For 26 years, the City of Akron had been managing their wastewater solids using the in-vessel Paygro Composting process. While the operations of the facility and the marketing of the finished compost was successful, the concern by the public over the odor generation, the steadily increasing operational costs and the need for a large capital investment to continue composting, as a result of the facility showing its years of wear from the operations, was the reason for the City to move forward with building a new 13,607 dry t (15,000 dry ton) anaerobic digestion (AD) plant. The new 13,607 dry t (15,000 dry ton) AD plant modeled the 4535 dry t (5000 dry ton) Schmack BioGas AG AD plant built in 2007, which was used to validate the operational process and understand the biogas generation prior to going full scale.

With the success of the 4535 dry t (5000 dry ton) AD plant, the City in 2011 entered into a long term contract with KB BioEnergy (formerly KB Compost) to build and operate the new 13,607 dry t (15,000 dry ton) (AD) plant on the same grounds that the composting facility existed now known as the Akron Renewable Energy Facility. The $32M plant would use the BIOFerm Energy Systems/Schmack BioGas AD technology. Three 600 kW MWM all-in-one packages from 2-G Cenergy were purchased to convert the biogas to electricity. In addition, an indirect paddle dryer by Komline Sanderson was installed to further process the solids coming out of the AD plant to produce material suitable for market. Construction of the facility began in September of 2011 with the plant being operational by the end of 2013.

To process the solids anaerobically, two 1000 cubic meter Eucos (horizontal plug flow tanks) and two 3000 cubic meter Coccus' (complete mix tanks) were constructed. One Euco and one Coccus act as a train capable of processing 6803 dry t (7500 dry tons) of solids. The feedstock consists of a liquid stream entering the Euco tanks at 5% solids and the solids stream where the dewatered sludge enters at 28–30% solids. Maintaining a temperature between 35°C and 37.8°C (95°F and 100°F) in the tanks, the material is slowly moved thru the Euco over the next 8 days where 50% of the biogas production occurs. The digested solids are then transferred to the Coccus where the retention time is approximately 20 days prior to being discharged to holding tanks. Material entering the holding tanks is between 9 and 11% solids. The digestate is then dewatered using centrifuges. This dewatered digestate cake is then the feedstock for the dryer where the solids enter at approximately 30% and leave at ≥92% solids.

The biogas production from the process is approximately 11.3 m³/min (400 cfms). This biogas can be used to fuel the three 600 kW MWM engines or the thermal boilers used with the dryer or a combination of both depending on the need. With some of the biogas used in the thermal boilers, a biogas conditioning skid by Unison Solutions was installed that removes hydrogen sulfide, siloxanes if present and moisture. Normal operations result in the biogas being consumed by the engines with a power production of 1.2MW of electric capacity and 1.3MW of thermal capacity. An enclosed flare was installed to manage up to 17.0 m³/min (600 cfms) of biogas.

After one year of operations, the 13,607 dry t (15,000 dry ton) AD plant has been able to process all the sludge generated by the Akron Water Reclamation Facility successfully. On average 12.0 m³/min (425 cfms) of biogas have been produced resulting in 1048 kWh generated or 87% design capacity. This paper will take a closer look at the operational numbers as well as the challenges that occurred along the way of commissioning the facility.

INTRODUCTION

The City of Akron's Water Reclamation Facility (WRF) is a single stage nitrification, activated sludge treatment facility with an average design flow of 340,687 m³/d (90 mgd). The average wastewater flow over the last three years has been 283,906 m³/d (75 mgd). As a result of this flow, there has been approximately 246,052 m³ (65 mg) of combined primary and waste activated sludge that has been managed by the Akron Renewable Energy Facility (REF), depicted in Figure 1. With the use of five 2.0 meter Andritz belt filter presses, the 5% solids are dewatered to approximately 28–30% solids. The last five years production numbers for the wastewater solids can be seen in Table 1.
Figure 1

The REF and the WRF.

Figure 1

The REF and the WRF.

Table 1

Production numbers for the wastewater solids

  Biosolids – Annual Input
 
Percent Dry Solids
 
Year Cubic Meters Processed Dry t Biosolids Filter Cake 
2014 229,131 13,417 4.82 28.87 
2013 241,623 14,104 4.63 28.81 
2012 240,979 14,489 4.85 29.29 
2011 257,522 12,889 5.07 30.14 
2010 278,001 12,315 4.91 28.39 
Average 249,451 13,443 4.86 29.10 
  Biosolids – Annual Input
 
Percent Dry Solids
 
Year Cubic Meters Processed Dry t Biosolids Filter Cake 
2014 229,131 13,417 4.82 28.87 
2013 241,623 14,104 4.63 28.81 
2012 240,979 14,489 4.85 29.29 
2011 257,522 12,889 5.07 30.14 
2010 278,001 12,315 4.91 28.39 
Average 249,451 13,443 4.86 29.10 

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.

