QUANTITATIVE COMPARISON OF WASTEWATER TREATMENT SLUDGE DEWATERABILITY A CASE STUDY

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1 QUANTITATIVE COMPARISON OF WASTEWATER TREATMENT SLUDGE DEWATERABILITY A CASE STUDY Skinner, S. J. 1, Stickland, A. D. 1, Cavalida, R.G. 1, Rees, C.A. 2, Devadas, M. 2, Scales, P. J 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 Increasingly stringent regulations on sludge disposal have heightened the need to understand sludge dewatering. This case study measured the dewaterability of five Melbourne Water sewage treatment plant sludges to quantitatively assess causes of poor filtration. The process involved labscale filtration tests and characterisation of dewatering behaviour using a compressive rheology approach. This approach was validated by prediction of the filtration behaviour of the sludges. Sludge dewaterability was quantified using a plate-and-frame filter press model under a standard set of conditions. The results indicate the choice of wastewater treatment process significantly impacts sludge filterability. Specifically, improved dewaterability correlates with extent of digestion and inversely with volatile solids. INTRODUCTION Improvement in water treatment standards worldwide has resulted in increasing quantities of solid-rich waste sludge for disposal. Advanced wastewater treatment methods involve removal of contaminants such as pathogens, nutrients and harmful chemicals from wastewaters to produce high quality water; the by-product is a solids rich sludge. The reduction in volume of this solid waste is termed sludge dewatering. Sludge dewatering is a key sustainability issue in wastewater treatment, due to the high energy usage associated with the process and enforcement of increasingly stringent environmental guidelines on sludge disposal (Hamer 2003). Filtration is frequently utilised for dewatering of wastewater treatment sludges, with common filtration equipment including plate-and-frame filter presses (Wakeman and Tarleton 1999). Sludges treated through anaerobic or aerobic processes are known to achieve differing degrees of dewatering, but quantitative comparisons of sludge filterability have previously been difficult due to operational differences across different facilities. Parameters such as the feed concentration and the applied pressure are critical to accurate comparison. However, methods now exist to characterise the dewaterability of extremely compressible materials such as wastewater sludges (Stickland et al. 2008). In addition, there are validated predictive models of filter presses (Stickland et al. 2006). Thus, the effect of treatment type and length of treatment on the filterability of the resulting sludge can be predicted. Melbourne Water s Eastern Treatment Plant (ETP) and Western Treatment Plant (WTP) sludge samples were used for the case study. MATERIALS AND METHODS Three sludge samples from WTP and two sludge samples from ETP were used to compare the effect of the different treatment processes on dewatering. The ETP process involves grit removal, primary sedimentation then an activated sludge section followed by relatively short-term (approx. 17 days) anaerobic digestion of the waste sludge. The WTP process involves an extensive lagoon system (anaerobic and aerobic) with sludge digestion in an anaerobic pond (up to years but typically 1-3 years) and in subsequent aerated lagoons, it also incorporates an extended aeration activated sludge process. Sludge samples were taken from the anaerobic pond, aerated pond 28 and the activated sludge plant at WTP, and the anaerobic digester and activated sludge process following thickening at ETP. The volatile suspended solids (VSS) content of each sample were measured at The University of Melbourne following the procedure in Standard Methods (APHA et al. 1995). The samples were characterised using batch settling, centrifugation and filtration based on a compressive rheology approach. The protocol developed by Stickland et al. (2008) was able to account for the extremely compressible nature of wastewater treatment sludges by fitting the compression region of a series of constant pressure filtration tests. A schematic of the filtration rig used in the experiments is shown in Figure 1. The approach considers only local material properties in terms of two parameters that are dependent on the suspended solids volume fraction of the material, ϕ (Buscall and White 1987). The two key parameters are the extent of dewatering or the compressive yield stress, P y(ϕ), and the rate of dewatering or hindered settling function, R(ϕ). The assumptions made to extract the dewatering parameters were validated using a one-dimensional model of filtration to predict the original filtration experiments (Stickland et al. 2005).

