Mitigating the effect of methane on the biological stability of drinking water Methane Removal in Drinking Water Treatment Systems

ion Reverse Osmosis Aeration IEX Remineralisation Figure 3 Treatment steps of the future treatment facility at ZS de Hooge Boom as currently planned by Oasen Methane Removal in Drinking Water Treatment Systems 14 Currently, Oasen is carrying out extensive research into the feasibility of this new approach and the capability to achieve the envisioned results. A pilot plant has been set-up, which contains all of the aforementioned processes. In this plant the capabilities of individual processes, the interaction between processes and the quality of the end product are being tested and optimised so as to guarantee a new standard in drinking water treatment. 1.4 Purpose of this research This document is part of the ongoing research into the feasibility and optimisation of the novel treatment approach. Its focus is on third post-treatment process which should: 1. Provide sufficient removal of volatile gasses (MBTE, vinyl chloride & cis-1.2 dichloroethylene) 2. Provide sufficient removal of methane (CH4) 3. Provide sufficient addition of oxygen (O2). The purpose of the document is to determine the most optimal post-treatment system in order to comply with these conditions. To determine the most optimal post-treatment system, however, this report focusses on determining the most optimal treatment system for the removal of sufficient amounts of methane because it is assumed that: 1. The required removal efficiency for methane is much higher than that required for the removal of volatile gasses and the addition of oxygen. 2. Volatile gasses are removed more easily than methane. Therefore, it is assumed that the system providing sufficient methane removal will also provide more than sufficient removal of volatile gasses and addition of oxygen. Methane Removal in Drinking Water Treatment Systems 15 2 PROBLEM DEFINITION & GOALS For over a century it has been known that methane can be present in groundwater. As water infiltrates through various layers of the sub-surface, bacteria will utilise oxygen, forming CO2. Depending on the amount of organic matter available this can happen quite rapidly, so that the oxygen is depleted after only a few meters of soil passage. Once depleted, nitrate is used as an electron donor (denitrification) to oxidise organic material in the soil after which the water is subjected to a variety of anaerobic processes. Anaerobic bacteria will continue to oxidise organic material, if present (e.g. in peat), producing methane (CH4) while utilizing carbon dioxide and bicarbonate as oxidators. Biological production of methane (Methanogenesis) is the last process in a sequence of redox processes and will only occur after manganese, iron and sulphates have been reduced (De Vet, 2011). Due to the high pressure of the water column pressing down, the water is able to dissolve much higher concentrations of methane than is possible under “normal” surface pressure conditions causing oversaturation. When the pressure of the water is reduced, for example by pumping it up to the surface, methane will escape back to the atmosphere (Helm, van der, 1998). In European countries which do not use chlorination as a residual disinfectant such as the Netherlands, Switzerland, Germany and Austria, the presence of methane poses a threat to the biological stability; these countries rely on the production of drinking water in which microbial regrowth is limited through limitation of nutrients essential for growth, usually focusing on organic carbon (Hammes, et al., 2010). Methane is a carbon source which could potentially be utilised by bacteria in drinking water under aerobic conditions. Research in the Netherlands has shown that the presence of methane in the influent of treatment plants has an impact on the number of Aeromonas found in the distribution system (Reijnen, 1994). The given reason for this is not that there was methane in the effluent, but rather that the biological breakdown of methane in the filtration steps led to high levels of biomass production which in turn was not sufficiently retained by the sand filters. The higher concentrations of biomass led to regrowth of Aeromonas in the distribution system; not the presence of methane itself. Finally, literature gives a wide range of biomass yields for methane oxidizing bacteria (methanogens); 0.5-0.8 g DW/g CH4 (Leak, et al., 1986). Another source states that the yield of bacteria on methane is 6 times higher than the yield on ammonium (Helm, van der, 1998). The aforementioned research, leads experts to believe that even very small amounts of methane could lead to significant growth in the distribution system. To the author’s knowledge, however, no attempt has been made to quantify the direct effect of the presence of methane on the biological stability of the produced drinking water. In addition, there is no current legislation dictating the maximum level of methane in drinking water. Conventional treatment of anaerobic groundwater usually involves some form of aeration as a pretreatment step to add oxygen to the water, which is needed in the subsequent treatment of the water in filters. During aeration, methane and volatile gasses are largely removed, leaving low concentration of methane in the water. Sand filters will break down remaining methane biologically by methane oxidizing bacteria (MOB) so that the effluent contains no more methane. By switching to RO membranes Oasen is confronted with a new challenge; it can no longer assume that all methane will be removed in the treatment as RO membranes do not remove dissolved gasses. A single post-treatment step is therefore introduced to provide sufficient removal of methane. This will have to be a chemical or physical removal technique, as biological removal would lead to recontamination of the already very pure RO filtrate. To remove all methane without biological treatment of the water is however virtually impossible as this would have a large impact on the cost of water production and also demand large amounts of energy. This begs the question how much methane should be removed from the water for it to be biologically stable without having to overdimension the removal step. Various removal techniques exist which from literature show the Methane Removal in Drinking Water Treatment Systems 16 potential to achieve high removal efficiencies for methane. The question is which of these techniques is capable of achieving the required removal to guarantee biologically stable water most optimally.