The Operator s Guide to Monitoring Secondary Clarifier Performance

Transcription

1 The Operator s Guide to Monitoring Secondary Clarifier Performance Heinrich O. Buhr * 1 Carollo Engineers, Talbert Ave., Fountain Valley, CA *To whom correspondence should be addressed. hobuhr@carollo.com. ABSTRACT The State Point Analysis (SPA) procedure provides exceptional insight into the expected performance of activated sludge secondary sedimentation basins. Yet, the operating guidance offered by the SPA is not applied regularly and with confidence particularly in day-to-day operations. Such lack of use may stem from the perceived complexity of the procedure, including the (unnecessary) need to perform graphical constructions. The paper seeks to encourage the application of SPA principles by presenting straightforward algebraic and spreadsheet calculations for assessing the risks of clarification or thickening failure in clarifiers. Clarifier performance is monitored by evaluating a Clarification Safety Factor, based on comparing the observed settling velocity of the sludge with the overflow rate in the clarifiers. Similarly, a calculated Thickening Safety Factor provides warning of impending process limitations due to deteriorating settleability or limited return sludge pumping capacity. The mathematical background is summarized and step-by-step calculations are detailed in the paper. KEYWORDS: Secondary Clarifiers, State Point Analysis, Settleability, Settling, Clarification, Thickening. NOMENCLATURE A = Clarifier surface area, ft 2 or m 2 A = Point on the SPA diagram representing settling flux at the clarifier feed concentration CSF = Clarification Safety Factor DSVI = Diluted SVI; basis used in correlating some reported settling constants exp ( ) = Exponential function, i.e., e raised to the power of the quantity in parentheses F n = Non-ideality factor (to account for deviation of observed results from theoretical predictions of the State Point Analysis) G = Solids flux at concentration X, lb/hr ft 2 or kg/hr m 2 k 1 = Units conversion factor from [concentration x velocity] to [settling flux] = in U.S. Customary units, or = 1.0 in SI Units k 2 = Units conversion factor from [Q/A] to [velocity] = 5,570 in U.S. Customary units, or = 1.0 in SI Units, or = 6,689 when using Imperial gallons rather than U.S. gallons ln = natural logarithm of the quantity shown mgd = Million (U.S.) gallons per day (= million Imperial gallons per day) n = Second settling constant in the Vesilind settling equation (1), 1/(mg/L) or 1/(kg/m 3 ) Q = Clarifier effluent flow rate, mgd or m 3 /hr Q ave = Daily average clarifier effluent flow rate, mgd or m 3 /hr Q sp = Maximum expected sustained one-hour peak flow rate, mgd or m 3 /hr Q p = Effective peak flow rate, used for evaluating the minimum required RAS flow, mgd or m 3 /hr; Q p = Q ave x CSF Q R = Underflow or RAS flow rate, mgd or m 3 /hr

2 Buhr Q R,p = Highest available RAS flow rate that would be provided by the RAS pumping system as currently configured (considering issues such as return ratio, units in service or pre-set pumping limits) Q Rmin = Minimum (or critical ) RAS flow rate required to avoid thickening failure, mgd or m 3 /hr Q Rmin,p = Minimum RAS flow rate required at the expected peak plant flow rate, mgd or m 3 /hr RAS = Return activated sludge. In this analysis, the term RAS represents the total clarifier underflow, including any wastage that may be taken from the underflow. S = State Point on the SPA diagram; intersection of the overflow operating line and the influent (feed) concentration SI = International System of Units (Système International d'unités). Note that a time unit of 1 hour is used in this paper. SPA = State Point Analysis SSVI 3.5 = Stirred SVI at 3.5 g/l; basis used in correlating some settling constants SVI = Sludge Volume Index, ml/g TSF = Thickening Safety Factor; equation (10) TSF p = Thickening Safety Factor at peak flow; equation (13) V 0 = First settling constant in settling equations (1) and (2), ft/hr or m/hr V F = Zone settling velocity at the clarifier feed concentration, ft/hr or m/hr V of = Vertical upflow (overflow) velocity in the clarifier, ft/hr or m/hr V of,p = Vertical overflow velocity at peak flow, ft/hr or m/hr V R = Zone settling velocity at RAS concentration, ft/hr or m/hr V X = Zone settling velocity at concentration X, ft/hr or m/hr X = Solids concentration, mg/l or kg/m 3 (Note: kg/m 3 = g/l) X F = Solids concentration in the clarifier influent (feed), mg/l or kg/m 3 X L = Limiting solids concentration at which slopes of the underflow operating line and settling flux curve are equal, mg/l or kg/m 3 X L1, X L2 = Starting estimate and next estimate in iteration to find X L (Appendix A) X M = Second settling constant in the settling equation (2), mg/l or kg/m 3 ; (X M is equivalent to 1/n) X R = Solids concentration in the RAS sample, mg/l or kg/m 3 INTRODUCTION Based on solids-flux principles, the State Point Analysis (Keinath, et. al., 1977) is well established in the wastewater industry as a procedure for design and operation of activated sludge secondary sedimentation basins. Although its value is recognized, the method is still not being used commonly and confidently (Narayanan, et. al., 2000a). The problem stems at least in part from uncertainty about the settling constants to be used in the analysis and the perceived complexity of the graphical construction. This paper seeks to simplify the process by providing simple algebraic procedures for applying SPA guidelines to monitor the clarification and thickening performance of secondary sedimentation basins. Zone Settling Velocity The settling characteristics of activated sludge are key to the analysis of clarifier performance. The zone settling velocity of a sludge sample may be readily measured, as summarized in a subsequent section. Settling velocities progressively decrease as the sample concentration increases (Figure 1). The relationship may be represented by an exponential equation of the form proposed by Vesilind (1968): V X = V 0 exp (- nx) (1) 2

