- Email: [email protected]
Long-term changes in sewage sludge stored in a reed bed
The Science of the Total Environment 297 (2002) 59–65
Long-term changes in sewage sludge stored in a reed bed Janusz Pempkowiaka,*, Hanna Obarska-Pempkowiakb a
Faculty of Construction and Environmental Engineering, Technical University of Koszalin, ul Raclawicka { 15y17, 75-620 Koszalin, Poland b ´ Faculty of Hydro and Environmental Engineering, Technical University of Gdansk, ul. Narutowicza 11y12, 80-952 Gdansk, Poland Received 10 August 2001; accepted 18 January 2002
Abstract The problem of the utilization and management of sewage sludge originating from small wastewater treatment plants is still unsolved. A common approach is to store the sludge in plots which in time turn into grassland. This investigation was aimed at evaluating the influence of the storage time in plots on the chemical properties of sewage sludge deposited there. Tests were carried out on samples obtained from discrete layers of stratified sludge that had lain in a hydrophyte facility disused for 7 years after 23 years of continuous sludge discharge. The age of the sludge was established by the lead-210 method. Moisture, organic matter, total nitrogen and total phosphorus contents were measured in samples of dated sewage sludge. The composition of the stored biosolids stabilized with respect to phosphorus, nitrogen and organic matter within 11, 15 and 17 years, respectively. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrophyte facility; Long-term storage; The lead method; Dating; Moisture; Organic matter; Nutrients
1. Introduction The treatment of sewage leads to the production of large amounts of a secondary waste product— sewage sludge. The quality of the sludge depends on the wastewater treatment technology, and excessive contents of pathogens, heavy metals and organic pollutants are of concern (Gray, 1989). While standard methods of processing sludge reduce its volume, efficient techniques for utilizing the dewatered sludge are still lacking. One possible solution is to use it as a soil fertilizer, but the use *Corresponding author. E-mail address: [email protected] (J. Pempkowiak).
of this is limited and in some cases even banned if the sludge contains heavy metals, persistent toxic organic substances and pathogenic microorganisms (Hoffman, 1990; Zwara and ObarskaPempkowiak, 2000). Only small quantities of sludge are used in the reclamation of post-industrial wastelands (Gray, 1989). In the rural areas of central Europe sewage has often been treated in small local plants, usually in septic tanks. In recent years, small biological wastewater treatment plants have been introduced. These are usually ground filters, filter-drainage systems and, for larger amounts of sewage, trickle filters and activated sludge systems. Nevertheless,
0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 0 2 3 - 2
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J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65
the primary and secondary sludge from these systems has to be disposed of. Since agricultural utilization is limited or banned, the filled-to-capacity storage plots remain unused, and in time turn into grassland (Zwara and Obarska-Pempkowiak, 2000). Recently, sludge storage in facilities colonized by plants, usually the common reed Phragmites communis, has been suggested as a means of resolving the problem. The presence of plants intensifies dewatering processes (Hoffman, 1990; ´ Lienard et al., 1995; De Maesener, 1997; ObarskaPempkowiak et al., 1997; Zwara and ObarskaPempkowiak, 2000). The hydrophyte method is applicable under a variety of conditions (Burgoon et al., 1997), and it has been suggested that the resulting biosolids could be used in agriculture after 10 years (De Maesener, 1997; Burgoon et al., 1997). Analysis of operation showed hydrophyte systems to be highly efficient in dewatering sludge within a matter of weeks, though few if any changes in the nutrient content were recorded (Nielsen, 1993; Reed et al., 1995). Within months, however, nutrients contents decreased, while pathogens increased (De Maesener, 1997; Zwara and Obarska-Pempkowiak, 2000). One question which has yet to be answered is the extent to which the dewatered sludge undergoes chemical changes in the course of prolonged storage. The aim of this study was to assess the changes in moisture, organic matter and nutrient contents in sewage sludge stored at a disused hydrophyte facility over a period from 7 to 21 years. The measurements were intended to indicate the importance of the time factor with respect to the extent of sludge dewatering, nutrient dynamics and decomposition of organic matter contained in the sludge. The actual age of the investigated sludge samples was established by means of the Pb-210 method and Caesium-137 profiles. 2. Methods 2.1. Sampling site The study was carried out in the former natural wetland system near Jastarnia (Hel Peninsula,
northern Poland), the location of the facility for treating liquid and semi-liquid sewage sludge brought in from Jastarnia, Jurata, Kuznice and ´ environs. Close to the shore of the Gulf of Gdansk, southern Baltic, the facility was in operation from 1968 to 1991. The storage fields were located on former wet meadows; beneath these, the soil consisted of fine-grained humus sands or sandy muds with admixtures of peat. Drainage ditches had been dug to enable percolating sewage to discharge into Puck Bay, while solids settled and remained in the fields. While the facility (15 ha in area) was operational, the daily discharge of sewage sludge was 1400 m3 yd (17.5 mmym2 d) in summer, and 400 m3 yd (4.7 mmym2 d) during the rest of the year, a total of 120 000 m3 per year (Zwara and Obarska-Pempkowiak, 2000). The load of solids is estimated at 1700 t per year (21.25 kgy m2 year). Self-seeding reeds (P. communis) entered the system while the facility was still in operation. Coverage was patchy—the reeds never formed a continuous, dense bed. Within a few years of the cessation of operations, however, grasses colonized the fields, and bushes are now slowly beginning to take over. The area is particularly attractive to birds. The sampling station for the stratified samples of stored material was located in the north-east corner of the field, used between 1978 and 1991. The area is bare of plants except for grass. Three vertical profiles—sludge cores—were collected from a 2 m2 area. Every effort was taken as to preserve the stratification of the sediments. Holes 40 cm deep were dug to expose profiles of the stored sludge and the underlying sand. Transparent Plexiglass tubes (8.0 cm in diameter=40 cm in length) were carefully hand-pushed into the sediments down to a depth of 30 cm. The cores were retrieved, the contents of the tubes were pistonpushed up, and cut into slices of suitable thickness on emerging from the tube (slices were 2.0 cm thick down to a depth of 20 cm below the airy solid surface interface, and 1.0 cm thick in the layer 20–24 cm). During collection and cutting, no compaction of the solids was noticed. Samples originating from the same layer were combined (some 50 g of wet material was collected from
J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65
Fig. 1. Moisture and organic matter vs. depth in the stratified sludge stored in a hydrophyte facility.
