CITY OF MORRO BAY WATER RECLAMATION FACILITY MASTER PLAN TECHNICAL MEMORANDUM 6: BIOSOLIDS TREATMENT EVALUATION

Transcription

1 CITY OF MORRO BAY WATER RECLAMATION FACILITY MASTER PLAN TECHNICAL MEMORANDUM 6: BIOSOLIDS TREATMENT EVALUATION B&V PROJECT NO Black & Veatch Holding Company All rights reserved. PREPARED FOR City of Morro Bay 30 OCTOBER 2015

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3 City of Morro Bay TM 6: Biosolids Treatment Evaluation Reviewed by: Signature Joerg Blischke Printed Name Date Reviewed by: Signature Bradley E. Hemken Printed Name Date Professional Engineer: Signature Jacob S. Holden Printed Name C73192 License No. Date Approved by: Signature Matthew L. Thomas Printed Name C52858 License No. Date BLACK & VEATCH Introduction i

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5 TM 6: Biosolids Treatment Evaluation City of Morro Bay Table of Contents 1.0 Introduction Project Overview Purpose of this TM List of Abbreviations and Acronyms Biosolids Regulations Federal Biosolids Regulations Exceptional Quality Biosolids Metals Limitations Pathogen Reduction Vector Attraction Reduction State and Local Biosolids Regulations Other State Regulations Relevant to Biosolids Treatment Existing WWTP Biosolids Processing Biosolids Treatment Technologies Sludge Thickening Technologies Gravity Thickener Dissolved Air Flotation Gravity Belt Thickener Rotary Drum Thickener (RDT) Screw Thickener Thickening Centrifuge Recommendation Digestion Aerobic Digestion Anaerobic Digestion Aerobic vs. Anaerobic Digestion Evaluation Recommendation Dewatering Technologies Sludge Drying Beds Belt Filter Press Screw Press Rotary Press Centrifuge Recommendation Composting ii OCTOBER 2015

6 City of Morro Bay TM 6: Biosolids Treatment Evaluation Windrows Aerated Piles In Vessel Systems Recommendation Thermal Drying Rotary Drum Dryer Fluidized Bed System Combination Dewatering/Vacuum Drying System Recommendation Biosolids Disposal and Reuse Disposal Reuse On Site Composting Contracted Biosolids Management Land Application Energy Recovery Internal Combustion Engines Fuel Cells Gasification Organic Waste Co Digestion to Increase Biogas Production Recommended Treatment Alternatives Alternative 1: Status Quo with Class B Digested Sludge to Liberty Composting Alternative 2: Un Stabilized Dewatered WAS only to Liberty Composting Alternative 3: Class A EQ In Vessel Compost of Anaerobic Digested Sludge With/Without Energy Production Alternative 4: Dewater/Heat Dried WAS only to Landfill or Other LIST OF REFERENCES BLACK & VEATCH Introduction iii

7 TM 6: Biosolids Treatment Evaluation City of Morro Bay LIST OF FIGURES Figure 2 1. Class A and B Pathogen Reduction Requirements (40 CFR )... 6 Figure 4 1. Typical Gravity Thickener (Tchobanoglous, Burton, & Stensel, 2003) Figure 4 2. Huber Packaged DAFT Unit (directindustry.com, 2013) Figure 4 3. Gravity Belt Thickener (Huber Corporation, 2013) Figure 4 4. Thickened sludge in RDT (Parkson, 2013) Figure 4 5. RDT Unit Figure 4 6. Huber Screw Thickener (Huber Corporation, 2013) Figure 4 7. Thickening Centrifuge Figure 4 8. Aerobic Digester Figure 4 9. Anaerobic Digester at City s WWTP Figure Drying Beds at WWTP (Carollo Engineers, 2007) Figure Belt Filter Press Figure Inclined Screw Press Cutaway (Hydroflux Industrial, 2013) Figure Rotary Press Figure Centrifuge Schematic Figure Windrows at Existing WWTP (Carollo Engineers, 2007) Figure Static Aerated Pile System (O 2 Compost) Figure In Vessel Composting System (DTEnvironmental) Figure Tunnel Like IVC with Spigot Aeration Piping embedded in the floor (left) and Duct Work and Air Heat Exchange in the Piping Chase Behind the Tunnel (right) (Zero Waste Energy) Figure Rotary Drum Drying System (Courtesy of Andritz) Figure Fluidized Bed System (Courtesy of Andritz) Figure J VAP System (Courtesy of Evoqua) Figure 7 1. Alternative Figure 7 2. Alternative Figure 7 3. Alternative Figure 7 4. Alternative iv OCTOBER 2015

8 City of Morro Bay TM 6: Biosolids Treatment Evaluation LIST OF TABLES Table 2 1. Pollutant Concentration Limits... 5 Table 2 2. Summary of Vector Attraction Reduction Options... 7 Table 2 3. WWTP Biosolids Hauling Data... 8 Table 4 1. Liquid Treatment Technologies Table 4 2. Thickening Technologies Table 4 3. Alternatives Evaluation Rating System Table 4 4. Digestion Evaluation Weighting Table 4 5. Qualitative Assessment of Digestion Alternatives Table 4 6. Matrix Evaluation of Digestion Alternatives Table 4 7. Dewatering Technologies Table 5 1. Disposal and Reuse Options Table 5 2. Landfill Total Disposal Cost BLACK & VEATCH Introduction v