3 The Operator s Guide to Monitoring Secondary Clarifier Settling Velocity, V x V X = V 0 exp (- nx) V X = V 0 exp (- X/X M ) Solids Concentration, X Figure 1 Variation of Activated Sludge Settling Velocity with Concentration/ In this formula, the settling constant V 0 represents the theoretical zone settling velocity at near-zero solids concentration. The constant n, however, does not offer a ready physical interpretation. This disadvantage often leads to confusion in ensuring that the units of measurement employed for n are consistent with those used for the solids concentration. To address this recurring inconvenience, an alternative form of the settling equation is proposed, as V X = V 0 exp (- X/X M ) (2) X M is simply the reciprocal of n. The advantage of this formulation is that X M is now more familiarly expressed in the same units as X, such as mg/l. Technically, X M is equivalent to the solids concentration at which the settling velocity is equal to the fraction 1/e (or 36.8%) of V 0. Significantly, it may also be shown mathematically that X M corresponds to the solids concentration for which the settling flux in the clarifier is at a maximum. The settling constants V 0 and X M form the basis for further analysis of the activated sludge settling process. State Point Analysis Solids flux theory, introduced by Coe and Clevenger (1916) and subsequently advanced by others (summary by Narayanan, et. al., 2000a), provides the foundation for the State Point Analysis (SPA), as formalized by Keinath, et. al. (1977). The SPA employs a graph of solids flux against solids concentration to analyze the operation of secondary clarifiers. Even though the purpose of this paper is to apply the principles of SPA without the need for graphical constructions, it is useful to review the diagrammatic background of the procedure. Components of the SPA Figure 2 summarizes the components of the SPA diagram. These include (1) the settling flux curve, (2) a line representing the clarifier influent (feed) concentration, (3) the overflow operating line and (4) the underflow operating line. The point of intersection of the overflow operating line and the influent concentration is referred to as the State Point, S. 3

4 Buhr Solids Flux, G (1) (2) A S (3) (4) (1) Settling flux curve (2) Clarifier feed concentration (3) Overflow operating line (4) Underflow operating line A Settling flux at feed concentration S State Point Solids Concentration, X Figure 2 - Components of the State Point Analysis diagram. The settling flux curve (1) represents the mass rate of solids that would settle per unit area (lb/hr ft 2 or kg/hr m 2 ) at a particular solids concentration, X. Settling flux may be expressed as the product of the solids concentration X and the downward settling velocity V X, or G = k 1 X V X = k 1 X V 0 exp(- X/X M ) (3) The constant k 1 is introduced to convert from the conventional units of mg/l and ft/hr, customarily used for sludge concentration and velocity in the U.S., to units of lb/hr ft 2 for settling flux. When quantities are expressed in SI ( metric ) equivalents of kg/m 3, m/hr and kg/hr m 2, the constant k 1 simply becomes 1.0. The U.S. Customary and SI units employed for each parameter are defined in the Nomenclature section. The clarifier influent (feed) concentration line (2) is drawn vertically at the concentration of the feed entering the clarifier, X F. The overflow operating line (3) corresponds to the solids flux that would be carried upwards by the upflow rate in the clarifier, at a given solids concentration. The line is formulated as G = k 1 X V of (4) and is drawn through the origin at a slope corresponding to the clarifier upflow (overflow) velocity. The overflow velocity is given by V of = k 2 Q/A, (5) so that Slope of the overflow operating line (flux/concentration) = k 1 k 2 Q/A (6) 4

5 The Operator s Guide to Monitoring Secondary Clarifier Q is the effluent flow rate from the clarifier. It does not include return activated sludge (RAS) flow. The constant k 2 is another units conversion factor, defined in the Nomenclature section. The clarifier underflow (or RAS) operating line (4) is drawn to pass through the State Point at a negative slope corresponding to Q R /A, where Slope of underflow operating line (flux/concentration) = - k 1 k 2 Q R /A (7) The term Q R, or RAS flow rate, is used to represent the total underflow rate from the clarifier and is not reduced to account for any waste sludge flow. The point at which the underflow operating line cuts the concentration axis corresponds to the return sludge concentration. Interpretation of the SPA The use and interpretation of the State Point Analysis diagram for examining and illustrating the expected behavior of a secondary clarifier has been comprehensively described in the literature (Keinath et. al., 1977; Ekama, et. al., 1997; Narayanan, et. al., 2000a). Among the insights to be gained from a State Point Analysis, the key principles for stable clarifier operation may be summarized as follows: 1. Clarification: The clarifier must separate solids from the influent liquid to produce a clear effluent. To meet this requirement, the State Point S on the SPA diagram must fall below Point A, on the settling flux curve. A location of S above the settling curve would represent unstable operation, with solids carried over the weir in the effluent. 2. Thickening: The separated solids must be adequately thickened to reach the appropriate return sludge concentration. On the SPA diagram, this condition would be met as long as the underflow (RAS) operating line does not cross the decreasing part of the settling flux curve. This criterion establishes a minimum acceptable RAS withdrawal rate. The Clarification Function The clarification function compares the location of the State Point S with the point on the settling flux curve directly above it (point A). For stable operation, the State Point must fall below the settling flux curve. The criterion may be easily evaluated without the need for actually constructing an SPA diagram. Any flux value on the SPA diagram effectively represents a solids concentration multiplied by a velocity (as seen in equations 3 and 4). Thus, the flux at the State Point is equal to the feed concentration multiplied by the overflow velocity in the clarifier. Similarly, the flux at point A represents the feed concentration multiplied by the settling velocity of the influent solids. Since the two concentrations (X F ) are the same, the requirement that the flux at point A must be greater than the flux at point S is equivalent to saying that the solids settling velocity must be greater than the overflow (i.e., vertical upflow) velocity in the clarifier. The ratio of influent settling velocity to overflow velocity may be defined as a Clarification Safety Factor (CSF), where: CSF = Zone settling velocity at the clarifier feed concentration / Overflow velocity = V F / (k 2 Q/A) (8) Referring to the SPA diagram, the CSF would be represented by the ratio of Point A to Point S. A CSF value of 1.0 would indicate incipient clarification failure, because point S would lie directly on point A, while a CSF greater than 1.0 would represent satisfactory clarification since the settling velocity of the influent feed would be greater than the upflow velocity (point A lies higher than point S). If the State Point, S, falls above the settling flux curve, the clarifier is said to be overloaded with respect to clarification. In this case the settling velocity would be lower than overflow velocity and the CSF would be less than 1.0. The solids entering the clarifier would not begin to move downward, but would be carried upward from the feed point and over the effluent weirs, resulting in clarification failure. 5

