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Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-II: laboratory characterizations of drainability and filterability
Process Biochemistry 40 (2005) 2435–2442 www.elsevier.com/locate/procbio
Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-II: laboratory characterizations of drainability and filterability Azize Ayola,*, Steven K. Dentelb a
Department of Environmental Engineering, Dokuz Eylul University, Kaynaklar Campus, 35160 Buca, Izmir, Turkey b Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA Received 19 February 2004; received in revised form 1 September 2004; accepted 25 September 2004
Abstract This paper provides an analysis of different measurements and derived parameters that may be used to characterize drainability and filterability dynamics. To better simulate the phenomena of gravity drainage and expression dewatering, specific devices and procedures were used in bench-scale conditioning and dewatering experiments. The setup included a plow simulator kit for drainage simulation and a mechanical dewatering unit, namely, the Crown PressTM, which simulates full-scale belt filter press dewatering. The effects of enzyme product additions prior to polymer conditioning of biosolids samples were investigated. The results showed that enzyme additions enhanced the drainability of the biosolids samples and this positively affected filterability of the samples in the press unit. According to these results, enzyme product addition for biosolids samples with polymer conditioning seems a promising new method of enhancing the performance of mechanical dewatering units in the future. # 2004 Elsevier Ltd. All rights reserved. Keywords: Biosolids; Anaerobic digestion; Dewaterability; Enzymes; Drainability; Filterability
1. Introduction The accurate prediction of biosolids dewaterability is important for the selection and optimization of conditioning chemicals, because full-scale trials are laborious and the polymers used for conditioning are expensive. Dewaterability is typically evaluated using a simple filterability test, such as qualitative or quantitative drainage tests, ‘‘time to filter’’ determinations, capillary suction time (CST) tests, or—for more exacting purposes—the specific resistance to filtration (SRF) test. Although the test methods give a general idea of dewaterability of conditioned biosolids, they cannot be claimed to simulate the full-scale dewaterability performance of mechanical dewatering units. Centrifuges, belt filter presses, and frame presses expend considerable * Corresponding author. E-mail addresses: [email protected], [email protected] (A. Ayol), [email protected] (S.K. Dentel). 0032-9592/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2004.09.024
amounts of energy in applying mechanical forces to the material, because this greatly improves water removal; yet, none of the lab-scale tests include this aspect of the process. Because the tests cannot indicate the response of conditioned biosolids to pressure or shear, they may fail to properly predict the relative performance of different conditioning chemicals. Furthermore, proper simulation of mechanical dewatering process should provide the imposed conditions in a sequence similar to that applied with the same process at full-scale. The small-scale test must therefore be processspecific. In these tests, dewatering by belt filter press was to be simulated, so the biosolids were subjected a sequence of four stages as may be considered to occur with this means of dewatering. The first stage that must be included is the chemical conditioning of the biosolids, in which a dewaterable floc should be obtained. This stage includes mixing, blending, and distribution up to deposition on the filter belt. The
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second stage is then gravity drainage, in which drainable water is removed from the biosolids as it accumulates on the belt surface. Plowing mechanisms should not be neglected, since they are generally assumed to improve the drainage rate. The biosolids are then layered between an upper and lower filter belts, and this ‘‘wedge zone’’ provides a lowpressure compressive dewatering stage, termed expression. Finally, increasing pressures (and shear) are applied at each successive roller, by decreasing the turn radius, to give continued expression and, ultimately, the final dewatered cake. These latter stages will provide unsatisfactory water removal, lateral escape of material off the belts, and loading limitations [1,2] if effective chemical conditioning is not effected in the initial stage. These problems should be observable in a good lab simulation procedure as well. For this work, conditioning, drainage, low-pressure expression, and high-pressure expression were simulated with small-scale procedures. Some of these procedures have already been compared to, and validated by, full-scale dewatering tests [3]. Here, we explored a broader range of quantitative measurements that can be obtained from the same procedures, in order to determine which were most meaningful. We varied chemical dosages in order to observe changes in these parameters, employing a flocculant polymer and an enzyme preparation as explained in the companion paper. The results reported here are therefore relevant in evaluating the relative efficacy of these two additives.
2. Objectives The objectives of the study were therefore: to determine whether dewaterability predictions using the more conventional tests (specifically, the classical CST test as presented in the companion paper) provide an adequate prediction of drainage and expression, as determined using a bench-scale dewatering device which simulates belt filter press dewatering; to determine which of the measurable parameters in the simulation procedures provide useful results, and which are either unique or redundant with others; and to utilize these data, with results from the companion paper, to determine relative efficacy and cost effectiveness of a flocculant polymer and an enzyme product for biosolids conditioning.
