INSIGHTS | PERSPECTIVES 1452 27 JUNE 2014 • VOL 344 ISSUE 6191 sciencemag.org SCIENCE Anticipating the next century of wastewater treatment Advances in activated sludge sewage treatment can improve its energy use and resource recovery WATER TREATMENT and the observed anisotropy could only be modeled if that boundary extended to the size of the crystallites. Liu et al. therefore provide evidence that disproportionation is not the domi- nant mechanism during fast cycling of nanosized LiFePO 4 . Instead, a metastable nonstoichiometric phase exists through- out the transition state accessed via the applied overpotential. Hence, instead of a local phase change, involving a progressive structural reorganization near a moving phase boundary, there exists a cooperative structural rearrangement across a wide re- gion of compositional variation as the struc- ture changes continuously from one form to the other with only a small degree of lattice strain. The latter fact explains why it is fast and reversible for thousands of cycles. The above model seems reasonable; in- stead of a phase boundary, there are con- tinuous variations in both the lithium concentration and the chemical potential (energy) of lithium in a one-phase mate- rial, as observed for normal electrodes. One problem remains: The spinodal region near the energy maximum still exists and can cause havoc—the diffusion coefficient can become negative, promoting rapid diffusion up concentration gradients and thereby forc- ing disproportionation. Maybe this effect is too weak or too slow to make a difference. However, there is another stabilizing influ- ence, which is that the moving species is a charged lithium ion; as a result, the electric potential, which increases with the current during discharge, adds another energy term to the equation ( 4). If the current is high enough, the miscibility gap, spinodal region, and anomalous diffusion effect all vanish and the behavior returns to normal as for a non–phase-transforming electrode. The value of the discovery by Liu et al. lies not only in the already optimized LiFePO 4 but also in the prediction of how it can be used to make better materials. The hope is that the single-phase transfor- mation pathway can be enabled in other phase-transforming electrode materials with high energy density, to reap the asso- ciated benefits of higher power and longer cycle life. ■ REFERENCES 1. A. K. Padhi, K. S. Nanjundaswamy, J. B. Goodenough, J. Electrochem. Soc. 144, 1188 (1997). 2. H. Liu et al., Science 344, 1252817 (2014); 10.1126/science.1252817. 3. R. Malik, F. Zhou, G. Ceder, Nat. Mater. 10, 587 (2011). 4. P. Bai, D. A. Cogswell, M. Z. Bazant, Nano Lett. 11, 4890 (2011). 5. N. Ravet et al., J. Power Sources 97-98, 503 (2001). 6. P. A. Johns, M. R. Roberts, Y. Wakizaka, J. H. Sanders, J. R. Owen, Electrochem. Commun. 11, 2089 (2009). 7. J. W. Cahn, J. E. Hilliard, J. Chem. Phys. 28, 258 (1958). 10.1126/science.1255819 R apid urbanization and industrial- ization in the 19th century led to unhealthy environments and wide- spread epidemic diseases. In re- sponse, research was undertaken that led to the development of sani- tation technology. Exactly 100 years since the activated sludge process was presented ( 1), it is still at the heart of current sewage treatment technology. Activated sludge is a mixture of inert solids from sewage com- bined with a microbial population growing on the biodegradable substrates present in the sewage. The settling and recycling of sludge inside treatment plants was the in- vention of Ardern and Lockett. The current demands from a rapidly growing human population and the need for a more sus- tainable society are pushing forward new developments for sewage handling. These developments have two main drivers: gen- eral process improvements and the contri- bution to the recycling of resources ( 2, 3). The activated sludge process, combined with a better drinking water supply, was the main factor behind the increase in av- erage life span in the previous century and for minimizing the environmental impact of human activities. Wastewater treatment is in itself a relatively low-cost process (in the Netherlands, 50 to 70 EUR per person per year), consuming limited energy (<7 W per person); its main limitations are the large upfront investment costs (usually to be re- covered from inhabitants within 20 years) and land area requirements (mainly needed for the gravity-based separation of flocculent activated sludge and treated wastewater). Attempts to intensify the separation process, e.g., by membrane separation of the sludge, have been technologically successful ( 4) but not widely used because of the additional en- ergy demand and capital costs. The morphogenesis of the microbial com- munities in activated sludge is a complex process based on the interaction of micro- biological, chemical, and physical processes ( 5). Only in recent years has it become pos- sible to engineer these microbial structures to allow bacteria to form a stable granular sludge instead of flocculent sludge (see the first figure) ( 6). This form of sludge makes gravity-based separation a compact process that can be integrated inside the treatment reactor and greatly reduces area require- ments and costs (by roughly 75 and 25%, respectively) ( 7). Activated sludge technology is based on a complex microbial ecology process, in 100 years of activated sludge—quo vadis? Two reactors at the wastewater treatment plant Garmerwolde in the Netherlands (A) using aerobic granular sludge technology (B) are treating the wastewater of 235,000 persons. By Mark C. M. van Loosdrecht 1 and Damir Brdjanovic 1,2 A Published by AAAS