Design Considerations for a Multiple-Reactor DEMON Process Treating Sludge Liquors

from a Thermal Hydrolysis Anaerobic Digester

Andrew Shaw, Black & Veatch

Peter Thomson, Black & Veatch

Beverley Stinson, AECOM

Salil Kharkar, DC Water

Sudhir Murthy, DC Water

Chris deBarbadillo, DC Water

Nick Passarelli, DC Water

Bernhard Wett, ARA Consult

Geert Nyhuis, Cyklar-Stulz

Blair Wisdom, Black & Veatch

Black & Veatch

8400 Ward Parkway

Kansas City, MO 64114

(913) 980 6318

ABSTRACT

The DEMON treatment process is rapidly becoming an established technology for sidestream

treatment due to its low energy use and no requirement for carbon addition to remove nitrogen

completely through deammonification. A new filtrate treatment facility (FTF) based on the

DEMON process and currently under design for the Blue Plains Advanced Wastewater

Treatment Plant (AWTP) has several unique design aspects and treatment challenges due to its

large scale and the fact that it will treat liquors from a digestion system that includes the CAMBI

thermal hydrolysis process. This paper outlines the design, highlighting the unique features and

the approach taken to overcome design constraints to meet the desired treatment performance,

including consideration of: (1) space constraints, (2) multiple-reactor operation, (3) potential

toxicity, (4) high temperatures, and (5) the aeration/mixing system.

Design of the Blue Plains FTF is due to be completed in 2013.

KEYWORDS

Sidestream treatment, deammonification, DEMON, sequencing batch reactor, SBR, thermal

hydrolysis

INTRODUCTION

The DEMON treatment process is an emerging technology that is gaining rapid acceptance as an

energy-efficient, nitrogen removal option for high-strength nitrogen wastes such as sludge

liquors from dewatering following anaerobic digestion. Since its initial full-scale development at

the Strass WWTP in 2004 there are already more than 12 DEMON plants in operation (Table 1)

and several more under construction around the world.

Table 1: DEMON Plants in Operation

Location Loading

kg N / d

Commissioned

Year

Strass (AT) 600 2004

Glarnerland (CH) 250 2007

Plettenberg (DE) 80 2007

Thun (CH) 400 2008

Gengenbach (DE) 50 2008

Heidelberg (DE) 600 2008

Etappi Oy (FI) 1,000 2009

Balingen (DE) 200 2009

Apeldoorn (NL) 1,900 2009

Limmattal (CH) 250 2010

Zalaegerszeg (HU) 160 2010

Alltech (Serbia) 2,400 2011

The DEMON process makes use of ammonia oxidizing bacteria (AOB) and ANaerobic

AMMonia OXidizing bacteria (Anammox) that, when compared to conventional

nitrification/denitrification processes, require less than half of the oxygen, no carbon addition

and, typically, no supplemental alkalinity. Neethling (2012) describes the considerable benefits

of using processes utilizing anammox bacteria and compares the different process configurations

based on anammox including ANAMMOX, DEMON and AnitaMox. Table 2 lists typical

performance ranges specifically for the DEMON treatment process for high strength ammonia

wastes such as digested sludge liquors.

Table 2: Typical Performance for DEMON

Performance Parameter Units Typical Values

Mass Loading Rates kg N/m3

/day 0.7 – 1.2

Nitrogen Removal % ~90% NH3-N

~85% TN

Energy Use kW hrs/ kg NH3-N

removed

1.0 – 1.3

BLUE PLAINS DEMON DESIGN

A new filtrate treatment facility (FTF) based on the DEMON process and currently under design

for the Blue Plains AWTP has several unique design aspects and treatment challenges due to its

large scale and the fact that it will treat liquors from a digestion system that includes the CAMBI

thermal hydrolysis process. This paper outlines the design, highlighting the unique features and

the approach taken to overcome design constraints to meet the desired treatment performance.

Compact Multiple-Reactor Design

Figure 1 is an aerial view of the Blue Plans AWTP showing the proposed location for the new

DEMPON FTF in orange and the new anaerobic digesters, CAMBI and combined heat and

power (CHP) system indicated in green.

Figure 1: Aerial Photograph of Blue Plains AWTP Showing Proposed Location of the New

DEMON FTF (orange) and New Digesters, CAMBI and CHP (green).

