High-rate Anaerobic Treatment of Biorefinery Wastewaters
Rintala, Kati (2022)
Rintala, Kati
2022
Ympäristö- ja energiatekniikan DI-ohjelma - Programme in Environmental and Energy Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2022-12-20
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202212149167
https://urn.fi/URN:NBN:fi:tuni-202212149167
Tiivistelmä
The demand of bio-based products is increasing in the future due to their renewable nature. Bio-based products are manufactured in biorefineries from organic feedstocks through several refining processes. During refining processes, wastewaters with high organic matter content and potentially inhibitory compounds are often generated. Wastewaters are treated before re-leasing them into water bodies to diminish the environmental impacts they cause for ecosys-tems. Anaerobic treatment has been found to be a suitable treatment type for removing organic matter and inhibitory compounds from high-strength wastewaters with simultaneous biogas production.
The aims of this thesis were to study anaerobic treatability of high-strength biorefinery wastewater in an expanded granular sludge bed reactor (EGSB) and the re-use of the reactor effluent in the treatment process. Effluent re-use reduces the need of fresh water improving the sustainable use of resources. Additionally, the treatability of the wastewater with lower influent pH (6-6.6, normally 7-7.5) was studied.
The experiments were implemented in three parallel reactors for 90 d runs. The studied pilot biorefinery wastewater was acidic (pH 4.2-4.6), contained high concentrations of organic matter measured as soluble chemical oxygen demand (sCOD 13-51 g/L) and high concentration of sulfate (1-2.5 g/L); thus, diluting was considered to be required before feeding the wastewater into the reactors. Wastewater was diluted with three different dilution waters, and the proportion of wastewater in the feed was increased from 11 % to 67 %, leading to increasing organic load-ing rate (OLR) in the reactors. The reactor effluent was re-used for diluting the wastewater in two reactors; in the other one the effluent was used as such and in the other one effluent was treat-ed with sulfide removal before use, to study the effects of high sulfur content on the reactor performance.
The results showed that the studied biorefinery wastewater could be treated in an EGSB re-actor with re-using the reactor effluent, and the anaerobic treatment process was not inhibited despite the high influent sulfate concentration. The amount of produced biogas increased from 1.4-2 L/d to ca. 17-20 L/d as the OLR was increased, the methane content and total sCOD re-moval were at high level (> 60 % and 60-75 %, respectively), and the effluent pH remained close to neutral in all the reactors at all OLRs. The non-biodegradable sCOD in reactor effluents decreased the total methane yields and total sCOD removals in reactors R2 and R3; however, the values calculated against the sCOD fed within the wastewater (sCODww) were similar (279-308 L-CH4/kg-sCODww, fed and 72-78 %, respectively) in all the reactors. The size distributions of granules were determined during the reactor runs, and no dispersion of the granules was no-ticed also indicating that no inhibition of the anaerobic treatment occurred. In addition, the wastewater was also successfully treated with lower influent pH.
Influent sulfate concentration varied from 98 to 1120 mg/L, and more than 90 % of sulfate in the influent was reduced most of the time, indicating the growth of sulfate-reducing bacteria in the reactors. Sulfate is reduced to sulfide (HS- and H2S) during anaerobic treatment. In this study, sulfide was assumed to be partially precipitated after exiting the reactor, as the sulfide concentrations in effluent were remarkably lower (63-85 %) than inside the reactors, and precipi-tations in the effluent lines were observed. The measured H2S contents in biogas were 1-1.5 % in all the reactors at OLRs of ca. 13-24 g-sCODww/L/d.
The recycling of reactor effluent as dilution water diminishes the amount of fresh water re-quired in a high-rate anaerobic wastewater treatment process, enhancing the overall water-efficiency in a biorefinery.
The aims of this thesis were to study anaerobic treatability of high-strength biorefinery wastewater in an expanded granular sludge bed reactor (EGSB) and the re-use of the reactor effluent in the treatment process. Effluent re-use reduces the need of fresh water improving the sustainable use of resources. Additionally, the treatability of the wastewater with lower influent pH (6-6.6, normally 7-7.5) was studied.
The experiments were implemented in three parallel reactors for 90 d runs. The studied pilot biorefinery wastewater was acidic (pH 4.2-4.6), contained high concentrations of organic matter measured as soluble chemical oxygen demand (sCOD 13-51 g/L) and high concentration of sulfate (1-2.5 g/L); thus, diluting was considered to be required before feeding the wastewater into the reactors. Wastewater was diluted with three different dilution waters, and the proportion of wastewater in the feed was increased from 11 % to 67 %, leading to increasing organic load-ing rate (OLR) in the reactors. The reactor effluent was re-used for diluting the wastewater in two reactors; in the other one the effluent was used as such and in the other one effluent was treat-ed with sulfide removal before use, to study the effects of high sulfur content on the reactor performance.
The results showed that the studied biorefinery wastewater could be treated in an EGSB re-actor with re-using the reactor effluent, and the anaerobic treatment process was not inhibited despite the high influent sulfate concentration. The amount of produced biogas increased from 1.4-2 L/d to ca. 17-20 L/d as the OLR was increased, the methane content and total sCOD re-moval were at high level (> 60 % and 60-75 %, respectively), and the effluent pH remained close to neutral in all the reactors at all OLRs. The non-biodegradable sCOD in reactor effluents decreased the total methane yields and total sCOD removals in reactors R2 and R3; however, the values calculated against the sCOD fed within the wastewater (sCODww) were similar (279-308 L-CH4/kg-sCODww, fed and 72-78 %, respectively) in all the reactors. The size distributions of granules were determined during the reactor runs, and no dispersion of the granules was no-ticed also indicating that no inhibition of the anaerobic treatment occurred. In addition, the wastewater was also successfully treated with lower influent pH.
Influent sulfate concentration varied from 98 to 1120 mg/L, and more than 90 % of sulfate in the influent was reduced most of the time, indicating the growth of sulfate-reducing bacteria in the reactors. Sulfate is reduced to sulfide (HS- and H2S) during anaerobic treatment. In this study, sulfide was assumed to be partially precipitated after exiting the reactor, as the sulfide concentrations in effluent were remarkably lower (63-85 %) than inside the reactors, and precipi-tations in the effluent lines were observed. The measured H2S contents in biogas were 1-1.5 % in all the reactors at OLRs of ca. 13-24 g-sCODww/L/d.
The recycling of reactor effluent as dilution water diminishes the amount of fresh water re-quired in a high-rate anaerobic wastewater treatment process, enhancing the overall water-efficiency in a biorefinery.