In the book (Chapter 13 Section 2 and Chapter 15 Section 4), I discuss the reuse of municipal wastewater as a strategy for water supply, but wastewater has value in at least three other areas: nutrients, energy, and information.
Nutrients
As a society, we expend vast amounts of energy and money on producing nitrogen and phosphorus fertilizers for use on croplands. Some of that N and P ends up in municipal wastewater, where it poses a eutrophication threat to local water bodies – so we spend more money and energy on tertiary treatment for nutrient removal. Could we start to “close the loop” by recovering wastewater nutrients for reuse as agricultural fertilizers?
The 380 km3/yr of wastewater generated around the world has a nitrogen content of 16.6 Tg, representing 14.4% of global N fertilizer use and a value of $9 billion; the comparable numbers for phosphorus are 3.0 Tg, 6.8% of P demand, and $2.3 billion. Recovery and reuse of these nutrients could, in theory, take place at different scales and stages of the wastewater cycle. There are already dozens of wastewater treatment plants (WWTPs) recovering P from sewage sludge, and new technologies for P recovery are being rapidly developed. Urine-diverting toilets can be very helpful in nutrient recovery, since urine contains about 80% of the N and 50% of the P in wastewater. However, urine can release significant ammonia gas unless stabilized (e.g., through acidification). An analysis for three US states found that urine diversion and conversion to fertilizer could reduce greenhouse gas emissions from wastewater treatment and fertilizer production by 29-47%.
Energy
WWTPs are increasingly taking advantage of the energy content of sewage sludge by installing anaerobic digestors on site. Anaerobic digestion reduces the volume and pathogenicity of the sludge, and converts much of the organic matter to biogas (mostly methane, though significantly contaminated with other compounds, like carbon monoxide and hydrogen sulfide), which can help meet the energy needs of the WWTP. Estimates suggest that most WWTPs could generate 60% of their energy needs through biogas production from sewage sludge, but the US lags behind Europe in wastewater-to-energy conversion, with only about 10% of WWTPs currently implementing the technology.
Jenfelder Au, a residential community of about 2,000 people in Hamburg, Germany, is implementing a similar approach at the local scale: Low-water-use vacuum sewer lines are used to transport concentrated blackwater from urine-diverting toilets to a digester, and the resulting biogas is used to generate heat and power, which are distributed back to homes.
Another form of energy in sewage is its heat content; for a typical European country, this represents 2800 MJ person-1 yr-1, considerably larger than the chemical energy of the wastewater’s organic matter. There are projects underway to tap this thermal energy using heat pumps, which can provide seasonal heating and cooling based on the relatively constant temperature of the wastewater.
Information
Just as water quality in a stream integrates pollutant contributions from the entire watershed, a community’s wastewater is a picture of the metabolism of all the individuals in that community, and can thus provide useful public health information. Wastewater-based epidemiology got its start in 2001 with a proposal to estimate use of illegal drugs from concentrations of those drugs in sewage (together with knowledge of their metabolism in the body and decomposition in the sewage system). Over the following two decades, a large number of studies from around the world improved the methodology and produced results that are useful in protecting public health. But the field really came into its own during the Covid-19 pandemic, when hundreds of scientists from around the world quickly developed techniques and began providing continuous data on community-level viral loads in wastewater. During 2021 alone, 230 scientific papers were published with titles that contained “SARS” and “wastewater.” More importantly, public-health officials were able to get early warnings of surges in their community. Future uses for wastewater epidemiology include assessing community exposure to third-hand cigarette smoke and synthetic organic chemicals like PFAS.