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ADVANCED ANAEROBIC DIGESTION PERFORMANCE COMPARISONS

Perry L. Schafer, P.E., DEE, Brown and Caldwell

2701 Prospect Park Dr., Rancho Cordova, California 95670Joseph B. Farrell, PhD, P.E., Environmental Consultant

Gary Newman, Brown and CaldwellScott Vandenburgh, Brown and Caldwell

ABSTRACT

Advanced anaerobic digestion processes are being developed and used at more wastewater

treatment facilities in recent years, especially in North America and Europe. These processes are

largely developed to achieve greater performance (i.e., greater volatile solids reduction andincreased digester gas production). This paper reviews the performance data and information

available for several of these advanced digestion processes, especially the ones most commonly

being implemented, evaluated, or proposed in North America. Comparisons are now possible formany of these processes showing the degree of improvement gained by the advanced digestion

process over the previous mesophilic digestion process. The paper also describes volatile solids

reduction calculations and the methods currently used for these calculations. Related issues are

also presented and discussed including product odor, dewatering information, and increasedammonia recycle.

KEYWORDS

Sludge, anaerobic digestion, phased digestion, staged digestion, thermophilic, mesophilic.

INTRODUCTION

Wastewater agencies are under pressure to minimize costs for sludge processing, yet create

improved biosolids products to satisfy users and the public. Since anaerobic digestion continues

to be the dominant municipal sludge stabilization technology in North America, improvementsand advancements in anaerobic digestion are occurring at a significant pace.

The primary objective of this work is to review the actual performance of advanced digestion

processes so that agency staff and engineers are armed with improved data when evaluating theseprocesses. Many authors are providing considerable information within technical papers and

publications about these processes, however, some papers do not provide complete data for

evaluation, and, therefore, the authors have included substantial additional data collected directfrom agency or POTW staff and plant operating data files. Whenever possible, longer-term

average performance data have been used to evaluate these processes (and presented here), rather

than providing results obtained from short-term testing periods. For the most part, full-scalefacility operating and performance data have been used for evaluation purposes. In some cases,

high-quality pilot testing program results are included here since comparative data between

processes can often be best represented by such work.



Performance of advanced digestion processes is evaluated primarily in terms of volatile solidsreduction (VSR) since in North America this tends to be one of the most common performance

criteria used and such values are almost always available for each plant’s digestion system.

Indeed VSR is a key parameter in regulatory compliance for most US agencies using anaerobicdigestion of sludge. Other performance criteria for anaerobic digestion processes such as gas

production, gas quality, biosolids product stability, and product odor, are more difficult toevaluate due primarily to infrequent or poor quality data.

PROCESSES EVALUATED

The advanced anaerobic digestion processes evaluated here are grouped into the followingclasses and are schematically represented in Figure 1:

• Thermophilic digestion. Although single stage thermophilic would be the simplest formof thermophilic digestion, several plants have moved to “staged thermophilic” digestion

so that improved pathogen destruction could be assured and, for several plants, Class A

product can be provided.

• Temperature phased digestion. This has been implemented most often with

thermophilic digestion as the first phase, followed by mesophilic digestion as the second

phase. The primary reason for this order is to produce a final product with minimal odorlevel since thermophilic digestion product has been described as odorous by some

authors.

• Acid/gas phased digestion. This category can include either phase (acid or gas) at

mesophilic or thermophilic temperatures, but is most often being implemented with bothphases at mesophilic temperatures due to simplicity in sludge heating and cooling.

• Three-phased digestion. Three-phase digestion (as implemented at the Inland EmpireUtilities Authority in California) is a combination of acid/gas phased digestion and

temperature-phased digestion.

Not included as part of this paper are various pre-digestion processes that are being used or are in

developing stages. These include the following:

• Pre-pasteurization, occurring in various types of tankage and configurations.

• Thermal hydrolysis processes.

• Pre-digestion physical or chemical treatment.

These pre-digestion processes are typically intended to improve digestion or dewatering

performance and/or create Class A biosolids.