each layer), homogenized, transferred into plastic bags, transported to the laboratory and kept in a deep-freeze for analysis. 2.2. Determination of physical and chemical properties The following physical and chemical properties of the samples were determined: moisture, loss on ignition (Fig. 1), concentrations of total nitrogen and total phosphorus, activities of 210Pb and 137Cs (Fig. 2). Since the measured contents of N–NOy 3 and N–NOy 2 indicated that these forms of nitrogen were not present in the sludge, the total nitrogen measurements were limited to the determination of Kjeldahl nitrogen. The water content (moisture) of the sludge was equal to the loss on drying at 105 8C (Hermanowicz et al., 2000). The total concentrations of organic and mineral substances were determined following ignition of the previously dried sludge samples at 500 8C for 6 h. It was assumed that the loss on ignition represents the content of organic substances and the residue after ignition corresponds to the mineral matrix content. Nitrogen was determined after the samples had been dried at 105 8C. The Kjeldahl nitrogen (the sum of organic and ammonia nitrogen) was determined after wet digestion with concentrated sulphuric acid catalyzed with CuSO4qK2SO4.
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Following wet digestion of the samples with a mixture of H2SO4 and HNO3, total phosphorus was determined colorimetrically using ammonium molybdate in the presence of glycine and SnCl2. The lead-210 method was used to determine the time that had elapsed since the deposition of the successive sludge layers. The activity of 210Pb was determined by measuring 210Po according to the method described in Pempkowiak (1991). In brief, the method consists of the following steps: 0.750 g d.m. of each sample is wet digested, (HNO3:HClO4:HFs3:1:3), excess acids are evaporated, the dry residue is dissolved in 0.1 moly dm3 HNO3, and 210Po is spontaneously deposited on silver discs. The activity of the deposited 210Po was measured in a P 1024 Multichannel Analyser (Polon) coupled with a Si surface barrier detector (Canberra). The yield, measured by 209Po (Amersham) addition prior to digestion, ranged from 83.2 to 110.4% (92.5%"6.2%). An equilibrium between 210Pb and 210Po was assumed to exist. Repeated measurements of 210Po in the samples after 7 months of ingrowth following the initial extraction of 210Po proved no statistically significant differences between the first and the second measurements, validating the assumption. 226Ra was measured using the 222Rn emanation method adopted form Mathieu et al. (1988). 137Cs was determined by g-spectrometry with HPGe detector with a relative efficiency of 30%.
Fig. 2. Activities of 210Pb and 137Cs vs. depth in the stratified sludge stored in a hydrophyte facility.
