Norwalk, CT Water Pollution Control Facility (WPCF) CSO Capacity and Treatment Evaluation. Subject: Task 3 Existing Grit Removal System Assessment

Transcription

1 A Technical Memorandum Date: Project: Norwalk, CT Water Pollution Control Facility (WPCF) CSO Capacity and Treatment Evaluation Subject: Task 3 Existing Grit Removal System Assessment This memorandum presents an assessment of the existing grit removal system at the Norwalk Water Pollution Control Facility (WPCF) Supplemental Treatment Building and considers alternative technologies and design modifications to the existing facilities to improve operations. Existing Grit Removal Systems The following is a description of the grit removal systems in the Supplemental Treatment Building and for the primary sludge. The focus of this memorandum is the process in the Supplemental Treatment Building. However, plant improvements being investigated under the plant-wide facilities planning study could impact and/or enhance primary sludge degritting operations. Supplemental Treatment Building The WPCF currently removes grit using horizontal flow grit settling chambers as the primary grit removal system for flow up to 30 million gallons per day (mgd). This system is located in the Supplemental Treatment Building. Each of the two horizontal flow chambers is 46 feet long with a depth of approximately 5.8 feet assuming a 9 foot operating depth in the wet well at the Main Pumping Station The grit channel width is 4 feet at the top and 3 feet at the bottom. The settled grit is removed from the bottom of the grit channels with buckets attached to two countercurrent grit elevators. The buckets lift grit from the grit channels on the lower level of the Supplemental Treatment Building to the grit conveyors on the main floor. Transverse screw conveyors then transfer the Horizontal Flow Grit Chambers

2 Page 2 grit to the grit washers for dewatering and then dewatered grit is discharged into a container for off-site disposal. Preliminary calculations show that at 30 mgd, with both chambers operating and a channel depth of 5.8 feet, the velocity through the chambers is about 1.08 feet per second (fps) and the detention time is 43 seconds. This velocity is within the typical design range of 0.8 to 1.3 fps for horizontal flow grit chambers, but the detention time is lower than the generally accepted design criteria of seconds (M&E 2003). If one channel is down for repair, the flow velocity through the remaining channel is doubled and the detention time is cut in half, which further reduces grit removal rates. Grit Processing Equiment in Supplemental Treatment Building Flows in excess of 30 mgd are diverted to the wet weather treatment train in the Supplemental Treatment Building. Two mechanical flow regulating gates are located upstream of the horizontal grit chambers and are intended to provide a means to control the amount of flow entering the plant for secondary treatment. Excess wet weather flow passes over a weir (just upstream of the regulating gates) and enters the wet well of the high flow pumping station. Wet weather flow (up to about 65 mgd) is pumped up to rotary drum screens, also located in the Supplemental Treatment Building. There are six drum screens, with two drums in each of three bays. The east bay contains screens with 74-micron openings and the central bay contains screens with 149-micron openings. The west bay is out-ofservice, but previously contained screens with 256- micron openings. After the flow is screened, it is disinfected using chlorine in a contact tank. The supplemental flow system was not designed with any grit handling but typically grit that does overflow into the wet weather treatment train Wet Weather Treatment Drum Screens collects in the drum screens, caught in the smaller micron screens and/or settles out in the wet well, the screen tanks or the chlorine contact tank, and is removed by a vacuum-truck after the storm. Most grit during wet weather events is conveyed along with the 30 mgd dry weather flow, is pumped through the Main Pumping Station and settles in the primary tanks.