Almost all DEMON facilities are single sequencing batch reactors (SBR) or dual reactors which

operate autonomously (i.e. they have their own individual blowers and feed pumps). However,

the Blue Plains facility will have an estimated filtrate flow of 4,200 m3/d (1.1 mgd) and liquor

ammonia-nitrogen concentrations of up to 3000 mg/L to give a loading up to 12,600 kgN/d

(27,000 ppd). A plant of this size requires multiple reactors and therefore 6 reactors were

selected for Blue Plains. Figure 2 shows the proposed layout of 6 deep filtrate reactors (labeled

FR1-6) using rectangular common-wall construction located in the tight space between existing

treatment stages shown on Figure 1. A maximum water depth of 7.9 meters (26ft) was selected

to make the best use of the available footprint and yet enable conventional blower systems to be

used for aeration. Figure 2 also shows the location of potential future filtrate reactors, feed

tanks, sedimentation tanks for pre-treatment of liquors to remove solids and phosphorus feed

tanks for nutrient addition.

Figure 2: Blue Plains FTF Layout Showing 6 SBRs (Also Showing Potential Future SBR

positions)

Designing for Flexible Multiple-Tank Operation

The feed system for a single DEMON reactor is straightforward and usually consists of a feed

tank and a single variable speed feed pump. However, a more complex system is required for

multiple reactors. For the Blue Plains FTF a feed system was developed which is based on a

single pumped flow loop with valves to each reactor, enabling the reactors to fill whenever they

call for flow. The flow loop is maintained at a constant pressure using a manometric leg with the

elevated top level at atmospheric pressure. This approach replaced an earlier concept which used

a pressure-sustaining valve which a HAZOP identified as being a critical single-point of failure

for the system. The new system has a better intrinsic reliability.

Sizing of the feed system and evaluating the sequencing of the filling and emptying of the

reactors was carried out using the spreadsheet shown in Figure 3. The example shown is for

SBRs operated as pairs, however other operating configurations were checked to determine the

maximum flows possible for the feed system. Unlike typical multi-reactor SBR systems that

share common blowers and fill in sequence, the feed design for the Blue Plains FTF SBRs allows

each reactor to fill "on demand" and individual reactors have their own aeration system which

allows them to operate semi-autonomously. In order to provide this flexibility in feed flow

ranges, a detailed system pump curves were developed and is shown on Figure 4.

The one restriction on the sequencing that was deemed necessary was that no more than two

SBRs can decant simultaneously so that the discharge piping would not have to be massively

oversized.

Figure 3: Spreadsheet Used to Investigate SBR Sequencing. Sequence Timings Entered in

Top-Left Table. Colored Chart at Top Right Shows 5-minute Intervals for SBR

Sequencing. Graph Shows Feed Flow (blue) and Decant Flow (red). Example Set Up Shows

Impact of Operating SBRs in Pairs.

Figure 4: Pump Curves Developed to Maintain an Even Flow in the Reactor Feed Line

Designing for Potential Toxicity

Pilot testing was carried out to test removal rates for sludge liquors from a CAMBI system in

order to determine if the DEMON process would be inhibited by them (Figdore, 2011). It was

found that significant inhibition did occur, most notably to the AOBs, but that diluting the

CAMBI liquors 1:1 with plant water reduced the inhibition to an acceptable level that enabled

the DEMON process to remove ammonia at a volumetric loading rate of 0.6 kgN/m3/d. A

dilution system using plant water (plant effluent) was included in the FTF design to facilitate

this.

Designing for High Temperature

The temperature of the sludge liquor is expected to be in the range of 30 - 35ºC, which is an

acceptable range for the DEMON process; however the slightly exothermic reactions coupled

with high ammonia concentrations may push the reactor temperature above 35ºC. Heat

calculations that included ambient conditions, dilution water temperature, blower air impacts and

other thermal considerations were carried out to determine the potential maximum temperature

for the reactors was 38ºC. This high temperature requires special consideration for the aeration

system design and was one of the factors in the decision to use the Invent aeration system rather

than membrane diffusers. Table 3 shows an example heat balance output for the reactors with

dilution water added in the summer to ensure the reactor temperature does not exceed 38ºC. The

most significant energy input is due to biological activity (327,000 MJ/d) and the major

contributors to reducing the heat energy are atmospheric cooling, addition of filtrate that is

marginally cooler than the reactor and addition of dilution water which is plant water assumed to

be at a summer high temperature of 27 ºC. The magnitude of the heat fluxes is such that cooling

using other means than dilution (e.g. chillers) was impractical.

Table 3: Example Heat Balance Inputs (top) and Calculated Heat Transfer Components

(bottom) for Summer Conditions. Note, negative “heat added” = heat removed from the

system.

