CHEMICALLY ENHANCED PRIMARY TREATMENT FOR A LARGE WATER RECLAMATION FACILITY ON A CONSTRICTED SITE - CONSIDERATIONS FOR DESIGN, START-UP, AND OPERATION ABSTRACT Jeffrey A. Mills, P.E., BCEE,* Roderick D. Reardon, P.E., BCEE,* C. Edward Chastain,** John L. Cameron,** Gregory V. Goodman, P.E., DEE* *CDM 3715 Northside Parkway, NW, Building 300, Suite 400 Atlanta, Georgia, 30327 **Cobb County Water System, Marietta, Georgia To provide greater treatment capacity and to meet new water quality criteria for discharge to the Chattahoochee River, Cobb County Water System (CCWS) construction a new 60 million gallon per day (mgd) liquid treatment facility at the R.L. Sutton Water Reclamation Facility (Sutton WRF). A restricted site (25 acres) combined with an effluent limit on total phosphorus of 0.5 mg/l resulted in the decision to use chemically enhanced primary treatment (CEPT) to both minimize facility footprint and to provide phosphorus removal. Based on performance between May 2005 and April 2006, the new CEPT process facility at the Sutton WRF is removing over 50% of the influent 5-day biochemical oxygen demand (BOD 5 ), over 60% of total suspended solids (TSS), and nearly 50% of the influent phosphorus. Jar settling tests performed during design indicated improved performance with chemical coagulation, and rapid mix and flocculation basins were provided prior to circular primary clarifiers. The primary coagulant is currently ferrous chloride, however, no polymer is used at this time. Although the facility is hydraulically under-loaded, it is meeting strict effluent limits with a low coagulant dose. KEYWORDS Chemically enhanced primary treatment, CEPT, chemical treatment, BOD 5 removal, TSS removal INTRODUCTION The Sutton WRF is the largest of four water reclamation facilities operated by CCWS in Cobb County, Georgia, a suburb of 600,000 people northwest of Atlanta. Effluent from the facility is discharged to the Chattahoochee River, which is the area s principal drinking water source. Continued growth in the area, restrictions on wet-weather discharges, and new water quality limits on discharges to the Chattahoochee River required that the liquid treatment facilities be expanded and upgraded from an average daily flow of 40 mgd to a maximum monthly flow of 60 mgd, with a peak hourly flow capacity of 120 mgd. Many of the new liquid treatment features allow for an ultimate peak flow of 160 mgd. National Pollution Discharge Elimination System (NPDES) permit requirements, during the time of design for the expansion, are outlined in Table 1. In July of 2005, the Georgia Department of Environmental Protection enacted more stringent effluent limitations at the Sutton WRF. These new limitations are outlined in Table 2. 1982
Construction of the facilities was divided into two construction packages, one for the expansion of the liquid treatment facilities and the second for improvements to the solids handling facilities. Construction of the liquid treatment facility commenced in July 2001 and was completed in December 2004. The liquid treatment process design is based on a single-stage nitrifying activated sludge system sized for complete year-round nitrification at maximum monthly flows. The effluent criteria for TSS and total phosphorus require filtration of the final effluent. Table 1 - NPDES permit requirements for Sutton WRF expansion during design Parameter Monthly Average Weekly Average CBOD 5, mg/l 6.7 10 Suspended solids, mg/l 13.3 20 Total phosphorus as P, mg/l 0.5 -- Total residual chlorine, mg/l 0.05 0.05 Fecal coliform, per 100 ml 200 400 Ammonia as N, mg/l a December to April 6.27 9.40 August 1.20 1.80 a Varies by month; lowest summertime limit shown Table 2 - NPDES permit requirements for Sutton WRF expansion enacted after start-up Parameter Monthly Average Weekly Average CBOD 5, mg/l 2.9 4.35 Suspended solids, mg/l 10 15 Total phosphorus as P, mg/l 0.75 -- Total residual chlorine, mg/l 0.02 0.02 Fecal coliform, per 100 ml 200 400 Ammonia as N, mg/l 0.5 0.75 SELECTION OF CHEMICALLY ENHANCED PRIMARY TREATMENT (CEPT) CCWS needed to construct a new 60-mgd liquids process train on an L-shaped, 25-acre site adjacent to its existing plant. The site is bounded on the west by a cemetery, on the south by the existing plant, and on the east by the Chattahoochee River. In addition, the plant staff desired circular primary and secondary clarifiers, as the existing plant s primary clarifiers were inconsistent in removing organic carbon and suspended solids. CDM recommended CEPT following rapid mix and flocculation to fit the new facility on the available land. Evaluations showed that CEPT minimized the process footprint by reducing the size of the primary clarifiers and aeration basins by providing higher and more consistent removal of organic carbon and suspended solids in the primary clarifiers at a higher design hydraulic loading rate. CEPT also assisted in the removal of phosphorus from the wastewater. Before CEPT system design began, jar tests using ferric chloride as a coagulant were conducted on the plant s primary influent. These tests examined the effects of ferric chloride addition, 1983
