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ReseaRch gRoup Jess Brown, Directorphone (941) 371-9832jbrown@carollo.com

eDIToRerin Mackey

DesIgn anD pRoDucTIonLaura corringtonKim LightnerMatthew parrott

ReseaRch SOLUTIONS

This publication is printed with soy inks on FSC®- certified 100% recycled content (60% post-consumer waste).

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Phoenix, Arizona Yuma, Arizona Fresno, California Inland Empire, California Orange County, California Pasadena, California Sacramento, California San Diego, California San Francisco, California Sunnyvale, California Ventura County, California Walnut Creek, California Denver (Broomfield), Colorado Denver (Littleton), Colorado Broward County, Florida Miami, Florida Orlando, Florida Palm Beach County, Florida Sarasota, Florida Boise, Idaho Kansas City, Missouri Omaha, Nebraska Las Vegas, Nevada Reno, Nevada Oklahoma City, Oklahoma Portland, Oregon Austin, Texas Dallas, Texas Fort Worth, Texas Houston, Texas Salt Lake City, Utah Seattle, Washington

Pasteurization has come a long way. Louis Pasteur might not recognize the tools, but the concept of pasteurization employed now in the City of Ventura, CA, is the same one that he perfected 150 years ago in France.

This latest incarnation of pasteurization began in 2003 by a small think tank, now called the Pasteurization Technology Group (PTG). In 2005, PTG brought their concept of pasteurization to our industry and, like many great inventions, it was met with insults. Unlike many others, Carollo and the City of Santa Rosa, CA, listened and worked with PTG. By 2007, pasteurization

had received the coveted California Title 22 approval for reclaimed water disinfection. It demonstrated that heating secondary effluent and filtered effluent to 155 to 180°F resulted in a robust kill of all studied pathogens. The next step was economic. Pasteurization works efficiently for two reasons. First, the heat source is waste heat. Second, the PTG pasteurization process captures and holds that waste heat. Simply put, cold water in and out, heat in the middle (Figure 1). For Figure 1, the waste heat is heat coming off of a gas turbine.

Because of the Title 22 approval and the potential low-energy (and low-cost) treatment, the City of Ventura hired Carollo

to evaluate pasteurization alongside other reclaimed water

disinfection technologies. The potential economic value was overwhelming. By installing gas turbines and burning natural gas (potentially supplemented by digester gas), the City could produce their own power, use the waste heat to disinfect their entire flow, and save between $2 to 6 million over a 20-year period compared to disinfecting with UV light. These results led to the construction and long-term operation of a 400-gpm PTG system.

As of today, the PTG reactor is living up to projections. The reactor is heating the filtered effluent to over 165°F, reliably disinfecting total coliform to below detection, and losing only 2 to 3 degrees of heat during the heat transfer process. Long-term testing is demonstrating how the system fouls and how to keep it clean. The last series of testing, soon underway, will look to repeat the rigorous Title 22 virus testing

previously done on a much smaller scale in Santa Rosa. Once this testing is complete, the total reactor performance will be analyzed and the economics for long-term and larger flow operation in Ventura will be closely examined.

Pasteurization of Secondary Effluent and Reclaimed Water – Optimizing an Old Technology for Low Energy Disinfection

KeY Team membeRandrew salveson, P.e. (asalveson@carollo.com)

Cutting the Cord

The Energy/Water Nexus

Modern Alchemy in Northern California: A Waste-to-Energy Success Story

Project Updates

Commentary

What’s New

In this Issue

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400-gpm PTG reactor in Ventura, CA.

220°F

175°F

70°FRawWater

Fuel (Gas or Liquid)

Air

PreheaterModule

PasteurizedWater

Exhaust

Waste HeatRecoveryModule

Waste Heat

GeneratorPower

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Figure 1. Schematic of PTG process (with example water temperatures).

WhaT’sNeW

Cutting th

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Shifting fr

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to Energy Production

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Energy and water are inextricably linked. The Sandia National Laboratories states: “The continued security and economic health of the United States depends on a sustainable supply of both energy and water.” This concept is not new. President Nixon (yes, Nixon) recognized the energy/water nexus when he formed the Project Independence Initiative in 1973. Yet, it remains a common view that energy requires water and water requires energy. While this is true for water, wastewater can flip this paradigm, with wastewater producing energy.

