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.