Lifting the Fog from FOG Receiving

Transcription

1 Lifting the Fog from FOG Receiving Rashi Gupta 1*, Daniel L. Meacham 1, Phil Parkins 2, Travis A. Peacock 3, Anthony L. Montoya 3 1 Carollo Engineers, Costa Mesa, California 2 Carollo Engineers, Phoenix, Arizona 3 Albuquerque Bernalillo County Water Utility Authority, New Mexico * RGupta@carollo.com ABSTRACT Fats, oils, and grease (FOG) often causes problems within collection systems, but represents a potentially high-energy feedstock for anaerobic digestion that can increase onsite energy production. Numerous facilities have implemented FOG receiving systems to reduce collection system overflows and increase digester gas production. However, as the number of such facilities has grown, so have reports of operational difficulties. Ten facilities were surveyed to determine a number of parameters, including operational practices, system configuration and equipment used, operation and maintenance (O&M) requirements, and benefits realized after implementation of FOG receiving. Digestion of FOG did increase digester gas production at most facilities, allowing several to meet or exceed most of their energy demands. However, FOG handling and system maintenance can be cumbersome and the delivered product can be highly variable in quantity, quality, and characteristics. Harnessing the energy inherent in FOG can significantly offset energy demand, but careful consideration of the practical aspects of FOG receiving is necessary for sustainable and successful operations. KEYWORDS: FOG Receiving, Operation and Maintenance, System Equipment, Materials of Construction, FOG Quality, Digester Gas Production, Lessons Learned INTRODUCTION A number of wastewater utilities have constructed receiving stations for fats, oils, and grease (FOG) where FOG haulers can discharge grease trap waste. Where FOG receiving has been practiced, the intent of such stations has been to remove the problematic FOG stream from the sewers and concentrate it in receiving tanks from which it can be metered into the anaerobic digestion system. Anaerobic digestion of this material, in addition to typical municipal sludge and other external feedstocks, can increase production of digester gas that can then be harnessed to produce power and/or heat for the utility. Some of these facilities have reportedly achieved net zero energy status by producing energy sufficient to meet or exceed their plants' demands. In addition to reducing power purchases from utilities by increasing the production of renewable energy, these facilities have also sought to reduce clogging of sewer piping and meet the demands of FOG haulers seeking to reduce hauling distances. However, as the number of such facilities has grown, so have reports of operational difficulties, inconsistent FOG availability, variable gas production, and higher than expected maintenance requirements. Preliminary feasibility studies have typically focused on FOG availability, tipping 1236

2 fees, and projected digester gas/energy production. Few studies have focused on the actual operation and maintenance (O&M) associated with these systems. Hence, the O&M expectations of plant staff are based on limited information and the actual O&M for FOG receiving typically becomes apparent only after start-up of the systems. METHODOLOGY For the reasons noted in the introduction, the Albuquerque Bernalillo County Water Utility Authority (the Water Authority) in New Mexico, while interested in the potential benefits of receiving FOG at its Southside Wastewater Reclamation Plant (SWRP), decided to study the operating histories of such systems at currently operating plants before constructing a FOG receiving station. To develop an up-to-date understanding of FOG receiving, the project team surveyed several FOG facilities in the United States that were being implemented or already in operation in The survey generally focused on the parameters listed in Table 1. This information was intended to aid the Water Authority in determining the feasibility of a FOG receiving station at the SWRP. Table 1. Information Sought During Surveys. Category Facility Overview Information Wastewater treatment facility capacity Size/capacity of FOG Receiving Station Project drivers Years in operation FOG Characteristics and Quality Control FOG sources Quantity received FOG quality Sampling parameters Quality control measures FOG Receiving System Process schematic Piping and valves Equipment Changes in system since installation 1237

3 Table 1. Information Sought During Surveys. Category Construction Costs and Schedule Operations and Maintenance (O&M) Information Costs Schedule Operating procedures Digester feed control parameters Reported changes in digester gas production O&M issues Staffing requirements Tipping fees/fog acceptance charges Of the 11 facilities shown in the map in Figure 1, 10 were successfully contacted and participated in the survey. Medium and large-capacity treatment plants were prioritized in the survey to better match the size of the SWRP. The survey was primarily conducted through telephone calls and electronic communication with operations staff, but some facilities were also visited in person. Due to the preference of some surveyed facilities, results are summarized in this paper as general conclusions and findings or provided without identification of specific facilities. Figure 1. Surveyed Facilities with FOG Receiving Stations. 1238