9 City of Morro Bay TM 6: Biosolids Treatment Evaluation 1.0 Introduction 1.1 PROJECT OVERVIEW The City of Morro Bay (City) is developing a plan to replace its existing wastewater treatment plant (WWTP) with a new water reclamation facility (WRF). Besides replacing the aging WWTP, the WRF will treat the wastewater to a quality for potential end uses that may include Title 22 restricted or unrestricted irrigation, irrigation of agriculture, indirect potable reuse through groundwater recharge, and potentially direct potable reuse in the future. This will serve the community by providing a sustainable approach to its local water resources and by enhancing the reliability of the City s broader water supply portfolio. Black & Veatch has been retained by the City to prepare a Water Reclamation Facility Master Plan (FMP). The FMP will establish treatment technologies and conveyance alternatives, design criteria, planning considerations, and recommendations for the WRF and its associated facilities. The following Technical Memoranda (TMs) are being developed to guide preparation of the FMP: TM 1: Summary of Existing Documents Reviewed TM 2: Influent Waste Characterization, Flow Projections, and Effluent Discharge Requirements TM 3: Morro Bay WWTP Decommissioning TM 4: Onsite Support Facilities TM 5: Offsite Facilities TM 6: Biosolids Treatment Evaluation TM 7: Liquid Treatment Evaluation TM 8: Future Potable Reuse Evaluation TM 9: Organic Waste Treatment Feasibility Study 1.2 PURPOSE OF THIS TM The purpose of this Technical Memorandum 6 (TM) is to evaluate options for the treatment and reuse or disposal of biosolids. Biosolids are produced in the liquid treatment process when solids and liquids are separated. The biosolids may either be disposed of in a landfill or reused for beneficial purposes. Disposal and reuse of biosolids is regulated at the federal, state, and local level, where one or more treatment processes are required to further reduce the liquid content of the solids and/or to stabilize the solids. Solids treatment processes discussed in this TM include: Thickening Digestion (with or without energy recovery) 1 Dewatering 1 Energy recovery will be further evaluated in the FMP. BLACK & VEATCH Introduction 1

10 TM 6: Biosolids Treatment Evaluation City of Morro Bay Composting Thermal Drying For each treatment process, different alternative technologies are discussed. Solids reuse and disposal alternatives include: Hauling to offsite composting facilities Landfilling On Site Composting Land Application One or more of the treatment processes will be required to meet regulations associated with each of the different reuse and disposal alternatives. TM 7 provides a similar evaluation of Liquid Treatment Process Alternatives. TM 9 evaluates treatment of organic waste at the WRF such as fats, oil, and grease (FOG); septage; and other green waste. The selected option or options for biosolids treatment and reuse, liquid treatment, and organic waste treatment will be developed in more detail as integrated treatment processes in the FMP. The FMP will include preliminary sizing of facilities and preliminary engineer s opinion of probable construction costs. A number of reference documents were used in developing this TM. These documents are listed at the end of this TM, and specific citations are noted within the TM. Among these, the Draft Technical Memorandum Analysis of Biosolids Treatment Alternatives, by Michael K. Nunley & Associates (MKN&A), dated October 2013 provided excellent descriptions of biosolids regulations and biosolids treatment and disposal alternatives. Some of the text in this TM is taken from that report, with permission from the author and the City. 1.3 LIST OF ABBREVIATIONS AND ACRONYMS The following abbreviations or acronyms are used in this and other TMs. ADC BFP CFR City CIWMB cy DAFT dtpd EPA EQ floc FMP FOG GBT GHG Alternative daily cover Belt filter press Code of Federal Regulations City of Morro Bay California Integrated Waste Management Board Cubic yards Dissolved air flotation thickening Dry tons per day Environmental Protection Agency Exceptional quality Flocculation Facility Master Plan Fats, oil, and grease gravity belt thickener Greenhouse gas 2 OCTOBER 2015

11 City of Morro Bay TM 6: Biosolids Treatment Evaluation hrs IVC kg mg mgd mi PC pph RDT RWQCB SWRCB THM Title 22 TM TPH UV VOC WAS WDR WDR WRF WWTP Hours In vessel composting Kilograms Milligrams Million gallons per day Miles Primary Clarifier, Pollutant Concentration Pounds per hour Rotary drum thickeners Regional Water Quality Control Board State Water Resources Control Board Trihalomethanes Title 22 of the California Code of Regulations Technical Memorandum Total petroleum hydrocarbons Ultraviolet light radiation Volatile organic compounds waste activated sludge Waste Discharge Requirements Waste Discharge Requirements Water Recycling Facility Wastewater treatment plant BLACK & VEATCH Introduction 3

