A mechanistic understanding of the nitrification sand layer performance in a nitrogen removing biofilter (NRB) treating onsite wastewater
Graphical abstract
Introduction
More than 60 million people and about one third of new houses in US are served by onsite wastewater treatment systems (OWTSs) (EPA, 2002b). Conventional OWTS including septic tanks and drain fields are not designed to remove nitrogen from domestic wastewater (Boyer and Rock, 1992). The excess nitrogen increases the risk of eutrophication in coastal areas and poses a human health problem (Kinney and Valiela, 2011; Wolfe and Patz, 2002). To address this issue, advanced soil based OWTSs have been installed in coastal areas and regions with shallow water tables to remove nitrogen with enhanced biological nitrogen removal processes. Advanced OWTSs have shown effective nitrogen removal (16–25 mg N L−1 in effluent) compared with conventional OWTSs (40–180 mg N L−1 in effluent) (Amador et al., 2018). Soil based OWTSs are typically installed near the ground surface (15–30 cm deep) to receive more diffused oxygen (Cooper et al., 2015). Among the different configurations of soil based OWTSs, nitrogen removing biofilter (NRB) is a group of passive biofilters that can facilitate nitrogen removal when the onsite wastewater flows through engineered media layers due to gravity (Waugh et al., 2020). The major nitrogen removing pathways involved in this process are i) nitrification of ammonium to nitrite/nitrate in the upper sand layer (aerobic condition) by nitrifying microorganisms, and ii) denitrification in the bottom saturated sand/lignocellulose layer (anoxic condition) by denitrifying microorganisms. As a non-proprietary technology, the NRB systems have been piloted in Long Island, NY and other coastal regions, and has demonstrated great potential for nitrogen removal from onsite wastewater.
Successful nitrogen removal in such an NRB highly depends on the extent of nitrification in the sand layer, since the nitrifying bacteria are autotrophic and grow much slower compared with the heterotrophic organic matter degraders (i.e., BOD degraders) (Benthum et al., 1997; Lamb et al., 1990; Okabe et al., 1996). Low hydraulic retention time (HRT), low dissolved oxygen (DO <2 mg L−1) (Beccari et al., 1992) or low temperature (Zhu and Chen, 2002) can cause nitrification failure in the upper sand layer (Rittmann and Carty, 2001). In addition, alkalinity availability is an important factor for complete nitrification due to: a) nitrification is proton generating process with a stoichiometric 7.14: 1 alkalinity to nitrogen ratio requirement for complete nitrification (Eding et al., 2006; Li and Irvin, 2007; Villaverde et al., 1997); b) nitrifying bacteria are sensitive to pH changes (EPA, 2002a), while the increased nitrification rate was reported to occur in the pH range of 7.0–8.0 (Cho et al., 2014). Previous studies have reported insufficient alkalinity negatively affected nitrification in biofilters treating municipal wastewater. A linear relationship between nitrification performance reduction and the alkalinity decrease was observed in plastic-media filters treating sea water (Kikuchi et al., 1994). Villaverde et al. reported that the nitrification became alkalinity limited when pH was below 5 in submerged biofilters treating domestic wastewater (Villaverde et al., 1997). The alkalinity level of 200 mg CaCO3 L−1 was recommended for wastewater treatment plants using nitrifying biofilters (Chen et al., 2006a), and the difference between influent and effluent alkalinity has been used to indicate the extent of nitrification / denitrification happening in a sequencing batch reactor (Li and Irvin, 2007).
The thickness of the upper sand layer in NRBs not only determines the space requirement and construction cost, it is also critical for the functionality of the system as it affects the HRT. The sand layer in current NRB designs has a depth of approximately 45 cm (~ 18 in.) and a hydraulic loading rate of 26.7 L m−2 d−1 (i.e., 0.65 gal ft.−2 d−1) to ensure sufficient HRT for complete nitrification (Langlois et al., 2020; Waugh et al., 2020). It has been reported that as the depth of intermittent sand filter increased from 25 cm to 65 cm, the filters performed better in organic matter removal in pond effluent treatment due to higher HRT (Torrens et al., 2009). The media depth of 20–80 cm has been reported for sand filtration systems similar to NRBs (Grantham et al., 1949; Rodgers et al., 2010; Widrig et al., 1996). In a previous study, the nitrogen removal performance of aerated leach field mesocosms did not change significantly at different depths (Amador et al., 2008). however, the other factors affecting the mesocosm performance and the distribution/abundance of associated functional species in the system has not been investigated. Hydraulic loading is another factor that governs the nitrification performance in the sand layer of the NRB, and its level is closely related to the filtration area. When the domestic wastewater hydraulic loading rate (HLR) increased from 100 to 700 L m−2 d−1, the nitrification efficiency of sand filters decreased from 89% to 49% for the influent containing 53.5 ± 13.5 mg N L−1 Total Kjeldahl Nitrogen (TKN) (Tonon et al., 2015). In a vertical flow constructed wetland (depth 42 cm), more than 60% of nitrification was observed with HLR of 80 L m−2 d−1 when treating synthetic wastewater with 49 mg N L−1 TN in the influent (Prochaska et al., 2007). When various HLRs (40 to 125 L m−2 d−1) were applied to treat domestic sewage by a pilot-scale subsurface wastewater infiltration system (depth 100 cm), higher NH4+ removal efficiency (> 85%) was observed at the low HLR range of 40–81 L m−2 d−1 (Li et al., 2012). The HLR applied for OWTSs (20.5–49.2 L m−2 d−1) was designed for single houses (NYSDEC, 2014) and is much lower compared with the studies mentioned above. A previous study also confirmed the flow rate of domestic wastewater was fluctuating and can result in low HLRs in OWTSs (8.4–13.7 L m−2 d−1) (Gill et al., 2007). In addition, a wide range of nitrogen loading rates (NLRs) from 0.04 to 1.3 mg N cm−2 d−1 were observed for domestic wastewater treatment with OWTSs due to the various HLRs and the fluctuation in influent water quality (De and Toor, 2015; Gill et al., 2007; Renman et al., 2008; Rodriguez-Gonzalez et al., 2020). In these studies, the nitrification efficiency of 59–99% was observed with the final effluent NH4+ concentration in the range of 0.5–13.3 mg L−1. Nitrogen loading for advanced OWTSs reported to be up to 9.6–96.2 g N d−1 per system (Amador et al., 2018) and the area of pressurized shallow drain fields (HLR = 21.2 L m−2 d−1) estimated to be 61.6 m2 (Amador and Loomis, 2019) which resulted in the typical NLR of 0.02–0.16 mg N cm−2 d−1 for advanced OWTSs.