Baselining Energy EfficiencyBefore we can measure improvements, we must set a baseline of energy use within a wastewater treatment plant (WWTP). As part of WateReuse Research Foundation (WRRF) Project 10-06, Challenge Projects on Low Energy Treatment Schemes for Water Reuse, Carollo defined a baseline of energy use for WWTPs. The baseline energy tabulations were segregated into WWTPs with “high” and “low” relative energy use. The high-power-demand WWTPs saw the largest power uses for pumping, activated sludge aeration, and disinfection (16, 27, and 34 percent, respectively). WWTPs with lower relative power demands showed a similar distribution (22, 38, and 25 percent for pumping, activated sludge aeration, and disinfection, respectively). In either situation, energy use at a WWTP is dominated by pumping, activated sludge aeration, and disinfection. Both ultraviolet light (UV) and sodium hypochlorite use a similar amount of energy, once the energy required to generate sodium hypochlorite is accounted for. Including the energy required to deliver sodium hypochlorite can result in higher energy use compared to UV.

Since the largest energy use is in the secondary and disinfection processes, incremental improvement of these systems would yield the largest gain in energy efficiency. Thus, WRRF 10-06 focused on documenting ways to decrease energy use in the secondary and disinfection processes and on ways to increase methane gas generation (and thus increase energy recovery).

Lower Energy TreatmentThe WRRF project team canvassed a wide range of newer technologies and energy reduction approaches. This list included both proprietary and non-proprietary technologies. Many of these technologies were well proven in the literature, while almost an equal number were in the early phases of development and lacked peer-reviewed data. These included:

Primary treatment technologies:M2 Renewables’ MicroScreen.•Salsnes Filter.•Chemically enhanced primary treatment •(CEPT).Veolia’s Actiflo™ enhanced high rate •clarification (EHRC).

Secondary treatment technologies:Shortcut nitrification denitrification •(Severn Trent Services using their deep bed filter system).Severn Trent Services’ TETRA• ® Denite® system.Anaerobic ammonium oxidation •(Anammox).Upflow anaerobic sludge blanket •(UASB)/expanded granular sludge bed (EGSB).Aerobic granular sludge (• e.g., the Nereda Process).Biologically active filtration (BAF).•Anaerobic membrane bioreactor (MBR) •with and without granular activated carbon (GAC).Low SRT ceramic MBR with physical •membrane scouring (e.g., Grundfos’ Biobooster).Emefcy’s Spiral Aerobic Bioreactor •(SABRE).Emefcy’s Electrogenic Bioreactor (EBR).•

The energy/WaTer nexus

Cutting the CordTom Gillogly, Ph.D., P.E.Special Editor

Energy is consumed through all stages of water and wastewater treatment; its rate of

use has been increasing as new regulations and issues have demanded energy-intensive solutions. Conservation, optimization, and preventative maintenance alone are not enough to stop this trend. However, combining these efforts with recovery of the energy inherent in these water and wastewater systems, the industry has started to slow and in some cases reverse this trend. A select few entities have been able to effectively “cut the cord” and shift from energy consumption to energy production.

This issue of Research Solutions highlights just a few of the ways in which Carollo is working to reduce energy demands and improve energy production, including:

Next Generation of Wastewater/Reuse •Treatment Alternatives Lowers Energy. Peering into the future of wastewater/reuse treatment, Carollo has begun the second phase of research into emerging secondary and disinfection processes.Power Generation Expansion Leads to •Energy Revenue. With the new turbine installation, East Bay Municipal Utilities District (EBMUD), CA, now generates enough excess power to yield at least $750,000 per year in energy revenue.Energy Recovery Turbines Reduce •Power Demand and Carbon Footprint. Carollo is using turbine-assisted interstage booster pumps in reverse osmosis facilities to reduce motor size and shrink greenhouse gas emissions.Raw Wastewater Characteristics •Establish Energy Production Potential. This article shows where the latent energy of raw wastewater ends up, compared to the system demands.Stepwise Approach Quantifies Real •Energy Available from Wastewater Solids. This article reveals how much energy may actually be captured through digestion and subsequent processing.Pasteurization Demonstration •Captures Heat and Potential Cost Savings. The long-term operation of PTG’s Title-22 approved pasteurization process is capturing total reactor performance to establish larger-flow economics for the City of Ventura, CA.

