Restoring Infrastructure without Capital Investment: Part I – How Optimizing Your Treatment Facility Can Help
Communities across the nation are facing a common challenge. The need for reliable, affordable utilities is growing along with the population, while utilities struggle to maintain aging infrastructure that cannot keep pace with operational demand. As wastewater infrastructure begins to fail or functionally decline, capital investment is often needed to repair or replace critical assets. This can be a particularly costly approach for a utility when its treatment facilities fall short of meeting acceptable benchmarks.
One way to combat this deterioration is to look for ways to optimize performance at our aging wastewater treatment plants (WWTP). WWTP optimization is a cost-effective way to achieve improved performance, reduce costs, and maximize the use of existing infrastructure. Through optimization, you can also solve non-compliance or capacity issues as well as achieve a higher level of treatment.
Optimization of WWTPs is a process that includes four major steps:
The specific details of the program depend on the optimization objectives. These objectives can be broadly-based, covering all aspects of the design and operation of the plants, or can be narrowly focused on mitigating a specific problem or problems. Optimization objectives might include the following, among others:
Wastewater Treatment Energy Best Practices
- VFD Applications - Variable frequency drives (VFDs), one type of variable speed technology, match motor output speeds to the load requirement and avoid running at constant full power, thereby saving energy.
- Reduce the use of fresh water consumption - Reducing the consumption of potable water by using final effluent (FE) in process applications may save energy by limiting the volume of water treated and/or pumped. The FE system should include a pressure tank and pump control system, where appropriate, and direct pumping where consistently high pressure is required (belt press).
- Optimize flow with controls - Assess variations in facility flows and apply control systems to address minimum, average and peak design flows. Although equipment must be designed to pump for peak flows, these designs are often not energy efficient for average existing flow conditions. Therefore, it can be beneficial to apply control strategies or equipment that more precisely meets low - and average - flow conditions and can shift system demands during off-peak power periods.
- Operational flexibility - Evaluate facility loadings and become familiar with the treatment systems to identify, plan and design the most efficient and effective ways to operate your system. This may include: operating fewer aeration tanks, installing variable frequency drives so equipment operation can match system loadings, installing dissolved oxygen monitoring and control equipment, idling an aeration tank during low-flow periods, reducing air flow to the aeration tanks during low-load periods (usually nights and weekends), waiting to recycle supernatant during lower-flow periods, avoiding periods of high organic loading, and operating diffusers or recycling backwash water during off-peak power demand periods.
- Staging of treatment capacity - When planning improvements, wastewater system personnel and designers should develop a team approach wherein they determine how modifications will effectively and efficiently meet current and projected conditions. Staging upgrades in capacity can help optimize system response to demand and reduce energy costs.
- Managing for seasonal / tourist peaks - Flexible system design allows a utility to adjust and operate more efficiently during peak tourist loadings as well as during the “off season.” In many areas, tourism-related loadings versus off season may reach as high as 10:1. This may require removing tankage that is used during tourist season from service during the off season.
- Flexible sequencing of tank use - The selection of basin sizes can have a large impact on the energy consumed at a facility during its lifetime. The facility design team should review the existing and projected organic loadings to identify the best selection of tank sizes. Typically, the use of smaller sized basins is beneficial so that initial loadings can be near the capacity of a smaller basin. The remaining basins can then be loaded sequentially until design capacity is met. This approach allows for energy efficient operation from start up to full design flow conditions.
- Recover excess heat from wastewater - prior to its treatment and/or discharge to use at or near the wastewater treatment facility. Some industrial wastewater systems have a large volume of low grade heat available in their wastewater (typically able to provide 20oF to 25oF). In northern climates, basins are often covered to prevent the contents from freezing. This practice reduces, or possibly eliminates, the energy used to thaw equipment or tanks.
- Optimize aeration system - Determine whether the aeration system is operating as efficiently as possible for the required level of treatment. Assess present loading conditions and system performance through a comparison of kWh/MG and other key performance indicators with those of other similar facilities. Consider the potential benefits and costs of improvements such as fine-bubble aeration, dissolved oxygen control and variable air flow rate blowers.
