The use of sludge drying pans is a common dewatering practice to stabilise and dry sludge in wastewater treatment (WWT) plants. This paper focuses on the first ...
IMPLEMENTATION OF ‘DRY STACKING’ OPERATION IN WASTEWATER TREATMENT SLUDGE DRYING PANS 1
1
1
1
Sriharini Chellappan , Shane P. Usher , Kithsiri B. Dassanayake , Anthony D. Stickland , 1 2 2 Peter J. Scales , Catherine A. Rees , Mukundan Devadas 1
Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Parkville, VIC, Australia 2 Melbourne Water Corporation, Melbourne, VIC, Australia
ABSTRACT The use of sludge drying pans is a common dewatering practice to stabilise and dry sludge in wastewater treatment (WWT) plants. This paper focuses on the first full scale implementation of ‘dry stacking’ as a novel sludge drying practice. To facilitate a stack formation for the sludge at low solids concentration, the use of biosolids embankments has been trialled in the existing drying pans at Western Treatment Plant, Victoria. A sloped partially dried stack was created with solids concentration from 10% (w/w) to 45% (w/w) through the length of the pan within 7 weeks. INTRODUCTION Melbourne Water’s Western Treatment Plant (WTP) uses drying pans to stabilise and dry sludge through natural sunlight and wind effects. The current operation at Western Treatment Plant requires approximately one hectare to dry 500 dry tonnes of sludge per year from around 3-5% solids to 70% solids. At WTP, the sludge from lagoons (anaerobic and aerobic) is dredged and pumped into sludge drying pans periodically in winter allowing solids to settle and the supernatant to be drained. The sludge is then dried during summer and harvested for stockpiling and future reuse. However, sludge drying using this method is affected by local rainfall (Stickland et al. 2013). Rainfall can lead to inadequate drying during summer and the sludge remains in the pans for an additional year, significantly reducing system throughput. Dry stacking is an alternative practice that has had a large impact on slurry drying in the mineral industries (Robinsky 1975, Sofrá and Boger 2002). In dry stacking, sludge is applied to the drying pan in very thin layers. Depending on the solids concentration, the sludge has a yield stress (Nguyen and Boger 1992) such that it stops flowing and the solids settle from the liquor and the supernatant is decanted. Each layer is then partially dried before the next layer is applied. The sludge layers are applied in such a way that a sloped sludge surface or ‘stack’ is formed. The sloped stack can drain rainwater and therefore increase net evaporation. Recently, initial validation of the dry stacking concept for wastewater sludge was provided by subjecting WTP sludge to various
laboratory drying and rheometry tests (Stickland et al. 2013). Using local weather conditions, the study estimated that dry stacking could improve pan throughput by 65-140%. However, dry stacking has not been tested at full-scale for wastewater treatment sludge. In the current study, full-scale trials of dry stacking of wastewater treatment sludge have been undertaken to explore the operational implementation of the technique in WTP’s existing sludge drying pans. A major challenge in implementing dry stacking at WTP is that the dredged sludge has variable and sometimes very low solids concentrations. At low concentrations, the sludge will flood the pan. The use of biosolids embankments has been trialled in the drying pans to minimise initial inertia and to maximise the time for the solids to settle and form a bed whilst the supernatant is decanted. METHODOLOGY The traditional practice at WTP is to fill the sludge drying pans with their outlets closed, allow the sludge to settle, decant the supernatant by opening the outlet valves at the appropriate height and then fill the pan with more sludge over an 8-10 day cycle. This cycle is repeated 8 to 10 times until the pan is full of settled sludge. The existing drying pans are rectangular in shape, 100 m wide and 200 m long, with a dredged pond sludge feed at one end and a valved outlet at the other end. The pans have a very mild slope of about 0.5 degrees, decreasing the base height by 50 cm over the length of the pan.
