Energy Efficiency for Wastewater Treatment Plants
Much has changed in recent years in the water and wastewater industries: evolving environmental regulations, increasing operating costs, technology advancements, and improved opportunities for load management, according to the Electric Power Research Institute (EPRI).
EPRI states that energy efficient operation is a primary goal in the US wastewater treatment industry, combined with approaches to recover energy from the wastewater stream. Between 1998 and 2008, the number of facilities employing processes greater than the secondary treatment grew by 48%. And as processing needs became more stringent, the energy required increased. A fuller discussion can be found in EPRI’s 2013 publication, “Electricity Use and Management in the Municipal Water Supply and Wastewater Industries.”
Energy efficiency is embedded in all of the changes EPRI cited above and wastewater treatment utilities have been taking advantage of improved technologies to reduce operating costs for more than 10 years and in some cases two decades. Not only EPRI but federal and state agencies have been documenting their experiences.
Advanced wastewater treatment usually includes aeration for removing dissolved organic matter and nutrients. Thus, EPRI writes, aeration is the principal energy-using process in wastewater treatment, representing half the cost of total wastewater treatment, followed by biosolids processing and pumping.
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Biosolids handling and treatment can account for one-third of overall wastewater treatment plant energy use. If energy recovery is not a solution, the goal will be to render the biosolids harmless so that they can be disposed of. If it is dewatered, requiring natural gas use, costs could be significant. If the biosolids are dried in beds, energy costs are small. If the biosolids are incinerated, the costs could again be significant, but EPRI notes that there are only about 100 wastewater treatment facilities with incinerators out of 15,000 publicly owned treatment works nationwide.
An example of a new technology is that developed by Smith & Loveless, a Kansas-based company that manufactures pre-engineered water and wastewater treatment and pumping systems for municipalities, industry, government, and military facilities. As noted above, one of the primary energy users in wastewater treatment lies with pumping systems, including those that remove grit in the headworks section before downstream treatment, explains Darby Ritter, marketing communications manager with Smith & Loveless. Once the grit is removed, it has to be pumped to the dewatering and washing area and later disposed of or sold.
The company developed its proprietary PISTA 360 grit removal system, to be more efficient than aerated grit chambers which use air diffusion. The PISTA 360 is a vortex-type system with a rotating device. Pumps, by their nature, are not efficient and they operate 24 hours a day, seven days a week, Ritter says. “You want a pump with some measure of efficiency.” Its efficiency can be measured by how much it runs. For example, a 10-HP pump running four hours per day uses less energy than a 5-HP pump running 24 hours per day.
The PISTA 360 maintains ideal velocity during low-flow and high-flow periods and only needs to rely on knowing the range of anticipated flows. On the other hand, a stacked-tray system relies on advanced particle sizing and settling rate analysis to achieve proper system sizing. Under these conditions, grit capture requires that the system pump operate continuously to remove the grit.
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PHOTO COURTESY OF EBMUD
East Bay Municipal Utility District upgraded its anaerobic digestors, where it processes trucked-in food waste, in 2014. Methane generated by the food waste added to the power generated onsite. Waste heat is used to maintain optimal anaerobic temperatures and for heating buildings.
LEADERSHIP AND PLANNING COME FIRST
The US Environmental Protection Agency (EPA), in its publication, “Energy Efficiency in Water and Wastewater Facilities,” reports that wastewater treatment plants are responsible for up to one-third of the energy costs a municipal utility must be responsible for. The EPA has also published the “EPA Portfolio Manager Tool for Wastewater Facilities.”
Other agencies have also published tool kits and guidelines offering guidance to water and wastewater treatment agencies on how to reduce energy use. These include the National Renewable Energy Laboratory (NREL), the New York State Energy Research and Development Authority (NYSERDA), and the California Energy Commission.
The EPA’s publication provides a guideline describing how managers can develop strategies for energy efficiency in their wastewater facilities. “The most effective way for communities to improve energy efficiency in their water and wastewater facilities is to use a systematic, portfolio-wide approach that considers all of the facilities within their jurisdiction,” says the EPA.
