“SECOND GENERATION” ATAD – A-TAD BETTER? – TWO CASE STUDIES OF
CONVERSION FROM “FIRST GENERATION” TO “SECOND GENERATION” ATAD
SYSTEMS
By
David W. Oerke, P.E. (Principal Author)
CH2M HILL, Denver, Colorado,
And
Chad Olsen, McMahon Associates; James Kirk, Grand Chute-Menasha West Sewerage
Commission, Neenah, Wisconsin; and Chris Maines, Eagle River Water and Sanitation
District, Vail, Colorado
ABSTRACT
Autothermal Thermophilic Aerobic Digestion (ATAD) is becoming more prevalent due to its
advantages over conventional aerobic digestion, including the generation of a Class “A” product,
a relatively small footprint, and reduced biosolids for end disposal.
Both the Grand Chute-Menasha West Wastewater Treatment Facility (WWTF) in Neenah,
Wisconsin and the Edwards Wastewater Treatment Plant (WWTP) operated by the Eagle River
Water and Sanitation District located in Edwards, Colorado have “First Generation” ATAD
systems and are in the process of converting the systems to “Second Generation” ATAD
systems.
The “Second Generation” ATAD systems include process modifications over the “First
Generation” ATAD systems such as better mixing and process control that have resulted in
higher volatile solids reduction, lower product volume, less odor generation and less sidestream
impacts at several installations. This paper explores the advantages of the “Second Generation”
ATAD systems over the original installations and the reasons why the two plants decided to
convert their systems to the newer ATAD systems.
KEYWORDS
Autothermal Thermophilic Aerobic Digestion, storage nitrification/denitrification reactor,
biofilter, odor control
INTRODUCTION
The handling, stabilization and reuse/disposal of biosolids are an increasingly costly portion of
the wastewater treatment plant (WWTP) operation. Given the trend of decreasing availability
and increasing cost of ultimate reuse/disposal options, WWTP residuals handling is becoming an
even greater challenge. Autothermal thermophilic aerobic digestion (ATAD) is becoming more
prevalent due to its advantages over conventional aerobic digestion, including the generation of a
Class “A” product, a relatively small footprint, and reduced biosolids for end disposal.
Both the Grand Chute-Menasha West Wastewater Treatment Facility (WWTF) in Neenah,
Wisconsin and the Edwards WWTP operated by the Eagle River Water and Sanitation District
located in Edwards, Colorado have “First Generation” ATAD systems and are in the process of
converting the systems to “Second Generation” ATAD systems. This paper explores the
advantages of the “Second Generation” ATAD systems over the original installations and the
reasons why the two plants decided to convert their existing systems to the newer ATAD
processing operations.
METHODOLOGY AND DISCUSSION
The ATAD process was first utilized in the United States in 1994 at the Grand Chute-Menasha
West WWTF. There are approximately 50 “First Generation” ATAD installations in Europe
and North America including designs based on equipment provided by Fuchs, Incorporated,
equipment provided by Jet Tech, and installations that used a pumped-venturi mixing system
designed by Dayton & Knight Consulting Engineers. Although some of the “First Generation”
systems have had impressive volatile solids destruction rates, many have also had a history of
significant odor problems, high ammonia concentration in sidestream recycle flows, foaming
issues and relatively high dewatering costs. The problems associated with the “First Generation”
ATAD systems have caused several facilities to discontinue operation.
ATAD is commonly referred to as “liquid composting” because it operates at thermophilic
temperatures comparable to conventional composting (50 to 70 degrees Centigrade [ºC] or 122 to
158 degrees [ºF]). The process is described as autothermal because, as it increases in temperature
during start-up, and then it requires no other heat source (other than mixing energy) to maintain
thermophilic temperatures. The heat released by organic decomposition during digestion
typically sustains the thermophilic operating temperatures. Increased operating temperatures
produce a more rapid digestion process when compared to conventional aerobic digestion. The
increased digestion rate results in a decreased digester volume. Results from operating facilities
show that the system, which is relatively simple, has low operation, maintenance,
instrumentation, and energy requirements.
A properly designed ATAD system has the following advantages over conventional biosolids
digestion processes:

Production of a Class “A” biosolids product

Cost-effective compact facilities

Significant volume reduction

Better biosolids dewaterability
The advantage of using a Class “A” stabilization process includes easier compliance, less
monitoring and record keeping, less odor in the final biosolids product, and maximum flexibility
for beneficial reuse and marketing options. A Class “A” biosolids material, similar to potting soil
is produced from a well designed and operated ATAD facility (see photo of “Second
Generation” ATAD Class “A” Product from Yorkville, IL in Figure 1). This material can be land
applied or distributed and marketed without the restrictions associated with Class “B” biosolids.
Figure 1 - Class “A” soil-like biosolids product from “Second Generation ATAD facility at
Yorkville, Illinois
ATAD is a very efficient stabilization process because its facilities are more compact (when
compared to conventional aerobic digestion). This compactness:

Reduces capital and some annual operation and maintenance costs. However, energy
costs can be higher for ATAD facilities since the high ATAD temperatures typically
provide poor oxygen transfer efficiency and the significantly higher degree of treatment
typically requires more energy)

Enables capture of off-gas for better odor control

Can provide those owners with existing available tanks significant tank capital cost
savings
The ATAD system has been successfully installed in existing retrofitted concrete and coatedsteel aerobic digester tanks. Compared to conventional aerobic digestion, ATAD is a more
effective digestion and pathogen and vector attraction reduction process. The ATAD process
typically destroys 50 to 70 percent of the volatile solids in the biosolids. This reduces
“downstream” solids support capacity requirements, such as dewatering facilities. Volume
reduction saves hauling costs associated with land application and distribution and marketing of
the final product.
To effectively operate the ATAD process, the feed material must be thickened to a minimum
40,000 mg/L (4 percent solids) or greater chemical oxygen demand (COD). Some ATAD
facilities with lower energy waste activated sludge (WAS) feed require a minimum of 5 to 6
percent solids concentration. Prior to ATAD, gravity belt thickeners, rotary drum thickeners, and
centrifuges have been successfully used for thickening solids. Feed material with a lower solids
concentration has been successfully treated by the ATAD process, but requires larger reactor
tanks and typically more process control to consistently meet Class “A” stabilization criteria.
“First Generation” ATAD Process Issues
The total ATAD process retention time recommended in the “First Generation” ATAD process
was only 5 to 8 days. This compares to a 20 to 60-day process retention time for aerobic
digestion. However, the design criteria for the “First Generation” ATAD systems with only 5 to
8 days hydraulic retention time (HRT) were originally used for only primary solids digestion.
Primary solids are much easier to “break down” compared to secondary solids. The shorter HRT
has been inadvertently applied to ATAD systems with combined primary and secondary feed
solids. Note that solids retention time (SRT) and HRT are the same since the ATAD process
tanks are designed to be completely mixed.
The “First Generation” ATAD design criteria generally required at least two to three reactors to
optimize process performance, stability, and flexibility. The temperature is typically maintained
at approximately 45ºC (113ºF) in the first “preheat” reactor and approximately 60ºC (140ºF) in
the second and third “process” reactors. A fourth reactor is typically required for cooling and
storage to enhance thickening and dewatering performance and supernatant/filtrate quality. Heat
is typically extracted from the ATAD biosolids through heat exchangers to heat the incoming
“cold” feed solids at the ATAD facilities with pumped-venturi mixing designed by Dayton &
Knight Consulting Engineers. The “First Generation” ATAD processes are typically operated in
batch, semi-continuous, or continuous mode. The “First Generation” ATAD systems typically
provide only one level of oxygen supply during the entire process, regardless of the level of
activity in the reactor.
Foam can serve as important insulation at the liquid-surface layer where the highest amount of
heat loss caused by evaporation can occur. Many times, ineffective process control in the “First
Generation” ATAD systems has led to excessive foam generation and numerous housekeeping
problems. Mechanical foam cutters were often used to control the foam generation.
Ammonia concentrations in the recycled centrate/filtrate from “First Generation” ATAD
facilities have also been significant (typical ammonia concentrations of 800 to 1,500 milligrams
per liter [mg/L]) depending on the degree of volatile solids reduction and the temperature. In
addition, impacts on the secondary treatment process had to be considered, especially at nutrient
removal WWTPs.
Odor generation was also a significant concern with “First Generation” ATAD facilities. The
“First Generation” ATAD facilities that do not have proper mixing and process control typically
