Various pilot-scale constructed wetland models have ...

2 downloads 0 Views 509KB Size Report
flow constructed wetlands are a new method for the treatment of activated ... wetlands. Their main advantages include low construction and operational cost,.
SWAT Conference 2008 ACTIVATED SLUDGE DEWATERING IN PILOT-SCALE, VERTICAL FLOW CONSTRUCTED WETLANDS

A.I. Stefanakis1, C.S. Akratos1, P. Melidis2 and V.A. Tsihrintzis1* 1

Laboratory of Ecological Engineering and Technology Laboratory of Wastewater Management and Treatment Technologies Department of Environmental Engineering, School of Engineering Democritus University of Thrace, 67100 Xanthi, Greece *E-mail: [email protected]; Tel./Fax: +30-25410-79393, 78113

2

ΠΕΡΙΛΗΨΗ Έντεκα πιλοτικές μονάδες τεχνητών υγροβιοτόπων κατακόρυφης ροής έχουν κατασκευαστεί και λειτουργούν από τον Σεπτέμβριο του 2007 για την επεξεργασία ενεργού ιλύος. Οι πιλοτικές μονάδες διαφέρουν μεταξύ τους στα κατασκευαστικά χαρακτηριστικά, όπως στη προέλευση και το μέγεθος του πορώδους υλικού, το είδος φύτευσης, τη φόρτιση λάσπης, την ύπαρξη σωλήνων αερισμού και την προσθήκη αλάτων χρωμίου. Περιέχουν ένα στρώμα στράγγισης από κροκάλες και δύο στρώματα από χαλίκι διαφορετικής διαβάθμισης. Στις πιλοτικές μονάδες εισάγεται ενεργός ιλύς από την Μονάδα Επεξεργασίας Υγρών Αποβλήτων (ΜΕΥΑ) της Κομοτηνής σε τρεις ρυθμούς φόρτισης (30, 60 και 75 kg/m2/yr). Η φόρτιση γίνεται για επτά συνεχείς μέρες ακολουθούμενες από τρεις εβδομάδες ανάπαυσης. Πριν την έναρξη κάθε περιόδου φόρτισης λαμβάνονται δείγματα από την ιλύ των πιλοτικών μονάδων για τον προσδιορισμό των αιωρούμενων στερεών (TS), των πτητικών στερεών (VS), του ολικού αζώτου (ΤΚΝ), του ολικού φωσφόρου (ΤP), και των νιτρωδών και νιτρικών ιόντων. Τα αποτελέσματα δείχνουν σημαντική αύξηση των TS, κάτι που υποδεικνύει ότι γίνεται ικανοποιητική αφυδάτωση της ιλύος και μείωση του όγκου της, καθώς και σημαντική μείωση των συγκεντρώσεων ΤΚΝ και TP, υποδεικνύοντας ότι επιτυγχάνεται ταυτόχρονα και ανοργανοποιήση της ιλύος. Λέξεις κλειδιά: Ενεργός ιλύς, τεχνητοί υγροβιότοποι κατακόρυφης ροής

1. Introduction Wastewater treatment using conventional technologies leads to the production of large amounts of activated sludge, whose treatment involves, among others, the removal of water to reduce volume for proper use or disposal. Vertical flow constructed wetlands are a new method for the treatment of activated sludge, which combines the traditional sludge drying beds and vertical flow constructed wetlands. Their main advantages include low construction and operational cost, simplicity and low-cost technology, and production of small volumes of wellcomposted biosolids, which can be used in agriculture. Sludge is fed onto the reed bed periodically, and becomes dewatered by percolation through earlier deposited layers of sludge and gravel drainage layers, transpiration by the reeds, and evaporation from the sludge surface (Edwards et al., 2001). With slow transfer of oxygen into the sludge layer, the sludge gradually becomes oxidized/mineralized, as observed through reduction in volatile solids, increase in percentage of fixed solids and gradual loss of moisture (Edwards et al., 2001). The percentage of total solids (TS) of the dewatered sludge can build up to 50% (Maesener, 1997). Most experience on reed bed dewatering comes from the work of Nielsen (1990; 1994; 2003), Lienard et al. (1990, 1994), De Maeseneer (1990), Hofmann (1990; 1995) and Edwards et al. (2001).

