LRWRP_Design_Report_Chapter_3.pdf

CHAPTER 3

HEADWORKS IMPROVEMENTS (TM 1)

Introduction

Design flows for the upgrade project were discussed and agreed to during the liquid stream facilities brainstorming session held at the Lompoc Regional Water Reclamation Plant PRWRP) on September 16,2004. The following design flows were agreed upon:

ADWF = 5.5 mgd PDWF = 9.5 mgd PWWF= l5mgd

It was further agreed during the meeting that the liquid stream facilities downstream of the new grit removal facilities would be designed to treat the PDWF hydraulic load and excess flows would be dtverted to the EHB and returned for processing as plant influent flows subsided.

Objectives

This technical memorandum addresses each of the following headworks improvements that were requested by the City, included in the scope of work, or proposed by Brown and Caldwell:

replacement of influent pumps, influent flow metering, Vactor truck unloading station, grit removal, and diversion of excess flow to the EHB. The following objectives are addressed in this technical memorandum.

Identify the design and construction issues that must be addressed to replace the existing influent pumps in the existing influent pumping station to serve the new treatment facilities, while keeping sufficient existing pumps in service to serve the existing treatment facdities. Identify one viable alternative to replacing the existing pumps. List the advantages, disadvantages and conceptual cost estimates for each alternative. Provide preliminary sizing criteria for each alternative and recommend one alternative for implementation.

Identify modtfications required to implement measurement of raw influent flows while taking into account internal plant recycle flows. Identify preliminary flow measurement design criteria.

Identify modifications required to incorporate a Vactor truck unloading station into the flow scheme. Identify preliminary design criteria and a candidate package unit.

Identify candidate grit removal processes and grit separation and dewatering equipment. Recommend a process for implementation and develop preluninary design criteria.

Identify method of diverting excess flow to the EHB. Develop preliminary design

. criteria for flow splitting.

I?\-Projects\126517 - l~OhlPOC\\~~\Project~1~eP~~~\I033979 Preliminary Design Report I'inal.doc February 2005

Headworks Improvements 3-2

Develop conceptual cost estimates for the proposed alternative headworks improvements.

3.3 Replacement of Influent Pumps

3.3.1 Existing Conditions

The existing influent pumping station at the LRWRP was upgraded in the early 1970s. Some of the mechanical improvements constructed at that time included the addition of two new influent pumps and construction of h e d draft tube inlets for all the pumps (two existing, two new, and one future).

The original pump drivers were eddy current couplings. Two bar screens with screenings grinders were installed upstream of the wet well in a new screenings room. Screenings were ground up and returned to the process flow stream.

Over the years, the pump drivers were replaced with variable frequency drives (VFDs) of various manufacturers, some of whch failed, and magnetic drives. At this time, two pumps are driven by VFDs and two pumps are driven by magnetic drives. These pumps have a rated capacity of 8.6 mgd at 50 feet TDH and have 125 hp motors. A description of current influent pumping equipment is included in Table 3.3-1.

Table 3.3-1. Existing Influent Pumping ~acilitied")

Item Influent Pumps

The original bar screens and grinders were replaced with Parkson Aqua Guard Traveling Filter Screens having 6 rnm openings, and having screenings washer/compactors. Screenings are now washed, compacted and discharged to drums for subsequent disposal. However, the Parkson screens have an inherently higher headloss across them, which has caused an increase in the water depth in the sewers directly upstream of the screens, with resultant lower velocities and some increased deposition in the sewers. Furthermore, the existing control system does not maintain a stable depth in the wet well as evidenced by plant personnel observations of occasional air aspiration into the pumps from the wet well; this causes reduced capacity and vibration that can result in

Value

Number of Pumping Units

Type Rated Capacity (mgd) Total Dynamic Head (ft.)

Motor Horsepower Number with VFD Drivers Number with Magnetic Drives

P:\-l'rolrcts\126517 - LOhPOC\\XP\l'roject-Report\I033979 Preliminan. Design lleporr Finaldoc

Vertical, dry pit with bottom suction

8.6

February 2005

(a) Information obtained from existing plant design drawings design data sheet G3 and visual inspection

Headworks Improvements 3-3

1 1 damage to pump shafts, seals, and bearings. The draft tube liners have corroded away for reasons the City is unable to determine, and some pump inlets also have corroded badly; the City has replaced one draft tube inlet.