By Andrew Salveson, P.E. (asalveson@carollo.com), Erin Mackey, Ph.D., P.E.

FeaTuResTORY High-efficiency blowers (• e.g., APG-Neuros’s Turbo Blower).American Water’s NPXPress.•Noram Engineering’s Vertreat Process.•

Tertiary filtration technologies:Water Tectonic’s Electrocoagulation and •Filtration.Parkson DynaSand Ecowash.•Schreiber’s Fuzzy Filter.•

Tertiary disinfection technologies:Pasteurization Technology Group’s •Pasteurization System.Aquionics/Dot Metrics LED UV •disinfection.

Energy recovery technologies:M2 Renewables Ultra High Temperature •Gasification.OpenCel Focused-Pulsed Technology.•Thermal hydrolysis (• e.g., Cambi Process).Fuel Cell Energy’s fuel cell. •Supercritical wet oxidation (• e.g., SCFI’s AquaCritox®).

Combined Treatment TrainsSeveral combined treatment trains were also considered. These systems were often propriety packaged systems or were closely coupled:

CEPT + microfiltration (MF) + BAF.•CEPT + IMANS• ®. Great Circle Industries’ i50 non-•biological satellite treatment.M2 Renewables – MicroScreen, BAF, •filtration, disinfection.UASB, facultative ponds, nitrifying •trickling filters, filtration, disinfection.

The technologies were sorted into three groups: Established Systems (proven and in use), Emerging Systems (partially proven, limited full-scale application), and Innovative Concepts (new ideas that show promise). The most promising technologies from the Emerging Treatment Systems category are now being carried forward for Phase II research; these include the anaerobic MBR, the full-stream anammox process, and pasteurization.

Anaerobic MBRThe anaerobic MBR has been demonstrated to be an effective treatment process by different research groups and with variations of the MBR technology. Kim et al. (2011) used a fluidized bed MBR with GAC to produce a high quality effluent (5 mg/L BOD and zero TSS) with minimal

membrane fouling. The same research estimated a ~ 50 percent decrease in secondary process energy and estimated a 75 percent increase in methane production. While promising, this research was done at a very small scale, in a warm climate, using synthetic wastewater, and did not address long-term membrane fouling concerns.

Researchers at the University of Michigan (WERF 2012) have been conducting anaerobic MBR research with the testing of a small-scale reactor (5 L) on both synthetic wastewater and municipal wastewater at temperatures down to 15°C. This work was done without the GAC scouring; biogas sparging was used to minimize membrane fouling. For the municipal wastewater work, an effluent BOD of <30 mg/L was achieved for extended periods of time. The next step for the anaerobic MBR is to design and operate a larger-scale system to evaluate its ability to meet stringent reclaimed water standards with subsequent disinfection. This larger system will be fully automated and use full-scale components.

Full-Stream AnammoxThe anammox process is currently applied as a side-stream treatment on digested sludge returns or waste streams with high ammonia and low carbon contents. Since the anammox process requires approximately equal parts of ammonium and nitrite to proceed, it is often implemented as a nitritation-anammox, or deammonification process in series to accumulate nitrite for use by the anammox bacteria as the electron acceptor (Gustavsson 2010). If the anammox process is preceded by nitrification, only part of the ammonia needs to be nitrified to nitrite, which helps to reduce cost compared to nitrifying all of the ammonia (Khin and Annachhatre 2004).