4 SURVEY RESULTS Facility Overview Surveyed wastewater treatment facilities have implemented FOG Receiving Stations for a variety of goals, including: Reduced utility costs through increased digester gas production and subsequent power/heat generation. Reduced clogging and impacts on collections systems. Improved sustainability and reduced greenhouse gas emissions by digesting rather than disposing of FOG at landfills. Revenue generation. Most of these facilities have been in operation from 3 to 11 years, but one system was awaiting start-up at the time of the survey. The total average dry weather flow (ADWF) treatment capacity, FOG receiving station capacity, and FOG storage capacity for the surveyed facilities are shown in Table 2. Table 2. Design Capacity of Surveyed Treatment Facilities and FOG Systems. Parameter Range Median Value Plant ADWF Capacity (mgd) a FOG Receiving Station Throughput Capacity (gpd) b 12, ,000 30,000 FOG Receiving Station Storage Capacity (gal) c 5,000-60,000 28,500 a mgd = million gallons per day b gpd = gallons per day c gal = gallons FOG Characteristics and Quality Control The FOG product is typically pumped out of restaurant grease traps and delivered to the receiving stations by private haulers. The material is considered "brown grease" rather than "yellow grease" and characteristics vary significantly from one source to another. Seven of the facilities regularly tested certain parameters to assess FOG quality or determine if haulers were in compliance with their permit requirements. However, there were also facilities that did not regularly monitor the characteristics of incoming FOG. Table 3 summarizes the testing intervals and reported values for regularly monitored parameters. Monitoring of material quality during delivery is typically limited to visual inspection and sometimes, ph measurements. Sampling is typically done with grab samples, either from each truck load or from the material stored in the receiving tanks. While ph can be quickly tested in the field with litmus strips, other analyses are conducted at facility laboratories after the material has already been delivered. 1239

5 Table 3. Reported FOG Characteristics, Monitored Parameters, and Monitoring Frequency. Parameter Number of Facilities Regularly Monitoring Typical Frequency of Measurement a Range of Reported Values b, (median) Quantity of FOG Received 9 With delivery or daily total 1, ,000 gpd, (18,500 gpd) ph 5 With delivery or daily composite Total Solids (TS) 5 With delivery or daily composite Volatile Solids (VS) 5 With delivery or daily composite 2-6, (4) 0.5% - 20%, (8.6%) Less than 20% to 100%, (79%) Chemical Oxygen Demand (COD) 2 With delivery or daily composite 5, ,000 mg/l, (50,000 mg/l) a Frequency of testing is based only on facilities that reported testing intervals and regularly monitored parameters. b Some facilities indicated that they test for these parameters, but did not report measured values. While most facilities prohibit the use of septage trucks for delivered FOG, it was reported that some haulers use the same trucks for both septage and FOG. This results in some contaminants typically associated with septage being delivered with the FOG. Many facilities found FOG to be abrasive, containing significant quantities of grit and other objects such as rocks, concrete, metal debris, and materials found in septage or restaurant waste. The FOG tends to be acidic, and facilities have reported that the low ph may cause equipment damage. Three of the surveyed facilities contract with single, local FOG haulers who are responsible for delivering a consistent, screened product with certain specifications for product ph, solids content, and volatile solids concentration. While this practice limits the number of haulers that can utilize the receiving station, it does allow for better control of FOG quality and consistency. Even with this level of control, the material can still be grit-laden and abrasive. Seven of the surveyed facilities require FOG haulers to apply for a permit before they can offload FOG at the receiving stations. This allows multiple haulers to offload and maximizes the utility of the receiving station for haulers within the area. However, it was also reported that this practice makes it difficult to monitor or control the quality of the FOG received. Permitted haulers that are found to deliver excessively contaminated product or product that causes process upsets due to its characteristics are typically warned and their permits can be revoked upon multiple infractions. In general, these facilities also reported wide variation in daily FOG deliveries. Several trucks might deliver on one day followed by one or two days of very few loads. One facility with a design capacity of 30,000 gpd received an average of 1,500 gpd over approximately one year, with many days without any delivery and a few days exceeding 1240