12 TM 6: Biosolids Treatment Evaluation City of Morro Bay 2.0 Biosolids Regulations In order to understand the selection of biosolids treatment technologies, it is important to understand that technologies are chosen to meet specific regulations, classifications, reuse and disposal goals, as well as aesthetic needs. The primary focus of current biosolids regulations is land application. Regulations that govern the classification and land application of biosolids include the treatment plant owner s Waste Discharge Requirements (WDR) issued by the State Water Resources Control Board (SWRCB), the U.S. EPA Sewage Sludge Regulations (Federal Register 40 CFR Part 503), the SWRCB Water Quality Order No DWQ (General Order), and any Biosolids Ordinance from the county where the biosolids are land applied. Any offsite facility that would receive biosolids must be permitted by the Regional Water Quality Control Board (RWQCB) through either the General Order or a site specific permit. This section will briefly summarize relevant biosolids treatment requirements for the WRF and the regulatory requirements for various biosolids treatment alternatives. 2.1 FEDERAL BIOSOLIDS REGULATIONS The Environmental Protection Agency (EPA) requires the processing of biosolids prior to land application or surface disposal such that adverse effects to public health or the environment are mitigated. Biosolids are classified into three categories: Class A; Class B; and sub Class B. Class A biosolids are more highly processed sludges that meet higher pathogen and vector reduction goals. These biosolids are typically land applied or surface disposed. Class B biosolids must meet one of three pathogen reduction alternatives to demonstrate significant reduction of pathogens. Biosolids that do not meet Class B requirements (sub class B) must be further processed prior to land application and/or surface disposal. Following is a brief summary of the Federal Standards that biosolids must meet in order to comply with the 40 CFR 503 Regulations Exceptional Quality Biosolids The designation of exceptional quality (EQ) refers to biosolids that meet both the Class A requirements and the maximum pollutant levels of part 503 relating to concentrations of various metals Metals Limitations The maximum allowable concentration of regulated metals for any biosolids to be land applied are found in 40 CFR , Table 1. Biosolids with pollutants greater than the limits defined in Table 1 cannot be applied to land. Biosolids with pollutants below the Table 1 ceiling limits, but above the Table 3 limits, can be applied to land but are subject to annual and cumulative pollutant loading limits. Biosolids below the Table 3 limits can be applied to land without regard to the annual or cumulative loading limits. The Table 1 and Table 3 metals limits are listed in Table 2 1. Currently, metal pollutant concentrations for biosolids produced at the WWTP are below the Table 3 limits. 4 OCTOBER 2015

13 City of Morro Bay TM 6: Biosolids Treatment Evaluation Table 2 1. Pollutant Concentration Limits POLLUTANT CEILING CONCENTRATION LIMITS FOR ALL BIOSOLIDS APPLIED TO LAND (MG/KG) 1 POLLUTANT CONCENTRATION LIMITS FOR EQ AND PC BIOSOLIDS (MG/KG) 1 Arsenic Cadmium Chromium 3,000 1,200 Copper 4,300 1,500 Lead Mercury Molybdenum 2 75 Nickel Selenium Zinc 7,500 2,800 Applies to: All biosolids that are land applied From Part 503 Table 1, Section Bulk biosolids and bagged biosolids 3 Table 3 Section Dry weight basis 2 As a result of the February 25, 1994, Amendment to the rule, the limits for molybdenum were deleted from the Part 503 rule pending EPA reconsideration. 3 Bagged biosolids are sold or given away in a bag or other container Pathogen Reduction 40 CFR defines performance based pathogen reduction standards and classifies biosolids as either Class A or Class B. The goal of Class A biosolids is to reduce pathogens to below detectable limits. Unlike Class A biosolids, in which pathogen levels are below detectable limits, Class B biosolids may contain some pathogens. Because of this, the Part 503 Rule contains restrictions associated with the application of Class B biosolids onto agricultural land designated for harvesting of crops and turf, for the grazing of animals, and for public contact until environmental conditions have further reduced the pathogens. The Class A and Class B pathogen reduction requirements are provided on Figure 2 1. BLACK & VEATCH Biosolids Regulations 5

14 TM 6: Biosolids Treatment Evaluation City of Morro Bay Figure 2 1. Class A and B Pathogen Reduction Requirements (40 CFR ) Vector Attraction Reduction The pathogens in biosolids present a health risk when they are brought into contact with humans. Vectors, which include flies, rodents, and birds can transmit these pathogens to humans through contact or by biologically supporting the pathogen. Reducing the attractiveness of biosolids to vectors reduces the potential of disease transmission. 40 CFR 503 specifies ten alternatives for meeting the vector attraction reduction requirements. One alternative must be met in order for biosolids to be land applied. These alternatives are provided in Table 2 2. The vector attraction reduction requirement for the existing WWTP biosolids is currently achieved by satisfying Option 1. The mass of volatile solids in the sludge is reduced by a minimum of 38 percent during the sludge treatment process using anaerobic digestion (Carollo Engineers, 2007). 6 OCTOBER 2015

15 City of Morro Bay TM 6: Biosolids Treatment Evaluation Table 2 2. Summary of Vector Attraction Reduction Options Requirements in one of the following options must be met: Option 1: Reduce the mass of the volatile solids by a minimum of 38 percent Option 2: Option 3: Option 4: Option 5: Option 6: Option 7: Option 8: Option 9: Demonstrate vector attraction reduction with additional anaerobic digestion in a bench scale unit Demonstrate vector attraction reduction with additional aerobic digestion in a bench scale unit Meet a specific oxygen uptake rate for aerobically treated biosolids Use aerobic processes at greater than 40 o C (average temperatures 45 o C) for 14 days or longer (e.g., during biosolids composting) Add alkaline materials to raise the ph under specified conditions Reduce moisture content of biosolids that do not contain unstabilized solids from other than primary treatment to at least 75 percent solids. Reduce moisture content of biosolids with unstabilized solids to at least 90 percent Inject biosolids beneath the soil surface within a specified time, depending on the level of pathogen treatment Option 10: Incorporate biosolids applied to or placed on the land surface within specified time periods after application to or placement on the land surface. 2.2 STATE AND LOCAL BIOSOLIDS REGULATIONS In 2004, the State Water Resources Control Board (SWRCB) adopted general Waste Discharge Requirements (WDR) for the discharge of biosolids as a soil amendment. These requirements exceed the requirements of the Part 503 Rule. The WDR is contained in Water Quality Order No DWQ (General Order) and was intended to streamline the regulatory process for land application sites statewide. In addition, application of biosolids is governed by the ordinances in the county where the biosolids are land applied. In San Luis Obispo County, only Class A EQ biosolids can be used on agricultural land within the limits established by the San Luis Obispo County Biosolids Ordinance. This ordinance limits the annual amount of land application of treated biosolids to 1,500 cubic yards (cy) until March 2018, and is an extension of the original 48 month moratorium (Ordinance No. 3080). This extension is intended to allow time for additional studies to evaluate impacts to food BLACK & VEATCH Biosolids Regulations 7

