Document 202337

back to basics
Anaerobic treatment: how to boost profits
Anaerobic
digestion is
ideally suited
to high
strength
biodegradable
industrial
effluent
treatment and
is even capable
of treating
chemicals
previously
regarded as
toxic to
biological
systems.
Robert Brookes
explains how
ONE OF the most widespread
applications of biotechnology in
industry is the treatment of
wastewater. This can be done in two
ways: aerobically and anaerobically.
In the main, industry favours aerobic
treatment, despite the fact that
anaerobic treatment can be
considerably more cost effective.
Sometimes, aerobic treatment is done
by the local water company in its own
treatment works and in such cases the
company producing the wastewater
pays for the service according to the
Mogden formula (see Box).
Aerobic treatment is ideal for
wastewater that is low in chemical
oxygen demand (COD). However,
many industrial wastewaters are of
high COD (>2000 mg/l). The water
companies are able to treat this high
COD material simply by diluting it
with the vast quantities of low
strength domestic effluent. As the
concentration of polluting load –
measured as COD and suspended
solids (SS) – increases, so does the
cost of treatment.
To treat high COD wastewater
aerobically on site is also expensive.
This is because of the energy needed
to supply the oxygen required to
maintain the aerobic bacteria and
because of the need to dispose of
large quantities of aerobic sludge.
However, if anaerobic treatment
can be used to achieve 80 to 95%
reduction in the concentration of COD
A modular plant capable of treating 1t/d of COD
Mogden formula
Pence/m3 of wastewater = R + V + (OT/OS)B + (ST/SS)S
where
R
V
OT
OS
=
=
=
=
B =
ST =
SS =
S
=
reception and conveyance charge (23.38 p/m3)*
volumetric charge (15.12 p/m3)*
COD of the wastewater mg/l
sewage COD – representative of the sewage received at the
sewage works (802 mg/l)*
biological charge (24.65 p/m3)*
suspended solids of the wastewater in mg/l
sewage suspended solids – representative of the sewage
received at the sewage works (313 mg/l)*
sludge charge (11.02 p/m3)*
*numerical data is for Wessex Water 2001/2002
in the wastewater going to sewer,
then the cost of discharge can be
vastly reduced, typically by around
two-thirds.
Anaerobic treatment has been in
use for many decades, primarily for
digesting municipal sewage sludge,
where it is used to stabilise and
reduce the volume of surplus
activated sludge arising from aerobic
treatment plants. Since the early
1970s anaerobic treatment has been
used for the treatment of industrial
wastewater. In the same way as
aerobic treatment, it relies on
bacteria digesting biologically
degradable carbon-containing
chemicals, but in the absence of
oxygen. So there is no need for
expensive air blowers. The product of
this degradation is primarily methane,
a valuable by-product, with a small
proportion of carbon dioxide and
hydrogen sulphide (collectively known
as biogas).
There are now more than 2000
engineered systems worldwide
treating industrial wastewater, and
more than 530 digesters in Europe
alone. But the UK lags behind its
continental European neighbours in
the adoption of anaerobic technology,
and the main reasons for this are
probably the frequent failures of
independently designed systems and
the persistent myths and
misconceptions surrounding anaerobic
treatment. They are chiefly: it smells;
it is slow and difficult to start up; and
anaerobic plants take up a lot of room
and cost more than aerobic systems.
All three may have been true in the
days of lagoon systems (large and
odoriferous) or contact systems (large
and slow to start up) but have
become less true with the
introduction of granular sludge
systems such as the UASB (upflow
anaerobic sludge blanket), EGSB
(expanded granular sludge bed) and
IC (internal circulation) systems,
where new plant can be seeded with
sludge from operating systems. Also,
tank-based systems are enclosed and
the generation of odours (often
resulting from the hydrogen sulphide)
should not be a problem. Even
modular systems able to treat as little
as 1 t/day of COD are available (see
photo).
As for size and cost, a common
misconception is that anaerobic
systems are much more expensive
than aerobic systems. This is true if
they are of equal physical size but in
reality a tank-based anaerobic system
will be much smaller than the
equivalent aerobic plant; the
treatment capacity of anaerobic
plants is typically tens times that of
aerobic plant.
Ideally suited to high strength
wastewater, with CODs in excess of
2000 mg/l, an anaerobic plant can be
used to treat effluents arising from a
variety of sources, ranging from sugar
and starch processing, soft drinks
manufacture, brewing and distilling,
through to dairy and cheese
production, vegetable processing and
cannery waste, paper, textile and
pharmaceutical production. It can
even be used to treat formaldehyde –
an infamous biocide – at up to 5000
or as much as 7000 mg/l.
