BioCycle December 2010, Vol. 51, No. 12, p. 45
Installation of a bioreactor to pretreat fats, oil and grease prior to loading into anaerobic digester yields high net energy production.
Richard York and Joe Magner
SIX years ago, the City of Millbrae, California embarked on an initiative to increase biogas production to offset energy purchases by generating electricity on-site at the wastewater treatment plant. By implementing a fats, oils and grease (FOG) receiving program to codigest trap waste and wastewater solids, biogas production more than doubled. The additional biogas enabled Millbrae to fuel a .25 MW Ingersoll Rand microturbine, providing electricity for the plant with heat produced by the generating equipment used to support wastewater treatment processes. Today, the Millbrae plant is effectively off-grid – only purchasing the minimum electricity required under regulatory requirements, saving ratepayers money and contributing to the city’s renewable energy goals.
The City of Millbrae owns and operates a Publicly Owned Treatment Works (POTW) serving a population of approximately 20,300. One of the smaller cities in the San Francisco Bay Area, Millbrae is about 17 miles south of San Francisco. Currently, the annual average daily influent wastewater flow to the plant is 1.8 MGD. The POTW, located just upwind of two high-end hotels, must process wastewater without releasing odors. Liquid stream treatment processes include: raw wastewater grinding and pumping; primary sedimentation, followed by conventional activated sludge incorporating anaerobic selectors; secondary clarification; chlorine disinfection; and dechlorination with final discharge into the San Francisco Bay. Solid stream treatment processes include: centrifugal grit removal; gravity thickening; thickened sewage solids screening; anaerobic digestion; and belt press dewatering with final land application of the biosolids.
For over 20 years, the Millbrae POTW used a combined heat and power (CHP) unit to generate electricity and heat from the digester biogas. Because of its age, obtaining parts for the unit had become difficult and the system was experiencing increased downtime. Additionally, the local air board had ratcheted down air pollution requirements so low that the engine could not meet them. Unless an alternative solution was found, the plant would have to begin paying full fare for electrical purchases because it could no longer generate part of its electrical need.
An energy services company (ESCO) was consulted to evaluate the treatment facility to determine if energy savings could be realized from conservation or production using modern CHP equipment. Neither was an option as all conservation measures possible had already been incorporated and the facility was deemed too small to make enough biogas to financially support a modern CHP unit that included complicated fuel cleanup equipment.
The obvious solution was to somehow make more biogas by leveraging the excess anaerobic digester capacity at the POTW. As a small wastewater plant, staff was cautious about selecting feedstocks that could drive a substantial increase in biosolids handling and disposal costs. The POTW chose to receive and process grease, a high-energy content feedstock proven to be digestible in a wastewater anaerobic digester. However, staff was wary of the substantial risks it could introduce into the operation.
FOG GREASE EVALUATION
Historically, POTWs have always received and processed grease from wastewater influent (called black grease), often arriving unexpectedly in large clumps. In fact, the US EPA’s 2008 report to Congress on combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs) identified that grease from restaurants, homes and industrial sources is the most common cause (47%) of reported blockages.
Making the case for separating grease from wastewater before it enters the treatment plant with the influent is simple. Because grease floats, sticks and stinks, it is a problematic and nearly impossible material to handle without clogs, odors or other problems. Grease imposes significant oxygen demand on aerobic wastewater treatment processes, driving up the cost to supply oxygen. Since grease floats it accumulates in undesirable locations in clarifiers and tanks causing them to lose efficiency and requiring significant and arduous labor to remove. Additionally, FOG-laden influent stimulates the growth of undesirable and harmful filamentous microorganisms in the activated sludge process that can result in poor effluent quality. Consequently, many communities have implemented programs to divert FOG from the influent to keep the material from clogging sewer systems and fouling wastewater treatment operations.
It is well documented that POTWs can stabilize grease anaerobically. Because it is not soluble in water and forms clumps, grease tends to separate from the sludge and float on the surface inside the digester where it remains until manual removal. Intense mixing can integrate floating grease back into the actively digesting sludge. However, even digester designs with intense mixing systems are limited in the quantity of grease that can be safely received and processed – the literature typically suggests digestion of less than a 30 percent grease to sludge ratio.
Despite the substantial risks of intentionally bringing FOG into the wastewater treatment plant, the fact that it is among the most energy-dense feedstocks available swayed senior staff at the Millbrae POTW to thoroughly investigate its use for digester feed augmentation. Laboratory research indicated that brown grease was substantially higher in Btu content when anaerobically digested than food waste, yellow grease, whey, cow manure and other feedstocks available in quantity. (See sidebar for a “Grease Glossary.”)