For simplicity, solids retention time (SRT) is used in this paper to describe both hydraulic andsolids retention times, since none of the processes described in this technical paper recycle

process liquor or solids from the digestion reactors, and we assume no significant buildup of

solids within the reactors.

Table 1 presents summary data and information for each of the treatment plant digestion systemsevaluated in this paper. They are grouped according to the process classes described above.Performance data shown are averages from plant operating data. If the performance data are

from a pilot plant, this is noted. Dewatering information is shown if the information is available

since dewatering performance is often a major consideration by agencies considering advanced

digestion systems. However, dewatering information presented here does not included detailson polymer dose and many of the finer details of dewatering needed to make decisions.

Dewatering information is presented to indicate the type of dewatering and general cake solids

performance being achieved.

Thermophilic Digestion

Several agencies have transformed their mesophilic digestion systems to thermophilic digestion

in recent years to improve volatile solids reduction (VSR) and pathogen control. As shown in

Table 1, the North American agency that has led this effort in the last decade has been the

Greater Vancouver Regional District (GVRD) in British Columbia, Canada. In the early 1990s,GVRD began 2-stage thermophilic digestion at the Lions Gate plant, primarily with the objective

of creating pathogen-free biosolids for beneficial use purposes. This experience provided the

impetus for the larger and more definitive 4-stage thermophilic digestion process at the AnnacisIsland plant in the mid-1990s. Both plants have seen significant increases in VSR, as indicated

in the table, as well as providing pathogen-free product for successful biosolids beneficial useprograms.

The OWASA facility (Mason Farm plant) is also showing large increases in VSR improvementby switching to a staged thermophilic process. However, part of this increase is due to longer

solids retention times within the process. This plant had difficulty in consistently meeting the 38

percent VSR regulatory requirement prior to the change to thermophilic digestion. A 22-hour/day batch operation in the second stage reactor of the new system insures meeting the

time/temperature requirements for Class A biosolids.

The single stage thermophilic digestion process at Aalborg, Denmark was reportedly chosen bythe City of Aalborg primarily for increased gas production benefits over the prior mesophilic

digestion system. In Denmark, there are significant financial incentives for agencies to produce

maximum energy from use of digester gas. Increased solids destruction and improveddewatering on belt presses are also significant factors in controlling final biosolids reuse costs for

Aalborg.

Two of these four thermophilic digestion facilities (Aalborg and Annacis Island) currently

recover at least part of the heat through sludge/sludge heat exchangers from thermophilic

digestion, and OWASA is planning to recover this heat to help heat incoming raw sludge.



Table 1. Advanced Anaerobic Digestion Process Information

Process/Plant Type of Sludge

HRT (days)

in Each Stage

Temps

(°C)

Reactor

Feeding

Process

VSRa

(%)

VSR for

Mesophilic

Digestion

Staged Thermophilic

Vancouver, Canada

Annacis Island Plant

P+TF/SC 17-Th

2-Th

2-Th2-Th

All stages

55 to 56

Continuous 62 to 63 47

Mason Farm WWTP

OWASA, NC

10% Fermented P

90% WAS

19-Th

10-Th

10-Th

All stages

56 to 57

10 hrs/day

1st stage,

Batch 2nd stage

65 43

Vancouver, Canada

Lions Gate WWTP

P only 22-Th

22-Th

55

Both stages

Continuous 67 to 70 57

SSb Thermophilic with Storage

Aalborg, Denmark

AalborgWest WWTP

P+WAS SS Thermo

15-Th

(plus short

meso storage)

52 to 55 Hourly Feed Sequence “Significantly”

greater than SS

Meso

Unknown

Temp-Phased Thermo/Meso

WLSSD,

Duluth, MN

WAS-only 8-Th

23-M

55-Th

37-M

Continuous 46 40

King Co., WA

Renton (Pilot Scale)

P+WAS 8-Th

16-M

55-Th

37-M

Nearly Continuous 68 58

Neenah-Menasha, WI P+WAS 16-Th

16-M

55-Th

36-M


Madison, WI Pilot (Lab Scale) P+WAS 5-Th

15-M

55-Th

35-M

Intermittent 64 58

City of Los Angeles Hyperion

WWTP (pilot-scale)