EPA goes on to explain that energy audits or monitoring devices that feed into the facility’s Supervisory Control and Data Acquisition (SCADA) will educate staff where energy is being used and will identify opportunities for energy efficiency improvements. These improvements can be found in equipment upgrades, operational modifications, and modifications to the facility buildings, including energy efficient lighting, windows, and cooling equipment. Furthermore, mechanical aerators, blowers, and diffusers used for wastewater treatment account for the largest share of energy use.
The American Council for an Energy Efficient Economy (ACEEE) recommends creating a leadership team in its publication, “Local Technical Assistance Toolkit: Energy Efficiency in Water and Wastewater Facilities.” The leadership team would be comprised of utility management and operations personnel which can maintain commitment and buy-in throughout the project lifetime. It points out that managers can be averse to implementing new measures which they may perceive to be risky. “By measuring baseline energy use, identifying cost-effective efficiency measures, and creating a plan to measure and verify savings . . . may lower the perception of risk,” says the ACEEE in the publication cited above.
EPRI noted in its study that there are significant variations in treatment approaches and treatment objectives across the US. Plans can vary significantly even within individual sites and are often driven by the condition of local water resources. EPRI includes detailed case studies of eight water or wastewater districts and their various approaches to energy reduction. Here are profiles of five wastewater treatment districts and their innovative methods for managing energy costs.
ENERGY AUDITS PAY OFF
Located in Central Solano County, CA, the Fairfield-Suisun Sewer District serves 135,000 customers and treats an average flow of 15 million gallons per day (mgd) during dry weather and 30 mgd during wet weather.
The sewer district’s electricity provider, Pacific Gas & Electric (PG&E) performed an integrated energy audit in 2009 and helped the district determine the critical energy users among the process and lighting equipment. PG&E provided $93,000 in incentives for the energy efficiency upgrades which had a simple payback of 2.7 years.
The sewer district converted two aeration basins from coarse-bubble diffusers to line-bubble diffusers. It installed premium efficiency motors for various pumps, mixers, and fans ranging in size from 1 HP to 100 HP for its secondary treatment expansion project. It also replaced one of its influent pump station motors with a high-efficiency motor.
During the secondary treatment expansion project, the district installed variable frequency drives to control several pumps with an aggregate load of 725 HP. Some equipment was downsized while controls were increased to better match the system to the wastewater flow. In 2011, it installed a low-pressure, high-intensity ultraviolet radiation system for disinfection and upgraded two more pump stations.
The district also replaced motor control center-panel lights with LED lamps and it relamped existing fluorescent lamps in the facility and remote sites with T8 lighting.
The sewer district was already generating electricity by burning digester gas in one 450-kW Superior reciprocating internal combustion engine. The power is distributed to dedicated loads within the treatment plant. Another 900-kW internal combustion Waukesha engine runs on a blended combination of natural gas and digester gas. Waste heat is recovered from the engines to heat the digesters and the maintenance building.
The sewer district also operates four wind turbines with a total installed capacity of 200 kW and a 1-MW photovoltaic solar system. They have been in operation since 2007.
PG&E also awarded $258,000 in incentives for the installation of the wind turbines. The sewer district is now saving 1.3 million kWh per year and reduced its electric demand by 206 kW.
GRANDDADDY OF EFFICIENCY
The Sanitation Districts of Los Angeles County operate 11 waste treatment plants and 10 water reclamation plants plus one ocean discharge facility known as the Joint Water Pollution Control Plant (JWPCP) in 23 independent special districts that serve about 5.4 million people within Los Angeles County.
Roya Phillips, a senior engineer at the Sanitation Districts says the aeration process is the highest electricity user in any waste treatment plant, followed by pumping and lighting, in agreement with EPRI. Aeration of digesters during the secondary treatment process continues 24 hours a day, seven days a week to oxygenate microorganisms that consume dissolved organic matter.