6 Buhr Equation (8) shows that, in practice, it is not necessary to construct an actual SPA diagram to evaluate the clarification function of the clarifier. The current CSF may conveniently be calculated using the measured feed settling velocity and the clarifier effluent flow rate, Q. This CSF may then be compared to a target safety factor, which is defined to include consideration of the peak flow ratios that may be encountered at the plant. The subject of selecting an appropriate target value for the CSF is discussed in a subsequent section. The Thickening Function The separated solids must move to the bottom of the clarifier by the combined action of solids settling under gravity and the downward velocity due to the underflow withdrawal rate. The combination of these two flux components represents the maximum solids flux that can be handled by the clarifier at a given underflow rate. When this limiting flux is only just sufficient to match the incoming solids loading to the clarifier, the secondary clarifier is said to be critically loaded with respect to thickening. At lower underflow rates, the entering solids loading would exceed the maximum achievable downward solids flux and cause an accumulation of solids in the clarifier, with a corresponding rise in the sludge blanket. Should this condition continue, the sludge blanket could eventually propagate to the surface and result in solids loss over the weirs. The minimum (or critical ) underflow rate, Q Rmin, that corresponds to a flux limitation is defined on the SPA diagram as the value at which the underflow operating line is just tangential to the settling flux curve. To avoid a limiting-flux condition, the actual operating RAS flow rate must be greater than Q Rmin. The minimum RAS flow rate may be estimated graphically, or it may be derived mathematically. By drawing an underflow operating line on the SPA diagram, passing through the State Point and tangential to the settling flux curve, the slope of this critical operating line (flux/concentration) may be measured and converted to a minimum RAS flow rate by: Q Rmin = - (Slope) A / (k 1 k 2 ) (9) As an alternative to graphical construction on an SPA diagram, Q Rmin may be derived mathematically by finding the point at which the slopes of the underflow operating line and settling flux curve would be equal. This computation involves an iterative solution for X L, the limiting solids concentration, with a subsequent calculation for Q Rmin. The required iterative calculation may be readily implemented on a spreadsheet and is described in Appendix A. The settling characteristics V 0 and X M must be specified, to define the settling flux curve. A Thickening Safety Factor (TSF) or RAS Safety Factor may now be defined for the thickening function in an analogous manner to the Clarification Safety Factor, as the ratio of the actual return sludge pumping rate to the required minimum: TSF = RAS pumping rate / Q Rmin (10) The Thickening Safety Factor would therefore give a measure of the extent to which critical operating conditions are being approached or avoided. A value of TSF = 1 would represent incipient thickening failure, while TSF > 1 would indicate that the RAS flow rate is above the danger point. It should be noted that the thickening function discussed here addresses only the potential limitation that might exist in moving solids through the liquid column to the floor of the clarifier. The analysis assumes that, once solids reach the floor, they will be removed in the underflow stream. In practice, this removal relies on a collection mechanism to pick up or move settled sludge from all points on the clarifier floor to the RAS withdrawal point. In addition to providing sufficient RAS pumping capacity, it is therefore also important to ensure that the sludge collection mechanism has sufficient capacity to avoid blanket buildup due to ineffective sludge collection (Albertson and Okey, 1990; Narayanan, et. al., 2000b). 6