produce an odorous off-gas that includes high concentrations of ammonia, amines, and reduced
sulfur compounds. Successful odor treatment systems for “First Generation” ATAD systems
include multiple odor treatment technologies in series, including wet scrubbers, ozone, threestage scrubbers and biofilters.
Grand Chute WWTF ATAD Facility
The first North American ATAD facility was installed at the Grand Chute-Menasha West
(GCMW) WWTF, in Neenah, Wisconsin in 1994. Co-settled and thickened primary sludge and
waste activated sludge (WAS) is pumped to the mixed sludge holding tank. Sludge is transferred
from the mixed sludge holding tank once/day, 7 days per week to the three ATAD reactors
operated in series.
The “First Generation” ATAD reactors consists of 3, 10.7-m (35-foot) diameter, 3 m (10 foot)
liquid depth insulated process tanks with a total of approximately 757 cu m (200,000 gallon)
capacity. Each process tank included: 1) four 9 kW (12 HP) Fuchs air-aspirating aerators, and
2) eight 1 kW (1.5 HP) foam cutters suspended from the tank roof. A gravity thickener is used to
co-settle the primary and WAS to provide a minimum feed solids concentration of 3.5 percent
solids. A gravity belt thickener was provided but is rarely used. An approximately 1,703 cu m
(450,000 gallon) Post-ATAD Storage Tank was also provided to decrease the process
temperature prior to dewatering. This facility has consistently achieved greater than 50 to 65
percent volatile solids destruction [measured from the initial tank through the belt filter press
(BFP)], and has never failed to meet Class “A” pathogen and vector attraction reduction
requirements since start-up in 1994. Fecal coliform levels in the BFP cake have typically
averaged 10-30 MPN/g TS (see table 1). Significantly levels of additional volatile solids
destruction have been achieved through the Post-ATAD storage tank. ATAD off-gases are
treated in a three-stage chemical scrubber followed by a biofilter with lava rock media. The odor
control system was design and installed after the initial ATAD facility start-up due to significant
odor generation.
The ATAD reactors were designed to handle approximately 5,715 kg/d (12,600 lbs/d) total solids
loading with a hydraulic retention time of 6.5 days at 115 cu m/d (30,250 gpd) at 3.5 percent
solids concentration. The ATAD reactors have been loaded at approximately 70 percent of the
design solids loading and 106 percent of the volumetric loading.
Post-ATAD biosolids are discharged into the circular sludge thickener tank. BFP feed pumps
draw suction from this tank to feed the BFPs. The dewatered solids cake concentrations of 23 to
29 percent have been achieved with a BFP. Previous cake solids concentrations for aerobically
digested biosolids were 14 to 17 percent using the same BFPs. However, a higher polymer dose
was required to dewater the ATAD biosolids. It should be stated that since the ATAD process
has a higher volatile solids reduction than aerobic digestion, comparably less material must be
dewatered. Therefore, the overall chemical dewatering cost is often times approximately the
same. For example, if only the ATAD process produces one-half the biosolids quantity but twice
the chemical dewatering dose is used, then the overall chemical use is approximately equal.
Table 1- “First Generation” ATAD Plant Performance – Grand Chute-Menasha West and
Edwards WWTPs
Plant
Year VSR
VSR
VSR
ATAD
Fecal
Fecal
Fecal
(%) – (%) –
(%) –
#3
Coliform Coliform Coliform
Across Through Through Reactor (MPN/g
(MPN/g
(MPN/g
ATAD PostBFP
Temp.
TS, GM)
TS, GM)
TS, GM)
Tanks ATAD
Cake
(degrees – Across
– Post– BFP
Storage
C)
ATAD
ATAD
Cake
Tanks
Storage
Grand
ChuteMenasha
West
WWTP,
Neenah,WI
1995 35.4
50.2
57.0
*
74
139
69
1996 31.6
53.6
60.6
*
65
185
22
1997 30.7
50.1
60.7
*
61
121
11
1998 34.1
52.5
62.5
*
74
188
29
1999 34.4
49.7
58.2
*
87
158
18
2000 30.5
47.6
54.7
*
82
173
19
2001 29.0
48.3
51.5
58
64
125
15
2002 33.2
52.4
57.1
60
64
119
16
2003 34.6
54.8
58.4
58
71
122
14
2004 34.9
58.7
62.3
60
61
95
12
2005 38.1
61.9
65.7
63
65
99
21
2006 38.8
60.2
63.3
64
66
102
12
2007 33.7
56.7
58.4
58
84
131
15
2008 36.6
58.5
60.0
60
67
95
28
Edwards
WWTP,
Edwards,
CO
2007
2008
*
*
52
39
*
*
44
44
*
*
VSR = Volatile Solids Reduction; GM = Geometric Mean; * Unknown
*
*
*
*
Despite excellent performance, there are issues with the current ATAD design that are being
addressed with the “Second Generation” ATAD facility modifications that are under
construction:
1.
Incomplete digestion which results in high reduced sulfide levels result from the short
retention times (only 5 to 6.5 days) and high temperatures. “Second Generation”
ATAD systems are typically designed for 12 to 14-day ATAD reactor retention time.
2.
Better volatile solids destruction and a more stable operation in the ATAD reactors is
anticipated for the “Second Generation” ATAD facility.
3.
The Fuchs design does not account for the significant oxygen demand immediately
after feeding the reactors, resulting in generation of reduced sulfide odor compounds.
“Second Generation” ATAD system design criteria matches oxygen demand with
oxygen supply, using ORP control to automate the system. This results in more
