BIOSOLIDS TO ENERGY CO-PROCESSING FOG AND SLUDGE TO INCREASE ENERGY RECOVERY POTENTIAL

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1 BIOSOLIDS TO ENERGY CO-PROCESSING FOG AND SLUDGE TO INCREASE ENERGY RECOVERY POTENTIAL Josef Cesca 1, John Kabouris 2, Yogesh Gokhale 1, Tim Shea 3, Bob Forbes 4 1. CH2M HILL, Sydney, NSW, Australia 2. CH2M HILL, Atlanta, Georgia, U.S.A. 3. CH2M HILL, Chantilly, Virginia, U.S.A. 4. CH2M HILL, Charlotte, North Carolina, U.S.A. ABSTRACT The acceptance and processing of fats, oils, and grease (FOG) wastes at municipal wastewater treatment plants (WWTPs) is of increasing interest because of the need to keep these materials out of wastewater collection systems, as well as the added energy recovery potential when FOG wastes are anaerobically digested. Adding FOG waste to anaerobic digesters is particularly attractive over other feedstocks such as food waste because of the high volatile content that can yield over one cubic metre of methane per kg and high degradation with minimal extra solids requiring disposal. The addition of FOG wastes to a WWTP usually requires combining the FOG waste with sludges from the WWTP, in a process that utilises the energy from FOG, such as anaerobic digestion. The manner in which this is done can impact the performance of the anaerobic digesters as well as other solids handling processes, and so must be done properly. The objective of this paper is to present a body of experience for assessing the feasibility of coprocessing FOG and other wastes with municipal sludge in anaerobic digesters. The paper also focuses on FOG waste receiving and process flow alternatives to minimise the problems that have been associated negatively with introduction of FOG wastes into a treatment plant. These problems have included: blockage of pipes and pumps; digester foaming; grit accumulation in digesters; stuck digesters (curtailed methanogenesis); clogging of gas collection and handling systems; and excessive downtime for maintenance. The authors have been involved in the evaluation and development of FOG waste acceptance and transfer systems for sewage sludge incinerators at Greater New Haven Water Pollution Control Authority (CT) and Hampton Roads Sanitation District (VA); and for anaerobic digestion systems at Gwinnett County (GA), Johnson County (KS), Pinellas County (FL) and elsewhere. These projects have contributed to the collective knowledge base presented in this paper. INTRODUCTION FOG is a generic term for a myriad of organic compounds that exhibit hydrophobic characteristics, liquefy when heated and yield methane when digested anaerobically or have a high energy content that is released in thermal oxidation. The acceptance and processing of FOG wastes from truck haulers at municipal WWTPs are of increasing interest because of the need to keep these materials out of wastewater collection systems, as well as the added energy recovery potential when FOG wastes are anaerobically digested or incinerated. The addition of FOG wastes to a WWTP usually requires combining FOG waste with WWTP sludges in a process that utilises the energy from FOG such as anaerobic digestion or incineration. The manner in which this is done can impact the performance of the anaerobic digesters or incinerator as well as other solids handling processes, and so must be done properly. The principal focus of this paper is not on energy recovery but rather on alternate methods of FOG waste acceptance and pre-processing that can be used, as these methods are an emerging practice. The state-of-the-art is not resolved, but the growing interest on energy recovery in our industry is pushing it forward. Thus, a thorough understanding of the types of FOG handled and its characteristics is needed to allow for a systematic approach to be developed. Therefore, the goal of this paper is to illustrate some of the FOG waste acceptance and pre-processing practices that have worked, and to stimulate discussion. Characteristics of FOG Wastes Generally, FOG varies in composition from 2-20% total solids (TS) of which anywhere from 80 to 99% of the TS content is volatile. When dewatered after chemical conditioning, the semi-solid FOG TS can increase to upwards of 50% TS, with 75-95% volatile content. Typical FOG characteristics from various sources are presented in Table 1. This range in FOG characteristics directly effects which FOG recovery scheme is most suitable for a specific WWTP. Typically, a FOG recovery scheme