16 TM 6: Biosolids Treatment Evaluation City of Morro Bay crops, issues related to contaminants of emerging concern, and address other concerns with widespread use of biosolids. Composted biosolids are currently exempt from the County s application limits, but are still required to be permitted and monitored if they exceed 5 cy in volume. In July 2004, the City was permitted by the California Integrated Waste Management Board (CIWMB) to conduct a pilot biosolids composting project at the WWTP with the goal of developing a cost effective technique for producing Exceptional Quality (EQ) biosolids that meet both metal and Class A standards. This successful pilot program produced compost that served as a high quality soil amendment for public use up until around 2011, when the City suspended biosolids composting operations at the WWTP. The City is not currently composting onsite. The City currently contracts with Liberty Composting, who hauls 100 percent of the City s biosolids to their facility in Kern County. See Section for more information on Liberty s operations. Table 2 3 provides recent biosolids hauling data. Table 2 3. WWTP Biosolids Hauling Data YEAR BIOSOLIDS VOLUME HAULED (CY) Notes: 1. The increase in biosolids volume in 2013 and 2014 was due to digester cleaning and lack of rain allowing more dry time for solids at the plant Other State Regulations Relevant to Biosolids Treatment There are three California assembly bills that may impact the WRF and biosolids treatment in the near future: AB 1826, AB 1594, and AB 341. A brief description of each bill is provided below. AB 1826 regulates commercial food waste collection and requires the state s commercial sector, including restaurants, supermarkets, large venues and food processors, to separate their food scraps and yard trimmings and arrange for organics recycling service in a phased approach to be fully implemented by The bill does not include biosolids, but the commercial organics will need to be sent somewhere other than a landfill. This could increase the volume of organic material available for composting or co digestion at the WRF and may be a possible source of revenue if a nominal disposal fee is charged. AB 1594 overturns a previous law that allowed yard trimmings to count as diverted material when they were used as alternative daily cover (ADC) at a landfill. Under AB 939, local waste jurisdictions are mandated to divert 50 percent of their waste material from landfills. Prior to AB 1594, yard trimmings used as ADC counted toward this 50 percent requirement. Now, the yard trimmings will count as disposed material, which could mean that local waste companies will be looking for places to dispose of their yard trimmings other than landfills. These yard trimmings could be used as a bulking agent and carbon source to aid with composting biosolids at the WRF. 8 OCTOBER 2015

17 City of Morro Bay TM 6: Biosolids Treatment Evaluation AB 341 is one of California s most recent recycling bills, which requires that not less than 75 percent of solid waste generated be source reduced, recycled, or composted by the year As organics represents one of the largest portion of the waste being currently landfilled, the goal is to divert organic waste (food, green waste, lumber, and other organics) from landfilling. There have been discussions to include biosolids in the 75 percent solid waste diversion goal. While biosolids were mentioned in the AB update report, no specific mentioning of biosolids in the recent staff report to the legislature. At some point, biosolids may be included in the term other organics, for landfill diversion. Based on this bill and others that are designed to divert recyclable waste from landfills, biosolids may eventually become targeted/mandated for landfill diversion. 3.0 Existing WWTP Biosolids Processing The existing WWTP is currently producing Class B biosolids through mesophilic anaerobic digestion by meeting time and temperature requirements within the digester. According to staff, biosolids produced at the WWTP typically meet Class A requirements via Alternative 4, as described in Section and on Figure 2 1, based on laboratory testing for fecal coliform, enteric virus, helminthology ova, and salmonella. Settled sludge from the primary clarifiers is pumped to one of two primary anaerobic digesters (Digester Nos. 2 and 3) on the plant site. Waste activated sludge (WAS) from the secondary clarifiers is sent back to the headworks so it can be thickened in the primary clarifiers. The primary digesters are heated to a temperature between 96 and 98 degrees Fahrenheit and mixed with digester gas. The primary digesters, which are also heated by digester gas, are operated in series with a secondary digester (Digester No. 1). Sludge is allowed to settle in the secondary digester and the supernatant is sent back to the headworks of the WWTP. Stabilized sludge is drawn from Digester No. 1 and sent to one of twelve sludge drying beds where it is allowed to solar dry to a solids concentration between 75 to 90 percent. Dried solids are removed from the sludge drying beds and stored in a concrete containment area. Since 1998, the City has been disposing of the bulk of its biosolids by having a permitted land applier, Liberty Composting, haul biosolids to their facility in the Central Valley for further treatment and land application. More details regarding Liberty Composting are provided in Section In the past, a small amount of biosolids were composted onsite to achieve Class A EQ standards using the open windrow composting method. During composting, the temperature of the biosolids was held at 55 degrees Celsius or higher, for 15 days or longer. During this time, the windrows were turned a minimum of five times. The City is not currently composting biosolids onsite at the WWTP. (Carollo Engineers, 2007) BLACK & VEATCH Existing WWTP Biosolids Processing 9

