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European Biosolids and Organic Resources Conference 15-16 November, Edinburgh, Scotland

www.european-biosolids.com Organised by Aqua Enviro

SLUDGE HYDROLYSIS: COMPARING PERFORMANCE OF BIOLOGICAL &

THERMAL ADVANCED DIGESTION FULL SCALE FACILITIES

Theodoulou, M.1, Bonkoski, N.1, Harrison D.2 and Keutgen H.2 1GE Water & Process Technologies, Canada, 2 GE Water & Process Technologies, United Kingdom

[email protected]

Abstract

Advanced digestion technologies facilitate maximizing the efficiency of existing anaerobic digester assets.

The incoming sludge is pre-treated and conditioned; enabling increased volatile solids destruction and

reduced digester retention times. Feed sludge concentrations can be effectively increased, reducing the

downstream digester volume and allowing flow and load increases in the existing assets.

As part of this study, full scale facilities owned and operated by a UK Water Utility will be evaluated,

representing two types of advanced digestion technologies, Biological hydrolysis and Thermal Hydrolysis.

Through the evaluation it was found that the Renewable Electricity output efficiency of the plants

equipped with these respective technologies was 0.65 – 0.86 MWh/tds for Biological Hydrolysis and

0.55 – 0.63 MWh/tds for Thermal Hydrolysis. It was also seen that the biogas yield efficiency was highest

in the Biological Hydrolysis, however, when digester utilization efficiency (MWh/m3) was considered,

although Biological Hydrolysis has an opportunity to have the highest utilization efficiency the gap

between Biological Hydrolysis and Thermal Hydrolysis was reduced. Thermal Hydrolysis benefits from

the intensity level of the process, namely in digester solids feed rate.

Keywords

Advanced Anaerobic Digestion; Sludge Hydrolysis; Biological Hydrolysis; Thermal Hydrolysis; Biosolids;

Renewable Energy

Introduction

The application of Anaerobic Digestion (AD) in wastewater treatment has been done for many years, with

the primary objective to reduce the amount of solids and to stabilize them such that they can be disposed.

In more recent years, the intent of beneficially using biosolids has been adopted, with biosolids regularly

destined for land application or composting. The main byproduct of anaerobic digestion, biogas, which is

a methane rich fuel, is also now being viewed as a source of renewable energy that can be leveraged.

Biogas can be converted into renewable electricity and heat when supplied as fuel to a Combined Heat

and Power (CHP) system. The enhancement of the anaerobic digestion process, with focus on the

increase of biogas production from sludge is a key to increasing the amount of renewable energy that can

be produced with wastewater primary solids and activated sludge being the feed.

The application of Advanced Anaerobic Digestion technologies to increase the biogas output and/or to

enhance biosolids quality has been applied widespread globally. Some advanced AD processes focus on

pre-treating sludge prior to the digestion process, or enhancing one of the digestion phases. One key

area of focus in this regard is sludge hydrolysis. In the AD process, hydrolysis of sludge is the first phase

of the digestion, and is commonly viewed as the rate limiting step. In the AD process, the hydrolysis

phase occurs based on the activity of the biomass which makes up a digestion process and is also

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impacted by the AD environmental conditions. Solutions providers will provide to plant owners

technologies and operating systems such to accelerate this hydrolysis process and in turn increase the

efficiency of the digestion process. For this evaluation, the impact the Biological and Thermal approach

to hydrolysis will be reviewed.

Overview of the Biological Hydrolysis Technology

Biological Hydrolysis (BH) is designed to be a non-invasive add on solution that can be installed upfront

of existing anaerobic digestion infrastructure to enable wastewater treatment plant owners to maximize

the efficiency of their digester assets. The primary motivators of plants owners to adopt a BH system are:

a) Maximize digester efficiency, allowing for either plant capacity expansion or importing sludges and

other organic wastes to digest in existing anaerobic digestion infrastructure;

b) Enabling repowering of the wastewater treatment plant by increasing biogas production from existing

infrastructure and producing electricity through combined heat and power systems to cover plant parasitic

loads; and

c) Produce an enhanced biosolids product that achieves up to a six log reduction in indicator pathogen

content.

