ASCE Los Angeles Section

Huntington Beach, CA

The Plant No. 2 Headworks Replacement Project is one of the largest and most complex projects undertaken by Orange County Sanitation District (OCSD) to date with a construction cost of $200 million dollars. The project provided a new modernized headworks facility for OCSD Plant No. 2 that replaced the 40-year-old facility that had become deteriorated and technologically outdated. The new headworks receives flow from five major trunk sewers that range in size from 78-inch diameter to 108-inch diameter and is capable of reliably processing a design peak flow of 340 million gallons a day (MGD) of raw sewage. The facility consists of 14 process structures including:

•An Influent Metering facility that can measure individual flows from each trunk sewer using magnetic flow meters ranging in size from 48-inches to 84-inches in diameter.

•A Screening facility including six bar screens, 8-feet wide and 60-feet tall, which remove debris from the raw sewage to protect downstream process equipment. Two hydraulic sluiceways convey screenings material to three washer/compactor units. Washed and dewatered screenings from the compactor units are conveyed by three shaftless screw conveyors to a truck loading facility for hauling off site.

•A 40-foot deep Influent Pump Station that lifts the screened sewage 20 feet above grade using seven 700 Hp VFD driven, vertical non-clog centrifugal pumps. The pump station design meets all Hydraulic Institute requirements (as was demonstrated by physical modeling) within a very compact footprint. This design resulted in significant cost savings. The pumps discharge to a uniquely designed channel that provides equal distribution of flow to the downstream grit basins regardless of which of the seven pumps are operating. CFD modeling was used to optimize the geometry and design of this unique channel, which was one of the factors contributing to this project earning the California Water Environment Association (CWEA) 2005 Engineering Research Award.

•Grit Basins and Handling facilities that separate and process grit from the wastewater using six vortex grit removal basins, grit slurry transfer pumps, and four grit washing cyclone and classifier dewatering units. The new grit removal system significantly increased the quantity of grit removed from the wastewater providing greater protection of downstream equipment. The new grit washing system also produces a much “cleaner” and drier grit product for disposal.

•A new Odor Control facility that includes the largest biological odor treatment facility of its kind in the country. The facility processes 188,300 cubic feet of foul air per minute from the trunk lines and headworks process areas through a two-stage system consisting of foam biotowers followed by wet chemical scrubbers. The biological odor treatment facility consists of 16 towers that are 10 feet in diameter and 42 feet high. The system is designed to achieve discharge hydrogen sulfide emissions that do not exceed 0.15 ppm, which is significantly lower than the South Coast Air Quality Management District (SCAQMD) imposed requirement of 0.5 ppm. Since the plant is located adjacent to residences, businesses, and popular open-use areas such as biking and hiking trails and the beach, a robust and reliable odor control system is critical to the District in their commitment to be a good neighbor.

Complicating factors in the design of the project included: deep construction (approximately 45 deep); high groundwater (within 5 feet of grade); highly corrosive soils; several seismic faults within the project site; corrosive air due to proximity to the ocean; corrosive atmosphere inside structures due to the nature of headworks processes; the characteristics of raw sewage (solids, rags, grit); hydraulic limitations due to existing upstream and downstream hydraulic grade lines; odor and noise control due to the proximity of neighboring residences; and the criticality of the headworks facility being at the head of the plant.

Special provisions for ease of operations and maintenance of this critical facility were included in the design. For example, every channel and piece of equipment (including every gate) can be isolated for maintenance without impacting normal operation. In addition, the control system for the new headworks facility provides full automation of equipment for unattended operation and integration with the existing plant-wide process control system.

An important part of the design involved developing very comprehensive and detailed construction sequencing, start-up and commissioning plans to allow construction, testing and start-up of the new facilities without compromising plant operations. Construction complexities included tie-in and diversion of flow from five different sewers ranging is size from 78-inches to 108-inches from the old headworks to the new headworks as described below.

The Headworks Replacement project is unique among other water/wastewater projects in many ways including its size, complexity, criticality, and high risk. The quality of the design and implementation of the project was also evident by tightness of the bids received and low change order rate.

SIZE

The new 340 mgd headworks facility is one of the largest in the country as OCSD is the third largest wastewater system west of the Mississippi River. The enormous scale of this project can be illustrated with some interesting facts:

•Construction of the new headworks required 200,000 cubic yards of soil excavation (the equivalent of two football fields dug 60 feet deep).