1

SWAT Conference 2008

2. Materials and methods 2.1 Experimental setup Various pilot-scale constructed wetland models have been constructed in our open-air laboratory, a facility with a total surface area of approximately 400 m 2 (Kotti and Tsihrintzis, 2006; Akratos and Tsihrintzis, 2007). A portion of the area is assigned to the experiments described here. Eleven circular pilot-scale, vertical flow constructed wetlands (S1-S11), of diameter 0.82 m and height 1.5 m, have been constructed using plastic cylindrical tanks (Figure 1). In all the units, a drainage layer made of cobbles (D50 = 90 mm), 15-cm thick, was placed first at the bottom of the tank. The drainage layer contains aeration tubes, which are PVC pipes (50 mm in diameter) perforated only within the drainage layer. Other porous media layers were placed on top of the drainage layer. For ten of the pilot-scale units (S1-S10), these layers include from bottom to top a 15-cm thick layer of medium gravel (D50 = 24.4 mm) and a 15 cm thick layer of fine gravel (D50 = 6 mm). For nine of these pilot-scale units the porous media were obtained from a river bed (S1, S3-S10) in the area (igneous rock material) and for one pilot-scale unit (S2) the porous media were obtained from a quarry (carbonate rock material). One pilot-scale unit (S11) has different sizes of porous media, which includes, above the cobbles layer, an additional 15-cm thick layer of cobbles (D50 = 90 mm) and a 15 cm thick layer of medium gravel (D50 = 24.4 mm). Two plant types were used, i.e., common reed (Phragmites) (S1, S2 and S5-S11) and cattails (Typha) (S3), and one unit was kept unplanted (S4). The pilot-scale units receive different rates of activated sludge, i.e., 30 kg/m2/yr (S5 and S8), 60 kg/m2/yr (S6 and S9) and 75 kg/m2/yr (S1-S4, S7, S10, S11). In three pilot-scale units, which receive different rates of activated sludge, chromium is added (S7-S9). Finally, one unit (S10) does not have aeration tubes. With this setup, conclusions can be drawn on the effect of activated sludge rate (comparison between units S1, S5, S6), effect of plant (comparison between S1, S3, S4), effect of porous medium material (comparison between S1, S2), effect of porous medium size (comparison between S1, S11), effect of aeration tubes (comparison between S1, S10), and effect of chromium presence (comparison between S7, S8, S9). The characteristics of the units are presented in Table 1. The units were constructed and planted in early June 2007; they were then filled with water, and left for the plants to grow and adjust to the new environment. From September 2007, activated sludge was introduced in the units and experiments started. Activated sludge was produced at the wastewater treatment plant (WWTP) of the municipality of Komotini, Rhodope Province, Greece, and transported to the laboratory at the start of each loading cycle. Activated sludge was introduced to the units for 7 days in daily equal portions, and then a resting period of 3 weeks was followed. 2.2 Sampling and analytical methods Sludge samples were collected at the end of the resting phase of every cycle using a core sampler. Samples were analyzed for TS (dewatering efficiency), VS (mineralization), TKN, N-NO3- and N-NO2- and TP, according to Standard Methods (APHA, 1998).

3. Results and discussion Results are presented here for 5 loading cycles (September 2007 until January 2008), for which the loading schedule was: 7 days loading / 21 days resting. Figure 2 presents time series charts for meteorological parameters (air temperature, solar heat flux and rainfall depth) and Figure 3 presents the time series charts for TS, VS, TP, TKN and nitrite and nitrate contents measured after each loading cycle in the whole sludge core sample. Although the pilot-scale units operated only in the winter months with rain and relative low temperature and solar heat flux values (i.e., low evapotransipiration), dewatering and oxidation processes were inhibited. TS values in all units showed an increase from the raw sludge value in all sampling dates with

2

SWAT Conference 2008 an exception of that of 28/10/07. The lower values of that day were due to the high rainfall during the previous days (Figure 2). The respective VS values did not show significant variation between the values in the raw sludge and in the pilot-scale units, which can be attributed again to rainfall and low evaportranspiration.

