Comparative life cycle assessment of aerobic treatment units and constructed wetlands as onsite wastewater treatment systems in Australia

Life Cycle Assessment was used to evaluate onsite wastewater treatment systems (OWTS): aerobic treatment unit (ATU) with reinforced concrete (C.ATU) and HDPE (H.ATU) tank; and constructed wetland (CW) with three biochar concentrations in the substrate (0%; 10, and 20% v:v), dubbed CW.BC0, CW.BC10 and CW.BC20, respectively. CML 2001 in SimaPro® was used to evaluate the impacts of the treatment of 1 m3 wastewater. The OWTS were compared on their overall environmental performance scores (OEP). ATUs have higher impacts on human toxicity, eutrophication, freshwater and marine ecotoxicity. The CW.BC20 has the lowest global warming impact (GWP) while CW.BC0 has the highest. Electricity consumption was the largest contributor to the impacts of the ATUs. PVC pipes, coir peat, geomembrane, and electronic devices were the biggest contributors to the impacts of the CWs. The OEP of the CWs were almost a third of the ATUs’ (6.07E-03). Changes in electricity sources were tested according to the 2030-Australian targets; increasing renewables share improves the OEP of ATUs by 39%; nevertheless, CWs continue to outperform the ATUs. Variations in biochar biodegradation has small effect on the OEP of CWs; being relevant only to GWP. This study provides a reference to policy makers for better evaluation of OWTS.

Comparison of Nitrogen Treatment by Four Onsite Wastewater Systems in Nutrient-Sensitive Watersheds of the North Carolina Coastal Plain

Wastewater may be a source of nitrogen (N) to groundwater and surface waters if not effectively treated. In North Carolina, onsite wastewater systems (OWSs) are used by 50% of the population for wastewater treatment, but most OWSs are not routinely monitored. There is a lack of information regarding the N contributions from OWSs to water resources. Four sites with OWSs were instrumented with groundwater wells near their drainfield trenches to compare N concentrations in groundwater to concentrations in wastewater and to determine the N treatment efficiency of the systems. Two OWSs (Site 200 and 300) were less than 1 year old, and two (Site 100 and 400) were more than 10 years old at the start of the study. Two OWSs (Site 100 and 200) used pressure dosing, while two OWSs (Site 300 and 400) used gravity distribution. The mean N treatment efficiency of the four OWSs was 77%. The new OWSs were more efficient (92%) relative to the older OWSs (62%) at reducing N concentrations. Similar N treatment efficiencies were observed when pooling data for the pressure dosed (77%) and gravity (79%) OWSs. Each OWS influenced groundwater by causing increases in N concentrations. It is important that new OWSs are installed at a shallow depth and with sufficient separation to groundwater to promote the aerobic treatment of wastewater. Remediation strategies including the installation of permeable reactive barriers or the use of media filters may be needed in some areas to reduce N transport from existing OWS.

Removal of Pathogens in Onsite Wastewater Treatment Systems: A Review of Design Considerations and Influencing Factors

Conventional onsite wastewater treatment systems (OWTSs) could potentially contribute to the transmission of infectious diseases caused by waterborne pathogenic microorganisms and become an important human health concern, especially in the areas where OWTSs are used as the major wastewater treatment units. Although previous studies suggested the OWTSs could reduce chemical pollutants as well as effectively reducing microbial contaminants from onsite wastewater, the microbiological quality of effluents and the factors potentially affecting the removal are still understudied. Therefore, the design and optimization of pathogen removal performance necessitate a better mechanistic understanding of the hydrological, geochemical, and biological processes controlling the water quality in OWTSs. To fill the knowledge gaps, the sources of pathogens and common pathogenic indicators, along with their major removal mechanisms in OWTSs were discussed. This review evaluated the effectiveness of pathogen removal in state-of-art OWTSs and investigated the contributing factors for efficient pathogen removal (e.g., system configurations, filter materials, environmental and operational conditions), with the aim to guide the future design for optimized treatment performance.

Water table dynamics beneath onsite wastewater systems in eastern North Carolina in response to Hurricane Florence

Onsite wastewater treatment systems (OWSs) are commonly used in eastern North Carolina. A vadose zone or vertical separation distance (VSD) between the OWS drainfield trenches and groundwater is required for effective aerobic wastewater treatment. Extreme weather events, including hurricanes, can deliver significant rainfall that influences groundwater levels and reduces the VSD, thus also influencing the treatment of wastewater by the OWS. Few studies have quantified the effects of storms on the VSD. Groundwater levels at three sites with the OWS were monitored before, during, and after Hurricane Florence. Groundwater rose over 1.5 m within 9 h at the sites in response to rain from the hurricane but took more than 3.5 weeks to return to prestorm levels. Groundwater inundated the drainfield trenches for several days at two sites leading to direct discharge of wastewater to groundwater. The hydraulic gradient and the groundwater velocity increased during the storm and the groundwater flow direction shifted, leading to greater dispersion of wastewater impacted groundwater. The wastewater treatment efficiency of the soil-based OWS in coastal areas may lessen over time because of rising water tables and reduced VSD. Individual pretreatment OWSs, elevated drainfields, or centralized sewage treatment may be required in regions with shrinking VSDs.

Efficiency of Iron- and Calcium-Impregnated Biochar in Adsorbing Phosphate From Wastewater in Onsite Wastewater Treatment Systems

This study evaluated the potential of biochar impregnated with Fe3+ or Ca2+, or mixed with Polonite®, as a filter material for removal of phosphate (PO4-P) from wastewater in onsite wastewater treatment systems (OWTS). Four treatments with biochar were investigated: unimpregnated biochar (UBC), biochar impregnated with iron Fe3+ (FBC), biochar impregnated with calcium oxide (CBC), and biochar mixed with Polonite® (PBC). In a batch experiment using phosphate solution at concentrations 0.5, 3.3, 6.5, 13, and 26 mg PO4-P L–1, adsorption of PO4-P in the different treatments was modeled using Langmuir and Freundlich isotherms. Column filters (5 diameter × 55 cm height) packed with UBC, FBC, CBC, and PBC were then furnished with raw wastewater over 148 weeks. During this experiment, adsorption of PO4-P was investigated in response to increasing hydraulic loading rate (HLR; 56, 74, and 112 L m–2 day–1) and increasing phosphate loading rate (PLR; 195, 324, 653, and 1715 mg PO4-P m–2 day–1). Among the materials, FBC had the highest maximum adsorption capacity (Qm) based on Langmuir isotherms (3.21 ± 0.01 mg g–1). FBC and CBC showed robust performance with increasing HLR, while increasing PLR increased the amount of PO4-P retained in all filters. After 148 weeks of operation, removal of PO4-P (averaged over the last 18 weeks of operation) was 13 ± 16% for UBC, 40 ± 20% for CBC, 88 ± 12% for FBC, and 30 ± 18% for PBC. The PO4-P amount retained in filters over the 148 weeks was 84.75, 221.75, 358.38, and 152.36 g m–2 in UBC, CBC, FBC, and PBC, respectively. The adsorption capacity of the filters after 148 weeks was 1.50, 4.02, 6.41, and 2.75 mg g–1 for UBC, CBC, FBC, and PBC, respectively. The adsorption capacity values and breakthrough curves showed that low concentrations (i.e., <2.6 mg L–1) of PO4-P in wastewater would allow the FBC filter to remain active for 58 months and the CBC filter for 15 months, before PO4-P removal declined to <70%. In conclusion, biochar impregnated with iron and calcium is a promising solution for removal of PO4-P from wastewater in OWTS.