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2.3. Dating method Radium-226 (half-lifes1.640 years), present in rocks, decays to radon-222 (half-lifes3.84 days). Some of this diffuses into the atmosphere and decays to lead-210 (half-lifes22.3 years), which is then rapidly flushed out to the Earth’s surface. This process is believed to bring about a roughly constant flux of 210Pb, the so-called excess 210Pb (Appleby et al., 1979). Deposited lead remains closely associated with solid matrices (Robbins, 1978). Once the layer of solids is covered by fresh material, the flux of excess 210Pb stops and this undergoes b-disintegration (T1y2s22.4 years). The current age since deposition can be calculated from the 210Pb activity vs depth profile (Appleby et al., 1979). The so-called lead-210 method has been successfully applied to the dating of lake sediments (Krisnaswami et al., 1971), near-shore marine sediments (Koide et al., 1973), ice cores (Crozaz and Langway, 1966), salt marsh sediments (Sharma et al., 1987), and growth rate of a deep-sea coral (Druffel et al., 1990). Dating solid materials accumulating at the Earth’s surface by the 210Pb method requires defining the flux of excess radiolead in time, F (Robbins, 1978; Oldfield and Appleby, 1984; Joshi and Shukla, 1991). Assuming a constant rate of sludge accumulation r, and taking into account the fact that the storage time t can be defined as tsOm yr (Om, the mass of sludge accumulated above a depth m), the activity of excess 210Pb at a depth m is given by the following equation: AmsFØeylOmyr yr where l is the disintegration constant of 210 Pb. Since, by definition, F is constant in time (Robbins, 1978), and assuming that the rate of sludge accumulation r is constant, the expression F yr yields the specific activity of excess 210Pb in the sludge at the surface (at instant ts0). A0sFyr The relationship between the surface load of the dry mass of sludge (Om), the excess 210Pb specific activity at the depth m, Am, the specific activity at
the surface, A0, and the age t of the layer m can be expressed by the following equation: 1 w Am z tsOm yrsy lnx |, l y A0 ~ while the disintegration rate of 210Pb is determined by the equation AmsA0ØeylOmyr. This equation determines the distribution of Pb in the vertical sludge profile, assuming a constant inflow of 210Pb, a constant sedimentation rate, and no mixing. In case of variable sedimentation of material having constant specific activity (A0sconst; r/const), the depth–age relationship remains the same (Robbins, 1978). If the 210Pb profile has been disturbed in surface layers, only part of the profile beneath the disturbed layer can be used for dating, and the least squares method is used for this purpose (Robbins, 1978). The age of a given layer of the undisturbed part of profile calculated in this way is then added to the time necessary for the disturbed layer to accumulate (Pempkowiak et al., 1995). The procedure was used successfully for undisturbed (Pempkowiak, 1991), and disturbed (Pempkowiak et al., 1995) marine, and lacustrine (Andersen and Pempkowiak, 1999), sediments. Calculations were performed on data from Table 1 except organic matter contents. Those were adjusted to the value measured in the uppermost sewage sludge layer to account for the loss of mass due to mineralization. The results showed that the sludge at depths between 7 and 19 cm of the sampled profile had been discharged over a period of 14 years. Since the facility had been disused for 7 years before the samples were collected in 1998, one can assume that it had lain there from 1977 (18–20 cm layer) to 1991 (6–8 cm layer). The sampling station where the sludge cores were collected was located at the edge of the depositional area where no sludge had been deposited in the period 1965–1978 and after 1991 (Zwara and Obarska-Pempkowiak, 2000). This is in good agreement with the results of the dating. 210
J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65 Table 1 Contents of water, organic matter, nutrients and activities of
137
Cs and
63
210
Pb in stratified sewage sludge stored in Jastarnia
Depth (cm)
Organic matter (% d.m.)
Moisture (%)
Ntotal (% d.m.)
Ptotal (% d.m.)
210
Pbexcess (mBqyg)
Cs (mBqyg)
0–2 2–4 4–6 6–8 8–10 10–12 12–14 14–16 16–18 18–20 20–21 21–22 22–23 23–24
40.6 40.1 48.5 50.2 45.8 41.9 38.6 37.0 36.6 36.8 32.5 18.2 14.3 16.4
46.7 48.5 49.7 48.6 44.8 44.1 40.2 38.7 39.6 40.9 36.1 28.0 26.3 30.1
3.05 3.10 3.81 3.86 3.25 2.85 2.54 2.52 2.44 2.60 2.52 1.94 1.70 1.72
0.35 0.34 0.55 0.51 0.34 0.28 0.26 0.24 0.23 0.24 0.22 0.16 0.14 0.12
58.8"5.9 46.2"4.3 38.9"3.1 31.1"2.1 25.8"2.0 23.6"2.7 21.5"2.0 20.3"1.3 19.4"2.0 18.1"1.5 20.6"1.7 35.9"2.0 32.7"2.4 24.4"2.3
18"3 18"6 20"4 23"3 26"5 28"3 28"4 32"1 32"4 26"3 n.b.a 12"5 n.b.a 16"4
a
137
n.b., not measured.
3. Results The measurements are given in Table 1. The organic matter content in the stratified sludge sample varies from 50.2% in the sludge layer collected from a depth of 6–8 cm below the surface to 14.3% in the 22–23 cm layer. The moisture content changes accordingly—it was highest in the 4–6 cm layer (49.7%), close to the maximum in the 6–8 cm layer (48.6%), and least in the 22–23 cm layer (26.3%). The value of Ntotal was largest (3.86%) in the 6–8 cm layer, and was least (1.70%) in the 22– 23 cm layer. The largest values of Ptotal (0.55 and 0.51%) were recorded in the 4–6 cm and 6–8 cm layers, respectively; Ptotal was smallest (0.12%) in the 23–24 cm layer. In general, the contents of organic matter, nutrients and radioceasium in the top 6 cm of the profile were smaller than those in the subsurface layer. The top 6 cm of the profile consists of reed stalks, tree leaves, dry grass, sand from nearby beaches and wind-driven dirt. The organic matter and nutrient contents were highest in the subsurface layer of sludge (6–8 cm). From that layer downwards, the contents decreased steadily as far as 20 cm. At this point there was a qualitative change in all the measured parameters: the material below the 20 cm boundary contained increasing
proportions of fine sand. The peak in 210Pb activity at the 21–22 cm layer may be caused by sorption of 210Pb from percolating sewage at the early stages of the facility operation. Therefore, neither the 0–6 cm nor the 20–24 cm layers of the profile were included in the discussion of the changes that sludge may undergo during storage. 4. Discussion From a depth of 6 cm below the surface downwards the organic matter content decreases at an ever slower rate to reach 36% at 15 cm. The organic matter deposited at this depth is 17 years old. Few if any changes occurred in the sewage sludge layer dated to 17–21 years (15–19 cm). It is therefore assumed that the organic matter at 15 cm becomes stable under the influence of temperature, microflora, moisture, and the prevailing sludge composition. The moisture content of raw sludge varies from 96 to 98%. After settlement, this content drops to 60% (Gray, 1989). In sludge that had lain in situ for 3 years, the moisture fell to 52%, and the organic matter content was approximately 35% (De Maesener, 1997; Zwara and Obarska-Pempkowiak, 2000). Little is known of moisture and organic matter changes in the course of further sewage sludge storage. In this particular study, the
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J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65
Fig. 3. Total nitrogen (Nog) and total phosphorous (Pog) vs. depth in the stratified sludge stored in a hydrophyte facility.
