Industrial Applications of Anaerobic Digestion

76 Pages·2001·0.57 MB·English

by  Matthew Hackett

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Anaerobic Digestion: Industrial Applications
D.Nally, S. Smith, F. Hackett, H. Mooney
Introduction
Anaerobic Digestion may be defined as a waste treatment in which liquor or slurried
organic wastes are decomposed biologically under strictly anaerobic conditions.
(Diaz et al, 1982)

Anaerobic digestion is used worldwide for the treatment of industrial, agricultural and
municipal wastewaters and sludges. Recently, it has also been applied to the
treatment of solid municipal wastes. Anaerobic treatment alone will usually be
insufficient to meet the effluent discharge standards and should therefore be combined
with a post treatment, which in most cases is an aerobic process. The anaerobic
process is often used as the first stage in the treatment of high strength organic wastes,
with the purpose being to reduce COD loads so that conventional aerobic processes
can accommodate the waste.

Anaerobic processes involve converting the organic matter in wastewater to methane
and carbon dioxide through a series of reactions involving a combination of
facultative and obligate anaerobic microorganisms. Although anaerobic digestion
involves a complex consortium of microbes, the biochemical processes as well as the
microbial species can be divided into three main categories. A schematic
representation of the reaction steps is outlined in Figure 1.0, appendix 1.
(Droste et al, 1997)

1. Hydrolysis
Hydrolysis is the breakdown of large complex soluble and insoluble molecules, for
example, starch, fatty acids, proteins and alcohols. The substrates are broken down
through enzymatic hydrolysis to form sugars, alcohols and amino acids.

2. Acetogenisis and Acid Formation
The same microorganisms that perform the hydrolytic reactions carry out the
fermentation through this stage. The end products of hydrolysis are fermented into
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organic acids, other low molecular weight compounds, hydrogen and carbon dioxide.
The primary product of this fermentation is acetic acid. The bacteria responsible for
this stage in the process are known as acetogenic bacteria. Such microbes are quite
resilient, having an optimum pH of 5-6. (Droste et al, 1997).

3. Methanogenisis
Methanogenisis is the production of methane, which is the ultimate product of
anaerobic digestion and occurs via two major routes. The primary route is the
fermentation of the products from the acid forming phase, ie. acetic acid to methane
and carbon dioxide. It has been estimated that approximately 70% of methane
production is derived from acetic acid. Bacteria that utilise the acetic acid are known
as “acetoclastic-acetophylic bacteria” and the overall reaction is as follows:
CH COOH CH + CO
3 4 2

Secondly, some methogens are capable of utilising hydrogen to reduce carbon dioxide
to methane (hydrogenophyilic methanogenisis) with an overall reaction of;
4H + CO CH + 2H O
2 2 4 2

It should be noted that the methane formers are considered the most environmentally
sensitive in a digester population. (Casey et al, 1997). Their rates of metabolism are
lower than that of the acid former and therefore methane production is often the rate-
limiting step in anaerobic digestion. The optimum pH for methanogens is
approximately 7.0 but the activity drops to very low values when the pH falls outside
the range of 6-8.

Thus the fermentation of methane in anaerobic digestion reduces carbon based
pollution of wastewater as expressed by BOD and COD, and at the same time the
process produces useable energy in the form of biogas (CH and CO ).
4 2

Fundamentals
Both physical and chemical variables influence the habitat of the microorganisms in
the reactor. Such variables include:
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• Temperature
It is common knowledge that temperature affects the rate of most reactions, ie. as
temperature increases, so too does the rate of reaction. In all, there are two
optimal temperature ranges in which methane can be produced: 30-40oC
(mesophilic) and 50-60oC (thermophilic). Although methane can be produced at
temperatures of less than 10oC, it is preferable, in order to obtain reasonable
methane levels, to keep temperatures above 20oC. Rates of methane production
double for each 10oC rise in temperature within the mesophylic range.
(Droste et al, 1997)

