Or ganic waste management by a smallscale innovative automated system of anaerobic digest ion
Supported by the European Commission under the research for SME associations theme of the 7th Framework Programme for Research and
Technological Development
1 st August 2012 to 31 st July 2015
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 Sensor technology
 Measurement strategy
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 Electronics and communication
 Pneumatics and sampling system
 Consumables
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The sensor module is an electronic system based on different arrays of sensors.
It automatically monitors the liquid biomass being converted and the biogas being produced by the digester, without any intervention of the end user.
The biogas composition is analyzed in terms of CH
4
, CO
2
, H
2
, H
2
S.
The parameters monitored from the digester effluent are the concentration of some ions of interest (Na + , Ca 2+ , K + , NH
4
+ ), the pH, the Oxidation-Reduction
Potential (ORP) and the Temperature.
The VFAs dissolved in the effluent (in particular the Acetic and Propionic
Acid) are analyzed by means of an array of broad selectivity conductive polymer sensors.
 The entire functioning of the systems relies on the cooperation between different arrays of selective and non-selective sensors.
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Chemical composition
1 2 3 .
.
.
.
.
.
j .
.
.
.
N
Chemical species
Selective sensor Non selective sensor
1 2 3 .
.
.
.
.
.
j .
.
.
.
N
Chemical species
𝑉 ≅ 𝑆 𝑗
∙ 𝐶 𝑗
Sensor response
1 2 3 .
.
.
.
.
.
j .
.
.
.
N
Chemical species
𝑁
𝑉 ≅ 𝑆 𝑖
∙ 𝐶 𝑖 𝑖=1
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A non specific sensor is unlikely to be useful when used by itself.
Its response is
correlated to most of the species it is surrounded with, and few can be done to discriminate or to quantify each of them.
The natural olfaction evolved to overcome this limitation. It uses a redundant array of
non specific sensors, able to generate peculiar response patterns to different odours. The olfactory system, then, is trained by experience to associate those patterns to the odours and their
concentrations.
The ORION sensor module uses a similar approach for the detection of the Acetic and
Propionic acid in the digester.
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Array response Different chemical states Array of non specific sensors
Sensitivities
1 2 3 . . . . . . j . . . . N
Sensor N
.
.
.
.
Sensor 3
Sensor 2
Sensor 1
1 2 3 . . . . . . j . . . . N
1 2 3 .
.
.
.
.
.
j .
.
.
. N
Chemical species
1 2 3 . . . . . . j . . . . N
Chemical state 1
Chemica l state 2
Sensors
Sensors
Sensors
Classification Chemica l state 3
Pattern Recognition Engine
(PARC)
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Three different sets of sensor arrays have been integrated in the sensor module.
A first set of commercial Ion selective electrodes monitors pH, ORP and ionic concentration of the effluent resulting from the digestion.
A second set of commercial electrochemical and infrared sensors measures the composition of the produced biogas.
A third set of custom developed conductive polymer sensors responds to the acetic and propionic acid contained in the effluent.
The following table summarises the analytes and the ranges of concentrations that the system can sense.
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Range of ion concentrations
Expected
Expected
Expected
Expected
1,E-06 pH: 0 to 14
(3 to 11, ±0.1)
1,E-05 1,E-04 1,E-03 1,E-02
Concentration of Ions [Mol/L]
ORP: ±2000 mV
1,E-01 1,E+00
T: -50 to 200°C ±0.2°C
10 to 65 °C ±0.1°C
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Range of gaseous compounds
Sensor Range measured
Expected
Sensor Range measured
Expected
Sensor Range measured
Expected
Sensor Range measured
Expected
1E-5 1E-4 1E-3 1E-2
Concentration
1E-1 1E+0
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From a liquid concentration to a gas measurement:
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The VFAs present in the effluent of the digester are in their ionised
form, at its usual pH value. In this form they have a low volatility and a direct measurement with a gas sensor is difficult to achieve because of their small concentration in the gas phase.
The sensor module overcomes this limitation sampling a small aliquot of the effluent from the digester and lowering its pH below the VFAs pKa. In this way the equilibrium changes to the unionised form and the VFAs concentration in the headspace increases.
 The headspace, in equilibrium with the effluent and rich in VFA
concentration, is collected and measured through the VFA sensor array.
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The VFA array is based on a chemoresistive sensor technology.
Different kind of conductive polymers, belonging to the families of polyanilines and polythiophenes, are deposited on an array of 8 interdigital transducers, microfabricated over a flexible plastic substrate.
The resulting array shows a broad and overlapping specificity to many vapors
(alcohols, esters, aromatic compounds, alkanes and carboxylic acids)
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The conductivity of these sensors is strongly related to temperature.
Moreover, the temperature is a key parameter in the adsorption phenomena, intrinsically modulating the sensitivity of a chemical sensor.
