A NEW METHOD FOR EVALUATING PRIMARY CLARIFIERS Alex Ekster and Cristina Pena San Jose/Santa Clara Water Pollution Control Plant City of San Jose ABSTRACT A new method has been developed for evaluating primary clarifiers that examines the individual effect of concentration of non-settleable solids, detention time, clarifier depth, sludge blanket depth, and design features of the clarifier on removal efficiency. This method was successfully used to determine priorities for clarifier improvements at the San Jose/Santa Clara Water Pollution Control Plant (SJ/SC WPCP). Maintaining the sludge blanket level within a narrow range (1.5-1.7 m (5 5.5 ft)) was found to be the most cost-effective method for increasing removal efficiency. KEY WORDS: PRIMARY TREATMENT, REMOVAL EFFICIENCY, SLUDGE BLANKET, CONTROL INTRODUCTION Primary clarification is one of the oldest methods of wastewater treatment and it is still popular today due to its simplicity and low operational costs. Primary clarification is commonly followed by more advanced treatment such as biological and/or filtration treatment. An increase of primary clarification removal efficiency (RE) usually leads to a reduction of air demand for activated sludge process, a decrease of solids loading on secondary clarifiers and waste activated
sludge thickeners, and an increase in digester gas production. RE is calculated using the following equation: C inf C eff RE = 100 * ----------------------- (1) C inf The efficiency of primary treatment has been traditionally assessed by solids removal efficiency and compared to the hydraulic loading rate. Using the traditional approach for assessing clarifier performance, the solids removal efficiency (RE) was compared to the hydraulic loading rate (Figure 1). The scattered data did not reveal any correlation between hydraulic loading and removal efficiency. Similar observations have been made by other researchers (Walberg et.al., 1997). It was hypothesized that the absence of a correlation could be explained by the fact that RE is affected by a number of factors such as the concentration of non-settleable solids, sludge blanket depth, and design features of the clarifier. Thus, the cumulative effect of these factors on RE may exceed the effect of hydraulic loading.
100 TSS Removal Efficiency, % 90 80 70 60 50 40 R 2 = 0.0173 30 20 10 0 40 45 50 55 60 65 70 75 Hydraulic Loading, m 3 /d-m 2 Figure 1. The effect of hydraulic loading rate on TSS removal efficiency. METHODS AND MATERIALS In order to evaluate the effect of concentration of non-settleable solids, sludge blanket depth, and design features of the clarifier on RE, it was decided to consider RE the product of four removal efficiencies: Removal Efficiency = RE1 * RE2 * RE3 * RE4 (2) Removal Efficiency 1(RE1) depends only on the fraction of non-settleable solids; Removal Efficiency 2 (RE2) depends on the clarifier design, short-circuiting and sludge blanket level; Removal Efficiency 3 (RE3) depends on hydraulic loading; and Removal Efficiency 4 (RE4) depends on clarifier flocculation ability. A method was developed to determine RE1-RE4.
RE1 RE1 is calculated using the following equation: C inf C ult/inf RE1 = 100 * ----------------------- (3) C inf where C inf is the concentration of TSS in primary influent and C ult/inf is the ultimate TSS equal to the concentration of non-settleable solids in primary influent supernatant after three hours of settling in a glass jar. RE1 does not depend on clarifier design nor on hydraulic conditions, therefore, RE1 can be increased only by chemical addition. Ultimate TSS was determined by measuring non-settleable solids concentration in the supernatant after quiescent settling of primary influent in a glass vessel 10 cm (4 inches) in diameter and 20 cm (8 inches) deep. Figure 2 shows the supernatant TSS results from samples collected from primary influent after settling 1, 2, 3, and 4 hours in a jar. Based on this test, it was determined that ultimate non-settleable TSS would be best represented after 3 hours of settling instead of the 1 hour settling time called for in Standard Methods (Section 2540F.3.b;AWWA, 1992).
Total Suspended Solids, mg/l 80 75 70 65 60 55 50 45 40 35 30 0 60 120 180 240 300 Detention Time, minutes Figure 2. Ultimate non-settleable solids after 1, 2, 3, and 4 hours settling. RE2 RE2 is calculated using the following equation: C inf C eff RE 2 = 100 * -------------------- (4) C inf C column where C inf is the concentration of TSS primary influent, C eff is the concentration of TSS in primary effluent, and C column is the concentration of TSS column supernatant after settling for a given detention time.
The settling column test is used to determine the maximum amount of solids that can be removed without interference from φ =0.1m 0101 hydraulic short-circuiting, sludge blanket resuspension or flocculation (deflocculation). Since solids removal depends on the depth of the settling tank; the height of the column (Figure 3) built for this test was exactly equal to the clarifier depth in the main part of the clarifier (downstream of the hopper). RE2 can be improved by retrofitting existing clarifiers(change weir design) and/or by optimizing sludge blanket depth and sludge withdrawal operation. H=2.4m Usually, improvement of RE2 is the most cost effective way to improve overall removal efficiency because it can be done without chemical addition or construction of new clarifiers. Sampling port Figure 3. Settling Column.