3.3.2 Proposed Conditions

As discussed during the September 16,2004 brainstorming meeting, the new treatment facilities will have a significantly lower than current hydraulic profile immediately downstream of the influent pumping station, and the plant liquid flow stream will be almost completely new. The existing primary clarifier and biofilter will be abandoned, significantly reducing the required influent pumping head. At the brainstorming session, it was proposed and accepted that two replacement influent pumps be sized to convey the entire PWWF of 15 mgd, and two identical pumps be provided as spares. Candidate pump drivers for the replacement pumps were identified at the meeting to be VFDs or magnetic drives.

Two alternative methods of upgrading influent pumping to accommodate the proposed new treatment facilities will be evaluated in order to identify the most effective alternative for implementation. X description of the proposed influent pumping design criteria is included in Table 3.3-2. Appendices C and D contain marked up drawings from the 1974 project that dlustrate the complexity of work required to implement Alternatives A and B as described in the following subsections.

I tern

Table 3.3-2. Proposed Influent Pumping Design Criteria

Alternative A - Existing Pump Station Number of Pumping Units

Type Rated Capacity (mgd) Total Dynamic Head (ft.)

Motor Horsepower Drives

Alternative B - New Pump Station Number of Pumping Units

Type Rated Capacity (mgd) Total Dynamic Head (ft.)

Motor Horsepower Drives

Value

Vertical dry pit with bottom suction 7.5

50-60 VFD or Magnetic

Vertical turbine solids handling

7.5

VFD

P:\-Projrcts\l26517 - I.Oh~OC\\~P\l'roject-1<ep~~rt\llI33979 I'rrliminnt). D e s i p Report Finaldoc February 2005

Headworks Improvements 3-4

3.3.3 Alternative A - Existing Pumping Station

Alternative A is replacement of the existing pumps and corroded discharge manifold piping in the existing influent pumping station. This is by far the most complex of the two alternatives, as the existing plant must be maintained in service until the new treatment facilities are successfully on line, yet sufficient pumping capacity must be installed in the existing influent pumping station to serve the existing facilities and to start up the new facilities. In addition, the cause of corrosion of the draft tube liners must be determined and the pump draft tube inlets must be replaced under this alternative. Note that Insituform lining is not a viable method of repairing the existing draft tube inlets; therefore, they must be demolished and reconstructed.

We have identified two phases of work required to implement Alternative A. For convenience in the following discussion, the existing pumping units will be referred to as occupying positions 1 through 5 starting from the west (i.e., Pump 1 [existing Pump P2010] occupies position 1).

3.3.3.1 Phase 1. Under Phase 1, the draft tube inlet for pump position 3 must be replaced first, and then Pump 1 or 2 and its motor, driver, pump shaft, starter and all its accessories must be relocated to position 3 to become the spare for Pumps 4 and 5, which will continue to serve the existing treatment facilities until the new facilities are successfully started. A knife gate valve will need to be installed in the discharge manifold between Pump 3 and the 36" manifold outlet that exits the building, in order to allow for additional Phase 1 construction without interrupting the existing treatment process. A brief plant shutdown will be required to shorten or replace the 24-inch pipe spool and install the gate valve.

Next, the remaining pump in position 1 or 2 must be removed and the draft tube inlets for pump positions 1 and 2 must be replaced. Following these replacements, the new Pumps 1 and 2 will be installed in these two positions, including motors, drivers, pump shafts, starters and all accessories;

and the pump discharges reoriented and the 24-inch manifold replaced completely between column line B and the west wall of the dry well; and another knife gate valve will be installed at that location.

The 24-inch pump discharge manifold will be extended out through the west wall of the pump station and extended to the new grit removal facilities inlet channel.

If VFDs are used as drivers for the replacement pumps, it will be necessary to construct a wall with a door along the east side of the MCC area to create a separate room for the MCCs and new VFDs.

An HVAC system will then be provided on top of the roof of Room 112 (Supply Fan Room) to serve the now isolated MCC room, and will include 3-stage filtration to protect sensitive electronic equipment. Minor modifications of the existing ventilation system will be required to serve the maintenance shop.