In this study, our major goal is to develop a mechanistic understanding of how environmental/operational changes impact nitrification performance in NRBs, in order to test the feasibility and the sustainability of such systems in areas with shallow groundwater table, limited space, and/or fluctuating influent water quality. The overarching hypotheses of this work are: i) the microbial community established in the sand layer of an NRB can sustain efficient nitrification at designed hydraulic loading and nitrogen loadings typical for NRBs; ii) nitrification occurs within the top few inches of the sand layer where the majority of the biomass is present, and the extent of nitrification is little impacted by the increased sand layer depth at the designed hydraulic loadings; and iii) the extent of nitrification is sensitive to the alkalinity availability in the influent, while oxygen is sufficient at the designed intermittent dosing pattern. To test the hypotheses, the objectives of this study are: i) Evaluate the effect of alkalinity and DO availability on nitrification performance at different depths using synthetic and real septic tank effluent (STE); ii) Compare the nitrification performance at various depths of the sand layer under selected hydraulic and nitrogen loadings; and iii) Investigate the abundance and distribution of microbial species within the sand layer at various depths under tested operation conditions.
Section snippets
Experimental setup
The aged nitrification filter matrix used in these experiments was taken from an NRB that has been continuously receiving STE for over two months (NH4+ave = 45.6 mg N L−1, TKNave = 51.8 mg N L−1, hydraulic loading = 16.4 L m−2 d−1) at the Massachusetts Alternative Septic System Test Center (MASSTC). The nitrification filter matrix was ASTM C33 (d10 = 0.15–0.30 mm, d50 = 0.45–1.18 mm, d60 = 0.60–1.20 mm, uniformity coefficient < 4.0, and hydraulic conductivity of 0.92 m s−1) (ASTM, 2018;
Nitrification performance as a function of depth
At all experimental stages, the oxygen partial pressure was between 0.20 and 0.23 atm at all sensor depths (6, 14, 22 and 30 cm) (Supporting Material, Fig. S2), which is close to the partial pressure of oxygen in air over the temperature range of the experiments (17–25 °C). Therefore, at all experimental stages aerobic conditions were observed throughout the columns. A drop in the oxygen level was observed right after the dosing event indicating oxygen consumption for biochemical oxygen demand
Conclusion
In this study, full nitrification was observed within the top 15 cm of sand filters when alkalinity was sufficient, and the quantitative microbial analysis confirmed the majority of the microbial community was present at the top layer (1–15 cm) of the sand filters. The microbial community originally present in the aged sand matrix could sustain effective nitrification, indicating the system was resilient to environmental changes (e.g., seasonal flow change or heavy rain/storm event) and could
Author statement
Zahra Maleki Shahraki: Conceptualization, Writing - original draft, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing - review & editing.
Mian Wang: Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing - review & editing.
Harold Walker: Investigation, Supervision, Writing - review & editing.
Frank Russo: Methodology, Resources.
Christopher Gobler: Funding acquisition, Resources, Supervision.
Xinwei Mao: Conceptualization, Funding
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by a grant to the Center for Clean Water Technology (CCWT) from the New York State Department of Environmental Conservation [NYS-DEC01-C00366GG-3350000].
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2022, Science of the Total EnvironmentCitation Excerpt :While there are many plant-induced factors that contribute to the lower pH of the surrounding soil, the extent of root-mediated pH changes is a direct function of the buffering capacity of the soil (Hinsinger et al., 2003). In sandy soil with low organic matter content, the buffering capacity of the soil is minimal and the corresponding extension of plant-induced pH changes is large (Hinsinger et al., 2003; Maleki Shahraki et al., 2021). This would perhaps explain the observed consistently lower soil pH throughout the entirety of the soil profile in planted systems.