May 17, 2010 | General

Anaerobic Digestion Of High Strength Organics Solves Treatment Plant Challenges

BioCycle May 2010, Vol. 51, No. 5, p. 45
Thinking outside the box at the Waco WWTP leads to big time improvements in processing HSO and FOG with less stress on the plant and decreased electrical usage.
Michael Jupe and Sally Brown

THE Waco Metropolitan Area Regional Sewerage System (WMARSS) Plant serves the people of Waco, Texas as well as neighboring municipalities. The average influent flow is 25 million gallons per day (mgd) with a rated capacity of 37.8 mgd. On the surface, the plant looks like many other regional wastewater treatment plants (WWTP) with primary clarifiers, aeration basins and secondary clarifiers for the liquids and digesters and heat drying for the solids. The plant used to function like most other WWTPs with the same headaches commonly experienced in these facilities.
WMARSS receives a lot of fats, oil and grease (FOG) and high strength organics (HSO) from food processing industries that discharge into the system. For example, average loadings of biological oxygen demand (BOD) into the plant are 47,200 lbs/day. Of this, 33 percent or 15,555 lbs BOD/day are from industrial dischargers. These influent streams regularly clogged pipes leading to the plant, resulting in overflows during storm events.
The HSO also caused problems inside the plant, with high oxygen demand during secondary treatment testing the capacity of the system and using large quantities of electricity to keep tanks oxygenated. These HSO were partially responsible for a 500 percent increase in energy costs since 2000. Digester capacity was also tested and the HSO moving through the system made the solids tough to dewater.
And here is what makes the WWTP in Waco unique: The operators, instead of just getting headaches from these challenges, thought outside the box and figured out a way to turn the headaches into opportunities. Working with Carollo Engineers, treatment plant managers and staff looked at the system with a cradle to grave approach. By controlling the quantity of organics entering the plant and where they enter the system, and viewing the solids processing and side stream flows as business opportunities, WMARSS has come up with a novel approach that can serve as a model for WWTPs across the country.

The city of Waco’s grease collection program has two parts. The first includes a FOG ordinance that regulates the food industry’s FOG trap sizing and removal frequency performed by local waste hauling companies. The city wanted to provide a local disposal site for the FOG but does not mandate that it be delivered to the WWTP. The second part is in the infancy stage. While large producers of used cooking oil usually have a service that takes care of their oil, residents and small producers usually do not. This can result, at times, in the used oil becoming either a sewer or storm water issue. The city set up used cooking oil drop sites for residents and provides pick up service for the small producers. This service has helped reduce the quantity of FOG dumped down drains and clogged pipes leading to the plant.
The primary innovation with HSO, in place since 2006, is that WMARSS has worked with the industrial dischargers in the area to provide an alternative means for organics to be introduced into the system. Rather than direct discharge into the pipes, the industries send trucks – an average of six per day – of the high strength organics to the plant. These trucks contain animal blood (turkey processing plant), grease trap waste, candy waste, peanut oil and biodiesel residuals such as glycerin. At the plant, these wastes are introduced into the anaerobic digesters, skipping secondary treatment completely. This uses the high BOD of these materials to the plant’s advantage rather than having them stress the secondary treatment system and increase electricity usage.
WMARSS has three primary digesters and one secondary digester, all mesophilic with capacity of 1.43 million gallons each. An unused 40,000-gallon tank was converted to a receiving station for the FOG, HSO waste and used cooking oils.
Each of these materials has significantly higher methane generation potential than the traditional digester feedstocks – primary and waste activated sludge. Carollo Engineers helped establish the digestibility of the various waste streams and predict the methane production. For example, after testing the candy waste, one daily 5,000 gallon shipment ramped up to five 5,000 gallon shipments per day.
By adding materials directly into the digesters, 30 percent more biogas is being generated. For example, the power potential of grease trap wastes is over three times higher than the sludge feedstocks. Keeping these materials out of the secondary treatment process also has reduced electricity usage. Since this diversion has taken place, overall electricity use and associated operating costs have decreased by 30 percent. Other measures also have resulted in decreased energy use, including optimizing efficiency of the aeration system in the secondary treatment. Total electricity use and associated savings post-HSO diversion are shown in Table 1.
Adding these materials directly to the digesters involved notifying and gaining approval from the state’s wastewater regulators. While there were questions initially on how direct addition of HSO to the digesters would affect total capacity, changes were made in the retention times for materials in the digesters, decreasing from 33 days to 29 days. This was due to the additional hydraulic loading and the fact that the HSO and other materials digest more quickly than the biosolids.
WMARSS is working to maximize the benefits associated with the additional biogas production. Fifty percent of the biogas from the digesters is used as fuel for the biosolids dryers, reducing use of natural gas in the furnace. The system is being upgraded so that the dryers can be fueled by 100 percent digester gas. The plant has three 500 kW Caterpillar cogeneration engines that provide electricity and heat. It plans to install a biogas conditioning system (primarily to remove hydrogen sulfide) this fall. Recovered heat from the gas engines provides all the heating requirements of the digesters. WMARSS is considering the potential to start selling power back to the grid.
The dried biosolids are sold to local farmers; demand for the biosolids exceeds supply. The plant is also looking into developing a composting operation.