3.3.3.2 Phase 2. Phase 2 construction will begin promptly after the new treatment facilities are successfully started. First the existing pumps in positions 3, 4, and 5 will be removed completely, and then the draft tube inlets for Pumps 4 and 5 must be replaced. Following these replacements, the new Pumps 4 and 5 will be installed in the corresponding positions, including motors, drivers, pump shafts, starters and all accessories. A section of the 24-inch manifold between column lines B and C will be replaced to serve future Pump 3 with a reoriented discharge; a brief new plant shutdown will be required to replace this section. The existing 36" WW outlet from the existing

P:\-l'rojccts\l26517 - LOM1'OC\\TT\l'rojrct~RePort\l~133'17~ I'reliminarg Drsibm Report Rnal.doc February 2005

Headworks Improvements 3-5

24-inch manifold wdl be removed to the north wall of the building and the opening in the wall will be plugged with concrete. Finally, the section of existing 24-inch manifold that included the 36-inch outlet wlll be removed and replaced with a 24-inch spool.

3.3.3.3 Pump Selection. Candidate bottom suction non-clog vertical pumps that appear to be able to meet the Altemative A pumping requirements listed in Table 3.3-2 include the following:

Fairbanks Morse 571 1, 880 rpm, 50 horsepower Cornell NHG, 880 rpm, 60 horsepower Morris 193V, 700 rpm, 60 horsepower

These pumps would be provided with either VFDs or magnetic drives. All electrical and control equipment would be located in the existing MCC area. Pump curves for the candidate pumps are included in Appendix C.

3.3.3.4 Conceptual Cost Estimate. The conceptual construction cost estimates for Alternative A with magnetic drives and VFDs are presented in Table 3.8-1.

3.3.4 Alternative B - New Pumping Station

Altemative B is construction of a new influent pumping station. Alternative B is by far the simplest and least disruptive of the two alternatives, as there would be minimal interfacing with the existing influent pumping stadbn and the existing plant operation would not be interrupted during construction of the new influent pumping station. In addition, replacement of the existing draft tube inlets would not be required and the existing influent pumps could be retired after the new treatment facilities are started. The existing Aqua Guard Filter Screens wdl not be affected and will remain in service.

The new influent pumping station would be located west of the existing influent pumping station building and north of existing Thickener 1. The new pumping station would consist of a slab covered trench type "self-cleaning" wet well (as described in ANSI/HI 9.8, Pump Intake Design), with four outdoor vertical turbine solids h a n h g (VTSH) pumps with VFDs. The wet well would be constructed 3 to 4 feet lower in elevation than the existing wet well for improved pump inlet conditions. A 36-inch sewer would connect the new wet well to the existing wet well. A temporary bulkhead would be installed in the existing wet well to enable connection of the new 36-inch sewer while maintaining three existing influent pumps in service to serve the existing treatment facilities.

3.3.4.1 Pump Selection. Candidate VTSH pumps that appear to be well suited to the Alternative B conditions listed in Table 3.3-2 include the following:

Fairbanks Morse 16-inch VTSH, 880 rpm, 50 horsepower Patterson 16-inch MPVT, 705 rpm, 50 horsepower

P:\-Pn,jects\126517 - LOhn'OC\\XP\l'mjectgrport\1033979 Preliminac). Design Report Finaldoc February 2005

Headworks Improvements 3-6

The VFDs and starters would be located in the nearby new MCC building required to serve the new dissolved air flotation thickeners (DAFTs). No separate MCC building would be required for the new electrical equipment nor would it be located in the existing influent pumping station MCC area.

Pump curves and information for the candidate pumps are included in Appendix D.

3.3.4.2 Conceptual Cost Estimate. The conceptual construction cost estimate for Alternative B is presented in Table 3.8-1.

3.3.5 Discussion and Recommendation

Based on the simplicity of implementing Alternative B; its minimal impact on existing influent pumping operations; the uncertain feasibility, cost and disruption of replacing the existing draft tubes; the superior wet well level control available by selecting VFDs over magnetic drives (the latter are slower to respond to changing wet well levels); the fact that the new wet-well would be simple to clean; and its lower conceptual cost, Alternative B is clearly superior and we recommend its implementation.

3-4 Influent Flow Measurement and Recycle Streams

3.4.1 Background

The Master Plan identified two alternatives - A and B - for influent flow measurement. Alternative A involved reconfiguring the influent sewers into the LRWRP to enable separate measurement of the flows from Vandenburg Village (VV), Vandenberg Air Force Base (VAFB) and Lompoc.

Alternative B involved a combined measurement and then calculating Lompoc's contribution. All measurements were to be made in new accessible vaults using Parshall flumes. The Master Plan presented the estimated costs at about $1.4 million for Alternative A and $130,000 for Alternative B.