There are several advantages to using the anammox process over conventional nitrification-denitrification treatment for removal of nitrogen from high ammonium content waste streams. For one, nitritation-anammox requires 57 percent less oxygen and 86 percent less carbon than conventional nitrification-denitrification. In addition, it is completely autotrophic, so nutrients and trace elements are not required. The low biomass yield means low sludge production. External carbon addition is not required. Energy costs are also low because oxygen is not required for the anammox process and only limited

oxygen is required for the partial nitritation (Gustavsson 2010), i.e., aeration is not needed. The anammox process also comes with several disadvantages. A pre-partial nitritation is required for converting part of the ammonia stream into nitrite prior to the anammox process (Ahn 2006). Anammox bacteria also have a slow growth rate, with a doubling time of 11 days at 32-33°C (Strous et al. 1998). This has the benefit of producing a low biomass but requires efficient sludge retention (long SRT) and long start-up times to get sufficient biomass concentrations (Jeffen et al. 1997). Conditions for anammox accumulation include long SRTs, stable operation, presence of nitrite, lack of oxygen, and lack of donors causing denitrification of nitrite (Rittmann and McCarty 2001).

Full-stream application could be extended to mainstream municipal sewage treatment if anammox can be applied at lower temperatures and lower nitrogen concentrations (Jeffen et al. 1997). A recent study (Winkler et al. 2012) has made the first step with a laboratory-scale study. A granular sludge sequencing batch reactor (SBR) mixed ammonium and acetate at a COD:N ratio of 0.5 using nitrate from the previous SBR cycle at 18°C. This work and other results from current research jointly conducted by Hampton Roads Sanitation District and DC Water can serve as the baseline for a Phase II anammox analysis. Because of the significant energy advantage of this process, the current plan is to implement anammox at demonstration-scale on the effluent of the anaerobic MBR.

PasteurizationThe pasteurization process is now constructed at demonstration scale (400 gpm) in Ventura, CA, to cost- effectively disinfect filtered secondary effluent to reclaimed water standards (Salveson et al., 2011). The demonstration unit is fully automated, with on-line monitoring of temperature, energy use, and control. This system is anticipated to run through the end of 2012 and into 2013 for Title 22 testing. Further details on pasteurization can be found in the article on page 8.

As the current and future generations of emerging treatment systems find their place in the market, the energy/water nexus will continue to help shift the industry from energy consumption to energy production. References available upon request.

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For more than 1,000 years, alchemists across the Middle East, Asia, and Europe struggled mightily to transform base elements into riches, yet were consistently disappointed by their inability to successfully combine science, magic, and philosophy to improve their lives.

Today’s modern alchemists carry on that tradition but enjoy a greater level of success, albeit with different methods. We can see their results all around us, be it sand into silicon, carbon into diamond, or in the case of East Bay Municipal Utilities District (EBMUD) in Northern California, waste into electricity.

EBMUD provides wastewater services to more than 650,000 customers across an 88-square mile area along the eastern side of the San Francisco Bay. The Main Wastewater Treatment Plant (MWWTP) processes an annual average daily flow of 65 mgd through primary and secondary treatment and, of course, solids handling.

In the mid 1980s, long before the phrases “climate change” and “carbon footprint” became part of the public’s everyday vocabulary, Carollo began helping EBMUD with its journey towards greater sustainability and energy independence. This took the form of a digester gas utilization study to determine the best way to make use of a resource that was traditionally flared off and wasted. The study led to Carollo’s design of three 2.15-megawatt (MW) engine generators — EBMUD’s first renewable energy production facility.

The benefits of the project were profound. By consistently self-generating more than 2 MW of electricity, EBMUD was able to reduce their outside energy purchases and cut air emissions from flaring off digester gas. EBMUD’s customers were well served by the lower energy costs, and the environment benefitted from the reduced emissions. Yet the full success of the Power Generation Station (PGS) was limited by one thing: the lack of enough digester gas to keep the PGS at full generating capacity.

That problem was partially solved at the beginning of the 21st century, when the industry discovered that adding high-strength organic wastes to digesters improved gas production. In 2002, EBMUD began accepting various types of such waste at the MWWTP, including discarded food, algae, non-edible crops, grease, and livestock processing “leftovers.”

The results were exciting: a nearly 200 percent increase in digester gas production, and a corresponding increase in EBMUD’s renewable energy production. Prior to adding the additional organic wastes, the PGS was generating around 2.5 MW using two engine generators. That production doubled to more than 4 MW by 2006, and the gas production often exceeded the capacity of the two generators. At that point, EBMUD made an operational decision: instead of running two generators at the PGS with one in standby (as originally designed), EBMUD began to operate all three generators simultaneously.

