Why am I getting variability in biogas FOS TAC results from an AT1000?

Document ID

Document ID TE8600

Published Date

Published Date 02/10/2017
Why am I getting variability in biogas FOS TAC results from an AT1000?
Causes of variable FOS and TAC results AT1000 Biogas application
The titration measures the start ph (if phi is selected as active – which is recommended to aid troubleshooting) and typically ph 8 ±0.3 but this can vary between sites.
The titrator adds acid until the sample ph falls to ph 5.0 to calculate the TAC and then continues adding further acid down to ph 4.4 to calculate the FOS.
Result 1 Is the alkalinity (mg/L CaCO3 as TAC) calculated from the volume of acid added to reach ph 5.0.
Result 2 is the volume A of acid required to lower the start pH (around ph8) down to pH 5.0 (TAC)
Result 3 is the volume B of acid required to lower the ph from 5 to 4.4 in ml (FOS)
Result 4 is the FOS in mg/L acetic acid CH3COOH using Result 3
Result 5 is the ratio of FOS divided by TAC in mg/L. This should be 0.2-0.35 for an efficiently fed digester. Less than 0.2 is underfed and above 0.4 is overfed.
If there are issues of variability it is important to monitor several runs from start to completion with attention to the following:
1.      Calibrate the electrode – ensure a minimum slope of 97% is achieved (if lower the glass indicator electrode needs cleaning for 2 hours in pepsin/HCl, followed by a thorough rinse in deionised water) and the offset is within ±30mv tolerance (if not the reference electrode is drifting and it is recommended to drain the filling solution and refill several times). When complete recalibrate the probe checking that the temperature sensor is recording sensibly.
2.      Make sure the sample is freshly mixed, tea strained and pipetted every 5ml taken (never
strain a large volume and leave to stand for subsequent aliquots). If the sample stands unmixed between repeats or is poorly mixed (eg with a beaker full to capacity of semi solid sample) and repeatedly used without re-mixing and straining, carbonates and FOS will drop out and give progressively lower results. This is also true of centrifuging the sample which causes lower results. Slight changes in either value have a large impact on the ratio after the division.
3.    Make sure the probe bulb and shoulders are immersed properly (without hitting the stirrer),
the electrode opposite the delivery tube and check that the filling hole of the electrode is not covered by the cone insert that sits in the electrode holder ring. Make sure the stirring is not so vigorous to create a vortex in which the probe sits
4.    Check the acid used is 0.1N H2S04 and nothing different.
5.    Run 3 repeats of the titration and watch the ph and volumes achieved for each stage – if any titration does not reach the required ph after the max volume is reached a reading of 0.00mg/L will result and the division is not plausible.
5.    If the start ph is variable on successive repeats there could be inconsistencies with the probe or poor sample mixing between runs. The acceptance criteria for the probe can be tightened from stability 50mph/min to 30mph/min. Other probe issues should emerge with the calibration.
6.    If the ph falls too quickly and records vol A <0.5ml the probe maybe poorly submerged, the start ph low (below 7.8), the acid too concentrated, the acid not mixing properly with the sample or the sample volume less than 5ml. It is critical not to alter the recommended sample volumes as the internal FOS calculation is strictly based on a 5ml sample volume. If the sample volume is halved (ie 2.5ml was used to speed up the titration) then the FOS equation must be doubled to meet the theory ie ((vol Bx8x1.66) –0.15)*500 and this program modification cannot be made by the customer – it must be modified and authenticated by internal Hach staff.
7.   If the ph falls too slowly the sample may have an unusually high start Ph or TAC value, the titrant maybe leaking or there could be a titrant delivery issue.