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OPTIMISATION OF THE ADVANCED DIGESTION PLANT AT AVONMOUTH
S. Bungay1,
, L. O’Hara2, M. I. Baloch
3
1 Principal Process Engineer, Monsal Ltd / Director, Helix Environmental Consultancy Ltd
2 Process Scientist, Geneco - Wessex Water Ltd
3 Senior Process Engineer, Jacobs Engineering, 1180 Eskdale Road, Wokingham, RG41 5TU
Corresponding Author Tel: 07796 172670 Email: [email protected]
Abstract
Anaerobic digestion involves a consortium of bacteria, with the degradation of complex
particulate sludge solids being described as a multi-step process of serial and parallel reactions.
Acid phase digestion (APD) or Enzymic Hydrolysis (EH) separates out the hydrolysis and
acidogenesis stages from the methanogenic stage, providing optimal conditions for hydrolysis
and acidification. The different microbial groups have different environmental and nutritional
requirements, and this is the fundamental premise for enzymic hydrolysis or two stage acid
phase digestion. Hydrolysis, acidogenesis, and acetogenesis proceed faster in an acidic
environment, and methanogenesis proceeds faster in a neutral environment. Under these
conditions, hydrolysis is no longer the rate-limiting reaction, and digestion becomes more
efficient.
In 2007, Wessex Water installed an APD plant upstream of six existing conventional Mesophilic
Anaerobic Digesters at their wastewater treatment works at Avonmouth, Bristol to maximise the
generation of renewable energy at the site. At the time of construction it was the largest
advanced digestion plant in the UK.
This paper discusses the integration and optimisation of the APD plant with the methane phase
plant at Avonmouth.
Key Words
Acid Phase Digestion, Methane Phase Digestion, Advanced Anaerobic Digestion, Biological
Hydrolysis, Enzymic Hydrolysis, Two-Phase Digestion.
Introduction
The Sludge Treatment Centre (STC) at Avonmouth treats a mixture of indigenous primary and
secondary sludge, imported liquid municipal sludge, and imported liquid commercial waste. The
STC treats the sludge using Mesophilic Anaerobic Digestion (MAD), before recycling to
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agriculture. The STC has undergone a number of expansions over the last few years. In 2007,
Wessex Water installed an Acid Phase Digestion (APD) plant upstream of six existing
conventional Mesophilic Anaerobic Digesters at the STC at Avonmouth, Bristol to maximise the
generation of renewable energy at the site, and to produce a conventionally treated sludge. At
the time of construction it was the largest advanced digestion plant in the UK.
Anaerobic digestion is unique amongst current treatment technologies in that it stabilises
sludge, reduces volume and odour, and generates biogas that can be used as a renewable
energy source. The flow sheet for a conventional anaerobic digestion plant is shown in Figure 1
below, and is a fair representative of the STC at Avonmouth before the acid phase pre-
treatment stage was installed.
Figure 1: Conventional Anaerobic Digestion
Anaerobic bioreactors generally comprise of four major components; a closed vessel; a mixing
system, a heating system; and a gas-liquid-solids separation system. The first tank is used for
digestion and is heated and mixed. The second tank is usually unheated and used principally for
storage and degassing of the digested sludge. In some installations the secondary digester is
covered and connected to the biogas system. The terminology when describing digesters varies
between America, Europe, and the UK. The flow sheet shown in Figure 1 is described in Metcalf
& Eddy (2003) as a two-stage digestion plant, where a high-rate digester is coupled in series with
a secondary digester or post-digestion tank. In the UK this flow sheet would be referred to as a
conventional digestion plant; not high-rate; and not always two-stage. For the purposes of this
paper, two-phase or phased digestion is used to describe a process where the digestion process
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is separated into different reactors (or phases) to optimise the process, such as acid phase
digestion, or enzymic hydrolysis.