P+WAS 10-Th

7-M

55-Th

35-M

Intermittent 66 48

Cologne, Germany Stammheim

Plant

WAS-only 7-Th

27-M

55-Th

35 to 38-M

Continuous 43 34

Sturgeon Bay, Wisconsin P+WAS ~ 17-Th

> 20-M

55-Th

35-M

Intermittent 65 62 (at much

longer SRT)

Acid/Gas Phased at Meso/Thermo

DuPage Co. Illinois WAS-only 2-Acid

16-Gas

(plus storage of 4 to 10 days)

37-Acid

52-Gas

(38 to 43storage)

Continuous 62 to 63 ~45

Acid/Gas Phased at Meso/Meso

San Bernardino, CA P+WAS 2 to 3 (acid)

25 (gas)

35 acid

35 gas

Continuous 57 53

Elmhurst, Illinois P+WAS 1-Acid

38-Gas

30-Acid

36-Gas

Batch Acid 50 36

DuPage Co., Illinois WAS-only 2 (acid)

16 (gas)

(plus storage of

4 to 8 days)

36 acid

36 gas

Continuous 59 ~45

Madison, WI

(Lab-Scale Pilot)

P+WAS 1 to 2-Acid

15 to 20-Gas

35-Acid

35-Gas

Twice/day 58 57

Acid/Gas Phased at Thermo/Meso

City of Los Angeles Hyperion

WWTP (pilot scale)

P+WAS 2 (acid)

12 (gas)

55-Th

35-M

Intermittent 50 48

Indianapolis, IN

Belmont WWTP

(IDI Pilot Scale)

P+WAS 2 (acid)

10 (gas)

55 & 60-Th

37-M

2 or 4 times a day 55 to 60 Unknown

Three-Phase Digestion

Inland Empire

Utilities Agency, CA

Reg. Plant #1

P+WAS 3-Acid

12/14-Th

14/16-M

33/38-Acid

53/55-Th

39/44-M


Notes:a VSR = Volatile Solids Reduction Mesophilic = Meso = M P = Primary Sludgeb SS = Single Stage Thermophilic = Thermo= Th WAS = Waste Activated Sludge

TF/SC = Trickling Filter/Solids Contact



Table 1. Advanced Anaerobic Digestion Process Information (continued)

Process/Plant

How VSR

Calculated Dewatering Performance

Extent

of Data Ref. Comments

Staged Thermophilic

Vancouver, Canada

Annacis Island Plant

Van Kleeck 32% cake solids content,

high solids centrifuges, 95%

capture.

3 years 1 Extensive data. Consistent long-term

performance. Class A process in British

Columbia.

Mason Farm WWTPOWASA, NC

Mass Balance No dewatering operations. 1 year 2 Meets Class A time/temp Batch Requirement in2nd stage.

Vancouver, Canada

Lions Gate WWTP

Van Kleeck Centrifuges, 37% cake

solids, 95% capture.

8+ years of

operation

1 Steady performance on primary sludge only.

Pathogen data suggest Class A product.

SSb Thermophilic with Storage

Aalborg, Denmark

AalborgWest WWTP

N/A Belt Filter Presses, 25%

cake solids, 10 kg/tonne

polymer

1 year 3 VSR is not calculated, but plant reports

“significant” more gas and power production

from thermophilic over prior mesophilic

operation.

Temp-Phased Thermo/Meso

WLSSD,

Duluth, MN

Mass Balance High-solids centrifuges, 28

to 30% cake solids, 99%

capture.

1 year 4 Large pulp/paper load on WWTP-So, WAS is

less digestible than normal WAS.

King Co., WA

Renton (Pilot Scale)

Mass Balance Lab-scale Belt Press got

16% cake solids – same as

full-scale mesophilic.

Several months 5 Well-run and extensive pilot test.

