Executive Summary of the West Mamala Bay Wastewater Facilities Plan

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1. PURPOSE

The purpose of the West Mamala Bay Facilities Plan is to provide a long-range strategy for accommodating the collection, treatment and disposal of wastewater from the West Mamala Subdistrict of the Mamala Sewerage District. Preparation of this plan was prompted by City and County of Honolulu General Plan (1992) objectives and policies encouraging development in this subdistrict. Specifically, the General Plan states the following:

1. Population

Objective C

To establish a pattern of population distribution that will allow the people of Oahu to live and work in harmony.

 

Policy 2

Encourage development within the secondary urban center at Kapolei and the Ewa and Central Oahu urban-fringe areas to relieve developmental pressures in the remaining urban-fringe and rural areas and to meet housing needs not readily provided in the primary urban center.

 

Kapolei, Ewa and the high-growth areas of Central Oahu, including Mililani, are within the West Mamala Subdistrict.

The West Mamala Bay Facilities Plan Study Area generally coincides geographically with the West Mamala Subdistrict of the Honouliuli Sewerage District. The Subdistrict extends from Halawa toward the Waianae Coastline and from Ewa to Central Oahu.

Most of the existing and planned developments within the Study Area are served by, or will be served by the Honouliuli Wastewater Treatment Plant (WWTP), including pockets of older developments that are still served by cesspools or septic systems.

1. APPROACH

   The West Mamala Bay Facilities Plan is being prepared through three intermediate documents that progressively lead toward final recommendations:

    

   The Interim Facilities Plan established the context for the subsequent development and examination of facility alternatives. Included in this plan is a discussion of the regulatory regime governing wastewater services, a summary description and assessment of the existing wastewater collection, treatment and disposal system, a projection of future wastewater disposal needs, and a review of alternatives to meet projected wastewater disposal needs. A public informational meeting was conducted on the Interim Plan to disseminate information contained in the plan and to solicit discussion and input.

    

   The Pre-Final Facilities Plan builds upon the Interim Plan by examining the most feasible alternatives for meeting projected wastewater disposal needs and making preliminary recommendations. Another public informational meeting will be conducted to disseminate information on the alternatives and preliminary recommendations and to solicit discussion and input.

    

   Following the preparation of the Pre-Final Facilities Plan, an Environmental Impact Statement (EIS) will be prepared to assess the overall environmental impacts of implementing the recommendations. The EIS will be prepared and processed in compliance with the requirements of Chapter 343, Hawaii Revised Statutes (Hawaii EIS Law) and will include opportunities for public and agency review of its scope and findings.

    

   The Final Facilities Plan builds upon the Pre-Final Plan by recommending a strategy for developing or improving wastewater collection, treatment and disposal facilities to meet projected demands. The strategy will include a preliminary timetable for implementation and an order-of-magnitude cost estimate. A third public informational meeting will be conducted to present the recommendations and solicit input.

    

   This Facilities Plan is being prepared in accordance with various Federal, State, and City and County of Honolulu (City) regulations and programs. Significant legal authorities affecting facilities plan preparation include the Clean Water Act, Environmental Protection Agency (EPA) facilities planning regulations, 208 Water Quality Management Plan, National Pollutant Discharge Elimination System, and Hawaii Administrative Rules, Title 11-23 relating to Underground Injection Control Program and Title 11-62 relating to Wastewater Systems, Water Reclamation and Solids Disposal. In addition, two consent decrees between the City and the State Department of Health (DOH) impose additional legal requirements affecting wastewater treatment and disposal at the Honouliuli WWTP.

2. STUDY AREA

   The Study Area encompasses the southern slope of the Schofield Plateau, the Coastal Plain of the Ewa District, the eastern and southern flanks of the Waianae Mountain Range and a leeward section of the Koolau Mountain Range. The topography of the Study Area varies from the relatively flat Coastal Plain to the gentle slopes of the Schofield Plateau to steep walls of deeply cut valleys in the Waianae and Koolau Mountain Ranges. The climate is considered subtropical with temperatures ranging from 60 to 85 degrees Fahrenheit. The median annual rainfall averages from approximately 20 inches along the Ewa coastal areas to about 300 inches at the summit of the Koolau Range.

    

   Comprising the leeward sloping portion of the saddle between the Koolau and Waianae Mountain Ranges, the Study Area includes seven perennial streams, all of which drain into Pearl Harbor. The Ewa Plain is characterized by the absence of perennial streams or conspicuous natural drainage features. Areas prone to flooding in the Study Area are mostly located along the coastline or adjacent to streams.