moisture content was highest in the 6–8 cm layer, which represents 7-year-old sludge. This is nevertheless somewhat lower than that in sewage sludge stored for 3 years in a reed bed, while the corresponding organic matter content is somewhat higher (Zwara and Obarska-Pempkowiak, 2000). This, very probably, reflects the changes imposed by a longer storage time andyor the specific origin and composition of the sludge. The content of Ntotal and especially of Ptotal decrease at a faster rate than that of organic matter (Fig. 3). This is best relected in the Org mattery Ntot profile shown in Fig. 4. In the uppermost
layers, the OMyNtot ratio increases from 12.7 to 14.1 as a result of the ammonification of labile organic nitrogen. In the bottom layers, the ratio stabilizes at 15.0"0.9, indicating that a pool of organic nitrogen more resistant to ammonification is consumed during the mineralization of sludge stored for approximately 15 years. Since little NNH4q was found in the samples it must be assumed that inorganic nitrogen had been washed out of the sludge profile, as in the case with sewage sludge stored for 3 years (Zwara and Obarska-Pempkowiak, 2000). The Ptot profile differs substantially from that of Ntot. There is a more than twofold decrease in Ptot during storage (Table 1, Fig. 3), while the respective decreases in Ntot and OM are 34 and 25%. This indicates that the pool of labile organic phosphorus susceptible to mineralization makes up nearly half of the sludge’s organic phosphorus content. Most of it is readily mineralized, since Ptotal decreases rapidly in the uppermost layer of sewage sludge. This is best manifested by the OMyPtotal and Ntotal yPtotal ratios (Fig. 4), both of which rise from 93"8 and 7.3"0.5 in the uppermost layer (7–14 years of storage) to 149"2 and 11.3"0.6 in the lowest layer of sewage sludge (17–21 years of storage). The 16–20 cm layer of the profile stored for 17–21 years, contains sludge which appears to be stable in terms of both organic matter and nutrients contents. 5. Conclusions A 21-year storage period leads to distinct changes in the chemical composition of sewage sludge. This is best manifested by the decrease in moisture (from 46 to 41%), organic matter content (49y 37%), Ntotal (3.86y2.5%), Ptot (0.51y0.23%) and increasing ratios of OMyPtot (to the value of 149"2), Ntotal yPtotal (20"4), and OMyNtot (15.0"0.9). References
Fig. 4. Ratios of Org.mat.yNtot, Org.matyPtot , and NtotyPtot vs. depth in the stratified sludge stored in a hydrophyte facility.
Andersen D, Pempkowiak J. Sediment content of metals before and after lake water liming. Sci. Tot. Environ. 1999;243: 107 –118. Appleby P, Oldfield F, Thompson R, Huttunen P, Tolanen K. 210 Pb dating of annually laminated lake sediments from Finland. Nature 1979;280:53 –55.
J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65 Burgoon P, Kirkbridge K, Henderson M. Reed beds for biosolids during in the arid northwestern United States. Wat. Sci. Technol. 1997;35:287 –292. Crozaz G, Langway L. Dating Greenland firn-ice cores with 210 Pb. Earth Plan. Sci. Lett. 1966;1:194 –196. De Maesener J. Constructed wetlands for sludge dewatering. Wat. Sci. Technol. 1997;35:279 –285. Druffel R, King L, Belastock R, Buesseler K. Growth rate of a deep-sea coral using 210Pb and other isotopes. Geochim. Cosmochim. Acta 1990;54:1493 –1500. Gray NF. Biology of wastewater treatment. Oxford: Oxford University Press, 1989. p. 828. ´ Hermanowicz W, Dozanska W, Dojlido W, Kozierski B. ´ ´ Fizyczne i chemiczne metody badania wody i sciekow. Warszwa: Arkady, 2000. p. 847. Hoffman K. Use of Phragmites in sewage sludge treatment. Adv. Wat. Pollut. Control 1990;11:269 –277. Joshi B, Shukla B. Ab initio derivation of formulations for Pb-210 dating of sediments. J. Radioanal. Nucl. Chem. 1991;148:73 –79. Koide M, Bruland K, Goldberg E. 228Thy232Th and 210Pb geochronologies in marine and lake sediments. Geochim. Cosmochim. Acta 1973;37:1171 –1187. Krisnaswami S, Lal D, Martin J, Meybeck M. Geochronology of lake sediments. Earth Planet. Sci. Lett. 1971;11:307 – 318. ´ ´ Ph, Gorni D. A study of activated sludge Lienard A, Duchene dewatering in experimental reed-planted or unplanted sludge drying beds. Wat. Sci. Technol. 1995;32:251 –261. Mathieu G, Biscaye P, Lupton R, Hammond D. System for measurement of 222Rn at low levels in natural waters. Health Phys. 1988;55:989 –992.