6 50,000 gallons. These fluctuations impact system economics, digester gas production, and subsequent gas utilization and cogeneration systems. Most surveyed facilities use their FOG receiving tanks, low-pressure digester gas storage, and/or digester feed control procedures to minimize fluctuations in gas production. FOG Receiving System Each surveyed facility had common process needs, but varied in specific equipment and material selection. Figure 2 illustrates a general process flow diagram that includes major process equipment used at various facilities and their frequency of use. Figure 2. General FOG Receiving Station diagram, showing major components used by surveyed facilities. Several types of equipment and various materials of construction are used for system components at the surveyed facilities. Table 4 lists the equipment types used for different components of the FOG receiving system and the major materials of construction as reported. Some facilities modified their FOG receiving stations after gaining experience with the delivered FOG product. Many of these changes were intended to increase equipment service life or reduce maintenance needs, and were driven by the contaminants, abrasion, and/or corrosive nature of the FOG material. Systems that did not originally have FOG screening were refitted with some type of screening or straining device, although this was noted as a particularly maintenanceintensive part of the system. Different types of elastomers were tried for valves and pumps, with 1241

7 Buna-N being the elastomer most often used. However, all reported elastomers were susceptible to some degree to damage from the FOG stream. Steel materials were generally replaced with stainless steel components to better handle corrosion. Some systems that had been installed without the ability to heat FOG initially were retrofitted with heat tracing or sludge recirculation systems to minimize clogs. No mechanical grit removal systems were reported at any of the surveyed facilities. It was found that grit settles in the system piping and FOG receiving tanks, and is periodically removed manually. Some facilities purposely use the tanks to settle grit and minimize grit carryover into the digesters. Table 4. Equipment Types and Materials of Construction Used by Surveyed Facilities. System Component Equipment and Materials of Construction Used at Different Facilities Number of Facilities Bar Screens/ Strainers Manual bar screen before inlet to storage tanks 2 Steel basket strainer with stainless steel internals 1 Rock Trap/ Macerators or Grinders Combination rock trap/macerator, hot dipped galvanized and hardened steel Combination rock trap/macerator, hot dipped galvanized and hardened steel housing with stainless steel blades and cutting screen Inline grinder, stainless steel internals or materials not reported Use of FOG storage tank as rock trap 1 Offloading Pumps Rotary lobe, NBR/Buna-N lobe casing with steel or stainless steel wear plates Rotary lobe, NBR/Buna-N lobe casing with tungsten carbide wear plates and housing 3 1 Rotary lobe, stainless steel lobes with rubber tips 1 Peristaltic hose, materials not reported 1 Centrifugal chopper pump, NBR/Buna-N elastomers with ductile iron casing and hardened cast steel impeller

8 Table 4. Equipment Types and Materials of Construction Used by Surveyed Facilities. System Component Above-Grade FOG Receiving Tanks Equipment and Materials of Construction Used at Different Facilities Steel tank in rectangular, cylindrical-flat bottom, or cylindrical-conical bottom shape Stainless steel tank in cylindrical-flat bottom or cylindrical-conical bottom shape Number of Facilities 2 1 Cylindrical FRP tank 3 Cylindrical polyethylene 2 Below-Grade FOG Receiving Tanks Rectangular concrete 2 Recirculation Pumps Heating System Rotary lobe, NBR/Buna-N lobe casing with steel or stainless steel wear plates (offloading pump used for recirculation) Rotary lobe, NBR/Buna-N lobe casing with tungsten carbide wear plates and housing (offloading pump used for recirculation) Rotary lobe, stainless steel lobes with rubber tips (offloading pump used for recirculation) Centrifugal chopper pumps with ductile iron casing and hardened steel impeller Centrifugal chopper pumps with all stainless steel internal components and casing Submersible centrifugal chopper pumps, with hardened steel internals, epoxy coating and lining Tube-in-Tube heat exchanger with FOG recirculation and hot water as heat source Heat tracing and insulation of piping and/or FOG tank 3 FOG delivered heated 1 Steam Injection (installed, but not used)

