Operational and environmental performance assessment of sewage sludge anaerobic digestion including ultrasound - thermal pre-treatment
Sewage treatment represents a cornerstone for public heath protection and conservation of aquatic ecosystems integrity. However, current treatment technologies represent a growing source of sewage sludge, which in the case of activated sludge facilities could be of 7 – 8 kg/inhab-year. Due to its so...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2018 |
| País: | Chile |
| OAI Identifier: | oai:repositorio.anid.cl:10533/210997 |
| Acceso en línea: | https://hdl.handle.net/10533/210997 |
| Access Level: | acceso abierto |
| Palabra clave: | Ingeniería y Tecnología Ingeniería Ambiental Ingeniería Ambiental y Geológica |
| Sumario: | Sewage treatment represents a cornerstone for public heath protection and conservation of aquatic ecosystems integrity. However, current treatment technologies represent a growing source of sewage sludge, which in the case of activated sludge facilities could be of 7 – 8 kg/inhab-year. Due to its source and related physic-chemical and microbiological characteristics, sludge management represents an important economical and environmental issue, representing up to 50% of the operational costs of sewage treatment plants (STP) and with potential impacts over soils, water bodies and population health. Anaerobic digestion is a biological stabilization technology that allows energy recovery through the transformation of organic matter into CO2 and CH4 (biogas). However, hydrolysis rates during digestion limits the process and its applicability in smaller STP (<50,000 equivalent inhabitants), and therefore there are only 6 sewage sludge digestion plants currently in Chile. In this scenario, different pre-treatment strategies have been proposed to hydrolyze sludge previous to anaerobic digestion, which results in process intensification and increased biogas production. In particular, the use of processes that involves the action of different hydrolysis mechanisms has gained notoriety, mainly due to its synergistic effects over sludge solubilization and methane yield during digestion. In this thesis, the influence of a sequential pre-treatment using ultrasound and thermal hydrolysis (55°C) over sewage sludge characteristics and the operational and environmental performance of anaerobic digestion was assessed. The objective of the process is sludge solubilization through physical and biological phenomena, which is expected to increase organic matter conversion and biogas production during anaerobic digestion. In order to asses if pre-treatment implementation results in trade-offs between the environmental impacts of the different steps of sludge management, an environmental performance evaluation of the process was done using a Life Cycle Assessment (LCA) perspective. Batch assays of pre-treatment were performed under different operational conditions, including application of ultrasound with specific energies (SE) of 500 – 30500 kJ/kgTS and 3 – 13 h (T) of thermal treatment at 55°C. Organic matter solubilization was assessed in terms of proteins, carbohydrates and chemical oxygen demand (COD). Moreover, the process influence on Volatile Fatty Acids (VFA) presence and endogenous protease and amylase activity in sludge was evaluated. The performance of anaerobic digestion after pre-treatment was assessed through kinetic and biodegradability batch assays, as well as during the operation of semi-continuous digesters (6 L reaction volume) under 3 operational conditions, corresponding to solids retention times (SRT) of 30, 15 and 7.5 days. The experimental period of the semi-continuous systems was of 303 days, including a biomass acclimation phase of 96 days. During this period, stability parameters of digestion (pH, alkalinity, VFA concentration, oxidation-reduction potential, NH4+-N concentration) were monitored, as well as the methane yield (mLCH4/gVS), COD, total solids (TS) and volatile solids (VS) removals and the transformation of organic matter associated to the hydrolysis, acidogenesis and methanogenesis phases. Moreover, COD balances for all experimental conditions were calculated. The influence of pre-treatment over digestate was assessed in terms of dewaterability (through centrifugation assays), the concentration of N, P, As, Cu, Hg, Ni, Pb, Se and Zn in digestate solids and supernatant, and the presence and logarithmic removal (logrem) of total coliforms, fecal coliforms and somatic coliphages. The LCA of the process was performed based on the results of digesters operation and operational data from the Biobío STP of Concepción (36° 48' S, 73° 08' W) for year 2015. The functional unit (FU) selected for the study was the treatment of 1 ton of sludge (dry basis), and the assessment of 6 potential impact categories was included: climate change (kgCO2eq), abiotic depletion (kgSbeq), acidification (molcH+eq) and eutrophication in terrestrial (molNeq), marine (kgNeq) and freshwater (kgPeq) ecosystems. It was supposed that after anaerobic stabilization, digestate was used as replacement of commercial fertilizers in wheat crops, and the results were compared to reference scenarios based on sludge alkalinization and disposal in landfill. Sequential pre-treatment to significant solubilization of organic matter, with increases in the soluble concentration of proteins (252 – 674%), carbohydrates (458 – 1030%) and COD (ƒ = 3.5 – 17.1%), besides 1.6 – 3.8 higher amylase and 1.8 – 4.3 higher protease activities in the soluble phase. COD solubilization was influenced by sludge concentration and the studied operational conditions (SE and T). Sequential pre-treatment led to increases in anaerobic digestion kinetics and sludge biodegradability, with increases of 30 – 80% in the maximum methane production rate (mLCH4/gVS-d) and 16 – 50% in the specific methane yield (mLCH4/gVS). During semi-continuous evaluation, increases of 19 – 29% in the specific methane yield were observed due to pre-treatment implementation, without affecting the process stability and with NH4+-N and NH3-N concentrations under the inhibitory threshold for all experimental conditions (≤1.8 gNH4+-N/L y ≤53 mgNH3-N/L). The highest increase in methane production was observed at 7.5 days SRT (3.6 gVS/L-d), associated with the higher kinetics of degradation during digestion. Organic matter balances showed that the increased methane production was proportional to solubilization during pre-treatment, which suggests that this is the main mechanism involved in the observed effects over anaerobic digestion operational performance. Both pre-treatment and SRT affected water recovery during digestate centrifugation, with 4.2% and 13.3 – 20.5% lower values associated to pre-treatment implementation and reduced SRT from 30 to 15 days, respectively. The pre-treatment/anaerobic digestion system showed 2.8 logrem for coliforms (total and fecal) and 1.5 logrem for somathic coliphages, besides increases of up to 58% in the concentration of As, Cu, Hg and Zn in the solids fraction of digestate. The environmental performance of anaerobic digestion including the sequential pre-treatment was similar to conventional digestion. The main differences were associated with a decrease in climate change potential and an increase in abiotic depletion potential, related to energy recovery from sludge and transport requeriments fro digestate. Both scenarios showed lower impacts in all studied categories compared to the reference scenarios. The results are dependent on the presence of fossil carbon in CO2 emissions and valorization of heat coming from the co-generation of biogas, and it was observed that parameters such as digestion SRT and sludge application rate in soil could have a bigger influence over the environmental performance of sludge management than the incorporation of pre-treatment. |
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