Although subsequent aerobic
treatment prior to discharge of
anaerobic effluent is not necessary,
back to basics
anaerobic treatment
technologies
(top) Figure 1:
Contact digester
and (bottom)
Figure 2: UASB
digester
EGSB Biobed at
British Sugar’s
York factory
concern over possible corrosion of
concrete sewers by dissolved hydrogen
sulphide means that ‘flash aeration’ is
usually provided. (Research has shown
that dissolved methane is unlikely to
present an explosion hazard.)
The optimum temperature for
anaerobic treatment is 37ºC, so a
heating source is needed if the
wastewater is cooler, but many
production processes often have a
source of low-grade heat. The process
will operate at lower temperatures, but
more slowly, thereby necessitating a
larger and more expensive plant.
There are a bewildering number of
digester types available, but only a
limited number of commercially
available designs. The performance or
capacity of biological treatment
systems is measured in terms of organic
loading rate (OLR), in kilograms of COD
removed per cubic metre per day
(kg COD/m3/d). An aerobic system will
typically have an OLR of 1 kg COD/m3/d
while the OLR of anaerobic plants,
albeit varying with the type of plant
and specific wastewater characteristics,
will be anything from 0.6 to 30+
kg COD/m3/d.
The most basic forms of digester are
the septic tank and anaerobic lagoon.
In its simplest form this is merely a
hole in the ground. It has a very low
3
OLR (0.6 to 1 kg COD/m /d).
Production and collection of gas can be
a problem, as can odours.
Variations on the anaerobic lagoon
include the anaerobic baffled reactor,
in the shape of a lagoon divided into a
number of compartments. This has a
high solids retention time and good
resistance to toxic shock. Rates of 20
to 30 kg COD/m3/d have been achieved.
ADI Systems of Canada produces
a low-rate digester (0.3 to
3 kg COD/m3/d) that is essentially a
low-rate upflow sludge blanket process,
as the bulk of the sludge is in one end
of the digester. An advantage of such
large digesters is their ability to accept
high levels of degradable solids and
even fats, oils and greases (FOGs) in
the influent. These stay within the
digester until fully broken down. The
difficulty of roof support is overcome
by using a floating membrane.
If space is not a limiting factor,
then a lagoon-based system can be a
cost effective solution – but beware of
the need to seal the roof adequately.
A relatively common type of tank-based
digester is the contact digester, also
known as the continuous stirred tank
reactor (CSTR), the suspended growth
reactor, or dispersed system. This
digester is shown in Figure 1. Here, the
biomass is kept in suspension by a
stirrer or by recirculation of biogas (or
both).
It has one of the lower loading rates
of tank based systems at 0.6 to 5.3
kg COD/m3/d. However, work by British
Sugar has shown that it is possible to
operate these digesters at up to
9 kg COD/m3/d – twice their design
rate – by increasing the volatile solids
in the digester. Although the digesters
have a high volume and any toxic
chemicals are therefore diluted, the
relatively long hydraulic retention time
does make this type of digester
susceptible to toxic shock. Hydraulic
retention times vary enormously – from
12 hours to 15 days. The long retention
time makes it suitable for handling
relatively high levels of organic solids
and FOGs.
The upflow anaerobic sludge blanket
digester, or UASB (Figure 2), is
increasingly popular for industrial
effluent treatment. Typically it will
have two to three times the treatment
rate of the contact digester for the
same influent and will therefore result
in a correspondingly smaller digester.
The UASB differs from the contact
system in that the biomass forms into
granules of 0.5 to 5 mm diameter, with
a high settling velocity (of up to
50 m/h). As a consequence, a dense
bed containing the majority of the
biomass forms at the bottom of the
digester with a layer of smaller particles
forming a blanket above the bed. This
high sludge density and even
distribution of the influent through the
sludge bed is key to the relatively high
rate of treatment achievable with the
UASB. Rates of 2 to 15 kg COD/m3/d
are common with rates up to
20 kg COD/m3/d on sugar industry
effluent being reported.
The UASB digester is typified by
having a built-in three-phase separator
for gas, biomass and liquid separation.
Although tolerant of solids (up to 2000
mg/l total suspended solids) they are
susceptible to calcium scaling if Ca2+
concentrations above 500 to 600 mg/l
are experienced without the addition of
anti-scalant.
Anaerobic packed beds or filters are
available as upflow and downflow
systems. These are tanks with random
or structured packing within which
(upflow type) or on which (downflow
type) the biomass grows. They should
not be used where calcium scaling
could be a problem.
The hybrid digester is a combination
of the UASB with the three-phase
separator replaced by packing. It has
the advantage of the granular sludge of
the UASB while maintaining good
sludge retention should de-granulation
of the sludge occur (a problem with
certain wastewaters). Loading rates up
to 20 kg COD/m3/d are achievable.
Most confusion over digester types
seems to revolve around fluidised bed
reactors (FBRs), expanded bed digesters
and expanded granular sludge bed
back to basics
(EGSB) digesters.