9 Table 4. Equipment Types and Materials of Construction Used by Surveyed Facilities. System Component Equipment and Materials of Construction Used at Different Facilities Number of Facilities Digester Feed Pumps Progressing cavity with NBR/Buna-N elastomers 5 Progressing cavity with custom silicone stators 1 Rotary lobe with NBR/Buna-N elastomers 2 Rotary lobe, NBR/Buna-N encasement with tungsten carbide-coated housing 1 Odor Control Carbon canister for tank vent 4 Chemical or biological odor scrubber with fans 1 Piping NBR/Buna-N hose 1 Glass-lined ductile iron 6 Unlined stainless steel 1 PVC 2 Isolation/Control Valves Plug values with NBR/Buna-N plug facing SST full-port ball valves with RTFE seats 1 PVC ball valves with RTFE seats and FKM/Viton O-rings 2 Construction Costs and Schedule Where reported, construction costs for the FOG receiving stations ranged from $250,000 to $4 million in the year of system construction. Costs for the seven facilities that reported a value are presented in Table 5. Construction durations were reported between 6 to 18 months. Construction schedule and costs were related to system size, redundancy, complexity, construction method, and intended life of the receiving station. Some systems were either constructed as "temporary" systems by plant staff or as part of larger expansion projects. Costs specifically for the FOG receiving station were estimated by survey respondents when constructed as part of a larger project. Some facilities received project funding through the American Recovery and Reinvestment Act and a few other agencies also took advantage of available state funding to construct their FOG receiving stations. Several of the "temporary" systems included in the survey were in operation for a number of years after being initially constructed by plant staff. These systems were often 1244

10 simpler than larger, permanent facilities and provided with less redundancy and lower cost materials of construction. Table 5. Reported Construction Costs for FOG Receiving Stations. Facility a FOG Storage Tank Capacity, gallons Reported Construction Cost b Year Constructed A 60,000 $3 million 2015 B 20,000 $2 million 2010 C 27,000 $2.1 million 2013 D 45,000 $2.9 million 2011 E 5,000 $0.25 million 2008 F 30,000 $2.3 million 2013 G 43,500 $3 million to $4 million 2011 a Facilities are not specifically identified due to preference of some survey respondents. Facilities identified alphabetically in this table may differ from those identified similarly in other tables. b Reported costs are presented for the year in which the facility was constructed. Operation and Maintenance Operating Procedures. Operational requirements and procedures differ based on type and size of FOG receiving system. Some facilities allow FOG deliveries only during staffed hours so that operations personnel are onsite to monitor deliveries, collect samples, and operate the FOG system. Operators at these facilities can track haulers and inspect delivered product so they are better able to control FOG quality than those facilities that allow material delivery during offhours. Some receiving stations that do not have staff onsite during delivery are fitted with key card systems that track hauler data and provide haulers access to the automated system based on unique access cards. However, gathering information such as FOG source and sampling necessary to capture FOG characteristics typically requires the presence of staff during deliveries. One surveyed facility has operated with deliveries during staffed and non-staffed hours. This facility found that FOG quality tended to be more consistent and the product less contaminated when deliveries were allowed only when plant operators were present. When plant operators are present at FOG delivery, they typically collect samples from each load for subsequent analysis. Without staff presence at each delivery, samples are typically collected from the FOG tank or piping system to provide a general picture of the FOG delivered throughout the day. Most facilities test for ph, total solids (TS) and volatile solids (VS) content. Very few facilities test for chemical oxygen demand (COD). Sampling results for these parameters help identify potential sources of problematic FOG and avoid digester upsets. 1245

11 Operations for FOG stations at the surveyed facilities vary based on system design. However, typical operations for a station with an above-grade receiving tank and components similar to that presented in Figure 2 include the following steps: 1. FOG Delivery: The FOG hauler checks in with an access card, a guard at the facility gate, or plant staff. The hauler connects the truck hose to a connection point at the receiving station. The offloading is started either by the hauler through an onboard air-assist system or at the receiving system's offloading pump. As the pump starts, the system rock trap/macerator also starts. The contents of the hauler's truck are transferred through the screen/strainer and rock trap/macerator into the FOG receiving tank. Valves within the system can be manual or automated. Manual valves are positioned by plant staff before system start and automated valves direct FOG flow per their control logic. If present, staff members collect samples for subsequent analysis. 2. FOG Recirculation: After the truck is emptied, valving is switched to allow a dedicated recirculation pump or the offloading pump to recirculate FOG within the tank. Some systems include a heat exchanger through which the FOG is pumped. Heat is typically supplied by hot water from the digester heating system. Systems with heat tracing typically turn the heat tracing system on at a setpoint low temperature. 3. Debris Removal: After the truck is emptied, the system screen/strainer and rock trap are cleared of debris and any spilled FOG is washed away to maintain a reasonably clean working area. Clearing debris from the screen/strainer is typically a manual and time consuming task that is completed by plant staff. Some strainer baskets become too heavy to lift without a hoist mechanism. Debris is typically transferred into covered trash bins that are periodically emptied for landfill disposal. 4. Digester Feed: FOG is typically blended into a sludge stream that is pumped into the digester. This sludge stream can be digester feed sludge (thickened primary or secondary sludge) or it can be the heated recirculation sludge that maintains digester operating temperatures. Separate FOG feed into poorly mixed areas of the digester can cause the FOG to float and develop into a mat rather than being properly entrained within the digester contents. Specific operational practices for how to control FOG feed to the digester are described below. Digester Feed Control. As noted earlier, most of the surveyed facilities reported wide variation in daily FOG deliveries. If carried into the digester feed cycle, these fluctuations can impact digester performance, digester gas production, and subsequent gas utilization and cogeneration systems. Most surveyed facilities equalize the feed within their FOG receiving tanks, meter FOG with variable speed digester feed pumps, and utilize digester feed control procedures to maintain acceptable operational parameters. However, based on survey responses, there was no "standard" way of controlling FOG feed to the digesters. Each of the various methods below was reportedly used by different facilities to control FOG feed to their digesters: FOG tank is emptied into digesters over an operator adjustable timeframe. FOG is fed into digesters based on the level in the FOG tank, with no digester-related controls. 1246