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Nielsen SM. Biological sludge drying in constructed wetlands. In: Moshihiri GA, editor. Constructed wetlands for water quality improvement. Boca Raton: Lewis Publishers, 1993. p. 549 –558. Obarska-Pempkowiak H, Zwara W, Cytawa S. Utilisation of sewage sludge in reed beds and willow plantations. In: Bien´ J, editor. Wastewater sludge: waste or resource?, vol. 2. Cze˛stochowa: Technical University, 1997. p. 217 –222. x Oldfield F, Appleby P. Empirical testing of 210Pb dating models for lake sediments. In: Haworth E, Lund J, editors. Lake sediments and environmental history. Minneapolis, MN: University of Minnesota Press, 1984. p. 93 –124. Pempkowiak J. Enrichment factors of heavy metals in the Baltic surface sediments dated with 210Pb and 137Cs. Environ. Inter. 1991;17:421 –428. ˙ Pempkowiak J, Kozuch J, Szymelfenig M. Meiobenthic organisms as indicators of mixing processes in the Baltic surface sediments. PSZNI Mar. Ecol. 1995;17:175 –178. Reed SC, Crites RW, Middlebrooks EJ. Natural systems for waste management and treatment. New York: McGraw-Hill, 1995. p. 454. Robbins J. Geochemical and geophysical application of radioactive lead. In: Nriagu J, editor. The biogeochemistry of lead in the environment. Amsterdam: Elsevier, 1978. p. 285 –393. Sharma P, Gardner L, Moore W, Bollinger M. Sedimentation and bioturbation in the soil marsh as revealed by 210Pb, 137 Cs and 7Be studies. Limnol. Oceanogr. 1987;32:313 –326. Zwara W, Obarska-Pempkowiak H. Polish experience with sewage sludge utilization in reed beds. Wat. Sci. Technol. 2000;41:65 –68.
Long-term changes in sewage sludge stored in a reed bed Janusz Pempkowiaka,*, Hanna Obarska-Pempkowiakb a
Faculty of Construction and Environmental Engineering, Technical University of Koszalin, ul Raclawicka { 15y17, 75-620 Koszalin, Poland b ´ Faculty of Hydro and Environmental Engineering, Technical University of Gdansk, ul. Narutowicza 11y12, 80-952 Gdansk, Poland Received 10 August 2001; accepted 18 January 2002
Abstract The problem of the utilization and management of sewage sludge originating from small wastewater treatment plants is still unsolved. A common approach is to store the sludge in plots which in time turn into grassland. This investigation was aimed at evaluating the influence of the storage time in plots on the chemical properties of sewage sludge deposited there. Tests were carried out on samples obtained from discrete layers of stratified sludge that had lain in a hydrophyte facility disused for 7 years after 23 years of continuous sludge discharge. The age of the sludge was established by the lead-210 method. Moisture, organic matter, total nitrogen and total phosphorus contents were measured in samples of dated sewage sludge. The composition of the stored biosolids stabilized with respect to phosphorus, nitrogen and organic matter within 11, 15 and 17 years, respectively. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrophyte facility; Long-term storage; The lead method; Dating; Moisture; Organic matter; Nutrients
1. Introduction The treatment of sewage leads to the production of large amounts of a secondary waste product— sewage sludge. The quality of the sludge depends on the wastewater treatment technology, and excessive contents of pathogens, heavy metals and organic pollutants are of concern (Gray, 1989). While standard methods of processing sludge reduce its volume, efficient techniques for utilizing the dewatered sludge are still lacking. One possible solution is to use it as a soil fertilizer, but the use *Corresponding author. E-mail address: [email protected] (J. Pempkowiak).
