design and operational considerations to avoid excessive anaerobic digester foaming

8/11/2019 DESIGN AND OPERATIONAL CONSIDERATIONS TO AVOID EXCESSIVE ANAEROBIC DIGESTER FOAMING

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DESIGN AND OPERATIONAL CONSIDERATIONS TO AVOID EXCESSIVE

ANAEROBIC DIGESTER FOAMING

Neil Massart*, Black & Veatch

Robert Bates, Louisville and Jefferson County Metropolitan Sewer District

Blair Corning, South Adams County Water and Sanitation DistrictGary Neun, Black & Veatch

*Black & Veatch8400 Ward Parkway

Kansas City, MO 64114

ABSTRACT

Excessive foaming in anaerobic digesters has been a problem for many years. All anaerobic

digesters will foam to some extent, but excessive foam can be problematic. Excessive foaming

is defined as foam that interferes with flow through the gas piping system and/or is not containedwithin the digester. Causes of foaming include presence of excessive filamentous bacteria,

extreme amounts of oils and grease, and the feed sludge composition (primary sludge (PS)versus waste activated sludge), but the chief cause of excessive foaming is inconsistent feed to

the digesters.

Many utilities have evaluated technological changes to the digester facilities. While suchchanges may reduce/control foaming, they often involve expensive additions to the facility in

terms of both capital and operation and maintenance (O&M) costs. Consequently, before

investing in technology changes, utilities should consider operational changes that could mitigatefoaming.

In addition, facilities that are installing new digesters should consider design elements that canreduce the potential for foaming as well as the effects of foaming on ancillary equipment.

Design considerations to avoid excessive digester foaming will be presented followed by three

case studies of unstable operations at existing digester facilities, and operational considerationsto avoid excessive foaming will be presented in relationship to the three case studies.

KEYWORDS

Anaerobic digester, foam, monitoring, primary solids, waste activated solids

INTRODUCTION

Foaming is a serious problem with many anaerobic digesters, and it seems to be becoming more

prevalent across the country. No one knows the exact cause, as there is no single common

thread; however, researchers believe that the increase in advanced biological treatment processesand the corresponding changes in the quantity and characteristics of the WAS are key factors.

Foaming is reported to occur more frequently if the ratio of WAS to total sludge exceeds 40

percent. There are usually multiple factors that contribute to digester foaming:

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Copyright 2006 Water Environment Foundation. All Rights Reserved


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Slug feed to the digesters

Intermittent feeding of digesters

Separate feeding or inadequate blending of WAS and PS

Insufficient or intermittent mixing of digesters

Excessive grease and scum in digester feed, especially if fed in batches WAS content exceeds 40-50 percent of the total amount of sludge fed

Filaments, specificallyNocadia, in activated sludge fed to the digesters

Presence of Organism B, an anaerobe filament believed to be generated in the digester,

and thriving under conditions of high volatile solids loadings

While some foaming always occurs, problematic foaming can result in loss of active digestervolume, structural damage if it causes the digester cover to rise, spillage, and adverse impacts on

gas handling equipment. Foaming is considered as long as it is contained under the digester

cover and does not interfere with the working of the gas piping system. Foam is defined asexcessive if it plugs piping and/or escapes the containment of the anaerobic digester. Excessive

foaming is illustrated on Figure 1.

Figure 1 - Excessive Digester Foaming

Facilities that are planning to build new digesters should consider design elements that canreduce the potential for foaming as well as the effects of foaming on ancillary equipment. Many

utilities have evaluated changes to their digester technology, such as adding ultrasound to pre-treat the WAS ahead of digestion or conversion to two-stage digestion. While such changes mayreduce or control the foaming, they often involve expensive additions in terms of both capital

costs and O&M costs. Consequently, before investing in technology changes to existing

facilities, utilities should consider operational changes that could mitigate the foaming.