The initial development of Biological Hydrolysis, previously referred to as Enzymatic Hydrolysis, was

performed by United Utilities. The commercialization of the technology was led by Monsal Ltd., now GE

Water & Process Technologies, and the initial product offering was referred to as Monsal 42, which is

depicted in Figure 1 below.

The installation of the Monsal 42 system consists of six serial reactor vessels whereby sludge is heated to

42°C in the first reactor, represented in Figure 1. Thermal energy is typically drawn from a plant hot water

loop, where the heat is provided by either a biogas boiler, or if the plant is equipped with combined heat

and power (CHP), from the CHP engine.

Figure 1: Monsal 42 Process Flow




The reactors operate in a semi-continuous reverse cascade batch system where sludge is batched once

per hour from R6 to the digester, then R5 to R6, R4 to R5 and so on to R1 to R2. Following this final

transfer, a fresh batch of sludge is transferred to R1. With each batch transfer, only a portion of each

reactor vessel is transferred forward. This seeds a new incoming batch with microorganisms and enzymes

that perform the conversion. The total design HRT within the Monsal 42 BH system is approximately 2-3

days. As sludge is only heated in the initial reactor, over the course of progression through the remaining

reactors, the temperature reduces to between 40-45°C prior to entering the methanogenic digester.

Through continuous improvement and expansion of the capability of the BH process, the Monsal 55 process

was developed. The same setup of six serial reactor vessels were used, however the operation of the last

three vessels were altered into a batch hold process.

Figure 2: Monsal 55 Process Flow

Referring to Figure 2 above, reactor vessels RV1 to RV4 operate in a semi-continuous reverse cascade

batch system where sludge is batched once per hour forward from RV3 to RV4, then RV2 to RV3, RV1 to

RV2. Following this final transfer, a fresh batch of sludge is transferred to RV1. With each batch transfer,

only a portion of each reactor vessel is transferred forward. RV1 through RV4 are designed to have a

minimum average hydraulic retention time of 12 hours. The batch transfer continues until RV4 is full. Within

RV4, the sludge temperature is increased to by way of a second stage heat exchanger. The temperature

for the second stage is defined dependent on the sludge solids concentration. Once the elevated

temperature is achieved in RV4, all or some of the contents of RV4 is transferred to either RV5 or RV6,

which are operated in parallel. This transfer occurs over the course of 5 hours.

Following the last reactor vessels, the sludge is transferred to the downstream mesophilic anaerobic

digesters. In the case of the Monsal 55 process, a cooling heat exchanger is used to bring the sludge

temperature down to between 40-45 Celsius.

While each of the reactors are completely mixed, due to the method of batch flow in a reverse cascade

fashion, short circuiting is minimized. This ensures that all sludge entering vessel R1(RV1) spends the




majority of the design HRT within the BH vessels, being fully hydrolyzed and acidified prior to forwarding to

the digester.

The BH process can effectively process sludges up to 10% dry solids. The higher the solids concentration,

the more efficient the BH process is and the greater the impact on efficiency of downstream digesters.

Overview of Thermal Hydrolysis

The concept of Thermal Hydrolysis (TH) was investigated in the 1970’s (Stuckey, 1978), and was first

commercialized in the mid 1990’s by the company Cambi out of Norway. The intent of thermal hydrolysis

is to use high temperature and high pressure to mechanically disrupt cells within primarily, secondary

sludge and in turn hydrolyze it. Achieving this essentially eliminates the need for digester residence time

to perform that hydrolysis step, which is largely recognized as the rate limiting step, in the multi-phase

digestion process (Appels, 2008).

The most commonly applied method of TH is a batch process up front of conventional mesophilic

Anaerobic Digestion. To make the TH process as efficient as possible, pre-dewatering of sludge to

between 14-18% dry solids is required. It has also been seen in practice where pre-screening of sludge

prior to pre-dewatering is implemented with TH.