As shown in Figure 1, Avonmouth is a fairly typical anaerobic digestion plant for treating
municipal sludge arising from wastewater and sewage treatment. The plant comprises of
primary and secondary digesters, biogas storage, and Combined Heat & Power (CHP). Digested
sludge is dewatered and recycled to agriculture. Prior to the APD plant being installed the
dewatered cake was limed to achieve the desired pathogen kill. The primary digesters are
operated at 35oC and utilise a combination of pump and gas mixing. The digesters are heated by
recovering heat from the CHP plant.
Two-Phase Anaerobic Digestion
The performance of anaerobic digestion can be improved by adding advanced pre-treatment
methods. Biological hydrolysis (enzymic or acid phase) using two-phase digestion enables the
hydraulic retention time (HRT) to be reduced; the digesters can be operated with a solids
loading as high as 4-6 kg VS/m3/d; biogas yields are increased; and reliable pathogen inactivation
can be achieved. This paper discusses the optimisation of the two-phase digestion plant at
Avonmouth. Figure 2 shows the configuration of a typical acid phase digestion plant.
Figure 2: Acid Phase Digestion
Phased biological hydrolysis; Acid Phase Digestion or Enzymic Hydrolysis separates out the
hydrolysis and acidogenesis stages from the methanogenic stage, providing optimal conditions
for hydrolysis and acidification. As shown in Figure 2, an additional reactor is installed upstream
of a conventional MAD. The hydraulic retention time of this acid-phase reactor is in the order of
3 days. Technically, Acid Phase Digestion is acid driven hydrolysis, and Enzymic or Enzymatic
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Hydrolysis is enzyme driven hydrolysis. However, sewage sludge involves a consortium of
bacteria, so in the context of anaerobic digestion they are both the separation of the hydrolysis
stage from the acidogenesis and methanogenesis stages, and the terms are almost
interchangeable. The APD plant installed at the Avonmouth STC was a Monsal Enzymic Hydrolyis
(EH) plant. The Monsal Enzymic Hydrolysis process utilises multiple CSTRs in series to harness
the benefits of plug flow batch treatment prevents short-circuiting. An advantage of the Monsal
EH Process is that in using multiple tanks a hydrolysis profile across the reactors develops
making the configuration more robust when treating variable sludge loads. The flow sheet for
Enzymic Hydrolysis is shown in Figure 3 below.
Figure 3: Avonmouth Acid Phase Digestion Plant
The APD plant at Avonmouth utilises six serial reactor vessels, with an overall retention time of
2-3 days upstream of MAD. The plant was designed to operate as a mesophilic system at 42oC
for optimum enzyme activity. Each APD vessel is mixed using gas mixing, and sludge is moved
through the plant in a reverse cascaded batch, via high and low-level gas lifts. Electrical energy is
generated via 5 no. biogas powered CHPs, and heat recovered from the CHP plant is used to
provide all the heating requirements of the process stream.
Avonmouth Sludge Treatment Centre
The Sludge Treatment Centre has undergone a number of expansions over the last few years;
before and after the installation of the APD. Sludge from the various locations is treated in two
process streams; Stream 1 (MAD1); and Stream 2 (MAD2). It is MAD1 that includes the APD
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plant upstream of six anaerobic digesters, treating up to 84tds/d to conventional treated
standards, and generating renewable energy via biogas. MAD2 consists of four conventional
anaerobic digesters, with the utilisation of MAD2 being a recent upgrade to the site. Four
redundant secondary digesters have been converted to primary digesters and now treat up to
33tds/d. Currently the site has the capacity to treat up to 84tds/d in MAD1, and 33 tds/d in
MAD2, giving a combined capacity of 117tds/d.
The STC is operated such that MAD1 takes priority over MAD2, with MAD1 treating a consistent
daily sludge load, and MAD2 treating the excess sludge not treated in MAD1, as MAD2 still
requires additional treatment using lime to achieve a conventionally treated sludge. However,
Wessex Water are currently reviewing the option to upgrade the STC such that all sludge can be
treated in the APD pre-treatment plant prior to anaerobic digestion so that additional treatment
is not required to achieve the desired pathogen destruction.