7 The Operator s Guide to Monitoring Secondary Clarifier TARGET SAFETY FACTORS The two safety factors CSF and TSF present the operator, planner and designer with a means of assessing the degree to which current or projected future operating conditions are safeguarded against failure. Calculating the Clarification Safety Factor, CSF, requires measurement of the mixed-liquor settling velocity, while the Thickening Safety Factor, TSF, requires knowledge of the settling constants V 0 and X M of the activated sludge. The observed Safety Factors may then be compared against established target values, to determine whether operating conditions provide a sufficient margin of safety against the risk of clarifier failure. The following guidelines may be used in the initial selection of minimum target values for the two safety factors. In time, these limits may be modified through experience and observation, if the selected values prove either too conservative or too aggressive at a particular facility. Allowance for Non-Ideality A concern about using the SPA procedure for evaluation of clarifier stability is that the analysis relies on a one-dimensional representation of flow in the clarifier the method assumes that settling and flow occur only in the vertical direction. Actual flow patterns in a clarifier are, of course, considerably more complex. The question therefore arises as to how well the one-dimensional flux analysis would approximate the stability characteristics of real-life clarifiers. This issue was examined by Ekama and Marais (1986), who compared failure values predicted by the SPA procedure with the results of a Dutch study (STOWa, 1981) that covered 23 plants and analyzed 45 cases of solids loading in full-scale circular clarifiers. They concluded that solids overload appeared to occur at loading rates amounting to about 80 percent of values predicted by the flux procedure. The 80-percent estimate was subsequently reinforced by hydrodynamic modeling (Ekama and Marais, 2004). This conclusion suggests that a minimum multiplier, F n, of 1.25 (or 1/0.80) should be included in the target safety factor, to allow for non-ideality in circular clarifiers. A similar analysis on results from 27 stress tests conducted on three rectangular clarifiers was described in an IAWQ report on secondary settling tanks (Ekama, et. al., 1997). This evaluation suggested that, in contrast to circular clarifiers, for which the permissible solids loading rate was 25% over-predicted by the flux procedure, the permissible flux appeared to be about 15% under-predicted for rectangular clarifiers. The interpretation of this result would be that a non-ideality multiplier of 0.85 might be used in rectangular clarifiers. The IAWQ report cautiously points out, however, that this result may be specific to the particular units studied and may not be universally applicable to all rectangular clarifiers. The selection of a multiplier to allow for non-ideality of the SPA procedure therefore remains up to the user. Until further substantiation, it may be prudent to select a non-ideality factor, F n, of 1.25 for both rectangular and circular clarifiers. Target Clarification Safety Factor As plant flow varies seasonally and during the course of the day, the State Point moves up and down the feed concentration line on the SPA diagram. As the State Point approaches the settling flux curve (and CSF decreases towards 1.0), solids at the feed concentration would no longer show a net downward movement, but would be carried upwards toward the weirs. A minimum target value for the Clarification Safety Factor should be selected to avoid solids overflow. It is convenient to base a target CSF on a selected average daily flow, such as annual average or maximummonth average, and then to choose the target CSF to account for flow variations in excess of that average. These variations would include diurnal patterns as well as any sustained peaks that may be expected due to seasonal conditions. The target CSF would consist of the selected peak flow ratio multiplied by the non-ideality factor discussed above: Target CSF = (Q sp / Q ave ) F n (11) 7

8 Buhr Here, Q sp typically represents the maximum sustained one-hour peak flow at which it is still intended to avoid clarifier solids overflow, while Q ave is the selected average daily flow. The reference to an intended maximum flow recognizes that there may be circumstances, such as severe storm flows, under which it is no longer economically feasible to provide increasingly larger clarifier surface areas to meet all flow conditions. To address extreme high-flow conditions, other options may be considered, such as offline storage, step feed (Buhr, et. al., 1984), or contact-stabilization operation. A typical Target CSF for plants not subject to excessive wet-weather flows may be 2.0 for large plants or 2.5 for smaller facilities. Should the average plant flow rate and expected peak flows vary substantially on a seasonal basis, different minimum CSF targets may potentially be selected for different seasons, at the option of the user. Target Thickening Safety Factor To develop a Thickening Safety Factor, it would be useful to compare the available return sludge pumping capacity against the minimum that would be required at the anticipated peak plant flow rate. The minimum required RAS flow rate is represented by the slope of a line drawn from the State Point on the SPA diagram, tangent to the settling flux curve. Because the settling plot curves, Q Rmin does not increase linearly (proportionately) as plant flow increases. This non-linearity means that the required Q Rmin cannot be calculated at the average plant flow rate and then proportioned up to represent higher flow rates. Q Rmin must be calculated directly at the expected peak plant flow rate, rather than at average flow. For calculation purposes, the effective peak flow rate may conveniently be represented by Q p = Q ave x Target CSF, (12) while the corresponding minimum return sludge flow rate at peak flow may be designated as Q Rmin,p. The quantity Q Rmin,p may be calculated on the SPA diagram, or by the spreadsheet method of Appendix A, noting that the flow rate used in the calculation must be the effective peak flow rate, Q p. By including the target CSF in the definition of effective peak flow, the previously noted non-ideality factor, F n, would be automatically recognized in the calculation. A Thickening Safety Factor for peak flow may now be re-defined as TSF p = Available RAS pumping capacity / Q Rmin,p (13) At a target TSF p of 1.0, the available return sludge pumping capacity would match the minimum required to avoid thickening failure at peak flow. In practice, experience shows that a case of thickening failure may often sneak up unexpectedly. With the clarifier full of sludge, it is useful to have a certain amount of spare RAS pumping capacity available, to rapidly return the built-up sludge to the aeration basin(s), instead of simply keeping pace with an overloaded condition. Operators may therefore prefer to select a higher minimum target TSF p (such as 1.25 for large plants to 1.5 for smaller plants), to provide a reserve for emergency return sludge pumping capacity. MEASURING SETTLING CHARACTERISTICS Calculating the CSF or TSF p both require information on the settling characteristics of the activated sludge. Zone Settling Velocity Zone settling is defined as the initial condition under which the activated sludge settles as a body, with a defined interface, before compaction begins. The zone settling velocity of a sludge sample may be measured by plotting the decreasing level of the sludge/water interface in a test vessel with time. One of the simpler test units is the 2-liter wide-mouth settleometer. A settling test procedure using this equipment has been detailed by Wilson (1993). Essentially, interface readings are taken at ½-minute to 1-minute intervals, depending on the rate of settling. A line is then drawn on the plot to approximate the steepest rate 8

