The influence of pre-treatment on biomat development in soil treatment units

Knappe, Jan; Somlai, Celia; Fowler, A. C.; Gill, Laurence

doi:10.1016/j.jconhyd.2020.103654

Cited by 12 publications

(34 citation statements)

References 42 publications

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“…Both sites were located in the west Ireland where the relatively strong maritime influence leads to cool and windy winters and mostly mild and less windy summers, with similar rainfall quantities across the year. Notably, during this study in summer 2018 there was an unusually long period of drought which lasted for approximately three months, which caused severe drying of the soil and soil moisture deficit at both sites (Knappe et al 2020). CO2 fluxes exhibited a clear temperature dependence with lower fluxes with narrower ranges during the colder months (from September until February) and higher fluxes with wider ranges over the warmer months.…”

Section: Seasonal Patternsmentioning

confidence: 72%

“…In the STU the high organic loading from the PE from the STs on both sites caused the development of an extensive biomat, spreading the lateral distribution of the effluent 15 m along the trenches from the inlet after approximately three years of operation. The biomat growth in the trenches receiving SE, however, did not extend to more than 7.5 m and 10 m at Site A and Site B respectively, reflecting more limited growth due to the lower availability of organics and nutrients in the effluent feeding these trenches (Knappe et al, 2020). The percolation of the effluent through the soil caused a significant reduction in organic C concentrations within the first 300 mm, as well as significant net total N removal.…”

Section: Effluent Qualitymentioning

confidence: 91%

“…A network of soil sensors (type EC5 and GS3, Decagon Devices, Inc., USA) installed within the STU and control soil provided detailed information on VWC content, soil temperature and pore water EC and was used to infer the development of a microbially active zone, i.e. biomat, along the base of STU trenches over time (see Knappe et al, 2020).…”

Section: Auxiliary Datamentioning

confidence: 99%

“…In STU trenches, a biomat forms at the infiltrative surface with time and gradually clogs soil pores. The development of this biomat layer is linked to the physical accumulation of suspended solids within soil pores and microbial growth (McKinley and Siegrist 2010; Thomas et al 1966;Beach et al 2005;Siegrist and Boyle 1987;Knappe et al 2020;Jones and Taylor 1965;Bouma 1975). As gradual biomat development results in a growing resistance to flow and reductions in hydraulic conductivity, ponding of effluent at the trench base can occur and anaerobic conditions may develop (Beach et al 2005;Hu et al 2007; Knappe et al 2020;Siegrist and Boyle 1987;Van Cuyk et al 2001).…”

mentioning

confidence: 99%

“…The development of this biomat layer is linked to the physical accumulation of suspended solids within soil pores and microbial growth (McKinley and Siegrist 2010; Thomas et al 1966;Beach et al 2005;Siegrist and Boyle 1987;Knappe et al 2020;Jones and Taylor 1965;Bouma 1975). As gradual biomat development results in a growing resistance to flow and reductions in hydraulic conductivity, ponding of effluent at the trench base can occur and anaerobic conditions may develop (Beach et al 2005;Hu et al 2007; Knappe et al 2020;Siegrist and Boyle 1987;Van Cuyk et al 2001). Within these ponded infiltration trenches and mature biomats, as well as other areas of the STU which lack oxygen such as saturated microsites, anaerobic organisms (methanogenic bacteria etc.)…”

mentioning

confidence: 99%

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Assessing the spatial and temporal variability of GHG emissions from different configurations of on-site wastewater treatment system using discrete and continuous gas flux measurement

Knappe

Somlai

Gill

2021

Preprint

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Abstract. Global emissions linked to wastewater treatment are estimated to account for up to 1.5 % of total greenhouse gas (GHG) emissions globally. However, few studies have measured GHG emissions from domestic on-site treatment systems (DWWTSs) directly. In this study, two DWWTSs were monitored for 446 days and > 42,000 gas flux measurements were conducted using both discrete spot measurements and continuous flux chamber deployments. The observed GHG fluxes from biological activity in the soil and water phase were found to be highly spatially and temporally variable and correlated to environmental factors, water usage patterns and system design. In total, the results show that a septic tank discharging effluent into a well-designed soil treatment unit is estimated to emit a net 9.99 kg-CO2eq cap−1 yr−1, with approximately 63 %, 27 % and 10 % of the total CO2-equivalent net emissions in the form of CO2, CH4 and N2O, respectively. Emissions from the septic tank surface contributed over 50 % of total emissions and tended to be strongly underestimated by one-off discrete measurements, especially when episodic ebullitive events are to be considered. Fluxes from the soil treatment unit (STU) stemmed from both the soil surface and the vent system, but were also found to be periodically negative, i.e. net uptakes. Soil fluxes were mostly influenced by temperature but peaked regularly under conditions of rapidly changing soil water content. Vent fluxes were mostly governed by effluent quality and a low number of high emission events was responsible for the majority of total observed vent emissions. Owing to the strong overall spatial and temporal heterogeneity of observed fluxes from DWWTSs across all modules, future studies should focus on continuous deployments of a number of flux chambers over discrete measurements to accurately assess GHG emissions from on-site systems. This study also provided insights into managing GHG emissions from DWWTSs by different system configuration design, as well as indicating that the current IPCC emission factors for CH4 and N2O are significantly overestimating emissions for on-site wastewater treatment systems.

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Section: Seasonal Patternsmentioning

confidence: 72%

Section: Effluent Qualitymentioning

confidence: 91%

Section: Auxiliary Datamentioning

confidence: 99%