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Thermofluidics successfully chlorinates borehole water containing less than 20mg/L of chloride ions to 3ppm with no added reagents

November, 2022

We have successfully completed a second UK trial of our novel PV-powered chlorinator. We were able to treat up to 225 litres per hour of “soft” borehole water to a free-chlorine concentration of 3ppm using solar power alone, on a rainy day in Devon, UK. The borehole water, normally used for cattle watering at Langley Farm in the Exe valley, contains only 16mg/L of chloride ions. This low chloride concentration is below that of approximately one third of water wells in the UPGro database; a publicly available resource covering over 400 wells distributed across Malawi, Ethiopia and Uganda. It is also more than 100 times less than the chloride levels encountered in our first “hard water” trials at our test well in Beckley, Oxfordshire in August 2022. The maximum “soft water” flow we were able to treat increased to at least 350 litres per hour at a lower target free chlorine concentration of 1.5ppm and it increases to over 2000 litres per hour at harder, higher-chloride sites.

Our Direct Contact Chlorinator (DCC) technology involves a novel approach to direct electrolysis of untreated groundwater, liberating naturally-present dissolved chlorides as free chlorine. This approach combines onsite chlorine production – without chemical feedstocks, including salt – and accurate chlorine dosing, into one seamless and fully-automated process. Alongside previous tests carried out, these latest tests also demonstrated that our approach is capable of treating water with rapidly varying flowrate and power availability. This is consistent with the varying power levels and flowrates associated with point-of-use treatment in off-grid/weak-grid public buildings, at community water sites, and in small piped-water networks in the global south. For even lower-chloride sites, we are working on a “Hypochlorite Generator Accessory (HGA)” which automatically dissolves and electrolyses crystalline salt from a salt-reservoir, and doses water to a settable target concentration on site. Whilst this accessory does require salt to be brought to sites, it avoids the need for expensive and short shelf-life chlorine products and skilled operator-intervention, providing a user-experience equivalent to the occasional “topping-up” of salt in a domestic dishwasher or water softener. This project draws on the latest generation C-programmable microcontrollers, expert inputs from Prof. Dan Rogers at Oxford University’s Energy and Power group, and our network of partners in the community water space around the world. It also sits in the context of rapidly decreasing photovoltaic module prices which is driving the African off-grid solar revolution more generally. This powerful combination can bring capabilities previously unthinkable below utility scales to distributed users off water and energy grids.  

Our next challenge is to refine our approach to on-site calibration, which enables us to achieve a settable target chlorine concentration without need for baseline analysis – and turn our prototypes into a robust and user-friendly product. To do this, we will draw on the experience we’ve gained on a pipeline of novel off-grid water pump products, commercially available through our subsidiary Impact Pumps. Subject to funding, we anticipate having a DCC ready for field trials with our partners in southern Asia and sub-Saharan Africa in Q2 2023.

Chlorination is by far the most common water sterilisation method used worldwide, and the only method providing residual protection against microbial contamination post-source. Post-source contamination is increasingly understood to be as significant a problem as contamination at source, particularly when water is collected in buckets or jerricans and carried to the home. However, its implementation is currently hindered by limited supply chains for chemical feedstocks, dependence on skilled operators, and routine service/maintenance requirements.

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