A criticism of behavioural nudges is that they lack precision, sometimes nudging people who – had their personal circumstances been known – would have benefitted from being nudged differently. This problem may be solved through a programme of personalized nudging. This paper proposes a two-component framework for personalization that suggests choice architects can personalize both the choices being nudged towards (choice personalization) and the method of nudging itself (delivery personalization). To do so, choice architects will require access to heterogeneous data. This paper argues that such data need not take the form of big data, but agrees with previous authors that the opportunities to personalize nudges increase as data become more accessible. Finally, this paper considers two challenges that a personalized nudging programme must consider, namely the risk personalization poses to the universality of laws, regulation and social experiences, and the data access challenges policy-makers may encounter.

In situ studies on waste stabilization ponds (in Portugal) showed that faecal coliform numbers were lowest at positions in the water column where pH, temperature, dissolved oxygen and algae were high. Numbers were not always lowest at the surface where light intensities were highest or in ponds where light penetration had increased through algal grazing by Daphnia. Laboratory studies showed that pH values approaching 9.0 or above increased faecal coliform die-off particularly under nutrient-poor conditions. Elevated temperatures enhanced the pH effect but the level of dissolved oxygen made little difference. Streptococcus, Salmonella and Campylobacter isolates all behaved similarly to the faecal coliforms. Daphnia grazing of the algal population in maturation ponds may reduce the microbial quality of the final effluent. These findings are discussed in relation to pond design.

Waste stabilization ponds are widely used in many European countries, such as France and Germany, for the treatment of wastewater from mainly small communities. There would seem to be no reason why they should not also be a suitable treatment process for populations of under 2000 in the UK. The relative advantages and disadvantages of ponds are compared to other small community treatment systems, such as rotary biological contactors, biological filters and reed‐bed systems, particularly in relation to effluent standards with respect to algae, nutrient removal and relative costs.

A recent development within nudge theory is the concept of sludge, which imposes frictions on decision-making. Nascent literature adopts a normative interpretation of sludge: nudge good, sludge bad. However, this normative interpretation leaves much to be desired. A clear definition and treatment of sludge remains absent from this literature, as is a complete understanding of ‘frictions’. Furthermore, the relationship between nudges and sludges is unclear. This paper proposes the concept of nudge/sludge symmetry in an attempt to advance the conceptual understanding of sludge. Building from the definition of a nudge, three types of friction permissible under nudge theory are identified: hedonic, social and obscurant. Sludge is then positioned, in terms of frictions, relative to nudge: nudges decrease relative frictions, sludges increase relative frictions. A consequence of this proposition is nudge/sludge symmetry – where a nudge decreases the frictions associated with a specific option, sludge is simultaneously imposed on all other options available to a decision-maker. Nudge/sludge symmetry subsequently challenges the normative interpretation of sludge, and so a new framework drawing on the literature on nudges in the private sector is offered, with the choice architect placed at the centre. This new approach to sludge and emphasis on the role of the choice architect, in turn, reaffirms the importance of transparency in public policy interventions.

In situ and laboratory experiments were performed on species of the waste stabilization pond algae Chlorella, Scenedesmus, Euglena and Chlamydomonas to investigate the utilization of various organic acids and sugars, light limitation on species dominance and the toxicity of ammonia and sulphide. Euglena showed the most flexible metabolism and was able to utilise butyrate. Scenedesmus did not grow on any of the substrates in the dark. Pond populations of Euglena and Chlamydomonas were able to incorporate acetate in situ in competition with the heterotrophic bacterial population but Chlamydomonas was the more successful. Light availability affected speciation in facultative ponds where rapid motility by flagellates was a clear advantage. All the algal species were sensitive to sulphide at µM concentrations in a tolerance sequence ChlamydomonasChlorellaScenedesmusEuglena. Ammonia was less toxic than sulphide and the tolerance sequence was ChlorellaScenedesmusChlamydomonasEuglena. It was the undissociated forms of both sulphide and ammonia which were toxic, Toxicity was influenced by pH and this would be a critical factor controlling ammonia inhibition of algal photosynthesis in ponds. The effects of these findings are related to species dominance and pond performance.