Herein we report a self-cleaning coating derived from zwitterionic poly(2-methacryloyloxylethyl phosphorylcholine) (PMPC) brushes grafted on a solid substrate. The PMPC surface not only exhibits complete oil repellency in a water-wetted state (i.e., underwater superoleophobicity), but also allows effective cleaning of oil fouled on dry surfaces by water alone. The PMPC surface was compared with typical underwater superoleophobic surfaces realized with the aid of surface roughening by applying hydrophilic nanostructures and those realized by applying smooth hydrophilic polyelectrolyte multilayers. We show that underwater superoleophobicity of a surface is not sufficient to enable water to clean up oil fouling on a dry surface, because the latter circumstance demands the surface to be able to strongly bond water not only in its pristine state but also in an oil-wetted state. The PMPC surface is unique with its described self-cleaning performance because the zwitterionic phosphorylcholine groups exhibit exceptional binding affinity to water even when they are already wetted by oil. Further, we show that applying this PMPC coating onto steel meshes produces oil-water separation membranes that are resilient to oil contamination with simply water rinsing. Consequently, we provide an effective solution to the oil contamination issue on the oil-water separation membranes, which is an imperative challenge in this field. Thanks to the self-cleaning effect of the PMPC surface, PMPC-coated steel meshes can not only separate oil from oil-water mixtures in a water-wetted state, but also can lift oil out from oil-water mixtures even in a dry state, which is a very promising technology for practical oil-spill remediation. In contrast, we show that oil contamination on conventional hydrophilic oil-water separation membranes would permanently induce the loss of oil-water separation function, and thus they have to be always used in a completely water-wetted state, which significantly restricts their application in practice.

Biological nitrogen removal through the nitrite pathway (NH → NO → N) is favorable for wastewater treatment plants without sufficient carbon sources. This study demonstrates an innovative approach for attaining the nitrite pathway based on sludge treatment using free ammonia (FA, i.e., NH). This approach is based on our innovative discovery in this study that FA at 210 mg NH-N/L is far less biocidal to ammonium-oxidizing bacteria (AOB) than to nitrite-oxidizing bacteria (NOB). A total of 22% of the activated sludge from the sequencing batch reactor (SBR) receiving synthetic domestic wastewater was treated in an FA treatment unit at 210 mg NH-N/L for 1 day. The FA-treated sludge was afterward recirculated back to the SBR. A nitrite accumulation ratio of above 90% was quickly achieved (in 40 days) and maintained stably in the SBR, indicating the establishment of the nitrite pathway. The NOB population and activity after implementing FA treatment was less than 5% of those without FA treatment, suggesting the washout of NOB. In contrast, the AOB population and activity in the SBR were not affected. The nitrogen-removal performance was significantly improved after incorporating the FA approach. The FA approach is a closed-loop approach and is economically and environmentally attractive.

Stable suppression of nitrite oxidizing bacteria (NOB) is one of the major bottlenecks for achieving mainstream nitrite shunt or partial nitritation/anammox (PN/A). It is increasingly experienced that NOB could develop resistance to suppressions over an extended time, leading to failure of nitrite shunt or PN/A. This study reports and demonstrates the first effective strategy to overcome NOB adaptation through alternating sludge treatment with free nitrous acid (FNA) and free ammonia (FA). During over 650 days reactor operation, NOB adaptation to both FNA and FA was observed but the adaptation was successfully overcome by deploying the alternate treatment strategy. Microbial community analysis showed Nitrospira and Nitrobacter, the key NOB populations in the reactor, have the ability to adapt to FNA and FA, respectively, but do not adapt to the alternation. Stable nitrite shunt with nitrite accumulation ratio over 95% and excellent nitrogen removal was maintained for the last 8 months with only one alternation applied. N2O emission increased initially as the attainment of nitrite shunt but exhibited a declining trend during the study. By using onsite-produced nitrite and ammonium, the proposed strategy is feasible and sustainable. This study brings the mainstream nitrite shunt and PN/A one step closer to wide applications.

Anaerobic ammonium oxidation (anammox) was discovered as a new microbial reaction in the late 1990s, which led to the development of an innovative energy- and carbon-efficient technologypartial nitritation and anammox (PN/A)for nitrogen removal. PN/A was first applied to remove the nitrogen from high-strength wastewaters, e.g., anaerobic digestion liquor (i.e., sidestream), and further expanded to the main line of wastewater treatment plants (i.e., mainstream). While sidestream PN/A has been well-established with extensive full-scale installations worldwide, practical application of PN/A in mainstream treatment has been proven extremely challenging to date. A key challenge is achieving stable suppression of nitrite-oxidizing bacteria (NOB). This study examines the progress of NOB suppression in both sidestream- and mainstream PN/A over the past two decades. The successful NOB suppression in sidestream PN/A was reviewed, and these successes were evaluated in terms of their transferability into mainstream PN/A. Drawing on the learning over the past decades, we anticipate that a hybrid process, comprised of biofilm and floccular sludge, bears great potential to achieve efficient mainstream PN/A, while a combination of strategies is entailed for stable NOB suppression. Furthermore, the recent discovery of novel nitrifiers would trigger new opportunities and new challenges for mainstream PN/A.