Partial nitritation/anammox (PN/A) has been one of the most innovative developments in biological wastewater treatment in recent years. With its discovery in the 1990s a completely new way of ammonium removal from wastewater became available. Over the past decade many technologies have been developed and studied for their applicability to the PN/A concept and several have made it into full-scale. With the perspective of reaching 100 full-scale installations in operation worldwide by 2014 this work presents a summary of PN/A technologies that have been successfully developed, implemented and optimized for high-strength ammonium wastewaters with low C:N ratios and elevated temperatures. The data revealed that more than 50% of all PN/A installations are sequencing batch reactors, 88% of all plants being operated as single-stage systems, and 75% for sidestream treatment of municipal wastewater. Additionally an in-depth survey of 14 full-scale installations was conducted to evaluate practical experiences and report on operational control and troubleshooting. Incoming solids, aeration control and nitrate built up were revealed as the main operational difficulties. The information provided gives a unique/new perspective throughout all the major technologies and discusses the remaining obstacles.

Aerobic ammonium-oxidizing bacteria (AerAOB) and anoxic ammonium-oxidizing bacteria (AnAOB) cooperate in partial nitritation/anammox systems to remove ammonium from wastewater. In this process, large granular microbial aggregates enhance the performance, but little is known about granulation so far. In this study, three suspended-growth oxygen-limited autotrophic nitrification-denitrification (OLAND) reactors with different inoculation and operation (mixing and aeration) conditions, designated reactors A, B, and C, were used. The test objectives were (i) to quantify the AerAOB and AnAOB abundance and the activity balance for the different aggregate sizes and (ii) to relate aggregate morphology, size distribution, and architecture putatively to the inoculation and operation of the three reactors. A nitrite accumulation rate ratio (NARR) was defined as the net aerobic nitrite production rate divided by the anoxic nitrite consumption rate. The smallest reactor A, B, and C aggregates were nitrite sources (NARR, >1.7). Large reactor A and C aggregates were granules capable of autonomous nitrogen removal (NARR, 0.6 to 1.1) with internal AnAOB zones surrounded by an AerAOB rim. Around 50% of the autotrophic space in these granules consisted of AerAOB- and AnAOB-specific extracellular polymeric substances. Large reactor B aggregates were thin film-like nitrite sinks (NARR, <0.5) in which AnAOB were not shielded by an AerAOB layer. Voids and channels occupied 13 to 17% of the anoxic zone of AnAOB-rich aggregates (reactors B and C). The hypothesized granulation pathways include granule replication by division and budding and are driven by growth and/or decay based on species-specific physiology and by hydrodynamic shear and mixing.

The nitritation/anammox process has been mainly applied to high-strength nitrogenous wastewaters with very low biodegradable organic carbon content (95% with added acetate while the nitrogen elimination rate stayed constant (275 ± 40 mg N∙L(-1)∙d(-1)). Anammox activity and abundance of anammox bacteria (AMX) were strongly correlated, and with increasing influent COD/N ratio both decreased steadily. At the same time, heterotrophic denitrification with nitrite and the activity of ammonia oxidising bacteria (AOB) gradually increased. Simultaneously, the sludge retention time (SRT) decreased significantly with increasing COD loading to about 15 d and reached critical values for the slowly growing AMX. When the SRT was increased by reducing biomass loss with the effluent, AMX activity and abundance started to rise again, while the AOB activity remained unaltered. Fluorescent in-situ hybridisation (FISH) showed that the initial AMX community shifted within only 40 d from a mixed AMX community to "Candidatus Brocadia fulgida" as the dominant AMX type with an influent COD/N ratio of 0.8 g COD∙g N(-1) and higher. "Ca. Brocadia fulgida" is known to oxidise acetate, and its ability to outcompete other types of AMX indicates that AMX participated in acetate oxidation. In a later phase, glucose was added to the influent instead of acetate. The new substrate composition did not significantly influence the nitrogen removal nor the AMX activity, and "Ca. Brocadia fulgida" remained the dominant type of AMX. Overall, this study showed that AMX can coexist with heterotrophic bacteria at elevated influent COD/N ratios if a sufficiently high SRT is maintained.

Sewage treatment relies mainly on conventional activated sludge (CAS) systems, reaching sufficiently low pollutant effluent levels. Yet, CAS has a low cost-effectiveness and recovery potential and a high electricity demand and environmental footprint. By 2050, globally we have to solve severe water and phosphorus shortages while significantly decreasing greenhouse gas emissions. In this review and opinion paper, the ZeroWasteWater concept is proposed as a sustainable centralised technology train to short-cycle water, energy and valuable materials from sewage, while adequately abating pathogens, heavy metals and trace organics. Electrical energy recovery from anaerobic digestion of the organics present in sewage and kitchen waste (KW) has a value of €4.0 per inhabitant equivalent (IE) per year. In addition to sewerage improvements and water conservation, prerequisites include an advanced physico-chemical and/or biological concentration step at the entry of the sewage treatment plant. In the side stream, the recovery of phosphorus and carbon-sequestrating biochar from the digested sludge and of nitrogen from the digestate has a value of €6.3 IE⁻¹ year⁻¹. Alternatively, recovery of biogas and materials can occur directly on source-separated black water. In the main stream, partial nitritation and anammox oxidise residual nitrogen. Moreover, two serial heat pumps recover thermal energy, valued at €6.9 IE⁻¹ year⁻¹, cooling the water by 5°C, and membrane technologies recover potable water at €65 IE⁻¹ year⁻¹. Interestingly, ZeroWasteWater is expected to be economically viable. Key steps are to incorporate water chain management into holistic urban planning and thus produce a cradle-to-cradle approach that society will find acceptable.