3 Page 3 Primary Sludge Degritting Primary sludge is pumped to three Wemco cyclone degritters to remove grit from the sludge prior to thickening. The grit passes through a classifier and is dewatered and discharged on conveyors before being deposited into containers for offsite disposal. Each cyclone/ classifier pair is designed to handle 270 gallons per minute (gpm) of primary sludge. The maximum input solids concentration is about 1 percent. It is important to note that, when the total influent flow to the primary clarifiers increases to approximately mgd during storm events, the primary sludge degritters are typically taken off-line as they cannot process the higher grit load. At these higher flow rates and higher grit loading conditions, the cyclones are less effective. First, grit removed from the cyclones under these conditions has higher, and unacceptable, amounts of organic material. Second, the cyclone centrate discharge line backs up, reportedly due to more friction because of the increased grit loading that is being discharged in the centrate. Accordingly, under high flow conditions, the primary sludge pumps discharge directly to the gravity thickeners where excess grit settles in the tank for later removal. Grit that is not captured in the primary clarifiers collects in the secondary aeration basins and is removed when the tanks are drained for diffuser maintenance. Existing Facility Assessment The city reports challenges in successfully operating the grit removal systems, which are the a result of a number of plant operating and plant and collection system design conditions. Under current conditions, the wet well of the Main Pumping Station operates at a fairly consistent and high water elevation. The current wet well operating level of about feet (USGS NGVD 1929) is about a 9 foot operating depth and was established based on a wet well depth that provided the best measurement control using the existing ultrasonic device (without creating a turbulent surface that caused inconsistent and inaccurate depth indication). The wet well stays fairly constant because the pumps use variable speed drives with a level set point control and the wet well is no longer operated in a draw-fill mode.

4 Page 4 The high water depth in the wet well normally results in a backwater elevation upstream of the Supplemental Building bar screens that surcharges the 72-inch influent interceptor, lowers pipeline and channel velocities, and causes sediment deposition to occur in the interceptors directly upstream of the building. During rain events, the high wet weather flow (and higher pipeline velocities) re-suspend the grit and solids that have settled on the bottom of the pipes and conveys the material into the Supplemental Treatment Facility. This slug of solids and grit tends to overwhelm the grit removal system, interfere with screening operations, and grit not captured by the grit system is carried through the channels into the Main Pumping Station wet well where it is pumped to the primaries. The marginal design of the horizontal flow grit chamber renders it ineffective during by this type of grit loading as grit is passing through to the primaries without much removal. Some operating conditions could be modified (such as installing the ultrasonic wet well level device within a stilling well to allow lowering the wet well level) to help improve the grit deposition/re-suspension condition. However, the level cannot be lowered sufficiently to impact velocities in upstream pipelines because of the existing interceptor invert elevations. For example, the 72-in pipe between Junction Box No. 1 and the Supplemental Building has a flat slope resulting in low velocities under all dry weather flow conditions. Variable grit loadings such as those experienced at Norwalk are also a typical problem in a combined sewer system. Thus, a grit removal system for the Norwalk WPCF needs to be sized and designed with adequate flexibility to handle these slug flow events. Grit capture is also reduced because of the design of the existing grit channels and flow control gates. The flow control gates include an orifice-type channel restriction that appears to have been used in conjunction with the gate mechanism to control flows. The exact dimensions of the opening are uncertain as the drawings do not provide sufficient detail. Field measurements require isolating the entire Supplemental Building. However exact measurement is not necessary as with the addition of VFDs at the Main Pumping Station, flow to the remainder of the plant is controlled by pump speed paced off a flow signal from the pump discharge flow meter. Thus the flow control gates are reportedly fixed in their most open position and are not adjusted. The channel restrictions even with no throttling may still be creating higher velocities into the grit chambers during higher flow events. There is also a concrete block installed in each grit channel, just upstream of where the grit collector buckets leave the channel and travel vertically up to the floor above. The blocks were installed to help dissipate channel entrance velocities created by the control gates. But the concrete blocks occupy 2 feet of a 4 foot wide channel leaving a one foot wide passage around each side of the block. The impact of these structures on channel velocities and grit bucket operation has not been fully investigated but it is theorized that vortexes may be created just downstream of the blocks that are causing the sediment within the grit buckets to be washed out as the buckets are lifted above the water surface.

