The Case for Source Separation
Source separation is a major intervention currently proposed in nutrient management with major potential to improve P reuse. Source separation refers to keeping liquid human waste separate from solid human waste at the source. Apparatus that can achieve this are varied, and my favorite one will be the topic of my next post. Perhaps a familiar example is a toilet where you pull up on the handle for “number 1”, and push down for “number 2”. Besides the additional benefit of water use optimization, this trick may be a great step to better utilize nutrients and improve P recovery.
The reason behind this is that urine contributes 50% of the P that reaches a wastewater treatment plant, not to mention 80% of the nitrogen and 90% of the potassium (Larsen 2001). As collected at the source, urine is easily sterilized and has a high concentration of the essential nutrients. This makes it very easy to achieve a high efficiency for any number of P recovery technologies. It can even be reused directly after a simple sterilization as a fertilizer and partially offset demand for chemical fertilizer.
However the current method of sanitation in many developed countries does not keep the high-value, nutrient-rich urine separate from the rest of the wastewater. It first mixes with fecal matter, which adds highly complex constituents with large organic content. These constituents interfere with P recovery technologies, drastically lowering their efficacy without some sort of pretreatment. Next the wastewater is highly diluted when mixed with shower and washing machine drains, not to mention many old cities that still have combined wastewater and stormwater sewers. This drops the nutrient concentration and again lowers nutrient recovery technology efficacy since the process has to look a lot harder to find the P molecules it is trying to catch. These challenges are just further complicated when industrial contaminants are considered.
Social, infrastructure and technical challenges do exist to implementing source separation. Source separation toilets can be unfamiliar to users and face social backlash. It may also require a large infrastructure investment to have piping and collection systems to maintain separation. This may indicate that dispersed treatment options such as on-site use or storage is more practical than centralized treatment.
Taken together, it makes strong sense from a nutrient reuse perspective to institute source separation. Increasing ability to recover P improves global food security through providing increasing availability of soil nutrients, reducing reliance on depleting P reserves, and having widespread availability independent of geopolitical influences.