2 Compressive yield stress, P y (ɸ) (kpa) (a) WTP Anaero. Pond Figure 1: Schematic of the lab-scale filtration rig with key components The material properties of P y(ϕ) and R(ϕ) were used as inputs to a validated model of a fixed cavity plate-and-frame filter press (Stickland et al. 2006). The filter performance for all samples was predicted under operating conditions similar to industry without requiring extrapolation from the experimental data. The selected fill pressure was 300 kpa, the initial concentration was 1 vol%, the handling time was 00 seconds and the cavity width was 1 cm. The fill pressure is lower than typical filter press operation (700-1,000 kpa) but avoids extrapolation from the maximum applied pressure for lab-scale tests of 500 kpa. The results are presented for quantitative comparison in terms of specific solids throughput (Q solids) versus average final solids concentration of the filter cake (<xf>). RESULTS Dewaterability Material Properties The dewatering material properties, P y(ϕ) and R(ϕ), of the five samples are shown in Figure 2. Although fundamental indicators of the dewatering behaviour of the material, P y(ϕ) and R(ϕ) are not straightforward indicators of operational performance. A material with greater compressibility (P y(ϕ) further to the right) and higher permeability (R(ϕ) lower) is the easiest to dewater, however the data in Figure 2 shows this is difficult to ascertain due to intersection of the curve fits and the complex interplay between material compressibility and permeability. Hence, another method for comparison is required. Hindered settling function, R(ɸ) (kg s -1 m -3 ) E+18 1.E+16 1.E+14 1.E+12 1.E+ 1.E+08 Solids volume fraction, ɸ (v/v) (b) WTP Anaero. Pond 1.E Solids volume fraction, ɸ (v/v) Figure 2: Material dewaterability properties for the samples from WTP and ETP including the compressibility given by (a) compressive yield stress, P y(ɸ), and (b) inverse permeability, the hindered settling function, R(ɸ). All curve fits are indicated by lines (dotted and solid) and experimental data points by markers. Figure 2 indicates that the WTP anaerobic lagoon sample is clearly the best to dewater as it the most compressible and most permeable material tested. However, due to the highly non-linear permeability behaviour and the different compressibility behaviour of the samples, direct comparison of the other samples is difficult and further modelling is required to perform a quantitative comparison.

3 Material Properties Validity Check The dewatering material properties were used to accurately predict the original filtration experiment results, which has previously proved difficult for wastewater sludges. The only inputs to the prediction were the initial sample height, initial solids concentration, applied pressure and material properties. The sample is the clearest example of this predictive capability, the results of which are shown in Figure 3. Time, t (s) 0,000 75,000 50,000 25,000 kpa 20 kpa 50 kpa 0 kpa 200 kpa Prediction Filtrate volume, V 2 (m 2 ) Figure 3: Example validation of material properties for Western Treatment Plant sludge sample taken from Pond 28 comparing experimental filtration results with simulations at a range of pressures. Figure 3 shows the comparison between predicted filtration behaviour from the material properties shown in Figure 2 and the experimental results on a typical plot comparing filter time, t, versus filtrate volume, V 2. The material properties were able to re-predict the experimental results to a reasonable accuracy for a wide range of applied pressures from -200 kpa. This process was repeated for all sludges with similar levels of accuracy. Re-prediction of the original experimental results to this level of accuracy over this wide range of pressures has not previously been achieved for sewage sludges using compressional rheology theory. Traditional filtration theory using average resistance to filtration has also been unable to adequately model sewage sludge filtration, although it is acknowledged that reasonable success can be achieved by incorporation of an osmotic pressure term and using parameters that depend on the local solids concentration (Christensen and Keiding 2007). The ability to repredict the original filtration experiment results with reasonable accuracy provides confidence in the validity of the material properties for all sludges. Quantitative Filterability Comparison The validated material properties were used as inputs into the fixed cavity plate-and-frame filter press model, thus allowing a quantitative basis