complete digestion and virtually eliminates odors from sulfur compounds.
4.
The BFP filtrate adds approximately 295 kg/d (650 lbs/d) of ammonia to influent
wastewater. The operating staff at the WWTF is currently having difficulties meeting
their effluent ammonia limits, especially on the days that the BFP is in operation.
“Second Generation” ATAD systems provide a Post-ATAD/Storage
Nitrification/Denitrification Reactor (SNDR) reactors should reduce the ammonia
concentration in the digested biosolids to below 500 mg/L and reduce the impact of
the ammonia in the dewatering filtrate on the secondary treatment process. The
installation of the Post ATAD/SNDR reactors has allowed the GCMW Sewerage
Commission to install a smaller secondary treatment system.
5.
With a SNDR tank - cooling, nitrification, and denitrification are provided. The
combination of cooling and post aeration to rebalance the cation to anion ratio in the
biosolids results in a drier dewatered product with less polymer addition. The
nitrification and denitrification process also reduces the nitrogen load returned in the
filtrate.
6.
Use of “Second Generation” ATAD odor control systems will include a water
scrubber followed by a biofilter that will allow for the elimination of the triplex
chemical scrubber odor control system, thus eliminating sulfuric acid, hypochlorite
and caustic addition.
7.
The addition of the new “Second Generation” ATAD system and related new solids
processing facilities allows the GCMW Sewerage Commission to handle more solids
production corresponding to an increased influent wastewater flow from growth
within the service area. The new facilities are designed to process an average annual
solids production of 7,606 kg/d (16,734 lbs/d).
Edwards WWTP ATAD Facility
The original design of the “First Generation” ATAD system for the Edwards Wastewater
Treatment Plant (EWWTP) was based on an average solids production of 2,517 kg/d (5,550
lbs/d); a maximum month solids production of 2,812 kg/d (6,200 lbs/d); and a peak week solids
loading of 3,402 kg/d (7,500 lbs/d). The peak week solids production of (3,402 kg/d (7,500
lbs/d) was used to size the tanks assuming a total HRT of 4.5 days. The recommended HRT
design value for other ATAD installations with lower odor generation compared to the EWWTP
is 12 days. The actual dimensions of the four ATAD tanks are as follows: total height – 5.86 m
(19.25 feet) to top of hatch; overflow height – 5.2 m (17 feet) for a capacity of 156 cu m (41,208
gallons); actual working height – 4.3 m (14 feet) to account for foam for a capacity of 129 cu m
(34,000 gallons).
The current pumped-venturi mixing system is inadequate. The staff typically can maintain an
ORP value between a low of –350 millivolts (mV) to a high of –200 mV that results in anaerobic
conditions that creates significant odor. The ORP values should be between –50 mV and +50
mV to keep the process aerobic and minimize odor generation.
The coarse bubble aeration system that includes a 186 kW (250 HP) positive-displacement
blower added by the District staff approximately 1 year ago is not effective in mixing the ATAD
solids above 3.5 to 4 percent solids concentration; however, it could be effective if the solids
concentration was decreased to approximately 1.5 to 2.5 percent solids.
The benefits for converting from the “First Generation” to the “Second Generation” ATAD
facility at the Edwards WWTP that is currently under construction:
1.
Compliance with Colorado Department of Public Health and the Environment
(CDPHE) regulations that state that the ATAD tanks must have a minimum 10 day
detention time to meet Class “A” standards. Stricter ATAD design criteria have been
adopted by CDPHE after the design of the initial “First Generation” ATAD facility at
Edwards WWTP.
2.
Additional solids processing capacity to handle future maximum month 6,033 kg/d
(13,300 lbs/d) solids production and eliminate the need to haul liquid raw sludge to
the Avon WWTP or to the Biosolids Containment Facility (BCF) operated by the
Eagle River Water and Sanitation District (ERWSD) in Wolcott, Colorado.
3.
To increase the capacity of the ATAD system, it was recommended that new larger
ATAD tanks with better mixing facilities be installed. The existing pumped-venturi
ATAD tank mixing system is inadequate and underpowered to completely mix the
thickened solids with a solids concentration in the 6 to 8 percent range. The staff
typically can maintain an ORP value between a low of –350 millivolts (mV) to a high
of –200 mV that results in anaerobic conditions (especially in the corners of the
square tanks) that creates significant hydrogen sulfide, mercaptans, and other odor
causing compounds. The ORP values should be between –50 mV and +50 mV to
keep the process aerobic and minimize odor generation.
4.
Over 25 operating “Second Generation” ATAD facilities provided by TPS have
shown that superior mixing, longer detention time, better process control to maintain
aerobic conditions result in significantly less odor generation, and better sidestream
treatment, including one at the Yorkville, Illinois WWTP. Several ERWSD staff
members have had the opportunity to tour this facility and have expressed positive