These alternatives were discussed during the brainstorming meeting on September 16,2004. Brown and Caldwell recommended Alternative B, but with a different flow meter: the MGD Technology ADFM Velocity Profiler installed within the 54-inch sewer. Operations staff noted problems with grit deposition in the 48-inch and 54-inch influent sewers and how this would impact on the accuracy of the MGD meter, and noted that plant recycle flows would be included in the 54-inch raw wastewater flows. The City asked Brown and Caldwell to look into other metering options that preferably would not include plant recycle flows.

3.4.2 Discussion and Recommendations - Recycle Flows

Brown and Caldwell investigated influent flow metering options and issues. There are three categories of plant drainage streams to consider: storm drains, sanitary waste streams, and process and tank drains (recycle streams). Plant site storm water runoff is currently collected and pumped directly to the Santa Ynez River. This arrangement is not a concern and will be retained. Sanitary waste is &charged from the Administration Building and the Influent Pumping Station via separate sewers to the manhole on the 54-inch sewer, and ths configuration also will be retained.

P:\-Prolects\126517 - LOhfl~OC\\\1~\Project-Keport\Jll33979 l'rcliminary Design Report Finddoc February 2005

Headworks Im~rovements 3-7

I Since many of the existing facilities wdl be retired from service, some of the current recycle flows will disappear (such as the recycle flows upstream of the Influent Pumping Station sanitary waste contribution), and new recycle flows will appear. A new tank drainage pumping station will be required to collect some new recycle flow streams, and we recommend that this facility, where feasible, also collect gravity recycle streams for existing facilities that will be retained. We further recommend that the existing tank drainage pumping station have its bscharges re-routed as noted below, and a small tank drainage pumping station be provided to divert process drainage from the digesters area. Rather than meter any recycle flows, we propose that all recycle flows be pumped into the raw wastewater flows downstream of the new metering facility and upstream of the new grit tanks, or diverted by gravity to the oxidation ditch.

3.4.3 Discussion and Recommendations - Influent Flow Measurement

We recommend that the raw wastewater influent flows be metered upstream of the discharge from the new 24-inch influent pumps hscharge manifold. This pipe will discharge into a new influent channel leading to the new grit removal tanks and the velocity of flow in the 24-inch pipe will be such that grit deposition will not occur. Raw wastewater sampling can be done at the 24-inch outlet to the new grit tank influent channel. We recommend that the MGD Technology ADFM Velocity Profiler (hot tap meter for closed pipes) be used for this application (see Appendix E for information on the meter). As noted during the brainstorming meeting, Brown and Caldwell's experience with the ADFM Velocity Profiler meters has been extremely good with no in-situ calibration required.

I The conceptual construction cost estimate for flow metering is presented in Table 3.8-1.

Vactor Truck Unloading Station

A Vactor truck unloading facility will be provided. It wiU include a ramp, containment walls, sump and an outlet pipe, and it will be sized to receive and contain the contents from a single Vactor truck tank discharge (1,000 gallons). The outlet pipe will convey the wastewater to a Lakeside Septage Acceptance Plant that contains a fine screen to remove debris and inorganic solids prior to hscharge. The unit will be sized to process 400 gpm and discharge effluent by gravity to the point where recycle flows join the grit tank influent. Appendix F includes information on this Lakeside equipment, which costs $80,000. The conceptual construction cost estimate is presented in Table 3.8-1.

Grit Removal and Processing

At present, grit is removed from primary sludge at the gravity thickener. However, the primary clarifiers and the gravity thickener are not included in the new treatment scheme so a separate grit removal facility will be required. When grit is not removed from the wastewater, it can settle out in downstream piping causing flow restrictions. What does not settle out in the piping is removed in-downstream processes and causes significant wear on mechanical equipment, and it often settles out in digesters, reducing dgestion capacity.

I>:\-l'rojects\126517 - I.Oh~OC\\~~P\Project-1lep1~rt\Ill33979 Preliminary Design Report Finaldoc February 2005

Headworks Improvements 3-8

1 There are two common primary grit removal processes: aerated grit removal and vortex grit removal.

3.6.1 Vortex Grit Removal

A vortex grit removal system is a circular tank located in the main flow stream of the inlet system.