9 The Operator s Guide to Monitoring Secondary Clarifier of settling during the initial settling period, ignoring any preliminary flocculation and the final compaction period. From the slope of the line, the zone settling velocity may be calculated in ft/hr or m/hr by making an appropriate conversion from settleometer scale readings to actual height per scale division. Various innovations in settling test equipment have been introduced to counter the potential influences of wall effects, sludge bridging or height of the settling column. These include addition of stirring and the use of wider and longer vessels. Such enhanced apparatus should certainly be employed when available; however, even the use of less sophisticated equipment is perfectly acceptable, especially if the alternative is to do no testing at all. Settling Constants The settling test will approximate the settling velocity of the activated sludge at the concentration of the initial sample. A range of concentrations for the same sludge may potentially be prepared by using a thickened sample, or diluting with secondary effluent. By repeating the settling test over a range of solids concentrations, a curve of settling velocity against solids concentration may be obtained. Usually, the resulting curve is well described by the settling velocity equation. The settling characteristics V 0 and X M may be estimated by expressing equation (2) as ln V X = ln V 0 - X/X M, (14) and plotting the settling data as (ln V X ) versus X. V 0 may be found from the intercept, and X M from the slope of the best-fit straight line through the data, as illustrated in Figure 3. Intercept = ln V 0 ; V 0 = exp(intercept) Best-fit straight line ln V X Slope = -1/X M ; X M = -1/Slope Concentration, X Figure 3 Determination of settling constants from settling data. = Measured data points Short-Cut Method for Settling Constants Generating the data needed for the multi-point data-fitting analysis suggested in Figure 3 is a timeconsuming process. In a busy operating environment this drawback may mean that the procedure will in practice be carried out infrequently or not at all. To encourage regular evaluation of settling characteristics, therefore, the following short-cut procedure is proposed: 9

10 Buhr Perform settling tests on only two samples: the clarifier influent feed and the return sludge. The information wanted is the settling velocity and the sludge concentration on each sample. Then apply the data-fitting concept of Figure 3 to these two data points to find V 0 and X M. Since only two data points are involved, an actual graphical plot is not necessary and the two settling constants may be calculated directly, as: X M = (X R - X F ) / (ln V F - ln V R ) (15) V 0 = exp (ln V F + X F /X M ) (16) The subscripts F and R refer to the Feed and RAS samples, respectively. Admittedly, the constants V 0 and X M calculated from only two data points may be less accurate than if they were found by a least-squares fit through a larger number of measurements. Nevertheless, the method has the advantage that it is relatively quick to carry out. This may make the difference between tests being done regularly, or not at all. By testing on a weekly basis, a valuable database of site-specific settling characteristics can be built up over time. If results from the two-sample method show unacceptable variability, accuracy may be enhanced by testing a third sample, at a concentration between that of the feed and RAS. Settling constants would then be derived from a best-fit straight line through the three data points, using the procedure illustrated in Figure 3. MONITORING PROTOCOL The recommended clarifier monitoring procedure is summarized as follows and illustrated in a Case Study with sample calculations in Appendix B: 1. Initial Setup Establish minimum target values for the Clarification Safety Factor (CSF) and Thickening Safety Factor (TSF p ) appropriate for the facility, as discussed under Target Safety Factors. Typical values might be for Target CSF and for Target TSF p. Review from time to time, based on experience for example, if limits are repeatedly hit without apparent adverse effect, minimum target values may potentially be decreased. 2. Monitor Risk of Clarification Failure For the clarification function, only one measurement is needed the zone settling velocity of the clarifier influent. The procedure is as follows: Daily (see Note 8 on Testing Frequency), measure the zone settling velocity V F on a sample of the clarifier feed, as described under Zone Settling Velocity, above. Also note the plant flow rate, Q, at the time of sampling. Calculate the Clarification Safety Factor, CSF, using: CSF = V F / (k 2 Q/A) (17) Check CSF against the minimum target CSF. A CSF well above the minimum target value provides the confidence that the clarifier system is not in danger of losing solids over the weirs due to hydraulic overload. A CSF at or below the target CSF, however, indicates that the system is at risk of clarification failure, and action is required (Note 5). 10

11 The Operator s Guide to Monitoring Secondary Clarifier 3. Estimate Settling Constants Once per week (see Note 8 on Testing Frequency), measure the zone settling velocity and solids concentration on a return activated sludge (RAS) sample in addition to the normal clarifier feed sample. The sample should be taken at a location prior to any RAS pumps, to avoid excessive destruction of the floc by pumping. If settling of the RAS sample is too slow to measure satisfactorily, the sample may be diluted with secondary effluent to obtain a more practical data point but remember to identify the velocity data point with the diluted concentration! Then apply equations (15) and (16) to find X M and V 0 from the Feed and RAS data points. 4. Monitor Risk of Thickening Failure Using X M and V 0, estimate X L and then Q Rmin,p by the spreadsheet method described in Appendix A. Note that the Q Rmin,p calculation should use the effective peak flow rate, Q p, as represented by equation (12): Q p = Q ave x Target CSF (12) Then calculate the Thickening Safety Factor at peak flow, TSF p, from equation (13): TSF p = Available RAS pumping capacity, Q R,p / Q Rmin,p (13) The available RAS pumping capacity, Q R,p, may be estimated as the highest flow rate that would be provided by the RAS pumping system under current control settings (such as return ratio or pumping limits), should flow increase to Q p. Check TSF p against the minimum target TSF p. A TSF p well above the minimum target value provides assurance that the clarifier system is not in danger of uncontrolled sludge blanket buildup and solids loss over the weirs. Of course, the sludge blanket level in the clarifier should in any case be monitored daily to ensure that RAS flow rate is sufficient to maintain a low blanket level. An operating TSF p at or below the minimum target value warns that the system is at risk of thickening failure at peak flow. 5. Interpretation Regular monitoring of the safety factors CSF and TSF p provides operators with practical early-warning indicators of the balance between current settleability of the sludge and the available capacities of their treatment facilities. Because operating measurements are inherently variable, a single miss of a safety factor may not be cause for immediate alarm. However, should safety factors consistently approach their minimum target values, this would call for placing more units on line, if available, or taking steps to improve the settling characteristics of the sludge. Such steps may include increased oxygenation, chlorination of the return sludge or, longer-term, construction of selector reactors (Jenkins, et. al., 1993; Wanner, 1994). 6. Recordkeeping It is highly desirable to maintain a record of the measured settling constants V 0 and X M, to build up a history of typical as well as worst-case settling characteristics experienced at the plant. This record would be valuable for comparing current with past operating experience and for future design of expansion facilities. 11

