Department Process Engineering

The research focus of the Process Engineering Department (ENG) ranges from current and future wastewater and drinking water treatment problems, as well as water pollution control and resource reuse. Our long-term goal is to develop sustainable concepts of the water and nutrient cycle in residential areas.

Using rainwater to water the garden is a sensible idea. (Photo: Adobe Stock)

July 21, 2026 –

When there is a prolonged dry spell and local authorities call on residents to save water, questions immediately arise as to whether it would make sense to reuse more water and collect rainwater. In light of recent events, here...

When there is a prolonged dry spell and local authorities call on residents to save water, questions immediately arise as to whether it would make sense to reuse more water and collect rainwater. In light of recent events, here are some answers from Prof. Kai Udert, Head of the Process Engineering Department at Eawag.

Bengaluru, India – a city on the cusp of a sustainable water future. (Photo: Christian Binz)

June 8, 2026 –

An Eawag project in Bengaluru, India, is opening up new ways to respond to droughts and water scarcity. The focus is on the decentralised reuse of treated wastewater. Technological solutions alone cannot resolve the complex water...

An Eawag project in Bengaluru, India, is opening up new ways to respond to droughts and water scarcity. The focus is on the decentralised reuse of treated wastewater. Technological solutions alone cannot resolve the complex water challenges. Social aspects must be considered, as well.

Thanks to sludge thickening, the Neugut WWTP no longer requires the addition of flocculants, which were previously used to increase sludge sedimentation. (Photo: Neugut WWTP)

March 3, 2026 –

An Eawag study at 10 WWTPs that use sludge densification shows that the process could improve capacity as well as nitrogen removal.

Ashta, N. M., Crosset-Perrotin, G., Moraz, A., Stoffel, J., Schilt, U., Ceglie, E., … Hueglin, C. (2026). Atmospheric deposition of microplastics: a sampling and analytical method including the associated measurement uncertainties. Atmospheric Measurement Techniques, 19(2), 371-388. doi:10.5194/amt-19-371-2026, Institutional Repository

Microplastics (MPs) are environmental contaminants of global concern, and the atmosphere may play an important role in their environmental distribution. In this study, we developed a tailored analytical chain – including sample collection, processing, and analysis based on optical microscopy and focal plane array μ-Fourier transform infrared spectroscopy (FPA-μ-FTIR) – to quantify 20–215 µm MPs (excluding tire wear particles) in wet and dry atmospheric deposition samples. We present a novel sampling setup to collect particulate wet deposition, which consists of an on-site precipitation filtration device. Validation of the sampling setup via spike-recovery experiments using surrogate standards resulted in average recoveries of approximately 90 %, suggesting limited MP losses. Additionally, we developed a custom software platform that combines the results from optical microscopy and chemical imaging obtained through FPA-μ-FTIR. Furthermore, an assessment of the total measurement uncertainty was made by addressing each step of the analytical chain individually. The resulting total expanded uncertainty was approximately 90 % for determining MP numbers in a single wet or dry deposition sample. The conversion of MP numbers and associated size information into MP mass was estimated to generate an additional systematic error of 50 %. Based on analyses of blanks, the critical level and the limit of detection, number-based thresholds for minimizing false positives and false negatives, were 29 and 58 MPs per analyzed subsample, respectively. The analytical chain was applied to quantify the MP content in wet and dry atmospheric deposition samples collected at a suburban site in Switzerland. The principles and methodology used in this study to calculate the uncertainties, recoveries and limits of detection are transferrable to other analytical methods intended for MP analysis. Such an assessment of method-specific uncertainties is an important step towards enhancing the comparability of MP (monitoring) data.

Breuninger, E. S., Beyrouti, E., Tolu, J., Le Bras, Z., Bouchet, S., & Winkel, L. H. E. (2026). Selenium speciation dynamics in the Baltic and North Seas: competing influences of marine and terrestrial inputs, biological activity, and redox conditions. Limnology and Oceanography, 71(7), e70460 (15 pp.). doi:10.1002/lno.70460, Institutional Repository

Britschgi, L., Jensen, N., Morgenroth, E., & Derlon, N. (2026). Mechanisms of granule detachment and impact on nitrifier growth and migration to flocs in aerobic granular sludge. Water Research, 301, 126003 (14 pp.). doi:10.1016/j.watres.2026.126003, Institutional Repository