Wastewater enters the tank tangentially, flows around the tank, and exits parallel to the inlet. The tangential motion of the water causes organic material to remain suspended while allowing the heavier grit particles to settle to the bottom of the main tank. The main tank has a sloped bottom, allowing the grit to migrate to a lower hopper area, where it is removed by pumping. A motorized paddle assembly is provided to impart additional rotational energy to ensure that the best rotating velocity is achieved.

There are several manufacturers who make vortex units, including Jones+Attwood (JetaTM Grit Trap), Smith and Loveless (Pista GritTM Chamber), and John Meunier (MectanTM Grit Tank).

Although different manufacturers promote slightly different designs, the basic principles are the same. The manufacturer primarily provides the mechanical paddle/propeller and motor, with the structure being constructed in the field out of concrete.

Typically each tank is designed to handle a range of flows: from minimum to peak flow rates. The removal efficiencies for finer grit particles are expected to increase as the flow rate decreases, due to the longer detention and settling time provided. Manufacturers claim that typical grit removal efficiencies range from 95 percent for 50-mesh particles down to 55 percent for 140-mesh particles.

In addition, recent independent evaluations by engineering consultants indicate that the rated capacity of vortex grit removal units may be as much as 50 percent overstated by the manufacturers (i.e. the capacity of the units should be half of what the manufacturers state).

3.6.2 Aerated Tank Grit Removal

In aerated grit removal tanks, the flow enters near the bottom of a rectangular tank (length:width ratio of about 3:1) and exits the other end at the top of the tank. Air is introduced along one side of the tank near the bottom, causing a spiral roll velocity pattern perpendicular to the flow through the tank. The roll velocity keeps organic material in suspension while allowing heavier grit particles to settle to the bottom. The grit is collected in hoppers from where it is removed using pumps (typically recessed impeller type). The roll velocity is constant regardless of flow through the tank, allowing high efficiencies over a wide range of flows. Plant personnel can vary the amount of air discharged into the tank to determine the most efficient amount of air. Although energy must be expended in providing the aeration air, the increase in grit removal efficiencies allow for a reasonably compact system.

3.6.3 Grit Separation and Dewatering

The grit slwry removed from the tank still contains many organic particles that cause odors when degrading and are best treated by the downstream treatment facilities. Also, the grit slurry contains only about 1 percent solids (i.e. mostly water). Therefore the organic material is separated from the grit and then the grit is dewatered, creating a product that is acceptable to landfills.

P:\-I'rojrcts\l26517 - LOhlljOC\\\'T\Project-Repofl\1033979 Preliminary Des ip Report Finaldoc February 2005

Headworks Improvements 3-9

\\ 3.6.3.1 Grit Cyclone and Classifier. One method of separation and dewatering that has been in use for decades is the grit cyclone and classifier. The grit slurry from the grit tank is pumped to a small cyclone that uses a hydraulically induced vortex to remove much of the organics from the slurry. Inlet pressure to the cyclone must be kept relatively constant at 7 or 8 psig as recommended by the manufacturer. After the cyclone, the grit slurry contains about 5 to 10 percent solids and the level of organic solids in the concentrated grit slurry is typically less than 15 percent. Variations in the design of the grit cyclone include replaceable liners made of a variety of metallic and nonmetallic materials, and replaceable inlet, outlet and main body parts.

The grit cyclone is usually mounted above a grit classifier/dewaterer so that the concentrated slurry can discharge from the cyclone directly into the classifier. The classifier is usually manufactured by the same company that makes the grit cyclone. The classifier washes and dewaters the grit slurry received from the cyclone, and is the final treatment step before storage and disposal of the grit. The grit slurry is washed of organic material and dewatered by action of water sprays and a rotating, inclined screw conveyor. The rotating screw transports grit up a trough. A gap between the screw and the trough causes a "bed" of grit to form, acting to prevent backwash of the grit particles down the trough and providing a path for the liquid brought up with the grit to drain. Liquid, containing suspended solids, collects in the lower part of the classifier and overflows a weir to return to the influent channel.

The dewatered grit is typically between 35 and 45 percent solids, although manufacturers do not seem willing to guarantee a certain solids concentration.

