Viewing Wastewater As a Resource

Prentiss Darden

Figure 1. Looking at wastewater as part of a wider urban metabolic system

Cities are like living organisms that take in resources, use them, then reject waste. This activity produces greenhouse gas emissions, truckloads of solid waste, and nutrient pollution in waterways on a daily basis. When material and energy flows are designed in a more integrated way, waste becomes a resource. The urban metabolic system becomes a process to transform cities into more livable places that are less harmful for present and future generations. In response to climate goals and costs associated with wasteful practices, San Francisco and other cities in the Bay Area are taking a systems approach to improve the impact on waterbodies, organic waste flows, and emissions of Bay Area cities. See figure 1. 

Figure 2. Conventional wastewater treatment generates waste in the form of carbon emissions and nutrient pollution


Contextualizing wastewater treatment plants within watersheds illustrates the physical landscape in which water moves, indicating a zone of collection, points of energy consumption, and nutrient pollution. The water-energy nexus begins to explain this by describing the interdependence of water and energy. Water is required to produce energy, more so for fossil fuel sources rather than renewable sources, and energy is required to treat and distribute water. Some municipalities in the US use up to 30% of their total energy budget on water and wastewater utilities, typically depending on the amount of energy needed to pump and treat water. When analyzing methods to decrease greenhouse gas (GHG) emissions produced in urban centers, it is vital to look at water infrastructure, and wastewater treatment in particular. See figure 2.

Understanding wastewater’s impact on human and ecological health illustrates the impact of wastewater beyond the energy and emissions story. When not handled properly, treated wastewater effluent has detrimental effects on the aquatic health of receiving waterbodies, causing algal blooms and dead zones, which are indicators of failing waterbodies. What goes into the water eventually enters human bodies, and vice versa. The efficacy of wastewater treatment technology to remove chemicals of concern is critical in removing these harmful compounds from the environment, keeping them out of human bodies, and minimizing disease and public health epidemics. Redesigning and implementing improved wastewater systems reduces GHG emissions, and the benefits multiply throughout local watersheds and our bodies, improving ecological, human, and climatic health.

water’s relationship to topography is much stronger than its relationship to jurisdictional boundaries

Mapping San Francisco’s water supply and wastewater treatment watersheds reveals aspects of the landscape that influence the way water flows through the urban metabolism. Understanding the relationship between water, humans, and infrastructure indicates points where design can be implemented to optimize urban metabolic operations to create more sustainable and resilient cities. 

Water:  Terrain and Infrastructure

Figure 3. The Tuolumne River Watershed supplies drinking water to San Francisco and other Bay Area counties


Rarely do urban water supplies originate within city boundaries, as water’s relationship to topography is much stronger than its relationship to jurisdictional boundaries. San Francisco’s water supply comes from snowpack and precipitation within the Sierras, as shown in figure 3. Water from this granite basin is clean and free of sediment, typically requiring minimal filtration and disinfection before being distributed throughout the city.

Figure 4. The Hetch Hetchy system delivers clean mountain water to San Francisco via three major pipelines and stores it in three reservoirs  

San Francisco’s water source is one of the cleanest in the country but unfortunately, most of the city uses clean mountain precipitation for all water uses, including flushing toilets, cooling buildings, and irrigating parks, streetscapes, and gardens. Most water is used only once before being rejected to the salty Pacific Ocean and brackish San Francisco Bay. Figure 4 shows a schematic of the city's drinking water supply.  

Figure 5. San Francisco's watersheds are formed by the city's terrain, moving water from high to low points in the landscape

San Francisco’s topography shapes the city’s watersheds, as shown in figure 5. Water supply lines are routed to the east and west of the city, avoiding major topographical extremes of the city. Topography also shapes the network of the city’s combined stormwater sewer system, as shown in figure 6. This pipe network is constructed to use the terrain’s slope to move water downhill towards the two treatment plants. Notice how areas of greater topographic change do not include sewer mains. These locations are served by lateral pipes, which have a smaller diameter and can be more easily suited to tight spots in steep slopes. Laterals in steep areas eventually connect to mains once the terrain is less steep. 

Figure 6. The city's combined stormwater and sewer system reveals patterns of the landscape, such as Golden Gate Park, and Twin Peaks

The ridgeline running north to south separates the stormwater and wastewater flows. Everything west of the ridge flows to the Oceanside Wastewater Treatment Plant located on the Great Highway by Ocean Beach, while everything east of the ridgeline flows to the Southeast Wastewater Treatment Plant in the Bayview-Hunters Point neighborhood. See figure 7. 

Instead of consuming energy and damaging aquatic habitat, the wastewater treatment process can recover nutrients, produce energy, and benefit watershed health. Collecting kitchen scraps and yard waste provides feedstock for waste to energy technologies to produce energy and soil amendments.

Figure 7. San Francisco's famous hills and ridge lines determine where wastewater is treated

Waste to Energy

Over one third of the energy that the San Francisco wastewater treatment plants use is generated onsite by capturing and using the methane produced during the wastewater treatment process. The San Francisco Public Utilities Commission beneficially reuses 100% of the biosolids created at the city’s wastewater treatment plants. Over in Oakland, the East Bay Municipal Utility District (EBMUD) digests wastewater solids, food scraps from local restaurants, and waste streams from nearby farms and wineries to generate energy to power their plant. In fact, their program was the first in North America to generate more renewable energy than it could use at its facility.

These are just a few examples of the many processes and technologies that can recycle water, produce energy from wastewater, and recover nutrients in several forms to reuse in agricultural, landscaping, and landfill applications. EBMUD’s wastewater treatment plant is a groundbreaking example of moving beyond the mindset of cleaning up the “problem” of municipal wastewater. They’ve shifted the lens to show how wastewater treatment plants can become regional renewable energy generators instead of energy sinks. This is the kind of practice that can be implemented at wastewater treatment plants across the country.


During a time when water infrastructure is aging, becoming vulnerable to sea level rise, and slipping into a past paradigm of urban design where waste is relegated to poor neighborhoods, there are many opportunities to rethink waste. Innovative partnerships and financing models to implement existing technologies are critical to the success of adjusting the material and energy flows of our cities to make greater use of the biological flows within our cities.

By retooling utilities to collect the organic solid waste streams of the city, urban metabolic operations are adjusted to benefit human, ecological, and climatic health. Wastewater treatment plants have historically been points of pollution, contributing to both air and water pollution. They are stinky, unsightly, and generally located in poor neighborhoods, whose residents bear the burden of being in close proximity to them. There are clear opportunities to transform typical wastewater treatment plants from points of waste to points of production. The effects of doing so have the power to improve human, ecological, and climatic health. 


Fall 2016 Volume 9 Issue 2

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