Measurement and understanding of methane emission from sewers

Abstract

Sewer emissions are a notorious problem that water utilities have to deal with. The production and emission of hydrogen sulfide is a well-known problem for decades, which is the primary cause of sewer odors and corrosion. However, hydrogen sulfide is not the only harmful emission from sewer networks. Methane can also be generated in sewers through methanogenesis. Methane is a highly potent greenhouse gas, which is significantly contributing to climate change. Over a 100-year horizon, 1 ton of CH 4 will induce a warming effect equivalent to 34 tons of CO 2 . It is also explosive with a lower explosive limit (LEL) of approximately 5% by volume, and thus poses a serious safety issue. Currently, very little attention has been paid to methane formation in sewer networks.Therefore, the overall aim of this thesis is to measure and understand methane emission from sewers.The limited studies conducted so far on methane measurement in both gas and liquid phases in sewers have relied on manual sampling followed by off-line laboratory-based chromatography analysis. These methods are labor-intensive when measuring methane emissions from a large number of sewers, and do not capture the dynamic variations in methane production.Therefore, the suitability of infrared (IR) spectroscopy-based online methane gas sensors for measuring methane in humid sewer air was investigated in both laboratory and field conditions. Under certain circumstances, humidity could become an issue, however, this can be solved by removing the humidity on the sensor probe surface. Also, IR sensors exhibit excellent linearity and can be applied with factory calibration. Furthermore, the detection limit of sensors is suitable for measuring methane gas in sewers. Field application of the sensors revealed that methane concentrations in sewer air are 3 -4 orders of magnitude higher than in the atmosphere, confirming that sewers are a source of methane. The continuous measurement also revealed that methane concentrations in sewer are highly dynamic.Complementary to the gas phase sensors, a new dissolved methane sensor was developed in this thesis. This device uses an online IR gas-phase methane sensor to measure methane under equilibrium conditions in a stripping chamber. The measured gaseous methane levels were then II converted to liquid-phase methane concentrations according to Henry's Law. The detection limit and range was noted to be suitable for sewer applications. Good linearity was also obtained during Contributions of sediments in gravity sewers to overall sewer emissions are poorly understood at present. Sediments collected from a gravity sewer were cultivated in a laboratory reactor fed with real wastewater for more than one year to obtain intact sediments for the study. Batch test results clearly showed significant sulfide and methane production from sewer sediments. Microsensor and pore water measurements of sulfide, sulfate and methane in the sediments, microbial community profiling along the depth of the sediments and mathematical modelli...