Neenah-Menasha, WI Van Kleeck 20% cake solids content

Belt Filter Press

2 years 6 Pleased with TPAD. Improved VSR.

Dewatering approved “a few” percentage points

in cake solid.

Madison, WI Pilot (Lab

Scale)

Van Kleeck Thickening tests conducted. Several months 7 Extensive pilot testing of many different

scenarios.

City of Los Angeles

Hyperion WWTP (pilot-

scale)

Mass Balance Not Reported 6 months

operation

8 City reported best results for VSR was obtained

with this process.

Cologne, Germany

Stammheim Plant

Van Kleeck Centrifuges, 33% cake

solids, 8 kg/tonne polymer

3 years average 9 Large plant using egg-shaped digesters. Limited

VSR due to all-WAS feed and long sludge age.

Sturgeon Bay, Wisconsin Mass Balance Belt Filter Press, 13 to 15%

cake solids.

4 years 10 Small plant with very long detention times.

Data 2001.

Acid/Gas Phased at Meso/Thermo

DuPage Co. Illinois Mass Balance 21% cake solids on belt

filter presses.

Several years of

full-scale data.

11,

12

Acid/Gas phasing solved the large foaming

problem and improved performance

significantly with WAS-only sludge.

Acid/Gas Phased at Meso/Meso

San Bernardino, CA Mass Balance Centrifuges 25% cake

solids, Belt Press 19% cake

solids

Several months 13 Poor mixing in acid digester has allowed debris

accumulation.

Elmhurst, Illinois Unknown Belt Filter Press, 20% cake

solids content.

Several months

of data.

14 Batch acid operation causing foam in gas

digesters. Acid reactors are new.

DuPage Co., Illinois Mass Balance Belt Filter Press, 20% cake

solids

Several months

of pilot and full-

scale data

11 Acid gas phased digestion provided

performance improvement and foam control.

Madison, WI

(Lab-Scale Pilot)

Van Kleeck Thickening tests conducted. Few weeks of

data.

7 Part of extensive pilot testing program.

Acid/Gas Phased at Thermo/Meso

City of Los Angeles

Hyperion WWTP (pilot

scale)

Mass Balance Not Reported Several months 8 Process did not appear to stabilize and was

discontinued after several months.

Indianapolis, IN

Belmont WWTP

(IDI Pilot Scale)

Mass Balance Not Reported Several months

at varying

conditions

16 Batch thermophilic acid phase proven as site-

specific PFRP equivalent.


Inland Empire

Utilities Agency, CA

Reg. Plant #1

Unknown Belt Filter Presses, 19%

cake solids (with 3-phase)

versus 17% cake solids with

SS Meso.

Several months

of data

15 Temperatures are not consistent. Final phase is

hotter than typical meso systems.



At Annacis Island and Aalborg the final product is dewatered at reduced temperature(approximately mesophilic temperature at Aalborg and about 43º C at Annacis Island), after

storage of only 1 or 2 days at these temperatures. This temperature reduction and short-term

storage helps to reduce odor emissions from the dewatering and cake loadout operations, as wellas odor level in the final product. The details of such storage (temperatures, variation in

temperatures, length of storage time, mixing conditions, etc.) are likely to be important indetermining the extent of odor reduction achieved.

At the Lions Gate plant, the digested product is dewatered at 55º C and the cake trucked off-site.

The dewatered cake at Lions Gate is reportedly more odorous than mesophilic digested cake

product, and the centrate is very odorous. At the OWASA plant, the thermophilic digestedsludge has been stored in large storage tanks for 20 days or longer. These tanks lose temperature

to the environment so that the final slurry product trucked to reuse sites is at approximately

mesophilic temperatures. This final reuse product at OWASA has reportedly little odordifference from the previous mesophilic digested slurry product.

Ammonia levels are higher in thermophilic digested sludge (and in dewatering filtrate/centrate)than in the previous mesophilic digested sludge from these plants. This is not a problem at

Vancouver, since ammonia removal is not required at the Vancouver plants. At Aalborg, the

higher ammonia level in the dewatering filtrate is treated within the liquid treatment system at

the Aalborg West plant. Figure 2 displays the performance change identified by agencies havingsufficient data.