12 A small quantity of FOG is fed to digesters and gradually increased over time, as digesters acclimate to the new food source. This was done after foaming issues, which were believed to be due to excess FOG loading into the digesters, causing an upset in the biomass. FOG is fed directly into digesters, maintaining an adjustable FOG:sludge ratio by volume within the digester. The reported volumetric ratio was approximately 30 to 40% FOG and 60 to 70% sludge. FOG is fed to the digesters while maintaining total solids loading into the digesters by reducing sludge feed. FOG is first mixed with an equal volume of primary sludge, then fed to digesters. FOG is fed solely based on maintaining the same volumetric loading of FOG in the digesters each day. FOG feed is determined by internally developed control system designed to optimize gas production. Among facilities with more than three digesters, two blended FOG with digester feed sludge in a tank or piping and pumped the blend to all digesters in the system. Three facilities fed FOG only to specific digesters within the facility. Facilities that had three or fewer digesters typically fed FOG to all digesters within the system. Reported Changes in Digester Gas Production. Of the facilities surveyed, half were able to quantify increases in digester gas production after the addition of FOG. The remaining facilities were not able to quantify the increase, but most reported that gas production did increase after FOG digestion was implemented. Two facilities even reported that digestion of FOG has played a key role in them achieving energy neutrality status. A third facility noted that its combination of FOG and food waste digestion, cogeneration, and solar power production has offset almost 90-percent of the plant's energy demands. On the other hand, one facility reported that FOG volume received at their station was too low to determine any impact on digester gas production and another plant indicated that they had expected a larger increase than what they actually experienced. As presented in Table 6, reported digester gas production increases were between 30 to 100%, depending on FOG quantity and quality. Some facilities accept external feedstock beyond just FOG, so some increases in digester gas production may be related to these other feedstocks. One facility that has separately accepted FOG and food waste indicated that digestion of FOG increased gas production significantly more than the food waste they received but did not provide quantitative data on the difference. 1247

13 Table 6. Reported Increases in Digester Gas Production. Facility a FOG Storage Tank Capacity, gallons Average Quantity of FOG Received, gal/day Reported Percent Increase in Digester Gas Production b Notes A 10,000 to 17, to 105,000 c Approx. 100% Note e B 20,000 4,000 to 9,000 Approx. 30% C 45,000 10,000 to 50,000 Approx. 57% Note e D NR d Approx. 25,000 Approx. 57% to 97% E 43,500 18,000 to 24,000 Approx. 100% Note e a Facilities are not specifically identified due to preference of some survey respondents. Facilities identified alphabetically in this table may differ from those identified similarly in other tables. b Reported percent increase relative to gas production before implementation of FOG receiving. c Range based on wide variety of FOG vessels used to deliver product to facility and acceptance of material during each of 3 shifts. Downstream blend tank used to further equalize FOG and produce an acceptable FOG:sludge ratio for combined digester feed. d NR = Not Reported e Facility accepts FOG and other feedstocks. O&M Issues. All facilities agreed that FOG is a difficult, sometimes odorous, material to handle that requires considerable attention and maintenance. In most cases, maintenance needs for these FOG receiving stations were higher than originally anticipated by O&M staff. Many of the previously noted changes in FOG system materials and configuration were made to mitigate high maintenance requirements. At the time of the survey, two of the facilities reported that they were considering some system changes to reduce system maintenance and increase equipment life. Even though maintenance needs have been high, most facilities indicated that the net benefits in terms of increased digester gas production were worth this effort. Staff members at these facilities had accepted the maintenance intensity and had accounted for the labor effort and materials replacement costs within their operational plans. Table 5 summarizes some of the reported maintenance issues, tasks performed, and task frequency. Table 7. Reported O&M Issues Experienced with FOG Receiving. Reported Issue Task Reported Frequency Screens/strainers clog with grease, rags, and debris Rock trap filled with debris Manual cleaning of screens/strainers Manual draining and cleaning of rock trap Daily or after each load After each load to weekly 1248