Figure 1: View from the inlets of trial drying pans SDP 7 and SDP 8 at WTP To test the concept of forming a dry stack from low solids concentration sludge, two pans were used over a 7 week period (SDP 7 and SDP 8) (See
Figure 1) starting March 2014. In SDP 8, the pan outlet was adjusted to remove the boards and allow free drainage of supernatant (see Figure 2). SDP 7 had its outlet open too, but also had biosolids embankments to direct the flow of the sludge throughout the pan (See Figure 2). Six embankments were constructed every 20 m along the pan. The embankments extended from alternate side walls to within 20 m of the opposite side (See Figure 2), creating a serpentine flow path for the sludge. The pans were filled with sludge simultaneously for about 1 hour for 3 days per week for 7 weeks with an approximate flow rate of 240 3 m /hr.
Turbidity measurements using a Hach 2100AN Turbidimeter were performed on liquor and settled liquor samples to assess the clarity of the supernatant of the inlet and the outlet. In dry stacking, as the sludge flows down the slope, the shear rate decreases such that the viscosity increases. Wastewater sludge is typically a shear thinning fluid, where the viscosity decreases as the shear rate increases (Dentel 1997, Slatter 1997, Baroutian et al. 2013). The network strength of the suspension at which the material yields (Nguyen and Boger 1983) is given as the shear yield stress, y. Shear thinning yield stress flow properties of the sludge is essential for dry stacking (Stickland et al. 2013). The yield stress is measured using a controlled strain-rate rheometer (Haake VT550) with a cup and vane. The dimensions of the cup and vane used in the rheometry experiments are shown in Table 1. The yield stress was measured as the peak torque at a fixed rotational rate of 0.2 rpm. Table 1: Cup and Vane dimensions Geometry
Diameter (mm)
Length (mm)
Cup
64
104
Vane
15
50
RESULTS AND DISCUSSION Figure 2: Design modifications in trial pans SDP 7 and SDP 8 Ten bollards with height indicators were placed at regular intervals along the centreline and one side of each pan to measure the height of the sludge bed. The bollards were also used as sampling locations. Photographs of the bollards at each position were taken every week to record the height of the sludge. In addition to photographs, the bed height was also physically measured with a higher degree of accuracy when sludge samples were collected. Samples were collected from the inlet and the outlet, nominally every hour for three hours. When the sludge in the pan was reasonably settled, bed samples were also collected from each bollard position. A portion of samples were allowed to settle and the supernatant liquor contents were separated. All liquor samples and sludge samples were then dried at 60°C to constant weight to measure the dissolved solids weight fraction of the liquid, A and dry weight fraction of sludge sample, B. The corrected or suspended solids weight fraction, X is given as (Usher 2002)
X
B A 1 A
(1)
Figure 3: Cumulative solids loading for solids concentration 1, 2 and 3% w/w The inlet solids concentration in the samples were in the range of 1-3 % w/w, lower than the expected range of 3-5 % w/w. This low solids concentration is periodically observed because of the difficulties in dredging. The drying pans were filled with sludge 22 times over 7 weeks. For a range of feed solids concentration 1, 2 and 3 % w/w loading at 240 3 m /hr for each drying pan, the cumulative solids loading per pan is shown (See Figure 3) with a total
solids loading range of 55-166 tonnes of solid over the 7 weeks. Considerable sludge beds had formed near the inlet after 3.5 weeks of dry stacking operation. In SDP 7, the embankments enabled good solids holdup. It was observed that most of the solids were contained within the third embankment and the material that flooded past the third embankment was primarily supernatant. However, the supernatant also had a tendency to pool in areas such that it could not drain. Designing proper draining ports along the sides of the pan could avoid pooling of the supernatant. In SDP 8, the sludge from the feed took a more direct path to the outlet.
the subsequent positions in the sludge bed. In SDP 8, a slope was formed (see Figure 4) even without any embankments. This was due to the limited flow regime compared to the normal flooding protocol, which allowed solids to settle before reaching the outlet. However, the height of the sludge bed in SDP 8 was less than SDP 7. It was also noted that the time taken for the supernatant to reach the outlet in SDP 7 was longer than in SDP 8. This suggests that embankments provided longer time for solids to settle preventing run off of feed to the outlet without settling. In SDP 7, the solids concentration progressively increased over 7 weeks, as shown in Figure 5. The solids concentration increased up to 10% w/w within the embankment zone and 45% w/w beyond the embankments. In SDP 8, the solids concentration was 10% w/w in the first half of the pan and over 32 % (w/w) in the second half of the pan (Figure 5) where the bed heights were less than 2 cm.