The dewatered sludge is pumped into the “Pulper” tank, which essentially acts as a pre-heat and staging

tank for the downstream reactor vessel. In a batch process, a fixed volume of sludge is transferred to the

pulper tank into the reactor. High pressure steam is injected into the reactor vessel, to bring up the

temperature and pressure within the vessel to upwards of 165 Celsius and 6 Bar(g) or 90 psig (Abu-Orf,

2010). Once the steam injection is completed, the sludge is held within the vessel for a fixed period

(typically 30 – 40 minutes). Following this hold time, half of the pressure is rapidly released in the reactor

and the steam is recycled back to the pulper to try and recover as much heat as possible. The residual

pressure then transfers the hydrolyzed sludge from the reactor to the downstream flash tank, sometimes

referred to as economizer. Within the flash tank / economizer, any other released steam is circulated

back to the pulper. From the flash tank, dilution water is added to control ammonia levels and to begin

cooling the sludge from the very high exit temperature of the TH. Sludge is then transferred to the

downstream digesters, via recirculation loops, with cooling heat exchangers to bring the temperature

down to 40 -45 oC. Typically feed solids concentrations into downstream mesophilic anaerobic digesters

are between 8-12% dry solids.

Figure 3: Sample Simplified Process Flow for Batch TH Process

PulperFlashTank

HX

Reactor

Dilution WaterDewatered

Solids

8-12% Solids to AD

Fresh Steam




Different variations of the Batch TH process are offered, by multiple companies. In addition, there are

some continuous TH processes offered. These continuous processes are designed to be plug flow, such

to ensure that the sludge being treated is subjected to the same level of time and temperature as would

be seen in a batch process.

Plug Flow (eg. Veolia’s Exelys) thermal hydrolysis is an adaptation of the industry proven batch

process. The evolution in the design revolves around keeping the hydrolysis reactor at a constant

temperature and pressure to reduce the energy needed by heating, cooling, and pressurizing

tanks. Dewatered cake (ranging from 16-25%) is fed into the hydrolysis reactor using a progressive

cavity pump. A second progressive cavity pump runs on the back end of the reactor to maintain the

required pressure in the system. Steam is then directly injected into the sludge downstream of the first

pump to preheat the sludge. Once at temperature and pressure, the sludge enters the hydrolysis

reactor. The hydrolysis reactor is a three pass, shell and tube style vessel where the raw sludge enters

the center of the three pipes. Second and third pass act as a thermal insulator to keep the temperature in

the reactor homogenous. The sludge velocity is low to ensuring that there is ample residence time in the

reactor to meet the time and temperature requirements set by the process. Once the hydrolyzed sludge

leaves the system, a series of heat exchangers recover heat to preheat the influent sludge and preheat

the water used for steam generation. The final hydrolyzed sludge also carries enough heat to the

digesters so they will not need supplemental heat for mesophilic operation.

Evaluation Methodology

The intent of the investigation conducted as part of this paper was to evaluate performance

characteristics of both Biological and Thermal Hydrolysis, not to directly compare plant to plant

performance, but to get a level of performance perspective. Many water utilities have deployed both

types of hydrolysis technologies, as such a selection of plants from one of these utilities was used, and

performance data provided.

For this evaluation, a total of four plants will be compared, two Biological Hydrolysis plants and two

Thermal. Of interest are the following evaluation points:

Key operating parameters for each of the plants, including: Hydrolysis temperature, Sludge Dry

Solids concentration, Digester Hydraulic Retention Time (HRT), and Digester loading.

Overall conversion of sludge to renewable energy. The evaluation of the net MWh electricity

output per tonne of dry solids of sludge fed to the advanced AD system.

Overall Biogas Yield Efficiency, considering not only the MWh/tds, but also the digester HRT to

which those renewable electricity outputs occur.

Digester Utilization Efficiency, which will look at each plants’ effectiveness of using digester

volume to convert into renewable energy.

The creation and consumption of Volatile Fatty Acids (VFAs) both in the hydrolysis process and in

the downstream AD.

Biosolids dewaterability.

Application of both Biological and Thermal Hydrolysis to produce Enhanced Biosolids, namely

Class A Biosolids as per the US Environmental Protection Agency.