14 Table 7. Reported O&M Issues Experienced with FOG Receiving. Reported Issue Task Reported Frequency Damage/wear of macerator blades Replace damaged cutting blades Once to twice per year Wear of macerator screen Replace screen Once per year Damage/wear of elastomers in system pumps Damage/wear of pump housings Damage/wear of pump wear parts Standard pump upkeep Damage/wear of elastomers in system valves Pipe clogs or FOG mat formation in receiving tanks, especially during cold weather FOG tank upkeep Replace elastomers or elastomercased pump components (lobes, lobe tips, stators, etc.) Replace pump housing, possibly with harder or more corrosion-resistant material Replace wear parts, possibly with harder material Inspection, pump component adjustment, and lubrication Inspect and replace valves or elastomer-cased valve components (plugs, flapper disks, etc.), possibly with different valve types and/or materials Pipe flushing and manual destruction of mats Draining, inspection, and removal of grit, debris, and rocks 3 to 6 months 6 to 36 months As needed Weekly to annually, depending on task 6 to 36 months During cold weather and as part of normal operations Semi-annually Staffing Requirements. The reported O&M requirements of FOG receiving stations require significant staffing. Specific staffing requirements are dependent on system size and complexity, level of automation, FOG acceptance protocols, quality control measures, and facility philosophy on FOG system operation. Reported staffing requirements for facilities that were able to provide this information are presented in Table 8. As shown, reported staffing requirements ranged from one full-time employee (FTE) at quarter-time to two FTEs. Depending on each system, staffing is required for the following tasks: Permitting FOG haulers and monitoring permit compliance Inspecting FOG loads, sample collection, and analysis 1249

15 Regular inspection and operation of FOG station valves and pumping, maceration, heating, mixing, digester feed, and odor control systems Removing captured materials from screens, strainers, rock traps, and tanks General housekeeping of FOG receiving area Regular, preventative maintenance of system components, including odor control Repair and replacement of damaged system components Collecting and monitoring FOG data for delivered material quantity, characteristics, impacts on digester feed, and impacts on digester gas production Monitoring of digester performance and troubleshooting as necessary Determination and collection of tipping fees Management of contracts, if any, with FOG haulers As noted previously, some facilities accept FOG only during manned hours under staff supervision so they dedicate staff members to the FOG system to inspect the FOG load and operate the FOG system. These facilities typically also require sampling and analysis of each FOG load. Other facilities allow FOG delivery during unmanned hours so staff time is not required at every delivery. Accurate projections of staffing requirements must consider each facility's staffing philosophy during FOG delivery and subsequent sampling and system operation. Table 8. Reported Staffing Requirements. Facility a FOG Storage Tank Capacity, gallons Reported Staffing Requirements b A 10,000 to 17,000 1 FTE B 20,000 1 FTE C 27,000 1 to 2 FTE D 45,000 2 FTE E 43, FTE a Facilities are not specifically identified due to preference of some survey respondents. Facilities identified alphabetically in this table may differ from those identified similarly in other tables. b FTE = Full-time employee equivalent Tipping Fees/FOG Acceptance Charges. In some cases, agencies were able to charge a tipping fee for haulers to deliver FOG to the treatment plant. As shown in Table 9, these fees ranged from $0.01 to $0.09 per gallon. Facilities located in areas with competing FOG receiving stations or disposal alternatives reported downward pressure on tipping fees. One facility that originally charged $0.11 per gallon has reduced the tipping fee to $0.08 per gallon. Tipping fees were also reported to impact the quantity of FOG delivered to receiving stations. If the fees are considered too high by the haulers, they typically find lower cost options and do not deliver the product to the receiving station. This has a significant impact on system economics, especially if a consistent revenue stream was expected during a feasibility assessment of the FOG system. As 1250