To avoid fluctuations due to temperature variability, the sensor array is kept at constant
temperature through a PID controller. The PID maintains the sensor array at ±0.1°C from the
set-point, rejecting ambient temperature drift.
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Debug and Maintenance
USB communication
Control electronics
(Master μC + PARC)
Internet
Control electronics
(Slave μC)
Commercial sensors for effluent and biogas
VFA sensors
Pneumatics
Pumps … Valves … Consumables
Digester effluent and biogas
Tap water line Waste
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200mL of the digester effluent are sampled by a peristaltic pump and fed in a measurement chamber. While the first set of sensors senses the ionic concentration, a headspace generates inside the chamber. When the headspace reaches an equilibrium, the valve V
1 switches from its normally open (NO) to its normally closed (NC) position and the CP sensors start to react to the humidity saturated headspace. Then, the effluent is acidified to increase the volatility of the fatty acids in such a way that their concentration in the headspace is increased and the
CP sensors can start to respond to them. Another pneumatic section handles the measurement of the biogas. The biogas enters the device through the NO input of the valve
V
2
, while the pump P
2 allows the sensors to recover cleaning them with ambient air. Another section of the system performs a recalibration of the Ion Selective Electrodes to maintain a standard degree of accuracy.
The device usually performs a biogas measurement every 2 hours and an effluent measurement 4 times per day.
A recalibration is generally needed once every 1-2 weeks.
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The sensor module operates utilising a few consumables:
 Chemical solution for calibration and deioniser filter

 These consumables are needed in particular to automatically recalibrate the ion selective electrodes from time to time
Low concentration acid solution:
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 used during the measurement of the effluent to increase the volatility of the volatile fatty acids.
The sets of sensor arrays
In the first prototype of the ORION AD integrated machine the amount of the consumables held in the reservoirs is enough to carry out around 100 measurements and 50 recalibrations.
Nonetheless, the ORION AD integrated machine is designed to be able to accommodate much larger quantities of these consumables.
The expected lifetime of the sensors is at least one year of standard operation of the
ORION AD integrated machine.
The deionised water filter should be replaced every two years.
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The end user do not need anything in order to make the system work.
The low level control system can autonomously decide to start one of the three main operation of the sensor module:
 Start a gas measurement:
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To have a measurement of the biogas composition
The required time is about 6-8 minutes
Start a liquid measurement:
 To have a measurement of the digester effluent
 The required time is about 25-30 minutes (max one every 2 hours)
Start a recalibration of the ion selective electrodes:
 The recalibration is needed once every 1-2 weeks
 The required time is about 25 minutes
The low level control system can increase the rate of the measurements if it needs to better follow the dynamics of the digestion.
An increase of the measurement rates means also a bigger consumable usage.
A usual rate is 1 biogas measurement every 2 hours and 4 effluent measurement per day.
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The sensor module connected to the
650L digester prototype
Schematic view of the 3000L prototype with the position of the sensor module
The sensor module needs few
simple connections:
 Pipeline connections to the
biogas, the effluent, and a waste line.
 A connection to tap water line
 An Ethernet connection for communication
 A power supply (240 Vac, max. 60W)
The sensor module is intended as an ORION subsystem, but it can be used as a stand-alone
module or integrated in other
AD situations.
The sensor module is enclosed in a box having dimensions of
50x50x30 cm (LxHxW)
Its final position in the ORION digester is in the fourth head
compartment.
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Simple for the user but intrinsically sophisticated, the automation module controlling the ORION digesters is divided into 2 parts:
This web application running on a Linux server, monitors and centralises the data from all the clients.
The received data are stored in a database and a user interface (HMI) allows scientists and maintenance people to visualise the data from each digester.
The server is also able to detect values out of limits and report warnings.
The server has one internet connection allowing the exchange of data with all the digesters and to provide an interface to the world (biologists, technician, etc.).
This program controls each digester as a standalone system and runs on an industrial PLC.
It is responsible for the regulation of the digester. It continuously acquires the data of all sensors and regulates the digester depending on their values.
It sends all the data to the server at defined intervals.
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The temperature of the digester is provided by two systems: an electrical mantle and a combustion biogas subsystem. The client is also responsible for managing these systems to keep the temperature at optimum level.
A connection with a terminal
(notebook, notepad, etc. or internet remote control) allows technicians to perform maintenance and commissioning of each digester.
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
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A sensor module has been developed, in order to autonomously gather essential information on the dynamics of the digestion process
The system is completely automatic and does not require any operation from the end user
An automation module (client side) is responsible for controlling the digester, taking decisions on the inputs received from the sensors
The server side of the automation module monitors and centralises the data from all the clients
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May you have any questions, please contact: krishna.persaud@manchester.ac.uk
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