18 TM 6: Biosolids Treatment Evaluation City of Morro Bay 4.0 Biosolids Treatment Technologies This section presents an overview of biosolids treatment technologies, discusses advantages and disadvantages of the technologies, and qualitatively compares the use of aerobic digestion without primary clarifiers versus anaerobic digestion with primary clarifiers. A summary of the biosolids treatment technologies considered is provided in Table 4 1. Table 4 1. Liquid Treatment Technologies TREATMENT CATEGORY Sludge Thickening Digestion Dewatering Composting Thermal Drying TREATMENT PROCESSES/TECHNOLOGIES Gravity Thickener Dissolved Air Flotation Gravity Belt Thickener Rotary Drum Thickener Screw Thickener Thickening Centrifuge Aerobic Digestion without Primary Clarifiers Anaerobic Digestion with Primary Clarifiers Sludge Drying Beds Belt Filter Press Screw Press Rotary Press Centrifuge Windrows Aerated Piles In Vessel Rotary Drum Dryer Fluidized Bed System Combination Dewatering/ Vacuum Drying System The recommendations made in this section are intended to provide focus for the development of a new WRF while at the same time allowing for creativity and innovation during the project implementation phase. The recommendations made in this section will also provide the basis to develop conceptual space planning and construction cost budgeting for the project. Proven technologies that have been used successfully in multiple, similar applications were considered in this evaluation. Capacity related design criteria are not included herein, but will be provided in the FMP. 4.1 SLUDGE THICKENING TECHNOLOGIES Thickening is a process used to increase the percent solids content by removing a portion of the liquid, which can reduce pumping and storage requirements. The solids content of sludge (including primary, activated, trickling filter, and mixed) from a municipal WWTP can differ greatly depending on the production process and the method of operation. Typical solids concentration values of unthickened sludge can be expected to be in the range of 1 6 percent total solids. Thickening is generally accomplished by physical means such as co settling, gravity settling, flotation, centrifugation, gravity belt, or rotary drum (Tchobanoglous, Burton, & Stensel, 2003). Thickened sludge behaves as a liquid and can be pumped. In general, thickening concentrates 10 OCTOBER 2015

19 City of Morro Bay TM 6: Biosolids Treatment Evaluation sludge to approximately 3 6 percent solids. Several thickening technologies are described in further detail below Gravity Thickener Gravity thickeners function much like settling tanks or clarifiers. Solids settle to the floor of the structure, while liquid flows upward, through weirs, and out to the next process. Settled solids are collected at the bottom of the tank using chainin flight or sludge scraper mechanisms, which move settled solids to a collection sump. The thickened sludge is then pumped to the digesters. Gravity thickening consumes very little energy, but is effective only for primary or stabilized sludge. A photo of a typical gravity thickener is shown on Figure Dissolved Air Flotation The objective of dissolved air flotation thickening (DAFT) is to attach a small air bubble to suspended solids and cause the solids to separate from the water in an upward direction. In the DAFT process, large quantities of air are introduced into a solution of sludge that is held at a higher pressure. Since air solubility increases with pressure, large quantities of air are able to be dissolved into the solution. The solution is then released into a chamber at near atmospheric pressure, which results in a rapid reduction in pressure of the solution. Accompanying the rapid reduction in pressure is a rapid reduction in solubility of the solution. Excess dissolved air then comes out of the solution, forming tiny bubbles that attach to suspended solids. These Figure 4 2. Huber Packaged DAFT Unit (directindustry.com, 2013) solids tend to clump together forming flocculation (floc) particles. These floc particles are carried to the top of the vessel where they concentrate as a blanket and are removed as thickened sludge. The energy requirement for the DAFT process is typically high compared to the other thickening technologies described in this section. A photo of a packaged DAFT system is shown on Figure Gravity Belt Thickener Figure 4 1. Typical Gravity Thickener (Tchobanoglous, Burton, & Stensel, 2003) The gravity belt thickener (GBT) is commonly used to thicken waste activated sludge (WAS) and aerobically and anaerobically digested sludge. Sludges are typically introduced at concentrations BLACK & VEATCH Biosolids Treatment Technologies 11

20 TM 6: Biosolids Treatment Evaluation City of Morro Bay less than 2 percent, and systems are designed to produce a maximum of 5 7 percent thickened solids. Thickened sludge must maintain sufficient liquid content to be pumped and mixed easily. A GBT requires less power than a DAFT unit. A gravity belt thickener consists of a porous belt, commonly configured in multiples of three foot wide belts (e.g., 3, 6, or 9 foot wide belts). Sludge is introduced into a feed box located at the head of the moving belt. Polymer is mixed with the sludge in the distribution box to improve the flocculation of the solids and increase thickening performance. The conditioned sludge is distributed onto the continuous belt, which moves over rollers driven by a speed controlled drive. The sludge is evenly distributed onto the belt with a series of plow blades that creates ridges Figure 4 3. Gravity Belt Thickener (Huber Corporation, 2013) and furrows, allowing the water in the sludge to pass through the porous belt. Drainage collects beneath the GBT unit in an area drain, and the thickened sludge remaining on the belt then collected at the discharge end of the thickener. GBT is a commonly used technology but it is difficult to contain odors and keep the system clean since they are typically open systems with no containment. A photo of a GBT system is shown on Figure Rotary Drum Thickener (RDT) Rotary drum thickeners (also called rotary screen thickeners) consist of a flocculation tank, polymer feed system, internal screw, a drum screen, and a motorized drive as shown on Figure 4 4 and Figure 4 5. The units are fed internally and allow free water to drain through a moving, porous media while retaining flocculated solids. Separated water passes through the screen, while thickened sludge rolls out the end of the drum. Drainage exits the screw through an outlet at the bottom of the unit. An external water source is typically required to spray wash the screens at regular intervals to prevent clogging, often several minutes per hour. RDTs can have fewer associated odor issues (compared to GBT) since the unit is typically enclosed and can be connected as a point source in an odor control system. 12 OCTOBER 2015