    

   The Study Area overlies the Pearl Harbor Aquifer Sector and a portion of the Wahiawa Aquifer System. Encompassing several types of groundwater resources, the most predominant is basal water which is formed where fresh groundwater floats on denser seawater within subsurface rock, creating a "basal fresh water lens." A second groundwater type occurs in coastal areas where basal water is confined by "caprock" which tends to restrict the seaward flow of freshwater, causing a thickening of the fresh water lens.

    

   Coastal waters of the Study Area, which extend from Ko Olina along the southwest coast, around Barbers Point including West Mamala Bay to the entrance of Pearl Harbor, are classified as "A" waters. Class A waters are recognized by the State Department of Health (DOH) with the objective that "their use for recreational purposes and aesthetic enjoyment be protected."

    

   Urbanized portions of the Study Area include eight neighborhood board areas: Aiea, Pearl City, Waipahu, Ewa, Mililani/Waipio/Melemanu, Makakilo/Kapolei, Mililani Mauka/Launani Valley, and the southern portion of Aliamanu/Salt Lake/Foster Village. The majority of development has occurred on the gently sloping Schofield Plateau, and more recently on the Ewa Plain.

    

   The Study Area is served primarily by the Honouliuli Wastewater Treatment Plant (WWTP). Wastewater is collected and conveyed to the plant through a network of gravity lines, as well as pump stations connected to force mains to gain elevation where necessary. The service area includes the following communities: Halawa, Aiea, Waimalu, Pearl City, Pacific Palisades, Waiawa, Waipahu, Mililani, Waipio, Village Park, Crestview, Waikele, Kunia, Kapolei, West Loch, Kalaeloa Community Development District (formerly Naval Air Station Barbers Point (NASBP)), Ewa Beach, Makakilo, Nanakai and Ko Olina.

    

   Despite rapid development in the Study Area, and resulting expansion and improvement of the wastewater collection system, there are four large unsewered areas remaining where public sewer systems are not available. The majority of unsewered areas are located within Ewa Beach, Honouliuli, Waimano and Aiea Heights.

    

   The Honouliuli Wastewater Treatment Plant is operated by the City and County of Honolulu and is the second largest wastewater treatment facility on Oahu. Located at the corner of North Hanson and Geiger Road, near the Kalaeloa Community Development District (formerly NASBP), the plant was put into operation in 1984. It has an average daily wastewater flow design capacity of 38 million gallons per day (mgd), and currently processes an average daily wastewater flow of about 26 mgd. All wastewater receives primary treatment and up to 13 mgd receives additional secondary treatment before exiting the plant via an 84-inch gravity line to be discharged at the Barbers Point Ocean Outfall located approximately 1.7 miles off-shore of Ewa Beach at a depth of 200 feet. Current solids treatment and handling processes includes thickening, storage/blending, stabilization, dewatering, and landfill disposal.

    

   Although the Federal Water Pollution Control Act Amendments of 1972 require secondary treatment for all effluent discharges, the City applied for and received a secondary treatment waiver for ocean discharge [301(h) waiver] in 1985 and 1991, the latter of which expired in 1996. The City reapplied for the waiver, but approval is still pending review by the United States Environmental Protection Agency (EPA). The plant continues to operate under the conditions of the 1991 National Pollutant Discharge Elimination System (NPDES) permit.

3. NEEDS ASSESSMENT

Existing and future needs related to wastewater collection, treatment and disposal system for the Honouliuli WWTP were assessed as the basis for facilities planning. These assessments were based on current population data and population projections prepared by the City Department of Planning and Permitting (DPP). The population projections extend through the year 2020 and, thus, establish the planning horizon for the West Mamala Bay Facilities Plan.

1. Collection System

   The wastewater collection system was assessed by inputting DPP's population data to the City's sewer flow analysis system (SFAS) model. The SFAS model was developed to determine the available capacity of the wastewater system to accommodate flows and includes a hydraulic assessment of the system's capacity. Since wastewater systems can be additionally taxed by inflows of runoff during storms, the effect of a 2-year 6-hour recurrence interval design storm on the wastewater system was modeled. The assessment also includes findings of a condition assessment conducted for a significant portion of the wastewater collection system. Identified are areas where age and corrosion have affected the structural integrity of the system.

    

   Deficient sewerlines and manholes are identified in the needs assessment. These include sewerlines that have inadequate capacity to accommodate existing and projected flows as well as manholes and sewerlines with structural or corrosion deficiencies. Pump stations and associated force mains with insufficient capacity to accommodate existing and projected wastewater flows are also identified.

    

2. Treatment System

    

   Assessment of the wastewater treatment system at the Honouliuli WWTP was based on receiving flows projected through the SFAS model.