2 can be integrated into a WWTP either within the existing liquid treatment or solids processing trains. Generally, FOG receiving schemes, when coupled with energy recovery, are designed to maximise the efficiency of the combined operation. Thus, from a hauling perspective alone, dewatering and hauling as a cake would be more cost effective. However, receipt as a liquid FOG waste is more common and requires less processing at the WWTP. WWTPs without specific receiving FOG facilities generally accept FOG as a liquid waste into the liquid treatment train where it is separated as primary scum, or a secondary scum that is usually returned to the primary sedimentation tank for removal as primary scum. At these facilities, FOG waste has been typically accepted as part of a septage receiving operation. Such stations will usually involve rock traps and coarse screening followed by transfer to the headworks and primary sedimentation tanks. FOG not captured as primary scum can be lost to the secondary treatment process, cling to the walls of basins, channels, or piping, form grease balls, etc. The results include a loss in energy-yielding material, an ongoing maintenance activity, and occasional overloading of the primary scum wells. Accurate estimates of FOG lost (i.e. not recovered) will require further study, but it can be assumed that at least 15% of FOG received in this manner is lost to the energy recovery systems. With the emerging interest in energy recovery, FOG wastes are being routed around the liquid treatment train directly to the solids processing train and into anaerobic digestion or thermal oxidation systems. Separate onsite receipt for anaerobic digestion will usually involve screening, heating, grit/rock removal, equalisation and transfer to anaerobic digestion either directly or after blending with other digester feed sludges. The loss of material in transit from acceptance to processing is minimal with dedicated facilities. These facilities can also support the fractionation of the waste into a FOG-rich supernatant to maximise volatile solids addition in conjunction with a FOG-lean underflow that would be returned to the liquid train. FOG acceptance facilities for energy recovery via thermal oxidation are more complicated and require one or more concentration steps to ensure that the water content is minimised before blending with the feed sludge and thermal oxidation. The steps can variously include: Scum/FOG concentration with heat addition to improve flow properties (at least 30-35C), the scum/fog can be concentrated to the 50% VS range after decanting. This material is labeled as concentrated scum (CS). Alternatively, with heat addition to the 65C range, and the decanting of concentrated scum/fog, decanted concentrated scum/fog (DCS) can be concentrated to 75 to 85% FOG VS. The CS or DCS is then either blended with or added to dewatered sludge cake before transfer to a thermal oxidation unit. The FOG-amended feed to the thermal oxidation unit will substantially increase the energy content of the feed, thereby reducing the supplemental fuel needed by multiple hearth incinerators and increasing the energy recovery potential in waste heat recovery boiler units in fluidised bed incinerators. METHODOLOGY Site surveys were used to gather information on multiple facilities, including those mentioned above as well as at Redwood City, Millbrae and Oakland (CA), Gresham (OR) and Gothenburg, Sweden. Each facility visited was surveyed with respect to: facility size; FOG quantities received; off-loading; transfer to storage; storage; mixing; heating; digester feeding; load tracking; wash down; and odour control. The survey results were analysed to develop a basis of design for several projects. REFERENCE FACILITIES Table 2 describes four facilities which exemplify elements of most FOG handling systems. Descriptions of the features of facilities outlined in Table 2 are provided below. Johnson County, KS The expansion of the Douglas L. Smith Middle Basin Wastewater Treatment Plant (Middle Basin WWTP) in Johnson County, Kansas to 55 megalitres per day (MLD) average capacity includes an environmentally friendly approach for the treatment of FOG wastes from local restaurants and industrial sources. FOG waste receipts are handled using a separate onsite liquid receiving facility consisting of the following: FOG waste unloading station; Rotary lobe receiving pumps with inline grinder and integrated debris/sediment trap; Storage tanks; Pumped mixing and heating; Heat exchangers; Progressing cavity digester feed pumps; and/or Odour control. Received FOG waste is first pumped through a debris/sediment trap then through a grinder and the heat exchangers to be pre-heated before entering the storage tanks. The receiving pumps will also continually mix the contents of the tank as well as maintain a temperature of about 27C to minimise issues with pumping the material. Separate pumps transfer the FOG waste to the anaerobic digesters.