Figure 4: Height of the sludge bed over 7 weeks in trial pans SDP 7 and SDP 8 The height of the sludge bed in SDP 7 decreased along the length of the pan beyond the first bollard (sampling point) (See Figure 4). This indicates that the embankments in SDP 7 formed the necessary slope to stack the sludge. Each week, the height of the sludge bed decreased at each corresponding bollard position. This may have been due to effective draining of the supernatant, evaporation from the sludge bed and local weather conditions. Due to high inlet inertia near the feed, the height of the sludge bed at the first bollard was lower than
Figure 5: Solids weight fraction, X, over 7 weeks in trial pans SDP 7 and SDP 8 The dissolved solids content, A, of the outlet samples ranged from 0.10-0.41 % w/w (1,0004,100 ppm) which is similar to the dissolved solids content in the feed sludge measured to range from
0.08-0.16 % w/w (800-1,600 ppm). There were no significant differences in the outlet dissolved solids content between the two pans. The liquor turbidity measurements at the feed and outlet every hour in SDP 7 and SDP 8 are shown in Figure 7. The results confirm that the solids had settled from the sludge, producing a good clarity in the outlet supernatant in both pans. However, the effect of loading pans at higher flow rates and for longer periods of time on supernatant clarity needs to investigated.
Figure 8: Yield stress curve over 7 weeks in trial pan SDP 7 and SDP 8 Higher yield stress values ensure good stack formation in the bed, preventing erosion of the sludge bed formed underneath. The shear yield stresses as a function of solids volume fractions for the samples of SDP 7 and SDP 8 over 7 weeks are shown in Figure 8. High yield stresses indicate good network strength of the sludge bed. Yield stresses were observed for samples as low as 2 % w/w. The yield stress values for both pans were similar with a power law fit of order 2.8.
CONCLUSION Full-scale trials of dry stacking in wastewater treatment sludge drying pans were implemented successfully for the first time. A small stack was formed without embankments through a reduced loading regime, however biosolids embankments were shown to help reduce feed inertia and to increase the path length to help solids settle. ACKNOWLEDGMENT Figure 7: Turbidity measurements in trial pan SDP 7 and SDP 8; FL-Feed liquor; O1L- Outlet liquor at 1 hr; O2L- Outlet liquor at 2 hrs; O3L- Outlet liquor at 3 hrs
We acknowledge the funding for this work from Melbourne Water, Smart Water Fund and the Australian Research Council. S Chellappan is a recipient of the Nancy Mills scholarship through WaterRA. REFERENCES Baroutian, S., N. Eshtiaghi and D. J. Gapes (2013). "Rheology of a primary and secondary sewage sludge mixture: Dependency on temperature and solid concentration." Bioresource Technology 140(0): 227-233. Dentel, S. K. (1997). "Evaluation and role of rheological properties in sludge management." Water Science and Technology 36(11): 1-8.
Nguyen, Q. D. and D. V. Boger (1983). "Yield stress measurement for concentrated suspensions." Journal of Rheology 27(4): 321-349. Nguyen, Q. D. and D. V. Boger (1992). "Measuring the flow properties of yield stress Fluids." Annual Review of Fluid Mechanics 24(1): 47-88. Robinsky, E. I. (1975). "Thickened discharge - A new approach to tailings disposal." CIM Bulletin 68: 47. Slatter,
P. T. (1997). "The rheological characterisation of sludges." Water Science and Technology 36(11): 9-18.
Sofrá, F. and D. V. Boger (2002). "Environmental rheology for waste minimisation in the minerals industry." Chemical Engineering Journal 86(3): 319-330. Stickland, A. D., C. A. Rees, K. P. M. Mosse, D. R. Dixon and P. J. Scales (2013). "Dry stacking of wastewater treatment sludges." Water Research 47(10): 3534-3542. Usher, S. P. (2002). “Suspension Dewatering: Characterisation and Optimisation”. Chemical and Biomolecular Engineering Melbourne, The University of Melbourne. PhD Thesis