    

   The liquid treatment stream was assessed based on capacity as well as operational and performance issues identified by plant operators and during site visits. With minor additions, the existing facilities are adequate to accommodate year 2020 Average Dry Weather Flows (ADWF). The plant, however, does not have sufficient capacity for the projected Peak Wet Weather Flows (PWWF). Improved treatment levels at the plant also need to be considered because of potential regulatory issues and future effluent reuse applications.

    

   The solids treatment stream was also assessed based on capacity as well as operational and performance issues identified by plant operators and during site visits. The needs assessment indicates that most of the existing solids treatment facilities are inadequate to accommodate projected year 2020 wastewater flows.

    

3. Disposal System

    

   Assessment of the wastewater disposal system was based on receiving flows projected through the SFAS model. Considered were regulatory issues that will affect the feasibility of alternatives for disposing of the projected flows, including those related to ocean disposal, water reclamation, and biosolids disposal and reuse.

    

   The effluent disposal capacity of the existing ocean outfall may be exceeded by the PWWF by the year 2001. However, the ocean outfall capacity can be increased to meet the projected year 2020 PWWF by constructing a pumping station. Other alternatives to increase outfall capacity or reduce the peak flow need to be considered. In addition, the structural integrity of the outfall needs to be assessed.

    

   The City has initiated a plan to evaluate available options for biosolid reuse. The plan is needed to determine the long-term feasibility of numerous options, including the trial composting project with the Navy.

    

    

    

4. Odor and Noise Control Systems

The needs related to odor and noise control must be addressed as the plant expands to accommodate future flows and as community development in the vicinity of the plant continues. While noise has not been a significant problem, odor control has always been an important component of the Honouliuli WWTP and will likely become even more important. Results of a 1989 odor survey at the plant demonstrated that hydrogen sulfide concentrations were within permit limits, but still can be detected by the human nose. Odors related to the heat treatment process also need to be considered. A comprehensive management and control program for odors and noise should be undertaken.

1. CONCEPTUAL ALTERNATIVES

During the Interim Facilities Plan phase, conceptual alternatives were developed that could address identified wastewater collection, treatment, and disposal system needs.

1. Collection System

Deficiencies in the wastewater collection system can be addressed through the following conceptual alternatives:

• Relief lines - Relief lines are installed to parallel a sewerline that has inadequate capacity and, thereby, provide additional capacity; 

• Replacement line - Replacement lines are installed to increase the capacity of a sewerline that has inadequate capacity or to replace one which is structurally deficient; 

• Rehabilitation - In-place rehabilitation of a structurally deficient line such as by installing a liner or coating within the line can be less expensive than replacing a line; 

• Flow diversion - Similar to a relief line, a flow diversion line can direct upstream flows around an area affected by capacity limitations into a downstream line with adequate capacity; and 

• Flow equalization - Flow equalization involves storing peaks in flows that could otherwise cause capacity problems and allowing those flows to feed back into the system at a rate that can be accommodated. Storage can be provided by constructing a new reservoir, by converting former treatment facilities into storage facilities, or backing flows up within upstream sewerlines.

1. Pump Stations

Wastewater Pump Station (WWPS) deficiencies can be addressed using the following conceptual alternatives:

• Upgrade - Pumps and other equipment can be upgraded to increase the capacity of a WWPS; 

• Replacement - When equipment upgrading requirements are too extensive to be economically feasible, replacement of the entire WWPS, possibly at a more advantageous location, may be preferable; 

• Flow diversion - Installation of a diversion line can direct some or all of the flow around a deficient WWPS to relieve its capacity problem or allow it to be abandoned; and 

• Flow equalization - As discussed previously, flow equalization can mitigate peak flow capacity problems by storing and releasing flows at a rate that can be accommodated at a WWPS.

1. Liquid Treatment

The alternatives pertaining to the liquid treatment stream include those to address (1) plant capacity and (2) plant treatment level.

Certain treatment processes were determined to have inadequate capacity at the future PWWF. Three conceptual alternatives were developed to address capacity deficiencies within the liquid treatment stream:

• Plant expansion - Plant expansion includes expanding the deficient existing liquid treatment processes, in addition to associated solids treatment processes, to meet the PWWF. The extent of the plant expansion would depend on the amount of flow equalization done in the collection system or at the plant; 

• Flow equalization in collection system - Flow equalization in the collection system involves storing peaks in flows that could otherwise cause capacity problems and allowing those flows to feed back into the system at a rate that can be accommodated. Storage can be provided by constructing a new reservoir, converting former treatment facilities into storage facilities, or backing flows up within upstream sewerlines. This would minimize the amount of plant expansion needed; and 

• Flow equalization of raw influent at Honouliuli WWTP - Similar to flow equalization in the collection system, flow equalization at the plant would store peak flows, allowing them to feed back into the system at a rate that can be accommodated.