3. Materials and methods 3.1. Experimental approach The simulation included all four steps described above. First, the biosolids were conditioned with polymer, enzyme product, or combinations of these, in a jar test procedure
with dosages and mixing conditions as described previously [4]. The second step was to simulate gravity drainage on a belt filter press. A bench-scale gravity drainage plow simulator kit was used, similar to that recommended by Severin et al. [2,5], to produce drained biosolids comparable to those produced by a full-scale belt filter press. Our device was improved by allowing the same filter cloth to be employed for both the drainage and pressuring stages, assuring that any interactions between the biosolids and the cloth (such as blinding) would be preserved. A schematic diagram of the test kit is given in Fig. 1. The test kit consisted of a 0.10 m 0.05 m PVC pipe bushing and 0.10 m PVC connector to hold a 0.16 m 0.50 m belt filter cloth. In order to prevent leakage, the assembly was clamped together with PVC plates. The drainage area was about 78.54 m2. Continuous plowing was applied using a plow simulator blade with a rotor, rotated at 10 rpm. The volume of drained liquid can be determined as a function of time (used here to find specific resistance values) and the properties of the drained liquid can also be evaluated. The low- and high-pressure stages were simulated using a manually operated mechanical press, available commercially as the Crown PressTM (Neogen, Lansing, MI). In the same manner occurring during cake compression in a fullscale belt filter press, the Crown PressTM allows controlled periods of mechanical pressure to be applied to drained biosolids between filter cloths. Fig. 2 shows the press used in this study. An upper cloth is placed over the biosolids and base cloth used for gravity drainage, and this is placed on a curved, porous surface known as the crown. A cam allows tension to be manually applied to the belts using a steel bar, with the user referring to a pressure gauge to determine the proper degree of force to apply. It is claimed that this duplicates both the normal and shearing components of pressure applied in a belt filter press [1,6]. Tests have shown this device to faithfully duplicate the dewatering effects of a full-scale belt press [3]. 3.2. Analysis Anaerobically digested biosolids were obtained from the Newtown Creek Wastewater Treatment Facility in New York City, and the Wilmington (Delaware) Wastewater Treatment Facility, denoted as NYC and WIL, respectively. After enzyme and/or polymer conditioning, a 300 mL sample was poured onto the filter cloth material mounted within the gravity drainage kit, and drainage volumes were measured at predetermined time intervals (5, 10, 15, 20, 30, 45, and 60 s) using a graduated cylinder. Drainage sample viscosity was determined using a Brookfield LVT viscometer with an Ultra-Low Adapter (Brookfield Engineering Laboratory Inc.). After drainage, the same filter cloth with drained cake was placed on the Crown PressTM as a bottom belt. The cake was then squeezed, slowly applying 45.4 kg (100 pounds) of
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Fig. 1. Gravity drainage plow simulator used in this study.
pressure, releasing, press rapidly to 59.0 kg (130 pounds), releasing, then rapidly to 77.2 kg (170 pounds) force. At each force, the pressing time was 10 s. After pressing, the pressed cake diameter was measured to estimate the cake
migration. Filtrate samples were also collected for other analyses. 3.3. Determination of specific resistance to filtration (SRF) The filterability of a sludge can be described by the SRF, which is derived from Darcy’s law for flow through porous media [7,8]. The rearranged form of this relation, in terms of the inverse filtrate flux, is given in Eq. (1). 1 dt mðSRFwV=AÞ þ Rm ¼A (1) ¼ q dV Dp
Fig. 2. Crown PressTM used in this study (Neogen, Lansing, MI).
where q is the liquid flow rate per cross-sectional area (m s1); V the filtrate volume (m3); t the filtration time (s); A the cross-sectional area (m2); Dp the applied pressure (Pa); m the liquid viscosity (Pa s); SRF the specific resistance to filtration (m kg1); w the mass of solids deposited in the cake per unit liquid volume (kg m3); and Rm the resistance of the filter medium (m1). This equation is conventionally used to determine the SRF for filtration conditions in which either a vacuum or pressure is applied to give a constant pressure difference during over the course of the process. The model was used here to describe gravity drainage, which differed in that the pressure was much less, and it also decreased over time. The above equation was integrated over this time period, while
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accounting for the pressure changes, and fit by non-linear regression to the V(t) data to obtain the SRF values [9]. When completed, each test of conditioning and dewatering included the measurements listed in Table 1. Some of these results are presented in the companion paper, and some in this paper. Several measurements proved not to be useful and are not reported.
4. Results 4.1. Specific resistance Fig. 3a and b show the gravity drainage results for enzyme-treated, polymer conditioned NYC and WIL samples, including controls without addition of polymer and/or enzyme. As with the preceding companion paper, results are shown in terms of isopleth graphs, in this case lines of constant log10 SRF values on axes of enzyme and polymer dosage (note that the scales are non-linear). The SRF values are seen to generally decrease with polymer dose until an optimum dose range is attained. Higher doses may actually cause poorer performance, particularly with the WIL biosolids. The enzyme doses to the biosolids also effectively reduced the SRF, indicating that either additive, or the two combined, improved the rate of drainage. The optimum enzyme dose is somewhat above 15 mg/L for both biosolids, with overdosing being deleterious. This is in good agreement with CST results reported in the companion paper, confirming that the drainage SRF measurement has meaning, and is probably related to degree of flocculation. It should be noted that the SRF values are at least two orders of magnitude less than SRF values commonly measured by the vacuum or pressure filtration methods. Ideally, specific resistance is a property of the matter being retained on the filter, but the effects of compression are much less here due to the low applied pressure. Significantly, the plow simulator was in operation, which may have reduced the SRF due to lateral shear applied to the cake as it was deposited. Other results [10] demonstrate that lateral shear improves dewaterability. Thus, these results may be valid in an absolute sense, and they can certainly be used in a relative
Fig. 3. Log10 SRF (m kg1) values as a function of polymer and enzyme doses: (a) NYC and (b) WIL.
