Environmental life cycle assessment of septic tanks in urban wastewater system – a case study for Poland

Results of life cycle inventory (LCI) and life cycle assessment (LCA) for septic tanks collecting domestic sewage were presented. The study included the whole life cycle: construction, use and end-of-life stages of septic tanks. The analyses were conducted basing on actual data concerning performance of 793 septic tanks in Żory. Environmental impact assessment of the life cycle of septic tanks was conducted with TRACi and ReCiPe methods. Greenhouse gas (GHG) emission, eutrophication, fossil fuel depletion and metal depletion indicators were calculated and determinants of LCA of septic tanks were analysed. The system boundary was from cradle to grave. It was concluded that at the construction stage, GHG emission and fossil fuel depletion indicators are determined by the amount of concrete, steel, polyester resin, polyethylene, cast iron and PCV. At the use stage, GHG emission is determined by the amount and type of electricity used to treat sewage in a wastewater treatment plant (WWTP). Untreated wastewater, introduced into the environment (leaking tanks and users discharging sewage), is a determinant of infl uence on eutrophication. Life cycle inventory and environmental assessment of septic tanks with life cycle perspective are presented in the literature for the fi rst time. The results highlight the importance of including each stage in the environmental assessment of elements of the urban wastewater system. Archives vol 45 no 4 a4 srodki_kor 1.indd 68 2019-11-14 11:06:44 Environmental life cycle assessment of septic tanks in urban wastewater system – a case study for Poland 69 showed that hitherto works have concerned the issue of soil pollution, and resulting groundwater pollution, caused by leaks or wrong use of septic tanks (Meile et al. 2010, Richards et al. 2016, Schaider et al. 2016, Swartz et al. 2006, Wilcox et al. 2009, 2010). Existing sanitation practices in the majority of developing countries rely mainly on on-site waste treatment approaches (Wang et al. 2014), including fl ush and waterless latrines connected to pit or septic tanks as a basic treatment of the waste (Anastasopoulou et al. 2018). According to Somlai et al. (2019) septic systems used for on-site wastewater treatment are potential sources of groundwater and atmospheric pollution. The study (Somlai et al. 2019) demonstrated that there are distinct spatiotemporal patterns for both CO2 and CH4 fl uxes observed over a septic tank soakaway driven by both environmental factors and subsurface effl uent dispersal. Onsite wastewater treatment is a potential source of greenhouse gas (GHG) emissions. Considering the large number of onsite septic tank systems in use internationally, potentially constituting a signifi cant source of GHG emissions, there has been a surprising lack of direct fi eld measurements of these fl uxes to the atmosphere. Most of the existing septic system emission models rely on load-based calculations or estimated emission factors. The IPCC provides guidelines on national GHG inventories following an organic load-based approach to estimate septic system emissions. These guidelines only consider CH4 emissions from anaerobic degradation in septic tanks. Direct CO2 emissions from septic systems are omitted in the GHG inventories as they are of biogenic origin. Numerous recent studies on septic systems are focused mainly on the attenuation of chemical and biological pollutants and the risk for contamination of groundwater (Keegan et al. 2014), wells or surface waters (Dubber and Gill 2017, Ockenden et al. 2014, Withers et al. 2012) from septic systems. However, there is a limited number of studies with a scope on quantifying gas emissions from septic systems. Determinants of environmental assessment of Polish individual wastewater treatment plants in a life cycle perspective were presented by Burchart-Korol and Zawartka (2019). Environmental life cycle assessment of wastewater treatment plant under Polish condition was shown by BurchartKorol et al. (2017a). Nevertheless, there are still only few studies presenting results of an environmental analysis for the life cycle of septic tanks, which are a signifi cant element of water and sewage management in Poland, especially in the areas of scattered site housing (Burchart-Korol et al. 2017b, Zawartka 2017). The aim of the research was to assess environmental impacts of construction, use and end-of-life stages of septic tanks with life cycle perspective. The article is the fi rst one to present results of potential environmental impact throughout the life cycle of septic tanks in urban