In the framework of lowering the environmental impact of municipal sanitation systems, while keeping costs comparable to existing levels, the black water source-separation system (SÉP-EN), which consists of the separation of black water (toilet water) from grey water (bathroom, kitchen and laundry water) in wastewater collection, is generally advocated as a viable alternative to the conventional sanitation system (CONV). However, current studies have not led to any formal conclusions on the environmental and economic advantages of one system over the other.
The overall objective of this thesis is to determine if SÉP-EN generates higher environmental and economic performances than those of CONV. The environmental life cycle assessment (ELCA) and life cycle costing (LCC) methods are used. The functional unit of the study is as follows: "To ensure wastewater and organic kitchen refuse collection and treatment and byproduct (digestate/sludge and biogas) recycling for one inhabitant for one year." The systems being compared were implemented in a new 50,000-inhabitant city in Quebec (Canada), except in the first part of the research work (article 1) where implementation scales of 500 and 5000 people have been considered in order to study the effect of decentralization on the economic performance of SÉP-EN-0 (baseline scenario). The environmental and economic foreground data have been obtained through a review of the relevant scientific literature, from the MATTEUS v.5.11 software (IREQ, 2008) and from technology suppliers and engineering firms. The environmental background inventory data is taken from the ecoinvent v.2.2 database (ecoinvent Center, 2007). The life cycle environmental impact assessment of the sanitation systems studied is conducted using the IMPACT 2002+ method, version 2.15 (Humbert et al., 2012; Jolliet et al., 2003) and the results are presented using four damage (endpoint) indicators, namely Human health, Ecosystem quality, Climate change and Resources. The environmental life cycle costing is based on a nominal discount rate of 4%, and the life cycle cost is expressed with the specific equivalent annual cost (per inhabitant) in 2010 US dollars.
The results from the LCC show that when SÉP-EN is implemented at a scale of 500, 5,000 and 50,000 inhabitants, SÉP-EN-0 is respectively 118%, 56% and 33% more costly than CONV (article 1). With regards to the ELCA results (at the 50,000-inhabitant implementation scale), SÉP-EN generates higher impact scores (potential environmental impacts) than CONV: 100%, 89%, 24% and 25% more for the Human health, Ecosystem quality, Climate change and Resources indicators, respectively (article 2). However, the conclusions change if different assumptions are used: 1) if metal emission impacts are accounted for, SÉP-EN-0 involves less impact scores than the ones of CONV for Human health and Ecosystem quality and 2) if the North American grid mix and the organic fertilizer transport distance of 20 km are chosen, SÉP-EN-0 posts lower impact scores than the ones of CONV for Climate change and Resources.
Under the initially defined conditions, the lower environmental and economic performances (higher impact scores) of SÉP-EN-0 compared to those of CONV, mainly due to digestate spreading that generates ammonia emissions and digestate management (transport, storage and spreading), can be improved with the modification of key processes or changes in the digestate handling approach (article 3). Among the six single-pathway development scenarios compared, the scenario that includes a vacuum toilet flow-volume reduction from 1 L to 0.5 L per flush achieves a significantly higher performance than the ones of CONV for the Climate change and Resources indicators, while involving a significantly lower performance with regards to Human health and a comparable cost. The scenario that entails digestate mass reduction with reverse osmosis and acidification prior to its transport to farmland posts comparable performances to the ones of CONV for all environmental indicators and cost. The scenario with digestate treatment by means of phosphorus precipitation (struvite) and nitritation–anammox reactors gives performances that are comparable to the ones of CONV for all indicators, except for Climate change, for which this scenario has a significantly lower performance if the Quebec resource context is considered. Among the four multi-pathway development scenarios (combination of single-pathway development scenarios) established, two achieve higher performance with regards to Climate change, Resources and Human health indicators compared to those of CONV, while maintaining a comparable cost.
Although this thesis has contributed to enriching knowledge regarding SÉP-EN, some limitations associated with inventory data quality and the impact assessment method have been identified. In addition to suggesting possible avenue of solutions regarding these limitations, the thesis proposed research options such as an analysis of factors that underline the performance of SÉP-EN and an integrated environmental and economic assessment using the concept of eco-efficiency. From this concept, a comparative evaluation approach based on optimized impact-cost score has been put forward. This approach could allow comparing systems based on their minimum score, which would be determined according to their treatment technology, in order to globally lower the environmental impacts and cost over the life cycle of the compared systems.
Ultimately, designers and decision makers involved in sanitation are better informed about the performance of SÉP-EN compared to that of CONV, and about the priorities for action that lead to enhance the systems' environmental and economic performance.
| Date | 29 Aug 2014 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Frédéric Monette (Supervisor) & Mathias Glaus (Co-supervisor) |
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