ANAEROBIC DIGESTION PROCESS

In late 2011, KB BioEnergy, Inc. (KBBE) entered into a 20 year contract to build and operate an AD plant that could process 13,607 dry t (15,000 dry tons) of solids from the WRF. The new plant would be modeled after the 4535 dry t (5000 dry ton) Schmack BioGas AD plant, shown in Figure 2, that was in operation since 2007. The new plant would include various upgrades in the design and it was required that as much of the existing infrastructure of the composting facility was to be used when constructing the 13,607 dry t (15,000 dry ton) plant. Lastly, with the governmental decision to completely shut down the composting operations permanently, consideration had to be given as to how to further process the digested solids. After reviewing a number of dryer installations, the decision to install the Komline Sanderson indirect dryer was made. With BIOFerm Energy Systems now representing Schmack BioGas AD in the U.S., the final design and construction was agreed upon with construction beginning in September 2011, in order to secure the Federal 1603 grant funds available.
Figure 2

The 4535 dry t (5,000 dry ton) AD plant.

Figure 2

The 4535 dry t (5,000 dry ton) AD plant.

The AD system by BIOFerm Energy Systems consists of two identical trains each capable of processing 6803 dry t (7500 dry tons) of solids. Each train consists of two digestion tanks, the Euco and Coccus, shown in Figure 3. The Euco is a plug flow digester that can accommodate high solid wastes (up to 18% DS) and a variety of different wastes since agitation is done by a heavy duty horizontal mixer that keeps layers from forming on the top or bottom of the tank. The Euco units built are 1000 m³ each holding 1014 m³ (268,000 gal). The Coccus is a common circular tank that provides a complete mix and incorporates the same style mixers as found in the Euco. The Coccus units built are 3000 m³ each holding approximately 2839 m³ (750,000 gal). The roof component of the Coccus consists of a wooden lattice arrangement that accommodates a desulphurization process.
Figure 3

A pair of Eucos and Coccus' in the 13,607 dry t (15,000 dry ton) AD plant.

Figure 3

A pair of Eucos and Coccus' in the 13,607 dry t (15,000 dry ton) AD plant.

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.

Biogas production was anticipated to be approximately 2.9 m³/min (210 cfms) per train. With the two trains, the combined biogas flow would be used to fuel two 600 kw CHPUs supplied by 2-G Cenergy. When surplus biogas is available, the operation has the ability to burn this excess in the thermal fluid boilers that are used for the indirect dryer or send it to the flare to be burnt. Figure 4 depicts the process flow design of the 13,607 dry t (15,000 dry ton) AD plant.
Figure 4

Process flow design for the 13,607 dry t (15,000 dry ton) AD plant.

Figure 4

Process flow design for the 13,607 dry t (15,000 dry ton) AD plant.

RESULTS

Commissioning of the 13,607 dry t (15,000 dry ton) AD plant, shown in Figure 5, began in late September of 2013. Approximately 2271 m³ (600,000 gal) of digested solids from the 4535 dry t (5000 dry ton) AD plant were transferred to the two Coccus' as seed sludge. In addition, another 3028 m³ (800,000 gal) of raw liquid sludge was pumped into the Coccus' and held there for two weeks to stabilize the biological conditions. Once the sludge was stable, both of the Eucos were filled to operating levels by the material respectively from each paired Coccus. In mid- November, 30% dewatered cake solids were fed to the system and within four weeks the biology of the system was stabile resulting in the rate of fed solids being at design. Biogas production was slow initially and any gas produced was flared in the months of December and January. Once the biogas volume was close to design the start-up of the Unison biogas conditioning equipment occurred. Conditioned biogas was then utilized in the 2-G Cenergy CHPUs. Two of the three 600 kw units became operational late January. The final piece of equipment to be brought on line was the Komline indirect dryer. The unit was bedded with material in late June and became completely operational the third quarter of 2014.
Figure 5

The 13,607 dry t (15,000 dry ton) AD plant in Akron, Ohio.

Figure 5

The 13,607 dry t (15,000 dry ton) AD plant in Akron, Ohio.