One early challenge experienced was the FOG that had been clogging pipes leading to the WWTP was now clogging pipes as the trucks discharged their loads directly into process piping at the plant. The solution was to use the heat from the sludge in the digester to liquefy the FOG. A closed loop system was designed. These wastes are pumped at rate of 100 gallons/minute (gpm) into a closed loop maintained at 98°F that flows at 500 gpm. The closed loop premixes the materials with sludge pulled from the digester, and then pipes the mix back into the digester. The 98°F sludge in combination with the bacteria keep material from sticking to the pipes, keeping them clean.
These HSO also added additional nitrogen to the digesters, nitrogen that previously had been lost via volatilization during secondary treatment. The nitrogen addition resulted in an unanticipated consequence – high ammonia concentrations in the filtrate from dewatering the digested solids. Currently the high ammonia liquid is treated using a trickling filter. A system is being installed to treat the excess nitrogen contents of the filtrate to reduce N concentrations in the digester effluent.
The new treatment process is based on the Ludzak Ettinger process. Water from the digesters (high ammonia) is combined with waste activated sludge (high bacterial content) and overflow from the primary gravity thickener (carbon source plus alkalinity). Initial anoxic conditions force the bacteria to use the nitrate as an electron acceptor, resulting in reduced BOD and denitrification. The liquids are then exposed to oxygen which further reduces BOD and oxidizes any ammonia in the system to nitrate. The high nitrate, low BOD water is directed to the main plant’s influent where the high nitrate and low BOD is beneficial for the treatment process. WMARSS plans to add a step to reduce phosphorus concentrations in the effluent as well in anticipation of more stringent regulations for effluent discharge.

The move to attract HSO directly into the plant had impacts outside of the existing client base. The tip fees charged to the industrial dischargers for bringing material directly to the plant were sufficiently competitive to attract additional waste streams. The volume of HSO materials brought directly to the plant increased from just under 20,000 gallons per day in June of 2008 to over 30,000 gallons per day in November 2008.
The treatment plant is planning to expand capacity for HSO in anticipation of future demand. Rotary drum thickeners (RDTs) have been installed to increase the solids content of the sludge being fed into the digesters. Material is currently 4 percent from the WWTP’s dissolved air floatation unit; the RDTs yield solids between 7 and 10 percent. The goal is to overthicken the plant sludge to compensate for the lower solids content (3 percent) but high BOD of the HSO and FOG, thus minimizing release of the high BOD in the filtrate water during thickening. This eliminates trying to thicken the many different types of HSO, which could require use of different polymers and thickening technologies. Thickening the plant sludges balances out the two streams.
Waco also plans to recommission three mothballed digester tanks to increase its capacity to process HSO, FOG and plant sludges. Upgrades are being made to the mechanical equipment – including mixers, pumps, piping, digester covers and heat exchangers – at a cost of $2.5 million. When finished, WMARSS will be able to accept an additional 200,000 gallons/day of HSO and FOG.
Recommissioning the digesters enables Waco to physically separate the stages of anaerobic digestion. Anaerobic digestion is a multistage process: 1) Hydrolysis, where a range of organics are partially decomposed in reactions that involve the chemical transformation of water; 2) Fermentation, which involves two reactions – acidogenisis and acetogenesis – producing acids and acetate; and 3) Methanogenisis, where methane is produced. Normally these reactions occur in the same vessel, sometimes simultaneously and often in sequence. This has the potential to expose the methane-producing bacteria to excess acid, resulting in a sour digester and cessation of methane production. This reality is the source of nightmares for digester operators.
The mothballed digesters are going to be used for the first two parts of the digestion reactions, hydrolysis and fermentation. The final step will take place in digesters dedicated to only methanogenisis. This will allow the first set of digesters to become acidic while maintaining a basic pH in the methane digesters. This should increase the efficiency of the reaction and reduce the potential for digesters going sour.
WMARSS also plans to install innovative equipment to reduce operating and maintenance costs of the recommissioned digesters. These include linear motion mixers and tube in shell heat exchangers with static mixers. The former will provide a more efficient means to mix materials in the digesters; the latter is a way to optimize transfer of waste heat to heat the newly commissioned acid digesters.

What does all of this innovation mean for the Waco WWTP? Increased revenues, reduced operating costs and more green energy is the short answer to that question. For example, current methane production powers one generator at 300 kilowatts. At 13.5 cents/kWh, this has resulted in annual savings of $340,200 – plus WMARSS is saving $120,000 in natural gas to heat the digesters, and offsetting 50 percent of the heat needed for the dryer operations.
It also has increased the WWTP’s capacity. In 2000, an outside plant engineering study recommended the treatment plant be derated to 23-25mgd because of the high influent loading and inadequately reduced ammonia levels in the effluent. The study estimated $36 to $50 million in plant expansions to maintain the 37.8mgd rating. Direct injection of HSO into digesters meant that the plant no longer had this high BOD waste stream to treat during the initial stages of wastewater treatment. This effectively increased the capacity of the plant without any need for an expensive expansion. The plant is slated to begin expansion from 37.8 mgd to 45 mgd in summer 2010. This expansion requires installation of peak flow storage with hydraulic automation. The 7.2 mgd increase would normally cost around $7.00 per gallon, but the optimizations, both implemented and proposed, have allowed for the increased capacity at a reduced cost of $6 million or $0.83 per gallon. This innovation also has improved performance of the plant from a regulatory perspective.
The operation in Waco shows how innovative approaches can simultaneously achieve the plant’s primary function while optimizing energy production and reducing its carbon footprint. By taking an innovative approach to finding a solution for problems resulting from large quantities of high strength organics entering the plant, these materials have turned from a detriment to an opportunity.

Mike Jupe is Program Administrator/Plant Superintendent for the Waco Metropolitan Area Regional Sewer System. Sally Brown is Research Associate Professor at the University of Washington in Seattle.

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