flocculation, flocculation time, and mixing intensity on settled water quality. Results showed an optimal coagulant dose between 45 to 60 milligrams per liter (mg/l) as FeCl 3, and demonstrated that rapid mix with flocculation produced lower settled water turbidity, BOD 5, and phosphorus levels, and allowed for a lower coagulant dose and the new facility to have higher primary clarifier overflow rates than the existing plant. Figures 1 and 2 show the historical removal rates for BOD 5 and TSS in the primary clarifiers at the Sutton WRF. Previous to the new liquids process facility, the removal of BOD 5 averaged 41% (ranging from 0% to 75%), and the removal of TSS averaged 53% (ranging from 5% to 88%) in the Sutton primary clarifiers. The solid line in these figures represents the average trending line. Removal of total phosphorus during this period averaged 43%. Figure 1 Historical BOD 5 removal in primary clarifiers 1984
Figure 2 Historical TSS removal in primary clarifiers DESIGN OF CEPT SYSTEM Successful work by CDM on CEPT systems for Massachusetts Water Resources Authority (Deer Island Treatment Plant) and the Hong Kong Drainage Services Department (Stonecutters Island Sewage Treatment Works) suggested that an annual average loading rate near 4.8 m/h (2,850 gpd/ft 2 ) is possible. In the assessment of a CEPT system for a facility in Massachusetts, Firmin (1993) reported on successful large facilities with BOD 5 removal efficiencies ranging from 40% to 80%, and with TSS removal efficiencies ranging from 50% to 90%. Based on this work, the design team conservatively used a peak hydraulic overflow rate of 5.1 m/h (3,000 gpd/ft 2 ) to size the primary clarifiers, a BOD 5 removal efficiency of 50%, and a TSS removal efficiency of 60%. The Sutton WRF design provided for an average metal salt dose of 30 to 40 mg/l along with a maximum polymer dose of 1 mg/l. The facility adds lime to supplement alkalinity after the primary clarifiers. Rapid mix times are 45 seconds at average flow and 19 seconds at peak flow while flocculation times are 20 minutes at an average daily flow, and 8 minutes at peak flow. Three flocculation compartments in series will promote a settleable floc. Slow-speed vertical turbine mixers equipped with two-speed drives were provided to allow for tapered flocculation. Water flows through the flocculation stages in a serpentine pattern to minimize short-circuiting. Table 3 presents the basis of design for the rapid mix process, Table 4 presents the basis of design for the flocculation process, and Table 5 presents the basis of design for the primary sedimentation process. 1985
Table 3 - Basis of design - rapid mix process Number of Basins 1 Length, feet 17 Width, feet 17 Side water depth, feet 12 Total volume, gallons 25,941 Detention time, seconds 40 MGD 56 60 MGD 37 120 MGD 19 Mixing unit Surface mounted Motor size, hp 3 (constant speed) Table 4 - Basis of design flocculation process Number of Basins 3 Length, feet 48 Width, feet 24 Side water depth, feet 30 Total volume, gallons 775,630 Detention time, minutes 40 MGD 30 50 MGD 22 60 MGD 19 120 MGD 9.5 Flocculating unit Surface mounted Motor size, hp 12 (two speed) Table 5 - Basis of design - primary sedimentation process Number of clarifiers 4 Diameter, feet 125 Side water depth, feet 12 Unit surface area, sf 12,272 Unit volume, MG 1.10 Hydraulic overflow rate, gpd/ft 2 40 mgd 815 60 mgd 1,222 120 mgd 2,444 SYSTEM START-UP AND OPERATION The new liquids process facility began full start-up in December 2004. Plant staff began operation with the rapid mix and flocculation basins bypassed, and with two primary clarifiers in service. They brought on a third clarifier and then sent flow through the rapid mix and 1986