sense in assessing the gravity drainage stage in belt press operation. 4.2. Solids migration Severin and Collins [6] observed that the area occupied by solids increased during the pressure application stages in the Crown PressTM, and quantified this as the increase in radial dimension of the consolidated material. Presumably, this could be related to lateral expansion of the solids between the belts, which leads to leakage if excessive. As an
Table 1 Measured and derived parameters for this study Procedure/measurement
Reference
Derived parameter(s)
Conditioning stage Capillary suction time (CST)
Standard Methods [11]
CST 1/CST
Drainage stage Filtrate volume vs. time Filtrate turbidity Suspended solids
This paper Standard Methods [11] Standard Methods [11]
SRF log10 SRF initial flux, A(dV/dt), %drainage @ 60 s Turbidity (NTU) Suspended solids (mg/L)
Expression stages Cake dry solids content Solids migration
Standard Methods [11] Severin and Collins [6]
Total solids (%) Absolute migration (cm), %area increase
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as indicated for highest cake solids, with no dramatically change of cake area in overdosing ranges. For both biosolids, the optimum dose combination is about 20 mg/ L enzyme followed by 250 mg/L polymer. The smaller cake areas appear to be related with higher cake solids (reported in the companion paper), simply because these samples had higher drained volumes, and thus, less solids migration. 4.3. Relationships and utility of measured parameters
Fig. 4. Area increase (%) values as a function of polymer and enzyme doses: (a) NYC and (b) WIL.
alternative, this work instead determined the percent increase in solids area between the filter cloths during cake formation. While readily determined from the increased radial dimension (if the original radius is known), the area increase may be related to percent increase in width of the deposited solids layer at full-scale. Fig. 4a and b show the percent increase in cake area with different dose combinations of polymer and enzyme. Enzyme addition and polymer conditioning reduced the cake area, used either separately or in combination. Decreased cake area is seen up to the optimal dose ranges
All parameters listed in Table 1 were compared to determine correlations. This was done by examination of graphical relationships, by determining Spearman rank order correlations, and by regression analysis of power-law relations between pairs of parameters. Selected graphs are shown to illustrate the more notable correlations, and all are summarized in Table 2. The CST measurement appears to be very justified as a commonly employed predictor of dewaterability. There was a clear correlation with specific resistance (particularly evident when plotted as log10 SRF versus 1/CST, Fig. 5a). The reciprocal of CST is a fundamentally appropriate parameter, since it is indicative of filtration rate, and it also showed good correlation with both turbidity and suspended solids measurements (Fig. 5b and c), cake solids (Fig. 5d), and—although less strongly—to solids migration (Fig. 5e). Table 2 only reports statistical results for 1/CST values because the results are essentially identical using CST instead. In contrast to CST, the specific resistance results were less useful. In addition to the CST relationship, the SRF correlated somewhat with filtrate suspended solids and turbidity and, to an even lesser extent, with cake solids and solids migration (figures not shown). It is noted that the SRF values were measured during gravity drainage only, and the performance in this early stage is evidently not relevant to the final expression process. In comparison, the initial slope of the drainage curve (converted to a flux rate, A(dV/dt)), was actually more successful in relating to other measurements. Obviously, it is also a more direct determination from the drainage data. This value was not only related to the amount of drainage
Table 2 Relationships between measured parameters Parameters
Log10 SRF
Initial flux
Drainage @ 60 s
Turbidity
1/CST Log10 SRF Initial flux Drainage @ 60 s Turbidity Suspended solids Cake solids
0.31 0.28
0.38 0.29 0.18 0.18
0.34 0.42 0.14 0.13 0.71 0.67
0.65 0.29 0.45 0.40
0.50 0.36 0.41 0.40
Suspended solids 0.72 0.32 0.36 0.30 0.85
0.64 0.37 0.32 0.32 0.86
Cake solids
%Area increase
0.50 0.61 0.24 0.22 0.46 0.47 0.52 0.57 0.490.50 0.45 0.48
0.58 0.25 0.48 0.51 0.37 0.32 0.68
0.52 0.27 0.37 0.58 0.38 0.35 0.69
Values in bold indicate squared spearman rank order correlation coefficients and values in italic indicate squared correlation coefficients (r2) for power fit relationships between parameters.
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Fig. 5. (a) Log10 SRF (m kg1) values as a function of filterability rate (1/CST); (b) turbidity (NTU) values as a function of filterability rate (1/CST); (c) suspended solids (mg/L) values as a function of filterability rate (1/CST); (d) dry solids (%) values as a function of filterability rate (1/CST); (e) area increase (%) values as a function of filterability rate (1/CST).
obtained at 60 s (Fig. 6a), but also to cake solids and (inversely) to turbidity, suspended solids, and solids migration (Fig. 6b). The latter correlation is evidently due to prevention of lateral solids movement when the suspending fluid is rapidly depleted. The percent drainage at 60 s may be considered as similar to the ‘‘time to filter’’ (TTF) measurement
sometimes used in lieu of the SRF. The TTF is the time required to remove 50% of the sample’s initial volume as filtrate, but the drainage measurements did not reach this amount of removal in all cases. The percent drainage was related to dry solids and to solids migration (Fig. 7a and b), because—in contrast to the initial slope—it is a metric of drainage extent near the completion of the process, so it
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Fig. 6. (a) Percent drainage @ 60 s and (b) area increase (%) values as a function of initial slope of the drainage plots.