Anaerobic digestion systems are complex and do not run on autopilot; they require daily

sampling, analysis, data review, and process control adjustments. Excessive foaming is

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controllable if the operator is provided the time and resources to monitor the system. The goal of

the facility operator should be to minimize the factors that exacerbate foaming rather than toeliminate foaming altogether. A dedicated and knowledgeable staff is essential to the operation

of the anaerobic digesters and related processes. It is imperative the staff be adequately trained

to control digester performance. Regular staff meetings to discuss data trends and digester

sampling protocol are essential for routine operation of the digesters.

DESIGN CONSIDERATIONS

Design considerations are essential to efficient operation of anaerobic digesters and control of

excessive foaming, such as adequate mixing of primary sludge and WAS ahead of the digester,

and in the digester; adequate digester volume; sludge feed and withdrawal schemes; andequipment such as foam separators, grit removal facilities, electrically operated drip traps, and

sloped gas piping systems. Because the extent of foam generation is uncertain, its elimination

and/or provision of gas utilization equipment that can handle foam must be a designconsideration.

The designer of an anaerobic digester system must think not only about the anaerobic digester

system, but also about the other systems within the plant that will affect digester operation. Forinstance, if the plant receives hauled waste, the waste should be stored and metered into the

digesters rather than being pumped at a high rate into the digester. Anaerobic digestion is not

merely an add-on process for sludge treatment, but an integral part of the treatment system.

The digester complex must be compatible with other components of the plant and the type of

sludge being processed in the anaerobic digesters. The sludge must be properly blended before itenters the digester to assure consistent solids concentration in the digester feed, and thus, a more

consistent feed of volatile solids.

Upstream process such as grit removal is vital to effective digester performance. If the grit is not

removed, it could pass into the primary clarifiers and be pumped to the digesters, where it couldsettle and effectively reduce the usable digester volume.

To ensure that all solids that enter the digester will leave the digester, adequate mixing is

necessary. Adequate mixing produces a uniform solids concentration throughout the digester,which is important for the reduction of volatile solids.

The feed to the digester should be well mixed, especially if it consists of a combination of WASand PS. The ratio of WAS to PS can affect the amount of foam formation, but the more critical

parameter for controlling foaming is the volatile solids loading rate which can be partially

controlled by proper blending of feed sludge. Withdrawal from the digester should be done byhydraulic overflow rather than by opening and closing a valve. The difference in hydraulics

caused by opening a valve to withdraw sludge will result in the level and the volume of the

digester contents to vary. By using a hydraulic overflow, the level of sludge in the digester andits volume will always be a constant.

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Other means of controlling foaming include ensuring that the gas piping is sloped properly and

installing electrically operated drip traps at key points in the piping to remove condensed waterfrom the piping system. Water in the gas piping can cause blockages that increase the pressure

in the system. When the blockage is removed, foaming can occur in the digester. This is

analogous to shaking a can of soda and then opening the top: The sudden decrease in pressure

within the can will cause the contents to foam. Thus, it is imperative that the system is designedto allow the condensate to be quickly and effectively removed.

Two other add-on features can be considered when designing a digester: ultrasonic pretreatmentof WAS and foam separator. While both treat the symptoms rather than the cause, they can be

effective in alleviating short-term process upsets.

Pretreatment of WAS with ultrasonic energy causes cavitation within the microbe cell membrane

and creates a soluble chemical oxygen demand (COD) that can be readily digested.

Conditioning of thickened waste activated sludge (TWAS) using ultrasound before anaerobicdigestion has been recently implemented in a few treatment facilities in both Europe and North

America to achieve more complete digestion and to enhance volatile solids reduction. Increasedvolatile solids reduction translates to increased biogas generation (of relevance where biogas is

reused), and decreased quantities of stabilized biosolids for disposal. In some cases, improveddewaterability of the sludge has also been reported. Many of the installations that use ultrasonic

conditioning have also made anecdotal references to observing a decrease and in some cases

complete elimination in foaming incidents.