Visual observations and analytical testing indicated that the optimization of the sludge blanket height could improve RE2. Visual observation indicated that the sludge stored in the hopper was resuspended by turbulence created by the inflow at the head of the clarifier. Preliminary tests (see Figure 4) confirmed that sludge is resuspended and re-distributed along the length of the clarifier, although the effect of the resuspension on effluent quality, wasted sludge concentration, and RE2 was still unknown. Total Suspended Solids, mg/l 900 800 700 600 500 400 300 200 100 0 Minimum Daily Flow Peak Daily Flow 0 10 20 30 40 50 60 Length of the Clarifier, meter Figure 4. Suspended solids profile along the length of the primary clarifier. Therefore, in order to understand the effect of the sludge blanket on effluent quality and RE2, two parallel clarifiers with different sludge blanket heights were tested. In the first clarifier, the sludge blanket was maintained at 1.37 m (4.5 ft) above the hopper floor; in the second clarifier the sludge blanket was maintained at 2.13 m (7 ft) above the hopper floor. Influent and effluent samples were collected and measured for TSS, BOD, and soluble BOD using Standard Methods (AWWA, 1992). In addition, the effect of the sludge blanket height on wasted sludge was
determined by measuring the sludge concentration when the sludge blanket height was at various levels above the hopper floor. RE3 RE3 reflects the effect of detention time (hydraulic loading) on overall removal efficiency. RE3 is calculated using the following equation: C inf C column RE 3 = 100 * --------------------- (5) C inf C ult/baffle where C inf is the concentration of TSS in primary influent, C column is the concentration of TSS of primary influent supernatant after settling for a given detention time in the column and C ult/baffle is the non-settleable solids in primary influent collected downstream of the inlet baffle. RE3 can be increased by chemical addition to increase settling velocity and constructing new clarifiers to increase detention time. RE4 RE4 reflects the ability of solids to flocculate or deflocculate in the clarifier. RE4 is calculated using the following equation: C inf C ult/baffle RE4 = 100 * ---------------------- (6) C inf C ult/inf
where C inf is the concentration of TSS in primary influent, C ult/inf is ultimate TSS equal to the concentration of non-settleable solids in primary influent supernatant after three hours of settling in a glass jar and C ult/baffle is ultimate TSS in primary influent collected downstream of the inlet baffle. RE4 can be increased by chemical addition or installation of flocculation baffles. Clarifier Description The primary clarifiers evaluated at the SJ/SC WPCP are rectangular clarifiers and have fully extended baffles in front of the inlet that run across the width of the clarifier so that influent flows over the baffle (Figure 5). Flights move sludge to the hopper located at the head of the clarifier. The sludge blanket is maintained between 1.5 m ( 5 ft) and 1.8 m (6 ft) above the hopper floor during the day and up to 2.1 m (7 ft) at night. Sludge is periodically withdrawn by a centrifugal pump and sent to anaerobic digesters. Influent channel weir Influent 2.44 m Effluent Hopper Inlet port baffle 1.22 m Sludge Withdrawal Pump 63.4 m Figure 5. Diagram of Primary Clarifiers at SJ/SC Water Pollution Control Plant
RESULTS Analytical Testing The total variation of grab samples was calculated using multiple regression analysis from grab samples collected from the same primary influent channel at two locations 5 feet apart. These samples were collected within 2 minutes of each other. Analytical variance for TSS was calculated from duplicate samples analyzed by the SJ/SC WPCP Process Lab; laboratory analytical precision was found to be better than Standard Methods (AWWA, 1992). The sampling error was calculated using the additive property of variances: s 2 = s 1 2 + s 2 2 so that, s 1 2 = s 2 - s 2 2 (7) (8) where s is the total variance, s 1 is the variance due to sampling and s 2 is the variance due to analytical analysis (Shernam et. al., 1996). Standard deviations for RE1 was calculated using the following equation: (y 1 y ) 2 σ = ----------------- (9) n - 1 Standard deviation for overall removal efficiency and RE2-RE4 values may depend on hydraulic load and cannot be considered as random values. As a result, instead of using equation 9, the standard deviation was calculated by the method described by Berthouex and Brown (1994) using partial differentials for each variable multiplied by the variance. For example the standard deviation of RE3 was calculated by the following formula:
σ RE3 = θ 2 inf σ 2 inf + θ 2 ult/baffle σ 2 ull/baffle + θ 2 column σ 2 column (10) where θ inf is the partial derivative δre3/δc inf and the sensitivity coefficient for influent TSS, θ ult/baffle is the partial derivative δre3/δc ult/baffle and the sensitivity coefficient for TSS ultimate/baffle, and θ column is the partial derivative δre3/δc column and is the sensitivity coefficient for TSS column. Table 1 summarizes the standard deviations of the water quality parameters as well as for removal efficiencies. Table 1. Standard deviation for influent, effluent TSS samples, and RE1 through RE4. Standard Deviation Water Quality Parameter No. Samples Sampling Analytical TOTAL Influent TSS, mg/l 35 9 14 16 Effluent TSS, mg/l 35 3 3 4 Ultimate TSS, mg/l 22 3 8 9 Column TSS, mg/l 8 3 7 8 Total Removal Efficiency, % 19 4 RE1, % 9 10 RE2, % 4 10 RE3, % 16 6 RE4, % 3 7
RE1 Results for non-settleable solids and RE1 calculated from wet and dry seasons are summarized in Table 2. Table 2. RE1 during dry summer weather and wet winter weather. Non-Settleable Solids, mg/l RE1, % Wet Season Dry Season Wet Season Dry Season Mean 63 61 74 75 Median 54 55 78 75 Min 40 50 52 61 Max 104 93 86 86 Effect of diurnal variation on TSS ult/influent and RE1 are shown in Table 3. Table 3. Results for RE1: minimum daily flow and peak daily flow. Non-Settleable Solids, mg/l RE1, % Minimum Daily Flow Peak Daily Flow Minimum Daily Flow Peak Daily Flow Mean 54 72 77 74 Median 50 71 80 75 Min 40 50 52 61 Max 86 104 83 86 RE2 Results of TSS column and RE2 are given in Table 4.
Table 4. RE2 using a quiescent settling column. Hydraulic Loading m 3 /d-m 2 Clarifier Effluent mg/l Total Suspended Solids Quiescent Settling Column mg/l RE2, % 40.7 90 95 104 63.2 95 90 97 77.4 130 140 107 81.5 130 130 100 The effect of the sludge blanket height on primary effluent is shown in Figure 6 and on wasted sludge is shown in Figure 7. Percent Difference, % 80 70 60 50 40 30 20 10 0 Hydraulic Loading Rate 32.6 m3/d-m2(800 gpd/ft2) 73.3 m3/d-m2(1800 gpd/ft2) TSS sbod BOD Liquid H2S Effluent Water Quality, mg/l Figure 6. Deterioration of effluent quality with an increase of sludge depth in the hopper from 1.4 m to 2 m.
Sludge Total Solids, %. 5.0 4.5 4.0 3.5 3.0 R 2 = 0.9624 2.5 2.0 1.5 1.0 0.5 0.0 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 Sludge Blanket depth, m Figure 7. Effect of sludge depth on the concentration of sludge pumped from primary clarifiers. RE 3 As mentioned earlier RE3 depends on the non-settleable solids and hydraulic loading. RE3 is calculated using equation 5. Since it was found that at lower sludge blanket levels RE2 is essentially 100% and the column test is labor intensive, it will be more convenient to use clarifier effluent to calculate RE3. Substituting C eff in equation 5 in place of C column, RE3 will be: C inf C effluent RE 3 = 100 * ----------------------- (11) C inf C ult/baffle Figure 8 shows RE3 for the clarifiers during the dry summer and wet winter weather.
Removal Efficiency 3, %. 100 90 80 70 60 50 40 30 20 10 R 2 = 0.84 Winter Summer 0 30 50 70 90 110 Hydraulic Loading, m 3 /d-m 2 Figure 8. Effect of hydraulic loading on RE3 during wet winter weather and dry summer weather. RE4 Results of flocculation (deflocculation) tests are shown in Table 5. Table 5. Sample Location Non-Settleable Solids, mg/l Influent 53 Before Baffle 63 Downstream of 66 Baffle Effluent 63
BOD Removal Equations similar to equations 3 through 6 were used to determine the removal of particulate BOD. RE1 and RE3 were slightly lower for particulate BOD than for TSS since settleability of inorganic material was a little better than organic. DISCUSSION Analytical Testing The standard deviation of the measured TSS is fairly high due to analytical and sampling errors, although those errors according to Standard Methods(1994) were in the normal range. The overall removal efficiency standard deviation is 4%. Individual standard deviations (RE1-RE4) are between 6% and 10%. It is believed that no less than 3 tests should be performed for determining individual efficiencies in order to gain confidence in the results. RE1 On average RE1 was 75%. The presence of non-settleable particles in primary influent at the SJ/SC WPCP on average reduces overall efficiency of primary clarifiers by 25% (Table 2). Statistical analysis revealed that RE1 varies by as much as 34%. The variations were not associated with diurnal flow variation or with wet weather conditions (Table 3). Since RE1 does not depend on clarifier design nor on hydraulic conditions, the only option considered to increase RE1 was chemical addition.