In practice MAD1 is fed 1,400m3/d at approximately 5.5 % d.s. (% w/w) giving an actual average
daily load of 77tds/d, and MAD2 treats 10 to 30tds/d at approximately 5.5 to 6% d.s. (w/w)
depending whether there is sludge available in the catchment. Therefore, the actual catchment
load is in the range 87 to 104tds/d. Table 1 below shows a comparison between the two
streams.
Table 1: MAD1 & MAD2 Comparison
MAD1 MAD2
No. of Digesters 6 4
Digester Vol (m3) 2,700 2,200
Digestion Volume (m3) 16,200 8,800
Feed Flow (m3/d) 1,400 500
Dry Solids (% d.s.) 5.5 6.0
Dry Solids (kg/d) 77,000 30,000
VM (%) 75 75
Volatile Solids (kg/d) 57,750 22,500
VS Load (kg VS/m3) 3.56 2.56
HRT (days) 11.57 17.60
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Stream 1 (MAD1)
MAD1 comprises of an APD Thickener Feed Tank; three Ashbrook Simon-Hartley gravity belt
thickeners; an APD Buffer Tank; six APD Reactors; six MPD reactors (Digs 5 to 10); a Post-
digestion Storage Tank; an External Buffer Tank; and three Centrifuges.
Stream 2 (MAD2)
MAD2 comprises of a Bellmer Thickener Feed Tank; two Bellmer gravity belt thickeners; a
Common Sludge Tank; four conventional MADs (Digs 1 to 4); a Post-digestion Storage Tank; a
Digested Sludge Break Tank; and three Baker Hughes Centrifuges. The dewatered cake is treated
in a Euroby Belt Drier. There is a facility to transfer the sludge from either stream (MAD1 or
MAD2) to the existing liming plant. Because the refurbishment of MAD2 is fairly recent, this
paper does not cover the performance and operation of Stream 2, but just the APD & MPD in
Stream 1.
Avonmouth APD & MPD Digestion Plant
The Monsal Enzymic Hydrolysis plant at Avonmouth was commissioned in October 2007. The EH
plant has become known as the APD, and existing MADs have become known as the methane
phase digesters (MPDs). The main drivers behind the APD installation were to maximise the
biogas production in order to generate more electrical power, and to achieve a conventionally
treated sludge to reduce the cost of liming the dewatered cake.
The APD plant is shown in Figure 4 below. At the time of construction, Avonmouth was the
largest advanced anaerobic digestion plant in the UK. The HRT in the APD is 3-days at maximum
flow, and the HRT of the MPD is 12-days, giving a combined minimum HRT of 15-days.
APD Process Description
Primary sludge gravitates to an internal pumping station sludge pit, and is pumped to the STC.
Secondary sludge is thickened and is either pumped to the STC; or gravitates to an internal
pumping station foul pit, and pumped to the primary tanks for co-settlement. If co-settled, the
secondary sludge is mixed with the primary sludge.
The imported liquid municipal sludge is discharged into the internal pumping station sludge pit
and mixed with the indigenous primary sludge. Liquid commercial waste is discharged into the
head of the main wastewater treatment works. Therefore, sludge arising from the commercial
waste stream will manifest itself in admixture with the indigenous primary and secondary
sludge.
