Comparative Environmental Performance of small scale wastewater treatment systems in Norway-A Life Cycle Analysis

Abstract

ABSTRACT Globally, the development of wastewater treatment systems evolved in order to treat wastewater so as to mitigate and reduce the public health issues as well as environmental impacts resulting from the discharge of untreated wastewater. To achieve this objective, treatment of wastewater is carried out with different technologies, some centralized and other decentralized. With further development in the wastewater management sector, sustainability of the wastewater treatment system with minimum environmental degradation became a global concern because all human individuals either living today or in future, have equal rights. Therefore, based on the sustainable development approach of wastewater treatment systems, various methods have been practiced to analyse and compare the wastewater treatment systems looking from the environmental, economic, technical and social point of view. Life Cycle Assessment (LCA) is one of them and has been successfully practiced globally, in order to analyse the environmental burdens and the potential impacts associated with a Wastewater Treatment (WWT) system. LCA is the compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product or system throughout its life cycle i.e. the stage from “cradle-to-grave” (ISO 144040:2006(en)). It has become a successful tool in identifying opportunities to improve the environmental performance hence playing an integral role in decision making towards sustainability. This study is focused on identifying and analysing the environmental burdens from three different decentralized WWT systems that are in operation in Norway using LCA. Kaja grey water treatment system is based on source separation technique which treats only the grey water generated by 48 persons, Høyås farm WWT system treats domestic wastewater only from a household of 8 persons and Vidaråsen WWT system that treats domestic wastewater from from 200 peoples along with wastes from a dairy, a bakery, a laundry, an animal husbandry, a food-processing workshop and a herb-garden. The boundary of the LCA study is limited only up to construction and operation phases and the functional unit considered was wastewater generated per person equivalent over duration of 20 years. CML2 baseline 2000 (Centre for Environmental Science, University of Leiden, The Netherlands) of SimaPro 7 software, has been used in analysing the environmental impacts limited to Acidification Potential (AP), Eutrophication Potential (EP), Ozone Layer Depletion Potential (ODP) and Global Warming Potential (GWP). vii For Høyås farm WWT system, AP (99.6% of the total value) is resulted during the construction phase. The main factor contributing to the impact is the production process of filter media “Filtralite-P” (41%) and pre-fabricated fibre glass components (31.3%). Likewise, ODP (98.9% of the total value) is also generated during the construction phase and the key resulting factor to the impact is the production process of filter media “Filtralite-P” (62.6%). Similarly, EP (96.55% of the total value) is resulted during the operation phase. Total-N (89.4%) and Total-P (7.2%) are the main elements contributing to the impact. GWP is resulted in both phases, 57.64% in operation phase and 42.24% in construction phase. Methane emission is the major greenhouse gas contributing 60.12% followed by carbon dioxide emission contributing 21.5% to GWP (Annex 8). For Kaja grey water treatment system, AP (99.9% of the total value) and ODP (99.63% of the total value) is contributed during the construction phase. The main factor contributing to both of the impacts is the production process of filter media “Filtralite-P” (78.9 % of total AP and 89.9% of total ODP). Likewise, EP (87.5% of the total value) is generated during the operation phase. Total-N (74%) and Total-P (13%) are the main element resulting to the impact. Similarly, GWP is generated in both phases, 55.38% in construction phase and 44.61% in operation phase. Methane emission is the major greenhouse gas contributing 45% followed by carbon dioxide emission contributing 42.9% to the impact. In the construction phase, GWP is caused by CO2 emission during the production of Filtralite-P (37.6%) and polystyrene foam (11.9%). Similarly, for Vidaråsen WWT system, AP (98.54% of the total value) and ODP (96.36% of the total value) is generated during the construction phase. The main factor contributing to both of the impact is the production process of filter media “Filtralite-P” (61.3 % of total AP and 82.6% of total ODP). Likewise, EP (98.04% of the total value) is contributed during the operation phase. Total-N (65.62%) and Total-P (32.3%) are the main elements in the effluent causing the impact. Similarly, GWP (98.58%) originates during the operation phase. Methane emission is almost 100% responsible for this impact and is contributed by the emissions from septic tank, facultative pond and constructed wetlands during the operation period. From the results of environmental impacts of all the three systems, it is seen that AP and ODP originate in the construction phase of every systems. The major factor contributing to these impacts in all the three treatment systems is the production process of filter media viii “Filtralite-P” (expanded clay). Productions of pre-fabricated fibre glass components are also responsible for these impacts. In all the three systems, EP is occuring during the operation phase and Total-N is the main element responsible for the impact. Likewise, GWP in two of the systems is mainly originated during operation phase but in one system it is originated in both the construction and operation phase. Greenhouse gases contributing to GWP are methane (CH4) emission from the treatment units during the operation stage and carbon dioxide (CO2) emission during the production process of Filtralite-P in the construction phase. Comparative assessment of three systems show that Kaja grey water treatment system is the system with best environmental performance. The system is based on source separation technique occupying a very small area with a low number of treatment units (a septic tank, a bio-filter unit and a horizontal flow constructed wetland) and treats grey water from 48 persons. The Kaja grey water treatment system contributes the least to EP and GWP among the three systems. However, the environmental performance scenario could be different if the system boundary is expanded to include the vaccum toilet system including the required plumbing elements. Høyås farm WWT system has the highest contribution to AP, EP and ODP among the three systems. This could be one of the findings that it is more reliable and environment friendly to treat wastewater from a group of houses or clusters rather than building up a treatment system only for a single household as in case of Høyås farm. Similarly, Vidaråsen WWT system has the highest contribution to GWP. This is resulted because the scale of the system is higher than other systems so it has higher operational greenhouse gas emissions. The important finding from this study is, though Filtralite-P has been regarded as a high quality filter media for phosphorus removal (ÁdÁm, et al., 2007), its production process has significant impacts in the environment regarding AP, GWP and ODP. According to (Roseth, 2000; Adam, et al., 2007) Filtramar (shell-sand) has higher phosphorus adsorption capacity than Filtralite-P. So recommendation can be made to analyse the environmental impacts, with Filtramar (shell-sand) used as an alternative filter media in on-site wastewater treatment systems. Environmental impacts associated with transport of sludge have minor contribution but still there could be options for reducing the sludge disposal cost and the potential impacts resulting during its transport. Like, in case of Vidaråsen WWT system, where the sludge ix volume generated is high, sludge drying reed-beds can be constructed near to the site so that a significant reduction in the sludge volume can be achieved.