21 City of Morro Bay TM 6: Biosolids Treatment Evaluation Figure 4 4. Thickened sludge in RDT (Parkson, 2013) Figure 4 5. RDT Unit Screw Thickener A screw thickening system consists of a flocculation tank, polymer feed system, rotating internal screw, a motorized drive, and a circular screen as shown on Figure 4 6. The units are fed internally and they allow free water to drain through the screen as the screw pushes the sludge to the end of the unit. An external water source is typically required to spray wash the screens and prevent clogging at regular intervals, often several minutes per hour. Similar to the RDT, these systems use considerably less power than a DAFT system. They also typically have a smaller footprint than RDTs and have less odor producing potential and less drainage issues than GBTs. Since the unit is typically enclosed, it can be connected as a point source in an odor control system. Figure 4 6. Huber Screw Thickener (Huber Corporation, 2013) BLACK & VEATCH Biosolids Treatment Technologies 13

22 TM 6: Biosolids Treatment Evaluation City of Morro Bay Thickening Centrifuge Centrifuge thickening is commonly used for WAS thickening. It is a self contained process that uses high speed centrifugal forces to separate suspended solids from the liquid. The solids are forced to the perimeter of the bowl, conveyed by a scroll to one end of the unit and discharged. The liquid flows through ports at the opposite end of the unit and is typically returned to the head works. An installed unit is shown on Figure 4 7. Figure 4 7. Thickening Centrifuge Centrifuges typically achieve solids concentrations ranging from 4 to 8 percent at solids capture efficiencies of 90 to 95 percent. They have higher power consumption than the other thickening technologies. Routine maintenance of centrifuges can be performed by the plant staff, but periodically the scroll/bowl assembly may have to be returned to the factory for maintenance. This can result in extended downtime for the equipment Recommendation Six different thickening technologies were considered for the WRF. For facilities similar in scale to the WRF, mechanical thickening processes are preferred over gravity thickening or DAFT, which occupy more space and are less efficient. The remaining four alternatives listed in Table 4 2 are suitable to be carried forward for detailed selection during the implementation phase of the project. Table 4 2. Thickening Technologies THICKENING TECHNOLOGIES Gravity Belt Thickener Rotary Drum Thickener Thickening Centrifuge Screw Thickener 4.2 DIGESTION The vast majority of treatment plants stabilize solids and biosolids to reduce odors, reduce pathogens, and reduce the potential for putrefaction. Additionally, stabilization can be effective at volume reduction, production of valuable gas, and improving the dewaterability of sludge. Depending on the type of digestion used, percent of the volatile suspended solids can be reduced through digestion, which significantly reduces the amount of biosolids requiring further processing or disposal (Metcalf & Eddy, Inc., 1979). Aerobic and mesophilic anaerobic digestion are the most common methods for stabilization and produce Class B biosolids. It is possible to produce Class A biosolids using thermophilic anaerobic digestion with adequate demonstration of retention 14 OCTOBER 2015

23 City of Morro Bay TM 6: Biosolids Treatment Evaluation time. Alkaline stabilization is an alternative process that utilizes a significant amount of alkaline chemical, which increases the biosolids volume compared to digestion. Handling, disposal, and reuse of the high ph biosolids can also be problematic. For these reasons, alkaline stabilization was not evaluated further in this TM Aerobic Digestion Aerobic digestion is a process through which volatile solids reduction and stabilization occurs in an oxygen rich environment. As the supply of available food is depleted, microorganisms begin to consume their own protoplasm, thereby reducing the volume of the processed biosolids. As shown on Figure 4 8, the aerobic digestion process takes place in an open top vessel, and is used primarily in smaller sized plants (<5 MGD) due to relatively easy control of the process, relatively lower capital equipment costs (the process occurs in an open topped vessel), Figure 4 8. Aerobic Digester and the lack of need to handle and process flammable gas. The final product can be an odorless, biologically stable end product. However, the aerobic process has a high recurring energy cost due to the need to supply oxygen. The volatile solids reduction is typically between percent (Metcalf & Eddy, Inc., 1979), which is lower than anaerobic digestion. The resultant biosolids are not readily dewatered, further impacting recurring costs Anaerobic Digestion Anaerobic digestion involves the decomposition of organic and inorganic matter in the absence of oxygen. Anaerobic digestion is considered to be the dominant approach for stabilizing solids as a result with emphasis on energy conservation and recovery, and the desire to obtain beneficial reuse of biosolids (Tchobanoglous, Burton, & Stensel, 2003). Additionally, anaerobic digestion can produce methane rich digester gas that may be able to offset some energy needs of the plant. Figure 4 9. Anaerobic Digester at City s WWTP Anaerobic digestion is most commonly utilized at larger WWTP facilities and facilities with primary clarifiers since primary sludge can enhance anaerobic digester performance. The volatile solids reduction is typically between percent (Metcalf & Eddy, Inc., 1979). The City s existing anaerobic digester is shown on Figure 4 9. Unlike aerobic digestion, anaerobic digestion entails recycling of high ammonia loads back to the liquid stream, which can impact the design of the biological treatment system. In addition, the primary clarifiers used with anaerobic digestion reduce the carbon concentration upstream of the BLACK & VEATCH Biosolids Treatment Technologies 15