Heat Balance Parameter Value

Filtrate Flow 4200 m3/d

Ambient Air Temperature 27⁰C

Filtrate Temperature 35⁰C

Dilution Water Temperature 27⁰C

Reactor Temperature 38⁰C

Required Dilution Water Flow 3400 m3/d

Heat Transfer Components

Total Biological Heat Added to Reactor 327000 MJ/d

Atmospheric Heat Added to Reactor -117000 MJ/d

Mechanical Mixing Heat Added to Reactor 13000 MJ/d

Filtrate Heat Added to Reactor -52000 MJ/d

Process Air Heat Added to Reactor -11000 MJ/d

Dilution Water Heat Added to Reactor -160000 MJ/d

Net Heat Transfer 0 MJ/d

Aerator/Mixer Design

A detailed assessment of different aeration system options was carried out. Ultimately it was

determined that the Invent Mixer/Aerator provided several benefits over a conventional diffused

aerator system, including: combined mixing/aeration functionality which gives good control of

the intermittently aerated SBRs; robust performance at high temperatures; lower maintenance

costs; ease of installation. Figure 5 is a schematic showing the main components of the invent

Mixer/Aerator and Figure 6 is a photograph of an example installation. Figure 7 is the proposed

layout for four 30kW (40hp) mixer/aerators with provision made for a possible fifth unit if

required in the future. In the mixing mode, the units will run at a slow speed, drawing

approximately 6.3kW (8.5 hp); in the aeration mode, air will be provided to the ring sparger

beneath the mixer/aerator by high-speed gearless turbo blowers and the unit will be run at a

higher speed.

Figure 5: Invent Mixer/Aerator Schematic (courtesy Invent)

Figure 6: Example Installation Photograph

for an Invent Mixer/Aerator System

(courtesy Invent)

Figure 7: Proposed Layout of

Invent Mixer/Aerators for the FTF

SUMMARY

The DEMON process is gaining in popularity and has rapidly become an established process

option for sidestream treatment. The DEMON plant proposed for the Blue Plains FTF was the

largest of its kind and the first to treat liquors from a CAMBI thermal hydrolysis system when it

was conceived. This has resulted in some unique design features including consideration of:

1. Space constraints

2. Multiple-reactor operation

3. Potential toxicity

4. High temperatures

5. Aeration/mixing system

Table 3 summarizes the FTF design parameters, noting the constraints and comments described

in more detail in the previous sections of this paper.

The design is due to be completed in 2013 with construction and commissioning starting soon

after. In parallel with the design, pilot testing is being conducted to investigate inhibition and

temperature effects using liquors generated from Blue Plains sludge.

REFERENCES

Figdore, B., Wett, B., Hell, M. and Murthy, S. (2011) “Deammonification of Dewatering

Sidestream from Thermal Hydrolysis-Mesophilic Anaerobic Digestion Process” Proceedings of

WEFTEC 2011

Neethling, J.B. (2012) “Deammonification Compendium” Water Environment Research

Foundation Report, December 2012

Table 3: Design Parameter Summary

Values Constraints/Comments

Flow 4200 m3/d (1.1 mgd) Design flow estimated from sludge

production and limiting ammonia

concentration in digesters to 3000

mg/L using dilution of Cambi treated

sludge.

Reactor Dimensions Number of reactors: 6

Length: 24.38 m (80 ft)

Width: 18.29 m (60ft)

Maximum SWD: 7.93 m (26ft)

Minimum SWD: 6.71 m (22ft)

Small-footprint available, therefore

deep tanks used. Depth limited to

enable normal blowers to be used.

Loading 0.6 kgN/m3.d (0.0375 lb/d/ft

3) Typical loading for DEMON is 1.0

kgN/m3.d, however toxicity concerns

require a reduced loading.

Acclamation may enable higher

loadings to be achieved than current

design values. Further pilot testing to

be conducted to check this.

Temperature Maximum 38°C Dilution water is plant effluent with a

temperature of 27°C in the summer.

Cooling of the reactors to 38°C is

feasible with a reasonable dilution of

up to 2.5 times the influent flow.

Aeration System Invent Mixer/Aerator

4x30kW (40 hp) Units

AOR = 404 kg/h (890 pph)

during aeration cycle

High temperatures and mixing

requirements for granular sludge,

amongst other factors made diffused

aeration less favorable than the Invent

system. Required AOR depends on

aeration period time and overall SBR

sequence timings. Longer cycles =

lower AOR but limits the flow of

filtrate that can be treated.

Feed System Max loop flow: 3600 m3/hr

(16000 gpm)

Minimum loop flow: 110

m3/hr (470 gpm)

The need to be able to feed up to 6

SBRs simultaneously over a wide

range of dilutions creates a wide range

of flows that have to be

accommodated by the feed flow loop

Equalization/Dilution Dilution range for inhibition

1:1

Dilution range for cooling:

2.5:1

Feed tank volume:

(0.58 MG)

2.5 times the influent flow maximum

available for temperature control.

Flows in excess of this would require

multiple decanters in the SBRs to

handle the extra flow.