of this is limited and in some cases even banned if the sludge contains heavy metals, persistent toxic organic substances and pathogenic microorganisms (Hoffman, 1990; Zwara and ObarskaPempkowiak, 2000). Only small quantities of sludge are used in the reclamation of post-industrial wastelands (Gray, 1989). In the rural areas of central Europe sewage has often been treated in small local plants, usually in septic tanks. In recent years, small biological wastewater treatment plants have been introduced. These are usually ground filters, filter-drainage systems and, for larger amounts of sewage, trickle filters and activated sludge systems. Nevertheless,
0048-9697/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 0 2 . 0 0 0 2 3 - 2
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J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65
the primary and secondary sludge from these systems has to be disposed of. Since agricultural utilization is limited or banned, the filled-to-capacity storage plots remain unused, and in time turn into grassland (Zwara and Obarska-Pempkowiak, 2000). Recently, sludge storage in facilities colonized by plants, usually the common reed Phragmites communis, has been suggested as a means of resolving the problem. The presence of plants intensifies dewatering processes (Hoffman, 1990; ´ Lienard et al., 1995; De Maesener, 1997; ObarskaPempkowiak et al., 1997; Zwara and ObarskaPempkowiak, 2000). The hydrophyte method is applicable under a variety of conditions (Burgoon et al., 1997), and it has been suggested that the resulting biosolids could be used in agriculture after 10 years (De Maesener, 1997; Burgoon et al., 1997). Analysis of operation showed hydrophyte systems to be highly efficient in dewatering sludge within a matter of weeks, though few if any changes in the nutrient content were recorded (Nielsen, 1993; Reed et al., 1995). Within months, however, nutrients contents decreased, while pathogens increased (De Maesener, 1997; Zwara and Obarska-Pempkowiak, 2000). One question which has yet to be answered is the extent to which the dewatered sludge undergoes chemical changes in the course of prolonged storage. The aim of this study was to assess the changes in moisture, organic matter and nutrient contents in sewage sludge stored at a disused hydrophyte facility over a period from 7 to 21 years. The measurements were intended to indicate the importance of the time factor with respect to the extent of sludge dewatering, nutrient dynamics and decomposition of organic matter contained in the sludge. The actual age of the investigated sludge samples was established by means of the Pb-210 method and Caesium-137 profiles. 2. Methods 2.1. Sampling site The study was carried out in the former natural wetland system near Jastarnia (Hel Peninsula,
northern Poland), the location of the facility for treating liquid and semi-liquid sewage sludge brought in from Jastarnia, Jurata, Kuznice and ´ environs. Close to the shore of the Gulf of Gdansk, southern Baltic, the facility was in operation from 1968 to 1991. The storage fields were located on former wet meadows; beneath these, the soil consisted of fine-grained humus sands or sandy muds with admixtures of peat. Drainage ditches had been dug to enable percolating sewage to discharge into Puck Bay, while solids settled and remained in the fields. While the facility (15 ha in area) was operational, the daily discharge of sewage sludge was 1400 m3 yd (17.5 mmym2 d) in summer, and 400 m3 yd (4.7 mmym2 d) during the rest of the year, a total of 120 000 m3 per year (Zwara and Obarska-Pempkowiak, 2000). The load of solids is estimated at 1700 t per year (21.25 kgy m2 year). Self-seeding reeds (P. communis) entered the system while the facility was still in operation. Coverage was patchy—the reeds never formed a continuous, dense bed. Within a few years of the cessation of operations, however, grasses colonized the fields, and bushes are now slowly beginning to take over. The area is particularly attractive to birds. The sampling station for the stratified samples of stored material was located in the north-east corner of the field, used between 1978 and 1991. The area is bare of plants except for grass. Three vertical profiles—sludge cores—were collected from a 2 m2 area. Every effort was taken as to preserve the stratification of the sediments. Holes 40 cm deep were dug to expose profiles of the stored sludge and the underlying sand. Transparent Plexiglass tubes (8.0 cm in diameter=40 cm in length) were carefully hand-pushed into the sediments down to a depth of 30 cm. The cores were retrieved, the contents of the tubes were pistonpushed up, and cut into slices of suitable thickness on emerging from the tube (slices were 2.0 cm thick down to a depth of 20 cm below the airy solid surface interface, and 1.0 cm thick in the layer 20–24 cm). During collection and cutting, no compaction of the solids was noticed. Samples originating from the same layer were combined (some 50 g of wet material was collected from
J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65
Fig. 1. Moisture and organic matter vs. depth in the stratified sludge stored in a hydrophyte facility.
each layer), homogenized, transferred into plastic bags, transported to the laboratory and kept in a deep-freeze for analysis. 2.2. Determination of physical and chemical properties The following physical and chemical properties of the samples were determined: moisture, loss on ignition (Fig. 1), concentrations of total nitrogen and total phosphorus, activities of 210Pb and 137Cs (Fig. 2). Since the measured contents of N–NOy 3 and N–NOy 2 indicated that these forms of nitrogen were not present in the sludge, the total nitrogen measurements were limited to the determination of Kjeldahl nitrogen. The water content (moisture) of the sludge was equal to the loss on drying at 105 8C (Hermanowicz et al., 2000). The total concentrations of organic and mineral substances were determined following ignition of the previously dried sludge samples at 500 8C for 6 h. It was assumed that the loss on ignition represents the content of organic substances and the residue after ignition corresponds to the mineral matrix content. Nitrogen was determined after the samples had been dried at 105 8C. The Kjeldahl nitrogen (the sum of organic and ammonia nitrogen) was determined after wet digestion with concentrated sulphuric acid catalyzed with CuSO4qK2SO4.
61
Following wet digestion of the samples with a mixture of H2SO4 and HNO3, total phosphorus was determined colorimetrically using ammonium molybdate in the presence of glycine and SnCl2. The lead-210 method was used to determine the time that had elapsed since the deposition of the successive sludge layers. The activity of 210Pb was determined by measuring 210Po according to the method described in Pempkowiak (1991). In brief, the method consists of the following steps: 0.750 g d.m. of each sample is wet digested, (HNO3:HClO4:HFs3:1:3), excess acids are evaporated, the dry residue is dissolved in 0.1 moly dm3 HNO3, and 210Po is spontaneously deposited on silver discs. The activity of the deposited 210Po was measured in a P 1024 Multichannel Analyser (Polon) coupled with a Si surface barrier detector (Canberra). The yield, measured by 209Po (Amersham) addition prior to digestion, ranged from 83.2 to 110.4% (92.5%"6.2%). An equilibrium between 210Pb and 210Po was assumed to exist. Repeated measurements of 210Po in the samples after 7 months of ingrowth following the initial extraction of 210Po proved no statistically significant differences between the first and the second measurements, validating the assumption. 226Ra was measured using the 222Rn emanation method adopted form Mathieu et al. (1988). 137Cs was determined by g-spectrometry with HPGe detector with a relative efficiency of 30%.