Figure 1. Photos from the pilot-scale unit construction: (a) 1st layer with medium gravel; (b) 2nd layer with fine gravel; (c) unit planted with common reed; (d) unit planted with cattails. Table 1. Pilot-scale unit characteristics. PilotPorous Plant Aeration scale media type tubes unit origin S1 River-bed Reed Yes S2 Quarry Reed Yes S3 River-bed Cattails Yes S4 River-bed Unplanted Yes S5 River-bed Reed Yes S6 River-bed Reed Yes S7 River-bed Reed Yes S8 River-bed Reed Yes S9 River-bed Reed Yes S10 River-bed Reed No S11 River-bed Reed Yes

Porous media size Fine Fine Fine Fine Fine Fine Fine Fine Fine Fine Coarse

Loading rate (kg/m2/yr) 75 75 75 75 30 60 75 30 60 75 75

Chromium presence

Yes Yes Yes

The excellent performance of mineralization in the pilot-scale units is indicated by the rather satisfactory decrease of TP and TKN contents in the accumulated sludge. Compared to the TP average value in the raw sludge, the TP content decreased in all pilot-scale units. The same trend also appeared in TKN concentrations, where TKN content decreased from the raw sludge in all pilot-scale

3

SWAT Conference 2008 units. Although an increase of the inorganic nitrogen contents would be expected as a result of the mineralization of the organic nitrogen, the concentrations of N-NO2and N-NO3- were also decreased in all units compared to the raw sludge, indicating a significant nitrogen up-take by the plants. Solar Energy Flux (W/m2)

35 Temperature (oC)

30 25 20 15 10 5 0 01/09/07 -5

01/10/07

31/10/07

30/11/07

30/12/07

250 200 150 100

29/01/08

50 0 01/09/07

Date

01/10/07

31/10/07

30/11/07

30/12/07

29/01/08

Date

70

Rainfall (mm)

60 50 40 30 20 10 0 01/09/07

01/10/07

01/11/07

01/12/07

01/01/08

Date

Figure 2. Time series charts for: (a) temperature; (b) solar heat flux; (c) rainfall depth. Concerning the effect of the different design parameters of the units on the quality of the accumulated sludge, it is obvious that even though the operation period is relatively short, some conclusions can be drawn. To compare the effect of porous media origin, pilot-scale units S1 and S2 are used. From Figure 3 it appears that S1 unit has only higher values of TS, while S2 shows lower values of VS, TKN, TP and nitrite and nitrate, i.e., the quarry material seems more effective. Comparing S1, S3 and S4 units the effect of plant type can be examined. S1 unit appears to have lower values of TKN, S3 has lower values of VS and TP, while S4 (the unplanted unit) has higher values of TS and lower values of nitrite and nitrate. The fact that the performance of the S4 unit is comparable with those of the planted units indicates that until now, because of the relative low temperature and solar heat flux values, the plants have not played yet a significant role in sludge treatment. In order to examine the effect of the presence of aeration tubes, a comparison between S1, S4 and S10 units is also made. From this comparison it is obvious that the planted unit without aeration tubes (S10) has higher values of TS and lower values of nitrite and nitrate while for the other pollutants the variations between these three units are not significant. These results indicate that the presence of the plants are imperative for these systems and cannot be substituted by aeration tubes. Comparing unit S1 and S11, results on the porous media size effect can be drawn. The S1 unit appears to have higher concentrations of TS and lower of VS and S11 unit appears to have lower concentration of TP, TKN and nitrite and nitrate. This can be explained by the fact that the coarser material in the S11 unit allows for better drainage of percolated fluid, resulting in lower concentration of the pollutants in the accumulated sludge and higher in the percolated fluid. In order to examine the effect of the three loading rates used in the experiment, a comparison between units S1 (75 kg/m2/yr), S5 (30 kg/m2/yr) and S6 (60 kg/m2/yr) is made. From Figure 3 it is obvious that S1 appears to have higher concentrations of TS, while S5 has lower concentration of TP, nitrite and nitrate. This result, especially when it occurs under relative low temperatures, leads to the conclusion that these systems can receive even higher loadings of activated sludge. For the three remaining units (S7, S8 and S9) which receive activated sludge with chromium added, results for the concentration of the heavy metals are not available yet. Comparing these three units with the others receiving

4

SWAT Conference 2008 the same loading rates (S1, S5, S6) for reproducibility it is obvious that with the exception of the first sampling date, where the function of the units was not yet stabilized, the performances of the units with the same loading rates (S1 and S7, S5 and S8, S6 and S9) are comparable. S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

In 100

90

90

80

80

70

70

60

60

VS (%TS)

TS (%)

In 100

50 40

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

50 40

30

30

20

20

10

10 0

0 14/10/2007

28/10/2007

25/11/2007

17/12/2007

14/10/2007

14/1/2008

28/10/2007

25/11/2007

In

S1

S2

S3

S4

S5

17/12/2007

14/1/2008

Date

Date

S6

S7

S8

S9

S10

S11

In

12

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

80 70

10 TKN (mg/g d.w.)

TP (mg/g d.w.)