Plants Investigated

For this paper, the sludge treatment of four different wastewater treatment plants were evaluated. Two

Biological Hydrolysis and two Thermal Hydrolysis plants were chosen, all owned and operated by the

same water utility. The names of the plants specifically are omitted from this paper. Table 1 below gives

an outline of the four plants evaluated:

Table 1: Evaluated Plants Outline

Plant Hydrolysis Type Average Sludge

Loading (TDS/day)

Sludge Blend

PS:SAS

Digester Capacity

(m3)

BH1 Biological – Monsal 55 90 60% : 40% (est.)

+ Cake Imports

20,470

BH2 Biological – Monsal 55 32 50% : 50% (est.)

+ Liquid Imports

12,000

TH1 Thermal – Batch 36 65% : 35%

+ Imports

7,500

TH2 Thermal – Batch 39 75% : 25%

+ Imports

8,150

All the plants have imported sludge. In the case of plant BH1, the vast majority of imported sludge comes

into the plant as ~25% dry solids cake. BH2 only receives liquid imports at present. On plants TH1 and

TH2, the import sludge is a combination of liquid sludge as well as sludge cake.

Results and Discussion

To interpret observations and results, it is essential to understand certain operational aspects for each of

the plants. While understanding the overall plant performance of producing renewable electricity, the

operational conditions surrounding that production is also important. As such, the key operational

parameters for each of the given plants are shown below in Table 2:

Table 2: Key Plant Operational Parameters

Plant Feed Solids

Concentration

(% Dry Solids)

Digester HRT

(Days)

Estimated Organic

Loading Rate

(Kg VS / m3-day)

BH1 7% 16 3.3

BH2 5% 19 2.0

TH1 9%* 18 3.8

TH2 10%* 19 4.1

*-Feed Solids Concentrations of Thermal Plants are the solids concentration going

into the digester recirculation loops, after dilution water is added




The stated organic loading rates (OLR) are estimated for each plant, assuming that the Volatile Solids

(VS) concentration of the sludge is 75%. The Feed Solids Concentration for the Biological Hydrolysis

plants is the sludge solids concentration entering the hydrolysis process. In BH, the sludge is not diluted

at any point, other than by steam condensation, so while there may be a reduction of solids content into

liquid form throughout the process, the total volume and loading on the digester is not significantly

changed. In the case of Thermal Hydrolysis, the TH plants add about 30% dilution water following the

hydrolysis process to bring the solids concentration down to the stated values.

Renewable Electricity Efficiency – MWh/Tonne Dry Solids Fed

Figure 4 below shows the average quarterly renewable electricity generation (MWh) per tonne of dry

solids (tds) fed into the front of the hydrolysis process from Q1 2014 through Q1 2016. Further Table 3

below shows the overall average for the four plants over this time.

Figure 4: Renewable Electricity Efficiency

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1

Re

new

able

Ele

ctri

city

Eff

icie

ncy

(M

Wh

/td

s)

Operating Quarters (Q1 2014 - Q1 2016)

Plant Renewable Electricity Efficiency (MWh/tds)

BH1

BH2

TH1

TH2




Table 3: Renewable Electricity Efficiency Average per Plant

Plant Average Renewable Electricity

Efficiency (MWh/tds)

BH1 0.86

BH2 0.65

TH1 0.63

TH2 0.55

It is apparent from the data that all plants have some variation to their outputs. This could be partly due

to varying feed amounts and fluctuations of sludge blends between primary and secondary sludge, as

well as potential maintenance occurrences to either hydrolysis or digestion equipment or combined heat

and power units.

The plant with the highest output is plant BH1, a Biological Hydrolysis plant. The output of this plant is

consistently between 0.80 and 1.00 MWh/tds, with an average over the entire observed period of 0.86

MWh/tds. The second plant, BH2, has a lower net output than BH1, with the overall average being 0.65

MWh/tds, however BH2 shows the widest fluctuations of output from 0.6 to 0.95 MWh/tds. There is one

data point that is significantly lower at 0.40 MWh/tds, which depresses the overall average, which if

ignored would result in an average of 0.68 MWh/tds. When comparing the two Biological Hydrolysis

plants, BH1 is fed with a sludge with a higher total dry solids percentage versus BH2. While no studies

were done to verify if feed solids concentration directly improves Biological Hydrolysis efficiency, one

hypothesis could be that the higher solids concentration leads to a higher loading rate in the hydrolysis

reactors, and the biomass present in the reactors are effective at this level and potentially have a higher

net community concentration.