According to the NPDES permit and the findings of the Mamala Bay Study Commission, primary treatment is adequate for the Honouliuli WWTP. However, while secondary and higher levels of treatment are not required at this time, five conceptual alternatives were developed should additional treatment be necessary and to provide a flexible plan for the future. The five alternatives included:

• No action - This alternative involves retaining the existing liquid treatment stream without any modifications or new additions to the treatment processes; 

• Chemically-Enhanced Primary Treatment (CEPT) - Incorporation of chemically-enhanced primary treatment would improve the settling capabilities of suspended solids in the primary clarifiers and enhance disinfection processes; 

• Expansion of secondary treatment - Because secondary treatment is a necessary element for effluent reuse, expansion of secondary treatment is considered; 

• Advanced (reuse) treatment - Advanced treatment processes to produce high quality reclaimed water is not required, but is an option for the future; and 

• Disinfection only - This alternative incorporates disinfection only to improve ocean discharge quality.

1. Solids Treatment and Disposal

Additional capacity is needed for solids treatment and disposal of projected future flows. In addition, the existing heat treatment process is near the end of its service life. As a result, selection of a solids treatment method should be expedited. Alternatives for solids treatment and disposal are mainly dependent on the ultimate disposal method or beneficial use of the biosolids or biosolids by-product. The following five solids treatment/disposal combinations were developed as alternatives:

• Thermal conditioning/dewatering and incineration - Thermal conditioning/dewatering and incineration is the historical treatment and disposal method used at the Honouliuli WWTP; 

• Thermal conditioning/dewatering and reuse - Because of the requirements of the 309 Consent Decree, biosolids reuse is the preferred disposal alternative. This alternative would match the existing solids treatment method with a beneficial reuse program. A number of biosolids processing techniques are available for beneficial reuse programs and include agricultural land application, composting (biological stabilization), chemical stabilization/fixation, and heat drying; 

• High-solids centrifuge raw sludge dewatering and incineration - High-solids centrifuges are compatible with on-site incineration because odors can be contained and treated; 

• High-solids centrifuge raw sludge dewatering and reuse - High solids centrifuges are not compatible with certain biosolids reuse applications, but this alternative may be viable depending on the biosolids reuse method selected; and 

• Anaerobic digestion/dewatering and reuse - Anaerobically digested biosolids could be used in a biosolids reuse program.

1. Effluent Disposal and Reuse

Two issues pertaining to effluent disposal and reuse must be addressed: peak flows and low flows. To convey the future PWWF for disposal through the oceanoutfall, the addition of a pump station will suffice. To supplement ocean disposal, a recharge trench at the plant could be used to replenish the Ewa caprock aquifer.

The outfall's low flow issue pertains to effluent being diverted for reuse purposes in the future. The outfall requires a minimum flow of 14 mgd to maintain proper design velocities within the pipe. Five effluent disposal alternatives were developed to address the low flow issues at the Barbers Point Ocean Outfall.

• Flow equalization in collection system - As discussed previously, flow equalization in the collection system can mitigate low flow capacity problems by storing and releasing flows at a rate that can be accommodated by the outfall; 

• Flow equalization of raw influent at Honouliuli WWTP - As discussed previously, flow equalization of raw influent at the plant can mitigate low flow capacity problems by storing and releasing flows at a rate that can be accommodated by the outfall; 

• Flow equalization of primary influent at Honouliuli WWTP - Flow equalization of primary effluent instead of raw influent would reduce the amount of odor control and maintenance needed. As discussed previously, flow equalization at the plant can mitigate low flow capacity problems by storing and releasing flows at a rate that can be accommodated by the outfall; 

• Periodic flushing of outfall - Periodic flushing of the outfall would be performed with the new outfall pump station to remove settled material in the outfall; 

• Plugging of diffuser ports - Plugging a number of the diffuser ports in the interim may help to maintain sufficient velocities in the outfall; and 

• Parallel outfall - A second, parallel outfall of smaller diameter would be constructed and used for peak and low flows, ensuring sufficient velocities are maintained within the pipe. An effluent pump station would not be constructed under this alternative, as the two outfalls would be capable of handling both peak and low flows.

1. Odor Control Considerations

There are four main methods in controlling odors produced in wastewater conveyance and treatment facilities:

• Control of odor production in the wastewater collection system (focusing on hydrogen sulfide and relating to corrosion protection and safety concerns as well as odor reduction); 

• Control of odor production in the wastewater treatment facilities through specific design and operational techniques; 

• Dilution of odors through proper design of odor release points, buffer zones, and other methods; and 

• Containment, collection, and treatment of foul odors.

An effective odor control program usually consists of a combination of these methods. The addition of a masking or counteraction agent is also an alternative, but is not considered a useful long-term solution.