The drawback of ultrasonic conditioning is nutrients are released which can increase by 10

percent the amount of nitrogen recycled to the head of the plant. An ultrasound conditioningsystem is illustrated on Figures 2 and 3.

Figure 2 Sonix Ultrasound System Figure 3 EMICO Sonolyzer

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The other add-on option, the foam separator, removes foam by drawing gas/foam from the top of

the digester and beating it down using a series of water spray nozzles. The excess water flowsthrough a P-trap to a drain leading to the headworks of the wastewater treatment plant. This

option is attractive where there is inadequate blending of the feed sludge or where the feed rate

to the digester is highly variable. A schematic and a picture of a foam separator are presented in

Figures 4 and 5.

Figure 4 Varec Foam Separator Figure 5 Foam Separator Schematic

OBSERVATIONS

Three case studies representing various process scenarios will be presented, demonstrating thatthe usual culprit for excessive anaerobic digester foaming is inconsistent feeding.

Case Study No. 1

The Morris Forman WWTP in Louisville, Kentucky, has a treatment capacity of 105 mgd. This

plant was upgraded in 2003 by converting the four sludge holding tanks to four 1.8 milliongallon anaerobic digesters, followed by five high rate centrifuges and four triple pass drum dryer

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systems. The digesters have an associated heating system consisting of a recirculation pump, a

spiral heat exchanger, and a heated water pump. Each digester is equipped with a gas mixingsystem consisting of a liquid ring gas compressor, gas distribution manifold, and 14 gas bubble

cannons.

Excessive foaming at the Morris Forman facility was a result of inconsistent organic loading(Figure 6). The four digesters were loaded with primary sludge plus an occasional load of

hauled-in septic solids received as hauled waste. Foaming occurred whenever the mass of

volatile solids to the digesters was significantly increased either by pumping too large a volumeor by not maintaining the feed solids concentration at less than 5 percent, which caused the

organic loading rates to exceed the targeted operating rate. Once the mass of volatile solids to

the digesters was controlled, the excessive foaming events ceased.

Figure 6 Excessive Foaming Event

At the Morris Forman facility, the anaerobic digesters perform two functions:

1. Produce digester gas for use as fuel by the dryer system2. Generate a uniform product for dewatering and drying

Monitoring the digester is vital to proper operation, and has greatly enhanced the performance ofthe digester. Figure 7 shows the digester performance in 2004.

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Figure 7 Digester Performance in 2004

2004 - Digester Performance

0

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Date

VSLoading(lbs/day)

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VolatileAcids(mg/L)

VSS fed to digester Volat ile Acids

Foaming occurred when the feed to the digester was inconsistent and the percentage of change in

the feed rate was greater than 20 percent. Once the feed rate was controlled, volatile acids

formation remained consistent and no excessive foaming events were reported. Figure 8represents a two-month period of inconsistent feed to the digester, and Figure 9 represents a two-month period of controlled feed rate.

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Figure 8 Inconsistent Feed to the Digester, July 2004 to August 2004

Digester Performance

July 2004 - August 2004

0

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VSS fed to digester Volati le Acids

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Figure 9 Consistent Feed to the Digester, October 2004 to November 2004


October 2004 - November 2004

0

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VolatileAcids(mg/L)

VSS fed to digester Volat ile Acids

Case Study No. 2

A WWTP in South Carolina was upgraded in 2000 to include three anaerobic digesters andancillary equipment. The digesters measure 85 feet in diameter, with a sidewater depth of 30

feet and a volume of 1.4 million gallons each, for a total volume of 4.2 million gallons. Total

design loading is 58,900 pounds per day under annual average conditions, with a detention timeof 29 days for 5 percent total solids (TS), and 75 percent volatile solids (VS). The digesters are

equipped with floating gasholder covers (arch type), which have a gas storage volume of 26,000

cubic feet, and are mixed by nine large-bubble gas cannons each. The digester gas is circulated

by liquid-ring type compressors.

