Thesis Defence: Anaerobic Semi-Continuous Flow Co-Digestion of Municipal Sludge with Ozone-Pretreated High-Strength Hydrothermal Liquefaction Aqueous Phase

Maryam Hosseinikhah, supervised by Dr. Cigdem Eskicioglu, will defend their thesis titled “Anaerobic Semi-Continuous Flow Co-Digestion of Municipal Sludge with Ozone-Pretreated High-Strength Hydrothermal Liquefaction Aqueous Phase” in partial fulfillment of the requirements for the degree of Master of Applied Science in Civil Engineering.

An abstract for Maryam Hosseinikhah’s thesis is included below.

Defences are open to all members of the campus community as well as the general public. Please email cigdem.eskicioglu@ubc.ca to receive the Zoom link for this defence.

Abstract

Municipal wastewater treatment plants (WWTPs) face escalating challenges in managing the massive quantities of sludge generated daily. While conventional anaerobic digestion (AD) stabilizes sludge and recovers biogas, this biochemical process is inherently slow and yields substantial volumes of residual digestate containing pathogens, heavy metals and micro-pollutants. Hydrothermal liquefaction (HTL) has emerged as a promising thermochemical alternative. By utilizing elevated temperatures and pressures, the technology efficiently converts high-moisture sludge directly into valuable biocrude oil and hydrochar without energy-intensive predrying requirement. However, integrating HTL to WWTPs is severely hindered by its primary byproduct: a high-strength aqueous phase (HTLaq) with high chemical oxygen demand (COD), ammonia and toxic nitrogen heterocyclic compounds that inhibit downstream biological wastewater treatment.

To overcome this bottleneck, this research investigated ozone pretreatment for detoxifying HTLaq derived from mixed municipal sludge for its enhanced biodegradation. The study evaluated the pretreated full-strength HTLaq’s biodegradability during semi-continuous flow anaerobic co-digestion alongside mixed sludge and its dewatering centrate. Initial batch biochemical methane potential (BMP) assays resulted in an optimal transferred ozone dosage of 0.18 g O3/g COD of HTLaq. Applying this dose increased mesophilic specific methane yields by 69%, increasing from 165 to 279 mL CH4/g COD of HTLaq.

Building upon these batch results, the investigation advanced to a long-term, 536-day semi-continuous flow co-digestion study. Both mesophilic (35°C) and thermophilic (55°C) systems were evaluated under incrementally increasing aqueous phase loadings. During lower organic loading conditions of up to 6% of total COD contributed by HTLaq, digesters exhibited robust stability and sustained biogas yields without any pretreatment. Nevertheless, as organic loading increased to the maximum tested condition (19-22% of total COD loading contributed by HTLaq) inhibition started. Thermophilic digesters utilizing non-pretreated HTLaq experienced a drastic 35% reduction in daily specific biogas production alongside significant volatile fatty acid accumulation. In contrast, digesters fed with the ozonated HTLaq maintained superior stability, successfully restricting the biogas yield decline to just 18%. Analytical profiling via mass spectrometry confirmed that ozonation actively degraded severe inhibitors, notably 3-methyl cyclopentanone and indole. Results show that ozone pretreatment enables for a better downstream treatment of HTLaq and facilitates integrating HTL into WWTPs.