Fig. 2. Activities of 210Pb and 137Cs vs. depth in the stratified sludge stored in a hydrophyte facility.
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2.3. Dating method Radium-226 (half-lifes1.640 years), present in rocks, decays to radon-222 (half-lifes3.84 days). Some of this diffuses into the atmosphere and decays to lead-210 (half-lifes22.3 years), which is then rapidly flushed out to the Earth’s surface. This process is believed to bring about a roughly constant flux of 210Pb, the so-called excess 210Pb (Appleby et al., 1979). Deposited lead remains closely associated with solid matrices (Robbins, 1978). Once the layer of solids is covered by fresh material, the flux of excess 210Pb stops and this undergoes b-disintegration (T1y2s22.4 years). The current age since deposition can be calculated from the 210Pb activity vs depth profile (Appleby et al., 1979). The so-called lead-210 method has been successfully applied to the dating of lake sediments (Krisnaswami et al., 1971), near-shore marine sediments (Koide et al., 1973), ice cores (Crozaz and Langway, 1966), salt marsh sediments (Sharma et al., 1987), and growth rate of a deep-sea coral (Druffel et al., 1990). Dating solid materials accumulating at the Earth’s surface by the 210Pb method requires defining the flux of excess radiolead in time, F (Robbins, 1978; Oldfield and Appleby, 1984; Joshi and Shukla, 1991). Assuming a constant rate of sludge accumulation r, and taking into account the fact that the storage time t can be defined as tsOm yr (Om, the mass of sludge accumulated above a depth m), the activity of excess 210Pb at a depth m is given by the following equation: AmsFØeylOmyr yr where l is the disintegration constant of 210 Pb. Since, by definition, F is constant in time (Robbins, 1978), and assuming that the rate of sludge accumulation r is constant, the expression F yr yields the specific activity of excess 210Pb in the sludge at the surface (at instant ts0). A0sFyr The relationship between the surface load of the dry mass of sludge (Om), the excess 210Pb specific activity at the depth m, Am, the specific activity at
the surface, A0, and the age t of the layer m can be expressed by the following equation: 1 w Am z tsOm yrsy lnx |, l y A0 ~ while the disintegration rate of 210Pb is determined by the equation AmsA0ØeylOmyr. This equation determines the distribution of Pb in the vertical sludge profile, assuming a constant inflow of 210Pb, a constant sedimentation rate, and no mixing. In case of variable sedimentation of material having constant specific activity (A0sconst; r/const), the depth–age relationship remains the same (Robbins, 1978). If the 210Pb profile has been disturbed in surface layers, only part of the profile beneath the disturbed layer can be used for dating, and the least squares method is used for this purpose (Robbins, 1978). The age of a given layer of the undisturbed part of profile calculated in this way is then added to the time necessary for the disturbed layer to accumulate (Pempkowiak et al., 1995). The procedure was used successfully for undisturbed (Pempkowiak, 1991), and disturbed (Pempkowiak et al., 1995) marine, and lacustrine (Andersen and Pempkowiak, 1999), sediments. Calculations were performed on data from Table 1 except organic matter contents. Those were adjusted to the value measured in the uppermost sewage sludge layer to account for the loss of mass due to mineralization. The results showed that the sludge at depths between 7 and 19 cm of the sampled profile had been discharged over a period of 14 years. Since the facility had been disused for 7 years before the samples were collected in 1998, one can assume that it had lain there from 1977 (18–20 cm layer) to 1991 (6–8 cm layer). The sampling station where the sludge cores were collected was located at the edge of the depositional area where no sludge had been deposited in the period 1965–1978 and after 1991 (Zwara and Obarska-Pempkowiak, 2000). This is in good agreement with the results of the dating. 210
J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65 Table 1 Contents of water, organic matter, nutrients and activities of
137
Cs and
63
210
Pb in stratified sewage sludge stored in Jastarnia
Depth (cm)
Organic matter (% d.m.)
Moisture (%)
Ntotal (% d.m.)
Ptotal (% d.m.)
210
Pbexcess (mBqyg)
Cs (mBqyg)
0–2 2–4 4–6 6–8 8–10 10–12 12–14 14–16 16–18 18–20 20–21 21–22 22–23 23–24
40.6 40.1 48.5 50.2 45.8 41.9 38.6 37.0 36.6 36.8 32.5 18.2 14.3 16.4
46.7 48.5 49.7 48.6 44.8 44.1 40.2 38.7 39.6 40.9 36.1 28.0 26.3 30.1
3.05 3.10 3.81 3.86 3.25 2.85 2.54 2.52 2.44 2.60 2.52 1.94 1.70 1.72
0.35 0.34 0.55 0.51 0.34 0.28 0.26 0.24 0.23 0.24 0.22 0.16 0.14 0.12
58.8"5.9 46.2"4.3 38.9"3.1 31.1"2.1 25.8"2.0 23.6"2.7 21.5"2.0 20.3"1.3 19.4"2.0 18.1"1.5 20.6"1.7 35.9"2.0 32.7"2.4 24.4"2.3
18"3 18"6 20"4 23"3 26"5 28"3 28"4 32"1 32"4 26"3 n.b.a 12"5 n.b.a 16"4
a
137
n.b., not measured.