12 Buhr 7. Some Notes on Flow Rates Strictly, the term Q represents the effluent flow rate from the clarifier. Because settling measurements are commonly somewhat imprecise, the front-end plant influent flow rate may often be used in calculations in place of Q, with little error. For improved accuracy, the estimated flow rates of internal recycle streams that originate from using secondary effluent as wash water (belt wash, filter backwash, and so on) may be added to the plant influent flow rate when estimating Q. Other recycle streams that originate elsewhere than secondary effluent (such as primary or secondary waste streams) would not be added, since they usually cancel out when they are returned to the plant flow. As flow to the activated sludge plant varies during the day, higher flows tend to lower clarifier influent concentrations, resulting in somewhat higher-than-average settling velocities (and vice versa). When calculating the operating Clarification Safety Factor according to equation (17), it is therefore preferred to use the actual flow rate at sampling time, rather than the daily average flow rate, to provide some degree of counterbalancing to the higher-flow/higher-settling-velocity effect. If sampling is routinely carried out near daily peak flows, and experience shows that the higher flow rates tend to generate false alarms in the CSF, it may be appropriate to moderately lower the Target CSF. The term RAS flow, or RAS pumping capacity, represents the total underflow rate from the clarifier(s). In cases where sludge wastage is taken from the clarifier underflow before its flow rate is measured, underflow would be the sum of the waste flow and the metered return flow. In many cases, the effect of including waste flow would be small. 8. Some Notes on Testing Frequency The frequency at which settling tests and safety factor calculations are be carried out may be selected by the operator. In selecting the testing frequency, it may be noted that a settling test on the mixed-liquor clarifier feed provides at least as much information as may be gained from an SVI test, which is carried out quite consistently at most treatment plants. The recommendation is to carry out mixed-liquor settling tests (and the resulting CSF calculations) at the same frequency and in parallel with any SVI testing. Operators may wish to first familiarize themselves with the CSF procedure before continuing to the next step of X M, V 0 and TSF p determinations. These tests require settling velocity at a return sludge concentration. Because concentrated sludges settle more slowly, the test may be more time-consuming and operators may wish to conduct these tests less frequently, such as weekly. Sludge settling characteristics can change surprisingly rapidly, however. Therefore, test intervals for this second part of the clarifier monitoring procedure should not be stretched out exceedingly. DISCUSSION For day-to-day plant operation, the aim is to determine safety factors as a means of quantifying the risk of clarification failure and thickening failure under current, short-term conditions. The calculations would be based on the settling characteristics currently experienced at the plant. Over the longer term, settling characteristics of activated sludge are not fixed, but tend to change over time. Although considered to be largely the result of varying concentrations of filamentous organisms in the mixed liquor (Jenkins, et. al., 1993), other changes in influent wastewater or operating conditions such as temperature (Wilson, et.al., 1997) also seem to affect settleability. For future planning and design, it is important to ensure that the facility will accommodate the poorest settleability reasonably expected at the 12

13 The Operator s Guide to Monitoring Secondary Clarifier plant. For this reason it is valuable to preserve a record of measured settling constants, so that appropriate planning values may be extracted from this wider, yet site-specific, history. Typical Settling Constants Numerous researchers have presented correlations for settling constants of activated sludge, based on analysis of their own data and those of others. Figure 4 presents a cross-section of relationships that have been proposed for V 0 and X M, as seen at facilities worldwide (X M as shown here represents the reciprocal of the Vesilind constant n). In the expectation that settling constants may be related to the traditional measures of sludge settleability such as Sludge Volume Index (SVI), Diluted SVI (DSVI) or Stirred SVI at 3.5 g/l concentration (SSVI 3.5 ), published correlations have generally been presented as functions of one or more of these index parameters. The data in Figure 4 have been converted to SVI as a basis. The relatively wide spread of the data shown in Figure 4 makes it appropriate to question whether a single, representative relationship between settling constants and SVI realistically exists. Indeed, it has been suggested that SVI measurements alone do not provide a good basis for deriving settling characteristics of activated sludge (Bye & Dold, 1999). Yet, for a lack of actual settling-constant data, many investigators have been obliged to resort to accumulated SVI data (and a selected settleability relationship) to estimate these constants. This concern about a shortage of good settling data emphasizes the need for regularly measuring the settling constants V 0 and X M for each individual facility, and maintaining a record from which site-specific information may be obtained for future planning. While the emphasis in this paper is on providing operators with straightforward SPA-related monitoring tools, an added bonus of implementing these simple techniques would be to contribute to the industry s sparse knowledge base of the range and variability of activated sludge settling constants. SVI measurements will no doubt continue to be employed as a broad-brush indicator of settleability at each plant. However, SVI alone does not provide a quantitative measure for monitoring failure risk. Therefore, it is important to supplement conventional SVI measurements by determining settling velocities, safety factors and the settling constants V 0 and X M. 13