for comparison. The results are shown in Figure 4 at the end of the paper. The sample with a higher specific solids throughput at a particular final solids concentration are considered more easily dewaterable. Similarly, a sample that achieves a higher final solids concentration at a particular solids throughput is also better to dewater. Two methods of comparison are presented to highlight the differences in filtration behaviour between the ETP and WTP sludges, and then the differences between the three WTP samples. The first method involves comparison at a fixed weight fraction; a typical requirement at a treatment plant is to achieve 20 wt% solids. For this first comparison, the required filter area for each sludge was determined at a solids throughput of 0.11 kg/hr, which was selected to simplify the analysis. The second method involves comparison at a fixed specific solids throughput of 0.05 kg/hr/m 2, as this throughput has proven to give industrially relevant final solids concentration values. The complete comparison is presented in Table 1, alongside characteristics of each sludge sample. Table 1: Quantitative comparison of the dewaterability of each sample in terms of (i) Filter area (A) required to achieve 20wt% solids at solids throughput of 0.11 kg/hr and; (ii) average final cake solids (<x F>) achieved at fixed specific solids throughput of 0.05 kg/hr/m 2 Sludge Name Sludge Type TSS (%) VSS (%) (i) A (m 2 ) (ii) <xf> (wt%) WTP Anaerobic Lagoon Anaerobic Digestion in Lagoon Mechanically Aerated in Pond Return Activated Sludge DAF Thickened Waste Activated Sludge Anaerobically Digested

4 Figure 4 demonstrates the expected behaviour that at low throughputs, a high final solids concentration can be achieved, limited only by the compressibility of the sludge. At higher throughputs, the final solids concentration achieved is reduced and the permeability of the sample becomes increasingly important. The optimum in the solids throughput is related to the handling time during the filtration process, the optimums appear at low concentration due to the difficult to dewater nature of the sludges. At the first set of fixed conditions, the filter area required to dewater each sludge highlights the difference between the WTP and ETP samples. The WTP anaerobic lagoon sample would require a filter area of 1 m 2 compared to the ETP anaerobic digester sample requiring 23 m 2 of filter area. It is clear that the digestion process has a significant impact on the dewaterability of the sludge produced. The comparison between the WTP and ETP samples indicates that the process constraints at ETP on the sludge digestion time results in sludge that is much more difficult to dewater. If both plants were to install new dewatering equipment then the capital and operating expenditure at ETP would be significantly greater. It should be noted that there is a primary grit removal process at ETP, but not at WTP. This results in sand, grit and other inorganics in the WTP anaerobic pond sample, contributing to the low VSS result. These inert inorganics should settle in the anaerobic pond and would be expected to have a negligible impact on the sample. It is still apparent that the filterability is significantly better than the ETP samples indicating that the effect of the digestion process is important. However, it is acknowledged that the inert inorganics could be another reason for the good filtration behaviour of the WTP anaerobic pond sample in addition to advanced digestion. At the second set of fixed conditions, the average final solids concentration at fixed specific solids throughput is used to compare differences between the WTP samples. Significantly different dewatering behaviour is predicted at different stages in the process at WTP, with the final solids concentrations ranging from 12.8 wt% to 27.9 wt%. The solids flow within the WTP process is complex, thus it is difficult to estimate the solids residence time of each sample. The VSS data indicate that the sample from the anaerobic lagoon has been digested to the greatest extent, although there is a possible influence from the inert inorganic fraction. Observation of the sample indicated that there was no clearly visible inorganic material, but it may influence the floc structure. The different dewatering behaviour of the WTP samples follows the general trend of improving dewaterability with extent of digestion. The WTP samples that have undergone greater VSS destruction over the longer period of digestion show the best dewaterability. Volatile Solids as an Indicator for