•The proximity of the project site to the ocean and the Santa Ana River resulted in the need to dewater approximately 1 billion gallons of water over a 24-month period.

•The reinforced concrete structures were constructed from 42 feet below grade to 55 feet above grade. Approximately 50,000 cubic yards of concrete was placed.

•The project added 330 new process electrical loads that can draw up to 12,000 hp of electrical power.

•Three 2,000 KW standby generators were added in order to support the vital processes during a power outage.

•The construction crews on the project enlisted 70 to 200 skilled workers at a time.

COMPLEXITY

One of the major challenges of this project was how to intercept the five major trunk sewers entering the plant and connect them to the new headworks facility. These trunk sewers range in size from 78-inches to 108-inches diameter. Since bypass pumping was impractical because of the high flows and high risk, a concept to construct junction boxes around the live pipes and make live tie-ins was developed during design and implemented during construction. This also required operating both the existing and new headworks facilities for an extended period of time until all influent and effluent connections to the new headworks were completed, and the existing headworks could be decommissioned and demolished at the end of the project.

The live tie-in process for each trunk sewer was successfully completed during the few hours of low flow in the early morning. The sequence for completing one of these tie-ins is depicted in the “Junction Box Tie-in” PDF attachment included in the Additional Information Section. A unique feature of each junction box is a precast concrete insert that was designed to provide a smooth flow path and minimal headloss through the box. It was installed in the box after removal of the pipe and the space around it was filled with grout. The first such tie-in was on a 96-inch trunk sewer and was completed in a way that allowed approximately 80 mgd of sewage flow to be either diverted to the new headworks or continue to be routed to the existing headworks facility. This was key to allowing extensive testing of the new headworks equipment including electrical and control systems in a fail-safe mode before permanently routing flow to the new headworks.

Diversion of the five influent trunk sewers had to be completed one at a time, over a period of several months as each tie-in significantly increased flow to the new headworks. Connection of pipelines carrying flow out of the new headworks to the downstream facilities had to be coordinated with influent tie-ins to maintain balance of hydraulic capacity in and out the new and existing headworks facilities, both of which were processing part of the flow (refer to “Tie-in Sequence” PDF attachment included in the Additional Information Section).

Other complexities of the project included:

•The majority of the work was completed within the oldest, most congested, and least documented area of the plant which required close coordination between OCSD, Carollo and the contractor and development of quick, innovative solutions to deal with unforeseen conditions.

•Temporary odor control facilities were required to prevent odor emissions during construction and start-up of new facilities.

•Additional design and construction difficulties are discussed under “obstacles” in the response to the next question.

CRITICALITY AND HIGH RISK

Because all flow entering the plant must pass through the headworks facility, it must remain in service at all times with sufficient reliability. The consequences of a failure at the headworks would be catastrophic and could result in flow backups and raw sewage spills in city streets. To manage risk during construction, the design phase included special workshops to develop a risk management plan; detailed construction sequencing plan; comprehensive testing, commissioning and start-up plan; and multiple constructability reviews. As stated by General Manager Jim Herberg, “The new headworks facility is one of the most important pieces of infrastructure in Orange County. This large and complex project required management of multiple risks during construction, and keeping the plant running during construction was like performing open-heart surgery on our treatment facility.”

Some of the greatest construction challenges occurred after the main process areas were constructed and passed the initial testing. The functionality of many of the process units such as screenings and grit equipment, could only be proven using high flow rates of actual raw sewage. To manage the risk of potential problems encountered during testing, the sequencing plan included the ability to divert flow back to the existing headworks if necessary. Only after all of the trunk sewers were diverted and the new headworks facilities fully tested at the rated capacities could the old headworks facility be demolished and the remaining site work completed.

QUALITY OF THE DESIGN

Despite the size and complexity of the project, all bids received were within a very tight range. In fact, the three lowest bidders were all within 1% of each other ($192.3M, $193.4M and $194.1M). This is a tribute to the clear and thorough bid documents. The District had properly budgeted for this project as the low bid came in 8% below the engineer’s estimate.