5 Page 5 The existing grit chambers could theoretically be improved if they could be modified to increase the length of the channels to improve detention time and/or to lower velocities. However, the channels cannot feasibly be lengthened or widened enough to improve either design parameter dramatically, with significant confidence that the improvement would handle the widely variable grit loadings. The entrance to the grit channels along with the concrete block would need to be modeled to determine the impact of this design and whether modifications to improve performance would be feasible. In addition, generally, this technology is considered antiquated by today s standards primarily due to the problems experienced at Norwalk which are typical for this type of system. Accordingly, CDM determined that trying to reuse or retrofit the existing horizontal grit chambers would not be a viable option because of the historically poor performance, the lack of system redundancy, and the limited ability to substantially expand the existing chambers within the Supplemental Treatment Building. A retrofit of the existing grit system will also not help to address the problem of heavy grit loads in the wet weather flow that is treated by the supplemental treatment system as those problems are inherent in the existing collection system design. Grit Removal Strategies Since the existing grit removal system could not be reasonably modified as is to meet project objectives, several options were developed and considered utilizing areas within the Supplemental Building. Grit removal improvements would have to be developed in coordination with future wet weather treatment options as available space within the building will only be created if/when the existing drum screens are abandoned. CDM considered various grit removal technologies/strategies to remove grit from the combined dry and wet weather flow and assessed the potential installation of these technologies within the existing building. Each technology was generally assessed for its capital and operating costs, space requirements, grit removal capability, ease of operation, and feasibility for use in Norwalk. In addition, another important consideration was maintenance of existing operations. The use of the technologies was discussed with CTDEP in a series of workshops. During a July 2007 meeting with Norwalk and CDM, CTDEP officials noted that the state would require Norwalk to maintain a primary level of treatment along with disinfection for the wet weather flow even during construction. The initial design flow for the grit removal system was set to about 95 mgd in order to treat the combined dry and wet weather flows, which reflected the approximate current capacity of the wet weather treatment system. In addition, a higher flow rate of 120 mgd was also considered in the event that additional wet weather treatment was necessary to minimize or eliminate CSO discharges from the Ann Street Siphon CSO. This design flow target may

6 Page 6 change as the future treatment plant flows are estimated as part of the larger facilities planning process. Grit Quantities and Characteristics Limited information was available regarding the quantities of grit captured at the plant and no information is available about its physical properties. As such, CDM engaged Grit Solutions to perform two days of continuous grit sampling in January 2008 of the plant influent so that a grit characterization could be performed. The main goals of the analysis were to determine the particle sizes, the actual settling velocity, and the relationship of grit size to settling velocity. The grit characterization study is finished and being analyzed at the time when this memorandum was being prepared. The results of the analysis will be used in the final evaluation of the various grit systems and in the design of the selected process. Grit Removal Process Performance Requirements The goal of a new grit removal system should be to remove about 95 percent of all grit particles with a specific gravity of 2.65 that are greater than or equal to 150 microns (100 mesh) in size. However, this design point may be refined after the grit characterization report is issued and reviewed. Potential Technologies CDM considered a variety of grit removal technologies for installation within the Supplemental Building at the Norwalk WPCF including horizontal flow grit chambers, aerated grit chambers, settleable solids concentrators, and vortex-type grit chambers. Horizontal Flow Grit Chambers Horizontal flow grit chambers are operated by maintaining a velocity of approximately 1 ft/s through a horizontal settling channel. The grit settles out in the channel and equipment such as a chain and flight system are used to remove the settled grit from the bottom of the channel. In general, the main advantages for this technology include its simple design and the flexibility allowed by controlling the outlet flow. The disadvantages include potential high head loss at the flow control structure, difficulty maintaining constant velocity during varying flow conditions, high organics removal if flow is too low, low grit removal efficiency if flow is too high, and large space requirements, as compared to other technologies. The existing horizontal flow grit chambers at the Norwalk plant are not effective at removing grit and there does not appear to be a feasible way to improve their capacity and efficiency. CDM therefore does not recommend horizontal flow grit chambers for further consideration because of their performance limitations, their inability to handle varying flows, and the problems with the existing chambers at the WPCF as described in this memo.