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!

Solids

After receiving 229,017 m³ (60.5 million gal) of 4.8% raw liquid solids from the WRF in 2014, the operations dewatered 168,451 m³ (44.5 million gal) and the remaining 60,567 m³ (16.0 million gal) were directly fed to the AD system. The solids input were 2903 dry t (3201 dry tons) of liquid and 10,513 dry t (11,589 dry tons) for the cake solids. These feed rates are comparable to the design rates, as shown in Table 2 and Figure 6.
Table 2

Feed Rates-Design versus 2014 operations

  Liquid Sludge to ADS (Dry t) Belt Filter Press Cake (Dry t) Raw Liquid Solids (%) Cake Solids (%) Combined Sludge to ADS (Dry t/day) 
Design 2,630 10,432 5.0 28.4 35 
2014 2,903 10,513 4.8 28.9 36 
  Liquid Sludge to ADS (Dry t) Belt Filter Press Cake (Dry t) Raw Liquid Solids (%) Cake Solids (%) Combined Sludge to ADS (Dry t/day) 
Design 2,630 10,432 5.0 28.4 35 
2014 2,903 10,513 4.8 28.9 36 
Figure 6

Dry t input for 2014.

Figure 6

Dry t input for 2014.

The system is designed as a high solids digester, that in addition to taking the 4.8% raw liquid solids, it took solids dewatered from 26.5% to 32.0% off of the belt filter presses. With the previously mentioned upgraded entry screw conveyor, the ability to meter solids into the Euco is extremely efficient. During the month of June, the overall solids inside the Euco were 11.9% and in the Coccus they were 11.33%. Figure 7 demonstrates that the year-end average was lower than these highs seen in June since the volume of solids produced at the WRF dropped off slightly in the second half of the year requiring the operations to increase the volume of liquid added which reduced the volume of cake solids, to maintain a reasonable retention time in the tanks. The Euco's annual average solids was 9.1% and for the Coccus 8.8% dry solids. Looking closer at Table 3, the volatile solids loading rate was 3.716 kg VS/day/1000 m³ (232 lbs VS/day/1000 ft³) and after the 29 day retention time there was a 55% destruction in those solids. With the conversion of solids to biogas, there was 227 m³ (59,954 gal) of digested solids discharged daily.
Figure 7

Digester solids input for 2014.

Figure 7

Digester solids input for 2014.

Table 3

Design versus actual digested solids

  Combined Sludge to ADS Dry Solids (%) Coccus Dry Solids (%) Retention Time (Days) Volatile Solids Loading (kg/day/m3Volatile Solids Destruction (%) Digested Sludge to Centrifuge (m3
Design 14.6 9.1 30 3,604 50 245 
2014 10.9 8.8 29 3,716 55 227 
  Combined Sludge to ADS Dry Solids (%) Coccus Dry Solids (%) Retention Time (Days) Volatile Solids Loading (kg/day/m3Volatile Solids Destruction (%) Digested Sludge to Centrifuge (m3
Design 14.6 9.1 30 3,604 50 245 
2014 10.9 8.8 29 3,716 55 227 

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.

The thermal oil used by the dryer is heated using a boiler that can be ran on natural gas or biogas, shown in Figure 8. To guarantee use of all biogas, the boiler heating system was designed with two boilers and each boiler has two burners. Each boiler can burn either natural gas or biogas. Any biogas not being consumed by the engines is redirected to a boiler for consumption. To meet the temperature requirement of the dryer, the second boiler can complement the first boiler on natural gas to guarantee that 204.4°C (400 °F) thermal fluid is delivered to the dryer. In addition to these boilers, there are heat exchangers off of each CHP unit specifically designed to heat the thermal fluid. During the warmer months of March thru October the operations anticipate being able redirect a large portion of the waste heat from the engines and use it to heat the thermal fluid.
Figure 8

Boilers used to heat the thermal oil.

Figure 8

Boilers used to heat the thermal oil.

Biogas

With 55.5% destruction of the volatile solids, approximately 12 m³/min (425 cfm) of biogas is generated or a 14% increase over the design generation of 10.6 m³/min (373 cfm). The biogas that is directed to the engines or the thermal fluid boiler is collected and cleaned by a biogas handling system supplied by Unison Solutions, Inc., depicted in Figure 9. The system consists of a chiller, heat exchanger, hydrogen sulfide removal vessel and siloxanes removal vessels. If biogas cannot be used by the engines or the dryer operations are not in service at the time, the excess biogas is flared. During the initial months of start-up and until the engines were in steady-state the facility did flare biogas. With the dryer not becoming fully operationally until the last quarter of 2014, all of the conditioned biogas was consumed by the engines. When taking into consideration that the engines were commissioned in January, the plant still managed to produce 8684 Mwh or 87% of the expected annual production.
Figure 9

Unison Solutions biogas handling system.