14 Buhr V 0 (m/hr) 10 8 (1) 6 (2) 4 (3) 10 2 (4) (5) ) (7) (6) SVI (ml/g) 6, V 0 (ft/hr) 5,000 4,000 X M (mg/l) 3,000 2,000 1,000 (7) (2) (4) (1) (5) (3) (6) SVI (ml/g) Figure 4 Published correlations for settling constants V 0 and X M vs. SVI. (1) Daigger (1995); (2) Ozinsky & Ekama (1995); (3) Koopman & Cadee ; (4) Wahlberg & Keinath (1995) - unstirred 1-liter cylinder; (5) Pitman (1984) # ; (6) Wahlberg & Keinath (1995) - stirred 1-liter cylinder; (7) Catunda & van Expressed as DSVI, assumed equivalent to SVI at low settled volume # Expressed as SSVI 3.5, assumed = 0.67 DSVI (Ekama & Marais, 1986) 14

15 The Operator s Guide to Monitoring Secondary Clarifier CONCLUSIONS State Point Analysis (SPA) provides valuable insight into expected performance for an activated sludge clarifier system. Among the important advantages to be gained from applying SPA principles is an evaluation of the risks of clarification failure and thickening failure based on the sludge settleability currently being experienced at the plant. These criteria may be readily checked by numerical calculations. For routine operation, it is recommended that the zone settling velocity of the clarifier feed be determined daily and the clarification safety factor (CSF) calculated. Comparison of the calculated CSF with a target value, set in accordance with the discussions in the paper, provides a measure of the degree of protection against clarification failure that is being maintained. Zone settling velocity should also be measured regularly on the return sludge. This information may be used to estimate the settling constants V 0 and X M. These, in turn, allow calculation of the thickening safety factor, TSF p. Comparison of TSF p against its target value will show whether sludge settleability and return sludge pumping capacity are adequate to guard against thickening failure and sludge blanket buildup. Incidentally, X M also denotes the feed concentration at which clarifier solids loading capacity would be at a maximum among other factors, such as desired sludge age and the like, this information may be useful in selecting an appropriate operating mixed-liquor concentration. SVI alone is not a sufficient indicator of potential clarifier problems. Calculated safety factors provide operators with a running indication of the balance between sludge settleability and the available capacities of treatment equipment. As operating safety factors approach their minimum target values, operators receive a timely warning that corrective action is needed. Measured settling constants should be recorded to build up a history of settleability experienced at the facility. ACKNOWLEDGEMENTS The contributions of J. Kabouris, B. Narayanan and C. Pretorius are gratefully acknowledged. REFERENCES Albertson, O.E.; Okey, R.W. (1990) Design rationale for secondary sludge collector mechanisms. Presented at 63rd Annual Conf., Water Pollut. Control Fed., Washington, DC. Buhr, H.O.; Goddard, M.F.; Wilson, T.E.; Ambrose, W.A. (1984) Making full use of step feed capability. J. Water Pollut. Control Fed., 56, Bye, C.M.; Dold, P.L. (1999) Evaluation of correlations for zone settling velocity parameters based on sludge volume index-type measures and consequences in settling tank design. Water Environ. Res., 71, Coe, H.S.; Clevenger, G.H. (1916) Methods for determining the capacities of slime settling tanks. Trans. Am. Inst. Mining Eng., 55, Catunda, P.F.C.; van Haandel, A.C. (1992) Activated sludge settling Part 1: Experimental determination of activated sludge settleability. Water SA, 18, Daigger, G.T. (1995) Development of refined clarifier operating diagrams using an updated settling characteristics database. Water Environ. Res., 67, Ekama, G.A.; Marais, G.v.R. (1986) Sludge settleability and secondary settling tank design procedures. Water Pollut. Control, 85 (1), Ekama, G.A.; Marais, P. (2004) Assessing the applicability of the 1D flux theory to full-scale secondary settling tank design with a 2D hydrodynamic model. Water Res., 38, Ekama, G.A.; Barnard, J.L.; Günthert, F.W.; Krebs, P.; McCorquodale, J.A.; Parker, D.S.; Wahlberg, E.J. (1997) Secondary settling tanks: Theory, modeling, design and operation, Sci. and Tech. Rept. No. 6; International Association on Water Quality (IAWQ): London. Jenkins, D.; Richard, M.G.; Daigger, G.T. (1993) Manual on the causes and control of activated sludge bulking and foaming, 2nd ed.; Lewis Publishers: Chelsea, MI. 15