3. Results The measurements are given in Table 1. The organic matter content in the stratified sludge sample varies from 50.2% in the sludge layer collected from a depth of 6–8 cm below the surface to 14.3% in the 22–23 cm layer. The moisture content changes accordingly—it was highest in the 4–6 cm layer (49.7%), close to the maximum in the 6–8 cm layer (48.6%), and least in the 22–23 cm layer (26.3%). The value of Ntotal was largest (3.86%) in the 6–8 cm layer, and was least (1.70%) in the 22– 23 cm layer. The largest values of Ptotal (0.55 and 0.51%) were recorded in the 4–6 cm and 6–8 cm layers, respectively; Ptotal was smallest (0.12%) in the 23–24 cm layer. In general, the contents of organic matter, nutrients and radioceasium in the top 6 cm of the profile were smaller than those in the subsurface layer. The top 6 cm of the profile consists of reed stalks, tree leaves, dry grass, sand from nearby beaches and wind-driven dirt. The organic matter and nutrient contents were highest in the subsurface layer of sludge (6–8 cm). From that layer downwards, the contents decreased steadily as far as 20 cm. At this point there was a qualitative change in all the measured parameters: the material below the 20 cm boundary contained increasing
proportions of fine sand. The peak in 210Pb activity at the 21–22 cm layer may be caused by sorption of 210Pb from percolating sewage at the early stages of the facility operation. Therefore, neither the 0–6 cm nor the 20–24 cm layers of the profile were included in the discussion of the changes that sludge may undergo during storage. 4. Discussion From a depth of 6 cm below the surface downwards the organic matter content decreases at an ever slower rate to reach 36% at 15 cm. The organic matter deposited at this depth is 17 years old. Few if any changes occurred in the sewage sludge layer dated to 17–21 years (15–19 cm). It is therefore assumed that the organic matter at 15 cm becomes stable under the influence of temperature, microflora, moisture, and the prevailing sludge composition. The moisture content of raw sludge varies from 96 to 98%. After settlement, this content drops to 60% (Gray, 1989). In sludge that had lain in situ for 3 years, the moisture fell to 52%, and the organic matter content was approximately 35% (De Maesener, 1997; Zwara and Obarska-Pempkowiak, 2000). Little is known of moisture and organic matter changes in the course of further sewage sludge storage. In this particular study, the
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J. Pempkowiak, H. Obarska-Pempkowiak / The Science of the Total Environment 297 (2002) 59–65
Fig. 3. Total nitrogen (Nog) and total phosphorous (Pog) vs. depth in the stratified sludge stored in a hydrophyte facility.
moisture content was highest in the 6–8 cm layer, which represents 7-year-old sludge. This is nevertheless somewhat lower than that in sewage sludge stored for 3 years in a reed bed, while the corresponding organic matter content is somewhat higher (Zwara and Obarska-Pempkowiak, 2000). This, very probably, reflects the changes imposed by a longer storage time andyor the specific origin and composition of the sludge. The content of Ntotal and especially of Ptotal decrease at a faster rate than that of organic matter (Fig. 3). This is best relected in the Org mattery Ntot profile shown in Fig. 4. In the uppermost
layers, the OMyNtot ratio increases from 12.7 to 14.1 as a result of the ammonification of labile organic nitrogen. In the bottom layers, the ratio stabilizes at 15.0"0.9, indicating that a pool of organic nitrogen more resistant to ammonification is consumed during the mineralization of sludge stored for approximately 15 years. Since little NNH4q was found in the samples it must be assumed that inorganic nitrogen had been washed out of the sludge profile, as in the case with sewage sludge stored for 3 years (Zwara and Obarska-Pempkowiak, 2000). The Ptot profile differs substantially from that of Ntot. There is a more than twofold decrease in Ptot during storage (Table 1, Fig. 3), while the respective decreases in Ntot and OM are 34 and 25%. This indicates that the pool of labile organic phosphorus susceptible to mineralization makes up nearly half of the sludge’s organic phosphorus content. Most of it is readily mineralized, since Ptotal decreases rapidly in the uppermost layer of sewage sludge. This is best manifested by the OMyPtotal and Ntotal yPtotal ratios (Fig. 4), both of which rise from 93"8 and 7.3"0.5 in the uppermost layer (7–14 years of storage) to 149"2 and 11.3"0.6 in the lowest layer of sewage sludge (17–21 years of storage). The 16–20 cm layer of the profile stored for 17–21 years, contains sludge which appears to be stable in terms of both organic matter and nutrients contents. 5. Conclusions A 21-year storage period leads to distinct changes in the chemical composition of sewage sludge. This is best manifested by the decrease in moisture (from 46 to 41%), organic matter content (49y 37%), Ntotal (3.86y2.5%), Ptot (0.51y0.23%) and increasing ratios of OMyPtot (to the value of 149"2), Ntotal yPtotal (20"4), and OMyNtot (15.0"0.9). References
Fig. 4. Ratios of Org.mat.yNtot, Org.matyPtot , and NtotyPtot vs. depth in the stratified sludge stored in a hydrophyte facility.
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