Excessive digester foaming was a recurring problem and became the focus of an evaluation

(Figures 10 and 11). The foaming caused sludge to overflow the walls of the digesters and toenter the biogas collection piping, blinding the flame traps and interfering with the operation of

the gas mixing systems.

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Although excessive amounts of grease and insufficient blending were contributing to thefoaming problems, they were determined not to be the primary cause of foaming. The primary

cause was thickened sludge, both primary and WAS (which was routinely injected into the

digester at concentrations greater than 5 percent) settling to the bottom of the digester, and theunstable operating conditions resulting from the inconsistent organic loading.

The following recommendations for reducing the foaming included evaluation of the upstreamprocesses, digesters, and digester support equipment.

1. Maintain solids concentrations in digester feed at 4.5 to 5 percent. Higher concentrations

reduce the effectiveness of the digester mixing systems.

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2. Maintain digester loading rates from 100 to 150 pounds per 1,000 cubic feet of digester

volume for mixed PS and TWAS.

3. Analyze the PS and TWAS characteristics and total solids, and volatile solids concentrations,

at least daily to track variations in organic loading rates to the digesters and track the flow

rates of PS, TWAS, and scum to the digesters.

4. Limit the daily variation in volatile solids load (organic load) to the digesters to 5-10

percent by trending volatile solids loading.

5. Trend the volatile solids reduction against the actual digester gas production, which typically

averages 13-15 cubic feet per pound of volatile solids destroyed. Foaming can reduce theeffluent volatile solids concentrations below these values, as a result of entrapment of solids

in the foam layer, causing abnormally high gas production rates.

6. Maintain lower sludge blankets in the primary clarifiers to improve removal efficiencies.

Improved removal of biochemical oxygen demand (BOD) in the primaries reduces secondarysludge production for digestion.

Since these recommendations were implemented, excessive foaming events have ceased. The

impacts of large variations in volatile solids feed on the anaerobic digester are depicted on Figure

12.

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Figure 12 Effects on the Digester Variations in Volatile Solids Loading


0

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1/24/2004 1/26/2004 1/28/2004 1/30/2004 2/1/2004 2/3/2004 2/5/2004

Time (Weeks)

VolatileSolids(lbs/day)

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VolatileAcids(mg/L)

Volat ile Solids Volat ile Acids

Consistent VSS Feed/ No FoamingInonsistent VSS Feed/

Excessive Foaming

The plot on Figure 12, which represents the digester performance for a two-week period,

illustrates the importance of controlling digester feed. Overfeeding the digesters caused

excessive foaming. Once the volatile solids loading was controlled, the excessive foamingceased. Trending the data became invaluable to the plant operating staff in controlling the feed,

thus stopping the excessive foaming.

Case Study No. 3

The Williams Monaco WWTP in Henderson, Colorado, is another plant where operationalcontrols were implemented to eliminate excessive foaming and to restore an upset digester to

proper operation. The plant consists of one primary and one secondary digester. PS and WAS

are pumped to a gravity thickener to be co-thickened. The thickened sludge is pumped to the

primary digester, and thickener overflow is pumped to the head of the plant. The primary

digester overflows by gravity to the secondary digester.

The gravity thickener is 45 feet in diameter, with a sidewall depth of 10 feet and a volume ofapproximately 136,000 gallons (Figure 13). The primary digester is 45 feet in diameter and 28

feet deep with a capacity of 344,000 gallons, and is equipped with three gas mixing cannons

(Figure 14). The secondary digester has a capacity of 321,000 gallons.

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Figure 13 Gravity Thickener

Figure 14 Primary Digester

Overloading and excessive foaming of the primary digester would occur due to the large

variations of thickened solids discharged from the gravity thickener (Figure 15).