wastewater system. Materials and methods Goal and scope of analysis The aim of the article was LCA of septic tanks from construction stage, through use stage to end-of-life stage, considering Polish conditions where septic tanks are one of elements of water and sewage management system. The basic function of the analyzed system is to neutralize sewage from a given area, through treating it before reintroducing into the environment. The area where the system operates includes an administrative unit with an urban agglomeration covering part of the area. A basic unit of the system function, i.e. a functional Fig. 1. Confi guration of sewage management system Archives vol 45 no 4 a4 srodki_kor 1.indd 69 2019-11-14 11:06:44 70 D. Burchart-Korol, P. Zawartka unit (FU), is population-equivalent (1 PE), which, is defi ned as load of biodegradable organic substances expressed as 5-day biochemical oxygen demand (BOD5) of 60 g oxygen per day (Burchart-Korol and Zawartka 2019). The functional unit of the analysis is 1 PE as a parameter which is universal for each urban wastewater system considering its diversity. The study was performed for an operating system of collecting, transporting and treating sewage in the city of Żory, Poland; including the area of the agglomeration of Żory with wastewater treatment plant (WWTP Żory) located in the catchment area of the Ruda river. The system consists of septic tanks which were analyzed. The system was accepted as the reference for analyses due to: – importance of tanks in the system of collecting, transporting and treating sewage in Żory (793 pieces), – differences in location, technology (material) and age of septic tanks, – plans to expand the system due to the development of residential housing and manufacturing and service facilities and the need to provide information to decide the best confi guration of the system. At the end of 2015, in Żory there were 793 operating septic tanks: 567 concrete ones, 155 HDPE ones and 71 GRP ones (UM Żory 2016). Average capacity of a tank is 10 m3 which allows a typical household to collect sewage for a month. Septic tanks in Żory serve 2,379 PE and are located mainly in the suburbs, in scattered site housing area where there is no sewerage system. Sewage from the tanks is transported in vacuum tankers to Żory wastewater treatment plant (WWTP Żory). There is a full placement inventory of septic tanks in the city, which are placed mostly in a few districts. The distance between septic tanks and treatment plant has been set as an average length of way which vacuum tanker transports the waste to treatment plant and it does include the way to the septic tank and from septic tank to treatment plant. The most common real life scenario has been taken under account that vacuum tanker drives out from the treatment plant and comes back. The average distance between a tank and the plant is approximately 8 kilometers. The system boundaries for given life cycle stages of septic tanks are presented in Figure 2. Life cycle inventory For given life cycle stages of septic tanks, Life Cycle Inventory (LCI), necessary to conduct life cycle assessment, was performed. The main data sources were: – data from registers of septic tanks (amount, material, capacity, users, frequency of sewage disposal, data for sewage balance) obtained from the City Hall of Żory (UM Żory 2016), – data concerning use of the tanks (consumption of materials, fuels, energy to transport and treat sewage, qualitative and quantitative parameters of sewage) – obtained from the operator (PWiK Żory 2015), – data from manufacturers of the septic tanks, – research and own analysis data concerning construction and use of septic tanks. Data identifi cation and inventory concerned the whole life cycle of septic tanks. It was assumed that, in accordance with the facts, the EU’s and common practice in the water and sewerage sector, the objects are built for a life span of 30 years and within the period their operating as intended is guaranteed. Data inventory for the construction stage of septic tanks considers consumption of materials and resources and earthworks, both for the tank itself and a section of Ø160 PVC pipe transporting sewage from a building to a tank. The inventory stage of construction septic tanks included also the impact on the environment of assembly works and earthworks with the use of machinery and equipment (diggers, trucks), to perform excavations and the reuse of unnecessary excess of land near the construction site. The data inventory also employed own research associated with implementation of the Fig. 2. System boundaries of life cycle of septic tank Archives vol 45 no 4 a4 srodki_kor 1.indd 7