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The APD is fed from an APD buffer tank. This tank receives thickened primary sludge; thickened
co-settled sludge, and thickened SAS. The APD consists of six completely mixed stirred tank
reactors (CSTRs) in series to approximate a plug flow reactor. All six reactors can be operated
within the mesophilic range between 32˚C and 42˚C. The reactors operate in a reverse cascade
batch system with 24 batches per day. This method of operation prevents short circuiting and
allows individual biological environments to be established in each reactor. The mesophilic stage
of APD commences the hydrolysis element of the plant. The first reactor vessel (APD1) is heated
by an external heat exchanger to (nominally) 42˚C and the hydrolysing sludge is batched in a
reverse cascade once per hour from APD5 to APD6, then APD4 to APD5 and so on to APD1 to
APD2. Following this final transfer a fresh batch of sludge is then fed to APD1 from APD buffer
tank. This batching process is split into hourly intervals with an approximate batch transfer of
58.3m3 at peak flow.
Figure 4: Avonmouth APD Plant
The reactors operate between a top sludge level and bottom sludge level of approximately
11.8m and 10.8m respectively. The difference in height between the top sludge level (TSL) and
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bottom sludge level (BSL) equates to the hourly batch flow volume. In effect each reactor acts in
a similar way to a sequential batch reactor. The system basically comprises of 7 sequences:
1. A batch of sludge is pumped from AP6 reducing the level from TSL to BSL. The pump rate
is such that it will take approximately 40 minutes to draw APD6 down from TSL to BSL,
providing an (almost) continuous feed to the digesters. This sequence is carried out in
parallel to Sequences 3, 4, 5, 6 & 7
2. A batch of Sludge is transferred from APD5 to APD6
3. A batch of Sludge is transferred from APD4 to APD5
4. A batch of Sludge is transferred from APD3 to APD4
5. A batch of Sludge is transferred from APD2 to APD3
6. A batch of Sludge is transferred from APD1 to APD2. This operation is only carried out if
APD1 is operating at set point temperature
7. A fresh batch of sludge is fed to APD1 from APD buffer tank and heated to the set point
temperature (32˚C to 42˚C) by the Stage 1 heat exchanger.
The system waits for the end of a 1 hour cycle time and repeats.
Performance & Optimisation
The initial commissioning of the APD plant was successful with the APD & MPD plant achieving
on average 52% volatile solids reduction (maximum 56%) during the months between February
2008 to July 2008, with an average and maximum gas production of 390 and 430 m3/tds fed
respectively. The feed rate to APD during this period was 78tds/d. However, after about 10-
months operation a number of operational problems were encountered. To remedy this a
programme of extensive investigation was undertaken to overcome these problems and to
subsequently optimise the performance of the plant. The operational problems were two-fold;
firstly there was a problem with the control of the volume of sludge fed from the APD to the
MPDs; and secondly there was a problem with a stable foam or mousse was forming in the
MPDs, which was restricting the volumetric throughput of the STC.
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APD Flow Control
The throughput of the APD is controlled via a Human Machine Interface (HMI) manually
inputted value. The target daily throughput is entered to the APD HMI, and the software
converts this figure to a batch volume, and sets the fill level in APD1 depending on the flow. As
the inputted flow goes up, the fill level of APD1 goes up accordingly until a top sludge level is
reached, at which point the APD is at its maximum hydraulic throughput. This bottom-up control
logic is exactly the same to that used in many wastewater treatment sequencing batch reactors.
At Avonmouth, there is a flow meter installed between the APD and MPD to measure the flow
pumped to the MPD. This flow meter would monitor the flow over 24-hours and a daily
adjustment would be made to the target level set-point in APD1 to compensate for any gain or
drift in the measured level in APD1. However, there was never parity between the calculated
flow based on level change in APD1, and the measured flow at the flow meter. The result of this
was that the flow pumped to the MPDs was always hunting around the set-point, and consistent
flows forward could not be guaranteed. After investigating the levels in the individual APD
reactors, it was discovered that there was actually a gain in volume of approximately 3.5 to
5.0%, across the six APD reactors as hydrolysis was starting, and the sludge was starting to gas,
and was becoming less dense. This phenomena is not unique to enzymic hydrolysis; a similar
phenomena is observed with the aeration of activated sludge. Figure 5(a) to 5(f) show the
individual APD levels and Figure 6 shows gain in volume across the APD process.