There are three technologies that are designed to do this: mechanical surface aerators that spray water over the mixture, and two types of submerged bubble diffusers. Process air compressors blow either course bubbles or fine bubbles of air through the secondary treatment tank. Fine bubble diffusers have more bubbles so there is more surface area per unit volume and greater oxygen transfer exchange, explains Phillips. They also require less energy than course bubble diffusers to run.
EPRI estimates that for an average plant flow of 100 mgd, a fine bubble diffuser will burn up about 30,000 kWh per day. For the same plant flow, a course bubble diffuser will use about 50,000 kWh per day. Brush aerators or high-speed splash aerators will use about 70,000 or 80,000 kWh per day.
At the JWPCP, Phillips says pure oxygen is used in the covered secondary treatment tank. In this case, mechanical technology is used to distribute the oxygen there. She says analysis has demonstrated the technique is more cost effective in this application.
The flow in digesters is not constant so variable frequency drives are used on compressors instead of control valves to reduce or increase air flow through the mixture as indicators determine the amount of air needed.
The JWPCP is the largest of the 11 treatment plants with a capacity of 400 mgd. It has operated a cogeneration plant for at least 25 years, making JWPCP electrically self-reliant. Three Solar Mars 90 turbines fired by digester gas, each coupled with an electric generator, generate 9.9 MW each. Turbine exhaust heat is used to heat water. It is also directed to heat recovery steam generators which produce steam used to generate more power in a steam turbine. Steam is also used for digester heating. Four digester gas-fired boilers provide backup with a natural gas-fired boiler available for emergencies. The most recent data available from December 2015 showed the power generation system produced 14,430 MWh that month.
Phillips says the Sanitation Districts are in the process of replacing pumps throughout the plants with higher efficiency pumps, based on age, application, and life cycle costs, taking into account maintenance costs on older pumps. “We always try to get the more efficient pump. It is our typical standard procedure,” she says.
The lighting in all the plants is being converted to LEDs. However, lighting energy use takes up just 5% of the budget, Phillips says.
FAIRFIELD-SUISUN SEWER DISTRICT
The Fairfield-Suisun Sewer District in Sonoma County, CA, expanded its secondary treatment plant and this is one of the two aeration basins converted from coarse-bubble to fine-bubble diffusers. Premium efficiency motors were also installed on various pumps, mixers, and fans.
PLANT IS NET ENERGY USER
The East Bay Municipal Utility District (EBMUD) in California handles wastewater for 650,000 customers in seven San Francisco Bay communities. Its wastewater treatment plant is located in Oakland. The district has been increasing its system’s energy efficiency since the early 1980s. It adopted goals of producing all of its energy with onsite resources like biogas by 2010 and reducing its greenhouse gas emissions by 10% from 2000 levels by 2015.
At EBMUD’s Special District 1 wastewater treatment plant electricity is being cogenerated using methane produced in digesters. Energy costs are being reduced by $1.7 million annually.
The district brought online three 2.3-MW engine generators in 1985 but operated no more than two at a time to generate about half of the facility’s energy needs. Then, in 2012, EBMUD brought a 4.6-MW gas turbine online. It has become the primary electricity generator, supplemented by one or more of the engines when additional biogas is available.
Together, the generation system produces on average 6 MW, with a peak capacity of 11 MW. The ability to produce additional power was aided by a growing supply of wastewater and trucked-in food waste treated in its anaerobic digesters. The waste heat recovered from the engine and turbine is used to maintain an optimal anaerobic digester temperature and to provide building heating.
With the addition of the turbine, EBMUD became the first wastewater treatment plant in North America to produce more energy than is required onsite, according to EPRI. It sells the excess electricity back to the grid.
In addition, EBMUD replaced five old 700-HP influent pumps and motors and four 1,000-HP effluent pumps with high-efficiency pumps and motors. Variable-frequency drives were installed on all nine motors. According to EBMUD, these upgrades have cut by 50% the cost to run the equipment without any impacts to treatment quality, saving $273,000 annually.