3 Insulated force mains with numerous several cleaning taps are provided in case one pipeline becomes clogged. Enhanced gas production from the anaerobic digesters will fuel a 2.12-MW biogas co-generation system that utilises two duty kw generators. Industrial FOG waste producers from several food processing facilities located within the county have or are planning to install dewatering equipment for their thickened FOG waste stream to reduce truck hauling costs. One food processor is currently dewatering thickened FOG waste using polymer and a plate and frame filter press. Another is dewatering using polymer and gravity draining in super-sacks. Other processors are evaluating belt filter presses. To capture these high energy waste streams, a dewatered FOG waste receiving station is under design for the Middle Basin WWTP. This facility will include equipment and processes to develop a slurry that will be pumped to the storage tanks in the liquid FOG waste receiving station for further processing and blending with the liquid FOG waste prior to being fed to the digesters. The facility will consist of two below-grade tanks with hoppers to receive the dewatered material from end-dump trucks or roll-offs. A high-pressure water jet will initiate the slurry formation and force the material through an in-line grinder prior to being pumped by rotary lobe pumps to the mixing tank. A pumped mix system in the mixing tank, consisting of nonpotable water, submersible chopper pump, and heat exchanger will precondition the material prior to being pumped to the liquid FOG waste receiving facility. Hampton Roads, VA The Hampton Roads Sanitation District (HRSD), Williamsburg WWTP utilises a multiple hearth incinerator (MHF) to incinerate biosolids. By blending liquefied FOG and scum into dewatered solids, the facility can recovery energy in the form of reduced natural gas usage. FOG is collected concurrently with septage at a receiving pit. The FOG is screened through the headworks and is transferred between processes before ultimately being captured in the primary sedimentation basins with the primary scum. A scum concentrator thickens the mixture where it collected in a hopper and heated to approximately 55C. The FOG rich float is decanted and pumped to the midpoint of the dewatered solids conveyor for blending prior to entering the incinerator. The aqueous subnatant from the scum concentrator is recycled back to the head of the scum concentrator. Pinellas County, FL The Pinellas County, FL South-Cross Bayou Water Reclamation Facility (SCBWRF) utilises anaerobic digestion and sludge drying for solids disposal. FOG is accepted at an offsite receiving facility where dewatering operations take place. Generally, the dewatering operations are as follows: The daily receipts are accumulated, mixed, and dosed at 1.5% polymer on a volumetric basis (15 gallons of polymer solution per 1,000 gallons of FOG waste). The polymer-conditioned waste is stored overnight in a fine-screen dewatering tank, resulting in about 80% reduction in the weight of the FOG due to dewatering. The dewatered FOG concentration is typically about 20-30% solids. The dewatered FOG is then transferred from the dewatering tank to a roll-on/roll-off container for transfer to the WWTP. The roll-on/roll-off container is equipped with a bottom drain and coupling such that when the container is received at the WWTP, the container is connected by hose to a rotary lobe pump for transfer to a storage tank. Once at the facility, the FOG is injected into one of the egg-shape anaerobic digesters. The biogas produced from the facility currently used by the sludge pelletiser. Planned upgrades include heating the stored FOG and feeding it to new acid-phase reactors, along with receiving, storing and feeding lime-dewatered FOG. It is anticipated in the future, that that biogas production will exceed sludge drying fuel requirements wherein a new cogeneration facility will be constructed to produce energy. Essex Junction, VT The Village of Essex Junction, Vermont operates a 12.5MLD permitted wastewater treatment facility (EJWWTF) that is currently treating 7.5 MLD on an annual average daily basis. The facility provides service to 30,000 residents within the Towns of Essex and Williston, VT as well as the Village. Waste primary sludge and GBT thickened WAS (TWAS) from the treatment process is stabilised within mesophilic anaerobic digesters. In late 2003, the EJWWTF installed a combined heat and power (CHP) system utilising two microturbines. The 30 kw microturbines utilised the digester gas of which nearly 50% was previously flared. Influent conditions at the facility indicated that FOG is an increasing problem. Without local resources for disposal of FOG, staff felt that it was not prudent to enforce existing ordinances without providing a disposal solution. Through discussions of how to potentially handle the FOG as a separate waste, it was decided that direct addition of FOG to the anaerobic digesters would be most beneficial as the facility had surplus hydraulic capacity in the digesters. As FOG is a carbon source and directly digestible, direct digestion was considered as it would increase methane generation and power generation by the CHP system. In early 2005, the facility began experimenting with FOG addition and by the June was accepting FOG on a regular basis.