• pH
The most important process control parameter is pH. As stated previously, the
optimum pH range for all methanogenic bacteria is between 6 and 8, but the
optimum pH for the microorganisms in the reactor as a whole is near 7. As a
result, the lower growth rates of the methanogens require that the process be run
at conditions most favourable to them. As such, the pH required for good
performance and stability in anaerobic systems ranges from 6.5-7.5, although
stable operations have been observed outside this range. The system must also
contain adequate buffering capacity so as to facilitate the production of volatile
acids and carbon dioxide that will dissolve at the operating pressure. As a result,
excess alkalinity or ability to control pH must be present to prevent against the
accumulation of excess volatile acids. The three major chemical sources of
alkalinity are lime, sodium carbonate and sodium bicarbonate.

• Mixing
Mixing is an important factor not only in pH control but also in the maintenance
of uniform environmental conditions. In the absence of adequate mixing,
undesirable microenvironments can develop. As mixing distributes buffering
agents within the reactor, the build up of high concentrations of intermediate
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metabolic products is prevented. Such metabolic products could otherwise inhibit
the activity of methanogenic bacteria.

• Ammonia and Sulphide Control
Inhibition of anaerobic metabolism at high concentrations can occur due to free
ammonia (NH ). Acclimatisation of anaerobes to high ammonia concentrations
3
can be achieved but large fluctuations can obstruct the process. At a high pH,
free ammonia is more toxic than the ammonium ion (NH +). The addition of acid
4
can control elevated levels of pH and ammonia which would otherwise lead to
system failure.

• Nutrient Requirements
Anaerobic digestion requires a number of trace elements such as Nickel and
Cobalt, which have been shown to promote methanogenisis. Excessive amounts
of these trace elements are found in all wastes. The low growth yield of anaerobes
from a given amount of substrate results in lower nutrient requirements compared
to aerobic organisms.

• Solids Retention Time
The most important advancement in the field of anaerobic digestion has been the
recognition of the central role of sludge age or solids retention time (SRT) in
controlling the process. The SRT is the average time that a solid particle stays in
the reactor. In suspended growth processes, the SRT is the same for all solid
particles but in other processes it may differ. In a suspended growth reactor,
without recycling and where the SRT and Hydraulic Retention Time (HRT) are
the same, a severe constraint is placed on the anaerobic process due to the slow
growth rates of the anaerobes and this factor results in the need for very large
reactors. The new anaerobic modifications on the other hand either control the
SRT independently of the HRT, or, by process design, promote the occurrence of
very high SRT’s.
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• Hydraulic Retention Time (HRT)
The HRT has been shown to be the single most important factor affecting the
treatment performance of upflow anaerobic filters. The HRT, which depends on
wastewater characteristics and environmental conditions, must be long enough to
allow metabolism by anaerobic bacteria in digesters. Digesters based on attached
growth have a lower HRT (1-10 days), than those based on dispersed growth (10-
60 days). The retention times of mesophilic and thermophilic digesters range
between25-35 days but can be lower. (Bitton et al, 1994)

The characteristics of recently developed anaerobic treatment processes will now be
examined: (see Figure 2.0, appendix 1)

1. Conventional Anaerobic Treatment
Conventional treatment consists of a well-mixed reactor without solids recycle.
The SRT is equal to the HRT but the reactor must provide a minimum HRT of 10
days at 35oC. Before the 1950’s, anaerobic digesters were frequently not supplied
with mixing systems and neither were they operated at elevated temperatures.
Modern Conventional Systems do however incorporate these features and are
often referred to as high rate digesters when compared to the older (conventional)
systems. Examples of Conventional Anaerobic Processes include septic tanks,
municipal sludge digesters and stirred tank reactors.