Fig. 7. (a) Dry solids (%) and (b) area increase (%) values as a function of percent drainage @ 60 s.
is more likely to predict expression effects. Thus, dry solids content was inversely related to solids migration (Fig. 8). The suspended solids values was closely related to turbidity, as might be expected (not shown). A weak correlation was also apparent to cake solids and to solids migration, with greater suspended solids preventing greater cake solids (e.g. by filter blinding) and producing greater lateral movement as a consequence. Overall, the drainage and Crown PressTM procedures were found relatively simple to use. In this research, comparisons to full-scale results were not done, but they have been carried out in the past with very consistent results [3,5].
reduction in biosolids management costs under many conditions, while Fig. 9 shows that the added costs due to this amount of enzyme use are extremely low under this scenario. Alternatively, by following the isopleths of equal cost in Fig. 9, while comparing to the performance-related isopleths in graphs, such as Fig. 4, the optimal performance for a specified total chemical cost can be determined.
4.4. Efficacy of enzyme pre-treatment Figs. 3 and 4 confirm observations made in the companion paper: use of the enzyme product at roughly 20 mg/L significantly improves both drainage and dewatering, particularly if a polymer is also used. For both biosolids, a 10 mg/L enzyme dose prior to a 250 mg/L polymer usage will increase the dewatered biosolids TS by approximately 50% (e.g. from 20% solids to 30%), which would decrease the mass and volume of dewatered material by about 33%. This will provide a significant
Fig. 8. The relationship between dry solids (%) and solids migration (%).
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The cost of using the enzyme product appears to be quite acceptable. The optimum dose for common, anaerobically digest sludges may be around 15 mg/L. Acknowledgements The first author was financially supported by the Technical and Scientific Research Council of Turkey (TUBITAK)-NATO Science Fellowship Program and the U.S. National Science Foundation under Award #0229293: Chemical, Rheological, and Physical Exploration of GelLike Behavior in Conditioning and Dewatering Processes. The authors thank the personnel of the Newtown Creek and Wilmington treatment facilities, and the Neogen Corporation, Lansing MI, for providing the Crown PressTM. Fig. 9. Cost per million liters of enzyme and polymer addition. Numbers are in US$ per milliliters of biosolids prior to conditioning. Costs used in developing this graph were: polymer, US$ 1.20 per lb (US$ 2.64 per kg) and enzyme, US$ 4.68 per lb (US$ 10.31 per kg), as provided by the suppliers.
5. Conclusions Enzyme product additions in advance of polymer conditioning enhanced the dewaterability of both NYC and WIL anaerobically digested biosolids samples, based on both drainability and expression test results. These findings were consistent with the CST, filtrate turbidity, suspended solids, and cake solids measurements, presented in the companion paper. For both biosolids used, the enzyme dose giving best preconditioning was approximately 15 mg/L, although this value varied somewhat based on the measurements used for dosage criteria. The relatively simple CST test was a very good predictor of most other test results. Although an SRF could be determined from the gravity drainage test, it was not as consistent with all other results. Cake solids content, which is very often the crucial parameter in selecting chemical dosages, was in moderate agreement with other performance parameters, but the correlation was not strong. Estimation of final cake solids may be the most important function of using the Crown PressTM.
References [1] Severin BF, editor. Owners manual. Neogen Inc., Crown PressTM simulator, Lansing. MI; 1993. [2] Severin BF, Grethlein HE. Laboratory simulation of belt press dewatering: application of the Darcy equation to gravity drainage. Water Environ Res 1996;68:359–69. [3] Graham MT. Predicting the performance of belt filter press using the Crown PressTM for laboratory simulation, M.Sc. Thesis. Clemson University; 1998 (Advisor: Dr. D.L. Freedman). [4] Ayol A. Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-I: performance evaluations. Process Biochem 2005;40(7):2427–34. [5] Severin BF, Nye JV, Kim BJ. Model and analysis of belt drainage thickening. J Environ Eng 1999;125(9):807–15. [6] Severin BF, Collins BH. Advances in predicting belt press performance from lab data. In: Water Environment Federation, 65th Annual Conference, Paper AC92-040-001. 1992. [7] Sorensen PB, Christensen JR, Bruus JH. Effect of small scale solids migration in filter cakes during filtration of wastewater solids suspensions. Water Environ Res 1995;66:25–32. [8] Novak JT, Agerbaek ML, Sorensen BL, Hansen JA. Conditioning, filtering, and expressing waste activated sludge. J Environ Eng 1999;125(9):624–816. [9] Dentel SK, Ayol A. A procedure for determining specific resistance from gravity drainage data, 2004, in preparation. [10] Ayol A, Dentel SK. Shearing effects on dewaterability of activated sludge and synthetic sludge: determination of dewatering kinetics using immobilization cell. Environ. Sci. Technol., 2004, submitted for publication. [11] APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC; 1995.
Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-II: laboratory characterizations of drainability and filterability Azize Ayola,*, Steven K. Dentelb a
Department of Environmental Engineering, Dokuz Eylul University, Kaynaklar Campus, 35160 Buca, Izmir, Turkey b Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USA Received 19 February 2004; received in revised form 1 September 2004; accepted 25 September 2004
Abstract This paper provides an analysis of different measurements and derived parameters that may be used to characterize drainability and filterability dynamics. To better simulate the phenomena of gravity drainage and expression dewatering, specific devices and procedures were used in bench-scale conditioning and dewatering experiments. The setup included a plow simulator kit for drainage simulation and a mechanical dewatering unit, namely, the Crown PressTM, which simulates full-scale belt filter press dewatering. The effects of enzyme product additions prior to polymer conditioning of biosolids samples were investigated. The results showed that enzyme additions enhanced the drainability of the biosolids samples and this positively affected filterability of the samples in the press unit. According to these results, enzyme product addition for biosolids samples with polymer conditioning seems a promising new method of enhancing the performance of mechanical dewatering units in the future. # 2004 Elsevier Ltd. All rights reserved. Keywords: Biosolids; Anaerobic digestion; Dewaterability; Enzymes; Drainability; Filterability
1. Introduction The accurate prediction of biosolids dewaterability is important for the selection and optimization of conditioning chemicals, because full-scale trials are laborious and the polymers used for conditioning are expensive. Dewaterability is typically evaluated using a simple filterability test, such as qualitative or quantitative drainage tests, ‘‘time to filter’’ determinations, capillary suction time (CST) tests, or—for more exacting purposes—the specific resistance to filtration (SRF) test. Although the test methods give a general idea of dewaterability of conditioned biosolids, they cannot be claimed to simulate the full-scale dewaterability performance of mechanical dewatering units. Centrifuges, belt filter presses, and frame presses expend considerable * Corresponding author. E-mail addresses: [email protected], [email protected] (A. Ayol), [email protected] (S.K. Dentel). 0032-9592/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2004.09.024
amounts of energy in applying mechanical forces to the material, because this greatly improves water removal; yet, none of the lab-scale tests include this aspect of the process. Because the tests cannot indicate the response of conditioned biosolids to pressure or shear, they may fail to properly predict the relative performance of different conditioning chemicals. Furthermore, proper simulation of mechanical dewatering process should provide the imposed conditions in a sequence similar to that applied with the same process at full-scale. The small-scale test must therefore be processspecific. In these tests, dewatering by belt filter press was to be simulated, so the biosolids were subjected a sequence of four stages as may be considered to occur with this means of dewatering. The first stage that must be included is the chemical conditioning of the biosolids, in which a dewaterable floc should be obtained. This stage includes mixing, blending, and distribution up to deposition on the filter belt. The
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second stage is then gravity drainage, in which drainable water is removed from the biosolids as it accumulates on the belt surface. Plowing mechanisms should not be neglected, since they are generally assumed to improve the drainage rate. The biosolids are then layered between an upper and lower filter belts, and this ‘‘wedge zone’’ provides a lowpressure compressive dewatering stage, termed expression. Finally, increasing pressures (and shear) are applied at each successive roller, by decreasing the turn radius, to give continued expression and, ultimately, the final dewatered cake. These latter stages will provide unsatisfactory water removal, lateral escape of material off the belts, and loading limitations [1,2] if effective chemical conditioning is not effected in the initial stage. These problems should be observable in a good lab simulation procedure as well. For this work, conditioning, drainage, low-pressure expression, and high-pressure expression were simulated with small-scale procedures. Some of these procedures have already been compared to, and validated by, full-scale dewatering tests [3]. Here, we explored a broader range of quantitative measurements that can be obtained from the same procedures, in order to determine which were most meaningful. We varied chemical dosages in order to observe changes in these parameters, employing a flocculant polymer and an enzyme preparation as explained in the companion paper. The results reported here are therefore relevant in evaluating the relative efficacy of these two additives.
2. Objectives The objectives of the study were therefore: to determine whether dewaterability predictions using the more conventional tests (specifically, the classical CST test as presented in the companion paper) provide an adequate prediction of drainage and expression, as determined using a bench-scale dewatering device which simulates belt filter press dewatering; to determine which of the measurable parameters in the simulation procedures provide useful results, and which are either unique or redundant with others; and to utilize these data, with results from the companion paper, to determine relative efficacy and cost effectiveness of a flocculant polymer and an enzyme product for biosolids conditioning.
