Ohemeng-Ntiamoah, Juliet

Dr. Juliet Ohemeng-Ntiamoah is a wastewater project professional at Jacobs Nashville office. She has expertise in biosolids management and residuals resource recovery with significant experience in designing and operating anaerobic digestion and co-digestion systems for improved biogas production. She is experienced in biosolids master planning and wastewater treatment capacity assessments.

Titles from this speaker

Description: Business Case Evaluation of Co-digestion for Wasatch Resource Recovery and Wasatch...

Business Case Evaluation of Co-digestion for Wasatch Resource Recovery and Wasatch Integrated Waste Management

Abstract

INTRODUCTION Wasatch Resource Recovery (WRR) is an existing anaerobic digestion facility located in North Salt Lake, Utah, next to the South Davis Sewer District's South (SDSD) Wastewater Treatment Plant. WRR has anaerobic digesters that became operational in March 2019. Figure 1 is a picture of WRR. WRR receives organic waste streams, such as fats, oils, and grease; food scraps; liquid waste; pre-consumer food waste; and bottled waste. The digesters are operated at mesophilic conditions (approximately ~95 degrees Fahrenheit) and are equipped with gas mixing. The biogas produced from the anaerobic digesters is treated to renewable natural gas (RNG) standards while the digestate is dewatered to achieve 14 to 16 percent cake solids and then hauled off for land application. The centrate from the dewatering process is sent to a sidestream ammonia recovery process before it is conveyed to SDSD WWTP for further treatment. Wasatch Integrated Waste Management District (WIW) owns the Davis Material Recovery and Transfer Facility (Figure 2), which is a mixed waste receiving and processing facility located in Layton, Utah. The facility processes residential waste generated in Davis and Morgan Counties. WIW is interested in diverting the organic fraction of the mixed waste from the landfill to WRR for anaerobic digestion and biogas production. This paper evaluates the feasibility of anaerobically digesting the organic fraction of municipal solid waste (OFMSW) from WIW's Recovery and Transfer Facility using the existing digester capacity at WRR. The Study Objectives were to: -Determine the required pre-digestion processing of the OFMSW for digestion at WRR -Conduct digester capacity analysis to determine the available digester capacity at WRR to accept OFMSW from WIW -Determine biogas and biosolids production and side-stream impacts from WIW's OFMSW -Determine if the business case is economically feasible for both WRR and WIW METHODOLOGY The MSW from WIW will need major pre-processing and additional polishing steps before it will be suitable as feed to anaerobic digesters. WIW recovers metals, plastics, cardboard, etc. from MSW and operates a trommel screen that results in 2-inch minus product. The organics would be recovered from the 2-inch product using an organics extrusion press (OREX) system to produce the OFMSW wet cake for trucking to WRR. Further processing would be required at WRR where an Organics Polishing System (OPS) would be added to further polish the wet cake to remove glass and grit prior to digestion. Figure 3 shows the proposed process flow diagram for the business case evaluation. The digester capacity was evaluated based on organic loading rate and hydraulic retention time (HRT). Specific chemical oxygen demand (COD) load limit of 0.5 (.i.e mass of COD loading per day per mass of volatile solids in digester) and minimum HRT of 15 days were used as the evaluation criteria for the digester capacity analysis. RESULTS AND DISCUSSION Analysis of the digestion capacity at WRR found that there was available digester capacity for acceptance of OFMSW. All of the OFMSW from WIW could be accepted because of its higher solids content. The higher solids content was compatible with the lower solids content of the FOG and food waste currently being fed to the digesters. Adding the OFMSW increased the biomass in the digesters and improved the food to microorganism ratio for stable digestion operation. The key costs and parameters for the base case of the business case evaluation is shown in Table 1. The key parameters that had significant impact on the business case were residual distribution cost, OFMSW material quality, RNG value, and the chemical cost used in sidestream treatment at WRR. The business case assumed the residual and rejects distribution cost for all scenarios would include WIW backhauling the rejects and residual from WRR. Since the cost of trucking the cake material from WIW to WRR included the round-trip mileage and costs for going from the Davis Facility to WRR to the landfill/compost facility to the Davis Facility, it was assumed that there were not any additional costs to backhaul. A sensitivity analysis was conducted to evaluate how the business case shifts with varying key parameters. The sensitivity analysis focused on the following parameters: -HSW (assumes the quality of the OFMSW could be improved) -Sidestream chemical costs are offset by revenue from recovered ammonia as ammonium sulfate -Increased RNG value CONCLUSION As shown in Table 2, the best scenario was the one with sidestream chemical costs being offset by ammonia revenue. This suggests that a reasonable cost for digestion of OFMSW is possible, with the right conditions. Distribution costs of residuals and rejects are a major cost and back hauling of materials is a crucial element of a successful project. Additionally, a higher value for RNG, as well as revenue from ammonia recovery, to offset the sidestream chemical cost are necessary. A tipping fee is necessary for profitable WRR operation, unless WIW takes the residuals and rejects as planned.

