Time series measurements and process modelling of methane production and release in two Danish septic tanks
Author
Konradsen, Carl Gustav
Term
4. term
Publication year
2021
Abstract
Dette studie undersøger metanproduktion og -frigivelse i to septiktanke i Nordjylland med fokus på kulstofomsætning over tid. Over flere døgn blev hovedrummets CO2 og H2S samt trykforskellen mellem tank og atmosfære målt som tidsserier for at belyse gasdannelse, udluftning og driftsrelaterede hændelser. Forsøg på at beregne luftudskiftningen ud fra trykforskellen viste sig upræcise, fordi tankene ikke var helt tætte. H2S lå for det meste ≤1 ppm med hurtige toppe på 10–20 ppm i forbindelse med pumpning, hvilket peger på turbulensdrevet frigivelse. CH4 i gasfasen lå på 6300–8700 ppm i den ene tank og 900–5200 ppm i den anden, i overensstemmelse med tidligere målinger samme sted, mens CO2 i hovedrummet varierede mellem 500–4000 ppm og 470–900 ppm. Den manglende direkte proportionalitet mellem CO2 og CH4 antyder, at andre processer end ren aceticlastisk metanogenese bidrager, eller at gæring og sulfatreduktion er mere fremtrædende i tank A. På basis af målingerne opstilles en begrebslig procesmodel med Monod-kinetik, der beskriver hydrolyse, gæring, sulfatreduktion og metanogenese samt gas–væske-fordeling ved Henrys lov og kulsyresystemets ligevægte. Begrænsningerne omfatter lange opholdstider, utætheder og manglende kontinuerlig CH4-logning, hvorfor CO2 anvendes som indikator for CH4. Arbejdet leverer nye tidsseriedata og et modelleringsgrundlag til at vurdere septiktankes klimaaftryk, men understreger også de væsentlige usikkerheder.
This study investigates methane production and release in two septic tanks in Northern Jutland, Denmark, with a focus on carbon transformation over time. Over several days, time-series measurements of headspace CO2 and H2S and the pressure difference between the tank and atmosphere were collected to capture gas generation, venting, and operational events. Attempts to estimate air exchange from pressure differences were inaccurate because the tanks were not properly sealed. H2S was ≤1 ppm for most of the period, with short-lived peaks of 10–20 ppm during pumping, indicating turbulence-driven release. Gas-phase CH4 ranged from 6300–8700 ppm in one tank and 900–5200 ppm in the other, consistent with earlier measurements at the sites, while headspace CO2 varied between 500–4000 ppm and 470–900 ppm. The non-proportional relationship between CO2 and CH4 suggests contributions from processes beyond purely aceticlastic methanogenesis, or more prominent fermentation and sulfate reduction in tank A. Based on the measurements, a conceptual process model with Monod kinetics is formulated to represent hydrolysis, fermentation, sulfate reduction, and methanogenesis, together with gas–liquid partitioning via Henry’s law and carbonate equilibria. Limitations include long residence times, tank leakage, and the lack of continuous CH4 logging, for which CO2 served as a proxy. The work provides new time-series data and a modelling framework to assess septic tank carbon footprints, while highlighting key uncertainties.
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