Figure 5: (a) (b)
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Figure 5: (c) (d)
Figure 5: (e) (f)
Figure 6: APD Sludge Volume Increase
The problem was that the increase in volume was not consistent, so a simple offset could not be
used in the flow trimming calculation. To overcome this, the daily adjustment calculation was
simply removed, and the flow meter between the APD and MPD was used for flow
measurement only, and not flow control. Once this feature had been disabled, the flow through
the APD stabilised, and the daily flow targets were achieved.
APD & MPD Foam Control
The installation of the APD upstream of the six existing digesters facilitated an increase in the
hydraulic and organic loading of Stream 1, improved the gas yield of the system, and produces a
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treated sludge suitable for recycling to agriculture. Therefore it was essential for Wessex Water
to overcome the operational problems and maintain reliable and robust treatment at all times.
Extensive investigations were initiated across APD and MPD plant. Initially anti-foam was used
to control the mousse, and a strategy was put in place to investigate the organic loading rate,
the hydraulic loading rate, the mixing regime, operational temperature, and any potential
inhibitory inputs to the STC. It was very difficult to establish any positive relationship between
any analytical sampling data, and the onset of the formation of foam in the MPD. The plant was
monitored for pH, total VFAs, individual VFAs, alkalinity, acid:alkalinity, dry solids, volatile solids,
and temperature. Results of the analysis are shown in Figure 7 below.
Figure 7: (a) pH (b) VFAs
(c) Alkalinity (d) TSS
(e) VSS (f) VSS & VSR
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(g) DS Feed v VSR (h) DS, VSR, Biogas
(i) VSR, Biogas (j) VFAs v Temp
The key to the solution was to try and find a relationship between the analytical data and the
operation and performance of the plant. As well as sampling the APD and MPD, the mixing,
feeding, and heating of stages were monitored and adjusted; microscopic analysis of the MPD
was undertaken; batch biogas testing was undertaken; and the liquid commercial waste treated
at the STC was scrutinised.
(k) MPD Alkalinity (l) MPD Acid:Alkalinity
Figure 7: Operating data and analytical parameters of APD and MPD
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The sample analysis was relatively inconclusive. The formation of the mousse in the digesters
coincided with rising VFA concentrations in the MPD. So the trigger in the elevated VFAs had to
be found. In accordance with the normal response to rising VFAs in a digester, the hydraulic and
organic loading to the digesters was reduced. At maximum flow to the APD, the HRT in the
MPDs assuming perfect mixing was actually under 12 days. Therefore, it was considered that the
organic and hydraulic loading to the MPDs may be the limiting factor in the process, so they
should be reduced in order for the digesters to recover. However, in practice, this actually made
the situation worse, and the digesters became more unstable.
MPD Mixing & Feeding Regime
During the investigations various changes were made to the mixing and feeding regime of the
digesters. The design of the MPDs is such that a variety of operational changes can be made to
them in order to optimise their performance. First of all the MPDs can be fed either direct from
the APD, or fed into the existing heating recirculation loop. Then via either route, the feed can
be directed to the surface, the middle, or the base of the digester. The MPDs are operated as
fill-and-spill reactors, and digested sludge spilling from the MPD can be drawn from near the
surface, the middle, or the base of the digester. Each digester then has two mixing systems; a
pumped system turning over the digester contents, and a gas mixing system using draught
tubes. The mixing systems are operated independently of each other, and independently of the
feeding regime. Both mixing systems operate on a run and dwell time. Details of the individual
MPD feeding and mixing configurations are illustrated in Figure 8 below.