A problem quickly became obvious. Without any generator redundancy, EBMUD was back to flaring gas whenever a generator was down for maintenance, and more frequent major engine overhauls further reduced generating capacity.

In 2004, EBMUD commissioned a study to figure out the best solutions to their capacity issues as well as the best use for their excess gas. The study looked at using the biogas as a transportation fuel, as a fuel cell energy source, and as an engine or microturbine power source. The study ultimately recommended using the gas on-site to increase the plant’s energy production, which meant expanding the PGS to accommodate energy production needs and still provide redundant generators to avoid flaring off excess gas.

In 2008, EBMUD once again turned to Carollo to expand the power generation station we had designed and built more than 20 years earlier. Carollo helped EBMUD refine the parameters and future needs of the PGS, as well as reduce operational risks and complexity. EBMUD ultimately determined that a large gas turbine would be better than an internal combustion engine because of the turbine’s lower emissions, smaller footprint, and higher efficiency. The PGS Renewable Energy Expansion project, which recently went into operation, added a 4.5-MW biogas turbine generator to the existing 6.5 MW of generating capacity, for a total of 11 MW. Carollo’s design allows for room to add another turbine in the future, increasing the total PGS generating capacity to 15.5 MW, should digester gas production continue to increase.

But perhaps the best news for EBMUD’s customers, the environment, and the industry is the fact that EBMUD has now become a net provider of electrical power to Northern California. EBMUD has worked hard over the past several years to cut power consumption at the MWWTP by installing more efficient process units,

upgrading power distribution systems, and replacing lighting with higher-efficiency alternatives. These efforts have reduced plant electrical demands from 5.4 MW in 2000 to 4.9 MW in 2010.

With the installation of the new 4.5-MW turbine, EBMUD is now consistently generating at least 6 MW of power, which is more than sufficient to meet the MWWTP’s average demands. The excess power is sent into the local electrical grid, helping energy providers reduce demands on their more traditional natural gas- and oil-fired power plants, which in turn cuts the regional carbon footprint and generates income for EBMUD. Indeed, EBMUD conservatively estimates that they will be able to consistently sell 1 MW of power to local energy providers, resulting in at least $750,000 per year in energy revenue.

EBMUD has a long history and commitment to renewable energy

By Tom Mossinger, P.E. (tmossinger@carollo.com), Paul Flick

Modern AlcheMy in northern cAliforniA: A Waste-to-Energy Success Story

generation. Their energy strategies are designed to protect the environment while saving money for their customers. As new wastewater regulations are put in place, EBMUD will undoubtedly see their energy demands rise. Converting their high-purity oxygen process to an air-activated sludge process to achieve nitrification will require a great deal more energy. Future limits on ammonia and phosphorus, as well as emerging contaminants of concern, will require both EBMUD and the greater wastewater industry to explore ways to meet these regulations while preserving sustainability and minimizing energy use. For now, EBMUD and Carollo can be proud of the way our modern alchemists have pushed the boundaries of science and technology to transform waste into a valuable commodity that keeps our homes, our environment, and our neighborhoods a bit cleaner … and brighter.

FeaTuResTORY

Fun Facts about EBMUD’s MWWTP

Average flow: 65 mgd.•Peak wet weather flow: 415 mgd.•Primary treatment capacity: 320 mgd.•Secondary treatment capacity: •168 mgd.

Liquid treatment processes include:Coarse and fine bar screens.•Aerated and vortex grit chambers.•Primary clarifiers.•Secondary clarifiers.•High-purity oxygen activated sludge •reactors.Secondary clarifiers.•Disinfection.•

Solid handling facilities include:Gravity belt thickeners.•Anaerobic digesters.•Dewatering centrifuges.•

EBMUD’s MWWTP consistently generates at least 6 MW of power, in part through acceptance of high-strength organic waste. (Photo used with permission of Paul Cockrell Photography.)

EBMUD’s MWWTP is now a net provider of electrical power.