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Figure 15 Dried Excess Foam

The concentration of solids in the gravity thickener ranged from 4 to 8 percent, with the depth of

solids varying accordingly, leading to organic overloading and excessive foaming. The gravitythickener not only thickened the solids, but also acted as a fermenter, producing volatile fatty

acids (VFA), which would not normally be necessarily detrimental to the process, except that thevolatile solids loading to the digester had been extremely erratic. To correct the problem,

operators established and maintained a consistent solids concentration in the gravity thickener

and prepared process control trend charts to correct any problems before the digester becameupset. Figure 16 illustrates data trending and how to recognize inconsistent volatile solids feed

to the digester.

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Figure 16 Volatile Solids Loading in the Digester

Primary Digester Performance

0

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0 5 10 15 20 25 30 35 40 45 50

Time (Weeks)

VSSFeedtoDigesterlbs/day

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VolatileAcidsinDigestermg/L

VSS fed to the digester Volatile Acids

Consistent VSS Feed/ No FoamingUpset Digester/

Excessive Foaming

Corrective

Action Taken

Inconsistent VSS Feed/

Excessive Foaming

From week 1 to week 19, inconsistent feed to the digesters caused excessive foaming and large

variations in the volatile acids concentration, which led to decreased gas production and inability

to restore the proper pH in the digester. As a corrective action, the feed to the digester wasdecreased to 20 percent of the design loading capacity to allow the methane formers to recover

and to reduce the volatile acids concentration in the digester. When the excessive foaming

ceased, the feed to the digesters was gradually increased to match the rate of feeding volatilesolids into the gravity thickener, as shown for week 26. The plant has continued to maintain the

volatile solids loading rate, operation of the digester has stabilized, and the excessive foaming

has stopped.

OPERATIONAL CONSIDERATIONS

The causes of excessive digester foaming include an excessive amount of filamentous bacteria,

excessive grease, and unstable operating conditions. In the three case studies presented, allexcessive foaming events were caused by unstable operating conditions.

Excessive amounts of filamentous bacteria in the WAS feed from secondary treatment process

were not considered the primary cause of foaming in any facilities studied. While such bacteria

may have been present and may have contributed to foaming, the intermittent nature of thefoaming suggests other primary causes. Experience has shown that the effects of filamentous

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1. Maintain solids concentrations in digester feed at 4.5 to 5 percent. Control thickened WASconcentrations or bypass the gravity belt thickeners. WAS at concentrations higher than

5 percent reduces the effectiveness of the digester mixing systems.

2.

Maintain volatile solids loading rates in the range of 100 to 150 pounds per 1,000 cubic feetof digester volume for mixtures of PS and TWAS (loading rates for PS only can be higher).

3. Analyze PS and TWAS for total and volatile solids concentrations at least once per day totrack variations in organic loading rates, and track PS and TWAS flow rates.

4. Limit daily variations in volatile solids load (organic load) to the digesters to 5 to 10 percentby analyzing the various types of sludges, and mixing as necessary. Trending of volatile

solids loading to the digesters is desirable to minimize variations.

5. Track volatile solids reduction from plant data against digester gas production. Typically,

the gas production averages 13 to 15 cubic feet per pound of volatile solids destroyed.Foaming can reduce the effluent volatile solids concentrations below the concentrations in

the digester as a result of entrapment of solids in the foam layer, which will cause higher-than-average gas production.

6. Mix the various types of sludge in a blending tank before pumping them to the digesters tomaintain a consistent feed.

7. Maintain the depth of sludge blankets in the primary and secondary clarifiers at less than 18inches at peak flow to improve removal efficiencies.

8. Develop trend charts, including upper and lower limits for each parameter. Develop process

control spreadsheets for weekly analysis and monthly summary reports. Some examples of

process control spreadsheets developed at other facilities are presented on Figure 17.

Combined sludge volatile solids loading and conversion to gallons.