4 This experiment was a precursor to the acceptance of brown grease, yellow grease and other suitable materials. Other materials include: Bottoms from grease that has been separated out as part of biodiesel generation Emulsified oils from a local natural beauty product manufacturer Settlement waste from cider manufacturer High Strength BOD/COD waste from a local recycling company FOG is accepted from trucks by gravity directly into the existing primary scum pumping pit. The existing primary scum pump (a double disc positive displacement pump), conveys the scum and FOG mixture directly to the anaerobic digesters. The pumping rate is based on staff experience to supplement the sludge (TWAS/PS) flows. This system currently has no automation. At the initial startup, FOG was fed at a weekly average rate between approaching 15% by volume s compared to total volume of sludge applied to the digester. The rate varies based on surplus waste material availability. Overall, the FOG addition has enhanced digester volatile solids (VS) reduction which previously was already performing well with VS reduction consistently greater than 65%. EJWWTF staff noted increased methane gas production and power generation. Currently, plant staff continues to study the effects of the direct digestion of these waste materials. DISCUSSION If a semi-solid FOG can be obtained from various sources, then this material could potential be added directly to a thermal oxidation process with little handling. This method produces the extra energy benefits without the need for biogas clean up, provided a reliable high concentration source of semi-solid FOG can be obtained. However, the thermal oxidation process will need to be operated under sustainable loading limits to avoid air emission excursions, which means that only a limited amount of FOG can be accepted. If not enough FOG is available to maximise energy production, then a concentrator would be necessary to dewater the liquefied FOG. An advantage could come from operating a separate FOG receiving facility with anaerobic digestion. This liquid FOG waste can be blended with sludge prior to entering an anaerobic digestion process with minimal handling. However, this facility requires screening, mixing, and heating facilities to insure the FOG maintains a liquid form for easy handling. An important consideration when adding FOG waste to anaerobic digesters is to monitor the performance of the digesters to ensure the first stage is not overloaded or produces excessive foam. Shock loading of the digesters must be avoided. The digester gas handling systems must be adequately sized to handle the additional biogas production and the biogas typically requires cleaning prior to beneficial reuse in cogeneration units. A major disadvantage can be seen from using dewatered FOG waste with anaerobic digestion if this would require re-slurrying of the FOG (i.e. add water) after dewatering (i.e. removing water) which may not make sense unless trucking costs are significantly reduced. Similarly, using a liquefied FOG in conjunction with thermal oxidation may not be advantageous unless scum is added as well given that a scum concentrator is required. Accepting FOG at a separate facility (either on- or offsite) versus mixing it with other received wastes generally would consolidate O&M in one location. However, it does require capital expenditure to install the facility versus using existing structures. While the extra operational cost for heating and mixture the FOG mixture may be undesirable to budgets, this cost may be less overall when compared to increased maintenance of influent screens, grit chambers, primary clarifiers, and scum pump stations. For the EJWWTP, the FOG addition location was chosen specifically at the primary scum pit to minimise impact to other facilities. A poor performing screen allowed for FOG to become entrapped within rag mats. These mats lead to an increase in the frequency of digester cleaning. The facility has since installed a new fine screen to hopefully reduce the cleaning frequency by means of better screening. If thickening of the FOG/scum is desired, then accepting the FOG at a separate location is ideal given that it isolates the material that the heat benefits: the FOG. If the facility accepts FOG with other waste receiving, then the mixture of material (FOG, scum, etc) would all need to be heated and mixed. The scum could include undesirable material (floatables) that may lead to increased maintenance on heating and mixing equipment. In addition, scum collection generally includes a lot of water for flushing/pumping. Heating this extra water would result in extra operational costs unless thickening/decanting occurs prior as utilised by HRSD. If enough excess hydraulic capacity exists in the digestion system, then the non-thickening approach utilised by the EJWWTF could allow for minimal capital investment from the handling and receiving facilities as a first-step approach. Once the digestion facilities reach capacity or a major upgrade is required, then a separate handling facility could be included. CONCLUSIONS With energy prices predicted to substantially increase in the near feature, the drive for energy recovery will motivate innovation in the handling of FOG wastes. Therefore, it is essential to build