3. Materials and methods 3.1. Experimental approach The simulation included all four steps described above. First, the biosolids were conditioned with polymer, enzyme product, or combinations of these, in a jar test procedure
with dosages and mixing conditions as described previously [4]. The second step was to simulate gravity drainage on a belt filter press. A bench-scale gravity drainage plow simulator kit was used, similar to that recommended by Severin et al. [2,5], to produce drained biosolids comparable to those produced by a full-scale belt filter press. Our device was improved by allowing the same filter cloth to be employed for both the drainage and pressuring stages, assuring that any interactions between the biosolids and the cloth (such as blinding) would be preserved. A schematic diagram of the test kit is given in Fig. 1. The test kit consisted of a 0.10 m 0.05 m PVC pipe bushing and 0.10 m PVC connector to hold a 0.16 m 0.50 m belt filter cloth. In order to prevent leakage, the assembly was clamped together with PVC plates. The drainage area was about 78.54 m2. Continuous plowing was applied using a plow simulator blade with a rotor, rotated at 10 rpm. The volume of drained liquid can be determined as a function of time (used here to find specific resistance values) and the properties of the drained liquid can also be evaluated. The low- and high-pressure stages were simulated using a manually operated mechanical press, available commercially as the Crown PressTM (Neogen, Lansing, MI). In the same manner occurring during cake compression in a fullscale belt filter press, the Crown PressTM allows controlled periods of mechanical pressure to be applied to drained biosolids between filter cloths. Fig. 2 shows the press used in this study. An upper cloth is placed over the biosolids and base cloth used for gravity drainage, and this is placed on a curved, porous surface known as the crown. A cam allows tension to be manually applied to the belts using a steel bar, with the user referring to a pressure gauge to determine the proper degree of force to apply. It is claimed that this duplicates both the normal and shearing components of pressure applied in a belt filter press [1,6]. Tests have shown this device to faithfully duplicate the dewatering effects of a full-scale belt press [3]. 3.2. Analysis Anaerobically digested biosolids were obtained from the Newtown Creek Wastewater Treatment Facility in New York City, and the Wilmington (Delaware) Wastewater Treatment Facility, denoted as NYC and WIL, respectively. After enzyme and/or polymer conditioning, a 300 mL sample was poured onto the filter cloth material mounted within the gravity drainage kit, and drainage volumes were measured at predetermined time intervals (5, 10, 15, 20, 30, 45, and 60 s) using a graduated cylinder. Drainage sample viscosity was determined using a Brookfield LVT viscometer with an Ultra-Low Adapter (Brookfield Engineering Laboratory Inc.). After drainage, the same filter cloth with drained cake was placed on the Crown PressTM as a bottom belt. The cake was then squeezed, slowly applying 45.4 kg (100 pounds) of
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Fig. 1. Gravity drainage plow simulator used in this study.
pressure, releasing, press rapidly to 59.0 kg (130 pounds), releasing, then rapidly to 77.2 kg (170 pounds) force. At each force, the pressing time was 10 s. After pressing, the pressed cake diameter was measured to estimate the cake
migration. Filtrate samples were also collected for other analyses. 3.3. Determination of specific resistance to filtration (SRF) The filterability of a sludge can be described by the SRF, which is derived from Darcy’s law for flow through porous media [7,8]. The rearranged form of this relation, in terms of the inverse filtrate flux, is given in Eq. (1). 1 dt mðSRFwV=AÞ þ Rm ¼A (1) ¼ q dV Dp
Fig. 2. Crown PressTM used in this study (Neogen, Lansing, MI).
where q is the liquid flow rate per cross-sectional area (m s1); V the filtrate volume (m3); t the filtration time (s); A the cross-sectional area (m2); Dp the applied pressure (Pa); m the liquid viscosity (Pa s); SRF the specific resistance to filtration (m kg1); w the mass of solids deposited in the cake per unit liquid volume (kg m3); and Rm the resistance of the filter medium (m1). This equation is conventionally used to determine the SRF for filtration conditions in which either a vacuum or pressure is applied to give a constant pressure difference during over the course of the process. The model was used here to describe gravity drainage, which differed in that the pressure was much less, and it also decreased over time. The above equation was integrated over this time period, while
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accounting for the pressure changes, and fit by non-linear regression to the V(t) data to obtain the SRF values [9]. When completed, each test of conditioning and dewatering included the measurements listed in Table 1. Some of these results are presented in the companion paper, and some in this paper. Several measurements proved not to be useful and are not reported.
4. Results 4.1. Specific resistance Fig. 3a and b show the gravity drainage results for enzyme-treated, polymer conditioned NYC and WIL samples, including controls without addition of polymer and/or enzyme. As with the preceding companion paper, results are shown in terms of isopleth graphs, in this case lines of constant log10 SRF values on axes of enzyme and polymer dosage (note that the scales are non-linear). The SRF values are seen to generally decrease with polymer dose until an optimum dose range is attained. Higher doses may actually cause poorer performance, particularly with the WIL biosolids. The enzyme doses to the biosolids also effectively reduced the SRF, indicating that either additive, or the two combined, improved the rate of drainage. The optimum enzyme dose is somewhat above 15 mg/L for both biosolids, with overdosing being deleterious. This is in good agreement with CST results reported in the companion paper, confirming that the drainage SRF measurement has meaning, and is probably related to degree of flocculation. It should be noted that the SRF values are at least two orders of magnitude less than SRF values commonly measured by the vacuum or pressure filtration methods. Ideally, specific resistance is a property of the matter being retained on the filter, but the effects of compression are much less here due to the low applied pressure. Significantly, the plow simulator was in operation, which may have reduced the SRF due to lateral shear applied to the cake as it was deposited. Other results [10] demonstrate that lateral shear improves dewaterability. Thus, these results may be valid in an absolute sense, and they can certainly be used in a relative
Fig. 3. Log10 SRF (m kg1) values as a function of polymer and enzyme doses: (a) NYC and (b) WIL.