EBMUD has also discontinued mixing its second-stage activated sludge. It used to be that the district mixed the sludge in a continuous process after adding microbes and oxygen to ensure microorganisms continued their activity. However, it found this mixing wasn’t necessary for sludge breakdown and discontinued it, saving the $223,000 annual cost to operate four 150-HP mixing motors around
The district added plastic balls to its oxygen production vaporizer pit to prevent evaporation and heat loss. It also inter-tied pipes on gas recirculation blowers to allow one blower to serve two mixing tanks.
Combined, these and other measures have reduced total plant energy costs by about 60%. They save the site approximately $3 million annually, thereby transforming it from an energy consumer to a net energy producer. The Plant also utilizes hydropower and a solar system. Excess power produced is sold to the grid.
INCREASE BIOGAS, STABILIZE RATES
Gresham Wastewater Treatment Plant, located near Portland, OR, reached net zero energy use in 2015, producing more energy than it uses. The plant is saving the City of Gresham $500,000 a year in electricity costs. Alan Johnston, the plant’s senior engineer, says the energy savings have stabilized rates which are some of the lowest in the northwest.
Biogas from the digesters is converted into electricity produced by two Caterpillar 400-kW engines and used in the plant. The heat coming off the engine is captured and used to heat the digesters. A one-acre solar photovoltaic ground-mounted system is capable of generating 420 kW as long as the sun is out, says Johnston. Most of the power is used onsite, but 10% is exported to Portland General Electric.
Over the past 10 years, Johnston says the staff has improved the lighting systems, installing LEDs in the streetlights around the plant and proximity motion sensors. Other projects have included high-efficiency air blowers and digester mixers. Diffusers used in the aeration basins are the biggest energy users, Johnston says.
Gresham created the fats, oil, and grease (FOG) program to boost its biogas production in the treatment plant. Private haulers had been hauling the FOG from thousands of restaurants in Portland, OR, to their own or private treatment plants, paying tipping fees as high as 21 cents per gallon. The Gresham plant decided to offer the haulers 8 cents per gallon in tipping fees. The haulers were quick to sign contracts and thereby reducing transportation costs.
The FOG goes into the plant’s digesters increasing biogas production 60%, Johnston says, allowing them to purchase the second engine. Tipping fees bring in $350,000 a year, he adds.
Johnston says before the changes, the plant was using 6.5 million kWh. Since the addition of the power systems, the plant has been using about 5.5 million kWh.
PHOTO COURTESY OF WASHINGTON SUBURBAN SANITATION COMMISSION
The solar array at Washington Suburban
BIO-ENERGY PLANT WILL CREATE REVENUES
Washington Suburban Sanitary Commission (WSSC) is a 99-year old water and wastewater utility serving 1.8 million residents in Prince George’s and Montgomery Counties in Maryland. It operates and maintains three reservoirs, two water filtration plants, and six wastewater treatment plants. It also has a cost-sharing agreement with DC Water which operates another advanced wastewater treatment plant.
WSSC is currently planning several energy-focused capital projects including a bioenergy project and energy conservation improvements at its wastewater treatment plants which include installing high-speed turbo blowers, submersible mixers, rebuilding major pumps, and a self-generating microgrid.
Rob Taylor, energy manager at WSSC, described the Piscataway Bio-Energy Project, currently in its design phase. It is being developed adjacent to the existing Piscataway wastewater treatment plant at an estimated cost of $185 million, according to Taylor.
When built and operating in late 2021, it will accept biosolids from all of WSSC’s wastewater plants. The biosolids will be processed through a thermal hydrolysis system, which breaks them down to make them more digestible to the bacteria in the downstream anaerobic digesters.
The methane generated in the anaerobic digestion process will be processed to remove water, carbon dioxide, and other impurities to produce fuel commercially equivalent to natural gas. The current plan is to sell the fuel to a facility for fueling buses. The original plan was to run a generator on the methane, but selling the fuel has become much more profitable.
Selling the fuel to the bus-fueling facility will produce renewable energy credits, Taylor explains. WSSC can then sell the credits to oil companies which need them to comply with clean air standards. Revenues from the sale of renewable energy credits will bring in $2 to $2.5 million annually he says.