Simply, FBRs and expanded bed
digesters are fluidised bed systems with
the degree of expansion varying from
10–20% (expanded bed) to 100% (FBR)
and rely on the biomass being attached
to a supporting media such as sand,
foam glass or pumice. The high liquid
velocities invariably necessitate a
recycle of wastewater around the
digester.
Rates for FBRs and expanded beds of
up to 50 kg COD/m3/d are claimed. In
reality there are few FBRs or expanded
bed digesters in use. There are two
FBRs in use in Germany – developed by
BMA in conjunction with the
Braunschweig Sugar Institute and
Zuckerverbund Nord. They were
designed specifically to cope with the
high calcium levels typical of German
sugar factory wastewater. The support
material used is pumice and this
gradually becomes scaled with calcium
carbonate. Once the granules of
pumice, biomass and scale become too
heavy, they are removed through a
special water lock. The BMA FBR is
rated at 50 kg COD/m3/d.
The EGSB – or Biobed (Figure 3) – on
the other hand, is a development of the
FBR by Biothane of the Netherlands.
Finding it difficult to maintain
attachment of the biomass to the
supporting media, it was decided to
remove the media completely. Using
granular sludge from UASBs,
performance was found to be better
than in both the FBR and UASB.
Subsequently a number of Biothane
FBRs have been converted to EGSBs
with rates up to 30 kg COD/m3/d being
achieved. The UK’s first EGSB was
Figure 6: EGSB biobed at British
Sugar’s Wissington factory
installed at British Sugar’s York site in
1996 (see photo on p40). This has
achieved a loading rate in excess of
23 kg COD/m3/d and is able to treat
14 t of COD/day in only 800m3. It is
the subject of a Bio-Wise case study.
British Sugar has since installed a
second, larger EGSB at its Wissington
site (see photo below). The plant,
supplied by Biothane, is designed to
remove up to 42 t/day of COD
(equivalent to serving a human
population of 700,000).
The rise rate in the EGSB ranges
from 3 to 6 m/h with a sludge settling
velocity of 60–80 m/h. This makes the
design more suited than the UASB,
(with a rise rate of about 1 m/h), to
treating wastewater with higher
concentrations of inorganic solids
which pass through, rather than
accumulating in the digester. The high
liquid superficial velocity that is
achievable also makes the design
suited to treating water containing
toxic but biodegradable compounds –
such as formaldehyde, as mentioned
earlier. The high liquid velocity permits
very high rates of recirculation of
anaerobic water, thereby diluting the
offending compound to below the toxic
concentration.
Another variation on the UASB is
the Paques Internal Circulation reactor
(Figure 4). This is essentially two
UASBs mounted vertically, one on top
of the other. The lower UASB is high
loaded while the upper is low loaded.
The influent is pumped into the
bottom of the reactor where it is mixed
with sludge and effluent circulating
from the top of the reactor. The bottom
of the reactor contains an expanded
granular sludge bed where most of the
COD is converted to biogas. This gas is
collected halfway up the reactor where
it creates a gas-lift for treated liquor
and sludge which are carried to the top
of the reactor. Here, the gas is
separated and removed from the
reactor while the liquor and sludge
return to the bottom of the reactor via
a downcomer, creating the internal
recirculation. Effluent from the first
stage rises to the second stage where
residual biodegradable COD is removed.
Biogas, sludge and liquor are separated
in a three-phase separator and effluent
exits the top of the reactor. Rates up
to 31 kg COD/m3/d have been
achieved.
Two Paques IC digesters have
recently been installed to pretreat
industrial wastewater at Anglian
Water’s Lowestoft wastewater treatment
Figure 3: EGSB
Biobed digester
Figure 4: Paques
Internal
Circulation
reactor
works. The key advantage of FBR, EGSB
and Paques IC digesters is the small
footprint of the digester itself,
particularly beneficial where space is at
a premium.
The running costs of anaerobic
systems – excluding any possible
heating of the influent – is generally
much lower than that for the equivalent
aerobic plant, simply because there are
no air blowers consuming expensive
electricity – only pumps and possibly
stirrers – and much lower sludge
disposal costs.
Also, the biogas produced has a
monetary value. The quantity can be
between 0.31 and 0.5 m3 per kg of COD
removed and will typically be 70 to
85% methane. It can be burnt in a gasfired boiler, displacing natural gas,
used as an energy source to provide
some pre-heating of the influent to the
digester, or even to power a gas engine
for onsite electricity production, and is
now classed as a renewable energy
source, attracting tax advantages.
The surplus sludge produced is
typically 5 to 10% of the equivalent
aerobic sludge – and if it is granular
sludge is sometimes a saleable
commodity for seeding new plants.
All things considered, anaerobic
digestion should be taken more
seriously when planning onsite
treatment of wastes. The cost savings
could be enormous. ■
A British Sugar case
study is available at
www.biowise.org.uk
Robert Brookes is a
senior process
engineer at British
Sugar and lectures
on anaerobic
treatment in the
UK and US
`