optimisation of the advanced digestion plant at · pdf file 15th european biosolids and...

15 th

European Biosolids and Organic Resources Conference and Exhibition

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Organised by Aqua Enviro Technology Transfer

OPTIMISATION OF THE ADVANCED DIGESTION PLANT AT AVONMOUTH

S. Bungay 1,

, L. O’Hara 2 , 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: stevebungay@monsal.co.uk

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

15 th

European Biosolids and Organic Resources Conference and Exhibition

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Organised by Aqua Enviro Technology Transfer

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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European Biosolids and Organic Resources Conference and Exhibition

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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 35 o C 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/m 3 /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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European Biosolids and Organic Resources Conference and Exhibition

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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 42 o C

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

15 th

European Biosolids and Organic Resources Conference and Exhibition

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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 slud