The six MPDs are identified on-site as Digs 5, 6, 7, 8, 9, and 10. They are fed in a sequential cycle
based on a batch feed and daily volume. During the period that hydrolysed sludge is being
pumped from APD6 to the MPD, a batch volume is fed to the first digester, the second digester,
and so on, in a cycle until the APD batch has been transferred. Changes were made to the
feeding and mixing regime to try and maximise the effective volume of the digesters. Initially the
target was to mix all MPDs constantly, feed the fresh hydrolysed sludge into the recirculation
line, and to feed into the middle of the digesters to avoid ‘hot’ sludge being pumped on top of
the digester. This was to prevent a layer of VFA rich sludge accumulating at the surface of the
digester, as it was suspected that this might be the cause of the foam or mousse.
Varying the run and dwell times of the mixing didn’t have any marked affect on the situation;
either improving or exacerbating it. Whereas, re-directing the fresh ‘hot’ sludge away from the
surface of the MPD actually made the situation worse. The benefit of the surface feed was that
each MPD was fed in four locations, and the action of the fresh sludge hitting the surface
actually helped to break up any surface foam.
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Figure 8: Primary Digester Schematic
So although it was felt that operating the MPDs at an HRT of less than 12-days might have been
the cause of the formation of the foam, ultimately, varying the operation of the MPDs made
very little difference to the overall performance of the STC.
APD Retention Time, Temperature and Microbial Reactions
The APD plant was designed to maximise the performance of the STC in terms of both
maximising the biogas and subsequent renewable energy generated in the plant, and to produce
a conventionally treated sludge product. The initial development of the Enzymic Hydrolysis
process by United Utilities demonstrated maximum enzyme activity at 42oC. This operating
temperature was the basis of the APD plant at Avonmouth. With regard hydraulic retention time
(HRT), the APD plant was designed to give an overall system HRT of 15 days. As the MPD HRT
was actually just under 12 days, the HRT of the APD was designed at 3 days to achieve the
overall target of 15 days. This meant that the HRT in the hydrolysis stage was longer than the
normal design standard.
Analysing the data, although there were no clear relationships between results and the onset of
foam formation other than the elevated VFA levels, there was inference that there might be a
relationship between the dry solids feed, and the VFA instability. It was surmised that at higher
MPD Feed
Recirc Pumps
HEX
Outlet Box
Gas Mixing
Digester Feed
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dry solids loading rates, hydrolysis was becoming more efficient, and consequently the APD
plant was actually working too efficiently, and that hydrolysis may be proceeding as far as
acteogenesis. From this it was postulated that the effectiveness of the APD could be controlled
by varying the operational temperature and HRT of plant.
Where there is process instability, there must be a breakdown somewhere in the chain of
biochemical reactions during digestion.. There are a number of microbial groups involved in
digestion:
1. Hydrolytic Bacteria
Produce extra-cellular enzymes to break down carbohydrates, proteins, and lipids.
2. Acid Forming Bacteria
The break down products of hydrolysis is fermented to acetate, propionate, butyrate,
and hydrogen by the acid-forming bacteria. There are two-groups of acid-forming
bacteria.
i. Acidogenic Bacteria
Acidogenic bacteria metabolise amino acids and sugars to the intermediary
products acetate, hydrogen, and carbon dioxide.
ii. Acetogenic Bacteria
Two distinct groups of acetogenic bacteria can be distinguished on the basis of
their metabolism. The first group, the obligate hydrogen producing acetogens
(OHPA) produce acetic acid, carbon dioxide (CO2) and hydrogen (H2) from the
major fatty acid intermediates (proprionate and butyrate), alcohols and other
higher fatty acids (valerate, isovalerate, stearate, palmitate, and myristate). The
second group are homoacetogens catalyzing the formation of acetate from
hydrogen and carbon dioxide.
3. Methanogenic Archea
Methanogens are divided into two groups.
i. Acetoclastic Methanogens
Which cleave acetic acid into methane and carbon dioxide
ii. Hydrogen Utilising Methanogens
Which utilise hydrogen and carbon dioxide to produce methane.
In simplistic terms, as one group of bacteria produce soluble compounds, they are quickly
degraded as the substrate by another group of bacteria, giving rise to an anaerobic food chain.
