorus of mineral form in filters of vertical and horizontal flow varied slightly, whereas phosphorus of organic form in vertical filters was removed by 3 times more, which was mostly influenced by the factor that wastewater treatment processes take place under aerobic conditions providing better decomposition of organic substances. Phosphorus removal efficiency in filters with respect to its form P total P-PO 4 P org. SVF Influent, mg l Effluent, mg l Removal efficiency, % SHF Influent, mg l Effluent, mg l Removal efficiency, % FWF Influent, mg l Effluent, mg l Removal efficiency, % Table 1 211

6 Conclusions Phosphorus removal efficiency depends on the construction and load of constructed wetlands according to total phosphorus for area unit of filter surface. Subsurface filters are distinguished by better phosphorus removal efficiency. It was determined that at the same load of all filters, i.e., TP 0.3 g m -2 d -1, the treatment efficiency of subsurface flow horizontal, vertical, and free water flow filters reaches 53.0, 46.5, and 28.0%, respectively. Aerobic wastewater treatment conditions, caused by the construction of filters vertical flow, result in a three times greater removal of organic phosphorus compared to horizontal filters. In order not to exceed the allowable concentration of 4.0 mg l -1 of total P in effluent, the load of horizontal flow filters (SHF) cannot exceed 0.38 and free water flow filters (FWF) 0.26 g m -2 d - 1. Filters of vertical flow (SVF) failed to reach the wastewater treatment level according to total P which has to be lower than concentration of 4.0 mg l -1, because the initial wastewater pollution with phosphorus was twice bigger than in other filters. In order to improve phosphorus removal efficiency, it is necessary to apply additional means. References 1. Andersson J.L., Kallner B.S. and Tonderski K.S. (2005) Free water surface wetlands for wastewater treatment in Sweden: nitrogen and phosphorus removal. Water Science and Technology, 51, (9), pp Arias A., Del Bubba M. C. and Brix H. (2001) Phosphorus removal by sands for use as media in subsurface flow constructed reed beds. Water Resources, 35, (5), pp Brix H., Arias C.A. and Bubba M. (2001) Media selection for sustainable phosphorus removal in subsurface flow constructed wetlands. Water Science and Technology, 44, (11-12), pp Ciupa R. (1996) The experience in the operation of constructed wetlands in north-eastern Poland. In: Preprints of Proceedings of the 5th International Conference on Wetland Systems for Water Pollution Control. IX/6, Sept. 1996, Vienna, Austria. 5. Cooper P. F., Job G. D., Green M. B. and Shutes R. B. E. (1996) Reed beds and constructed wetlands for wastewater treatment. WRc Publications, Medmenham, Marlow, UK, 206 p. 6. Del Bubba M. C., Arias A. and Brix H. (2003) Phosphorus adsorption maximum of sands for use as media in subsurface flow constructed reed beds as measured by the Langmuir isotherm. Water Resources, 37, (14), pp De Busk T.A. and Dierberg F.E. (1999) Techniques for optimizing phosphorus removal in treatment wetlands. In: Reedy et al. (Eds.). Phosphorus Biogeochemistry in Subtropical Ecosystems. Lewis Publishers/CRC Press, pp Faulkner S.P. and Richardsson C. J. (1989) Physical and chemical characteristics of freshwater wetland soils. In: Hammer D.A. and Freeman R.J. (Eds). Constructed Wetlands for Wastewater Treatment. Lewis Publishers, Chelsea, Michigan, pp Kadlec R.H. and Knight R.L. (1996) Treatment wetlands. CRC Press/Lewis Publishers, Boca Raton, Florida, 893 p. 10. Kim S. and Geary P.M. (2000) The impact of biomass harvesting on phosphorus uptake by wetland plants. In: KR Reddy, RH Kadlec (Eds.) Proceedings of the 7th International Conference on Wetland Systems for Water Pollution Control. November 11-16, Florida, USA. pp Nuotekų tvarkymo reglamentas (2007) (Regulation for Wastewater Treatment). Valstybės žinios, Nr (in Lithuanian). pp Richardson C.J. and Craft C.B. (1993) Efficient phosphorus retention in wetlands: fact or fiction? In: G.A. Moshhiri (Eds.) Constructed Wetlands for Water Quality Improvement. CRC Press, pp Verhoeven J.T.A. and Meuleman A.F.M. (1999) Wetlands for wastewater treatment: opportunities and limitations. Ecological Engineering, 12, pp