1. ALTERNATIVES ASSESSMENT WASTEWATER COLLECTION SYSTEM

During the Pre-Final Facilities Plan phase, the most feasible conceptual alternatives developed in the Interim Plan phase were assessed toward making preliminary recommendations. These alternatives are presented in two chapters: Chapter 5, one for the collection system; and, Chapter 6 for the treatment and disposal system at the Honouliuli WWTP.

The alternatives selected for assessment are capable of eliminating spills, accommodating peak flows from storm events up to the design storm, and meeting all requirements pertaining to effluent and sludge reuse.

1. Collection System Deficiencies

The alternatives available to address deficiencies in the collection system, as identified in Chapter 3 Needs Assessment are divided by the nature of the deficiencies into four classifications:

• West Interceptor System Deficiencies - For the purposes of the alternatives assessment, the West Interceptor System is the 30-inch gravity line conveying wastewater flows from areas generally west of the Honouliuli WWTP, to the plant.

Deficiencies of the West Interceptor System are addressed as a unit because its entire length is projected to be inadequate to accommodate projected wastewater flows from the growing Kapolei and Ewa areas.

 

• East Interceptor System Deficiencies - For the purposes of the alternatives assessment, the East Interceptor System is the primary trunk system of gravity lines, pump stations and force mains conveying flows from areas generally east and north of the Honouliuli WWTP, to the plant. Communities served by the East Interceptor System extend from Halawa to West Loch and north through Central Oahu, up to and including Mililani.

Deficiencies in the East Interceptor System are addressed as a unit because they are interrelated. Alternatives addressing the entire system as well as individual deficiencies in the system are assessed.

• Isolated System Deficiencies - Isolated system deficiencies are those not located along either the West or East Interceptor Systems. Most of these existing capacity and structural deficiencies are being addressed through the City's on-going Sewer Rehabilitation and Infiltration & Inflow Minimization Plan (I/I Plan). Alternatives for addressing projected future deficiencies are assessed in the Facilities Plan.

• Unsewered Areas - In Chapter 2 Existing System Overview, four unsewered areas were identified in the Study. Two of these areas are presently planned for connection to the collection system. Alternatives for addressing the remaining two areas are assessed.

1. Alternatives Assessment

• West Interceptor System Deficiencies

  The West Interceptor system serves the projected high-growth areas of Ko Olina, Makaiwa Hills, Makakilo, Kapolei and Ewa. Toward addressing the associated increase in wastewater that will be generated, the City has approved the Sewer Master Plan for the Regional Interceptor Sewer System for the Developments of the Estates of James Campbell in the Kapolei Area, May 1992. The Sewer Master Plan collectively involves various developers of these areas to relieve the existing 30-inch line with a proposed 48- to 54-inch relief line.

   

  The Sewer Master Plan will address the projected future deficiencies when it is implemented. Therefore, the preliminary recommendation is for the City to ensure that the planned improvements are implemented in a timely manner to meet wastewater collection demands as planned developments proceed.

   

• East Interceptor System Deficiencies

  The East Interceptor System serves virtually all of the remainder of the Study Area not served by the West Interceptor System. Capacity deficiencies in this system are related to the accommodation of existing and projected peak wet-weather flows (PWWF).

   

  Three primary alternatives for addressing these deficiencies are assessed. The first alternative, Alternative A - Upgrade System, is to upgrade each of the deficient components of the East Interceptor System by increasing its capacity to accommodate peak wet weather flows and by implementing repairs, modifications or replacements at structurally deficient and corroded facilities.

   

  The second alternative, Alternative B - Replace System is to replace virtually the entire East Interceptor System with a new interceptor line following a different route to the Honouliuli WWTP. The third alternative, Alternative C - Flow Equalization Within System, is to construct flow equalization facilities along the East Interceptor System to capture and store PWWF return and feed them back into the existing system at a rate that can be accommodated.

   

1. Alternatives Assessment/Preliminary Recommendation

   The three alternatives for the East Interceptor System are assessed in three steps. All three steps use an assessment process adapted from one being used for the City's I/I Plan. The process involves rating alternatives as good (+), fair (0), poor (-), or unacceptable (X) against criteria for operation, reliability, constructability, community nuisance, and construction cost.

   The first step involves a comparative assessment of the alternatives. Based on this assessment, a preferred alternative is identified.

   The second step involves dividing each of the three alternatives into comparable component sections, assessing each of the alternate component section to identify a preferred alternative for the component, then linking them to create a hybrid alternative, assuming that components of more than one alternative would be favored.

   The third step then compares the preferred alternative identified in the first step to the hybrid alternative, if any, developed in the second step. The resulting favored alternative then becomes the preliminary recommendation.