This paper was presented at the WEF/IWA Residuals and Biosolids Conference, May 16-19, 2023.

SpeakerOhemeng-Ntiamoah, Juliet

Presentation time

14:00:00

14:30:00

Session time

13:30:00

16:45:00

SessionSession 14: Co-Digestion

Session number14

Session locationCharlotte Convention Center, Charlotte, North Carolina, USA

TopicDigestion & Stabilization

Author(s)

J. Ohemeng-Ntiamoah

Author(s)J. Ohemeng-Ntiamoah1, D. Parry2, M. Myers3, N. Rich4,

Author affiliation(s)Jacobs1; Jacobs2; Wasatch Resource Recovery3; Wasatch Integrated Waste Management4

SourceProceedings of the Water Environment Federation

Document typeConference Paper

PublisherWater Environment Federation

Print publication date May 2023

DOI10.2175/193864718825158845

Volume / Issue

Content sourceResiduals and Biosolids

Word count15

Can Microbial Acclimation Work to Avert Inhibition During Fog Co-Digestion?

Abstract

INTRODUCTION Anaerobic digestion is utilized by water resource recovery facilities (WRRFs) to stabilize wastewater residuals and recover renewable energy in the form of biogas. Recently, anaerobic co-digestion of wastewater residuals with high strength organic waste has gained much popularity in the wastewater industry due to it's potential to improve methane yield. Fats, oils and grease (FOG) is one of the desirable co-substrates due to its higher methane production potential and degradability (compared to wastewater residuals) (Salama et al., 2019). However, some studies (Amha et al., 2017; Wu et al., 2018) have shown that higher FOG loadings can be inhibitory to the digestion process or the complex consortium of microorganisms involved. Therefore, it is critical to explore and understand how FOG impacts the co-digestion process and evaluate methods to avoid inhibition. One of such methods examined in this work was to evaluate if microbial acclimation through stepwise introduction of the co-substrate can avoid inhibition during FOG co-digestion. The goal of this study was 1) to determine the impact of co-digestion of waste activated sludge (WAS) with FOG on methane yield and microbial communities using short term Biomethane Potential (BMP) tests and long-term bench-scale reactors; and 2) to determine if microbial acclimation through stepwise introduction of FOG can be implemented to avert inhibition by comparing step-load and shock-load conditions in bench-scale reactors. METHODOLOGY Substrate and inoculum collection: Anaerobic inoculum was obtained from the Nashville Metro Biosolids WRRF's anaerobic digester which treats primary sludge and secondary scum. The WAS was obtained from City of Cookeville WRRF. The FOG used in this study was pure fats and grease obtained directly from a pork roasting grill at a local restaurant. BMP tests design and operation: Three experimental phases were carried out in 160 mL serum bottles at 35 °C. Phase 1 involved mono-digestion of WAS, as well as the co-digestion of WAS with FOG fractions of 25%, 50% and 75% (volatile solids basis). Phase 2 involved the co-digestion of 50% FOG with the 25% FOG digestate from Phase 1 as inoculum, while Phase 3 involved the co-digestion of 75% FOG with digestate from Phase 2 as inoculum. Phase 1 lasted for 20 days, while the phase 2 and 3 lasted for 13 days each. Bench-scale experimental design and operation: The experiments were conducted using two identical 10 L bench-scale reactors with a working volume of 6 L and operated at 35 °C. During the startup phase, all the reactors were fed with 100% WAS until they reached steady state. The designated control reactor was fed with WAS throughout all experimental phases, while the test reactor received various fractions