Ecological assessment of selected alternative sanitation concepts via Life Cycle Assessment

Conventional and alternative sanitation concepts are compared in terms of their ecological performance via the methodology of LCA. Alternative scenarios include urine separation, faeces composting and digestion, and greywater treatment in a soil filter or technical plant. Urine and faeces are used as secondary fertilizers in agriculture after adequate treatment. Data for Life Cycle Inventory is compiled from pilot projects, literature and databases. Results of normalized ecological indicators show that alternative scenarios can offer substantial advantages over the conventional system, especially in the categories of eutrophication and terrestrial ecotoxicity. Energy-related indicators also show less energy demand for the alternative scenarios, although their contribution to the overall eco-profiles is relatively small. The construction phase can be neglected in simplified LCA studies of sanitation systems. Introduction In conventional wastewater treatment, different wastewater fractions such as toilet wastewater and wastewater from kitchen, laundry, and personal hygiene (=”greywater”) are mixed and transported to a wastewater treatment plant in a sewer system. Here, the mixed wastewater is treated with mechanical, biological, and physicochemical processes to eliminate biodegradable organic carbon and the plant nutrients nitrogen and phosphorus. Besides the high energy demand which is necessary for the aeration of the activated sludge, this approach suffers from the waste of nutrients contained in anthropogenic excretions. Dissolved nitrogen is converted into nitrogen gas via nitrification and denitrification, while phosphorus is incorporated into the sewage sludge by chemical precipitation or biological phosphorus elimination. Residual nutrients in the wastewater treatment plant effluent can trigger excessive growth of phytoplankton in the receiving waters and lead to oxygen depletion and eutrophication. In recent years, alternative sanitation concepts have been developed which are based on separate collection and treatment of the different wastewater flows. The distribution of nutrients and organic carbon (measured as chemical oxygen demand, COD) between greywater, human urine and faeces shows that these partial flows are highly different in quantity and quality (Table 1). The greywater flow has a high volume and relatively small loads of COD and nutrients, while urine and faeces are low in volume but contain the majority of nutrients N and P. Pathogenic microorganisms mainly derive from the faeces fraction, while undiluted urine is basically sterile when leaving the human body. The distribution of nutrients led to the idea of nutrient recycling via the use of human excreta for agricultural fertilizing. Industrially produced mineral fertilizer could be substituted with these fertilizers from secondary resources. Thus, the energy-intensive production of nitrogen fertilizer (fixation of atmospheric nitrogen via the Haber-Bosch process) and the import of raw phosphate rock potentially contaminated with toxic heavy metals such as cadmium (Cd), chromium (Cr), and uranium (U) could be avoided. Additionally, the separation of the nutrient-rich fractions from the remaining wastewater leads to a significant reduction of the nutrient loads entering the wastewater treatment plant and hence to a reduction in effluent loads to surface waters. Table 1: Composition of different wastewater flows (average values from literature) Greywater Urine Faeces Specific flux [kg/(pe*d)] 29200 548 51 Nitrogen load [g/(pe*a)] 1.3 10 1.5 Phosphorus load [g/(pe*a)] 0.5 1 0.5 Potassium load [g/(pe*a)] 2 2.6 0.55 COD load [g/(pe*a)] 60 15 35 Pathogenic germs ++ – ++++++ The present work deals with an ecological assessment of these alternative sanitation concepts and their comparison with the conventional system. Ecological advantages of the new conceptual approach should be quantified with an integrated evaluation method together with potential hotspots of the alternative systems. Therefore, the methodology of Life Cycle Assessment as defined in [1] is adopted for this work. A detailed substance flow model is set up which describes all relevant flows of materials and emissions. Beside the operational processes (wastewater treatment, energy production, transport etc), this LCA also includes the expenditures for the construction of the necessary infrastructure. The resulting flows of materials and emissions are evaluated with a set of environmental indicators based on the LCA guide of CML [2]. Methodology The framework of this LCA study is outlined roughly in Figure 1. The primary function of the investigated systems is the drainage and treatment of human urine, faeces, greywater, and biowaste. Preceding processes like the supply of energy, drinking water for flushing or the production of auxiliary chemicals are included into the system boundary as well as subsequent processes like the treatment of solid or liquid wastes. The functional unit of this LCA is the performance of the above mentioned primary function for one person during one year. The different sanitation scenarios for the conventional and the selected alternative sanitation concepts are listed in Table 2. The reference scenario consists of the drainage of the mixed wastewater and its treatment in an activated sludge plant with anaerobic sludge digestion and sewage gas usage for energy production. Biowaste is collected and transported to a composting plant prior to application as agricultural fertilizer. Two of the alternative scenarios use gravity separation toilets, where the undiluted urine is separated from the faeces and the flush water. While the urine is directly applied as a fertilizer in agriculture after a certain storage period (six months), faeces are separated from the flush water and stabilized aerobically in a composting process together with the biowaste before application as a fertilizer. The remaining greywater and the faeces filtrate are treated either in a Figure 1: Simplified LCA framework of this study soil filter or in an activated sluge plant (sequencing batch reactor, SBR). The other two alternative scenarios rely on vacuum separation toilets, where the faeces are drained in a vacuum system together with a small volume of flush water. They are treated in an anaerobic digestion process together with the collected biowaste to produce biogas which can be used for energy production. The digester residual can also be applied as an agricultural fertilizer after an aerobic stabilization. The separated urine and the greywater are treated in the same way as in the composting scenarios. For proper system comparison, the reference scenario has to be expanded with the functions of industrial fertilizer production and energy production to account for the secondary functions of the alternative systems. The respective amounts of fertilizer equivalents to human urine and faeces are produced industrially as nitrogen and phosphorus fertilizer in the reference scenario. Thus, the corresponding resource usage and emissions are imputed to the conventional sanitation scenario. A similar procedure is adopted for the additional energy which is produced from the biogas during faeces digestion. Table 2: Reference scenario and alternative sanitation scenarios Scenario Faeces Urine Greywater Biowaste System expansion Reference Conventional activated sludge plant with sludge digestion and use of sewage gas Composting Fertilizer Mineral fertilizer + energy Comp_Nat Soil filter Comp_Tech Gravity separation toilets + composting fertilizer Separation fertilizer SBR Composting Fertilizer Energy