feedback on the facility performance.
5.
With a SNDR tank - cooling, nitrification, and denitrification are provided. The
combination of cooling and post aeration to rebalance the cation to anion ratio in the
biosolids results in a drier dewatered product with less polymer addition. The
nitrification and denitrification process also reduces the nitrogen load returned in the
centrate.
6.
Potential for different and more efficient odor control technologies (mist scrubber and
two-stage biofilter) to replace existing biotrickling filters, ozone oxidation and threestage chemical scrubber resulting in less odor control chemical consumption (now at
$30,000 per year), lower operating costs and less non-potable water recycle flow
(currently 1,136 cu m/d (300,000 gallon/d) to the Edwards WWTP that takes valuable
plant capacity.
7.
Reduce ammonia loading in the recycle to the aeration basins during peak diurnal
loading.
8.
The equalization of centrate allows for the centrate flow to be recycled over a 24-hour
basis instead of the full load being applied during the operation of the centrifuge.
9.
Recycle of nitrate to the aeration basins will decrease the overall oxygen demand for
the aeration basins resulting in significant electrical savings and will increase
alkalinity resulting in less soda ash addition.
10.
Additional WAS storage is provided to improve WAS equalization and to allow for
thickening and dewatering equipment operational flexibility and significant reduced
labor and polymer costs.
Overall Advantages of the “Second Generation” ATAD Process
The “Second Generation” ATAD ThermAer™ process differs substantially from the “First
Generation” systems mentioned previously. Thermal Process Systems (TPS), Crown Point,
Indiana, has patented the “Second Generation” system and introduced it in 1997. TPS has 25
successfully operating “Second Generation” facilities and has converted seven “First
Generation” facilities to “Second Generation” ATAD facilities. The ATAD process design has
evolved during this time, especially in regard to thermophilic/mesophilic staged operation,
detention time, reactor covers and foam control.
Previous studies (Scisson, 2009) have shown good to exceptional solids reduction and
dewaterability from 8 operating installations. The municipal plants range in size from 0.1 cu m/s
(2.5 MGD) to approximately 0.53 cu m/s (12 MGD). Information and process performance
collected by Scisson from several “Second Generation” ATAD facilities is shown in Table 2.
Table 2 - “Second Generation” ATAD Facility Characteristics
Plant
Three
Rivers,
MI
Yorkville,
IL
Bowling
Green,
OH
Capacity Date in Process
New or
Dry
Service replaced
Retrofit
Mg/D
(TPD)
3.6 (4)
2002 Anaerobic Retrofit
Digestion
Liquid or
cake
storage/
reuse
Cake
1.8 (2.2)
2004
Cake
7.3 (8)
2005
Aerobic
Digestion
Aerobic
Digestion
New
Retrofit
plus
New
Tanks
Anaerobic New
Digestion
Liquid
storage
plus
dewatering
Cake
Cake
Morehead, 2.7 (3.3)
KY
2005
Delphos,
OH
4.1 (4.4)
2006
Aerobic
Digestion
Heart of
the
Valley,
WI
Marshall,
MN
10
(13.5)
2007
Anaerobic Retrofit Liquid
Digestion
storage
5.4 (6)
2006
Anaerobic Retrofit Liquid
Digestion
storage
New
Type of
solids
processed
Total
Solids
Reduction
(percent)
45
Volatile
Solids
Reduction
(percent)
55
50
Unknown
61
72
unknown
Unknown
55
65
56
63
Co-settled 60
and
Thickened
Primary +
WAS
65
Primary +
TWAS +
septage
TWAS
Co-settled
Primary +
WAS +
septage
Co-settled
Primary +
WAS
TWAS
from
MBR
Co-settled
Primary +
WAS
Updated After Scission, 2009
.
Excellent TS and VS reduction values for all sites is shown in Table 2 with exceptional TS and
VS values from the Bowling Green, Marshall and Heart of the Valley WWTPs. These three
plants all process primary and secondary biosolids. Primary solids are more degradable than
secondary solids and are assumed to contribute to the superior performance. The good
performance at all these plants indicates that the “Second Generation” ATAD system
performance should perform well at other treatment plants. The lower TS and VS reduction data
from the Three Rivers WWTP is probably due to the very thin feed solids concentration that
results in a shorter than desired HRT. Much of the thin solids concentration is due to the
relatively high percentage of septage received at the Three Rivers WWTP (approximately 56.8 to
94.6 cu m/d (15,000 – 25,000 gallons/d).
The Marshall ATAD facility operates only two to three days per week with co-settled and
thickened primary and WAS material from a gravity thickener. When the ATAD operating
temperatures drop to below 58 degrees C (135 degrees F), then the staff co-thickens the primary
and WAS in a gravity belt thickener to 5 to 6 percent solids concentration. The Morehead ATAD
facility also experienced ATAD operating temperatures drop to below 51 degrees C (120 degrees
F) when co-settled primary and WAS material to only 3 percent solids concentration. The
Morehead WWTP staff has decided to install a gravity belt thickener to increase the feed solids
concentration and address the operating issue.
The “Second Generation” ATAD systems have the following overall advantages over the
original “First Generation” ATAD processes:

A longer solids retention time (12 to 14 days) compared to the “First Generation” ATAD
process of 5 to 8 days. This allows for better stability and volatile solids reduction,
especially for the more difficult to digest secondary waste activated sludge (WAS) feed
material.

Jet aeration that thoroughly mixes the ATAD process tanks “from the bottom up” and
keeps aerobic conditions rather than aspirating or pumped-venturi systems that tend to
create anaerobic and high odor conditions at times. The mixing system also uses
conventional out-of-basin pumps and compressors with variable-speed drives so that the
mixing energy can be varied to match the actual mixing energy required (See “Second
Generation” Delphos, OH equipment gallery photo in Figure 2).
Figure 2 - Delphos, OH “Second Generation” ATAD Facility Equipment Gallery Photo
Showing Mixing Pumps with Variable Speed Drives

Use of a single reactor with oxidation-reduction potential (ORP) control that matches
oxygen supply to variable process demands and provides a more stable and complete
digestion process with minimal odor generation.

A unique and patented foam Splashcone™ system that controls foam with hydraulic
energy and has no internal moving parts to maintain (see figure 3). The Splashcone is a
cone that is suspended from the ATAD roof. Biosolids are recirculated from the ATAD
reactor and piped to the Splashcones™. The cone disperses the biosolids around it, and
the biosolids “beat down” the foam layer, controlling foam depth.
Figure 3 - Splashcones™ For Foam Control on “Second Generation” ATAD Facility

