Reduce, redirect, or recycle? Quantifying opportunities to increase household nutrient sustainability

Background/Question/Methods

Excess nutrient loading to the environment can degrade ecosystem functions and impact human health, while at the same time depleting nonrenewable nutrient sources by moving them into nonrecoverable pools. In this study, part of the Twin Cities Household Ecosystem Project (TCHEP), we examine carbon (C), nitrogen (N), and phosphorus (P) output fluxes from ~2,700 residential households in the Minneapolis-Saint Paul metropolitan area (Minnesota, USA). We tracked nutrient outputs through transformations in waste streams to their eventual fates as pollutants, inert forms, or recycled nutrients. We quantified opportunities for changes in nutrient management strategies to reduce nutrient pollution and increase nutrient recycling. By examining C, N, and P fluxes simultaneously, we also considered synergies and tradeoffs among the three nutrients when evaluating management options.

Results/Conclusions

Study households' output fluxes (annually per household) were 8126.7kg C, 54.58kg N, and 4.70kg P. The bulk of C fluxes were fossil-fuel CO₂ (83%). The main fates of N were pollutants (54%, primarily NO_x from travel and household energy) and inert forms (40%, primarily N₂ from incinerating solid waste and wastewater sludge). Most P fluxes ended up as inert forms (78%), primarily landfilling of household garbage and incinerator ash. Only 5% of N and 9% of P are currently recycled via food-to-livestock, compost, and land-applied septage.

We found few opportunities to redirect pollutant fluxes to non-polluting forms; reducing household nutrient pollution must rely primarily on reducing demand. Previous TCHEP studies have addressed the largest nutrient fluxes from travel, household energy, and fertilizer use. In this study, we targeted additional pollutant fluxes (e.g. wood smoke, non-CO₂ greenhouse gases) and identified opportunities to focus efforts on a small proportion of households with disproportionately large pollutant fluxes. Our findings highlight the importance of including waste-management practices (e.g. sewer vs. septic) and spatial data (estimating human exposure or environmental impacts) when designing pollution-reduction efforts.

In contrast, household N and P recycling can be substantially increased and could potentially exceed household inputs of N and P in food. Doing so would require recycling nearly all food waste and yard waste (to livestock, cropland, or gardens), implementing industrial-scale P recovery from incinerator ash, and separating urine for fertilizer use. To address the challenges of nutrient pollution and nutrient scarcity, it is important to reduce household consumption of C, N, and P while also improving waste management to greatly increase nutrient recycling.