IWRA Proceedings

1. Introduction In communities with low population densities and dispersed households, decentralised systems for wastewater treatment can be a long-term solution and a reliable and cost-effective option (Massoud et al., 2009). In addition, the Landfill Directive (Council Directive 1999/31/EC) requires from Member States to gradually divert organic wastes, such as domestic organic waste (DOW) away from landfills, towards material and energy recovery. Furthermore, in order to achieve a 'low-carbon footprint' society, waste treatment technologies should be maximize resource recovery (Nakakubo et al., 2012). The main objective of this study was to assess the environmental performance of a system designed for the decentralised co-management of wastewater and DOW in a small community of 2,000 inhabitants. The integrated system under assessment consists of anaerobic treatment of wastewater in an upflow anaerobic sludge blanket (UASB), the fermentation of DOW to produce the carbon source rich in short-chain fatty acids (SCFAs) required for nutrient removal in a sequencing batch reactor (SBR) and a composting unit for the treatment of the fermented solid and the dewatered waste activated sludge produced from the biological process to be further applied to agricultural land. Alternative scenarios were examined and evaluated in order to identify potential environmental improvements. The methodology selected in order to perform the environmental assessment was the life cycle assessment (LCA). LCA results in the quantitative evaluation of the impact of a certain activity on the environment throughout its whole life cycle (ISO 14040, 2006). 2. Description of the base system and alternative scenarios under assessment The 'baseline' treatment system (Scenario 1a) includes the treatment of wastewater in a UASB, where biogas is produced and subsequently converted into heat. DOW is fermented in order to produce a carbon source, which is then fed to the SBR where the denitrification via nitrate is performed. After the SBR process, the final effluent is discharged into river. The sludge produced along the system is sent for composting and the produced compost is applied to land as soil conditioner. Furthermore, it was considered that both the production of heat and the use of compost within the examined system would replace the production of heat from natural gas and the use of peat as a soil conditioner (Saer et al., 2013). As previously mentioned, all the fermented liquid is fed into the SBR as carbon source for nitrogen removal via nitrate as baseline case (Scenario 1a). The environmental consequences of different nitrogen removal options were assessed: short-cut nitrification-denitrification (SCND) (i.e. nitritation/denitritation) (Scenario 1b) and partial nitritation-anoxic ammonium oxidation (anammox) (PN-ANM) (Scenario 1c). Therefore, these three nitrogen removal options were evaluated considering that a fraction of the fermented liquid (35%) is recirculated to the anaerobic reactor (Scenarios 2a, 2b and 2c). The functional unit was selected for the treatment of wastewater and DOW produced for 2,000 inhabitants per day. Within the environmental assessment, the production of inputs and outputs from the management 'baseline' system were included, as well as, for the proposed alternative scenarios. 3. Results and discussion The environmental profile was estimated using the characterisation factors provided by the ReCiPe Midpoint methodology (Goedkoop et al., 2009). The impact categories selected were climate change (CC), ozone depletion (OD), terrestrial acidification (TA), freshwater eutrophication (FE), marine eutrophication (ME) and fossil depletion (FD). Concerning the impact categories of CC, OD, TA and POF, the major environmental burdens were related to the SBR process and the composting unit. The SBR highly affected CC, OD and POF due to the increased electricity requirements. With regard to TA, ammonia emissions from the composting process highly affected this impact category. As expected, the process that was mostly contributing in the case of ME and FE was the discharge of the treated effluent into the river, due to phosphorus and nitrogen releases. Owing to this treatment system, environmental impact in eutrophication related categories has decreased compared with the impact produced if wastewater would be discharged without any treatment. In detail, it is 56% lower in FE due to phosphorus removal in the final effluent and 79% lower in ME because of nitrogen removal. Finally, environmental credits improved the environmental profile of the system, especially due to avoidance of heat production from natural gas. As mentioned, five alternative scenarios were analysed in order to determine the most viable options to be applied at a decentralized level. In all impact categories, the partial recirculation of a fraction of the fermentation liquid to the anaerobic reactor (Scenarios 2a, 2b and 2c) results in better environmental performance compared with the simplest schemes (Scenarios 1a, 1b and 1c) due to the higher production of biogas and consequently higher credits are expected since the use of natural gas to produce heat is avoided. Concerning the removal of nitrogen, each option entails different emissions of nitrous oxide and carbon dioxide, as well as electricity consumption due to aeration requirements. Despite these emissions, electric consumption is in this case key factor in the definition of the environmental profile. Denitrification via nitrate (Scenarios 1a and 2a) requires 30% more aeration than SCND (Scenarios 1b and 2b), while SCND requires 30% more aeration than PN-ANM (Scenarios 1c and 2c). Thus, the best environmental results are obtained for PN-ANM process. However, there is not specific treatment for phosphorus removal. 4. Conclusions This study examined the environmental performance of the combined management of wastewater and DOW for a small community. The results showed that the production of electricity is the major hotspot for almost all impact categories under examination. However, regarding FE and ME, the discharge of the effluent is the main contributor. The production of valuable products, such as biogas and compost enhanced the environmental profile through the contribution of environmental credits. The recirculation of part of the fermentation liquid and PN-ANM in the SBR was the best option from an environmental point of view. 1. Directive 1999/31/EC, 1999. Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste. 2. Goedkoop, M., Heijungs, R., Huijbregts, M., Schryver, A. De, Struijs, J., Zelm, R. Van, 2009. ReCiPe 2008, A Life Cycle Impact Assessment Method Which Comprises Harmonised Category Indicators at the Midpoint and the Endpoint Level. University of Leiden, Radboud University Nijmegen, RIVM, Bilthoven, Amersfoort, Netherlands. 3. ISO 14040, 2006. Environmental Management-Life Cycle Assessment- Principles and Framework, Geneve. 4. Massoud, M.A., Tarhini, A., Nasr, J.A., 2009. Decentralized approaches to wastewater treatment and management : Applicability in developing countries. J. Environ. Manage. 90, 652–659. doi:10.1016/j.jenvman.2008.07.001 5. Nakakubo, T., Tokai, A., Ohno, K., 2012. Comparative assessment of technological systems for recycling sludge and food waste aimed at greenhouse gas emissions reduction and phosphorus recovery. J. Clean. Prod. 32, 157–172. doi:10.1016/j.jclepro.2012.03.026 6. Saer, A., Lansing, S., Davitt, N.H., Graves, R.E., 2013. Life cycle assessment of a food waste composting system: environmental impact hotspots. J. Clean. Prod. 52, 234–244. doi:10.1016/j.jclepro.2013.03.022