of FOG. Phases 1 to 3 were conducted at volatile solids loading rate (VSLR) of 2 g-VS/L/d, and involved co-digestion of 25%, 50% and 75% (VS basis) of the co-substrates, respectively. Due to digester failure in Phase 3, Phase 4 was experimented as a recovery phase by feeding the test digester with only WAS. Phase 5 involved the co-digestion of 75% co-substrate at higher VSLR of 4 g-VS/L/d without prior exposure to the co-substrate, and therefore without prior microbial acclimation. Reactors were operated at 20 days SRT and each experimental phase was run for 3 SRTs. The digesters were monitored weekly by measuring parameters such as biogas volume and methane content, pH, and VS removal. The methanogenic community present within the different experimental phases were also examined through 16s rRNA gene sequencing to understand the impact of the FOG fractions on the methanogenic communities. RESULTS & DISCUSSION Results from the study revealed that compared to the mono-digestion of WAS, co-digestion with FOG can improve specific methane yield (i.e the total methane volume per gram VS added) up to 16.5-fold. The specific methane yields of all the tested FOG fractions were higher at the BMP level than the ones observed in the bench-scale as shown in Table 1. In the BMP study, inhibition was not observed even at the 75% FOG fraction; however, 75% FOG was found to be inhibitory in the bench-scale experiment as seen in the decline in weekly average biogas production rate in Figure 2. The BMP tests were loaded only once, which made it less likely to cause inhibition as compared to the continuous loading in the bench-scale experiment which resulted in organic overload and subsequent inhibition. At the microbial scale, the overall methanogenic community was dominated by Methanolinea, a hydrogenotrophic methanogen, and its abundance remained similar across all the tested FOG fractions at the BMP level. However, significant reduction of methanogenic population was observed during the inhibitory conditions at 75% FOG fractions in the bench-scale experiment, as shown in Figure 3. The results suggest that bench-scale experiments provide more realistic methane yields with detectability of inhibitory thresholds of FOG as well as reveal methanogenic community response to different FOG fractions under stable and unstable conditions. The bench-scale experiment also showed that step-wise increment of the FOG could not achieve microbial acclimation to avert inhibition in this study. Further, FOG co-digestion was only viable with up to 50% FOG addition at the 2gVS/L/d loading rate. At the higher loading rates, inhibition occurred due to accumulation of volatile fatty acids above 2,000 mg/L and decline in pH. It should be noted that the 50% FOG threshold observed in this study may have been influenced by the inoculum, the FOG characteristics or lack of continuous pH adjustments within the experimental phases. CONCLUSION The study shows that FOG can be used as a co-substrate to improve methane yield when up to 50% FOG fraction is used at an organic loading of 2gVS/L/d. The study also provides insights about the influence of operational modes and stepwise acclimation strategy on FOG co-digestion and highlights the need for other strategies to avoid inhibition in FOG co-digestion systems.

The following conference paper was presented at Residuals and Biosolids 2021: A Virtual Event, May 11-13, 2021.

SpeakerOhemeng-Ntiamoah, Juliet

Presentation time

11:00:00

11:15:00

Session time

11:00:00

12:15:00

SessionOptimizing Digestion and Co-Digestion

Session number3

Session locationSimu-Live

TopicAnaerobic co-digestion, FOG, Microbial Acclimation