( 1 3.6.3.2 SlurryCupTM and Grit SnailTM System. Eutek manufactures two units which perform the same function as the grit cyclone and classifier. They call the proprietary units the SlurryCupTM and the Grit SnailTM. The SlurryCupTM also uses vortex action to remove the organics from the grit slurry. Grit settling velocities generated by the vortex action can be significantly higher than those for gravity settling, which allows for higher tank overflow rates and, hence, a smaller effective tank area. The grit slurry enters the SlurryCupTM where high fluid velocities generate a swirling action, causing grit to settle to a lower chamber where either pumping or blow down transports the grit to the Grit SnailTM for dewatering. Operation of the SlurryCupTM requires hydraulic energy to generate the swirling action and, as a result, incurs a typical headloss of 45 inches (1.6 psi) or greater.

The Grit SnailTM provides additional washing and dewatering of the grit slurry from the SlurryCupTM. Grit is collected on an inclined reciprocating belt as it settles out of the slurry, and is carried up to the discharge chute. Grit capture is accomplished by sizhg the main tank surface area to limit the effective overflow rate to 16,000 gpd/ft2 to capture 140-mesh grit. Expected grit removal efficiencies are 95 percent for particles 140-mesh and larger. Units with capacities of up to 4 wet tons per hour of dewatered gnt are available. The washed and dewatered grit is guaranteed by the manufacturer to be greater than 50 percent and typically about 60 percent solids.

3.6.4 Grit Storage and Disposal

After grit is removed from the flow stream and dewatered, it wdl be deposited in a container until the grit is hauled offsite. At a peak dry weather flow of 9.5 mgd, it is expected that about 0.7 cubic yards per day of grit (average) w d be generated. The grit storage should be covered to control odor.

I':\grojccts\126517 - LOh~OC\\T1'\Projectf1teport\lIl3.?979 Preliminary Design lieport Finaldoc February 2005

Headworks Improvements 3-1 0

Grit transport is extremely harsh on equipment and sho~dd be minimized. We propose locating the grit separation and dewatering equipment at a level that will allow the dewatered grit to drop directly into a storage container without needing additional transport. Grit can be combined with screenings so the combined concentration can be about 50 percent solids, for offsite disposal.

3.6.5 Discussion and Recommendations

Due to low velocities in the upstream sewers close to the LRWRP, even under PDWF conditions, and the buildup of solids depositions in these sewers as a result of the low velocities, we would expect severe grit loading to occur during peak flow events, especially at the beginning of these flow events. This type of grit loading would tend to overwhelm a vortex grit removal system, with its single grit pump per tank. Accordingly, we recommend a covered aerated grit removal tank with three cells for the LRWRP, with enclosed stainless steel Eutek grit processing equipment suitable for outdoor installation and to acheve a grit concentration guaranteed to be above 50 percent solids as currently required for landfill disposal as a solid waste. Design criteria for the aerated grit removal tank are presented in Table 3.6-1. The conceptual construction cost estimate for this facility includes flow diversion to the EHB and is presented in Table 3.8-1.

Table 3.6-1. Proposed Aerated Grit Removal Tank Design Criteria

Item

Grit Tanks Number of tanks Number of cells or hoppers per tank Wastewater average depth, ft Cell width, ft Total tank length Floor cross slope, degrees Agitation air per ft of tank length, scfm Diffuser spacing, ft Detention time at PWWF, rnin

Grit Agitation Air Blowers Number, duty + standby

Type Capacity, each, SCFM Discharge pressure, psig, approx

Value

1 + 1 future + bypass channel 3 6

4 at inlet, 2.4 at outlet 1.75 3.0

1 + 1 Positive displacement

P:\-Pn,jccts\126517 - LOhPOC\\YP\Project-Report\1033979 Preliminary Design Report Find.doc February 2005

Headworks Improvements 3-1 1

Grit Pumps Number per cell, total for tank Capacity, gpm

Type

Item

1 , 3

Recessed impeller

Value

Grit Separation and Dewatering Slurry Cup sTM Grit SnailsTM

Flow Diversion to Emergency Holding Basin

In order to optimize treatment and minimize construction and operating costs, the secondary treatment processes downstream of grit removal will be designed to treat the PDWF as the peak hydraulic load (see TM 2). To achieve this objective, raw wastewater flows in excess of the design PDWF (9.5 mgd) will be diverted downstream of the grit removal process to the existing Emergency Holding Basin.