The third plant TH1 shows to average over the entire observed period an average of 0.63 MWh/tds, and

TH2 shows and overall average of 0.55 MWh/tds. It should be noted that plant TH2 does typically utilize

a portion of the biogas in the steam boiler, partially due to an export limit to the grid for electricity. It is

apparent that the output of both Thermal Hydrolysis plants are more consistent throughout the observed

period, however plant TH2, shows a decline starting in Q1 2015.

Biogas Yield Efficiency

The overall conversion of the solids to biogas of each of the respective plants, is not just attributable to

the technology which is applied. Both the digester organic loading rate (OLR) as well as digester

hydraulic retention time (HRT) can affect the conversion efficiency. In order to explore this impact, the

rate of renewable electricity output (MWh/tds) is compared to the digester retention time.

Considering the provided feed solids concentrations given in Table 2 above, total dry solids feed mass

and digester volume, the digester HRT can be estimated. With the HRT for each given data period

known, a Biogas Yield Efficiency (BYE) can be determined. The digester efficiency factor is calculated

using the following equation:

BYE = MWh/TDS

HRTdigester




The BYE for each of the four plants is shown in Figure 5 below:

Figure 5: Biogas Yield Efficiency (BYE)

From Figure 5 above, the plant with the highest BYE is BH1. The efficiency factor is above 0.05 at all

times over the observed period, with the average being 0.055. In comparison, BH2 has a lower BYE,

with the factor around 0.04 during the steadiest period between Q3 2014 and Q3 2015. The sludge feed

solids concentration of BH2 is only at 5%, which has a direct impact on the BYE, as digester volume is

being used up by liquid.

The TH plants have a BYE average of 0.036 and 0.029 over the entire period for TH1 and TH2

respectively. TH2’s plant average is depressed due to a lower value in Q1 2016, which if not considered

would elevate the average to 0.031.

Digester Utilization Efficiency

In addition to Biogas Yield Efficiency, the efficiency of utilization of the installed digester capacity at each

of the respective plants is a consideration. Many wastewater treatment plants which would adopt either

BH or TH technology will have existing anaerobic digestion assets. Based on this, realizing how much

renewable electricity which can be generated by those assets is important to plant owners. For this

consideration, Digester Utilization Efficiency (DUE), which is the generation of renewable electricity per

unit digester volume (MWh/m3) was evaluated.

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0.080

0.090

0.100

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1

BY

E(M

Wh

/td

s/ H

RT

Day

s)

Operating Period (Quarters Q1 2014 - Q1 2016)

Biogas Yield Efficiency (BYE)Q1 2014 - Q1 2016

BH1

BH2

TH1

TH2




Figure 6: Digester Utilization Efficiency (DUE)

As can be seen from Figure 6 above, plant BH1 shows the highest DUE. The average over the observed

period is 0.35 MWh/m3, and consistently above all other plants. The other Biological Hydrolysis plant in

contrast has the lowest DUE. With an average over the entire period of 0.16 MWh/m3, the DUE of this

plant is clearly impacted by the operating condition of the most dilute feed concentration of the four

plants, resulting in the lowest Organic Loading Rate. The two TH plants TH1 and TH2 have a DUE in

between the two BH plants, with the average DUE being 0.29 and 0.26 MWh/m3 for TH1 and TH2

respectively. The TH process is a more intensive process than BH, and the feed solids into the digesters

are higher than either of the BH plants. Even though, the Renewable Energy Efficiency and the BYE for

the TH plants are lower than the BH plants, operating the digesters at a higher solids concentration, and

in turn a higher organic loading rate, improves the utilization efficiency of the digesters.

