Removing Carbon from the Water and Wastewater Systems

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Overview

Water conveyance, distribution, storage, end use and waste treatment/discharge are among the largest single users of energy for most municipalities. Water pumping is energy intensive, with much of it occurring during peak electricity periods. Wastewater treatment requires energy for aeration, pumping, and increasingly for ultraviolet disinfection. Water systems and wastewater treatment plants are often older infrastructure, and were not designed with energy efficiency in mind

 Water demand in Sonoma County, although positively impacted by water conserving measures, is increasing and so is the energy associated with it.  Energy use for wastewater is increasing much faster than the rise in water demand, because of increasingly stringent regulatory requirements for discharge and reclamation.

 Greenhouse gas emissions due to energy use by the water/wastewater system are generally proportional to population. See Figure 5 below. Santa Rosa, the largest city in Sonoma County, emits over 14,000 tons of GHGs annually due to water and wastewater operations.

Figure 5

However, not all water and wastewater systems are created equal. If the greenhouse gas emissions per unit of water volume (million gallons) processed is calculated, the differences between systems is evident. See Figure 6 below. Two primary variables distinguish different systems in terms of their greenhouse gas emissions:

• Power purchasing practices
  • Peak demand management (Peaking power plants generally emit more GHGs)
  • Power provider selection (Hydropower plants have lower GHG emissions, and emissions are lower across PG&E’s service area.  In Sonoma County, Healdsburg obtains its electricity from WAPA, a mostly hydropower utility.)
• Process variations
  • Pumping efficiency (Mostly the ability to adjust or curtail pumping according to demand)
  • Process energy intensity (For example, the choice between high and low intensity UV disinfection; minimizing aeration needs by maximizing primary treatment)

Figure 6

The Problem

Today in Sonoma County, water/wastewater operations are highly energy intensive. Water and wastewater systems were designed in an era when energy was cheap and plentiful. As a result, design tradeoffs were made that reduced upfront capital costs (less expensive treatment plants) with higher operating costs (more energy intensive processes. Any increase in demand for water was met with investment in infrastructure to increase supply, which traditionally has meant more energy to move more water over longer distances. Similarly, more stringent wastewater discharge standards were met with increasingly energy intensive processes.

 Projected population growth in Sonoma County will result in greater GHG emissions from water and wastewater systems unless new strategies are undertaken to decouple them from population growth by minimizing demand and maximizing efficiency. 

Toward Lower Energy Requirements

Technical measures for reducing greenhouse gas emissions from water and wastewater systems fall into four basic categories:

• Reduce demand for water
• Maximize pumping energy efficiency
• Minimize energy intensity of wastewater treatment processes
• Maximize energy recovery
• Cut peak power demands

Non-technical measures for reducing greenhouse gas emissions exist as well. There are institutional barriers to implementing deep reductions in energy use, particularly in the peak demand category. In particular, budgets for implementing energy efficiency and demand reduction are in a different category from capital projects for increasing capacity. Organizational change strategies that enable shifting budgets from capacity building to demand reduction measures could be very cost-effective.

In the energy field, applying demand side solutions to reduce consumption has been shown to be far more cost-effective than building new supply. This is known as “nega-watts.” The ability to apply this concept to water, “nega-gallons”, is often limited by jurisdictional and organizational barriers. In San Bernadino County, the Inland Empire Utilities Agency has overcome some of these barriers. The IEUA has implemented an “integrated municipal utility agency” that deals with water, wastewater and municipal solid waste. It is through this integrated approach that the IEUA can implement innovative strategies for reducing energy and water demands, increasing efficiency, and maximizing opportunities for energy recovery. 

Investing in Demand Reduction

A study of energy efficiency options for the Sonoma County Water Agency’s main supply pumps, conducted in 2003 by Provimetrics Corporation found that a $2 million investment could save $600,000 per year (out of the $5 million annual electricity cost in 2002).   Agency staff declined to proceed, citing (a) the need for water retailers to first improve their own demand mangement before SCWA could make adjustments, and (b) a new contract with WAPA was about to significantly reduce electricity costs. However, the SCWA has between $300 million and $600 million budgeted for increasing capacity. Based on the Provimetrics report, Dr. John Rosenblum estimates that the planned 68% increase in water supply from 2002 levels[1] would increase annual energy costs by about $3 million, mostly in the summer months even with the new electricity rates.

 Dr. Rosenblum found that if large-scale outdoor water efficiency measures were implemented instead of increasing supply, not only would the increase in energy expenditure be cut in half, but also a large portion of the capital cost of increasing system capacity could be avoided.  Figure 7 shows that combining off-the-shelf irrigation efficiency measures with a very modest expansion of reclaimed wastewater distribution to urban areas could eliminate about 60% of the planned increase in water supply, and the same fraction of mostly peak electricity use associated with urban irrigation (the resulting increase in electricity costs would be only 26%, instead of 63%).  Dr Rosenblum also notes that eliminating so much of the urban irrigation demand will undoubtedly reduce the large fraction of capital costs required to meet peak supply capacity.

 Fig.3 demonstrates that even modest water efficiency improvements can have a large impact on infrastructure needs.  Adding more aggressive efficiency meaures, including indoors, with site-specific details for each SCWA contractor will reveal an optimal economic balance between supply and demand.  Investments in indoor water efficiency and the expansion of reclaimed wastewater can be shared by water and sanitary agencies for the mutual benefits of all ratepayers.

 Organizational structures currently limit the ablity to capture such system-wide savings and their associated reductions in GHG emissions.  On the other hand, any opportunity to reduce the over $1 billion investment planned for water/wastwater infrastructure should not be ignored, at least not without a detailed evaluation.  Focusing the evaluation on GHG emissions might allay some of the organizational and jurisdictional constraints that have impeded progress so far.