16 noted previously, one facility with a design capacity of 30,000 gpd received an average of 1,500 gpd over approximately one year, with many days without any delivery and a few days exceeding 50,000 gallons. The expected tipping fees from this system have not been realized and system economics have been dramatically impacted by the limited FOG deliveries. Table 9. Reported Tipping Fees. Facility a Reported Tipping Fees Reported Annual Tipping Fee Revenue Notes A $0.08/gal - Notes b and c B $0.09/gal - Note d C $0.03/gal - Note b D - $300,000/year Note b E $0.01/gal - F $0.08/gal - G - $300,000/year Note b a Facilities are not specifically identified due to preference of some survey respondents. Facilities identified alphabetically in this table may differ from those identified similarly in other tables. b Facility accepts FOG and other feedstocks. c Reported fee is specifically for FOG. d Facility reported that delivered FOG volumes were significantly lower than expected. DISCUSSION OF RESULTS Wastewater treatment facilities have implemented FOG receiving stations for various reasons. Beyond sustainability goals and reducing collection system maintenance, the prospect of increasing digester gas production and offsetting energy demand is a major driver for FOG receiving. Digestion of FOG has played a part in allowing two of the surveyed facilities to produce enough energy onsite to become energy-neutral (or energy-exporting) plants. A combination of FOG receiving, additional external feedstock, anaerobic digestion, and cogeneration comprise the energy portfolio for those energy-neutral facilities. Many of the other plants with FOG receiving stations also reported increases in digester gas production that partially offset total energy costs. While these facilities reported some increase in digester gas production, the increases were sometimes lower than expected and gas production varied with the actual quantity and quality of FOG received. In addition to the actual digester gas and energy production that is realized after implementation, tipping fees significantly impact system economics. Tipping fees are highly variable and can change due to market conditions. Receiving stations that are in areas with competing facilities may not receive expected volumes of FOG or associated revenue. Similarly, plants that charge relatively high tipping fees compared to other options for the local haulers may drive the market 1251

17 elsewhere. However, those facilities that are located in areas without major competition can successfully charge market-rate tipping fees and realize additional revenue to improve their overall bottom line. Appropriate tipping fees must balance market conditions with the operating costs and revenue goals of the wastewater treatment plant. System economics are heavily dependent on both the cost savings from energy production and revenue generated from tipping fees. Hence, conservative planning based on a potential range of FOG quantities, tipping fees, and projected gas production can help facilities make well-informed decisions. System economics are also impacted by initial capital costs and the ongoing cost for O&M. Economic analysis should consider not only the FOG receiving station but also the facility's digestion system. Implementation of FOG receiving at facilities with excess digestion capacity is more likely to be economically feasible than for facilities that require construction of new digesters. Adding new digestion volume would significantly impact capital costs. When analyzing the capacity of the digestion system to handle FOG, the ancillary systems associated with the digesters should also be considered in terms of capacity and suitability. The digester heating, gas, and mixing systems would all be impacted by the addition of FOG. Digester gas piping, utilization, and flaring systems should have sufficient capacity to handle projected gas production. Heat demand will increase with any additional digester feed, and the mixing system should be capable of entraining FOG into the digester contents rather than allowing it to float and form a mat within the tank. If any of these systems require modifications to accommodate FOG, the associated capital and O&M costs should be included in the system economics. Receiving stations can vary in size, complexity, redundancy, level of automation, equipment, and materials of construction. All of these factors affect both the capital cost and staffing requirements. Construction costs for most of the FOG receiving stations included in the survey exceeded $2 million and staffing requirements ranged from one FTE at quarter-time to two FTEs full-time. When considering costs associated with staffing, all of the tasks required for FOG receiving should be accounted for, from initial permitting and contract development to system operations, sampling, monitoring, and maintenance. The overall cost of the system can then be compared to potential cost savings and revenue generation to determine system economics. If the initial economic analysis does not reflect the potential O&M costs for these types of systems, proper staffing may not be available, and system operation may suffer after the station is built and started up. The quality and characteristics of FOG can vary greatly, and the material is often delivered with numerous contaminants such as rocks, grit, metal, and concrete debris. The abrasiveness, low ph, and chemical composition of this material have been reported as contributing factors in the damage of FOG system components. As a result, FOG receiving stations can be O&M-intensive and may require additional staffing to sustain successful operations. The mechanical components, the methods of handling FOG, and the operation of FOG receiving stations differ from one facility to another. In general, equipment life for FOG system components is significantly shorter than equipment life for the same components in sludge or similar applications. Though some materials may work well with one aspect of FOG, such as low ph, the same material may not be suitable for other characteristics like abrasion. The most suitable materials balance performance in abrasive and corrosive environments. However, various materials and equipment types all experience some level of wear and eventually require 1252

