Steps to circularity: Impact of resource recovery and urban agriculture in Seattle and Tacoma, Washington

Abstract

Capturing the value in urban residuals (food scraps and wastewater) is a critical component of urban sustainability and a circular nutrient economy. Food production in urban areas has also been recognized as an important component of urban health. Data from two cities (Seattle and Tacoma, WA) with active resource recovery and community garden programs were used to quantify nutrient recovery and food production potential. Yield data from growth trials conducted using soil amendments produced from locally generated organic residuals were used to model yields in existing urban agriculture programs. Our survey showed much lower than expected volume of food scraps from both residential and multifamily housing for both cities. Nutrient generation rates from food scraps were estimated as 0.55–0.67 kg N and 0.09–0.11 kg P capita−1 yr−1. Recovery rates for Seattle with an established food scrap collection program were 0.21 kg N and 0.006 kg P capita−1 yr−1. Nutrient recovery from wastewater biosolids was higher; 1–1.67 kg N and 0.23–0.76 kg P capita−1 yr−1. Data on effluent quantity and nutrient concentrations from these programs suggests that effluent has a high potential for nutrient recovery (4.03–5 kg N and 0.3–0.5 kg P capita yr−1). Yield was modeled for kale (brassica oleracea) considering the number of people that could be fed per hectare for one year using a 67 g portion by comparing yields from synthetic fertilizer and residuals-based amendments in both high and low quality urban soils. The Tacoma biosolids potting soil yielded enough for 310 and 736 people ha−1 yr−1 for the high and low quality soils, respectively. The modeled food/yard compost produced from the food scraps yielded sufficient kale for 148 to 353 people ha−1 yr−1. Relative yield from fertilizer for the low and high quality soils was 15 and 263 people ha−1yr−1, respectively. Considering yield, enough biosolids are produced to meet 6.7–29.2% of the vegetable needs of each city. These results suggest that significant nutrients can be recovered using existing infrastructure. With enhanced nutrient capture from wastewater effluent, sufficient nutrients could be recovered to meet the N and P needs for food crops for the residents of each city.