sense in assessing the gravity drainage stage in belt press operation. 4.2. Solids migration Severin and Collins [6] observed that the area occupied by solids increased during the pressure application stages in the Crown PressTM, and quantified this as the increase in radial dimension of the consolidated material. Presumably, this could be related to lateral expansion of the solids between the belts, which leads to leakage if excessive. As an
Table 1 Measured and derived parameters for this study Procedure/measurement
Reference
Derived parameter(s)
Conditioning stage Capillary suction time (CST)
Standard Methods [11]
CST 1/CST
Drainage stage Filtrate volume vs. time Filtrate turbidity Suspended solids
This paper Standard Methods [11] Standard Methods [11]
SRF log10 SRF initial flux, A(dV/dt), %drainage @ 60 s Turbidity (NTU) Suspended solids (mg/L)
Expression stages Cake dry solids content Solids migration
Standard Methods [11] Severin and Collins [6]
Total solids (%) Absolute migration (cm), %area increase
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as indicated for highest cake solids, with no dramatically change of cake area in overdosing ranges. For both biosolids, the optimum dose combination is about 20 mg/ L enzyme followed by 250 mg/L polymer. The smaller cake areas appear to be related with higher cake solids (reported in the companion paper), simply because these samples had higher drained volumes, and thus, less solids migration. 4.3. Relationships and utility of measured parameters
Fig. 4. Area increase (%) values as a function of polymer and enzyme doses: (a) NYC and (b) WIL.
alternative, this work instead determined the percent increase in solids area between the filter cloths during cake formation. While readily determined from the increased radial dimension (if the original radius is known), the area increase may be related to percent increase in width of the deposited solids layer at full-scale. Fig. 4a and b show the percent increase in cake area with different dose combinations of polymer and enzyme. Enzyme addition and polymer conditioning reduced the cake area, used either separately or in combination. Decreased cake area is seen up to the optimal dose ranges
All parameters listed in Table 1 were compared to determine correlations. This was done by examination of graphical relationships, by determining Spearman rank order correlations, and by regression analysis of power-law relations between pairs of parameters. Selected graphs are shown to illustrate the more notable correlations, and all are summarized in Table 2. The CST measurement appears to be very justified as a commonly employed predictor of dewaterability. There was a clear correlation with specific resistance (particularly evident when plotted as log10 SRF versus 1/CST, Fig. 5a). The reciprocal of CST is a fundamentally appropriate parameter, since it is indicative of filtration rate, and it also showed good correlation with both turbidity and suspended solids measurements (Fig. 5b and c), cake solids (Fig. 5d), and—although less strongly—to solids migration (Fig. 5e). Table 2 only reports statistical results for 1/CST values because the results are essentially identical using CST instead. In contrast to CST, the specific resistance results were less useful. In addition to the CST relationship, the SRF correlated somewhat with filtrate suspended solids and turbidity and, to an even lesser extent, with cake solids and solids migration (figures not shown). It is noted that the SRF values were measured during gravity drainage only, and the performance in this early stage is evidently not relevant to the final expression process. In comparison, the initial slope of the drainage curve (converted to a flux rate, A(dV/dt)), was actually more successful in relating to other measurements. Obviously, it is also a more direct determination from the drainage data. This value was not only related to the amount of drainage
Table 2 Relationships between measured parameters Parameters
Log10 SRF
Initial flux
Drainage @ 60 s
Turbidity
1/CST Log10 SRF Initial flux Drainage @ 60 s Turbidity Suspended solids Cake solids
0.31 0.28
0.38 0.29 0.18 0.18
0.34 0.42 0.14 0.13 0.71 0.67
0.65 0.29 0.45 0.40
0.50 0.36 0.41 0.40
Suspended solids 0.72 0.32 0.36 0.30 0.85
0.64 0.37 0.32 0.32 0.86
Cake solids
%Area increase
0.50 0.61 0.24 0.22 0.46 0.47 0.52 0.57 0.490.50 0.45 0.48
0.58 0.25 0.48 0.51 0.37 0.32 0.68
0.52 0.27 0.37 0.58 0.38 0.35 0.69
Values in bold indicate squared spearman rank order correlation coefficients and values in italic indicate squared correlation coefficients (r2) for power fit relationships between parameters.
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Fig. 5. (a) Log10 SRF (m kg1) values as a function of filterability rate (1/CST); (b) turbidity (NTU) values as a function of filterability rate (1/CST); (c) suspended solids (mg/L) values as a function of filterability rate (1/CST); (d) dry solids (%) values as a function of filterability rate (1/CST); (e) area increase (%) values as a function of filterability rate (1/CST).
obtained at 60 s (Fig. 6a), but also to cake solids and (inversely) to turbidity, suspended solids, and solids migration (Fig. 6b). The latter correlation is evidently due to prevention of lateral solids movement when the suspending fluid is rapidly depleted. The percent drainage at 60 s may be considered as similar to the ‘‘time to filter’’ (TTF) measurement
sometimes used in lieu of the SRF. The TTF is the time required to remove 50% of the sample’s initial volume as filtrate, but the drainage measurements did not reach this amount of removal in all cases. The percent drainage was related to dry solids and to solids migration (Fig. 7a and b), because—in contrast to the initial slope—it is a metric of drainage extent near the completion of the process, so it
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Fig. 6. (a) Percent drainage @ 60 s and (b) area increase (%) values as a function of initial slope of the drainage plots.
