Assessing the potential phytotoxicity of digestates during processing of municipal solid waste by anaerobic digestion: Comparison to aerobic composts

As a preliminary evaluation of the progression of mixed municipal solid waste processing, we assessed digestates sampled at various stages of an enhanced anaerobic digestion process (Primary digestates: unwashed [PU] and washed [PW]; Steam disrupted primary digestate [ST]; Secondary digestate [SD]; and Finished 'peat-like' product [FP] for their potential phytotoxicity to the germination of cress (Lepidium sativum L. cv. Peppergrass) and radish (Raphanus sativus L. cv. Champion). A germination bioassay was conducted on both undiluted and diluted (3 and 10x) saturated extracts and expressed in terms of a germination index with all values relative to water (100%). For comparison, extracts from aerobically composted source-separated organic fraction of municipal solid waste (City of Guelph, GMC), yard waste compost (City of Toronto, TYC), and a commercial growing mixture (Sunshine Mix #2, SSM) were also included. The electrical conductivity (EC, dS/m; an indicator of soluble salts concentration) for undiluted extracts was highest for GMC (17.7) and lowest for FP (0.8), while SSM has an EC of 1.2. Germination index was negatively correlated with EC (r= -0.65^*, p<0.05, n=24). In undiluted extracts, the highest germination index occurred in SSM (141), followed by PW (108) and TYC (107), then ST (72), PU (71), FP (67) and SD (42), while GMC had the lowest (2). High soluble salts in the extracts appear to represent the major component of phytotoxicity.