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Everyone knows that reverse osmosis (RO) is an energy-intensive process. Combining this energy use with a high capital cost usually makes desalination one of the last water supply solutions pursued by a water provider. However, Carollo has been working to improve energy efficiency for our projects, saving both operating costs and reducing greenhouse gas emissions. Below are profiles for two recent projects where Carollo implemented an innovative energy recovery turbine process to reduce the motor size for interstage booster pumps:

Lee County Utilities, Fort Myers, FL, North Lee County WTP

Completed in •2011, turbine- assisted interstage booster pumps were added to four RO trains to make a combined 10 mgd.Saves up to $37,000 per year (60-kW •power reduction).Reduces greenhouse gas emissions by up •to 360 tons of CO2 per year.

Jordan Valley Water Conservancy District, West Jordan, UT, Southwest Groundwater Treatment Plant

Completed in •2012, turbine- assisted interstage booster pumps were included on all three RO trains to make a combined 7 mgd.Saves $12,500 per year (36-kW power •reduction).Reduces greenhouse gas emissions by •233 tons of CO2 per year.

Energy recovery may not make sense for every RO plant. A number of economic and social benefits should be considered. Power cost fluctuations and annual average production flow are two key factors that should be evaluated. Carollo takes a rigorous approach to determine if energy recovery is a good fit.

Carollo Reduces RO Impacts by Using Energy Recovery Turbines

pRoJecTUPDaTes

Electrical energy consumption for an activated sludge wastewater treatment facility ranges from about 1,400 to 1,900 kWh per million gallons (kWh/MG) for a 5-mgd facility, to approximately 1,000 to 1,600 kWh/MG for a 100-mgd facility, depending on final effluent discharge limits (WERF, 2010). Aeration and wastewater pumping consume the majority of this power. As the cost of energy has increased — and, in some cases, as energy supplies have become limited — the wastewater treatment utility perspective on raw wastewater has changed. Some are now viewing it as an energy source that can help them reduce or eliminate purchased power.

Chemical oxygen demand (COD) can be used to estimate the latent energy of raw wastewater; 1 kg COD is equivalent to 0.35 m3 of methane, which has a heating value of 35.8 MJ/m3 at standard temperature and pressure (0°C, 1 atm) (Rittmann and McCarty, 2001). The calculated latent energy of a “typical” North American raw wastewater — 500 mg/L COD, 245 mg/L cBOD, and 195 mg/L VSS — is 23,700 MJ/MG. This value represents approximately 6,600 kWh/MG, the power available if all the raw wastewater chemical energy could be converted to electricity.

While the total raw wastewater energy is significantly greater than the benchmark energy consumption of a typical plant, it cannot all be recovered. The energy recovery realized in the field depends on the raw wastewater characteristics, the type

of treatment used, and the treatment process efficiency (at converting

this energy into a usable form). The first step in estimating potential energy recovery is to understand the raw wastewater characteristics. The figure below shows that the raw wastewater energy content comprises soluble, colloidal, and particulate fractions (Figure 1). In addition, the soluble and particulate fractions are divided between biodegradable and unbiodegradable portions. The colloidal fraction is assumed to be wholly biodegradable.

Using a benchmark electrical energy consumption of 1,700 kWh/MG, typical of a 20-mgd activated sludge facility with nitrification, the latent energy of raw wastewater exceeds this value by nearly a factor of four. The companion article, “Recovering My Energy,” goes on to describe how the latent energy of raw wastewater can be recovered, focusing on anaerobic digestion of wastewater solids.

ReferencesRittmann, B.E. and P.L. McCarty. Environmental Biotechnology. McGraw-Hill, 2001.

WERF. Energy Efficiency in Wastewater Treatment in North America: A Compendium of Best Practices and Case Studies of Novel Approaches (OWSO4R07e), 2010.

Where Is My Energy?KeY Team membeRsRon appleton, Jr., P.e. (rappleton@carollo.com) sudhan Paranjape, P.e.andrew salveson, P.e.

Figure 1. Raw wastewater energy content comprises soluble, colloidal, and particulate fractions. The soluble and particulate fractions are divided between biodegradable and unbiodegradable portions. The colloidal fraction is assumed to be wholly biodegradable.