7 Page 7 Aerated Grit Chambers Aerated grit chambers have a long history of use at major wastewater treatment facilities with well-established design criteria. The aerated grit process creates a spiral rolling motion with the addition of air through coarse diffusers located along one side of the chamber. The diffused air addition is used as a method of controlling particle velocities within the chamber, which in turn controls of the size of grit particle allowed to settle and be removed. Because of the spiral air flow pattern, grit particles make several passes across the tank bottom thus improving the chances for removal. The addition of diffused air keeps the lighter organic particles in suspension and they are discharged over the effluent weir. This is a proven process with numerous installations nationwide. A typical section through an aerated grit chamber is shown in Figure 1. The aerated grit chamber also provides additional storage within the process, which enables further flexibility for varied flow and load conditions as experienced in a combined sewer system. Typical process design criteria for aerated grit tanks are listed in Table 1 and Table 2 below along Figure 1: Typical Section Through Aerated Grit Chamber (Source: M&E 2003) with the proposed dimensions for a new aerated grit removal tanks. CDM established deeper basins to minimize the footprint of the aerated grit tanks. Aerated grit chambers also create odors and these tanks would have to be covered and have odor control/mitigation.

8 A Table 1: Aerated Grit Design Criteria 90 mgd Typical Criteria Proposed for Norwalk Number of Tanks for 90 mgd As Required - Min. of 2 3 Hydraulic detention time 2 to 5 minutes at peak flow 3 minutes Overflow Rate for 100 Mesh 42,000 gal/day/sq ft 42,000 gal/day/sq ft Side Water Depth 7 to 16 ft 12 ft Width (each tank) Varies to Suit 20 ft Length (each tank) Varies to Suit 40 ft Length to Width Ratio 1:1 to 5:1 2:1 Air Supply 3 to 8 cfm/ft of length 6 cfm/ft Table 2: Aerated Grit Design Criteria 120 mgd Typical Criteria Proposed for Norwalk Number of Tanks for 120 mgd As Required - Min. of 2 4 Hydraulic detention time 2 to 5 minutes at peak flow 3 minutes Overflow Rate for 100 Mesh 42,000 gal/day/sq ft 42,000 gal/day/sq ft Side Water Depth 7 to 16 ft 12 ft Width (each tank) Varies to Suit 22 ft Length (each tank) Varies to Suit 44 ft Length to Width Ratio 1:1 to 5:1 2:1 Air Supply 3 to 8 cfm/ft of length 6 cfm/ft Historically, grit has typically been removed from aerated grit chambers using grit screws at the tank invert in conjunction with bucket elevators or in conjunction with recessed impeller centrifugal pumps in a drywell adjacent to the grit tank. A small number of plants utilize a clamshell bucket mounted on a two way bridge crane to lift grit from the bottom of the tanks. The advantage to this is that the grit is somewhat dewatered as the clamshell is lifted out of the flow stream.