Figure 9

Unison Solutions biogas handling system.

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.

Commissioning

With the start-up of any new plant, there always come challenges and some became real evident once the cold weather of December and January set in. The cold weather exposed areas of the plant that required more insulation. Many of the biogas lines to and from the Unison Solution system as well as components related to the flare needed additional insulation. In conjunction with the biogas it was necessary to upgrade the flare biogas blowers, shown in Figure 10. The old units did not allow for removal of moisture and thus ice would build up inside locking them up mechanically. Just the opposite of cold weather issues there were problems associated with the days of warm weather. Inside a number of the buildings, where the heating loops ran, the temperatures inside the rooms became very warm. This became an issue with some of the instrumentation on various pumps and pipes. The need for additional insulation on the piping used in the transfer of heat in the tanks was also necessary.
Figure 10

Upgraded flare blowers.

Figure 10

Upgraded flare blowers.

Once the dryer, shown in Figure 11, was commissioned and some run time had occurred with the unit, the initial challenge was fine tuning the operational parameters. Initial runs with the dryer resulted in a finished product with 99% dry solids which was too dry. This material became very powdery and dusty creating a messy situation in the load out area of the dryer. Learning to run the bed of the dryer a little cooler allowed for the material to get into the lower 90% dry solids making the material more manageable. Work is ongoing with an improved design for the chutes that deliver the dried solids to the bed of the trucks. Telescoping chutes that will minimize the distance that the dried material free falls into the bed is being considered.
Figure 11

Indirect solids dryer and ancillary equipment.

Figure 11

Indirect solids dryer and ancillary equipment.

Another area of concern with the dryer operations was the hardness found in the nonpotable water that is being used for the cooling conveyors and heat exchanger. With the rise in pH of the water, as a result of the ammonia present in the digested solids and the subsequent temperature of the dryer, the calcium in the water precipitated out as calcium carbonate. In order to control the precipitant from taking place, an antiscalant is now being used to distort the crystal formation of scale, shown in Figure 12.
Figure 12

Scale buildup in the dryer unit (left) – after antiscalent introduced (right).

Figure 12

Scale buildup in the dryer unit (left) – after antiscalent introduced (right).

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.

In the design of the overall plant and more specifically with the 2-G Cenergy engines, it was decided to incorporate an exhaust heat exchanger off of the CHPs that would be used for the thermal fluid used with the dryer, depicted in Figure 13. Any heat not necessary to maintain the digester tanks at 35 °C to 37.8°C (95°F to 100°F) would be redirected to the thermal fluid. Many fail safe procedures were required to be installed on the system to protect the integrity of the equipment if by chance there was an electrical outage knowing this fluid was at 204.4°C (400°F). As a result, this particular system will only come on line in June of this year. The bottom end of the heat exchanger is being changed out to account for the higher psi found in the thermal fluid loop as well as an improved check valve that can work with 204.4°C (400 °F) fluid.
Figure 13

Exhaust heat exchanger on CHPs.

Figure 13

Exhaust heat exchanger on CHPs.

CONCLUSION

Operational data

After commissioning of the equipment, the operations were able to work towards a steady state accounting for the fluctuations in the volume of solids available to be processed. A look at how some of the key operational parameters and the costs associated with them will be reviewed as the operations transitioned from 100% compost to 100% AD. In Figure 14 a breakdown of the method used to dispose of the yearly solids over the last 10 years is provided. Reviewing Figure 14 it can be seen that in a number of the years prior to the ADS facility being 100% operational, the raw and digested solids had to be landfilled. In 2011 and 2012, the need to landfill was a direct result of the fire which did not allow composting at times to occur. Then in 2013, the composting operations were shutdown permanently to make way for some of the new equipment resulting in most all of the solids going to the landfill. Finally, in 2014, even with the ADS plant at 100% operations, all of the digested solids were dewatered and sent to the landfill until the dryer was commissioned and fully operational in the fourth quarter. The goal for 2015 is to process all solids thru the dryer with no landfill disposal costs.
Figure 14

Final solids disposal methods.