Temperature Phased Digestion

About a dozen agencies in the US have now implemented temperature phased digestion, andperformance data for several of these plants are included in Table 1. In addition, data from three

pilot scale programs for temperature-phased digestion are included in the table because of the

availability of comparative performance results. The comparative results are plotted on Figure 3to better display the performance change from prior mesophilic digestion to temperature phased

digestion. These temperature phased digestion facilities are all achieving increased VSR and gas

production.

It is interesting to note the large variety of SRTs being used by these temperature phased

digestion plants. SRTs in each phase are often dictated by the size of digesters available at the

plant since the only temperature-phased plant listed in Table 1 with new digesters is the WesternLake Superior Sanitary District plant at Duluth, Minnesota. In most cases for full-scale

operations, the SRTs are substantially longer than required for stable operation.

Thermophilic sludge heat recovery is practiced at most of the full-scale temperature phased

digestion plants listed in Table 1, however the methods of heat recovery are different. Direct

sludge/sludge heat recovery is practiced at Sturgeon Bay. The Cologne-Stammheim plantutilizes a sludge/water/sludge heat recovery system with concentric tube heat exchangers. At

Duluth, the thermophilic heat is recovered as hot water for building heating.



Agencies operating temperature phased digestion report that the dewatered cake product is well-stabilized and has the typical odor of mesophilic digested biosolids, although a frequent

comment is that ammonia odor from the product is stronger than from the previous mesophilic

digested product. This appears to occur because of the following: (1) higher ammoniaconcentrations in temperature phased digested biosolids, and (2) higher pH of the biosolids,

resulting in a greater percentage of ammonia in the molecular form. The greater ammoniaconcentrations in dewatering centrate/filtrate are causing concern in some of these plants inmeeting current or expected future ammonia discharge limits from the wastewater treatment

plant.

Acid/Gas Phased Digestion

In this process, the acid phase has a short SRT, with pH generally below six, and low methane

content in the digester gas. The methane phase has fully developed methanogens levels withlonger SRT and full methane production. Table 1 includes performance data from plants or pilot

plants utilizing three different temperature configurations:

• Mesophilic acid phase followed by thermophilic gas phase (acid/gas phased at

meso/thermo)

• Mesophilic acid phase followed by mesophilic gas phase (acid/gas phased at meso/meso)

• Thermophilic acid phase followed by mesophilic gas phase (acid/gas phased atthermo/meso)

The wastewater plant with the longest operating experience with acid/gas phased digestion is theWoodridge-Greenevalley plant of DuPage County, Illinois. This plant operated with both phases

at mesophilic temperature many years ago, but has operated under the meso/thermoconfiguration for almost 10 years. There is a reactor following the thermophilic phase that is

primarily a digested sludge storage tank. The temperature of the sludge in this tank is typically

at elevated mesophilic temperatures, but some limited additional biosolids stabilizationreportedly occurs in this tank. The reported performance data for this digestion system does not

take this storage reactor into account.

The acid/gas phased digestion process performance at DuPage County has been extremely

beneficial, resolving a substantial foaming problem in the digesters and providing major

improvement in VSR. The high ammonia recycle in the dewatering filtrate has been a significant

problem for the plant in meeting its ammonia discharge standards.

Several plants are operating in the meso/meso configuration and data from some of the plants

and one pilot program are shown in Table 1. Some of the comparison work is showing relativelylittle VSR improvement from acid/gas phased digestion, while other plants show significant VSR

improvement. No factor or reason has been identified to account for this variation in

performance.

It is clear that the acid phase digester needs to be designed to account for the specific needs of

this process. This includes adequate precautions for grit and debris accumulation or removal. Inaddition, handling of the gas from this phase needs careful consideration since it will probably



contain low methane content and perhaps high hydrogen sulfide content. It has been suggestedthat plug-flow movement through this reactor could be beneficial for process performance. In

summary, more detailed development of the operating requirements for the acid phase portion of

this process would be helpful for agencies seeking to implement this process. Figure 4 providesa review of the performance of acid phase digestion systems.


