The effects on biological stability and later growth of mixing treated water from alternative sources and conventionally produced drinking water

Several drinking water utilities are currently working on using alternative sources for drinking water production – such as brackish groundwater or surface water – with the aim of meeting future demand for drinking water. They are also looking at the possibility of mixing the drinking water obtained from these new sources with drinking water produced using conventional methods before it is distributed.

Mixing effects

Previous and ongoing research have already shown that a mixing effect may occur when drinking water from conventional treatment approaches is mixed with treated water from an alternative source. Mixing with water produced using new treatment methods such as NF/RO membrane filtration and a conventional treatment can also lead to a mixing effect. Mixing often results in the deterioration of the biological stability of the mixed water compared with the original water (van der Wielen & van der Kooij, 2008; Hootsmans et al., 2016). It is supposed that this deterioration is attributable to a range of factors limiting growth in the two, separately produced, types of drinking water.

Biological stability

When these two types are mixed, those limiting factors may be eliminated, resulting in the increased growth of microorganisms in the mixed water. By measuring the biological stability of the mixed water and individual waters from different locations and different sources, the potential risks of mixing for biological stability can be identified better and it may be possible to identify mixing risks for specific sources.

Water holding tanks

However, it is still unclear whether mixing effects are attributable to the mixing of different water types or to the fact that the water is kept in a water holding tank. Regardless of whether drinking water is mixed in a water holding tank, sediment and biofilms also form in those tanks: both factors can affect the biological stability of the drinking water during its residence time in a tank. Since different types of water are often mixed in water holding tanks, the observed mixing effect could be associated with the tank itself. By conducting biological stability measurements of drinking water from a location where mixing does not take place (before and after storage in a water holding tank), the effect of the tank on the biological stability of water can be identified. The results obtained can then be used to determine whether observed/yet to be observed mixing effects are attributable to mixing or to the residence time in a water holding tank.

Results

With this research, we aim to

• acquire insight into the possible influence of residence time in water holding tanks on the biological stability of drinking water;
• acquire insight into the effect of mixing on the biological stability of the mixed water;
• identify risks that may occur when mixing water from specified new water sources.

Armed with this knowledge, drinking water utilities can decide whether drinking water types from alternative and conventional sources can be mixed and distributed without difficulties, or whether additional research is required to look at the biological stability of the mixed water before that water can be used. In this way, they can also determine whether, and which, management measures can be taken to limit mixing effects.