Fig. 7. (a) Dry solids (%) and (b) area increase (%) values as a function of percent drainage @ 60 s.
is more likely to predict expression effects. Thus, dry solids content was inversely related to solids migration (Fig. 8). The suspended solids values was closely related to turbidity, as might be expected (not shown). A weak correlation was also apparent to cake solids and to solids migration, with greater suspended solids preventing greater cake solids (e.g. by filter blinding) and producing greater lateral movement as a consequence. Overall, the drainage and Crown PressTM procedures were found relatively simple to use. In this research, comparisons to full-scale results were not done, but they have been carried out in the past with very consistent results [3,5].
reduction in biosolids management costs under many conditions, while Fig. 9 shows that the added costs due to this amount of enzyme use are extremely low under this scenario. Alternatively, by following the isopleths of equal cost in Fig. 9, while comparing to the performance-related isopleths in graphs, such as Fig. 4, the optimal performance for a specified total chemical cost can be determined.
4.4. Efficacy of enzyme pre-treatment Figs. 3 and 4 confirm observations made in the companion paper: use of the enzyme product at roughly 20 mg/L significantly improves both drainage and dewatering, particularly if a polymer is also used. For both biosolids, a 10 mg/L enzyme dose prior to a 250 mg/L polymer usage will increase the dewatered biosolids TS by approximately 50% (e.g. from 20% solids to 30%), which would decrease the mass and volume of dewatered material by about 33%. This will provide a significant
Fig. 8. The relationship between dry solids (%) and solids migration (%).
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The cost of using the enzyme product appears to be quite acceptable. The optimum dose for common, anaerobically digest sludges may be around 15 mg/L. Acknowledgements The first author was financially supported by the Technical and Scientific Research Council of Turkey (TUBITAK)-NATO Science Fellowship Program and the U.S. National Science Foundation under Award #0229293: Chemical, Rheological, and Physical Exploration of GelLike Behavior in Conditioning and Dewatering Processes. The authors thank the personnel of the Newtown Creek and Wilmington treatment facilities, and the Neogen Corporation, Lansing MI, for providing the Crown PressTM. Fig. 9. Cost per million liters of enzyme and polymer addition. Numbers are in US$ per milliliters of biosolids prior to conditioning. Costs used in developing this graph were: polymer, US$ 1.20 per lb (US$ 2.64 per kg) and enzyme, US$ 4.68 per lb (US$ 10.31 per kg), as provided by the suppliers.
5. Conclusions Enzyme product additions in advance of polymer conditioning enhanced the dewaterability of both NYC and WIL anaerobically digested biosolids samples, based on both drainability and expression test results. These findings were consistent with the CST, filtrate turbidity, suspended solids, and cake solids measurements, presented in the companion paper. For both biosolids used, the enzyme dose giving best preconditioning was approximately 15 mg/L, although this value varied somewhat based on the measurements used for dosage criteria. The relatively simple CST test was a very good predictor of most other test results. Although an SRF could be determined from the gravity drainage test, it was not as consistent with all other results. Cake solids content, which is very often the crucial parameter in selecting chemical dosages, was in moderate agreement with other performance parameters, but the correlation was not strong. Estimation of final cake solids may be the most important function of using the Crown PressTM.
References [1] Severin BF, editor. Owners manual. Neogen Inc., Crown PressTM simulator, Lansing. MI; 1993. [2] Severin BF, Grethlein HE. Laboratory simulation of belt press dewatering: application of the Darcy equation to gravity drainage. Water Environ Res 1996;68:359–69. [3] Graham MT. Predicting the performance of belt filter press using the Crown PressTM for laboratory simulation, M.Sc. Thesis. Clemson University; 1998 (Advisor: Dr. D.L. Freedman). [4] Ayol A. Enzymatic treatment effects on dewaterability of anaerobically digested biosolids-I: performance evaluations. Process Biochem 2005;40(7):2427–34. [5] Severin BF, Nye JV, Kim BJ. Model and analysis of belt drainage thickening. J Environ Eng 1999;125(9):807–15. [6] Severin BF, Collins BH. Advances in predicting belt press performance from lab data. In: Water Environment Federation, 65th Annual Conference, Paper AC92-040-001. 1992. [7] Sorensen PB, Christensen JR, Bruus JH. Effect of small scale solids migration in filter cakes during filtration of wastewater solids suspensions. Water Environ Res 1995;66:25–32. [8] Novak JT, Agerbaek ML, Sorensen BL, Hansen JA. Conditioning, filtering, and expressing waste activated sludge. J Environ Eng 1999;125(9):624–816. [9] Dentel SK, Ayol A. A procedure for determining specific resistance from gravity drainage data, 2004, in preparation. [10] Ayol A, Dentel SK. Shearing effects on dewaterability of activated sludge and synthetic sludge: determination of dewatering kinetics using immobilization cell. Environ. Sci. Technol., 2004, submitted for publication. [11] APHA, AWWA, WEF. Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC; 1995.