Brown and Caldwell recommends that the diversion to the EHB be accomplished by installing and throttling two parallel 20-inch ball valves on parallel 24-inch pipelines downstream of the grit tank effluent channel, to control the maximum flow rate to the oxidation ditch. Two ball valves will be provided for redundancy. We recommend that an MGD Technology ADFM flow meter be installed on a common 24-inch discharge pipe downstream of the control valves, and the flow meter output signal be used to automatically regulate the maximum flow through the ball valves. This will cause flows in excess of the PDWF (the secondary treatment maximum flow rate will be operator selectable), to back up in the grit tank effluent channel and be diverted over a weir to flow by gravity via a new approximate 18-inch diameter sewer (size to be determined during detailed design) to the EHB. Up to 6 mgd could be diverted in this manner. Enclosed pipe metering and control valves are being proposed, rather than open channel devices such as flumes and modulating gates, to limit odor and hydrogen sulfide release.

When flows subside below the PDWF rate, wastewater in the EHB will be pumped in a new force main back to the point where the recycle flow streams join the grit tank influent. Pumping will be provided by a new pumping station with one duty and one standby pump.

The pipehes to the flow control valves will have submerged inlets, which will cause floating material to accumulate at the downstream end of the grit tank effluent channel, above the entrances to the outlet pipes. A sinall scum sump will be provided at that location to collect scum and pump it to the new dissolved air flotation thickeners.

P:\-l'rolects\126517 - LOMPOC\\SP\l~roject~lIeport\Il~3.7979 Preliminary Design lleport Fial.doc February 2005

Headworks Improvements 3-12

I The conceptual construction cost estimate for the Emergency Holding Basin pumping station is presented in Table 3.8-1. Table 3.8-1 also includes conceptual costs for the diversion structure as part of the grit removal/flow diversion facility, and conceptual costs for the piping as part of the sitework.

Comparative Costs of Conceptual Alternatives

A conceptual layout of the headworks facilities is presented on Figure 3.8.1, and the location of the new influent pumps and grit tank are shown on Figure 1.12.1 - Site Plan. Conceptual construction costs have been estimated for the recommended and alternative headworks improvements described in this Chapter. These costs include electrical and instrumentation, and allowances for contractor overhead and profit, taxes, insurance, other miscellaneous contractor costs, and a 25 percent allowance for construction contingencies. Estimated costs are based on recent similar construction projects on the west coast where available, or the latest edition of Mean's Construction data. Table 3.8-1 presents a comparison of the conceptual construction cost estimates.

The estimated construction costs for the headworks improvements incorporating the influent pumping station Alternative X with magnetic drives or with VFDs, or Alternative B with VFDs are $4.7 million, $4.6 million, and $4.3 million respectively. The major differences between these three alternatives are in the cost of the influent pumping station itself. Alternative B, the new pumping station, has the lowest estimated construction cost and will be the least disruptive to implement.

Table 3.8-1. Comparison of Conceptual Construction Costs

Item

Influent Pumping Station Yard Piping Emergency Holding Basin Return Pump Station Flow Metering Grit Removal/Flow Diversion Facility Vactor Truck Unloading Station Sitewide Allowances

Subtotal - Unburdened Cosl Markups '

Subtotal - Construction Cosl Construction Contingency, 25%

Total Construction Cosl

Modifications to Existing Pumpi

Alternative A with magnetic drives

($1 1,05 1,456

262,162 39,173 56,723

657,668 1 27,669 378,236

2,573,087 1,299,887 3,872,974

833,974 4,700,000 g Station Alternative A with VFDs ($1

971,521 262, I62 39,173 56,723

657,668 128,426 393,196

2,508,869 1,255,942 3,764,811

810,683 4,600,000

4ew Pumping Station Alternative B with VFDs ($1

854,299 264,092

39,173 56,723

657,668 127,669 400,236

2,399,860 1,147,989 3,547,849

763,964 4,300,000

Note: I. Labor, materials, subcontractor and equipment markups; taxes; shipping; escalation; general conditions; startup, training and O&M; insurance; and bonds.

P:\-l'n~jects\126517 - l~.0hWOC\\Tl'\Projectf1Zeport\Il133979 Preliminary D e s i p Report Finxl.doc February 2005

Headworks Improvements 3-14

Preliminary Construction Cost

The preliminary estimate of probable construction cost for the recommended headworks improvements described above is $3,482,000 as indicated for TM 01 in Table 2.1-2. A detailed breakdown of this estimate is provided in Appendix B. The construction cost of $3,482,000 excludes costs for the yard piping and instrumentation and controls.

P:\-Projrcts\126517 - LOMPOC\\TP\Project-Repo~\I~l3i979 Preliminaq Design Report Finddoc February 2005