Integration of Microalgae Systems at Municipal Wastewater Treatment Plants: Implications for Energy and Emission Balances Eve Menger-Krug, †,‡, * Jutta Niederste-Hollenberg, † Thomas Hillenbrand, † and Harald Hiessl † † Fraunhofer Institute for Systems and Innovation Research ISI, Breslauer Strasse 48, 76139 Karlsruhe, Germany ‡ Karlsruhe Institute of Technology KIT, Institute for Geoecology and Geography, Kaiserstraße 12, 76131 Karlsruhe * S Supporting Information ABSTRACT: Integrating microalgae systems (MAS) at municipal wastewater treatment plants (WWTPs) to produce of bioenergy offers many potential synergies. Improved energy balances provide a strong incentive for WWTPs to integrate MAS, but it is crucial that WWTPs maintain their barrier function to protect water resources. We perform a prospective analysis of energy and emission balances of a WWTP with integrated MAS, based on a substance flow analysis of the elements carbon (C), nitrogen (N), and phosphorus (P). These elements are the main ingredients of wastewater, and the key nutrients for algae growth. We propose a process design which relies solely on resources from wastewater with no external input of water, fertilizer or CO 2 . The whole process chain, from cultivation to production of bioelectricity, takes place at the WWTP. Our results show that MAS can considerably improve energy balances of WWTPs without any external resource input. With optimistic assumptions, they can turn WWTPs into net energy producers. While intensive C recycling in MAS considerably improves the energy balance, we show that it also impacts on effluent quality. We discuss the importance of nonharvested biomass for effluent quality and highlight harvesting efficiency as key factor for energy and emission balances of MAS at WWTP. ■ INTRODUCTION Microalgae as a source of bioenergy have evoked interest in the economic and scientific community, due to their potential high energy yields. 1 But due to upstream burdens, sustainability of MAS has been questioned in recent life cycle based studies. 2-6 Integrating MAS and wastewater treatment is often recom- mended for improved sustainability. The idea of integrating MAS and wastewater treatment dates back to the 1950s 7,8 and offers many potential synergies. In theory, all resources required for algae growth are available at WWTPs. Wastewater provides a growth medium rich in macro and micro nutrients, 9-13 CO 2 can be supplied from flue gas on site. 14 Another synergy is the energy offset from (partial) wastewater treatment, as algae remove nutrients from wastewater during growth. 9-13 Harvested biomass can be used energetically for production of biofuels, 15-17 or for electricity generation via biogas 17-19 or via direct combustion of dried biomass. 6 Despite these potential synergies, only a few pilot projects of MAS running with wastewater have been described, mainly located in the U.S. 10,19 and New Zealand. 20-22 They confirm the technical feasibility of the concept. Today, with fossil resource depletion and climate change, WWTPs are searching for ways to improve their energy balance. While striving for improved energy balances, it is crucial for WWTPs to maintain their barrier function, to safeguard water resources and aquatic ecosystems from eutrophication and anthropogenic pollutants. We perform a prospective substance flows analysis (SFA) of the elements carbon (C), nitrogen (N), and phosphorus (P) at a WWTP and a WWTP with MAS, accompanied by an analysis of energy flows, to show implications for energy and emission balances. CNP are the main ingredients of wastewater and the key nutrients for algae growth. We propose a process design which relies solely on resources from wastewater, with no external input of water, fertilizer, or CO 2 . The whole algae process chain, from cultivation to production of bioelectricity, takes place on site. ■ MATERIALS AND METHODS System Description and Substance Flow Analysis (SFA). As an example, we chose a WWTP dimensioned for 20 000 population equivalents (p.e.). Figure 1 shows the system boundaries for analysis. They include the processes for wastewater and sludge treatment, including biogas use on site (water and sludge pathway). For the WWTP with integrated MAS, system boundaries additionally include the process steps cultivation, harvest and codigestion of harvested biomass (algae pathway). The WWTP and the WWTP with MAS receive identical input: raw wastewater with a typical composition and a daily Received: May 16, 2012 Revised: October 4, 2012 Accepted: October 10, 2012 Published: October 10, 2012 Policy Analysis pubs.acs.org/est © 2012 American Chemical Society 11505 dx.doi.org/10.1021/es301967y | Environ. Sci. Technol. 2012, 46, 11505-11514