Some benefits represent a quantifiable savings to the utility, while other benefits cannot be quantified in the same way. When building a business case, it is important to account for both tangible, or quantifiable, economic components and intangible, or nonquantifiable, components. Developing an inventory of costs for energy, chemicals, and labor can be useful in identifying areas that have the highest cost; it also focuses automation efforts on areas with the highest savings potential.
2.1 Tangible Economic Benefits
Table 2.1 shows the relative magnitudes of operating costs for a typical WRRF. The proper application of automation can have an effect on several of these cost categories. The following sections highlight key tangible automation benefits. The value of tangible savings can change over time because of inflation or escalation; this should be accounted for in the overall economic analysis.
2.1.1 Labor Savings
There are many facilities with a significant manual operational component in which automation can result in reduced labor. Increased automation can
• Reduce instances of “walking the facility” and visiting remote facilities to check and control the processes;
TABLE 2.1 Operating costs for a typical WRRF (Hamilton et al., 2009).
• Eliminate repetitive tasks such as filling tanks and adjusting chemical flows;
• Minimize the need for staff presence at a particular place at a specific time;
• Reduce time for the collection of operational data;
• Reduce laboratory work collecting and processing samples; and
• Reduce the time for compiling regulatory, operational, and management reports.
A fully automated process area would automatically handle ordinary events and alarm an operator when an unusual event occurs.
Labor savings can be estimated by itemizing individual tasks performed by operations staff with estimates of the time and frequency of each task, both before and after automation improvements. It is important to account for additional training, testing, and verification activities needed to confirm that automation improvements are achieving the desired goals.
Labor savings opportunities can be substantial for some facilities. The City of Anchorage Water and Wastewater System in Anchorage, Alaska, increased facility capacity by 62% during a 10-year period while staff was reduced by 15% (47 people) during the same period. A significant component of this savings was attributable to automation (Patrick, 1997).
Estimated reductions in labor should be partially offset by anticipated increases in maintenance and calibration labor for the additional automation components.
2.1.2 Energy Savings
Benefits from energy management at a WRRF typically come from automatically controlling a process to reduce wasted energy and shifting the time of day when energy loads are used. Energy usage data from a survey of 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