Granule detachment in aerobic granular sludge (AGS) systems is considered an important aspect for both granulation and nitrification performance, yet it has received little attention. This study aimed to identify the dominant detachment mechanisms (erosion, abrasion, sloughing, breakage) and quantify their respective kinetics under operational shear conditions. Ultimately, the impact of detachment on nitrifier migration and growth conditions in flocs was also investigated. Granules from two full-scale AGS wastewater treatment plants (WWTP) were exposed to operational shear (157 s^-1) for up to one week in a Couette-Taylor reactor. The distribution of size-classes in terms of total suspended solids (TSS) and nitrifiers was measured, and detachment kinetics and solids retention time (SRT) in flocs, accounting for detachment, were estimated. Under the tested shear rate, shear-induced erosion from the granule surface was the main detachment mechanism with a median detachment rate of 0.017 gTSS_detach gTSS_granule^-1 d^-1 (25th–75th percentile: 0.014–0.027 gTSS_detach gTSS_granule^-1 d^-1). In contrast, granule breakage was not observed within 15 detachment tests and therefore does not appear to be a continuous process, indicating that re-growth on breakage debris is not a main driver of granulation as previously proposed in literature. Nitrifiers were found to migrate almost exclusively to biomass <0.2 mm, i.e., flocs. While the migration rates of ammonia- and nitrite-oxidizing bacteria (AOB and NOB) were similar in autumn, distinct values were quantified in winter, with two to six-fold lower NOB migration rates, likely reflecting a temperature-dependent stratification of nitrifiers within granules. Consequently, the influence of nitrifier migration on the growth conditions in flocs varies across nitrifying groups and seasons. The floc SRT of AOB was estimated to be above their minimum SRT throughout the year, indicating migration is not critical for AOB to persist in flocs. In contrast, the minimum SRT of NOB increases significantly at low temperature, such that migration becomes crucial to prevent NOB washout. Migration of NOB from granules to flocs may sufficiently increase the SRT of NOB in flocs, if they are predominantly enriched in granules rather than in flocs. Overall, flocs provide favourable growth conditions for AOB throughout the year, whereas this is only intermittently the case for NOB, which ultimately rely on migration from granules, particularly during cold winter months.

Britschgi, L., Rohrbach, S., Lauber, C., Morgenroth, E., & Derlon, N. (2026). The benefits of promoting small granules to enhance nitrification in aerobic granular sludge. Water Research, 305, 126434 (13 pp.). doi:10.1016/j.watres.2026.126434, Institutional Repository

In aerobic granular sludge (AGS) systems, nitrification occurs across flocs and granules of different sizes, yet maintaining robust nitrification can remain challenging. Small granules (0.2–1 mm) have received little attention in AGS research, although previous observations suggest they provide a favourable niche for nitrifier growth and activity. While small granules naturally exhibit higher per-biomass activity than large granules (>1 mm) due to their higher surface-area-to-volume ratio, it is unclear whether they can outperform flocs. This study investigated whether nitrifier enrichment and activities in small granules can be promoted through targeted solids retention time (SRT) strategies, and how such strategies could be implemented in practice. Lab-scale sequencing batch reactors (SBR) were operated for 3–6 months to assess the effects of decoupling SRTs between flocs and small granules, as well as the influence of floc SRT on the nitrification capacity of small granules. Small granules provided a favourable niche for nitrifiers: ammonia oxidizing bacteria (AOB) thrived in both flocs and granules, irrespective of SRT conditions. In contrast, nitrite oxidizing bacteria (NOB) predominantly enriched in small granules, an effect that was further amplified upon SRT decoupling. Under decoupled SRT operation, median ammonia and nitrite oxidation rates in small granules reached 2.7 and 2.4 mgN gTSS⁻¹ h⁻¹, respectively, corresponding to 1.7–3.1-fold higher rates than observed in flocs and around four-fold higher rates than in large granules. These results indicate that SRT decoupling between flocs and small granules is required to fully exploit the nitrification potential of small granules. To evaluate strategies for selectively promoting small granules in full-scale AGS SBR, lab- and full-scale settling and sludge stratification tests were conducted. Flocs and small granules consistently settled together, forming an intertwined sludge bed. Consequently, the selective retention of small granules through settling and sludge stratification alone is not feasible. Alternative operational approaches should therefore be explored to enhance the abundance and activity of small granules in AGS systems.

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