24 TM 6: Biosolids Treatment Evaluation City of Morro Bay biological treatment system. Because a carbon source is needed for denitrification, a supplemental source of carbon may need to be added Aerobic vs. Anaerobic Digestion Evaluation This section presents the methodology and results for qualitative evaluation of selecting aerobic digestion versus anaerobic digestion. Criteria were established to evaluate these two alternatives. A weight (% out of 100) was then assigned to each criterion. Criteria with greater impact to the project were given a higher weight. Alternatives were given scores for each criterion using a scale of 1 to 5 as shown in Table 4 3. The criteria, weighting, and scoring used for the evaluation were developed by and with City staff during a workshop on September 16, Criteria descriptions and weighting are provided in Table 4 4. Table 4 3. Alternatives Evaluation Rating System SCORE DEFINITION 5 Satisfies project objectives with significant noted advantages 4 Satisfies project objectives with noted advantages 3 Satisfies project objectives 2 Satisfies project objectives with noted disadvantages 1 Satisfies project objectives with significant noted disadvantages Treatment of external organic waste at the WRF such as FOG, septage, and other organics can impact the costs and benefits of anaerobic digestion. For the purpose of this TM and the evaluation of aerobic vs. anaerobic digestion, these types of external sources were not considered and will be further evaluated as discussed in Section 1.2. Table 4 4. Digestion Evaluation Weighting CRITERIA DESCRIPTION WEIGHT % Digestion Facilities Primary Clarifiers (PC) Secondary Load Reduction via PC s (smaller tankage) Digestion Secondary Load Reduction via PC s (smaller tankage) Capital Costs This criterion compares the relative capital costs of the digestion facilities. The alternative with the lower capital cost receives a higher score. This criterion compares the relative capital costs of the primary clarifiers. The alternative with the lower capital cost receives a higher score. This criterion compares the relative capital costs savings for secondary treatment facilities when primary clarifiers are used. The alternative with the lower secondary treatment capital cost receives a higher score. Operational Costs This criterion compares the relative energy costs for operation of the digesters. The alternative with the lower energy cost receives a higher score. This criterion compares the relative energy cost savings for operating of secondary treatment facilities when primary clarifiers are used. The alternative with the lower energy cost receives a higher score OCTOBER 2015

25 City of Morro Bay TM 6: Biosolids Treatment Evaluation CRITERIA DESCRIPTION WEIGHT % Solids Volume Reduction (reduced hauling cost) Odor Potential Energy Recovery Potential This criterion compares the relative cost of hauling digested sludge based on the total volume produced. The alternative that has a higher solids volume reduction receives a higher score. This criterion compares the relative operational costs for odor treatment. The alternative that produces less odor impact receives a higher score. This criterion compares the relative benefits of recovering energy from the digestion process. The alternative that has the higher potential for energy recovery receives a higher score. Table 4 5 presents a qualitative assessment of the evaluation criteria for each biological treatment alternative along with a rationale for the assessment. The qualitative assessment is used to develop the scoring used in Table 4 6. Table 4 5. Qualitative Assessment of Digestion Alternatives CRITERIA AEROBIC DIGESTION WITHOUT PC S ANAEROBIC DIGESTION WITH PC S Digestion Facilities 5 3 Primary Clarifiers (PCs) 3 1 Secondary Load Reduction via PC s (smaller tankage) 1 3 Digestion 3 4 Secondary Load Reduction via PC s (smaller tankage) Solids Volume Reduction (reduced hauling cost) Odor Potential 4 3 Energy Recovery Potential 1 3 Capital Costs Operational Costs COMMENTS Aerobic digesters do not require the associated gas handling and digester heating equipment and have a lower capital cost than anaerobic digesters. Aerobic digesters do not require primary clarifiers. Anaerobic digesters require primary clarifiers, which increases the capital cost. PC s used with anaerobic digesters remove organic load and result in smaller secondary treatment aeration basins as compared to aerobic digesters. Aerobic digesters require aeration which results in higher energy costs than anaerobic digesters, which only require mixing and not aeration. PCs used with anaerobic digesters remove more organic load than aerobic digesters without PCs, which reduces secondary treatment aeration cost. Anaerobic digestion with PCs has a higher reduction of solids volume than aerobic digestion without PCs. Anaerobic digesters produce more offensive odors compared to the musty odor associated with aerobic digestion. Anaerobic digester gas can be captured and used for energy generation unlike aerobic digesters BLACK & VEATCH Biosolids Treatment Technologies 17

26 TM 6: Biosolids Treatment Evaluation City of Morro Bay Table 4 6 presents the numerical scoring for the digestions alternatives. Table 4 6. Matrix Evaluation of Digestion Alternatives WEIGHT CRITERIA AEROBIC WITHOUT PRIMARY CLARIFIERS Raw Score Capital Costs Weighted Score ANAEROBIC WITH PRIMARY CLARIFIERS Raw Score Weighted Score 15% Digestion Facilities % Primary Clarifiers % Secondary Load Reduction via PC s Operational Costs 15% Digestion % Secondary Load Reduction via PC s % Solids Volume Reduction % Odor Potential % Energy Recovery Potential Total Recommendation Evaluation of the digestion alternatives resulted in anaerobic digestion with primary clarifiers receiving the highest score. 4.3 DEWATERING TECHNOLOGIES Dewatering is a method of solids concentration and volume reduction. Generally, dewatering concentrates sludge to higher than 15 percent solids concentration, which allows the solids to be trucked in most cases. Dewatering stabilized sludge prior to disposal or reuse results in volume reduction, which reduces hauling, handling, and disposal costs. Dewatering also better prepares biosolids for composting or thermal treatment at elevated temperatures and/or pressures, further reduces odors, and allows sludge to be handled more easily (as a solid). Dewatering processes can include passive processes, such as sludge drying beds (or solar drying), and mechanically assisted processes, such as presses or centrifuges. 18 OCTOBER 2015