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This chain starts with carbohydrates, proteins, and lipids, and ends with methane, carbon
dioxide, and water. The difficulty begins with the interaction of the different microbial groups
which have different environmental and nutritional requirements. This is the fundamental
premise for acid-phase digestion. The theory is that hydrolysis, acidogenesis, and acetogenesis
proceed faster in an acidic environment, and that methanogenesis proceeds faster in a neutral
environment. Therefore, hydrolysis is no longer the rate-limiting reaction, and digestion
becomes more efficient, and digester can be smaller.
The separation of the hydrolysis and acid forming stages from methanogenesis must not
compromise the obligate syntrophy between the acetogens and the methanogens. This relates
to the production and consumption of hydrogen. Acetogens are hydrogen producers, and
methanogens are hydrogen consumers. Hydrogen is central to the production of acetic acid as
the major end product of acidogenesis. However, when the partial pressure of H2 is high, the
reactions leading from long chain fatty acids, volatile acids, amino acids and carbohydrates to
acetic acid will not proceed, and instead fermentation will occur. The bacteria that produce H2
are obligately linked to the methanogens that use it. Only when the methanogens continually
remove H2 by forming methane will the H2 partial pressure be kept low enough to allow
production of acetic acid and H2 as the end products of acidogenesis.
The OHPAs produce acetate and hydrogen. However hydrogen inhibits the activity of these very
same OHPAs. So for the biochemical pathway to follow acidogenesis instead of fermentation,
hydrogen has to be consumed at the same rate it is being produced, i.e., the methanogens have
to consume hydrogen as fast as the acetogens produce it. If acidogenesis is occurring in an
upstream reactor, H2 will accumulate and inhibition of the acetogens will occur.
So theoretically, if the retention time in the acid phase is too long, it is possible that anaerobic
digestion could proceed too far and intermittent inhibition of the acetogens occurs as described
above. This would drive some of the biochemical reactions to fermentation. However, it is very
unlikely that hydrogen inhibition actually occurs in practice, as the pH in both the acid stage and
the methane stage would be driven down, and digester souring would occur. Whereas, acid
phase digestion generates very high levels of alkalinity which will buffer any severe pH changes,
and in practice, the operating pH in the MPD will actually be higher than traditional mesophilic
anaerobic digesters.
Although, inhibition of acetogens is unlikely to occur, unstable anaerobic digestion could occur
with acid phase digestion. If the hydraulic retention time of the acid stage too long, transient
acteogenesis may occur i.e. the reactions in the acid phase proceed as far to allow intermittent
growth of acidogenic and acetogenic bacteria, resulting in unstable digestion. Therefore, two-
phase digestion should be operated such that it forces acetogenesis into the MPDs to prevent
this. This is achieved by keeping the retention time shorter than the doubling time of the
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acetogens, and if the feed sludge hydrolyses very rapidly, reducing the operating temperature to
increase the reaction time of hydrolysis.
The APD plant at Avonmouth is six vessels in series, and it was impractical to take one vessel out
of the flow sheet to try reducing the overall HRT. Therefore, instead of reducing the HRT, the
operational temperature was reduced, to reduce the biological activity within the APD, and to
force acetogenesis into the MPDs. The operational temperature of the APD was reduced to
33oC. In doing this, it actually improved the overall heat balance of the STC, as the sludge to be
treated no longer had to be raised to 42oC. The MPDs now required heating from 33
oC to 35
oC,
but this is easily achieved using the existing hot water ring main and heat exchangers.
Following the reduction of the operational temperature in the APD, the performance of the APD
and MPD stabilised, the foam or mousse in the MPDs disappeared, and the STC could
consistently treat 1,400m3/d. Figure 9 (a, b, and c) show the performance of the APD from
commissioning to the present day. The optimisation of the process took place during the period
June 2008 to January 2009, and throughout that period a lot has been learnt regarding the
hydraulic and organic loading of both the APD and MPD plant.