16 Buhr Keinath, T.M.; Ryckman, M.D.; Dana, C.H.; Hofer, D.A. (1977) Activated sludge unified system design and operation. J. Environ. Eng., 103, Koopman, B.L.; Cadee, K. (1983) Prediction of thickening capacity using diluted sludge volume index. Water Res., 17, Narayanan, B.; Buhr, H.O.; Leveque, E.G.; Fraser, J.S.; Johnson, W.; Shepherd, W. (2000a) State point analysis - A neglected operational tool for activated sludge systems. Presented at 73rd Annual Conf., Water Environment Fed., Anaheim, CA. Narayanan, B.; Buhr, H.O.; Leveque, E.G.; Lee, M.C.; Dielmann, D. (2000b) Could your secondary clarifier sludge removal mechanism be a process bottleneck? Presented at 73rd Annual Conf., Water Environment Fed., Anaheim, CA. Ozinsky, A.E.; Ekama, G.A. (1995) Secondary settling tank modeling and design Part 2: Linking sludge settleability measures. Water SA, 21, Pitman, A.R. (1984) Settling of nutrient removal activated sludges. Water Sci. Tech., 17, STOWa (Stichting Toegepast Onderzoek Waterbeheer) (1981) Hydraulische en technologische aspecten van het nabezinkproces: Rapport 1-Literatuur; Rapport 2-Ronde nabezinktanks (Praktijkonderzoek); Rapport 3-Ronde nabezinktanks (Ontwerpgegevens en bedrijfservaring); Postbus 8090, 3503 RB Utrecht, The Netherlands. Vesilind, P.A. (1968) Design of prototype thickeners from batch settling tests. Water Sew. Works, 115, Wahlberg, E.J.; Keinath, T.M. (1995) Development of settling flux curves using SVI: An addendum. Water Environ. Res., 67, Wanner, J. (1994) Activated sludge bulking and foaming control; Technomic Pub. Co.: Lancaster, PA. Wilson, T.E. (1993) Application of ISV test to the operation of activated sludge plants. Water Res. 17 (6), Wilson, T.; Carrio, L.; Fillos, J.; Bailey, W. (1997) Getting more information with less work: Application of the SSV test to the operation activated sludge plants. Presented at 70th Annual Conf., Water Environment Fed., Chicago, IL. 16

17 The Operator s Guide to Monitoring Secondary Clarifier APPENDIX A Calculation of Q Rmin and Q Rmin,p by Iteration The procedure for finding the minimum ( critical ) underflow rate, Q Rmin, for any clarifier flow rate, Q, is based on determining the limiting solids concentration, X L, at which the slopes of the critical underflow (RAS) operating line and the settling flux curve are equal. To find the value of Q Rmin,p, the minimum underflow rate at peak flow, the calculation shown below should be applied at the effective peak flow, Q p, defined as: Q p = Q ave x Target CSF. The slope of the critical underflow operating line (CUOL), which passes through both the State Point and the limiting solids concentration X L, is given by Slope (CUOL) = k 1 [X L V 0 exp(-x L /X M ) - X F (k 2 Q p /A)] / (X L - X F ) (A1) By differentiation of equation (3), the slope of the settling flux curve (SFC) at X L is Slope (SFC) = k 1 V 0 (1 - X L /X M ) exp(-x L /X M ) (A2) A solution for X L is achieved when (A1) and (A2) are equal, or Slope (CUOL) = Slope (SFC) (A3) The non-linear equation (A3) may be solved by selecting a starting estimate for X L, calculating a projected adjustment and repeating the calculation with the new adjusted estimate ( next estimate ) until the initial and next values converge. The technique is known as the Newton-Raphson method of successive approximations. The calculations may be implemented on a spreadsheet, using the following sequence: 1. X L1 (starting estimate) = previous value of X L2, from line 3. X M [(V of,p /V 0 ) exp(x L1 /X M ) (X L1 /X M )(1 - X L1 /X F )] 2. Adjustment = (V of,p /V 0 ) exp(x L1 /X M ) X L1 /X F where V of,p = k 2 Q p /A 3. X L2 (next estimate) = X L1 (starting estimate) - Adjustment. To set up the spreadsheet, calculation cells should be allocated to store values for the input variables Q or Q p, A, V of,p, X F, V 0 and X M. Spreadsheet options should be set to allow iteration, so that the calculation can successively converge to a final value for X 1 and X 2. This would be the desired solution for X L. In practice the iteration may oscillate, but will generally converge to a stable result. A prototype Excel spreadsheet may be obtained from the author by . Minimum Underflow (RAS) Rate Once a value for X L has been found by the iterative procedure, the minimum underflow rate, Q Rmin,p, may be calculated by: Q Rmin,p = (1/k 2 ) A V 0 (X L /X M -1) exp(-x L /X M ). 17

18 Buhr APPENDIX B Case Study and Sample Calculations Scale Divisions Return Sludge Clarifier Feed (Mixed Liquor) Minutes Figure B1 Settling Velocity Plots. The straight lines shown on the plot are drawn by eye through the steepest part of the corresponding settling curve, ignoring initial flocculation and the final compaction period. Table B1 Sample Calculations Part 1 - Initial Setup US Customary Units SI ("metric") Units Select Target Safety Factors (refer to Text for Guidelines) Peak flow ratio PFR Non-ideality Factor F n Target Clarification Safety Factor, CSF Target = PFR x F n Target Thickening Safety Factor at Peak Flow, TSF p,target Part 2 - Evaluate Clarification Safety Factor, CSF Settling Velocity for Clarifier Feed Derived from steepest slope on settling curve (Fig. B1) Scale divisions settled over 1 minute s 200 div/min 200 div/min Settleometer scale: Inches [mm] per division h 5.9/1,000 in/div 150/1,000 mm/div Settling rate per minute SR = s x h 1.18 in/min 30.0 mm/min Settling velocity in ft/hr V F = SR x 60/ ft/hr or, Settling velocity in m/hr V F = SR x 60/1, m/hr Solids concentration in Clarifier Feed sample X F 2,400 mg/l 2.40 kg/m 3 (only needed for X M & V 0 determination) Clarifier overflow (upflow) velocity Clarifier Effluent Flow Rate at sampling time Q mgd 2,200 m 3 /hr Total Clarifier Area A 31,420 ft 2 2,919 m 2 Units conversion factor k 2 5, Clarifier overflow velocity V of = k 2 x Q/A 2.47 ft/hr m/hr Clarification Safety Factor, CSF = V F / V of Target CSF (from Part 1) Compare: CSF > Target? OK OK continued/ 18