Americans generate 198 million tons of organic waste each year (including both sewage and organic solid waste). While currently treated as a liability that causes numerous environmental problems, we envision these flows as a vast resource waiting to be tapped.

Communities can mine their sewers and waste streams, using the alchemy of anaerobic biodigesters to create value: biogas, that can be burned for heat and electricity; biosolids, which is a rich fertilizer; and clean water, which can be used for irrigation or purified to potable quality.

This system can be used to revitalize and empower communities. Biodigesters can be retrofitted incrementally onto the existing system. They can be inserted into existing neighborhoods, slowly replacing the existing centralized order, in which costs and benefits accrue inequitably across the supply chain, with a small-scale decentralized model that allows communities a say in the design of the system, and allows benefits to accrue to local communities. Biodigesters can support local agriculture, create free energy, provide public amenity and education, and provide green jobs.

This system is flexible enough to work in multiple contexts:

• The urban core becomes a forest: small, modular biodigesters are located in neighborhood centers. The fertilizer can be used for local greenery, which in turn provides an additional source of carbon and helps to maintain an optimal carbon:nitrogen ratio.
• The streetcar suburb becomes a filter: the large available land area (76.1%) can be utilized to recycle 100% of wastewater by pairing biodigesters with ecological machines and artificial wetlands. This water can be used for irrigation and/or drinking water.
• Suburban sprawl becomes farmland: biodigesters help to create a demand for agricultural waste and food scraps, as well as providing high-quality fertilizer. This lowers the cost of agriculture, and helps to spur the transformation of suburbs (including areas such as disused shopping malls) to farmland.

Biodigesters operate best with a carbon-to-nitrogen ratio around 23:1. Over time, this ratio will begin to impact urban design, creating demand for green cities, policy, and planning.

• Urban greenery provides much-needed carbon and absorbs stormwater, increasing the efficiency of sewer mining.
• Urban agriculture provides much-needed carbon and benefits from local fertilizer production.
• Transit-oriented development helps to create pockets of density that frees land area elsewhere for agriculture or habitat; and also provides concentrated nitrogen production.
• Mixed-use development creates a more even flow of sewage throughout the day, allowing biodigesters to operate more efficiently.

Over time, a new urban morphology begins to emerge: one with pockets of dense urban fabric surrounded and infiltrated by greenery in the form of farmland and habitat. A need for carbon translates to a demand for greenery; if/when carbon and nitrogen become balanced, the demand will shift toward maintaining this balance — in other words, balancing new development and population increases with additional greenery. Thus the shift in the flow of waste has a much larger effect on the city as a whole, helping to incentive many sustainable urban strategies that have been long sought-after in the sustainable design community.