It’s a fact the water industry whether it be in drinking water treatment or in wastewater treatment produces a lot of sludge. A quick look through the Internet will give you a ball park estimate but to save you the time it is about 1.4 million dry tonnes in the UK and about 7 million dry tonnes in the USA per annum in the wastewater industry alone.
In terms of operational efficiencies the two biggest areas of efficiencies that can be
realised is in Activated Sludge (already discussed in previous blogs) and in sludge
management, something that I will discuss.
The first thing that needs to be known about sludge is how much sludge you have
and in what state it is in. The day to day accounting of sludge stocks it is safe to say
is somewhat of a “black art” in the wastewater industry and this is probably one of the
industries greatest problems.
So how do things get accounted for and why is this of use to the operators of sludge
treatment facilities and in what form does the data need to be. All valid questions
which are simple if you have a single wastewater treatment works but highly complex
if 10 works, 100 works or even a 1000 or more. First things first is, as always,
find what information that you need. In sludge terms this is relatively simple. The
questions are how much have I got, how thick is, what is it and where is it.
Firstly to answer the where is it. All of the UK water sewage companies have multiple
treatment works serving one sludge treatment facility. The sludge at these sites is
in relatively small quantities and not too fresh as they have been stored for week
or sometimes several weeks. The handy thing at these sites would be to know
when they are capacity and when sludge needs to be collected and ideally in what
quantity. The physical quantity can be relatively easy calculated if store in a sludge
or settlement tank using levels. When the sludge is collected on collection both
the volume of sludge and the dry solids content should be measured by passing
the sludge being picked up through a flow meter and dry solids meter onboard the
tanker. Quick calculations on board the tanker by the computer can calculate the
mixture of sludges that are picked up. This on arrival at the sludge treatment centre
can be imputed into the system automatically at discharge at the sludge treatment
facility. This accounts for all imported sludges.
With the sludge imports then the nature of the sludge should be known (usually
combined sludges) and with this final part of the jigsaw the nature, volume and dry
solids in imported sludges are known.
So what is left to know? Well naturally the nature and type of sludge at the sludge
treatment facilities (and corresponding wastewater treatment works) themselves.
Hopefully this is know already as the quality and quantity of the sludge should
already be know via measurement at the wastewater treatment works (simple flow
and dry solids monitoring).
With the nature of the imported and indigenous solids known at a sludge treatment
centre then the rest should be relatively simple as a combination of level monitoring,
flow meters and dry solids meters should be able to track the sludge inventory as it
flows through the sludge treatment process whatever it maybe.
With proper sludge inventory management knowing what sludge you have, in
what volume and what mass is something that sounds relatively simple. There are
technological difficulties in terms of flow and dry solids measurement of thick sludge,
especially sludge cake but as the treatment of sludge progresses to the point where
it is more of a production industry (arguably it already is) then this becomes more and
more important. No other production industry would not know precisely how much
product that they are producing, unfortunately this is the case for the wastewater
industry. Actual quantities are known but certainly not down to the kilogramme
as arguably it should be. Wastewater sludge is being used more and more as a
resource for producing energy, in some cases producing nutrients and in a lot of
cases as a resource for the agricultural industry with all of the HACCP procedure that
are necessary for this.
In this case why don’t we know exactly what gets processes, especially as it is
something that can be measured and controlled pretty much automatically.
In next weeks blog I will start looking at the problems associated with the various
sludge treatment processes.