2. Floc Based Digesters
a) Contact Processes
To maintain biomas density, sludge recirculation was originally introduced
into anaerobic digestion as the “anaerobic contact process”. These systems are
collectively known as floc based digesters, whereby sludge is recirculated
from a settling tank back into a fully mixed digester with the aim of increasing
the SRT. Suitable HRTs’s range from 1-7 days, depending on the waste
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characteristics. The process was originally developed for the treatment of
biological wastes ie. the food industry.

b) Upflow Anaerobic Sludge Blanket Reactors (UASB’s)
The UASB process was developed in the Netherlands and is the most
commonly used high rate anaerobic process. The reactor relies on the
development of a dense, active sludge mass in the lower portion. The UASB
is reported to be more sensitive than other processes to waste composition,
with startup being more difficult and requiring special considerations to
develop the sludge blanket. (Droste et al, 1997).

3. Fixed Film Reactors
In these reactors, biomass is retained by attachment to an inert support medium
and hence remains in the reactors for times longer than the HRT. (Foster et al,
1987).
a) Upflow Fixed Film Reactors (UFF’s)
UFF reactors are “packed biological reactors”, filled with rocks or plastic
modules that provide a multitude of random channels and a large surface area.
Upflow reactors retain some solids in suspension in the pore spaces and the
treatment is derived from a combination of both suspended and fixed film
growth. (Droste et al, 1997)

b) Downflow Fixed Film Reactors
With downflow reactors, the downflow geometry has the advantage of the
biogas rising against the flow, aiding effective distribution without the
expense or complexity of such a distribution arrangement. (Foster & Wase et
al, 1987).

Both of the afore mentioned result in the rapid rate biomethanogenic
conversion of soluble wastes. Suspended solids concentration of up to 3% can
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be accomodated by the downflow reactor. (Ghywoweth & Isaacson et al,
1987).

4. Expanded and Fluidised Bed Reactors
The fluidised or expanded bed reactor is the most recent innovation in anaerobic
treatment technology. (Droste et al, 1997).
Waste water flows upwards through a sand bed which provides a surface area for
the growth of bacterial biofilms. The flow rate is high enough to obtain an
expanded or fluidised bed. (Bitton et al, 1994).
These processes have been applied to; a) the reduction of BOD and COD in both
weak and strong wastewaters, b) denitrification and most importantly, c) the
pretreatment of industrial wastewater to sewer acceptance standards. (Foster &
Wase et al, 1987).

5. Two-Phase Anaerobic Digestion
This system adapts the two stage nature of anaerobic metabolism. Separate
reactors are designed for acidogenesis and methanogenesis, which means that the
conditions in each reactor can be optimised for either group of microorganisms.
As it is impossible to optimise the conditions for both groups of bacteria, phase
separation reduces the instability of performance caused by fluctuations in feed
stock loading, pH and toxic feed components. (Chywoweth & Isaacson et al,
1987).

Anaerobic metabolism is inherently more unstable than aerobic metabolism and in
general, anaerobic treatment is more unstable than aerobic treatment. However,
aerobic treatment is not without problems, and some of the new anaerobic processes,
particularly the fixed film options, are virtually as stable as conventional aerobic
systems. Anaerobic systems will generally require more time to recover from
poisoning by toxic waste than aerobic systems because of the lower synthesis rates of
the anaerobes. (Droste et al, 1997).
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In general, comparison between aerobic and anaerobic treatment processes should be
undertaken with caution. Each individual case has peculiarities that may make only
certain processes feasible. In many cases, both aerobic and anaerobic should be used
together for optimal treatment. (Droste et al, 1997).

As an individual unit, an anaerobic digestion system displays the following
advantages:
• Low production of biological sludge
• High treatment efficiency
• Low capital costs
• No oxygen requirements
• Methane production (a potential source of fuel)
• Low nutrient requirements
• Low operating costs
(Wheately et al, 1990).

The following report deals with the industrial applications of anaerobic digestion and
will be discussed under the following headings:
1. Anaerobic Digestion in the Treatment of Municipal Waste
2. Anaerobic Digestion Technology in the Food Industry
3. Anaerobic Digestion in the Treatment of Wastewaters from the Pharmaceutical
Industry.
4. Applications of Anaerobic Digestion in the Treatment Wastewaters from the
Chemical Industry.
5. Sources and Uses For Biogas
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