A question often posed by operators of anaerobic digesters is “what should I be looking for?”. This is often in the context of process plant with a myriad of expensive instrumentation and computer control. I have always been of the view that anaerobic digestion is a simple process looked after by nature and with its slow reaction rate needs little attention, but in the words of the management guru: “measure more by measuring less”.

Ratio 1 – Gas : Sludge
There are 2 ways of measuring gas production : the instrumentation way and the operator way. If you can rely on your gas meter, please use it, but there is another way. Nowadays, for safety reasons and to reduce greenhouse gas emissions, all biogas is burned, either in boilers, gas engines or waste gas burners. The trick is to calibrate the consumption rate of each gas appliance (m3/hr) and to ensure that there is an hours run meter for each appliance. There is no need to monitor gas consumption more regularly than daily, so the daily gas production from the digester(s) is the sum total of the hours run by each appliance and its gas consumption rate. A log book and a calculator or a spreadsheet will do the job for you.
Likewise, there are 2 ways of measuring the raw sludge feed to the digester : the instrumentation way and the operator way. If you are unhappy with reliance on a series of flowmeters, calibrate your feed pump(s), record the hours run, and there you are with the daily raw sludge feed volume.
Now you can calculate Ratio 1 – divide the daily (or weekly or whatever time interval you choose) gas production by the raw sludge feed volume. What is the significance ? If your raw sludge is consistent, then the ratio will be. If the ratio is not consistent day by day, either your raw sludge dry matter is varying or your digester is not performing, because Ratio 1 is approximately equal to:

(Raw Sludge % TS) x (% VS) x (% Volatile Destruction) / 1000

For example a raw sludge with a TS of 6%, a VS of 80% and a VSD of 45% will give a value for Ratio 1 of 21.6.

Ratio 2 – Gas : Digester Capacity
Again monitor the gas production, expressed as m3/day. Divide this figure by the digester capacity (m3) to give you your second ratio. This tells you how hard your digester is working.
In the example above a digester working on a 15 day retention time will give a value for Ratio 2 of 1.44. A figure less than 1.0 means that you could be working your digester much harder; a figure more than 1.5 is good for a conventional sewage sludge digester; high solids digesters have been known to achieve a figure of more than 4.

Ratio 3 – CH4: CO2
There are two ways of monitoring gas quality – yes, you guessed it, using an expensive instrument, either permanent or portable, to measure the CH4 content of the gas, or a “Draeger” tube with a portable set of bellows to measure the CO2 content; it can be assumed for this ratio that the biogas comprises only CH4 and CO2. Whether you measure CH4 or CO2 the important issue is that Ratio 3 is consistent from day to day; for a sludge digester it is normally about 1.5. The alarm bells should ring if Ratio 3 falls because this indicates a rising CO2 content, which in turn indicates that the methane forming stage of the anaerobic digestion process is becoming less stable, which might lead to failure. Measuring the pH will tell you that your digester has already failed and measuring VFA’s is more complex.

Measure the 3 ratios regularly; if your digester is happy the ratios will not vary.
Anaerobic Digester
Diagram of anaerobic digester

Michael Chesshire, Greenfinch Ltd, Burford House, Tenbury Wells, Worcs, WR15 8HQ (Tel: 01584 810777)