Bridging the gap: next-generation characterisation of AOC

A small proportion of the organic carbon in drinking water is broken down by micro-organisms into compounds such as carboxylic acids. This can lead to regrowth problems. Given the fact that the concentrations in question are low, no chemical methods are yet available that can quantify these biodegradable organic substances separately. As a result, it remains unclear which substances in Dutch drinking water cause regrowth problems. To remedy this, two chemical methods for measuring individual carboxylic acids in drinking water up to 0.20 – 6.3 µg C/l were implemented and validated in the laboratory as part of the exploratory study. However, using them for drinking-water-related samples showed that these methods are also still too insensitive to quantify individual carboxylic acids in drinking water. It is therefore concluded that, for the time being, the current AOC or BPP methods will continue to be the standard in this respect.

Unclear which organic compounds are broken down in drinking water

There are relatively high regrowth levels at most locations in the distribution system in drinking water prepared from surface water after passage through a reservoir. This leads to exceedances of the legal standard for Aeromonas. That increased level of growth is probably attributable directly or indirectly to particulate and/or high-molecular organic carbon (PHMOC) and AOC since research has identified relatively high concentrations in the drinking water type referred to here. Because PHMOC and AOC are bulk parameters, it remains unclear which individual organic substances cause the increased microbial growth. If more was known, it would be possible to intervene more specifically in the treatment process in order to remove these substances. The aim of this study was therefore to determine whether and which chemical methods are available to characterise and quantify biodegradable organic carbon (BDOC) in drinking water.

Testing of most appropriate chemical methods

The first step was to conduct a brief literature search to identify the appropriate chemical methods for detecting and quantifying individual BDOC compounds in drinking water. On the basis of the results, the three most appropriate methods were tested and validated in the laboratory. Finally, these methods were applied to drinking water samples from practice in order to determine their suitability for identifying the individual carboxylic acids in the AOC fraction in drinking water.

Sensitivity too low for the detection of carboxylic acids in drinking water

The results of the literature review show that, at present, only chemical methods have been developed for the adequate measurement of carboxylic and amino acids in drinking water. Two of the three chemical methods tested in the laboratory in the implementation and validation studies detected most of the fifteen selected carboxylic acids. The detection thresholds were generally higher than the desired 1 µg C/l. Acetic acid, propionic acid and oxalate acid cannot be reliably detected in drinking water. The two selected chemical methods were also used for drinking water samples from the treatment and distribution system of different Evides production sites. The results show that all 12 measured carboxylic acids are below the detection threshold of 0.20 to 6.3 µg C/l. The methods are therefore still too insensitive to be used for this purpose in Dutch drinking water.