Filterability The data from the second fixed conditions for achievable final solids concentration compared to VSS content are presented in Figure 5. There appears to be a very strong negative correlation between filterability and VSS content. This observation is consistent with the concept that the volatile extracellular polymeric substance (EPS) fraction of the sludge is the cause of the poor filtration behaviour (Neyens et al. 2004). The general trend observed is that the higher the VSS concentration (as a surrogate for EPS fraction), the poorer the filterability. Volatile suspended solids, VSS (%) VS = <x F > R² = 0.84 ETP WTP Final average cake solids, <x F > (wt%) Figure 5: Predicted average final solids concentration from a fixed cavity plate-and-frame filter press versus volatile solids for five Melbourne Water sewage sludges. The fit is a linear regression of the data. The regression values in Figure 5 are arbritrary and depend on the basis for comparison, but the general trend is that VSS appears to be an indicator of poor dewatering. Although the observed trend is almost linear, there is no reason for this to be the case. A true correlation would require a controlled experiment following a single sludge as it degrades. There are a number of other limitations including the fact that small changes in VSS do not result in large dewaterability changes. Furthermore, EPS composition varies significantly depending on the type of digestion method (Neyens et al. 2004), which could produce inconsistencies in the results if VSS is used as a surrogate. This may be a reason for the lower VSS ETP sample showing such poor filterability. Overall, it appears that VSS can also be used as a good and simple performance indicator of filterability. Additionally, the dewaterability of sludges can be improved by increasing the extent of digestion, which has important implications on wastewater treatment process design.

5 CONCLUSION The dewaterability of sludges from two Melbourne wastewater treatment plants was quantitatively compared. The samples showed significantly different behaviour during filtration, which was able to be modelled using compressive rheology theory. The quantitative comparison of Western Treatment Plant and Eastern Treatment Plant samples showed that improved dewaterability correlates with decreased volatile solids. This case study indicates the wastewater treatment process has a significant effect on sludge dewaterability and increasing the extent of digestion can improve wastewater treatment sludge filtration. ACKNOWLEDGEMENTS Samuel J. Skinner acknowledges receipt of the Elizabeth and Vernon Puzey Bequest Scholarship for PhD study. Infrastructure support is acknowledged from the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council. REFERENCES APHA, AWWA and WEF Standard Methods for the Examination of Water and Wastewater. Washington D.C., APHA. Buscall, R. and White, L. R The consolidation of concentrated suspensions. Part 1. The theory of sedimentation. Journal of the Chemical Society. Faraday Transactions I: Physical Chemistry in Condensed Phases, 83(3): Christensen, M. L. and Keiding, K Filtration model for suspensions that form filter cakes with creep behavior. AIChE Journal, 53(3): Hamer, G Solid waste treatment and disposal: effects on public health and environmental safety. Biotechnology Advances, 22(1 2): Neyens, E., Baeyens, J., Dewil, R. and De heyder, B Advanced sludge treatment affects extracellular polymeric substances to improve activated sludge dewatering. Journal of Hazardous Materials, 6(2 3): Stickland, A. D., Burgess, C., Dixon, D. R., Harbour, P. J., Scales, P. J., Studer, L. J. and Usher, S. P Fundamental dewatering properties of wastewater treatment sludges from filtration and sedimentation testing. Chemical Engineering Science, 63(21): Stickland, A. D., De Kretser, R. G. and Scales, P. J Nontraditional constant pressure filtration behavior. AIChE Journal, 51(9): Stickland, A. D., de Kretser, R. G., Scales, P. J., Usher, S. P., Hillis, P. and Tillotson, M. R Numerical modelling of fixed-cavity plate-and-frame filtration: Formulation, validation and optimisation. Chemical Engineering Science, 61: Wakeman, R. J. and Tarleton, E Filtration: equipment selection, modelling and process simulation, Elsevier. Specific solids throughput, Q solids (kg/hr/m 2 ) <xf> = 20 wt% WTP Anaerobic Pond Q solids = 0.05 kg/hr/m Final average cake solids, <x F > (w/w) Figure 4: Quantitative dewaterability comparison of sewage sludge samples showing fixed cavity plate-andframe filter press simulation results.