If the value of the credits drops, the gas will be fed, along with some supplemental natural gas, into new onsite generators to produce 2.5 MW to 3 MW to power equipment at the Piscataway plant.
Currently, WSSC hauls the biosolids from its wastewater treatment plants to land sites for farm applications at an equivalent of 20 trucks per day, five days per week, Taylor says. Those transportation costs will be eliminated.
WSSC has two 2-MW, ground-mounted solar installations, one each at its Seneca and Western Branch Wastewater Treatment Plants. They were commissioned in November 2013. Washington Gas Energy Systems owns and operates the solar plants under a 20-year power-purchase agreement (PPA). The solar plants provide an average 17% of the electricity required to operate the two wastewater plants, saving ratepayers approximately $3.5 million over the lifetime of the agreement.
Since 2008, it has been buying wind power generated by a wind farm in southwestern Pennsylvania under a 10-year PPA. By 2011, wind power accounted for 28% of WSSC’s total electric consumption. Taylor says a new wind plant will be built so electricity can be exported, probably to the Potomac Water Filtration plant.
Taylor says WSSC is now negotiating with an energy services company to replace existing 600-HP and 450-HP centrifugal blowers at the Piscataway plant with new smaller 400-HP and 300-HP high-efficiency turbo blowers. In addition, WSSC will replace nearly 50 submersible mixers at the Piscataway plant with mixers nearly one-third the size. A similar mixer replacement upgrade will also be carried out at the Parkway Wastewater Treatment Plant.
Taylor estimates $750,000 will be saved, mostly by replacing the blowers which operate 24 hours a day, seven days a week. “We are also replacing a large 3,000 HP pump at the Potomac water filtration plant,” he adds.
WSSC staff is now planning to construct a self-generating microgrid also at the Potomac plant, Taylor says. It will include a gas-fired 10- to 13-MW generator and a solar system, not yet sized. “We plan to buy power from the developer who will operate it,” he says.
The motivation for the microgrid, says Taylor, is to have the capability to separate from the existing grid and provide reliable power in times of heavy storms.
Taylor explains the reason the microgrid is planned for the water filtration plant instead of at one of the wastewater plants is that all of the latter are too small. Most of the wastewater plants have backup generators and WSSC will be installing more at those plants that don’t currently have them, Taylor says.
DEMAND-SIDE MANAGEMENT WORKS
One of the most notable case studies in EPRI’s report is that of the Eastern Municipal Water District of Southern California which receives annual demand response payments of $600,000.
During critical power need periods when its third-party aggregator requests a power reduction, EMWD reduces non-essential energy use, shutting down pumps and turning to its biogas-fired and natural gas-fired onsite generators. It has about 12.2 MW enrolled in the various demand response programs available to it, representing approximately 33% of its peak demand. EMWD’s electricity use costs it more than $14 million a year and demand response payments help offset that cost.
In its report, “Energy Efficiency in Municipal Wastewater Treatment Plants,” NYSERDA identifies demand-side management opportunities including energy-efficient lighting, high-efficiency motors, electric load controllers, and adjustable-speed drives as methods to reduce electricity costs. Generating electricity via outfall hydropower or onsite generation can reduce the amount of electricity purchased from the utility.
Noting that aeration of activated sludge can account for 30 to 80% of total plant electricity demand, NYSERDA says energy use can be reduced by either using fine-pore diffused air systems and aeration process controls or lowering the sludge treatment age from 10 to 12 days to three to four days. Alternately, some plants may be able to treat normal flows in off-peak hours to take advantage of time-of-day electricity pricing when the cost of electricity is at its lowest rate.
NYSERDA notes that the normal diurnal sewage flow patterns into a waste treatment plant closely parallel an electric utility’s system demand and energy cost curves. Shifting the electrical load from on-peak to off-peak hours or leveling electricity use throughout the day may require temporary storage of influent wastewater either at the plant or within the sewage system.