The final construction cost was $199,250,000 which was within OCSD’s budget for the project and only 3.6 percent above original bid price. The cost increase above bid price was primarily due to unknown conditions encountered at this congested “old” part of the plant and design changes and enhancements. Due the long duration of this project from start of design to completion of construction (approximately 11 years), several changes and enhancements to the project were incorporated during construction to reflect current wastewater practices and OCSD’s latest design standards to improve safety and efficiency. Working closely with the contractor, the construction management and design teams were successful at incorporating changes and project enhancements during construction while keeping the project cost well within OCSD’s budget.

RELATIONSHIPS

During the design phase, Carollo worked closely with OCSD engineering and O&M staff to select the best available technologies for Plant 2 and incorporate design features that would enhance operability, maintainability, and reliability of the new facilities. The process and equipment selection included workshops with plant staff, site visits to other plants, and equipment pilot testing. Input from plant O&M staff was incorporated into the design to provide desired access and equipment to improve efficiency and safety in performing operation and maintenance tasks. This included access platforms (both fixed and moveable); adequately sized and located access hatches to covered process units; bridge crane, monorail and other lifting devices; and gates and valves to provide easy isolation of equipment. In addition, Carollo design engineers worked closely with OCSD automation group to optimize control strategies and with OCSD construction management group to confirm construction sequence, and constructability issues.

During the construction phase, close collaboration between OCSD’s construction management team and Carollo’s design and construction services team contributed to the success of this large and complex project. The coordinated efforts resulted in the best technical and contractual decisions for resolving the numerous challenges during the construction and commissioning phases of the project. The other key to the project’s success was the strong dedication by both the District and General Contractor to find resolutions to all conflicts. Their solid working relationship and mutual respect enabled all parties to be guided by the common goal of project success.

OBSTACLES OVERCOME

The Headworks Replacement Project was the most complex project undertaken by OCSD to date. Complicating factors in the design and construction of the project included: deep construction; high groundwater; highly corrosive soils; several seismic faults within the project site; corrosive air due to proximity to the ocean; corrosive atmosphere inside structures due to the nature of headworks processes; the characteristics of raw sewage (solids, rags, grit); hydraulic limitations due to existing upstream and downstream hydraulic grade lines; odor and noise control due to the proximity of neighboring residences; and the criticality of the headworks facility being at the head of the plant. Some of these obstacles are described in more detail below:

•Site and hydraulic constraints:   The facility had to be constructed at a congested site to accommodate the locations and hydraulics of existing sewer interceptors and the existing primary treatment facilities.

•Earthquake faults:   Major high activity branches of the Newport-Inglewood strike-slip fault run through the project site, which is located in Seismic Zone 4. A subsurface exploration program to better define the locations of the fault splays and a site-specific seismic response study were conducted. The results were used for positioning of structures and developing the structural design criteria. All major hydraulic structures and critical facilities such as the electrical building were located outside a 50-foot setback from identified fault lines.

•Highly corrosive atmosphere:   Materials of construction were carefully selected to accommodate salt-water (beach) atmosphere and the corrosive nature of raw wastewater.

•High groundwater:   One of the major challenges for the project was the high groundwater table (within 5 feet of grade) due to the low elevation of the site and proximity to the ocean, coupled with the need for deep structures (45 feet deep) that could intercept the existing trunk sewers. This dewatering challenge was complicated by the existence of a confined pressurized aquifer that is located just below the elevation for the deepest excavations. Successfully pumping one billion gallons of groundwater from the site over a two-year period required installation of numerous dewatering wells and elaborate drain systems.

•Old site, unknown conditions:   The location of the Headworks Replacement Project is within the 40-year old section of OCSD’s Plant No. 2. As a result, numerous pipes, electrical ductbanks, and buried structures were encountered, many of which did not appear in as-built drawings from previous projects. While this was anticipated by OCSD and processes were in place for the Contractor to pot-hole prior to scheduled construction activities, rapid resolutions to these conflicts was critical. OCSD, Carollo, and the contractor worked together to develop cost effective and practical solutions to the conflicts caused by these unknown conditions.

•Operational testing of the new headworks:   Operational testing of the new headworks facilities was another major challenge. Multiple levels of testing were needed to test automated controls, equipment functionality, and system operation. Due to the size and nature of the facilities, testing with clean water was not practical so a large flow of wastewater was needed for testing. However, the existing headworks facilities also needed to remain fully operational throughout the testing and start-up period so flow could be routed back to it if necessary. This challenge required construction of uniquely designed junction boxes on the existing trunk sewers and a detailed construction sequencing and testing plan as described above.