KeY Team membeRTom seacord, P.e. (tseacord@carollo.com)

PrimaryTreatment

Waste ActivatedSludge5.3 MMBTU/MG(5,500 MJ/MG)

RawWastewater

22.5 MMBTU/MG(23,700 MJ/MG)

SecondaryEffluent1.1 MMBTU/MG(1.2 MJ/MG)

PrimarySludge9.1 MMBTU/MG(9,600 MJ/MG)

SecondaryTreatment

Soluble (unbiodegradable)SolubleColloidal (biodegradable)ParticulateParticulate (unbiodegradable)

Legend

{

cOmmeNTaRY

The companion article, “Where Is My Energy?,” describes the typical electrical energy demands for various levels of wastewater treatment and the latent energy of the solid and liquid wastewater fractions that can help meet these demands. This article describes how the latent energy of wastewater can be realized and recovered, focusing on anaerobic digestion of wastewater solids.

According to the laws of physics, energy and mass cannot be created or destroyed, simply transformed. If we could perfectly characterize the system, the sum of the total energy and mass contained in the products would exactly equal the sum of the total energy and mass contained in the feed stocks. However, energy balances in solids processing for wastewater have been anything but clear and simple. Our industry is plagued with claims of improvements and efficiencies that defy reason and the laws of nature. The purpose of this article is to identify how much energy is contained in the biosolids produced at a wastewater treatment plant and highlight how energy can be extracted from it.

Step 1: Determine how much energy is in the raw sludge.There are no two sewer systems that are the same or treatment facilities that process everything the same way. Therefore, no two sludges have exactly the same composition. In general terms, primary sludge (PS) has 8,000-10,000 BTU/lb and waste activated sludge (WAS) has 6,000-8,000 BTU/lb. All this energy comes from the volatile fraction.

To find out how much energy is in a given sample of sludge, it can be burned under controlled conditions in a bomb calorimeter. The energy released (i.e., heat of combustion) is quantified via a rise in water temperature and can be easily measured. This value represents the ultimate amount of energy the sludge can produce if all of the organic matter is oxidized to carbon dioxide.

Step 2: Determine how much total energy is available for biological degradation.Of the total energy content, only that fraction associated with biodegradable

compounds is available to microorganisms to convert to methane (CO2 has zero energy value). A degradability test can provide information on how much of the organic fraction of the sludge can be biodegraded and how long it will take. Carbohydrates degrade very quickly, lipids have to be hydrolyzed to become available, and proteins take a long time to metabolize due to their large, stable structures. Typically, municipal sludges only have between 68-80 percent of the organic matter available for biological decomposition. If an anaerobic digester is getting 65-percent destruction of the volatile solids (PS only) and the sludge had 80-percent biodegradable fraction, that anaerobic digester is already only 81 percent biologically efficient, as 20 percent of the organic matter was not available for biological breakdown to begin with.

Step 3: Determine the energy output of sludge processing.Continuing with the example from Step 2, an anaerobic digester operating on primary sludge should produce 5,850 BTU/lb of methane gas, assuming 65 percent conversion of a 9,000 BTU/lb influent. This leaves 3,150 BTU/lb in the solids. While the digested sludge has over one-third the latent energy of the raw sludge, most of this remaining energy is associated with unbiodegradable volatile solids. Of the biodegradable volatile solids, over 80 percent have been biologically converted to methane.

Step 4: Identify the limits of performance.After digestion, if the solids have 3,150 BTU/lb, only approximately one third of these could be converted to biogas. We cannot (at reasonable temperatures and pressures) convert non-biodegradable organic molecules into degradable ones. When a technology is said

Recovering My Energyto increase performance by 25 percent, the question is: 25 percent of which value? As advanced technologies are evaluated, consider this limit. No matter what we do (aside from adding additional organic molecules) we cannot extract more energy than the energy we started with.

