Wastewater facilities are faced with a myriad of challenges: aging infrastructure, rising labor costs and difficulty in getting rate increases to properly fund maintaining their operations. This has focused many utilities on looking for smart ways to cut costs without negatively affecting their ability to perform their mission.
Treating wastewater is an energy intensive process. And because the process is energy intensive, it is cost intensive as well. This is why many treatment facilities have looked to lower costs by reducing the amount of energy they purchase. This can be done in two ways: reducing the amount of energy the facility uses and incorporating power generation sources into the facilities themselves.
The treatment process?s high energy use comes from two activities ? powering the system of pumps that are required to lift the sewage water at the front of the plant and the aeration process. Aeration, which removes the oxygen demand of the effluent, can account for roughly half of a treatment plant?s energy usage. The sewage is aerated by the spinning of massive turbines that add air into the sewage.
Lowing Energy Costs
The first step in lowering energy costs is to do a comprehensive audit of the facility to identify where and when energy is used and what changes can be made to lower energy usage. This includes the scrutiny of all equipment to determine whether or not it can be operated for fewer hours than current practice. It is also important to see if there is poorly functioning equipment that can be replaced with a more energy efficient substitute.
Reducing a facility?s overall energy usage goes hand-in-hand with adding the ability for a facility to generate power on its own. Once the system is installed, it will likely be connected back into the local electrical grid. In some areas, this allows facilities to sell back any excess power that they don?t use through a process called net metering. So the less power a facility uses, the more they can sell back.
The energy demand of a treatment facility stays fairly flat throughout the day while the price of energy that it purchases from a utility fluctuates. This allows treatment facilities with solar generation to trade peak power during the day for more units at night when the value is lower.
To meet energy generation needs, many treatment facilities have turned to solar power. Solar and treatment facilities make a good fit for a number of reasons. Solar isn?t just for the rooftop. Treatment facilities often have unused, open land where ground-mounted solar systems can be installed. This also allows facilities to modularize the size and expense of their system, allowing them to potentially add panels as their energy demand increases. These open spaces are also likely to be areas without trees, so shading of the panels, which cuts their power output, is not an issue.
Solar panels generate their peak power output during the day, also when electricity prices are at their highest. This allows plants to sell back energy at the most cost-effective time. Generating energy from solar, rather than relying on the grid, provides budget stability, as the price of the fuel ? sunlight ? is free. This stability serves as a good hedge against the threat of rising fossil fuel energy costs.
Third-Party Ownership
Since early 2010, the average cost of a photovoltaic solar system has dropped by more than 46 percent, with the average price of a solar panel declining by 51 percent. This significant drop in price, combined with higher efficiency panels have created a market where there is financing available to amortize the up-front expenses into a payment schedule that is lower than a typical energy bill. This is especially helpful for facilities that are facing tight budgets. Through third-party ownership, treatment facilities can host and receive power from a solar system at a fixed cost that is lower than what they would pay the utility. Importantly, this system is managed by the third-party as well, meaning that facilities do not have to train their own staff to maintain the system. The third-party owner also assumes project risk should there be cost overruns or underperformance of the system. This relationship is known in the industry as a power purchase agreement (PPA).
The rise of third-party ownership has been a boon to the solar industry, allowing for the construction of many projects that would not have otherwise been built because the end users of solar lacked resources to pay for more than two decades worth of energy all at once. This ownership model has also put a premium on equipment selection because it changes going solar from a simple product choice into an investment decision. As a result, the companies that finance these projects value reliable solar panels, because those are the ones that provide the best return on their investment.
A PPA is how the Oro Loma Sanitary District incorporated a 468 kW, 2,000-plus panel system into its facility that serves more than 180,000 residents on East Bay of the San Francisco Bay Area. Oro Loma wanted to go green and save money and was able to do so by incorporating solar into its facility.
The addition of a solar system was preceded by a thorough review of the needs of all of the plant?s operating equipment. This led to operating its aerated grit blower only during nighttime, low-flow hours. The plant also reduced the daily runtime on its digester mixers from running constantly to running for only 12 hours. Oro Loma also shut down 12 percent of its 24 aerators and reduced the use of spray water for foam control when no foam is present. It raised the influent wet well level set point in its main influent wet well during dry weather and reduced its return activated sludge flows to the lower limits of the prescribed process range.
Project Challenges
The project faced two major technical challenges.
The system had to accommodate a previous solar system design that ultimately proved unworkable. Additionally, the solar system could not interfere with two on-site power generation systems. These co-generation systems have sensitive controls and protective relays that the solar system could not interfere with in the event that power from the electrical grid was to be taken offline. To solve this problem, engineers custom designed a fiber optic-based interlock to immediately shut off the inverters should power from the utility be cut. Even with this challenge, the system was completed on time and
on budget. ?
The treatment facility also has a bio co-generating unit that, combined with the solar array, allows the facility to produce 100 percent of its energy needs. This isn?t to say that all of the power the plant uses is produced on site. Instead, the plant sells the energy it generates during the day to the local utility at a higher price than what it purchases from it for power at night. The facility has found that, on average, it is able to trade one kW hour worth of power generated during the day for 2.5 kW hours worth of power when the plant uses that power.
In total, the solar system is expected to save more than $2 million for the Oro Loma Sanitary District over the term of the 20-year PPA. The initial PPA rate provided an immediate 25 percent savings from what the facility was paying to the local utility. In the year that the system has been operating, the facility is proud to report that the solar array was integrated well with its existing power generation and utility sources. From the time the district flipped the switch, the system has operated without any attention by Oro Loma employees, other than the manager who spends time reporting the cost savings of the project to his board on a monthly basis.
Victoria Vestal is a marketing manager for Trina Solar North America.