A patented post digestion nitrification/denitrification reactor (SNDR) that provides
optimum temperature, pH, alkalinity, and aeration conditions for nitrification and
denitrification of processed ATAD biosolids. The cooled biosolids are sprayed down into
the tank headspace, losing heat and dissolving ammonia from the air in the headspace
back to the tank liquid contents. The SNDR decreases ammonia and soluble COD while
reducing the overall dewatering chemical and polymer consumption. The SNDR has been
shown to reduce ammonia concentrations from 800 to 1,500 parts per million (ppm) to
200 to 300 ppm or less, which can minimize sidestream impact on the secondary
treatment process. pH set points are used to track and control the nitrification and
denitrification process by creating an aerobic or anoxic condition, as warranted. In
addition, the SNDR provides additional detention time that improves solids reduction.
The use of a SNDR has shown to improve biosolids dewatering in two ways. First, the
ammonia reduction improves the effectiveness of polymer. Polymer dose is affected by
the ratio of monovalent cations to divalent (or multivalent) cations, with divalent cations
being more desirable. By reducing the concentration of monovalent cations by
denitrification, the water chemistry for polymer effectiveness is improved. Secondly, the
reduction of COD in the SNDR can potentially eliminate the need for the use of a metal
salt coagulation in the dewatering step. It has demonstrated that thermophilic digested
biosolids have high polymer demand for effective dewatering. These high polymer doses
seem to correlate with the biopolymers lysed from the cells during thermophilic
digestion. Biopolymer concentration in biosolids correlates well with the soluble COD
concentration. To reduce polymer dose, the biopolymers have to be reduced or
coagulated. Iron or aluminum salts often are used to coagulate the biopolymers to reduce
polymer dose for dewatering. The SNDR reduces the soluble COD by 65 percent thereby
reducing the biopolymers in the digested biosolids, and reducing the required polymer
dose.
The SNDR can also act as part of the odor control system if the off gas from the ATAD
reactor is piped to the SNDR headspace. The off-gas is directed to the spray from the
Splashcones™ that provide up to a 50 percent reduction in ammonia and humidify the off
gas for further treatment in the biofilter.

A combined water scrubber and two-stage inorganic/organic biofilter BiofiltAer™ odor
control system (see Marshall, MN odor control system photo in Figure 4). High ammonia
concentration, sometimes created during upset conditions if not significantly reduced in
the SNDR, can be further reduced in the water scrubber. The remaining off-gas, typically
no higher in concentration than 300 ppm ammonia is then sent to the biofilter. The
biofilter airflow distribution, temperature, humidity and pH are controlled by controlling
various set points such as influent velocity, ammonia scrubber saturation and air
temperature, and the washing of the media periodically with plant effluent water. The
two-stage biofilter with both inorganic and organic media removes primarily hydrogen
sulfide and other reduced sulfur compounds.
Figure 4 – BiofiltAer™ Odor Control System Showing Water Scrubber and Two-Stage
Biofilter at the “Second Generation” ATAD Facility at Marshall, MN
CONCLUSIONS
The benefits of the ATAD process vary depending on the size, layout, and configuration of the
WWTP. A site-specific WWTP evaluation is recommended that includes both biosolids
stabilization and reuse/disposal issues and costs. Several of the projects that have been retrofitted
with the “Second Generation” process have reported a 60 to 70 percent reduction in biosolids
product hauled from the WWTP site due to both greater mass destruction and drier cake solids
concentration (less water) as compared to conventional aerobic and anaerobic digestion
processes. The “Second Generation” ATAD systems has proven to be an effective biosolids
conditioning process, producing total solids reductions as high as 60 percent and reliably
producing exceptional quality biosolids products.
The ATAD process has been historically most applicable to small- to medium-sized WWTPs.
However, the “Second Generation” ATAD process has been installed in WWTPs up to 1.3 cu
m/s (30 MGD) with combined primary and secondary feed solids and up to 4.4 cu m/s (50 MGD)
with 100 percent secondary feed solids. ATAD makes sense for many communities that have
little available onsite land. A Class “A” product is often necessary to increase access to end user
markets. Some of the municipal clients that have installed either the ”First Generation” and
“Second Generation” ATAD systems give away the biosolids product as topsoil and others sell
the product to topsoil producers and farmers. In a time when stricter regulations, growth, and
higher unit costs are making biosolids disposal/beneficial reuse more difficult, ATAD should be
considered.
ACKNOWLEDGEMENTS
We would like to acknowledge James R. Kirk, Grand Chute-Menasha West Sewerage
Commission, Neenah, Wisconsin and Parker Newbanks and Candance Burbridge, Eagle River
Water and Sanitation District, Vail, Colorado, for collection of the “First Generation” ATAD
performance data from their respective WWTPs. I would also like to acknowledge Jim Scission
for the summary of data on “Second Generation” ATAD facilities that is included as Table 2.
REFERENCES
Scisson, J. (2009) As Good as the Hype: An Overview of the Second Generation ATAD
Performance. Proc. WEF Residuals and Biosolids Specialty Conference 2009, Water
Environment Federation, May 3-6, Portland, OR, USA.