9 Page 9 The traveling bridge grit removal system is a newer technology to the market that is well suited to handle the high peaks associated with wet weather flow and is suitable for use in aerated grit chambers. Submersible pumps are mounted on a bridge system that moves along the chamber in both directions and continuously removes grit collecting in the troughs at the bottom of the tanks. A typical section through an aerated grit chamber with traveling bridge pumps is shown in Figure 2 below. Vortex Type Separators Vortex type separators include low energy, medium energy and high-energy variations. The difference in variation is the method used to induce and maintain a rotary vortex motion over the range of design flows for the unit. The typical range of flow for an individual low and medium energy induced vortex type unit is about 4:1. Figure 2: Typical Section Through Aerated Grit Chamber with a Traveling-Bridge-Type Grit Removal System (Source: M&E 2003) High-energy units rely solely on velocity to maintain the induced vortex and have limited flexibility in design flow rates. High-energy units such as a cyclone separator are typically used as a secondary classification device to separate water from collected grit and are not applicable as a primary grit removal system for Norwalk. Low energy vortex grit collectors use a mechanical paddle in the center of the tank to maintain rotary motion and induced vortex action. Vortex units are widely known by the proprietary names of Pista Grit by Smith & Loveless or JETA by Jones & Attwood/Eimco, but are also manufactured by WesTech, Waste Tech, Lakeside, and others. These units have the lowest head loss of the induced vortex processes, typically a few inches or less. Medium energy units rely on a velocity control device instead of a mechanical paddle to maintain rotary motion over the 4:1 flow range and thus have higher headloss, typically in the 9 to 12- in range. Grit is collected in a lower hopper section of the tank, and removed with pumps suitable for handling a grit-water slurry. The pumps may be top-mounted suction-lift style, or can have a flooded suction if space is available to construct a below-grade gallery or basement.

10 Page 10 Figure 3 shows a typical section through a vortex grit chamber. Table 3 contains model information and planning level sizing for 30-mgd and 100-mgd Pista Units for conceptual level design. Figure 3: Typical Section Through Vortex Unit (Source: M&E 2003) Table 3: Vortex Unit Data Model 100.0A Flow (mgd) 100 Upper Chamber Diameter 32'-0" Upper Chamber Depth 12'-8" Lower Chamber Diameter (min) 8'-0" Lower Chamber Depth (min) 12'-0" Inlet Channel Width 8'-0" Outlet Channel Width 8'-0" Model 30.0A Flow (mgd) 30 Upper Chamber Diameter 18-0 Upper Chamber Depth 9-2 Lower Chamber Diameter (min) 5-0 Lower Chamber Depth (min) 7-0 Inlet & Outlet Channel Widths 4-6 Most vortex units require a pre-screening size no larger than ¾- inches. The proposed influent screens will be finer than this spacing. Because of the variable flows during a storm and for operational flexibility, the better design approach is to use multiple 30 mgd units to meet the treatment capacity required for dry and

11 Page 11 wet weather flows. Each unit must have the appropriate turn down ratio to meet low nighttime flow requirements. In addition, multiple units provide some flexibility to maximize redundancy. However, the disadvantage of multiple units is that space requirements increase for the additional units, connecting piping, and workspace/clearances. Headcell Settleable Solids Separators The Eutek Headcell Settleable Solids Separator is a type of vortex unit that operates under differential pressure (as shown in Figure 4). The modular, multi-tray concentrator has a small footprint and can be designed to fit into existing grit chambers or basins. Flow feeds into the concentrator tangentially, establishing the vortex flow pattern. Solids settle into the boundary layer of each try and are drawn down the center to the collection chamber. The Headcell provides fine grit removal down to 50 microns. The headloss in 30-mgd units is 12 inches. The Headcell requires a considerable amount of service water; however if the Norwalk WPCF plant water meets the reuse water requirements, then this requirement could be incorporated into the system to avoid using city water. Figure 4: Headcell Concentrator Diagram (source: Eutek brochure)

