Implementation of Acid/Gas Digestion Approach to Maximize Volatile Fatty Acids Production from Municipal Sludge and a Techno-economic Feasibility Study

<p>Acid digestion, which is the first step of two-phase acid/gas anaerobic digestion process, was studied to identify the optimum operating conditions that would maximize the production of volatile fatty acids (VFAs). Batch and semi-continuous acid digesters were operated using combined primary sludge and waste-activated sludge as the feedstock from New York City (NYC)’s Wards Island water resource recovery facility (WRRF). Operating conditions tested in batch acid digesters for maximizing VFAs production included temperature (35-70°C), inoculum-to-substrate ratio (0.0-0.5 mgVS/mgVS), thickening (3-13%TS), and contact time (0-10 days), while simultaneously inhibition mechanisms on VFAs production were investigated. Once the optimum operating conditions for VFAs production were determined, thermophilic (50°C) semi-continuous acid digesters were operated at three different solids retention times (SRTs) (1.5, 2.0, and 2.5-days) to determine the optimum SRT that yielded the highest VFAs production. Subsequently, a semi-continuous thermophilic (50°C) acid digester (SRT 2.0-days) was coupled with a mesophilic (35°C) gas digester (SRT 15-days), whose results were compared with a simultaneously operated single-phase mesophilic (35°C) anaerobic digester (SRT 15-days). The performance indicators for the comparison of the two anaerobic digestion configurations included tCOD removal (%), volatile solids destruction (%), biogas methane content (%), and biosolids quality (Class A or B).Finally, experimental performance data from the two-phase digesters was used to conduct a techno-economic assessment for NYC’s Wards Island WRRF as a case study. The cost-benefits of the two-phase technology were compared to that of thermal hydrolysis pretreatment (THP) coupled single-phase anaerobic digestion technology, a comparable alternative sludge stabilization process, while using the current single-phase anaerobic digestion configuration at Wards Island WRRF as the baseline.</p> <p>Findings revealed that inoculum addition does not improve VFAs production in batch acid digestion. The optimum thermophilic temperature for VFAs production was 50°C with VFAs yield (mg(sCOD)/mgVSfed) and fermentation specific kinetic rate constants (d-1) of 0.27 and 0.52, respectively. Although mesophilic fermentation achieved approximately 15% higher VFAs yield than that of thermophilic fermentation, the latter has the potential to yield Class A biosolids as designated by Code of Federal Regulations (CFR) Title 40-Section 503, a more valuable end-product with wider options for land application than Class B biosolids, typically produced in mesophilic systems. Two-phase digesters improved methane content from 53% to 63%. Low pH (pH</p> <p>Subsequent to the experimental work performed, techno-economic assessment projected an annual operational cost saving of 10% for two-phase digestion with VFAs recovery, compared to 12% for THP-coupled single-phase digestion for a full-scale implementation at Wards Island WRRF. Since both alternatives yielded similar cost savings in operations, the basis of selection of the appropriate technology will hinge on additional drivers such as available footprint and availability of digester capacity, biosolids handling costs or supplemental carbon procurement expenses.</p>

Name thesis:degree_name

Doctor of Philosophy (Ph.D.)

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Civil Engineering

Year dc:date.available

2024

Author dc:creator

• Sayin, Aykut

Contributors dc:contributor

• John Fillos

• anaerobic digestion
• fermentation
• volatile fatty acids
• two-phase digestion
• wastewater treatment
• techno economic assessment
• Environmental Engineering

Repository record dc:identifier

https://academicworks.cuny.edu/cc_etds_theses/1295

OAI identifier oai:identifier

oai:academicworks.cuny.edu:cc_etds_theses-2265