Akron, Ohio – a high solids anaerobic digestion process replaces a successful In-Vessel composting operation

In 2010, 62,387.7 m³ (81,600 yd³) of Exceptional Quality compost was generated which was very representative of the previous 25 years' worth of production. In August of 2011, the facility experienced a fire that shut down its composting operations. As a result, any solids not being processed by the 4535 dry t (5000 dry ton) anaerobic digestion (AD) plant were dewatered and hauled to a landfill. It was not until June of 2012, that the facility was able to compost once again but was limited to 50% of its normal operations. Any solids not able to be processed either thru the AD process or the smaller composting operations footprint were sent to the landfill. At the beginning of 2013, the composting operations was shut down permanently to accommodate a change in design of the 13,607 dry t (15,000 dry ton) AD facility that required removal of equipment used in the composting operations to make room for the indirect dryer. Solids once again were landfilled until the commissioning of the new AD plant which occurred in October of 2013.

Upgrades from the 4535 dry t (5000 dry ton) plant to the 13,607 dry t (15,000 dry ton) plant are evident in the Euco design as well as the Coccus design. In the Euco, a new heating arrangement inside the tank was deployed. The digester tanks are maintained at temperatures between 35 °C to 37.8 °C (95 °F to 100 °F) using the exhaust heat from the combined heat power units (CHPUs) that convert the biogas to electricity. In the original 4535 dry t (5000 dry ton) plant, the Euco tank had two means to maintain the ideal temperatures of 35°C to 37.8°C (95 °F to 100 °F) which consisted of using a tube and tube heat exchanger where hot water from the engine was used to maintain the temperature of the solids in the tank or heating occurred along the mixer shaft where heat radiated out from the apex tubing that was placed in the hollow mixer shaft. During the cold months of winter it was not possible to maintain ideal temperatures and on average the best was 29.4°C (85°F). Use of a tube and tube heat exchanger and the tubing inside the mixer shaft are still a part of the heating transfer but additional Brugg tubing has been installed along the walls of the new Euco tanks to guarantee meeting the optimum temperatures for digestion. The corrugated tubing provides more surface area as well as better transfer of heat since it is made out of stainless steel. Another improvement to the tank design was the feed hopper used in delivering the 30% dewatered solids. The previous hopper consisted of three screws that would auger the material into the tank. The new design feed system consists of a single screw that is positioned at an angle that allows the material to freely drop into the tank. This change in design allows for a better flow rate of material to the digester overall.

The overall design of the Coccus did not change. The difference was in the choice of gas holders used for the dome. For the 4535 dry t (5000 dry ton) plant a single membrane dome was used that was affected by the various changes in the weather. The wind, the amount of cloud cover and the volume of snowy/icy conditions all made it difficult at times to control dome pressure and ultimately resulted in fluctuations on the engine running. With the new plant a double membrane dome was installed. This new arrangement results in minimal affects from the environmental weather changes and as a result delivers a more steady flow of biogas to the engines. It is worth mentioning as well that the apex tubing inside the Coccus was also upgraded to the Brugg tubing like in the Euco for a higher efficiency in heat transfer between the water and the solids in the tank.

With the need to incorporate as much as possible of the existing infrastructure, the digested solids once leaving the system would be pumped back to three existing wells that had a total capacity of 852 m³ (225,000 gal). These three wells, at one time, held raw solids but were reconfigured now to take the digested solids. Likewise, the dewatering room that houses the belt filter presses for the dewatering of the raw solids was also reconfigured to house three centrifuges that are used in the dewatering of the digested solids. This would allow the use of the existing polymer system for raw solids to be used for the digested solids since it was found that the type of polymer being used would work on both type of solids and produce a cake solids very favorable for the indirect dryer operations.

The daily operational design for the feed solids is 66m³ (17,500 gal) of 5% raw solids delivered to each train as well as 54 wet t (60 wet tons) of 28% to 30% dewatered solids. At this feed rate, 20% of the total solids comes from the 5% sludge that is pumped into the units over a 24 hour period. In addition, there is multitude of times that material from the Coccus is scheduled to recirculate back to the Euco. At the end of the day approximately 123 m³ (32,500 gal) of digested solids is transferred out of each Coccus back to holding wells that will be dewatered again prior to introduction into the dryer. With the daily production of 246 m³ (65,000 gal) of digested sludge the dryer operations would produce between 26.8 and 30.6 m³ (35 and 40 yd³) of pellet-like material five days a week.