Step 5: Deal with the water.Even after digestion, the sludge still has energy left. The challenge our industry faces is that it comes suspended in a large amount of water, and water is an excellent heat sink. It takes 1,800 BTU to evaporate a pound of water. So even if the sludge is dewatered using high solids centrifuges it will still be close to 40 percent solids; for every pound of biosolids this very good sludge cake would still have 1.5 pounds of water. To recover the remaining energy (burn the solids) we would have to evaporate the water first. So to recover the 3,150 BTUs in the solids we would have to expend 2,700 BTUs to dry the sludge, leaving an energy positive of 450 BTU/lb, only 5 percent of the original primary sludge.

In conclusion, there is no free lunch when it comes to energy and every processing step reduces the amount of energy available in the biosolids due to inefficiencies and required additional energy inputs. Consider your end product and plan your processing steps accordingly to maximize the benefit, be it in the biogas or in the solids from a biosolids-to-energy project.

KeY Team membeRRudy Kilian, P.e., PmP (rkilian@carollo.com)

Time (days)

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Sludge ASludge B

Figure 1. Sludge degradability tests can determine the amount of time it takes for microorganisms to generate a quantity of gas.

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ReseaRch gRoup Jess Brown, Directorphone (941) 371-9832jbrown@carollo.com

eDIToRerin Mackey

DesIgn anD pRoDucTIonLaura corringtonKim LightnerMatthew parrott

ReseaRch SOLUTIONS

This publication is printed with soy inks on FSC®- certified 100% recycled content (60% post-consumer waste).

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Pasteurization has come a long way. Louis Pasteur might not recognize the tools, but the concept of pasteurization employed now in the City of Ventura, CA, is the same one that he perfected 150 years ago in France.

This latest incarnation of pasteurization began in 2003 by a small think tank, now called the Pasteurization Technology Group (PTG). In 2005, PTG brought their concept of pasteurization to our industry and, like many great inventions, it was met with insults. Unlike many others, Carollo and the City of Santa Rosa, CA, listened and worked with PTG. By 2007, pasteurization

had received the coveted California Title 22 approval for reclaimed water disinfection. It demonstrated that heating secondary effluent and filtered effluent to 155 to 180°F resulted in a robust kill of all studied pathogens. The next step was economic. Pasteurization works efficiently for two reasons. First, the heat source is waste heat. Second, the PTG pasteurization process captures and holds that waste heat. Simply put, cold water in and out, heat in the middle (Figure 1). For Figure 1, the waste heat is heat coming off of a gas turbine.

Because of the Title 22 approval and the potential low-energy (and low-cost) treatment, the City of Ventura hired Carollo

to evaluate pasteurization alongside other reclaimed water

disinfection technologies. The potential economic value was overwhelming. By installing gas turbines and burning natural gas (potentially supplemented by digester gas), the City could produce their own power, use the waste heat to disinfect their entire flow, and save between $2 to 6 million over a 20-year period compared to disinfecting with UV light. These results led to the construction and long-term operation of a 400-gpm PTG system.

As of today, the PTG reactor is living up to projections. The reactor is heating the filtered effluent to over 165°F, reliably disinfecting total coliform to below detection, and losing only 2 to 3 degrees of heat during the heat transfer process. Long-term testing is demonstrating how the system fouls and how to keep it clean. The last series of testing, soon underway, will look to repeat the rigorous Title 22 virus testing

previously done on a much smaller scale in Santa Rosa. Once this testing is complete, the total reactor performance will be analyzed and the economics for long-term and larger flow operation in Ventura will be closely examined.

Pasteurization of Secondary Effluent and Reclaimed Water – Optimizing an Old Technology for Low Energy Disinfection

KeY Team membeRandrew salveson, P.e. (asalveson@carollo.com)

Cutting the Cord

The Energy/Water Nexus

Modern Alchemy in Northern California: A Waste-to-Energy Success Story

Project Updates

Commentary

What’s New

In this Issue

2

6

8

6

4

2

400-gpm PTG reactor in Ventura, CA.

220°F

175°F

70°FRawWater

Fuel (Gas or Liquid)

Air

PreheaterModule

PasteurizedWater

Exhaust

Waste HeatRecoveryModule

Waste Heat

GeneratorPower

Gas Turbine

73°F

180°F

950°F

Figure 1. Schematic of PTG process (with example water temperatures).

WhaT’sNeW

Cutting th

e Cord

Shifting fr

om Energy Consumption

to Energy Production