Waste
energy to
Two pressing needs in remote
communities in the developing
world are sanitation and
energy supply. Using the
so-called ‘bush toilet’ can
pass on disease and spoil local streams
used for drinking water, while the lack of
availability of fuel for cooking can prevent
people from eating properly.
A water treatment invention of design
engineer Thomas Fudge (pictured, right)
might be able to tackle both problems
at the same time. The patent-pending
system, called SaniWASE, capitalises on
bacteria that breaks down organic waste
in the process of anaerobic digestion,
producing three phases of matter: biogas
for fuel; grey water that can be used for
irrigation (but not for drinking), and solid
fertiliser pumped out by truck. Compared
to anaerobic digesters, the system is said
to process waste ten times faster, and to
produce biogas with a signi cantly higher
proportion of methane – its combustible
fraction – at fractions of up to 80%.
Fudge’s biological electrochemical
system increases the performance of the
aqueous, anaerobic digestion process
with electricity, delivered by electrodes
inside a 1m reactor vessel that is
capable of processing up to 150 litres of
sludge, or up to 3,000 litres of municipal
wastewater per day, which is said to be
suf cient for processing the toilet waste
of 50 people. Doing so requires about
10W of power, provided by a solar panel.
As the process
relies on the
growth and
establishment
of a colony of
microorganisms,
the reactor can
take from one to three
weeks to start up from an
appropriate local feedstock. The
start-up time depends on the size of the
system and its operating temperature. A
range of 35-38° is said to be ideal for the
anaerobic digestion process; although
tests have shown that it still works at
ambient temperatures as low as 10°,
though lower temperatures reduce biogas
production (more tests are planned). Also,
although the bacteria is not affected by
small amounts of cleaning products, large
amounts of harsh chemicals would kill it.
In its current implementation,
the system, which has no moving
parts, monitors pH, gas ow rate and
composition (particularly the percentage
of methane), as well as the performance
of internal components for maintenance
purposes. The main marker of the organic
content of the out ow water is chemical
oxygen demand – since most organic
compounds can be oxidised (burned) in
air to create CO2. The system cannot
monitor this, since no such real-time
sensor exists; instead, measuring COD
involves a technician analysing a sample
in a process that takes up to 2.5 hours.
monitoring total organic carbon;
however, they cost two to four times
the total price of initial production units
(£5,000; though with economies of scale,
Fudge hopes to get their price down to
£2,600, and eventually below £500).
At the moment, the system uses
a simple gravity feed to bring in the
ef uent; pumps might be required in
some cases. And although wastewater
ef uent has been thin enough to react
well, installations that process greater
quantities of food or animal waste would
require a macerator to chop it up; the
team at WASE is currently doing uid
dynamics research to explore this.
The treatment rate depends on the
amount of organic material in the in uent
liquid. Broadly speaking, the system
can be optimised for two outcomes: to
treat wastewater, or to maximise biogas
generation. In the rst instance, the
system can reduce the waste’s chemical
oxygen demand as low as 125mg/l,
meeting the European standard for
ef uent, though doing so slows down the
system’s processing, and ends up costing
slightly more energy than the system
generates, according to Fudge.
A young designer’s new refi nement of anaerobic
digestion aims to offer off-grid sanitation as well as
high-quality biogas
A more expensive solution
would be to t a sensor
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