Volatile Fatty Acid Formation and Consumption

As discussed in the introduction of this paper, the digestion process is multi-phased, to which it is well

known that the hydrolysis step is rate-limiting. Once hydrolysis is achieved, sludge still needs to go through

Acidification prior to being converted into methane rich biogas in Methanogenesis. For the two processes

that are subject to investigation, BH is designed to achieve both the hydrolysis and acidification prior to

mesophilic AD. In the case of TH, it is specifically for hydrolysis, and does not attempt to acidify.

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1

Dig

est

er

Uti

lizat

ion

Eff

icie

ncy

(M

Wh

/m3)

Operating Quarters (Q1 2014 - Q1 2016)

Digester Utilization Efficiency (MWh/m3)

TH1

BH1

BH2

TH2




Table 4 below summarizes the volatile fatty acid (VFA) formation throughout the hydrolysis processes. In

addition, the VFA concentration in the downstream mesophilic AD is indicative as to whether full

consumption of the VFAs created has occurred within the digester retention time.

Table 4: Volatile Fatty Acid formation and Consumption

Plant Feed Sludge VFA

(mg/l)

BH Vessel 1

Discharge VFA

(mg/l)

BH Vessel 3

Discharge/Dig.

Feed VFA (mg/l)

Digester VFA (mg/l)

BH1 3,500 5,194 7,300 200

BH2 1,500 N/A 4,200 200

TH1 N/A N/A N/A 1,150

TH2 N/A N/A N/A 1,175

Of the BH sites, BH2 blends both indigenous and imported sludge in liquid form, and the feed sludge VFA

concentration is representative of the blended feed sludge. BH1 blends import sludge cake at 25% dry

solids with the indigenous liquid sludge. As such it is expected that some anaerobic activity has

commenced on the import cake in travel, causing the high starting VFA concentration. In both BH1 and

BH2 cases, the VFA concentrations are significantly higher after BH vessel 3 than the feed concentration.

Both sites do not measure VFAs across BH vessels 4 through 6 as they are both Monsal 55 sites, and the

batch hold process at thermophilic temperatures has been seen to maintain VFA concentrations, but not

increase them. The digester concentration of VFAs is consistent at both sites averaging around 200 mg/l.

This is indicative that essentially all the VFAs are converted into biogas in the anaerobic digester. For the

two TH plants, VFA concentrations are not recorded either in the TH process or prior to entering the

digesters. Considering the TH process, and its purpose to thermo-mechanically hydrolyze sludge, there is

no enabler in the TH process to acidify. As such, it is assumed that while the sludge entering the digester

has been fully hydrolyzed, retention time in the mesophilic digesters is required to perform the acidification

phase of digestion. The VFA content of the digesters are however tracked, and the VFA content is 1,150

mg/l and 1,175 mg/l for TH1 and TH2 respectively. Based on these values, potentially a longer digester

HRT would aide in fully consuming the VFA’s which are created within the digesters.

Digested Biosolids Dewaterability

Dewatered Biosolids Cake values both in terms of percent dryness and of polymer consumption was not

specifically given for this evaluation. The plant owner however provided commentary as to their

experience with Digested Biosolids Dewaterability. From all the plants discussed, the plant owner stated

that transportation cost of the resultant cake is not a primary driver for dewaterability efficiency

improvements, rather in all cases the Biosolids have a strong value in the marketplace, and can achieve a

premium revenue for the product as cake. As such, from an optimization of biosolids management cost,

the focus is more on polymer consumption and centrifuge run time.

In previous discussions with the plant owner, typical polymer consumptions used are 7.0-7.2 kg / tonne

dry solids (DS). In comparing the dewaterability between the biosolids from the two types of plants, there

is little difference between the two, but perhaps a percentage point higher for the TH plants, with the

Biosolids Cake from a BH plant being around 26% DS, and the TH plants 27% DS. At those DS




contents, the plant owner sees great value in the end product, and does not need to spend more money

on polymer to achieve a higher DS percentage.

