however, complications in the measurement techniques that make this a difficult
proposition, especially as the sludge from the primary and secondary treatment
processes moves from a Newtonian to a non-Newtonian fluid (usually around the
5–8% dry solids mark).
The processes within sludge treatment often see sludges that start at 2–3% dry
solids and are thickened to around 8% dry solids. If this is on a sludge treatment
centre, it is more about processing the sludges for use in subsequent treatment steps,
but often this is done on satellite sites to ensure that as small amount of water as
possible is transported via tankers.
Once thickened sludges are produced it becomes very difficult, although not
impossible, to measure what is actually happening. This is especially the case
when sludges are de-watered in a centrifuge process up to dry solids between
23–28%. At this level they resemble a powder. This depends upon what is being
done to the sludges in terms of treatment steps.
The most common treatment step in the UK is mesophilic anaerobic digestion
(MAD) although quite often nowadays there is an Advanced Thermal Digestion step
before MAD to speed up the throughput of the process [29]. In MAD, sludge is kept
in an anaerobic environment normally for upwards of 12 days at 37 Æ 2
C. This
converts a proportion of the sewage sludge to methane gas as it goes through the
anaerobic digestion process. The temperature of the process is very critical, as is the
mixing and the complete absence of oxygen. Thus, temperature monitoring is an
absolutely critical part of the process and is normally carried out at three points
within the digester. However, this is not the only critical factor as the heating loops
for the digester heating have to be maintained and monitored precisely to ensure the
maximum output of the gas quality. Measurement of the gas quality is becoming
more and more common to measure the performance of the overall system.
What is essential for the digestion process is consistency and, in an ideal world, if
the same volume of sludge is fed to the digesters consistently throughout the day ad
infinitum the process would run without error. However, this consistency is not
typical due to problems within treatment processes and general operational challenges that are the normal daily routine within the water industry. Thus checking the
anaerobic environment becomes a priority. For example, when a digester is overfed,
the amount of acids that are produced as part of the digestion process can build, and a
digester will turn acidic. It is in these circumstances where monitoring the pH as well
as the output gas quality can become important.
Taking it back a stage, the whole process can be controlled by a greater level of
monitoring and control at the sludge thickening (or dewatering stage if using thermal
hydrolysis), to get a consistent output in the sludge quality feeding the digesters and
basically processing on an on-demand basis. This level of consistency is where an
automated measurement and control system comes into its own. Increasingly, the
industry is seeing the use of advanced process control on sludge thickening and
dewatering systems using sludge flow measurement into the thickening stage, as
well as the dry solids content. This approach allows a consistency of application of
the polymers used in the treatment process. In advanced systems, the flocculation
stage, where the polymer and sludge are mixed, is optimised. Also, changing the
134
O. Grievson
proposition, especially as the sludge from the primary and secondary treatment
processes moves from a Newtonian to a non-Newtonian fluid (usually around the
5–8% dry solids mark).
The processes within sludge treatment often see sludges that start at 2–3% dry
solids and are thickened to around 8% dry solids. If this is on a sludge treatment
centre, it is more about processing the sludges for use in subsequent treatment steps,
but often this is done on satellite sites to ensure that as small amount of water as
possible is transported via tankers.
Once thickened sludges are produced it becomes very difficult, although not
impossible, to measure what is actually happening. This is especially the case
when sludges are de-watered in a centrifuge process up to dry solids between
23–28%. At this level they resemble a powder. This depends upon what is being
done to the sludges in terms of treatment steps.
The most common treatment step in the UK is mesophilic anaerobic digestion
(MAD) although quite often nowadays there is an Advanced Thermal Digestion step
before MAD to speed up the throughput of the process [29]. In MAD, sludge is kept
in an anaerobic environment normally for upwards of 12 days at 37 Æ 2
C. This
converts a proportion of the sewage sludge to methane gas as it goes through the
anaerobic digestion process. The temperature of the process is very critical, as is the
mixing and the complete absence of oxygen. Thus, temperature monitoring is an
absolutely critical part of the process and is normally carried out at three points
within the digester. However, this is not the only critical factor as the heating loops
for the digester heating have to be maintained and monitored precisely to ensure the
maximum output of the gas quality. Measurement of the gas quality is becoming
more and more common to measure the performance of the overall system.
What is essential for the digestion process is consistency and, in an ideal world, if
the same volume of sludge is fed to the digesters consistently throughout the day ad
infinitum the process would run without error. However, this consistency is not
typical due to problems within treatment processes and general operational challenges that are the normal daily routine within the water industry. Thus checking the
anaerobic environment becomes a priority. For example, when a digester is overfed,
the amount of acids that are produced as part of the digestion process can build, and a
digester will turn acidic. It is in these circumstances where monitoring the pH as well
as the output gas quality can become important.
Taking it back a stage, the whole process can be controlled by a greater level of
monitoring and control at the sludge thickening (or dewatering stage if using thermal
hydrolysis), to get a consistent output in the sludge quality feeding the digesters and
basically processing on an on-demand basis. This level of consistency is where an
automated measurement and control system comes into its own. Increasingly, the
industry is seeing the use of advanced process control on sludge thickening and
dewatering systems using sludge flow measurement into the thickening stage, as
well as the dry solids content. This approach allows a consistency of application of
the polymers used in the treatment process. In advanced systems, the flocculation
stage, where the polymer and sludge are mixed, is optimised. Also, changing the
134
O. Grievson