Assessing biological stability of drinking water after reverse osmosis and remineralization

Mohaned Sousi (PhD Candidate IHE Delft), Maria Kennedy (promotor IHE Delft), Walter van der Meer (promotor), Sergio Salinas (supervisor IHE Delft ), Gang Liu (supervisor Research Center for Eco-Environmental Sciences (RCEES), Chinese Academy of Sciences (CAS), Antoine Kemperman (supervisor)

Duration: 2014-2020

Funding:  Dutch drinking water companies Oasen and Evides

Biological stability of drinking water is a vital topic due to its relevance to the public health where drinking water is considered biologically stable when bacterial regrowth, thus pathogenic bacteria e.g., Legionella, in the distribution networks is limited.

Two approaches are practiced to produce biologically stable potable water: (i) limiting the growth-supporting nutrients in the final water, and/or (ii) addition of disinfectant residual to the distributed water. However, in some countries, the Netherlands for instance, disinfectant residual addition is not practiced due to the risk of carcinogenic disinfection by-products formation. Therefore, growth-supporting nutrients have to be limited to very low concentrations in order to prohibit bacterial regrowth in the distribution networks. For this purpose, water treatment systems have to be designed to produce drinking water with a very high quality. Therefore, conventional water treatment processes are not always sufficient to achieve the targeted water quality - especially in terms of biological stability - and this raises the need for applying advanced water treatment technologies - e.g. membrane technology - in drinking water sector. Reverse osmosis (RO) systems are believed to be one of the potential solutions to produce ultrapure and biologically stable drinking water due to their ability to remove a large portion of growth-supporting nutrients. However, RO membranes should be coupled with other water post-treatment systems to add the basic minerals needed for the human body to the RO permeate (remineralization process). As a result, water post-treatment might have a negative impact on the biological stability of the original RO permeate.

Technological challenges
Several methods can be applied to assess the biological stability of drinking water. Nevertheless, the bacterial growth potential (BGP) method is mainly used for this research. The BGP method implies collecting and analyzing water samples in carbon-free glassware with laboratory-based batch tests by monitoring bacterial counts over time using the flow cytometry (FCM) tool to perform simple, rapid and accurate analysis of cell count. However, the BGP method needs to be adapted for low-nutrient water e.g., RO permeate, which is sensitive to handling procedures.

Research goals
This research will focus on:

  • developing and adapting the BGP method for low-nutrient water including lowering the limit of detection (LoD) of the method
  • assessing the impact of RO filtration and remineralization on biological stability of drinking water as compared to the conventional water treatment processes
  • evaluating the effect of different remineralization processes on the biological stability of the produced RO permeate
  • identifying the factors affecting the bacterial growth, e.g. organic and inorganic nutrients availability (carbon, phosphorus and nitrogen) or other water characteristics (pH, mineral content and temperature).

The case study of this research is the drinking water treatment plant at Kamerik, the Netherlands (belongs to OASEN Drinking Water Company) where two water treatment lines are installed: 

  1. a full-scale conventional treatment line where elevated bacterial counts are observed in the currently distributed water
  2. a pilot-scale advanced treatment line with reverse osmosis and remineralization units which is proposed to improve the biological stability of the produced water.