flocculation basins. Current operation includes the use of three out of the four primary clarifiers, with rapid mix and flocculation in service. In analyzing plant operational data for the period of May 2005 through April 2006, BOD 5 removal in the primary clarifiers averaged 51.1% (ranging from 12% to 97%), TSS removal averaged 67.8% (ranging from 19% to 99%), and phosphorus removal averaged 48.8% (ranging from 1% to 79%). During this period, ferrous chloride was used as the primary coagulant at an average does of 12.9 mg/l as FeCl 2. The facility has provisions to use polymer as a secondary coagulant, but the plant staff has chosen not to use polymer at this time. The facility met all of its effluent permit parameters in this period, including the more stringent permit limitations enacted in July of 2005. Operating conditions and CEPT removal data from this period is shown in Tables 6 and 7. Unfortunately, temperature is only measured on the plant effluent, after the aeration basins, secondary clarifiers, filters and ultraviolet disinfection, so the temperature recorded is not the temperature the CEPT system sees. Although the average removal efficiencies exceed the design parameters, a narrower or more consistent range of daily removal efficiencies would be desirable. Some scum removal difficulties may have helped widen the range of removal efficiencies. Figure 3 shows the BOD 5 removal during this study period, and shows the 30-day average trending line. Figure 4 shows the TSS removal during this study period, and the 30-day average trending line. Figure 5 shows the total phosphorus removal during this study period, and the 30- day average trending line. All three parameters (especially TSS and phosphorus) show a dip in removal efficiency during the winter months. However, this dip is not directly proportional to water temperature, as the decline in efficiency begins before the temperature declines. Table 6 Primary clarifier operating conditions May 2005 through April 2006 Parameter Average Maximum Minimum Design Daily flow, mgd 32.7 50.9 25.5 60 a Hydraulic overflow rate, gpd/ft 2 888 1,383 693 1,222 Iron salt dose, mg/l 12.9 23.9 4.8 30-40 Polymer dose, mg/l 0 0 0 1.0 Effluent water temperature, o C 22.3 25.2 14.4 14 b a Maximum month Minimum 1987
Table 7 CEPT performance May 2005 through April 2006 Parameter BOD TSS Phosphorus Primary influent, mg/l Average 230.9 342.7 8.48 Range 82 512 102-1168 2.9 17.5 Design 154 331 4.8 Primary effluent, mg/l Average 113.0 110.4 4.34 Range 6-239 5-480 2.3 10.1 Design 77 132 2.8 CEPT removal, % Average 51.1 67.8 48.8 Range 11.8 96.7 19.0 98.7 1.0 79.4 Design 50 60 -- Figure 3 BOD removal May 2005 through April 2006 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 4/8/05 5/28/05 7/17/05 9/5/05 10/25/05 12/14/05 2/2/06 3/24/06 5/13/06 7/2/06 Percent BOD Removal 1988
Figure 4 TSS removal May 2005 through April 2006 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 4/8/05 5/28/05 7/17/05 9/5/05 10/25/05 12/14/05 2/2/06 3/24/06 5/13/06 Percent TSS Removal 7/2/06 Figure 5 Phosphorus removal May 2005 through April 2006 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 4/8/05 5/28/05 7/17/05 9/5/05 10/25/05 12/14/05 2/2/06 3/24/06 5/13/06 7/2/06 Percent Phosphorus Removal 1989
RESULTS AND CONCLUSIONS The CEPT system at the Sutton WRF is meeting and exceeding the design goal of 50% removal of BOD 5 and 60% removal of TSS. However, with the hydraulic load to the plant only averaging approximately 55% of the maximum month design flow, the system has yet to be fully challenged. However, since only three primary clarifiers are in service, the hydraulic overflow rate is within the design parameters. As the hydraulic load increases, it is anticipated that the iron salt dose will have to be increased closer to design levels (at least doubling current dosing levels). Polymer as a secondary coagulant may also need to be added, to increase removal efficiencies (in winter months or year-round), and to help meet the new effluent limits at higher flow rates. Figures 6-8 show pictures of the CEPT system and the Sutton WRF. Figure 6 Sutton WRF flocculation basins 1990
Figure 7 Sutton WRF primary clarifier Figure 8 Sutton WRF aerial view of new liquids process facility 1991
ACKNOWLEDGEMENTS Personnel from the R.L. Sutton Water Reclamation Facility, Cobb County, Georgia, provided the facility operating data evaluated. REFERENCES Firmin, A.C. (1993) South Essex Sewerage District: Assessment of Chemically Enhanced Primary Treatment; New England Water Pollution Control Association Annual Meeting. Camp Dresser & McKee International Inc.; Montgomery Watson, Strategic Sewage Disposal Scheme Stage 1: Principal Collection and Treatment Systems, Impact of Revised Design Criteria for the Primary Sedimentation Tanks at the Stonecutters Island Sewage Treatment Plant Final Report; Camp Dresser & McKee International Inc.: Cambridge, Massachusetts, 1995. 1992