18 replacement. Regular inspections and preventative maintenance are necessary to identify maintenance needs early and sustain successful operations. The chemical characteristics of FOG also impact system performance and digester gas production. If material is delivered with high water content relative to volatile organics content, it may simply add to the hydraulic load on the digester without providing a high-strength feedstock that increases digester gas production. Facilities that are able to monitor and control FOG quality are more likely to have consistent and stable digester performance. One way to control FOG quality is to contract with a single FOG hauler and require that hauler to provide material within specifications. This method can reduce variability in the FOG quality and quantity received, but it also limits the use of the receiving station. Facilities that prefer to maximize use of their receiving stations can permit multiple local haulers and revoke permits if delivered product causes operational problems. Having staff present during FOG delivery allows for initial material inspection and sample collection that can also improve the quality and consistency of received FOG. While there was no "standard" way of controlling FOG feed to the digesters, all of the facilities monitored digester performance to avoid process upsets. Some FOG may have surfactants or other characteristics that exacerbate foaming within the digester. Slowly metering the material into the digester feed or sludge recirculation lines is a common practice that may minimize sudden impacts on the digester, reduce foaming, and allow for better FOG entrainment. Three of the surveyed facilities currently control FOG feed by maintaining a specific FOG:sludge ratio by volume (approximately 30:70 to 40:60). Some facilities also track FOG as a percent of total solids or volatile solids feed to the digestion system. However, a feed regime based on COD may ultimately be the best control parameter to optimize digester performance and track digester gas production. For better tracking of FOG quality and feed control, FOG samples should be regularly analyzed for ph, TS, VS, and COD. Better tracking of digester gas production with and without FOG would provide more comprehensive data that can be used to project impacts of FOG digestion on gas production. Some facilities accept both FOG and other high-strength feedstocks like food processing wastes (FPW). In general, FOG has higher energy value than most FPW but these other waste products tend to be less contaminated and more consistent than FOG. If a facility is weighing FOG receiving against co-digestion of FPW, the potential digester gas production and associated O&M requirements for both products should be considered relative to facility goals. Food processing wastes also differ from FOG in carbon-to-nitrogen ratios. While FOG primarily consists of carbon-rich lipids, certain FPW contain high levels of nitrogen that can impact the liquid treatment train through ammonia in the returned dewatering filtrate stream. The relative impacts of FOG or FPW digestion on overall solids production and dewatering processes may also differ. These types of process impacts should be considered if a facility is considering digestion of FOG, FPW, or other organic feedstocks. CONCLUSIONS While FOG receiving and digestion can be maintenance intensive, it also represents a means to increase onsite production of renewable energy, reduce collection system overflows, and provide 1253

19 local disposal options for FOG haulers. Many lessons have been learned from operating facilities about how to plan, design, operate, and maintain a sustainable FOG receiving system. The necessary amount of O&M effort is dependent on system design and FOG characteristics, and the required staff, labor, and investments in equipment and materials must be seriously considered during feasibility assessment. The associated costs in time and material must be studied and weighed against the potential benefits in terms of digester gas production, tipping fees, and improvements for the collections system. Implementation of these lessons learned and improvements to current operating procedures will yield program success as more FOG receiving systems are considered and brought online. Making a well-informed decision on FOG receiving requires: Careful definition of the facility s economic, environmental/sustainability, and operational goals. It is important for all stakeholders to achieve consensus on these goals and their relative weights/priorities. Determination of the quantity and quality of FOG potentially available in the local area. Understanding of the local FOG market, current FOG disposal options, and associated costs to FOG haulers. This is necessary to determine tipping fees that are competitive and sustainable. Conservative planning based on a range of FOG quantity, tipping fees, and projected gas production. Determination of existing FOG hauler quality control and what quality control measures the haulers would be willing to implement if required for acceptance at the FOG receiving station. Acceptance of the O&M intensity of FOG receiving systems and provision of required staffing. Incorporation of lessons learned and successful design features into FOG receiving system designs. The Water Authority is currently surveying the local FOG haulers to determine FOG availability, quality, and current market conditions. The final decision regarding implementation of a FOG receiving station will be made collectively by all stakeholders, including the O&M personnel that will be required to sustain the system. ACKNOWLEDGEMENTS The project team thanks all of the facilities that participated in the survey. 1254