Figure 9: (a) APD Alkalinity
Although optimal enzyme production occurs at 42oC, this optimisation process at Avonmouth
has demonstrated that the operational temperature is site specific and ideally the APD plant
should be operated at the minimum temperature required for volatile fatty acid production
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(VFA) and stressing pathogen indicator organisms. So now the APD plant is operated at 33oC,
and the MPD plant is operated at 35oC. This has not affected the overall performance of the
plant with regard to volatile solids destruction, biogas production, or pathogen reduction.
In an effort to ‘sweat the asset’, Wessex Water are always looking to push the process harder.
Operating the APD at 33oC has given them a good benchmark to do this from. Wessex Water is
always looking to increase the throughput of the plant. Currently the average dry solids in the
feed to the APD is 5.5% d.s. w/w corresponding to an average daily solids load of 77tds/d. From
this average the APD can tolerate peaks up to 84tds/d, but if the peak is prolonged for a period
of time, the plants starts to suffer from elevated VFA levels again.
(b) APD pH
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(c) APD Volatile Acids
Figure 9: APD Alkalinity, pH and Volatile Acids
So at Avonmouth, given the reactivity of the sludge, the optimum operating temperature of the
APD is 33oC, and given a hydraulic throughput of 1,400m
3/d, and the HRT of the MPDs, the
optimum dry solids feeding the APD is 5 to 6% dry solids.
There is scope to combine the Streams MAD1 and MAD2. By prorating the sludge between the
two streams, the performance of MAD1 could improve further, as the VS load and the HRT are
adjusted back to within the normal design range for anaerobic digesters. The HRT of the APD
plant would be reduced so it could be fed with a higher dry solids feed, and it would also give
Wessex Water the facility to take one digesters out-of-service for routine maintenance.
Conclusions
• The optimisation process of Acid Phase Digestion (enzymic hydrolysis, biological
hydrolysis, or acid-phase) at the STC at Avonmouth, Bristol, has provided a useful insight
on the operation of APD and MPD process.
• The volume of the sludge fed to the APD increases by about 5% across the APD reactors.
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• At Avonmouth, changing the mixing and feeding regime on MPDs did not affect the
overall efficacy of the digestion process.
• Two-phase digestion should be operated such that it forces acetogenesis into the
methane phase digestion, to prevent the onset of transient acetogenesis.
• The performance of acid phase digestion plants should be optimised by adjusting the
hydraulic retention time, the operational temperature, and solids inventory to suit the
sludge feedstock on a site-by-site basis.
• The APD plant was optimised at Avonmouth by reducing the operational temperature to
33oC, and restricting the dry solids feed to 5 to 6% d.s. w/w.
Acknowledgements
The opinions expressed in this report are those of the author(s), and do not necessarily reflect
the views of the organisations involved. The author(s) would like to offer sincere thanks to
Wessex Water and Geneco for their co-operation in researching and producing this paper.
References
Bungay. S. R., (2009) Operational Experience of Advanced Anaerobic Digestion. 14th
European
Biosolids Conference. Aqua Enviro, Leeds.
Bungay. S. R., Abdelwahab. M. (2008) Monsal Enzymic Hydrolysis – New Ideas and Lessons
Learnt. 13th
European Biosolids Conference and Workshop. Aqua Enviro, Manchester.
De Lemos Chernicharo. C.A. Biological Wastewater Treatment Series, Volume 4, Anaerobic
Reactors, IWA, 2007.
Leslie Grady. C.P., Daigger. G.T., Lim. H.C. Biological Wastewater Treatment. 2nd Ed, CRC, 1999.
Speece. R.E. Anaerobic Biotechnology – For Industrial Wastewaters. Vanderbilt University, 1996.
Tchnobanoglous. G., Burton. F.L., Stensel. H.D. Wastewater - Engineering Treatment and Reuse.
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