5 experience into standards of design practice to ensure that these systems are successful. As with any new practice in wastewater treatment, it will be several years before such experience is brought together into a body of knowledge and standards of practice. This paper will reduce the time required to move the information to those needing it today. REFERENCES Bailey, R. S Anaerobic Digestion of Restaurant Grease Wastewater to Improve Methane Gas Production and Electrical Power Generation Potential. Proceedings of the 80th Annual Water Environment Federation Technical Exposition and Conference; San Diego, California, Oct 15 17; Water Environment Federation: Alexandria, Virginia. Downing, L., Shea, T., Chung, G., and Kabouris, J Direct Addition of High-Strength Organic Waste to Municipal Wastewater Anaerobic Digesters. Technical Practice Update Document. Gabel, D., Pekarek, S., Nolkemper, D., Kalis, M Sustainability Incorporated into the Solids Handling Improvements of the Douglas L. Smith Middle Basin Wastewater Treatment Plant. Presentation at the WEF Residuals Biosolids Conference, Portland, OR. Kabouris, J. C.; Tezel, U.; Pavlostathis, S. G.; Engelmann, M.; Dulaney, J.; Gillette, R. A.; Todd, A. C The Ultimate Anaerobic Biodegradability of Municipal Sludge and FOG. Proceedings of the 80th Annual Water Environment Federation Technical Exposition and Conference; San Diego, California, Oct 15 17; Water Environment Federation: Alexandria, Virginia. Shea, T., Cooley, D., Burrowes, P Feeding FOG to Thermal Oxidation Units. It s all in the Presentation. Presentation at the WEF Residuals Biosolids Conference, Savannah, GA. ACKNOWLEDGMENT The authors would like to acknowledge the following for their contributions: David Cooley, Hampton Roads Sanitation District, VA Susan Pekarek and Doug Nolkemper, Johnson County Wastewater, KS Mike Englemann, Pinellas County Utilities, FL Jim Jutras, Village of Essex Junction, VT Wastewater Treatment Facility

6 Table 1: Characteristics of liquid and semi-solid FOG wastes 3 Restaurant Biodiesel Interceptor glycerin FOG Waste FOG Component Polymer Dewatered Lime Dewatered FOG Total Solids, %TS Volatile Solids/Total Solids, % Chemical oxygen demand, g/l 1,160 1,211 1,030 Total nitrogen, g/l 5.4 Total phosphorus, g/l ph Volatile solids destruction potential 1, % Methane content of generated gas, % Methane (CH 4 ) potential yield 2, m 3 /kg (16.8) (14.8) (scf/lb) feedstock volatile solids Biogas potential yield 2, m 3 /kg (scf/lb) feedstock volatile solids (22.4) (19.9) 1. Maximum potential volatile solids destruction and yield based on long-term (120 days batch) 2. At 60 F and 1 atm 3. From Bailey (2007); Parry et al. (2009); Kabouris et al. (2007) Table 2: Reference facilities with specific elements of FOG handling 1 Pinellas County, Parameter EJWWTF, VT FL 2 HRSD, VA3 JCW, KS 4 Facility Size 3.3 mgd 33 mgd 22.5 mgd 14.5 mgd FOG Quantities Received 2,000 gpd average day and 9,500 gpd peak day. 2,000 6,000 dry lb/day of dewatered FOG, 5 days/week 4,600 gpd average and 6,805 gpd peak 12,400 gpd average day and 30,000 gpd peak day Off-loading Transfer to storage Storage Quick connect hose connection By gravity to primary scum pit Existing primary scum pit (3,000 gallons) Quick connect hose connection Rotary lobe Pump 5,000 gal. dewatered FOG storage tank at SCBWRF Mixing None Recirculation of stored FOG using rotary lobe pump Heating Digester or Thermal Oxidation Feeding Load Tracking Wash down None Positive displacement pump feeds FOG & scum mixture to digesters Monitor haulers Plant water hose bibb available, if needed. Temporary heating used during winter. Rotary lobe pump used to pump to digesters recirculation pipeline. Monitoring FOG volume received at FOG dewatering facility Plant water available. Combined with Septage Receiving Collected with primary scum in primary clarifiers Within Scum Concentrator and Hopper None Hot water heat exchangers to maintain FOG temperature to abut 130ºF Pumped to mix point of dewatered solids conveyor to feed MHF Monitor haulers Plant water available Quick connect hose connection Combination in line grinder/rotary lobe pump Three 15,000 gallon fibreglass reinforced plastic or polyethylene tanks inside a building Recirculate FOG waste using rotary lobe pump through the heat exchanger and grinder; Hot water heat exchangers to maintain FOG temperature to about 80ºF Progressing cavity pumps will feed FOG to digesters. Panel containing key pad access; track by company, driver and load; security camera at unloading station. Power sprayer available. Concrete access area; wash- down water drains

7 Table 2: Reference facilities with specific elements of FOG handling 1 Pinellas County, Parameter EJWWTF, VT FL 2 HRSD, VA3 JCW, KS 4 to plant drain system. Odour Control None. Scum pit is covered. Cartridge filter installed on the vent line of the storage tank None 1. Personal communication with Jim Jutras, Personal communication with Mike Engelmann, Shea et al, Gabel et al, 2009 Biotower with carbon