Staff also learned there is a significant difference in the chemical composition between black and brown grease, although both are incredibly nasty to handle. Black grease, often arriving at the plant in a slug after the collection crew has cleaned a blocked pipe, is brown grease that has reacted inside the collection piping to become a biologically stable, soap-like material, much of which passes through the anaerobic stabilization process into the dewatered biosolids untreated. On the other hand, brown grease separated at the source offers the prospect that its addition to an anaerobic digester feeding regime can help meet biogas production goals.
While controlled receipt of fresh brown grease was clearly preferable to the black grease that enters the plant in a slug, any type of grease is still a difficult feedstock to handle. POTW staff was confident all the problems could be overcome with a well-planned methodology and well-designed system comprised of FOG receiving, preconditioning and continuous feeding.
Lastly, a quick survey of local haulers showed that brown grease was available in significant quantity since most communities required programs to remove FOG from the collection system at its sources – restaurants, hotels and commercial food processing facilities. The survey found considerable demand for a local Bay Area disposal site, as the material has not been suitable for any other energy conversion process, such as biodiesel production.
BUILDING A SUCCESSFUL FOG PROGRAM
The Millbrae plant’s uniqueness gave the program a number of advantages out of the gate. Since the facility was adjacent to a major freeway and centrally located in the San Francisco Bay Area, obtaining trap waste in quantities would be straightforward. Additionally, the plant had ample anaerobic digester capacity with which to experiment with varying levels of feedstock augmentation. It was calculated that digester capacity (hydraulic retention time (HRT) of approximately 71 days) would enable the introduction of 3,000 gallons/day of processed FOG while reducing digester detention time to a still substantial 58 days.
Investigation of sites with existing grease programs found systems had issues with odors and/or shock loaded digesters with batches of trap waste, or required substantial manual intervention to manage clogging issues. Data on digestion times and yields was insufficient. Other systems required haulers to preprocess or render the trap waste, or pressurize trucks at additional cost. None of these issues were acceptable at Millbrae, so it was decided to create a new design to address the key challenges in implementing a FOG program.
The design process centered on the following key priorities: 1) Change the FOG waste biologically to make it miscible (capable of being mixed) and easily digestible prior to introduction into the digester to avoid the need for heated tanks, added chemicals or enzymes; 2) Fully automate trap waste offloading so drivers do not need assistance from plant staff; in addition, no pretreatment, dewatering or rendering prior to delivery would be required; 3) All piping contacting the FOG must remain free from clogs to assure uninterrupted process capability; 4) System must feed the digester continuously to avoid shock loading or digester upset and encourage development of healthy biology to digest grease with steady biogas production; 5) No odors can be produced; and 6) System must be integrated operationally to allow system monitoring and adjust feeding rates according to conditions in the plant.
Based on these priorities, the following system was installed (Figure 1):
FOG Receiving Station: The receiving station is located outside the plant perimeter for security and safety reasons, i.e., keeping truck traffic outside the plant. The station has an ATM-like user interface, enabling drivers to swipe a security card and enter a pin number to start the offloading process. Once activated, a PLC (programmable logic controller) takes over, and completely automates the offloading process and documentation of the truck. To avoid clogs, the receiving station is designed to preflush warm, actively digesting sludge (“ADS”) from the high-rate circulation loop through the receiving pipes.
The high rate circulation loop (pipe) begins at the tail end of the digestion process (the bottom of the second of two digesters operated in series). It carries actively digesting sludge to the receiving station then to the bioreactor – from which the flushing/filling volume is taken and into which the miscible slurry is metered – then returns back into the normal digester feed introduction location. (Ideally, the raw feed sludge will be mixed into this loop ahead of the digester so it too becomes inoculated and incorporated into the miscible slurry.) Figure 1 illustrates this high-rate circulation loop.
FOG Bioreactor: Trap waste and warm ADS flow to a bioreactor where aggressive mixing reduces grease to a particle size that enables conversion of the entire load of FOG – grease, water, carbohydrate, protein and lipids – into a completely miscible slurry (does not separate, float, or stratify). Odor is controlled by neutralizing the pH of the acidic FOG. Methane production within the reactor is prevented by introducing a small amount of outside air into the process to inhibit the activity of methanogenic microorganisms. As a second level of odor control, all of the air exiting the system passes through a biofilter before release to the atmosphere.
Continuous Feeding System: Conditioned loads stored in the bioreactor – inoculated with the very microorganisms that will soon become active in the anaerobic environment of the digester – are evenly fed into the high-rate circulation loop of warm ADS to promote optimal digester health. Because trap waste deliveries vary in volume and frequency, the operator and PLC controlled variable rate introduction pumping system ensures that material is continuously and evenly introduced into the digester environment on a 24-hour a day basis. At Millbrae, the operators can meter in as little as one gallon per minute (gpm) up to 15 gpm of conditioned FOG, further diluting the conditioned FOG with actively digesting sludge. This also prevents adding extraneous fluid into the digester to dilute its contents.