•Odor control: OCSD has a strong commitment to being a good neighbor; as such their goal is to exceed the requirements imposed by the SCAQMD. Since the influent sewers and headworks facility are some of the greatest potential sources of odor at the plant, odor control is a very important part of this project. To answer this challenge, the P2-66 Project has one of the largest two-stage biotower odor control systems in the United States that treats 188,000 cubic feet of foul air per minute to well below the regulatory requirements. The odor control system consists of foam biotowers, which remove the bulk of the hydrogen sulfide, followed by wet chemical scrubbers for final “polishing.” Design criteria for the odor control system were based on approximately two years of pilot testing completed by OCSD. Using biotowers has reduced chemical use at the plant, reducing costs while improving operator and public safety.

•Construction sequencing and trunk sewer tie-ins:   These obstacles and associated solutions were described in detail above.

AWARDS

2005 – Engineering Research Achievement Award (SARBS)
2005 – Engineering Research Achievement Award (CWEA)
2011 – Engineering Achievement Award (SARBS)
2014 – Public Works/More Than $100 Million (CMAA Southern California)

QUOTES

see “Quotations” PDF attachment included in the Additional Information Section

The design included hydraulic modeling through the entire facility. Computational fluid dynamic (CFD) modeling of the bar screen inlet structure and pump station discharge/grit basin feed channel was completed to optimize the geometry of these hydraulic structures to promote even distribution to multiple process units. CFD modeling results for the pump station discharge channel for one operating scenario is presented in the “CFD Model” PDF attachment included in the Additional Information Section. Physical modeling of the pump station wet well was also completed to confirm compliance with Hydraulic Institute (HI) standards for the anticipated flow range and under various operating scenarios.

The Headworks Replacement Project at Orange County Sanitation District’s Treatment Plant No. 2 was a complex undertaking to replace one of the District’s most critical components of infrastructure. The new 340 mgd headworks facility will provide reliable and efficient service for many years to come and help the District in its “good neighbor” commitment. Design and construction of the project included many innovative and unique features to improve reliability, operability, safety and efficiency.

The Plant No. 2 Headworks Replacement Project is one of the largest and most complex projects undertaken by Orange County Sanitation District (OCSD) to date. The new 340 mgd headworks facility consists of 14 process structures including influent metering, bar screens, screenings handling, influent pumping, grit removal and handling, primary flow splitting and metering, odor control, chemical feed facilities, electrical and control building, and standby power generation.

Complicating factors in the design and construction of the project included: deep construction; high groundwater; highly corrosive soils; several seismic faults within the project site; corrosive ocean air outside and corrosive atmosphere inside structures from raw sewage; hydraulic limitations due to existing upstream and downstream hydraulic grade lines; the characteristics of raw sewage (solids, rags, grit); odor and noise control due to the proximity of neighboring residences; and the criticality of the headworks facility being at the head of the plant. Because all flow entering the treatment plant must pass through and be pumped by the headworks facility, the headworks must remain in service at all times with sufficient reliability as a headworks failure could result in raw sewage backups and spills in city streets.

An important part of the design involved developing very comprehensive and detailed construction sequencing, start-up, and commissioning plans to allow construction, testing, and start-up of the new facilities without compromising plant operations. One of the major design challenges was intercepting the five major trunk sewers entering the plant and connecting them to the new headworks facility. These trunk sewers range in size from 78-inches to 108-inches diameter. Since bypass pumping was impractical because of the high flows and high risk involved, a concept to construct junction boxes around the live pipes and make live tie-ins was developed during design and implemented very successfully during construction. This concept also allowed operational testing of the new headworks facilities with raw sewage while the existing headworks facilities remained operational throughout the testing and start-up period so flow could be routed back to it if necessary.

Design and construction of the project included many innovative and unique features to improve reliability, operability, safety and efficiency. For example, the new odor control facility includes the largest biological odor treatment facility of its kind in the country, treating almost 190,000 cfm of foul air from the trunk lines and headworks to well below regulatory requirements. The innovative two-stage system consists of foam biotowers that remove the bulk of the hydrogen sulfide followed by wet chemical scrubbers for final polishing. The system has reduced chemical use at the plant, reducing costs while improving operator and public safety, and helped OCSD in its commitment to being a “good neighbor.”

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