Poo, Pathogens and Plastics: Home composting domestic canine faeces for use in edible gardens

Dog faeces in the environment are a known source of nutrient, pathogen and plastic pollution that can harm human and ecosystem health. Home composting may be a more environmentally sustainable method of managing dog faeces by reducing pollution and recovering organic matter for use as a soil conditioner. Composting is a well-established method for treating faecal pathogens in animal manures and recycling nutrients into low-risk soil conditioners for food production. However, little is known about home composting dog faeces and whether the resulting compost could be safe for use in domestic gardens growing food for human consumption (Chapter 2). This project investigated the feasibility of composting dog faeces and compostable plastic dog waste bags under typical small scale, outdoor household conditions. Home composted dog faeces were assessed for suitable use in backyard edible gardens by evaluating compost process parameters and soil conditioning properties, reduction of zoonotic dog faecal pathogens and disintegration of compostable plastic bags. Initially, a nationwide stakeholder survey was conducted to determine how dog owners currently home compost their dog(s)’ faeces (Chapter 3). Data from Australian dog owners (n = 1054) provided insight into their household demographics, how they collect and dispose of dog faeces, their home composting practices, and their attitudes, experiences, and concerns regarding home composting dog faeces. Within households, most dog faeces were collected using tools such as shovels and scoops more than single use bags. General waste bins were the most common home disposal location followed by organic waste bins, home compost, and burial. Few participants included compostable plastic dog waste bags in home compost. Most composted dog faeces were applied to non-edible plants within gardens. Over half of surveyed dog owners viewed home composting as a potentially effective disposal method and many wanted to home compost dog faeces but were not yet doing so. Reported challenges included a lack of reliable compost information, hygiene concerns, and uncertainty how canine de-worming treatments affect compost. Survey findings were used to inform dog faecal home compost experiments that reflected current practices and addressed dog owners’ main concerns and challenges (Chapters 4, 5, 6). A series of three home compost trials were conducted to compare the effects of compost treatments on the process efficiency and end quality of dog faecal home composts under backyard conditions. Concentrations of zoonotic, foodborne dog faecal pathogens E. coli, Salmonella spp., Campylobacter spp., Ancylostoma spp. and Toxocara canis in fresh and home composted dog faeces were determined (Chapter 5). Quality of final composts were evaluated according to Australian Standards for industrial compost (AS4454) (Chapter 4) and certified home compostable plastic (AS5810) (Chapter 6). Two pilot scale trials compared compost treatments typically used in commercial dog faecal home compost systems, effective microorganism (EM) amendment and Eisenia fetida worms (VM). Other pilot trial treatments were frequent aeration (AT) and control (C). Pilot trial 1 ran for 7 months between May and November 2019 and Pilot trial 2 ran for 15 months starting from January 2020 to May 2021. Pilot trial test bins (25 L) contained fresh dog faeces, sawdust, mature compost, and compostable plastic bags. Temperature and pH were measured throughout both trials with moisture measured only in Pilot 2. Pilot 1 final composts showed low/moderate stability (Solvita index 5 - 5.5) with concentrations of E.coli (185 - 3873 MPN/g) and Salmonella spp. (present in 25 g) exceeding maximum sanitation limits. Pilot 2 final composts were stable (Solvita index 5.5 - 7) with no detectable bacteria. No Campylobacter spp. or soil transmitted helminth ova were detected in the final compost of either pilot trial. Home compostable plastics bags did not meet minimum disintegration requirements of 90% mass loss in 6 months, with mass loss by weight between 8.2 - 66.5% in Pilot 1 and 14.4 - 46.9% in Pilot 2. None of the pilot trial treatments (EM, VM, AT) improved compost process parameters or end quality compared to control so these treatments were not tested in the subsequent household trial. A household scale compost trial, informed by pilot trial findings, compared the overall process efficiency and final quality of dog faeces co-composted with food waste versus a food waste only control. The household trial ran over 12 months between September 2021 and September 2022, where feedstocks were added to compost bins in weekly batches during the first 3 months. Six compost bins (160 L) contained dog faeces, food waste, sawdust, and mature compost. Three of these bins (BP1 - 3) also contained samples of 6 types of compostable plastic bags. One control bin (FWC) contained food waste, sawdust, mature compost and the same plastic bag samples used in BP bins. Compost process parameters of temperature, moisture, pH and biological stability were measured. Compost quality was determined by maturity, phytotoxicity, nutrients and contaminants. Final household trial composts were stable (Solvita index 6 - 6.5) and showed undetectable concentrations of Campylobacter spp., E. coli, Salmonella spp. and canine soil transmitted helminth ova despite compost temperatures below thermal disinfection criteria. Compost stability, rather than heat, appeared to be the principal factor influencing pathogen reduction in dog faeces composted at ambient temperatures (Chapter 5). All household scale dog faecal composts (BP and NP) complied with AS4454 compost quality standards, while FWC compost under the same conditions did not comply with phytotoxicity criteria suggesting that composts containing dog faeces would better support plant growth when applied to soil (Chapter 4). Samples of compostable plastic bags did not meet AS5810 minimum disintegration requirements with average mass change by weight of certified home compostable bags ranging from +1.51 to -81.28%, but most appeared to meet the disintegration requirements visually. All samples of certified industrial compostable bags showed an average mass increase of 10.90 - 35.04% during composting, likely from the presence of biofilms. Microplastic fragments < 2mm and macroplastic fragments > 5mm were recovered in all composts (Chapter 6). This is the first study to demonstrate dog faeces, with or without food waste, home composted in variable outdoor conditions can be safely treated for use in domestic food gardens with sufficient processing time to attain a stable compost. However, it is not recommended to include compostable plastic dog waste bags in home compost systems if the compost will be applied to soil as an amendment. Future studies can expand on the methods and findings from this research to investigate quality, sanitation and plastic contamination risk in dog faecal home composts using different feedstock ratios and in varying climates. This research provides essential knowledge to guide best practices for sustainable domestic dog faecal waste management.