The additional dilution step of feeding conditioned FOG continuously into the high rate circulation loop reduces the VSS (volatile suspended solids) concentration of the introduced conditioned material to a level well within the customary range of high rate anaerobic digester loading. This eliminates the customary dependency on digester mixing, ensuring rapid evolution into biogas and avoiding potential problems from introducing concentrated feedstock directly into the digester. An additional benefit is that the receiving station, bioreactor and continuous feeding system could be located any distance from the digester. In Millbrae’s case, that was more than 400 feet from the anaerobic digesters – important for its already crowded plant property.
HOW DID MILLBRAE FARE?
The Millbrae POTW has documented numerous process improvements using the FOG receiving methodology – some as planned and some pleasant surprises. With the FOG program in place for nearly four years, Millbrae has more than doubled biogas production, and raised methane concentration from 65 percent to greater than 72 percent. The problems of odors and plugged piping were completely avoided, and the digesters are very healthy. With this large volume of high quality biogas, Millbrae has been able to offset grid purchases of electricity and natural gas, effectively going “off-grid” excepting for minimum regulatory purchases.
Beyond increased biogas production, there were additional benefits that were not initially predicted. With improved biology in the digesters, Millbrae has fully documented substantial residual biosolids destruction – an average improvement of 23 percent. Reducing the biosolids a facility handles and disposes by 23 percent reduces dewatering needs and significantly reduces costs associated with the land application program, adding to the environmental and economic benefits of the system.
As designed, the FOG receiving system installed at the Millbrae facility is fully automated and requires very little operator input and has received positive feedback from the hauling community. Millbrae has a long-term contract with Liquid Environmental Solutions that assured regular deliveries of the raw feedstock in the amount needed for continuous feedstock augmentation. The contract also is a good tool to manage and assure feedstock quality and consistency and stable tipping revenue. In addition, working with a consistent set of drivers employed by one hauler minimizes the risk of spills, tainted feedstock or improper use of the system.
RAPID RETURN ON INVESTMENT
The FOG program at Millbrae was part of a $6.3 million facility upgrade that included the bioreactor and receiving station, new biogas conditioning and methane storage equipment, a new microturbine, digester mixing system, and switchgear upgrades in addition to replacement of existing wastewater treatment plant infrastructure like the digester hot water heater and old main switchgear. The increased biogas production directly resulting from the FOG system installed creates sufficient revenue to cover the debt service for the entire capital project (including the needed wastewater plant infrastructure).
Based on initial estimates of the value of anticipated, FOG volumes and biogas yield, the project was expected to pay back in roughly 12 years (not including the replacement wastewater treatment plant infrastructure). With the significantly better than expected biogas yield and the ability to safely raise inbound grease levels to over 50 percent of VSS digested, coupled with the savings from biosolids reduction, the entire program is anticipated to pay back in less than 8 years. When looking at the FOG receiving station and bioreactor costs only, the initiative paid for itself in less than two years.
Richard York is retired Plant Superintendent of the Millbrae, CA WWTP and Chief Technologist of FOG Energy Corporation. Joe Magner is WWTP Plant Superintendent at the City of Millbrae.
FOG: Acronym for “Fats, Oils, and Grease,” often interchanged with trap waste
Yellow Grease: Deep fryer grease or oils
Brown Grease: Grease found floating in a restaurant grease trap
Black Grease: Grease congealed inside sewer pipes
Trap Waste: Sewage (water and organics) and brown grease from a grease trap, often used synonymously with FOG
IKG: Acronym for Inedible Kitchen Grease
POTW SUPERINTENDENT TO ENTREPRENEUR
THE positive results of the FOG Program at the Millbrae, California POTW led system designers and Millbrae WWTP staff Richard York and Joe Magner to turn their invention into a company. FOG Energy Corporation markets the bioreactor York and Magner invented. They were awarded a U.S. patent in February 2009 for the unique method of receiving, processing and metering FOG into a digester. FOG Energy began operations and is working across the United States in partnership with major engineering firms and energy service companies to implement the FOG Energy System. The company enjoys the backing of experienced Silicon Valley technology investors, and has taken the initial invention from Millbrae and scaled it to meet the needs of wastewater treatment plants of any size. Current and proposed installations will process as little as 3,000 gallons/day of FOG up to more than 70,000 gallons/day.
December 22, 2010 | General
FOG Boosts Biogas Production At Small POTW
BioCycle December 2010, Vol. 51, No. 12, p. 45