Conversion of gallons of WAS feed to a gravity belt thickener to the percentages ofpercent solids and liquid (gallons) discharge rate to the digesters.

Moving average spreadsheets, 7, 15, and 30 days.

Trend charts with upper and lower control points for all anaerobic digestion process

control data points.

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Figure 17- Example of Volatile Solids Loading Rate SpreadsheetVolatile Solids Feed Calculations

Volatile solids dose to be used INSTRUCTIONS ON

HOW TO USE THIS SPREADSHEET

Average 1. Blue cells are for data entry.

VS loading 0.13 lb/ft3/day 2. Yellow cells are for calculated results.

Percentage of Feed as TWAS 30 % 3. Enter volatile solids loading.

4. Enter VS concentration for primary and TWAS.Plant Flow 5. Enter amount of VS feed in lbs to be TWAS

6. Enter the digester volume.

Primary VSS conc mg/L 47900 TWAS VSS conc 48000 mg/L 7. Spreadsheet calculates dosing rates based

Digester Volume MG 6.6 on 24 hour pumping.

8. If required enter different pumping rate less than

24 hours.

Dose to use in digester 9. Spreadsheet calculates pumping rates based

Volatile Solids 114271.4 lbs/day on hours of operation to achieve VS loading rate.

VSS Flows from Primary Only VSS Flows into Digester From TWAS Only

Primary Flow into Digester 0.20 MGD TWAS Flow into Digester 0.09 MGD

Pumping rates Pumping rates

gpm 139.05 gpm 59.47

gpd 200,232 gpd 85,635

Pumping rates for less than a 24 hour day operation Pumping rates for less than a 24 hour day operation

Hours per day in operation 24 Hours Hours per day in operation 24 Hours

gpm 139.05 gpm 59.47

gpd 200,232 gpd 85,635

HRT = 23.00018 days

9. Perform system monitoring.

The key to preventing instability of digestion is continuous monitoring to assess the health of the

digester. The data collected should be used to set up trend charts in the form of a spreadsheet to

monitor the solids loading, pH, and volatile acids and alkalinity in the digester; volatile acids toalkalinity ratio; and volatile solids destruction. The trends will show the condition of the digester

and the effects of operational changes. Data collection is noted in Table 1.

Table 1. Data CollectionData Item Frequency

Flow rateFeed

Digested solids

Daily

Daily

Total solidsFeed solids

Digested solids

Daily

3/week

Volatile solidsFeed solids

Digested solids

Daily

3/week

Alkalinity Digested solids 3/week

Volatile Acids Digested solids 3/week

pH Digested solids Daily

Temperature Digested solids Daily

Ammonia Digested solids 1/week

Gas production Digester gas Daily

Gas Quality Digester gas 1/week

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The data collection listed in Table 1 is the minimum required for good digester operation.Additional monitoring would be necessary during digester upset or when sludge feed is

unpredictable, such as hauled waste flows or intermittent thickening that interface with

continuous feed to the digesters.

Primary ClarifiersSeptic conditions in the primary clarifier are conducive to excess VFA formation which will

upset the digester operation. It is important to control sludge flow rate of the primary clarifier tomaintain the depth of the sludge blanket during peak flow, as measured at the middle of the

clarifier bridge at 12 inches or less. This will mitigate VFA formation that could upset the

digester and cause excessive foaming.

Digester Gas Collection SystemThe operation and maintenance of the digester gas collection system directly affects digesteroperation.

During normal operation, significant amounts of condensate or water carryover accumulate

in the digester gas piping system. Manual drip traps are small and must be emptiedfrequently. The drip traps can be fitted with electric actuators that operate on timed cycles

to automatically drain the liquid. The digester waste gas flare and gas relief valves must be set at the proper pressure. For

instance, if the system is to operate at a pressure of 9 inches of water column, the waste gas

burner should be set to fire at 9.2 inches of water column (Figure 18). The pressure relief

valves should be set for a pressure approximately 5 percent lower than the design pressure.For instance, if the design pressure is 11 inches of water column, the relief valves should be

set for 10.5 inches of water column.

