Abstract
Wastewater is a major source of anthropogenic nitrogen (N) pollution that causes groundwater contamination and eutrophication in coastal ecosystems. The negative effects of excess N from wastewater on human and environmental health have led the United States Environmental Protection Agency (USEPA) and many state and local agencies to set maximum N concentrations for treated wastewater before it can be discharged to ground and surface water bodies. Wastewater treatment systems that include biological nitrogen removal (BNR) can help meet these standards by promoting microbial N removal in centralized wastewater treatment plants (WTP) as well as decentralized, onsite wastewater treatment systems (OWTS; i.e., septic systems). Nitrogen removal in BNR wastewater treatment is accomplished by sequential nitrification in oxic zones and denitrification in hypoxic/anoxic zones. Wastewater treatment, including BNR, can produce the greenhouse gases (GHGs) CO2, N2O, and CH4 as by-products, potentially threatening air quality. In Manuscripts 1 and 2 of this dissertation, I investigated the dynamics of GHGs and microbial communities in OWTS that have a lignocellulose-amended, N-removing layered soil treatment area (STA). These systems are designed to promote sequential nitrification and denitrification by stratifying the two processes into layers that promote microbial N removal. Layered, nonproprietary STAs are a cost-effective alternative to proprietary, advanced N-removal OWTS. In Manuscript 3, I compared the microbial community structure and composition of nine proprietary, advanced N-removal OWTS and a WTP with BNR. Although BNR in OWTS and WTPs is designed to promote the same microbial processes – and their microbial communities assumed to be the same for OWTS management purposes – their nitrifying and denitrifying microbial communities have not been compared. In Manuscript 1, I describe CO2, N2O, and CH4 emissions from the septic tanks, Layered STAs, and Control STAs from three OWTS serving homes in southeastern MA, USA. Emissions did not differ significantly between Layered and Control STAs at any of the sites, and were controlled chiefly by temperature, soil moisture, and subsurface GHG concentrations. Per capita average emissions for these systems were higher than those reported by others, with mean values ranging from 0 to 1835 gCO2e capita-1 day-1 and from 48 to 1400 gCOe2 capita-1 day-1 in septic tanks and STAs, respectively. These results suggest that Layered STAs are unlikely to have a negative impact on air quality compared to conventional STAs. In Manuscript 2, I investigated the diversity, structure, and composition of ammonium oxidizing and nitrous oxide reducing bacterial communities in
Highlights
Coastal communities threatened by nutrient pollution from septic systems often require advanced onsite wastewater treatment systems (OWTS) that reduce C and N loads to sensitive ground and coastal waters to a greater extent than is possible with conventional OWTS (Bellone et al, 2017; FDOH, 2018; MassDEP, 2013; Murray et al, 2018; RIDEM, 2020)
We found that the Layered soil treatment area (STA) at Acushnet had higher greenhouse gases (GHGs) levels in the subsurface than the Control STA, but this did not translate to a difference in flux between the STAs, likely because of GHG consumption near the ground surface
These findings suggest that using lignocellulose-amended STAs to remove N from wastewater is unlikely to have adverse effects on air quality compared to other STAs with low-profile dispersal systems
Summary
Coastal communities threatened by nutrient pollution from septic systems often require advanced onsite wastewater treatment systems (OWTS) that reduce C and N loads to sensitive ground and coastal waters to a greater extent than is possible with conventional OWTS (Bellone et al, 2017; FDOH, 2018; MassDEP, 2013; Murray et al, 2018; RIDEM, 2020). With biological nitrogen removal (BNR) can lower the concentration of N in effluent before it is discharged to receiving waters [1], lowering the public health and environmental risks associated with N pollution of ground and surface waters [2–4] In both cases the BNR process employs some type of microbial growth surface and achieves removal by engineering conditions that promote sequential nitrification (NH4+ → NO3-) in an oxic zone and denitrification (NO3- → N2O, N2) in a hypoxic/anoxic zone. Nitrogen removal is maximized by recirculation of wastewater between the oxic and hypoxic/anoxic zones Because they are designed to promote the same microbial processes and conditions, OWTS with BNR – commonly referred to as advanced N-removal systems – are considered to be a scaled-down version of a WTP with BNR [1], with designs explicitly based on engineering principles underlying for a WTP [5,6]. The validity of this assumption can have consequences for effective management of OWTS with BNR
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