- Fine bubble aeration - Assess the feasibility of implementing fine bubble aeration at activated sludge treatment facilities. This practice provides energy efficient treatment of wastewater. It can be installed in new or existing systems. The technology usually improves operations and increases the organic treatment capability of a wastewater treatment facility. For optimum performance, combine this practice with dissolved oxygen monitoring and control, and a variable capacity blower.
- Aerobic digestion options - Assess your aerobic digester operation to determine if a smaller blower would provide better control of airflow using fine-bubble diffusers and equipment with adjustable airflow rates. Many facilities operate aerobic digesters with surface aerators or coarse-bubble diffusers with limited ability to modify or control air flow delivered to the process. First, consider fine-bubble diffusers, which allow for variable airflow rates, for digester applications. Second, choose equipment and/or controls with adjustable airflow rates. Often, air for the digestion process is bled from the aeration system, allowing little or no control over the airflow delivered.
- Biosolids processing options - When planning new facilities or expansion, assess the energy and production impacts of various biosolids process options. Standard aerobic digestion of biosolids is energy intensive compared with fine-bubble diffusers with dissolved oxygen control and a variable air-flow rate blower. Some locations currently turn off the air-flow to the digester over extended periods of time to further reduce energy costs. Anaerobic digestion requires detailed assessment. While the capital cost of an anaerobic system is considerably greater than for an aerobic system, an anaerobic system can produce biogas for energy production and can help offset capital costs. Both types of system should be considered.
- Biosolids Mixing options (aerobic) - Biosolids mixing is an energy intensive task that should be addressed in aerobic digestion. Mixing is generally provided by aeration, mechanical mixing, pumping or a combination of these methods. Aeration of the biosolids mass is required to destroy volatile solids and control odor. However, aeration may not be the most energy-efficient way to provide complete mixing in a digester, especially if constant aeration is not required. Evaluate the energy costs of available options to identify the best technology for the site. A combination of mixing methods that will permit the system to be completely turned off periodically may be most practical.
- Variable blower air flow rate (aerobic) - Require that aeration system and aerobic digester blowers have variable air supply rate capability. The range of variability should respond to the specific requirements a site needs to precisely match system demands. The blower system should be able to supply the minimum air flow required to meet existing low-load conditions and to meet the high loads of design conditions.
- Dissolved oxygen control (aerobic) - Consider dissolved oxygen monitoring and control technology which will maintain the DO level of the aeration tank(s) at a preset control point by varying the air flow rate to the aeration system.
- Biosolids mixing options (anaerobic) - The contents of an anaerobic digester must be mixed for proper operation, the destruction of volatile suspended solids, and the production of biogas. Mixing is generally accomplished by injecting biogas into the bottom of the digester and having it pass through the contents of the tank. Some sites also continually pump the contents to provide mixing. Mechanical mixing can also be used to achieve a higher level of volatile solids destruction and greater biogas production.
- UV disinfection options - Consider various ultraviolet disinfection (UV) system redesign options that can be configured by reducing the number of lights, bulb orientation, bulb type (pressure and intensity), turn-down ratio (bank size and lamp output variability) and dose-pacing control (system output automatically controls to disinfection requirement).
- Final effluent recycling - Reuse final effluent to replace potable water use for wash down of tanks and process related applications. The installation should include a pressure tank so the recycle pump will not operate continuously. Additional applications are possible with an inline filter prior to each application.
More than Nuts and Bolts?
But optimizing a WWTP isn’t just about the nuts and bolts mechanics of the plant; true optimization also takes into consideration the people who operate and manage the facility. The work of WWTP operators is critical to public health; however, some may lack the specific training or background needed to efficiently operate the plant, or they may be unfamiliar with the latest technology that can have a positive impact on the plant’s performance. So, if you’re considering a review of your facility, it’s important to consider adding a training component for operations staff once the optimization study is complete.
As engineers, WK Dickson understands the science behind WWTP treatment and operations. But we are also not your typical engineer. Our specialized team includes a group of engineers who are not only wastewater operator certified but have also spent significant portions of their career as wastewater operators. They have spent time in your shoes and know firsthand the challenges you face. This unique perspective can bridge the gap between what works on paper and what works in the real world so that you get the best solution.
Restoring wastewater infrastructure in the U.S. is vital to our environment and our own health. But with a few changes, as recommended by an optimization study, treated wastewater can be processed and discharged more effectively, at a lower cost and result in a cleaner product released into the environment.
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