The rivers and seas are polluted with human sewage, which does not decompose properly because the microbes in water are different from those on land and cannot break down animal waste. The result is foul-smelling water and algal blooms. The microbes that have evolved to break down animal manure are primarily found in soil and on organic matter, not in aquatic environments. Human faeces is a very rich source of nutrients and can be an excellent fertiliser. Until about 1875, it was routinely applied untreated to farmland outside London, a practice that was eventually made illegal due to the obvious health risks, since untreated sewage can contain dangerous pathogens. Proper composting, however, creates optimal conditions for the microbes that have evolved over billions of years to efficiently break down human waste, producing a safe material that can be used to grow food after sufficient processing. We have long ignored, hidden, and made taboo the act of defecation, breaking the natural nutrient cycle. This elective ignorance has created a range of environmental and public health problems. So why don’t we stop polluting our rivers and start working with the natural decomposition process instead? How hard can it be, even on a houseboat? I started designing such a system over a year ago, and the process led me through chemistry and mathematics to plant biology. In the rest of this talk, I will explain how I set up and run a self-sustaining micro sewage treatment system on a houseboat. The system had to maximise the speed of decomposition whilst minimising smell and space, which meant the process needed to be as efficient as possible. The physical components of the system are a composting toilet, a macerator, cardboard, rabbit bedding, a hot composter, and yellow bamboo grown in pots. ============================================================================ So the structure of my talk would be 1. Sewage in rivers and why decomposition fails in water 2. Night soil and historical nutrient cycles 3. Composting microbiology 4. Social taboo and broken nutrient cycle 5. Design constraints on a houseboat (space, smell, speed) 6. Chemistry (C:N ratio, moisture, oxygen) 7. Maths (volume, decay rates, steady state) 8. Plants as nutrient sinks (bamboo) 9. System overview and results