60 8 6 4

50 40 30 20

2

10 0

0 14/10/2007

28/10/2007

25/11/2007

17/12/2007

14/10/2007

14/1/2008

28/10/2007

25/11/2007

17/12/2007

14/1/2008

Date

Date

In

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

S11

1600

1200

-

1000 800

-

NO2 +NO3 (μg/g d.w.)

1400

600 400 200 0 14/10/2007

28/10/2007

25/11/2007

17/12/2007

14/1/2008

Date

Figure 3. Time series charts for: (a) TS; (b) VS; (c) TP; (d) TKN; (e) N-(NO2-+ NO3-).

4. Conclusions Eleven pilot-scale, vertical flow constructed wetlands were constructed and are in operation. Current experimental results show several positive effects of constructed wetlands on wastewater sludge treatment, including increased TS content of the accumulated sludge and decreased concentrations of TP, TKN, nitrite and nitrate. Although the units were new and operated in a period of low temperatures, the percentages of sludge dewatering and the oxidation of organic compounds (TP and TKN) were at a satisfactory level. The accumulated sludge also showed a rather satisfactory mineralization, as the concentrations of TP and TKN were significantly decreased compared to the raw sludge values. Acknowledgements The study was funded by the General Secretariat of Research and Technology (GSRT) of Greece, as part of the project “Integrated Management of Sludge from Wastewater Treatment Facilities, and Wastewater Treatment Using Natural Systems”, Operational Program of the Region of East Macedonia–Thrace, 2000-2006.

5

SWAT Conference 2008 References Akratos C.S., V.A. Tsihrintzis, 2007. Effect of temperature, HRT, vegetation and porous media on removal efficiency of pilot-scale horizontal subsurface flow constructed wetlands. Ecol. Eng., 29: 173-191. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th edition. De Maeseneer J., 1990. Experience concerning the use of constructed wetlands for drying of wastewater sludge. Specialist Group on the Use pf Macrophytes. Water Pollution Control Newsletters, 3: 33-35. De Maesener J.L., 1997. Constructed wetlands for sludge dewatering. Wat. Sci. Tech., 35: 279-285. Edwards J.K., 2001. Reed bed dewatering of agricultural sludges and slurries. Wat. Sci. Tech., 44: 551-558. Hofmann K., 1990. Use of Phragmites in sewage sludge treatment. In: Constructed Wetlands, P.F. Cooper and B.C. Findlater (eds.), pp. 269-277. Pergamon Press. Hofmann K., 1995. Entwasserung und Vererdung von Klaerschlamm in Schlifbeeten. In: Naturnahe Abwasserreinigung, Bahlo K. and Wach G. (eds.), pp. 137, Oekobuch, Staufen bei Freiburg. Kotti E.P., V.A. Tsihrintzis, 2006. Surface-flow constructed wetland pilot-scale units for wastewater treatment, Proceedings of Conference Decentralized Wastewater Treatment Plants, 8-9 April, Portaria, Greece. Lienard A., P. Duchene, D. Gorini, 1994. A study of activated sludge dewatering in experimental reed-planted or unplanted sludge drying beds. Wat. Sci. Tech., 32: 251–261. Lienard A., D. Esser, A. Deguin, F. Virloget, 1990. Sludge dewatering and drying in reed beds: an interesting solution? In: Constructed Wetlands in Water Pollution Control, P.F. Cooper and B.C. Findlater (eds.), pp. 257-267, Pergamon Press, Oxford. Nielsen S.M., 1990. Sludge dewatering and mineralisation in reed bed systems. In: Constructed Wetlands in Water Pollution Control, P.F. Cooper and B.C. Findlater (eds.), pp. 245-255, Pergamon Press, Oxford. Nielsen S.M., 1994, Biological sludge drying in reed bed systems - six years of operating experiences. Proceedings of 4th International Conference on Wetland Systems for Water Pollution Control, Guangzhou, China, 6–10 November, 1994, pp. 447–457. Nielsen S.M., 2003. Sludge drying reed beds. Wat. Sci. Tech., 48: 101-109.

6