Figure 14

Final solids disposal methods.

When looking at the costs associated with the following key operational parameters: amendments, sulfuric acid, polymer, electric, natural gas and digester nutrients, the cost per dry ton to process pre ADS solids ranged from $101 to a low of $65, as shown in Figure 15. When reviewing the chart a little closer, the composting years of 2005 thru 2010 show the costs trending upwards. Any time after 2010, once landfill became a part of the final disposal means, the operational costs increased from the composting years ranging from $93 to $144 per dry t. With the objective in 2015 of eliminating the landfill costs, it is estimated that the cost per dry t will be in the range of $60 to $70 when taking these five operational parameters into account.
Figure 15

Cost per dry t.

Figure 15

Cost per dry t.

Taking a closer look at each of the contributing operational parameters, it can be seen in Figures 1618 that the yearly cost for amendments, sulfuric acid, and electric all trended downward to almost a no cost associated with it in the processing effort. With the composting operations being phased out, the need for amendments decreased accordingly until compost ended and there was no need for amendment. The use of sulfuric acid in the odor control system to scrub the ammonia compounds from the process air decreased significantly over the years, again directly related to the shutdown of the composting operations.
Figure 16

Yearly costs for Amendments.

Figure 16

Yearly costs for Amendments.

Figure 17

Yearly costs for Sulfuric Acid.

Figure 17

Yearly costs for Sulfuric Acid.

Figure 18

Yearly costs for Electric.

Figure 18

Yearly costs for Electric.

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.

Where amendments, sulfuric acid and electric all had a positive impact on the overall operational cost to dispose of the solids, the supplemental nutrients, polymer and natural gas had just the opposite in that they increased the costs. In Figure 19, the nutrients associated with the digestion process increased but only proportional to the fact that we are processing 100% of the solids anaerobically. The cost of the MetSource DG nutrients is worth the biological stability of the operations as evident by a steady flow of biogas and a decrease in biogas then when not used.
Figure 19

Yearly costs for Nutrients.

Figure 19

Yearly costs for Nutrients.

Polymer usage, shown in Figure 20, has increased as a result of not only dewatering the raw solids but also the digested solids. Using a centrifuge, digested solids are dewatered to 28% to 30% dry solids for the dryer operations. As history has shown in the industry, digested solids thru a centrifuge typically will require double the dosage of polymer per dry t. The potential to improve upon the polymer usage will be reviewed when a new polymer system is considered to replace the existing polymer blending system that is approaching 30 years old.
Figure 20

Yearly costs for Polymer.

Figure 20

Yearly costs for Polymer.

The second parameter with a negative impact on the cost per dry t is natural gas, as shown in Figure 21. Even with the unit cost of natural gas being at a historical low, the volume required to operate is significantly high annually. It is projected that if the thermal boilers are operated fully on natural gas the annual costs will be between $400,000 and $600,000. As stated earlier, the operations will utilize the exhaust heat exchangers on the engines to heat the thermal fluid for the dryer. This will reduce the volume of natural gas necessary. In addition, biogas will be directed as well to the dryer to lessen the need for natural gas.
Figure 21

Yearly costs for Natural Gas.

Figure 21

Yearly costs for Natural Gas.

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.

Future

With the new AD plant capable of processing all of the solids generated by the City of Akron's WRF, consideration is now being given as to how to move forward with the initial 4535 dry t (5000 dry ton) AD plant. It has long been thought that implementing some upgrades to this plant would allow for the operations to take in other high organic materials that could be processed and allow us to treat it like a merchant facility for the surrounding community. Before that would happen a number of improvements need to be made to the 7 year old facility, shown in Figure 22.
Figure 22

The original 4535 dry t (5000 dry ton) AD plant – Decommissioned in 2013.

Figure 22

The original 4535 dry t (5000 dry ton) AD plant – Decommissioned in 2013.

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.

Now that the $32 million AD facility is operational, many of the residents can look back at some of the promises made to them and actually recognize them daily as they ride the bike path along the scenic Cuyahoga River as seen in Figure 23. The odor that was so prevalent when the composting operations occurred is no longer detectable and this is evident by the fact that no registered complaint has occurred since the facility went operational. Second promise upheld, using ‘green technology to convert solids once considered a waste into renewable energy. This renewable energy in the form of electricity helps in stabilizing costs to manage the solids long term for present generations and generations to come.
Figure 23

The REF situated next to the bike path.

Figure 23

The REF situated next to the bike path.