Applicability for Enhanced Biosolids / Class A

With the application of advanced anaerobic digestion, in addition to the benefits surrounding increased

biogas production and reduction in required digester retention time, the biosolids product can meet

enhanced specifications. The applications of the two described technologies, Biological Hydrolysis and

Thermal Hydrolysis can be designed and operated to meet the most stringent regulations for biosolids

quality, specifically for pathogen reduction. The US Environmental Protection Agency (EPA), 40 CFR

part 503 rule surrounding the treatment of biosolids is often viewed as the most stringent requirement for

the treatment and subsequent disposal of biosolids. Specifically regarding pathogen reduction, the Part

503 rule outlines very specific requirements to achieve either Class B, or Class A Biosolids. Biosolids

treated through Anaerobic Digestion, Class B, is viewed as the standard level of treatment, and Class A

an enhancement or a premium product.

To achieve Class A biosolids, with respect to pathogen reduction, two things must be achieved: 1) The

treatment of sludge through one of six specified alternatives for Class A pathogen requirements (outlined

in Figure 6 below), and 2) meeting the requirements of vector attraction reduction.

Figure 7: Table 5-1 Summary of Six Alternatives to meet Class A Pathogen Requirements

With respect to the two processes investigated, biological hydrolysis and thermal hydrolysis both are

capable of producing Class A biosolids as per the 40 CFR part 503 rule.




Class A Biosolids from Biological Hydrolysis

The application of Biological Hydrolysis has been implemented with two process flows previously, where

in the Monsal 42 design, all six reactor vessels are operated in series. In the second design of the Monsal

55, the six reactor vessels are operated in two sets of three where the first three operate in series with a

reverse cascade batch flow into the fourth reactor vessel. Reactor vessel four then is the staging vessel

for the second step where the temperature of the sludge is raised to an elevated temperature for

pathogen kill. The contents of vessel four is then transferred to either vessel 5 or 6 which operate in

parallel and provide a batch hold at temperature.

Where Biological Hydrolysis is being designed to produce a Class A biosolids, the hold time of minimum 5

hours is done at a temperature to be adequate to meet the Alternative 1: Thermally Treated Biosolids, to

meet Class A pathogen requirements. For sludges that are below 7% dry solids content, Equation 3 of

section 503.32 of the EPA Part 503 rule is applied. For sludges above 7% dry solids content, Equation 2

of section 503.32 is applied.

Figure 8: Biological Hydrolysis Prior to Discharging RV4 to RV6.

The temperature to which sludge is elevated to for the hold process is defined in Table 5 below:

Table 5: Minimum Hold Temperatures

Sludge Solids DS% Hold Temperature Minimum Hold Time EPA 40 CFR Part 503

Calculation

< 7% DS 60⁰C 5 Hours

𝐷 =50,070,000

100.14×𝑡

(Equation 3 of section

503.32)




> 7% DS 63⁰C 5 Hours

𝐷 =131,700,000

100.14×𝑡

(Equation 2 of section

503.32)

Sludge is held within RV5 or RV6 for a minimum of 5 hours at the stated temperature in Table 5. While

either RV5 or RV6 is being held at temperature, the other tank is being filled.

When the batch hold is complete in either RV5 or RV6, the entire contents are transferred to mesophilic

anaerobic digestion. Along the way, the sludge is cooled down to between 40-45⁰C. Each reactor in the

BH system is mixed utilizing unconfined gas recirculation, drawing biogas from the head space of the six

reactors, compressing and injecting it into the base of each reactor sequentially through mixing pipes.

Within the mesophilic digester, a CSTR type digester design is utilized. The hydraulic retention time of

the overall system including the digester is a minimum of 15 days to ensure requirements for vector

attraction reduction are met.

Class A Biosolids from Thermal Hydrolysis

From the described batch TH process above, the sludge is held for a fixed period within the reactor

vessel. Typically, the sludge will be processed at higher than 7% DS, as such to meet alternative 1 in

Figure 7 above, equation 2 of section 503.32 will need to be satisfied. Considering that the reactor

temperature in a Batch TH process is 165 Celsius, the hold time to meet the time/temperature calculation

is minimal and far exceeded under typical TH operating practice.