The Inland Empire Utilities Authority has implemented three-phase digestion, as depicted on

Figure 1. This process has provided some increase in VSR, and one of the primary benefits

reported by the process has been the improvement in mechanical dewatering of the product. Thethird phase of the system at Inland Empire is not operated at a specific temperature, but is

allowed by seek its own temperature from radiant heat loss to the environment. The result is

that this phase often has temperatures between typical mesophilic and thermophilic temperatures.Biosolids dewatering on belt filter presses is not reported to be an odor problem at Inland

Empire.

PERORMANCE MEASUREMENT BY VSR

Volatile solids reduction calculations are important for proper performance assessment work in

anaerobic digestion. Several methods are available for calculating VSR, as described by theUSEPA (1999). The two most frequent methods used are the Van Kleeck and Mass Blance

methods. These methods often do not provide the same calculated VSR and the reasons for this

are not always clear. However, one of the primary distinctions between these calculationmethods is the assumption by the Van Kleeck method, that fixed solids remain “fixed” through

the digestion process. The discussion below provides understanding on this issue.

At most facilities that utilize anaerobic digestion, volatile solids reduction is calculated and used

as a control parameter for judging the performance of the process. Typically, total and volatilesolids concentration of the feed and product streams are determined and the fraction volatile

solids is determined at inlet and outlet. Then volatile solids reduction is calculated, using the

Van Kleeck equation:

VSRVK = (VSF - VSP) / (VSF – VSF x VSP) (1)

Where VSR is volatile solids reductionVS is fraction of volatile solids to total solids

Subscripts F and P refer to feed and product

Subscripts VK refers to the Van Kleeck equation



Volatile solids reduction can also be calculated by a mass balance. This calculation is extremelysimple when volumetric flow rate entering and leaving are the same and no decant stream has

been removed from the digester. The relationship is as follows:

VSRMB = (vF – vP) / vF (2)

Where v is concentration of volatile solids, typically in grams / literSubscripts F and P refer to feed and product

Subscripts MB refers to the mass balance equation

Despite the ease of calculation, volatile solids reduction is seldom calculated in this fashion.One reason may be that the volatile solids concentration fluctuates with time more than the

fraction of volatile solids. Actually, when propagation of error in Equations 1 and 2 is examined,

it is found that for the same percentage deviation in fraction of volatile solids and volatile solidsconcentrations, the variation in VSRVK is greater than the variation in VSRMB. Thus, in cases

when the volatile solids concentration is reasonably constant, VSR calculated by either method

will show about the same random variation.

With the advent of federal regulation of biosolids utilization, the volatile solids determination has

become more than a useful control test. The adequacy of the digestion process is evaluated by

many facilities by the volatile solids reduction, which must be at least 38%. Severalinvestigators in the past have observed that the basic assumption of the Van Kleeck equation,

that fixed solids are conserved, is not strictly true. For example, Benefield and Randall (1980)

cite a number of papers by Randall and coworkers that document a decrease in fixed solidsduring aerobic digestion. Thus, it is highly unlikely that VSR calculated by the two methods will

agree. Switzenbaum et al. (2002A), in their evaluation of the protocols used in the Part 503regulation, contacted many facilities practicing aerobic and anaerobic digestion, and found

several of them reporting substantial differences between results of the two methods. This

prompted these investigators to establish the relationship between the two methods. Theydetermined that the two methods are mathematically related by the following equation:

VSRVK / VSRMB = 1 – (1/Rf –1)/ (1/Rv –1) (3)

Where Rf is the ratio of the fixed solids concentration in the product to the feed

Rv is the ratio of volatile solids concentration in the product to the feed

The derivation of this equation is provided in a publication by Switzenbaum et al (2002B). To

illustrate the use of this equation, assume that we have obtained the following data on the feed

and product from a digester:

Feed: vF = 50 g/L, f F = 20 g/L, VSF = 0.7143

Product: vP = 30 g/L, f P = 15 g/L, VSP = 0.6667

Where “f” is the concentration of fixed solids (e.g., in g/L).