RE2 Determined at different hydraulic loading, RE2 was always 100% when sludge level in the hopper was below 1.7 m (5.5 ft). This indicated that clarifiers were properly designed and shortcircuiting, even if it existed, did not have any effect on the removal efficiency. An increase of sludge level in the hopper above 2.0 m (6.5 ft), however, reduced RE2 by as much as 15%. Calculated total BOD removal efficiency showed that an increase of sludge level above 2.0 m (6.5 ft) reduced overall BOD RE by as much as 28%. Reduction of BOD removal efficiency was partially caused by an increase of particulate BOD associated with TSS. In addition, effluent soluble BOD was higher than influent soluble BOD due to solubilization of particulate BOD stored in the hopper (Figure 6) and that process had detrimental affect on overall BOD removal efficiency. In addition, the higher sludge blanket also increased the generation of H 2 S by 50%. Conversely, if the sludge blanket was maintained below 1.4 m (4.5 ft), concentration of withdrawn sludge reduced drastically (Figure 7) and potentially could cause digestion problems since primary sludge is pumped directly to the digesters. RE3 Figure 8 showed that an increase of hydraulic loading rate by a factor of 3(from 35 to 105 m 3 /dm 2 ) reduces efficiency by 40-45% (from 90% to 50%) during wet winter weather. No correlation between RE3 and hydraulic loading which ranged between 35 m 3 /d-m 2 and 75 m 3 /dm 2, was observed during dry summer weather. The difference in performance between winter and summer months could be caused by water temperature that differs by 12 F between summer and winter. A decrease in water temperature caused a decrease in settling velocity so the effect of hydraulic loading on removal efficiency is more pronounced during wintertime.
RE4 Comparing non-settleable solids in samples collected from the influent channel, before the inlet baffle, downstream of the inlet baffle, and in the effluent, revealed that ultimate TSS didn t change in the clarifier, indicating that neither flocculation nor deflocculation occur in the clarifier (see Table 5). Options to increase RE4, such as chemical addition and installation of flocculation baffles, were also considered. CONCLUSION A new method has been developed for evaluating and optimizing primary clarifiers that examines the individual effect of concentration of non-settleable solids, detention time, clarifier depth, sludge blanket depth, and design features of the clarifier. RE1 depends only on a fraction of the non-settleable solids. On average RE1 was 75 ±10%. Variation of RE1 had no correlation with diurnal flow variation nor with wet weather conditions. RE2 depends on the clarifier design, short-circuiting and sludge blanket level. RE2 was always 100% when the sludge level in the hopper was below 1.7 m (5.5 ft). As sludge depth increased, the effluent BOD, both soluble and total, and TSS start to increase. On the other hand a decrease of sludge depth below 1.4 m (4.5 ft) reduced the concentration of waste sludge. RE3 depends on hydraulic loading. An increase of hydraulic loading rate by a factor of 3(from 35 to 105 m 3 /d-m 2 ) reduces RE3 by 40-45% (from 90% to 50%) during wet winter weather. No correlation between RE3 and hydraulic loading, which ranged
between 35 m 3 /d-m 2 and 75 m 3 /d-m 2, was observed during dry summer weather. Difference between seasonal performance is believed to be associated with temperature. RE4 depends on clarifier flocculation ability. Comparing non-settleable solids in samples collected from the influent channel, before the inlet baffle, downstream of the inlet baffle, and in the effluent, revealed that ultimate TSS didn t change in the clarifier, indicating that neither flocculation nor deflocculation occur in the clarifier. Using the described method of calculating RE1-RE4, the effect of each of the following factors can be compared: influent matrix, operational parameters, and specifics of clarifier design on overall RE. As a result an opportunity is provided to determine priorities for RE improvements. For the SJ/SC WPCP maintaining the sludge blanket level within a narrow range (1.5-1.7 m (5 5.5 ft)) seems to be the most cost-effective method for increasing RE. If further increase of efficiency is needed, the described method of removal efficiency evaluation will be used to choose among the remaining options: chemical addition, construction flocculation baffles or constructing new clarifiers. REFERENCES AWWA, APHA, WEF (1992) Standard Methods: For The Examination of Water and Wastewater. 18 th edition. P.M. Berthouex and L. C. Brown. (1994). Statistics for Environmental Engineers. CRC Press, Inc. Metcalf & Eddy and G. Tchobanoglous (1991). Wastewater Engineering: Treatment, Disposal, and Reuse. 3 rd Edition. McGraw-Hill. E.J. Wahlberg, J.K. Wang, and M.S. Merrill (1997). Primary Sedimentation: It s Performing Better Than You Think. Proc Wat Environ Fed 70 th Annual Conf & Expo. Vol.1, pp.731-741.