By Rudy Kilian, Kristy Wolter and Mike Jupe
As many municipal wastewater facilities have discovered, especially those that provide centralized regional treatment, high-strength organic wastes are major contributors to grease blockages and corrosion, which often result in sanitary sewer overflows in the collection system. Also, when high-strength organic materials are discharged into sewer systems, they stress loading to the liquid stream and increase a treatment plant's electrical demands–in addition to requiring more time and higher costs for maintenance.
The staff at Waco Water Utility Services, serving the City of Waco and surrounding communities, looked for an alternative disposal method to reduce the amount of high-strength organic wastes in their service area's sewers. They ultimately identified a multifaceted solution–permitting local industries and restaurants to truck their materials in concentrated form directly to the Waco Metropolitan Area Regional Sewerage System (WMARSS) Wastewater Treatment Plant (WWTP) for the material to be treated in the anaerobic digestion facility. Currently being upgraded to 45 mgd from 37.8 mgd, the WMARSS WWTP has an annual average plant flow of 29.32 mgd.
On average, 26,000 gallons per day of high-strength organic waste are hauled to the WMARSS WWTP on a five-day per week basis.
Carollo Engineers was selected by the City of Waco to provide consulting engineering services to technically support this initiative using high Btu organic waste, including fats, oils and grease (FOG), aimed to increase methane production for the cogeneration of "green energy" through a newly modified digester process. This waste-to-energy initiative would help the plant reduce its reliance on purchased electricity and natural gas and reduce its operating costs, both of which keep customer rates manageable.
The implementation of this waste-to-energy project is the culmination of several years of research and testing with the digesters at the heart of the WMARSS WWTP system. In 2006, Carollo Engineers initiated the set-up of a new digester operation as the foundation for the WMARSS high Btu organic waste disposal and energy reuse initiatives. The firm prepared a pilot test to identify the protocols needed to receive and feed this waste into a newly modified digestion complex, and allow the collection of data from the digesters to determine the net effect.
Later that same year, when a Waco-area candy food processor was also searching for alternative disposal options to reduce its costs, WMARSS used the manufacturing plant's waste in tests for digester compatibility. The high Btu organic materials were fed directly into the digesters. The results: (1) a considerable increase in digester gas to fuel a solids dryer and reciprocating engines for electricity generation of 0.5 MWh per mgd and (2) a decrease of 1.1 MWh per mgd in the amount of purchased electricity per treated wastewater volume.
In 2007, WMARSS and Carollo completed the construction of a high Btu organic waste receiving station and modifications to its existing digesters to receive the organic waste on a daily basis. As a protocol, high-strength organic waste is hauled to the WMARSS Central WWTP and discharged into a 45,000-gallon waste holding tank. The waste holding tank contents are ground, pumped into a digester sludge recirculation line, and distributed to three primary digesters, each with a capacity of 1.4 million gallons.
Primary sludge undergoes two-stage thickening (gravity followed by rotary drum) prior to anaerobic digestion. Secondary waste-activated sludge (WAS) is thickened in the rotary drum thickener (RDT) prior to digestion. Filtrate from the sludge dewatering is combined with gravity thickener supernatant prior to sidestream treatment. Presently, sidestream treatment is accomplished in a nitrifying trickling filter and a final solids clarifier, although a new denitrifying basin for sidestream treatment is under design. Sludge from the sidestream treatment final solids clarifier is thickened (by gravity and/or by rotary drum) prior to anaerobic digestion.
There are three methods of final sludge disposal: (1) the sludge dryer/pelletizer; (2) surface disposal units; and (3) landfill. Digested sludge is pumped to a belt filter press or directly to the surface disposal units. The sludge dewatered by belt press is dried to 92 percent solids in a sludge dryer/pelletizer, which produces a pelletized sludge. The landfill option is only used under emergency conditions.
From the start of the high-strength organic waste co-digestion project, there have been no digester upsets, and the feed process has been very successful. Feeding to the digester sludge recirculation loop prevents clogging of pipes and provides good incorporation with the digester contents. In the early stages of the project, however, there were slight digester foaming problems resulting from not effectively operating the digesters. Also, FOG that entered the plant's influent tended to clog the sludge processing piping. These development challenges were addressed and solved by enhanced monitoring of the digester operations and by using the methane gas more effectively.
On average, 26,000 gallons per day of high-strength organic waste are hauled to the WMARSS WWTP on a five-day per week basis. The digesters continue to show good treatment performance, with an average volatile solids reduction of 56 percent. The dewatering performance of the belt filter press units was not affected by the high-strength organic waste co-digestion, producing a cake with an average solids concentration of 15.3 percent.
The co-digestion operation has been running full-scale since 2007. Currently, the WMARSS WWTP does not have digester gas meters. However, the digester gas available for the cogeneration engines and the solids drying facility increased substantially. As a result of the increased gas availability, the City of Waco modified the solids dryer fuel system to be able to operate with 100 percent digester gas. The previous fuel system operated with a 50/50 blend of digester gas and natural gas. The dryer is rated at 17 MMBtu per hour and can process 24 dry tons of biosolids per day. WMARSS operates three 500-kilowatt reciprocating engines. The heat recovered from the engines' water jacket is used to heat the digesters.
Along the same timeline of this project, WMARSS started working with other Waco-area industries that discharge high-organic loadings into the collection system as a cooperative venture to reduce plant loading and increase methane production. Many of these industries are now working directly with WMARSS to receive a variety of organic wastes, such as blood, food waste, FOG, and bio-diesel waste.
Anaerobic digesters with gas holding covers full of gas. Since 2007, digester gas available for the cogeneration engines and the solids drying facility have increased substantially.
The waste-to-energy project is continually growing with new initiatives. For example, Carollo and WMARSS are conducting pilot tests for pretreatment and co-digestion of localized byproducts, such as algae and feathers.
City Promotes FOG Disposal Program
When the City of Waco initiated a citywide FOG disposal ordinance for grease trap waste, it recognized that it also needed environment-friendly disposal areas for industrial and restaurant FOG waste. The three locations the city designated for haulers to dispose of FOG were the area's two landfills and one location in Fort Worth. This also helps the WMARSS industrial receiving station save traditional treatment capacity in the plant, which helps postpone the need for expansion.
The City of Waco wanted the FOG program to also have re-use benefits to reduce the costs associated with treatment. These benefits would assist WMARSS participating cities in maintaining affordable sewage rates for customers. WMARSS took the same approach for what they did for the high Btu organic process. Carollo designed a new sludge thickening facility for the WMARSS plant and a two-phase digester complex to increase its digester capacity (currently under construction).
An ongoing campaign that was inspired by the industrial collection program is a unique residential cooperative aimed at reducing household disposal of FOG into the wastewater system. There are five yellow-oil recycling stations located throughout the city, which provide participants with a free and environmentally responsible method for disposing of their used oil. The campaign's initial outreach, named the "Green Turkey Initiative," started only three days before Thanksgiving and has collected more than 500 gallons of used cooking oil to date.
About the Authors: Rudy E. Kilian is Process Lead and Manager for Carollo's Advanced Digestion Group. He has 16 years of experience in wastewater design, operation and construction. Michael Jupe is the Wastewater Treatment Administrator of the Waco Metropolitan Area Regional Sewerage System. Jupe holds a Class A wastewater license and has 24 years experience in wastewater treatment plant management, operations and plant improvements. Kristy Wolter is the Operations Program Manager of the Waco Metropolitan Area Regional Sewerage System and oversees collection, distribution and wastewater treatment.
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