    

2. Preliminary Recommendations

    

   In the first step, based on the non-cost assessment factors and estimated construction cost, as summarized in Table A, Alternatives A and C appear offer significant advantages over Alternative B. Although regarded as the most reliable of the three alternatives in the long term, Alternative B would also entail the greatest construction impacts affecting the most people, and presents the most complex construction challenges. It is also the most expensive to construct with cost estimates more that twice that of Alternative A and more than two and a half times that of Alternative C.

   Comparing Alternative A and C, the construction cost estimate favors Alternative C over Alternative A, which is 20 percent higher. This construction cost advantage of Alternative C is reflected in the non-cost assessment of its constructability. In the long-term, however, the non-cost assessment also suggests that the higher operation and maintenance costs associated with Alternative C would somewhat offset its initial construction cost advantage. Alternative C is also regarded as less reliable for accommodating peak flows exceeding a single-event design storm.

   The non-cost assessment factors pertaining to construction impacts and community acceptance between Alternatives A and C are difficult to compare. Alternative C has less temporary construction-related impacts as the duration of construction would be shorter and fewer people would be affected by traffic disruptions, noise and other inconveniences than for Alternative A. This would be expected to reflect positively in community acceptance of Alternative C, at least in the short-term. On the other hand, in the long-term, Alternative A would likely be perceived by the community as returning to conditions more similar to those before its implementation than Alternative C. Although both Alternatives A and C involve underground construction with few outwardly visible features after completion, Alternative C would be a new type of facility while Alternative A involves modifications and improvements to existing facilities. As a new type of facility, the equalization storage facilities in Alternative C could be perceived as a threat of change in quality of life, including odor generation and health hazard if wastewater were released.

   Based on the alternatives assessment, neither Alternative A nor C emerge as a clear preferred alternative. Hence, the preliminary recommendation is to consider both as leading alternatives for assessment in the environmental impact statement (EIS) process. Following assessment of their impacts through the EIS process, which includes the opportunity for agency and public comment, a more informed final recommendation can be made in the Final Facilities Plan.

   In the second step, Alternative A - Upgrade System and Alternative C - Flow Equalization Within System were divided into four comparable components. Alternative B - Replace System was eliminated from further consideration because it is a totally different system that could not be logically divided into comparable components and because it was identified as the least favorable alternative in the first step.

   The comparative assessment of alternative components was inconclusive in identifying a preferred hybrid alternative to compare with the lead alternatives identified in the first step. Therefore, the preliminary recommendation is to consider the possibility of developing a hybrid alternative based on an assessment of the lead alternatives in the EIS process. Following the EIS process, which includes the opportunity for agency and public comment, a more informed final recommendation, which could involve a hybrid alternative, can be made in the Final Facilities Plan.

   The third step, involving a comparative assessment of the preferred alternative from the first step to a hybrid alternative in the second step will be conducted after the EIS process. The results of the assessment will be presented in the Final Facilities Plan.

3. Isolated System Deficiencies

    

   Most of the isolated deficiencies in the collection system are being addressed through the on-going I/I Plan. Upgrades and improvements recommended by the I/I Plan will be based on accommodating projected future flows to the year 2020 as developed through this Facilities Plan.

4. Unsewered Areas

    

   The two unsewered areas assessed are in the Honouliuli and Ewa Beach areas. The alternatives considered for each are intended to eliminate reliance on cesspools pursuant to the City's 208 Water Quality Management Plan. Alternatives considered include: Deferral of any action in anticipation of future development in the vicinity of an unsewered area extending the collection system to a point where a connection can be made; Conversion to individual septic tank/leach field system; Conversion to individual low pressure system; Installation of a gravity collection system and WWPS; and Installation of a gravity collection system and package WWTP.

   Based on the alternatives assessment, the preliminary recommendation for the

   Honouliuli Unsewered Areas is deferral of any action, since future development is anticipated to extend the sewer system nearer to the area. Future connections to that system will provide a cost effective solution.

   For the Ewa Beach Unsewered Area, the preliminary recommendation is to construct a new gravity collection system since the existing sewer collection system is relatively close and can accommodate additional wastewater flows.

5. Summary of Preliminary Recommendations

Table B summarizes the preliminary recommendations and their respective estimated costs to address collection system deficiencies identified in Chapter 3, Needs Assessment. These preliminary recommendations will be assessed in the EIS process. The Final Facilities Plan will present the final recommendation as may be modified following the EIS. The final recommendations will include a phasing schedule for implementation and an associated budget.