27 City of Morro Bay TM 6: Biosolids Treatment Evaluation Sludge Drying Beds Sludge drying beds are the most widely used method of sludge dewatering in the United States (Tchobanoglous, Burton, & Stensel, 2003). This process utilizes solar energy and the evaporation of moisture in the solids into the environment. This process, although slow, represents the lowest capital cost method when land is readily available. The use of solar drying is often not an option for facilities in close proximity to residential and/or commercial development because of odor concerns. In San Luis Obispo County, sludge drying beds are generally paved to prevent percolation into the ground and meet the requirements Figure Drying Beds at WWTP (Carollo Engineers, 2007) of the RWQCB. As a result, seasonal climate changes can greatly impact the effectiveness of the drying beds. Many agencies utilize drying beds during the dry months, and switch to mechanical dewatering during the winter months. The paved drying beds at the WWTP produce a solids content of approximately 80 percent using a sludge bed design loading criteria of 16 pounds of dry solids per square foot per year (Carollo Engineers, 2007). Figure 4 10 shows a photo of the drying beds at the existing WWTP Belt Filter Press A belt filter press (BFP) is a continuous feed dewatering device utilizing a permeable belt and mechanically applied pressure to achieve dewatering. An example is shown on Figure Similar to a GBT, BFPs are available in belt sizes ranging from 1.5 to 11.5 feet in width. Polymer conditioned sludge is introduced onto the BFP in a gravity drainage section where it is thickened by gravity. Next, pressure is applied to the thickened sludge by pressing the sludge between two porous belts. Water is squeezed out of the sludge and the final dewatered sludge cake is removed from the belts by scraper blades. Total solids concentrations for BFPs range between percent. Since the process is an open arrangement, it can be difficult to control odors. Figure Belt Filter Press BLACK & VEATCH Biosolids Treatment Technologies 19

28 TM 6: Biosolids Treatment Evaluation City of Morro Bay Screw Press The screw press is a continuous feed operation utilizing gravity drainage at the inlet end of a helical feed screw that reduces the volume of the material being dewatered as it is conveyed from the inlet to the discharge end of the screw press. There are two primary configurations of screw presses: horizontal and inclined. An example of an inclined configuration is shown on Figure Some screw presses also utilize the addition of lime and heat to dewater solids and reduce pathogens, which produces biosolids that meet Class A standards set forth in 40 CFR 503. A flocculation vessel (or floc tank ) is located upstream of the press. Polymer is combined with solids in the floc tank to enhance dewaterability of the sludge. A portion of the water is removed from the solids by gravity drainage at the inlet to the press. The screw then squeezes free water (filtrate) out of the solids by the screw which progressively reduces the volume available for the solids to occupy. The water is released through screens or perforations that surround the body of the screw. Solids exit at the screw s discharge outlet as dewatered cake. The total solids concentration for screw pressed sludge is similar to that of BFPs, which is around percent. Because the process is enclosed, controlling odors is less difficult than with a BFP. Figure Inclined Screw Press Cutaway (Hydroflux Industrial, 2013) Rotary Press Rotary press dewatering is a relatively new technology for the wastewater industry, but interest is growing due to its low energy requirements. The principle of rotary press operation is shown on Figure Low concentration solids are fed into the dewatering channel and are moved along the channel by a rotating element on the central shaft. As the solids travel the length of the channel, the pressure builds and forces water from the cake. The filtrate passes through metal screens on either side of the channel and is discharged at the bottom of the press. Dewatered cake is discharged at the bottom of the press. A flocculation unit is included upstream of the press to allow the solids to flocculate after polymer addition. The press capacity is based on the number of channels attached to the central shaft. Rotary presses have up to six channels. Rotary press performance is variable, with reported performance ranging from 14 to 25 percent total solids. 20 OCTOBER 2015

29 City of Morro Bay TM 6: Biosolids Treatment Evaluation A SLUDGE FEED A RESTRICTION ZONE CAKE EXTRUSION Figure Rotary Press Centrifuge Centrifugal dewatering is a technology that utilizes centrifugal force to separate the liquid from sludge. Solid bowl type devices are preferred for municipal dewatering applications and offer operational flexibility, good odor containment, a compact design, and produce a relatively dry sludge cake. Disadvantages CLARIFIED EFFLUENT BOWL DEWATERED SOLIDS include a requirement for skilled maintenance, high suspended solids content in centrate, and a requirement for INLET grit removal. Dewatering centrifuges can normally achieve a 20 to 30 percent cake solids concentration with a 90 to 95 percent solids capture. SCROLL CONVEYOR Figure Centrifuge Schematic However, the efficiency of dewatering is very sensitive to sludge characteristics, sludge conditioning, and sludge feed rate. A schematic of a dewatering centrifuge is shown on Figure Recommendation Four different dewatering technologies were considered for the WRF. Sludge drying beds were eliminated due to odor concerns, inconsistent performance caused by seasonal fluctuations, and site space requirements. The remaining four alternatives listed in Table 4 7 are suitable to be carried forward for final selection during the implementation phase of the project. Table 4 7. Dewatering Technologies DEWATERING TECHNOLOGIES Belt Filter Press Screw Press Rotary Press Centrifuge BLACK & VEATCH Biosolids Treatment Technologies 21