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Figure 18 Waste Gas Flare

A properly operated sediment trap provides sufficient protection system against carryover

of water from the digester to the gas piping system (Figure 19). Operators should monitor

the sediment trap sight glass (liquid level in tank) and observe the color of the liquid. Aclear liquid indicates normal conditions; brown or black indicates foam carryover and the

need to check the feed sludge VS loading.

Figure 19 Sediment Trap

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The manometers located throughout the digester gas header system must be properly

maintained, cleaned, and filled with the proper fluid. The operating staff should comparethe manometer readings with the electronic pressure indicators to confirm proper operating

gas pressures.

Digester Mixing SystemsDigester mixing is critical for a homogeneousmixture in the digester. The digesters evaluated in

all three case studies were equipped with gas cannon mixing systems. These systems are

effective for use with feed solids concentrations of 5 percent or less. At higher feed solidsconcentrations, their effectiveness is reduced. It is also important to minimize downtime of the

mixing system, as there may be insufficient mixing energy to re-suspend large accumulations of

settled solids.

Mixing system recommendations are as follows:

Operate the digester gas mixing compressors with the bypass valve completely closed and

with all cannons producing approximately one bubble every 5 to 10 seconds. Thecompressors should always operate continuously, except during maintenance. The digester

gas bypass valve should remain closed. Examples of a gas compressor and a manifold arepresented on Figures 20 and 21.

Figure 20 Digester Gas Compressor

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Figure 21 Gas Manifold

The gas mixing system can be turned off for up to 8 hours without disrupting the

biological process or the VS loading cycle. For periods longer than 8 hours, the VSloading should be stopped until the gas mixing system is re-started and operating for at

least 12 hours. Before feeding is resumed, digester contents should be sampled to

confirm the operating conditions, and the VS loading feed rates adjusted as needed.

Spreadsheets similar to that presented on Figure 17 should be provided for calculating theVS loading to allow operators to determine the correct loading rates.

CONCLUSIONS

The root cause of most cases of excessive digester foaming is unstable operation. There are

measures that can be utilized in the design of a digester process to mitigate foaming, but

ultimately continuous monitoring and trend charting will ensure that the process operates at peakefficiency performance. While some foaming is to be expected, excessive foaming is a serious

problem. Digester operation is not an automatic process. It requires time and resources tomonitor the operation and evaluate the data generated.

An adequate and well-trained staff is essential. It is not enough to simply collect data. It is

important to understand the data and compare them with past records to improve digesterperformance and ensure good gas production and volatile solids reduction. Thomas Edison,

Americas most prolific inventor, best describes this philosophy: Machines are no better than

the skill, care, ingenuity, and spirit of the men who operate them. All three plants evaluated inthe case studies are fortunate to have well-qualified operators who, once the problems of the

unstable operation were recognized, were able to meet or exceed Mr. Edisons expectations, byoperating and maintaining their digester systems through monitoring and establishing operationalprotocols.

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REFERENCES

Massart, Neil, Robert Bates, Blair Corning, Gary Neun (2006). The Root Cause of Excessive

Anaerobic Digester Foaming.Proceedings of the Residual and Biosolids Management

Conference, Water Environment Federation, Cincinnati, Ohio, March 2006.

Sandino, Julian, Hari Santha, Steve Rogowski, Wendy Anderson, Shihwu Sung, Ferit Isik

(2005). Applicability of Ultrasound Pre-Conditioning of WAS to Reduce Foaming

Potential in Mesophilic Digesters, Proceedings of the Residuals and BiosolidsManagement Conference, Water Environment Federation, Nashville, Tennessee, March

2005.

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design and operational considerations to avoid excessive anaerobic digester foaming

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