Nitrogen (N) removal from high-strength wastewater can be accomplished in single-stage combined nitritation-anammox reactors with suspended growth biomass composed of floccular sludge, granular sludge, or of any mix of these 2 different sludge fractions. To date, the influence of floccular biomass on granular sludge reactor performance and stability has not been investigated experimentally or numerically. To address this knowledge gap, two 1D multi-species models were developed in Aquasim to assess the importance of small levels of flocs in putatively granular sludge combined nitritation-anammox reactors for different bulk oxygen concentrations and organics loads. The models included the growth and decay of aerobic ammonium-oxidizing organism (AOO), nitrite-oxidizing organisms (NOO), heterotrophic organisms (OHO), and anammox organisms (AMO) in exclusively granular sludge reactors, and in granular sludge reactors with small levels (∼5% of total biomass) of flocs. While maximum N removal efficiencies were similar for both model structures, floc addition led to a lower optimal dissolved oxygen concentration (DO) as well as a narrower maximum N removal peak, suggesting that small levels of floccular material may decrease process robustness to bulk oxygen changes. For some DO levels, this led to drastic efficiency drops. Furthermore, floc addition also led to substantial segregation in activity and microbial population distribution, with AOO, NOO and OHO concentrated in flocs and AMO concentrated in granules. Increased organic loading (COD:N = 4:3) improved maximum N removal efficiency in both model structures, but yielded substantially different predictions for optimal DO setpoint and process robustness to variations in DO. Taken together, our results indicate that even small levels of floccular biomass in biofilm reactors can have profound implications for reactor performance and optimization and for segregation of linked microbial processes, and suggest that the common practice of neglecting small levels of floccular material in biofilm models and in practice may lead to erroneous predictions.

A model describing a given system should be as simple as possible – but not simpler. The appropriate level of complexity depends both on the type of system and on the intended use of the model. This paper addresses the critical question of which purposes justify increased complexity of biofilm (reactor) models. Additional model features compared to conventional models considered are: (1) the inclusion of microbial diversity, distinguishing between different species performing the same function; and (2) the distinction between flocs and granules in putatively granular sludge reactors. With a multispecies model considering interspecies diversity, it was demonstrated that a given macroscopic reactor performance does not necessarily reflect steady state conditions on the microscale. In a second case study, it was shown that the addition of a small level of flocs can have a significant impact on macroscale process performance and on microbial population and activity distributions in granular sludge reactors. It was concluded that increased complexity in biofilm models, concerning microbial diversity or mesoscale aggregate architecture, is likely more useful when the focus is on understanding fundamental microscale outputs, but under specific conditions, these additional model features can be critically informative for bulk reactor behavior prediction and general understanding.

Groundwater toxicity caused by the dissolution of fluoride from rocks affects more than 200 million people worldwide, mainly in rural areas of developing countries. An excess of fluoride in drinking water causes an incurable illness named fluorosis.
Although numerous methods have been investigated for fluoride removal from water, few of them are successfully implemented in these areas. Filtration with bone char and the \contact-precipitation" technique (addition of calcium and phosphate to the raw water prior to filtration with bone char) are among the most promising methods: they are effective, the filter material is not toxic and is locally producible. Nevertheless, the charring of the bone is a complex process and the use of bones is not well accepted by some communities. An interesting alternative to bone char is apatite, which is the major component of bone.
The present project aimed to investigate the fluoride uptake performances of three apatites synthesised by aqueous precipitation. Preliminary defluoridation experiments showed that the most effective apatite had a specific surface area of 82 cm²/g, a Ca/P ratio of 2 and a CO₃ content of 6%. In granulated form, this apatite had an effciency comparable to that of bone char: the fluoride uptake by apatite was 1.8 mg/g for a fluoride equilibrium concentration of 1.5 mg/l (guide-line value recommended by the World Health Organisation), which is 70% of that of bone char. The defluoridation kinetics experiments revealed that 45% of the initial fluoride was removed within one hour by apatite, for a initial fluoride concentration of 10 mg/l, which is in agreement with values observed for bone char.
These preliminary encouraging results support further investigations using apatite as a filter medium for water defluoridation. For example, a simplification of the synthesis should be tempted as well as the use of apatite in the contact-precipitation method.