12 Page 12 The Headcell has the advantages of efficient grit removal, a small footprint, and a modular design. The disadvantages include high capital cost, higher headloss than other technologies, and potential siting issues due to the equipment s height and weight. This is also a newer technology to the market with limited installations at the design flows for Norwalk to judge its performance, maintenance, and reliability. Typical design parameters are shown in Table 5. The grit washing and dewatering units are typically installed paired with each Headcell Concentrator. Table 5: Headcell Unit Data Parameter Headcell Concentrators Slurrycup Grit Washing Units Grit Snail Dewatering Units Flow per Unit 30 mgd 30 mgd 60 mgd Number of Units Required for 30 mgd Number of Units Required for 60 mgd Number of Units Required for 90 mgd Number of Units Required for 120 mgd Footprint 17'x17' each 2 washers, 1 dewaterer: 12'x17' Height 21' 12' Headloss 12" 16.8' - As discussed with the vortex grit removal technology, multiple smaller units would probably be recommended to meet the variable flows and to maximize operational flexibility and redundancy. However, the disadvantage of multiple units is that space requirements increase for the additional units, connecting piping, and workspace/clearances. Space is also required for various accessory equipment associated with grit removal technologies. These include recessed impeller pumps and bucket elevators, odor control measures, and grit treatment systems such as cyclones, grit classifiers, and grit washers. Conclusions Based on the preliminary design criteria and equipment dimensions discussed above, efforts were made to fit the new grit removal systems into the existing Supplemental Treatment Building (assuming that the existing rotary drum screens for wet weather treatment would be removed). Because of existing unstable soil conditions at the site, all of the major piping and facilities at the Norwalk WPCF are supported on piles. This means that lowering existing floors or

13 Page 13 adding substantial weight with new equipment in the existing building is problematic. Adding piles under the existing floor to support new weight may not be practical or cost effective. In addition, space within the bays of the rotary drum screens is marginal at best to be able to fit in the multiple vortex or Headcell type grit removal units. The footprint for an aerated grit removal facility also exceeds the available dimensions of the bays. Substantial removal of concrete to increase space within the existing bays may not be practical or costeffective considering the structural issues. Accordingly, most of the units would have to be installed at the main floor levels. Finally, the hydraulics of adding grit removal within the Supplemental Treatment Building is also difficult to address. Dry and wet weather flow would have to be pumped into the grit removal facilities in order to maintain downstream hydraulics. Increasing the capacity of the existing wet weather flow pumps meet peak wet weather flow conditions has several disadvantages including: The existing wet well for the wet weather pumping station may not meet hydraulic standards if flow is increased by at least 30 percent. The existing wet weather flow turbine type pumps may not be the appropriate pumping application for dry weather pumping conditions (as opposed to intermittent wet weather conditions), especially for the combined sewer flow with higher grit loadings. The issue of screening adequacy for all flows still remains. These alternatives and the benefits and disadvantages of each were discussed in a series of workshops with CTDEP and the city during Based on the discussions, there was agreement that it was not practical to install a new grit removal system within the existing Supplemental Treatment Building. Primary Sludge Solids Degritting One other alternative would be to eliminate grit removal operations entirely before the primary clarifiers. By this approach, all degritting of dry weather flow would be accomplished using sludge degritting equipment. Also, grit removal for wet weather flow would not be performed. The Norwalk WPCF currently utilizes Wemco cyclone degritters and grit washers to remove grit from primary sludge during dry weather. The degritters can handle flows up to approximately mgd. The existing degritting operation could potentially be upgraded to handle the peak dry weather flow (30 MGD) during normal operations.

14 Page 14 This approach eliminates the cost of building, operating, and maintaining separate grit units for preliminary treatment. The disadvantages include increased wear on the primary clarifier equipment, increased pumping costs, and increased primary sludge handling. In addition, many communities that practice this approach have had continued difficulties in consistently removing grit from the primary sludge. Recommendation Accordingly, based on the evaluation of the existing grit removal system limitations in the Supplemental Building and the performance limitations associated with the existing primary sludge degritting operations, CDM recommends that new appropriately sized grit removal facilities in a new wet weather preliminary treatment building be installed at the Norwalk WPCF to remove the grit as a standard preliminary treatment process. This approach will mitigate existing grit system performance problems in the Supplemental Building, provide a means to remove and process high wet weather grit loads which will continue to arrive at the plant due to the design limitations of the existing upstream conveyance system, eliminate pumping grit into the primary clarifiers and associated primary sludge degritting problems, and eliminate grit carryover into the aeration tanks.