The percent volatile solids have been calculated from the volatile and fixed solids concentrations.The volatile solids concentrations by the two methods can be calculated:

VSVK = 0.1999, VSMB = 0.400

The ratio, VSVK / VSMB, can also be calculated by Equation 3, whereby:

Rf = 15/20 = 0.750, Rv = 30/50 = 0.600

Substituting these values into Equation 3 gives a ratio of 0.500, which agrees with the ratio of

individual volatile solids reductions for the two methods.

The difference between the two calculation methods is large in this example because the fixed

solids concentration drops so much from the feed to the product. If the fixed solidsconcentration dropped only from 20 g/L to 19 g/L from feed to product, the ratio of the two

calculation methods becomes 0.921 instead of 0.500 in the above example.

It is important to note that the basic information needed to calculate volatile solids concentrations

and percent volatile solids can be used to calculate both VSRs. However, the relationship

between the VSRs cannot be calculated if only volatile solids concentrations are provided or if

only percent volatile solids are provided. Most published data does not provide fixed solidsconcentrations, so the comparison generally cannot be made. It would be very helpful in the

future if data reported on volatile solids reductions in either anaerobic or aerobic digestion

provide this additional information.

FINDINGS AND CONCLUSIONS

The key findings and conclusions from this evaluation of advanced anaerobic digestion processes

are as follows:

1. Advanced anaerobic digestion processes are creating improved digestion performance,

particularly as measured by volatile solids reduction. The degree of improvement isvariable, as seen by the data discussed here.

2. Increased gas production is also occurring from operation of advanced digestion

processes, although confirming the amount or percentage of increased gas production isoften difficult.

3. In most cases, the odor in the final product from advanced digestion processes isconsidered similar to the odor from mesophilic-digested products. However, greater

ammonia odor is sometimes associated with advanced digestion products and a different

odor character is noted at times for thermophilic digested products.

4. The processes involving thermophilic digestion, in particular, are showing high rates of

pathogen destruction. Innovative configurations are occurring to confirm pathogendestruction meeting Class A requirements of EPA’s Part 503 regulations.



5. Dewatering improvements are also being identified by many of the plants using advanced

digestion processes. Usually, this is noted as a small improvement in cake solids content.

More work is required to identify the detailed impact on dewatering.

6.

Higher ammonia recycle loads (from dewatering) are occurring with advanced digestion.In some cases, these loads are causing problems in meeting ammonia dischargerequirements for the treatment plant.

7. Calculations for VSR are becoming more important as process performance becomes an

increasingly dominant factor for agencies. More comparison work between VSRcalculation methods is needed.

ACKNOWLEDGEMENTS

We wish to acknowledge all staff from wastewater agencies providing data for this evaluation.

In particular, the following individuals provided important information beyond that available inprior publications: Kevin Buoy (DuPage County, IL), Walter Gottschalk (OWASA, NC), Jim

McQuarrie (GVRD, Canada), Kathy Hamel (WLSSD, MN), Gerald Hernandez, (City of Los

Angeles, CA), Randy Much (Neenah-Menasha, WI) Doug Drury (Inland Empire, CA), Steve

Schultz (City of San Bernardino, CA), and Søren Højsgaard from Krüger (Søborg, Denmark).

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2. Orange Water and Sewer Authority, 2002. Operating data and information on digestionand solids processing system at Mason Farm WWTP at OWASA, North Carolina.

3. City of Aalborg, 1999. Operating data for digestion and related solids processingfacilities at Aalborg West WWTP in Aalborg, Denmark.

4. Western Lake Superior Sanitary District, 2002. Operating data for solids processing

system at the District WWTP in Duluth, Minnesota.

5. King County, 2001. “Thermophilic-mesophilic Anerobic Digestion: Pilot Testing at

South Treatment Plant”, Draft report published October 2001 by King CountyTechnology Assessment Program, King County, Washington.

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improvements in anerobic digestion

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