47 wastewater utilities (Jones, 2006) shows the following breakdown: in-facility pumping, 38%; aeration, 26%; effluent reuse pumping, 25%; and “other”, 11%. Thus, pumping and aeration accounted for approximately 89% of total energy use within the surveyed wastewater facilities. These values serve as an example that data are site-specific and can vary widely from facility to facility. For example, data indicate that aeration can range from 25% to as much as 60% of total energy use (WEF, 2009).
The classic energy savings example in a WRRF is to measure the aeration dissolved oxygen and automatically adjust the blowers to maintain a dissolved oxygen setpoint. Installing automatic dissolved oxygen control can potentially save 15 to 40% of aeration energy (Hamilton et al., 2009). This is one example where automation can be used to improve aeration efficiency; although other opportunities exist for the operation of blowers over their operating range, these are beyond the scope of this chapter.
The use of variable frequency drives (VFDs) in conjunction with automation can further improve energy efficiency in some applications. Generally, systems that have varying flow demands and use valves for flow control or systems that use full-speed motors where the motor speed can be reduced while meeting the demand can provide potential energy savings applications with VFDs. For equipment that is operated occasionally, automation can be used to schedule the equipment to run during off-peak hours or sequence the operation of multiple units to reduce demand charges.
Electric utilities often provide several types of rate structures, including flat rates and rates that vary based on the time of day. Demand charges at a WRRF can account for one-third of the overall bill. Rates will vary for each electric company and will also vary by customers’ facility characteristics. Some electric utilities will provide real-time pricing rates that change daily. Energy-bill savings from automation can often be realized by shifting facility and equipment operation to periods with lower energy and demand charges.
The ability to identify power savings and load-shifting opportunities often begins by measuring real-time power usage throughout a facility. Supervisory control and data acquisition (SCADA) systems can include power monitors to monitor and track real-time and historical power consumption; they also provide displays, trends, and dashboards for understanding how the system is performing. By tracking overall power consumption, real-time control of demand can be implemented. Supervisory control and data acquisition systems can be programmed to alarm when demand is approaching the current monthly peak, and can perform preprogrammed load-shedding and load-shifting operations. The 136-ML/d (36-mgd) Encina Wastewater Treatment Plant in Carlsbad, California, modified operations to shut down selected high-demand equipment during peak hours, resulting in an annual savings of $50,000 per year (CEC, 2003).
Incorporating improvements in energy efficiency to a project can provide opportunities for energy utilities or other entities to assist in paying or financing a portion of project costs. It is important to point out that energy conservation measures need to take into account any potential effects to process performance or reliability.
2.1.3 Chemical Savings
Many WRRFs could save chemicals by implementing closed-loop control of chemical dosing. Closed-loop control is where an instrument monitors the process output (such as residual chlorine) and a controller adjusts a process input (such as hypochlorite pump speed) to maintain a desired process output. This type of control can often provide better performance with less wasted chemicals. For example, in the chlorination process, changes in flow and effluent quality result in varying chlorine demand. If the automation system can match this demand, substantial savings in chlorine are possible. Accurate estimates of these savings, however, are needed to make good design decisions for automation; typically, closed-loop control of chemical addition saves 10 to 20% of chemical costs.
Facility size can affect the economics of implementing closed-loop chemical control. Reducing chemical usage by 1.0 mg/L of chlorine at a 378-ML/d (100-mgd) WRRF, for example, could justify the cost of several chlorine analyzers, a controller, and a part-time technician, with substantial savings left over. Reducing chemical usage by 1.0 mg/L of chlorine at a 0.4-ML/d (0.1-mgd) WRRF, however, may not justify the installation of any automation equipment.
An optimization project implemented at the Morris Forman Water Quality Treatment Center in Louisville, Kentucky, modified the polymer dosing control strategy for the treatment center’s centrifuges, where the polymer dosing was adjusted based on total suspended solids in the sludge feed and centrate. This system resulted in a savings of 11.7% of polymer usage, lower recycle rates, a higher percentage of dry solids in the cake, and higher consistency in the cake (Bates and Montoya, 2010).
2.1.4 Additional Savings
There are other areas of a WRRF that represent potential savings that depend on unique characteristics of the utility. Effective automation can reduce capital costs by allowing staff to maximize existing treatment capacity and facilities, enhance maintenance, reduce equipment wear and damage, reduce chemical usage resulting in reduced chemical storage requirements, and minimize chemical hazards.
Energy efficiency improvements can also yield environmental benefits by reducing greenhouse gas emissions and improving a utility’s carbon footprint. The magnitude of the effect of energy reduction measures on a utility’s carbon footprint is highly dependent on local conditions and the amount of greenhouse gases attributed to the electrical energy source. For example, utilities that obtain electricity from coal-fired power plants will typically have a higher effect than those that obtain a significant amount of energy from renewable sources such as hydroelectric generation.
2.2 Intangible Economic Benefits
2.2.1 Compliance Monitoring
Monitoring of several key process parameters is required under water-quality regulations. State and federal discharge permits, for example, require WRRFs to record their daily flowrates and report average monthly and maximum daily flows. Most WRRFs also must collect flow-weighted composite samples for analysis. The samples may be collected by an operator and composited manually based on the flowrate during sampling or collected by an automatic sampler paced by a flow meter’s signal.
2.2.2 Improved Process Performance and Reliability
Automation can allow for more accurate and consistent control of a process, resulting in improved performance. Improved performance can take the form of better yields, in terms of the amount of wastewater treated for a given unit of chemicals or energy, or fewer unwanted byproducts. While an operator can occasionally adjust a process based on changing facility conditions, automation can continually adjust the process to more closely track an optimal operating condition. Reliability can also be improved as regular adjustments to alarm conditions and fault handling can be programmed into the control system.
As an example, the Water Environment Research Foundation (2002) presented five case studies of facilities that implemented automatic solids retention time (SRT) control. Significant benefits were demonstrated by the facilities, including improved process performance. Figure 2.1 shows an example of the effects on several process parameters of changing from manual control to automatic SRT control of an activated sludge process. The graph shows more consistent mixed liquor suspended solids and mass of wasted sludge. This affects not only the activated sludge process, but also sludge consistency, thereby improving operation of downstream thickening.
Another advantage to the improved control is the potential to maximize unit performance, potentially delaying upgrades.
2.2.3 Improved Responsiveness
With automation, operators can often respond