For plug flow TH processes, it is a little unclear. In principle, the same satisfaction of the

time/temperature calculation is met by the way in which the process is operated, however it is not a batch

hold process, and therefore a verification that there is no ability for short circuiting would be necessary.

For processes, which do not explicitly meet one of the six stated alternatives in Figure 7 above, specific

processes can be reviewed by the EPA’s Pathogen Equivalency Committee to potentially get recognized

under Alternative 6: Biosolids treated in a process equivalent to a PFRP (Process to Further Reduce

Pathogens).

Conclusions

From the analysis performed within this paper, the following conclusions can be made:

1. All four plants produced renewable electricity from the sludge fed, both indigenous and import.

Plant BH1 had the highest renewable electricity efficiency output with typical range of 0.8-1.0

MWh/tds, with an overall average of 0.86 MWh/tds. Plant BH2 had the second highest average

with a range of 0.6-0.8 MWh/tds, however was likely lower than plant BH1 due to more dilute

operating conditions and high SAS content in the blend. The two TH plants showed very

consistent renewable electricity efficiency output, however slightly lower than BH2.

2. When looking at the overall Biogas Yield Efficiency, BH1 is significantly higher than all other

plants with an overall average of 0.055 MWh/tds/days HRT. Again, BH2 was second around

0.04, which would look to be hampered due to the relative dilute sludge. This would indicate that




the operating condition of BH1, in terms on feed solids concentration, digester retention time and

OLR would be close to ideal for the Biological Hydrolysis technology.

3. The TH plants were slightly lower than the BH plants in overall Biogas Yield Efficiency with values

of 0.036 and 0.029-0.031 MWh/tds/days HRT for TH1 and TH2 respectively. The two TH plants

did have a higher OLR than either of the BH plants, which could have been a contributing factor

to the lower efficiency factors.

4. With respect to Digester Utilization Efficiency, plant BH1 shows to have the highest efficiency of

all plants with an average over the entire period of 0.35 MWh/m3. This combined with the high

level of performance both on Renewable Electricity Efficiency (MWh/tds) and Biogas Yield

Efficiency (MWh/tds/ days HRT) would indicate that plant BH1 is close to optimum operating

conditions for the BH technology. The two TH plants showed good Digester Utilization

Efficiencies of 0.29 and 0.26 MWh/m3 for TH1 and TH2 respectively. The benefit is seen based

on the feed solids concentration and resultant organic loading rate. Plant BH2 had the lowest

Digester Utilization Efficiency, evidently impacted by the dilute digester feed.

5. In terms of VFA creation and consumption, it is evident that VFAs are created during the

Biological Hydrolysis process, and that those VFAs created were consumed in the respective

digesters with VFA concentrations in both BH1 and BH2 plant digesters ~ 200 mg/l. With respect

to the TH plants, only VFA concentrations within the digester are tracked, and the values are

1,150 and 1,175 mg/l for TH1 and TH2 respectively. This would indicate that further VFA

consumption is possible, if a longer digester HRT were possible.

6. Both the TH and the BH process are capable of being designed to meet the US EPA 40 CFR part

503 rule for Class A biosolids. The BH plants reviewed however were not originally designed to

meet this requirement, and to meet this regulation, the operating condition of the BH plant would

need follow what was prescribed in the respective section above.

Acknowledgements

The authors of this paper would like to thank Anglian Water for providing information about their operating

facilities as well as context surrounding their operations.

References

Abu-Orf, M. (2010), Conceptual Design of Thermal Hydrolysis Processes for Enhanced Solids Reduction

from Anaerobic Digestion, Powerpoint Presentation.

Appels, L., Baeyens, J., Degreve, J., Dewil, R. (2008), Principles and Potential of Anaerobic Digestion of

Waste-activated Sludge, Progress in Energy and Combustion Science 34, 755-781.

Stuckey, D. C., McCarty, P. L. (1984), The Effect of Thermal Pretreatment on the Anaerobic

Biodegradability and Toxicity of Waste Activated Sludge, Water Res. Vol. 18, No. 11, 1343-1353.


sludge hydrolysis: comparing performance of …€¦ · be seen in a batch process. plug flow (eg....

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