1. ALTERNATIVES ASSESSMENT - WASTEWATER TREATMENT AND DISPOSAL

1. Liquid Treatment

To address deficiencies in plant capacity, the alternatives of plant expansion, flow equalization in the collection system, and flow equalization at the plant were considered. Of these three, plant expansion is the preliminary recommendation as it is uncertain to what extent flow equalization within the collection system will be pursued and how that could address projected PWWF at the plant. Flow equlization at the plant may be too limited to reliably reduce the peak flows and could cost more than increasing the plant capacity. Other concerns such as odors and clean up makes this option less attractive.

Table B - Summary of Preliminary Recommendations West Mamala Bay Facilities Plan (Page 1 of 2)

Table B - Summary of Preliminary Recommendations West Mamala Bay Facilities Plan (Page 2 of 2)

For plant expansion, the capacity of the influent pump station, associated force mains, grit/preaeration inlet channel, primary clarifiers, and effluent discharge lines would be increased to meet the year 2020 PWWF. Plant expansion requirements may be reduced if flow equalization is provided. Although equalization in the collection system is not recommended to specifically address plant capacity issues, it can address collection system deficiencies.

The appropriate level of liquid treatment was also assessed. These treatment levels included chemically-enhanced primary treatment, expansion of secondary treatment, advanced (R-1) treatment, and disinfection for ocean discharge as well as the "minimal action" alternative, which would maintain the current treatment level. Of these, the preliminary recommendation is minimal action because the 301(h) waiver is expected to be renewed and the existing liquid treatment level will likely be sufficient to meet all anticipated requirements of the forthcoming NPDES permit.

Four liquid treatment alternatives were assessed to address various treatment level and expansion options. These alternatives were developed from those identified in the Interim Plan. Each alternative assumes sufficient plant expansion to meet capacity requirements. Liquid treatment facilities have been developed for expansion stages through an ADWF of 51 mgd. A base treatment level of 51-mgd primary, of which 13-mgd receives secondary and R-1 treatment, was assumed for all alternatives. The four alternatives are as follows:

• No additional treatment - No treatment level improvements, such as additional secondary, CEPT, or disinfection, would be provided. The capacity of the existing treatment units, including the influent pump station, associated forcemains, and primary clarifiers, would be expanded to meet the year 2020 PWWF and permit requirements; 

• 25-mgd secondary/25-mgd R-1 (total) - If effluent reuse demand exceeds 13-mgd, an additional 12-mgd secondary and R-1 treatment capacity would be provided for reuse purposes; 

• 51-mgd secondary/51-mgd effluent disinfection (total) - Secondary treatment capacity would be expanded to meet the year 2020 flows. Disinfection of the secondary effluent for ocean discharge would be provided; and 

• 51-mgd secondary/25-mgd R-1/51-mgd effluent disinfection (total) - Secondary treatment would be expanded to meet the year 2020 flows. If effluent reuse demand exceeds 13-mgd, an additional 12-mgd R-1 treatment capacity would be provided. Disinfection of secondary effluent for ocean discharge is also included.

The "No Additional Treatment" alternative meets all expected permit and legal requirements while minimizing additional costs associated with increasing the treatment level. However, to maintain a flexible plan accounting for any changes in permit conditions or legal requirements, cost estimates and scenarios for alternatives were assessed for all alternatives.

1. Solids Treatment and Disposal

Five alternatives identified during the Interim Plan phase were evaluated to address solids treatment and disposal requirements. They were evaluated by combining solids disposal alternatives with solids treatment alternatives.

• Thermal conditioning/dewatering and incineration; 

• Thermal conditioning/dewatering and reuse; 

• High-solids centrifuge raw sludge dewatering and incineration; 

• High-solids centrifuge raw sludge dewatering and reuse; and 

• Anaerobic digestion/dewatering and reuse.

Anaerobic digestion/dewatering and reuse is the preliminary recommendation because it meets all anticipated permit and legal requirements and has flexibility for various biosolids reuse applications. This combination should be pursued as soon as possible because the existing solids treatment facilities are at the end of their service life. Costs associated with incorporating this new solids treatment process are comparable to expanding the existing thermal conditioning system. Cost estimates were provided for the alternatives with thermal conditioning because it is the existing process and the plant staff is comfortable and experienced with it.

1. Effluent Disposal and Reuse

An effluent pump station is recommended to address the peak flows. Six alternatives identified during the Interim Plan Phase, were assessed to address the low flow issue:

• Flow equalization in collection system; 
  

• Flow equalization of raw influent at Honouliuli WWTP;
  
• Flow equalization of primary effluent at Honouliuli WWTP; 
  
• Periodic flushing of outfall; 
  
• Plugging of diffuser ports; and
  
• Parallel outfall.

