Socio-environmental consideration of phosphorus flows in the urban sanitation chain of contrasting cities

Understanding how cities can transform organic waste into a valuable resource is critical to urban sustainability. The capture and recycling of phosphorus (P), and other essential nutrients, from human excreta is particularly important as an alternative organic fertilizer source for agriculture. However, the complex set of socio-environmental factors influencing urban human excreta management is not yet sufficiently integrated into sustainable P research. Here, we synthesize information about the pathways P can take through urban sanitation systems along with barriers and facilitators to P recycling across cities. We examine five case study cities by using a sanitation chains approach: Accra, Ghana; Buenos Aires, Argentina; Beijing, China; Baltimore, USA; and London, England. Our cross-city comparison shows that London and Baltimore recycle a larger percentage of P from human excreta back to agricultural lands than other cities, and that there is a large diversity in socio-environmental factors that affect the patterns of recycling observed across cities. Our research highlights conditions that may be “necessary but not sufficient” for P recycling, including access to capital resources. Path dependencies of large sanitation infrastructure investments in the Global North contrast with rapidly urbanizing cities in the Global South, which present opportunities for alternative sanitation development pathways. Understanding such city-specific social and environmental barriers to P recycling options could help address multiple interacting societal objectives related to sanitation and provide options for satisfying global agricultural nutrient demand.

     

The regulation of a spatially heterogeneous externality: Tradable groundwater permits to protect streams

Groundwater pumping can reduce the flow of surface water in nearby streams. In the United States, recent awareness of this externality has led to intra- and inter-state conflict and rapidly-changing water management policies and institutions. Although the marginal damage of groundwater use on stream flows depends crucially on the location of pumping relative to streams, current regulations are generally uniform over space. We use a population data set of irrigation wells in the Nebraska portion of the Republican River Basin to analyze whether adopting spatially differentiated groundwater pumping regulations leads to significant reductions in farmer abatement costs and costs from damage to streams. We find that regulators can generate most of the potential savings in total social costs without accounting for spatial heterogeneity. However, if regulators need to increase the protection of streams significantly from current levels, spatially differentiated policies will yield sizable cost savings.     

Evaluation of entanglement properties of crushed automobile shredded residue and detachment of entrapped particles

Automobile shredded residue is a potential solid fuel when its metal and chlorine-containing parts have been removed. The authors have investigated physical separation of automobile shredded residue to obtain the components that may be used in solid fuel and found that agglomerates of entangling fiber-like components are formed during crushing, making removal of metals and chlorine-containing parts difficult. In this paper, a reattachment ratio, entrapment ratio, and disentanglement ratio are proposed to evaluate the entanglement properties of crushed automobile shredded residue. It is found that products smaller than 16 mm were not reattached easily in the sieving process, and an index based on entanglement properties proposed here is useful to predict suitable crushing conditions. Wet detachment of entrapped particles from agglomerates of entangling materials was also applied to separate particulate matter from crushed ASR.     

Jig separation of crushed automobile shredded residue and its evaluation by float and sink analysis

Automobile shredded residue is a potential solid fuel when its metal and chloride-containing components have been removed. The authors have investigated physical separation of automobile shredded residue to isolate components that may be used as solid fuel and reported that agglomerates of entangling fiber-like material are formed during crushing. These agglomerates make removal of metals and chloride-containing components difficult, and the paper proposes a treatment flowchart consisting of separate treatments of the coarse size fraction containing entangling fiber-like components and the smaller size fraction containing particulate matter. This paper reports the treatment of the smaller size fraction treated with jigging to obtain a low-ash and low-chloride-content product. A new washability curve was developed based on float and sink analysis test results, and it was applied to estimate the gravity separation performance of treatments such as jigging and dense medium separation. The estimated results show good agreement with the experimental results of jig separation.