Water treatment facilities are large users of electricity and thus are also large emitters. All equipment in water supply systems like pumps and motors operate throughout the day and every single day. Typically, 35% of the energy in municipal energy budgets in the US is allotted to water and wastewater (discussed in the next section) systems (EPA, 2013). In fact, the water supply system of Durham uses the most energy compared to all other local government operations in the City. Increasing the efficiency of these systems can contribute greatly towards reducing the emissions of a city. Efficiency improving measures can be applied at various points in the treatment and distribution process. There are three categories of efficiency improving measures- equipment upgrades, operational modifications, and modifications to facility buildings (EPA, 2013). Besides reducing emissions, higher efficiency also reduces air pollution and cost of electricity through lowered demand and lower cost per kWh. This section attempts to find the effect of efficiency increasing measures adopted at the water treatment facilities.
The Brown treatment plant and the Williams treatment plant are the two water treatment facilities that are operated by the City of Durham. Their combined capacity is 52 million gallons per day (MGD) and the Brown plant has a capacity about twice that of the Williams plant. MGD is a measure of flow, i.e. volume per unit time. The Williams plant is a base-load plant and it operates continuously on an average of 6 MGD. The water treatment process is similar in both plants and is described here in brief. First, water goes through the raw water pumps which use about 2 to 4% of the total energy. The water then passes through filters to remove particle media. Backwash pumps of about 150 HP are placed near the filters and they also use 2 to 4% of the total energy used. These pumps pump clean water back through the filter to clean them and prevent buildup of particle media. After filtration, the water is chlorinated and pumped to clear wells where the water is stored. Finished water pumps of about 1500 HP pump the water into the distribution system. There is also a diesel pump for backup in the case of a power failure. Booster pumps are placed at several points along the system. Both the plants use electrical energy and there is no significant use of natural gas (Senior Engineer, 2013).
The Brown plant has variable frequency drives (VFDs) installed on the finished water pumps and the back wash pumps. They were installed at the end of the calendar year 2010. VFDs are an example of equipment upgrades to increase efficiency. These control the pumps based on variable water requirement as demand varies through the day. VFDs on backwash pumps were installed to regulate the amount of water pumped back through the filters. There was a large amount of clean water pumped back unnecessarily before the VFDs were installed. VFDs are 97% efficient but they also use air conditioners for cooling. The water treatment facilities are run more during off-peak hours than during peak hours in an effort to reduce emissions and costs. Reducing peak demand reduces emissions because during peak hours, dirtier energy is used. This is an operational modification to reduce emissions.
We met in person with an engineer from the Department of Water Management who provided us with details of the process as well as monthly flow, energy and financial data for the facilities from the fiscal year 2011. We also obtained annual energy and financial data for the facilities from the Durham City County Sustainability Office for other years. Data was not available for all years since 2007, the year of the last report. Monthly data and flow data was available only since the fiscal year 2011.
Available data for this analysis were the monthly bills of the treatment facilities and the total treated water in MGD (Senior Engineer, 2013). The total treated water data from the Brown plant is not entirely reliable as the meters are outdated and have been subject to much wear. The opinion of the engineer was that the readings most likely had large errors. On the other hand, the Brown plant has extremely accurate magnetic meters and the reported values for this plant are very reliable. Table 11 and 12 show the annual flow, electricity, financial and emission data for the Brown and Williams plants, respectively.
We see that the total electricity used by both plants has no clear trend. Efficiency cannot be calculated prior to 2012 but it was almost constant in the last two fiscal years. The effect of the VFDs cannot be quantified with the data provided to us. The devices were installed in late 2010 (calendar year) but flow data is only available from January 2011. From the ICLEI report, we know that efficiency was 1.2 tons of carbon dioxide per MGD in 2007. But this was the overall efficiency of the two plants and not just the Brown plant. The overall efficiency in the year 2012 to 2013 was 1.03 tons of carbon dioxide per MGD.
Table 11 Water Flow, Energy Consumption of Brown Water Plant from FY 2009
|Year||Flow (MGD)||Electricity (MMBtu)||Amount (USD)||
CO2e Emission (Tons)
Table 12 Water Flow, Energy Consumption of Williams Water Plant from FY 2009
|Year||Flow (MGD)||Electricity (MMBtu)||Amount (USD)||CO2e Emission (Tons)||Efficiency (MMBtu/MGD)||Efficiency