All alternatives except the Parallel Outfall require construction of an effluent pump station to accommodate future peak flows. It is necessary to accommodate future peak flows including any bypassed secondary effluent and/or reuse flows to the outfall. To address the existing low flow issue, the temporary plugging and installation of check valves on the diffuser ports is the preliminary recommendation until a minimum flow of 14 mgd to the outfall can be sustained. By the year 2020, all diffuser ports will be open. This alternative requires no new construction at the plant, which is important because of the limited plant space. Constructing a new permanent facilities specifically to address the low flow issue is not recommended because it is a temporary problem. An outfall study to evaluate structural (corrosion) and capacity issues is also recommended, particularly at the location of the hydraulic jump along the land portion. If major repairs are needed on the land portion, a parallel line could be constructed.

For effluent reuse, the U. S. Filter project is assumed to be sufficient to meet the consent decree reuse requirements through the planning horizon. If additional demand is identified in the future, the U. S. Filter facilities will likely be expanded.

1. Odor Control Program

The preliminary recommendation for odor control program includes the following:

• Reduction in the amount of hydrogen sulfide produced in the collection system. Chemical addition should be considered for long force mains; 

• Acquisition of land adjacent to the plant site as an additional buffer zone. Although it may not be feasible, a buffer zone of 1,000 to 2,000 feet would be desirable; and 

• Improvements in foul air containment, collection, and treatment. A preliminary recommendation is to consider odor treatment consisting of a two-stage process involving (1) a wet-scrubber, with pH adjustment to maximize hydrogen sulfide removal, and (2) a bulk-media biofilter, consisting of bark, compost, soil, and/or other materials to maximize organic compound removal.

To confirm the appropriateness of this program, and to develop specific design and operating criteria, an odor analysis of the Honouliuli WWTP is recommended to identify the types and amounts of odorous material being produced.

1. Alternative Wastewater Management Scenarios

• Capital costs and operation and maintenance expenses for the individual wastewater treatment and disposal components selected for further assessment were estimated. These components are combined to develop complete alternative wastewater treatment and disposal scenarios. Five alternative scenarios are developed and evaluated. Each scenario includes an improved on-site laboratory, odor control program, effluent pump station, and the plugging of diffuser ports of the outfall. Effluent and biosolids reuse to meet consent decree requirements are also included for all scenarios.

• Minimum Improvements - This alternative scenario focuses only on expanding capacity to meet year 2020 flows and to satisfy legal requirements. The existing liquid and solids treatment processes would be unchanged;

• Moderate Upgrade - Anaerobic digestion would replace the thermal conditioning process. The liquid treatment processes would be expanded only to meet year 2020 flows; 

• Full Secondary Treatment - Secondary treatment would be expanded to 51 mgd. Anaerobic digestion would replace thermal conditioning. Effluent disinfection for up to 51 mgd would be incorporated; 

• Maximum Beneficial Use - An additional 12-mgd of secondary and R-1 treatment capacity would be provided to expand the total effluent reuse capacity to 25 mgd. Anaerobic digestion would replace thermal conditioning. Effluent disinfection for up to 51 mgd would be incorporated; and 

• Maximum Improvements - This alternative combines the features of the "Full Secondary Treatment" and "Maximum Beneficial Use" scenarios.

Each alternative scenario was assessed based on both cost and noncost criteria. In general, costs increase with the level of treatment and amount of reuse. While in turn, provide additional benefits relative to the noncost criteria. Specific noncost criteria selected for the assessment of alternative scenarios included conformance with constraints, ease of operation, protection of the environment and public health, and recycling level. Each scenario was rated using weighted cost and noncost criteria. Each scenario was rated with respect to the criterion and selected weighting factors.

The "Moderate Upgrade" scenario received the highest score and is the preliminary recommendation. This scenario provides a reasonably attainable level of beneficial use for both reclaimed water and biosolids that would meet requirements of the 309 Consent Decree. In addition, this scenario was estimated to incur the lowest annual costs. It consists of the following:

• Common Components - An improved on-site laboratory and odor control program; 

• Liquid Treatment -51-mgd primary, 13-mgd secondary (existing), 13-mgd advanced reuse (U. S. Filter facility); 

• Solids Treatment and Disposal - Anaerobic digestion/dewatering and biosolids reuse (replaces existing thermal conditioning); and 

• Effluent Disposal/Reuse - 51-mgd ocean discharge, 13-mgd reclamation (U. S. Filter facility, a new effluent pump station and plugging of the outfall diffuser ports.

The process flow diagram of the recommended scenario is presented in Figure A, while the plant layout for this scenario is shown in Figure B. The potential for a buffer zone outside the fenceline is not shown.

Figure A; Recommended Process Flow Diagram for Honouliuli WWTP

Figure B; Recommended Layout for Honouliuli WWTP

Department of Environmental Services