We will take a closer look into the last twelve months of operation and see how close the operations are to the design of the facility. In addition, a look at the challenges that occurred during the commissioning process and some unanticipated operational problems that required attention will be discussed. Finally, a look at how some promises made to the residents of Akron actually did come true!

As part of KBBE's contractual requirement to the City, it was necessary that the end product meet EPA's Class A Exceptional Quality standards. With this in mind, all solids discharged from the AD tanks are then dewatered using a centrifuge. These digested dewatered solids, in the range of 28% to 30% dry solids, are fed to the Komline Sanderson indirect dryer. The dryer design rate is 3764.8 kg/hr (8300 lbs/hr) operating 24 hours per day, five days a week. The dryer containing dual agitators with hollow paddles and shafts in a jacketed trough have 204.4°C (400°F) thermal fluid oil passing through it. The agitators slowly mix the sludge and as it passes through the dryer there is sufficient contact with the heating surfaces that produces a dried product with a minimum of 92% dry solids at a discharge temperature of 121.1°C (250°F). On the days the dryer is in operation there is approximately 22.9 m³ (30 yd³) of pellet like material being produced.

With the REF interconnected electrically to the WRF, any excess electrical generation not consumed on site is exported to WRF for their use in the operations. With the bi-directional meters operational for just the last third of 2014 it was shown that 36% of the power generated was exported back to WRF or 1149 Mwh. Using the average rate of $0.0799/kwh this was a savings of $91,800 just for those four months that the City recognized in their operations. It was estimated that in 2014 the WRF experienced a $245,000 savings in electrical costs. At the same time, the REF recognized an electrical savings of approximately $400,000 for the year.

Lastly, a decision to not house the variable frequency drive (VFD) units for the dryer motor and thermal fluid pumps showed to be a poor decision. They were positioned in the same room as the dryer. With the fine dust from the dryer and cooling conveyors this became a problem for the cooling fans on the VFDs. To rectify the situation, the facility is presently constructing a new operator room off of the existing motor control center that will make way for these drives to be in their own room out of the elements.

Electric usage during the composting years averaged 7000 Mwh annually for a cost of approximately $490,000. Reviewing Figure 18, the overall cost of electric started to trend downwards in 2011 when the facility had the composting operations shut down due to the fire. Likewise in 2012, the composting operations were only sustained for the last six months of the year before being shut down again to make areas of the plant ready for the AD equipment being installed. With the engines commissioned in early 2014, the need to acquire electric from the grid was being reduced significantly. At steady state, the facility now generates more than consumed and as a result, the excess generation is being used by the operations at WRF.

Using 2015 as the year to develop the efficiencies in the operations, KBBE believes greater savings are still possible in the operations. But early indications are that the cost to process a dry t of solids has been stabilized and this was one of the major reasons for shifting away from composting and getting into the AD process.

The present heating arrangements for both the Euco and Coccus would be upgraded to incorporate Brugg tubing. This would guarantee that the temperatures in the tanks could be maintained at 35°C to 37.8°C (95°F to 100°F) since this was an operational shortcoming during the colder weather with the existing heating arrangement. The biogas lines would be sized from 152.4 mm to 203.2 mm (6 in to 8 in) and tied into the 13,607 dry t (15,000 dry ton) AD plant to make use of the Unison biogas conditioning system as well as the upgraded flare. By tying into the conditioning system, the ability to divert more biogas to the thermal fluid boilers for the dryer would make good economic sense. Finally, the dome of the 1893 m³ (500,000 gal) Coccus would be converted to a double membrane that has better control over the biogas pressure.

In addition to the upgrades needed to the 4535 dry t (5000 dry ton) AD plant, other operational upgrades are being considered as well that would enhance both plants. An additional storage tank for the digested solids is necessary since the limit for holding digested solids is presently 852 m³ (225,000 gal). Having the ability to be able to store up to 1893 m³ (500,000 gal) of digested solids would allow more downtime for routine maintenance on the dryer. A tank that could hold between 946 m³ and 1325 m³ (250,000 and 350,000 gal) would be considered. Finally, what type of solids should be considered for this merchant facility? Talks have centered on taking in FOG which will require its own receiving station along with other high organic liquids. In addition to a receiving station, there would need to be an additional tank for storage of these liquids that is equipped to manage the odors.