Solar-driven Drinking Water Disinfection via On-site Chlorine Generation – SuMeWa|OCG

The SuMeWa|OCG system for on-site chlorine production consists of the following components and process steps, as illustrated in Fig.1 below:

1. A saturated NaCl solution is prepared by dissolving salt tablets and stored in a brine tank.
2. A NaCl solution with a certain, fixed concentration is prepared by taking water from the brine tank and mixing it with treated water or clean water from another source. Both are fed into the chlorine reactor tank (CRT).
3. In the CRT, the chloride of the NaCl solution is oxidized to chlorine gas, which hydrolyses to hypochlorous acid (HOCl). Chlorine production takes place in batches.
4. The HOCl solution produced is stored in the Chlorine Stock Solution Storage Tank (SSST). Depending on the system configuration and chlorine demand, the SSST can have a volume of 30 to 300 litres.
5. Whenever the drinking water needs to be disinfected, chlorine solution from the SSST is dosed to the water in proportion to the flow.
6. A sensor measures the chlorine concentration in the disinfected water and allows the chlorine dosage to be adjusted.
7. To protect the chlorine reactors from scaling, a softener reduces the hardness level in the water used to produce the chlorine stock solution.
8. The system operates fully automatically. Only NaCl in the form of salt tablets needs to be refilled into the brine tank from time to time.
9. A SCADA (Supervisory Control and Data Acquisition) system allows remote monitoring.
10. The system is 100% powered by solar electricity.
11. A module for the removal of ammonium, arsenic, iron and manganese is optionally available. Neither additional chemicals nor media exchange are required for this module.

The capacity of the disinfection plant, which is available in a range from 10 to 1.000 m³/d (cubic metres per day), is adapted to the local requirements and especially to the chlorine demand. This is done by adjusting the number of chlorine reactors as well as the volumes of the chlorine reactors, and the saline- and the chlorine stock solution. The capacity of plants with requirements for the removal of ammonium, iron and/or manganese are limited to about 100 m³/d.

Fig.1. AUTARCON’s Sun Meets Water On-site Chlorine Generation (SuMeWa|OCG) unit as installed in Lavale, Maharashtra, India. Source: AUTARCON (2022)

This type of disinfection technology is ideally suited for the application for locations that are not connected to any centralized water supply system (see Network Design and Dimensioning). These are for example, rural areas, villages, hospitals, marginalized townships, and new building blocks. The here presented disinfection process requires source water from boreholes (see Drilled Wells), springs or bank filtration or any other fresh water which is nearly free of turbidity and other potential contaminants such as iron, arsenic, ammonia, or elevated organic loads, respectively, has undergone according to primary treatment. In case contaminants are present, the SuMeWa|OCG unit can be equipped with additional modules to remove turbidity, iron, manganese, arsenic, ammonia, and nitrate. The removal will neither require addition of chemicals (except NaCl) nor the exchange of a filtration media.

In combination with a solar system, the station can be operated sustainably in very remote regions where consumables are not easily available. Prior to any system installation the capacity of a new or existing water supply scheme must be determined through real measurement campaigns. With each SuMeWa|SYSTEM installed a water meter will be installed which will allow to (online) monitor the water volumes disinfected.

The stations can either be shipped fully assembled to start operation immediately or in single modules to be mounted on-site. This task usually takes around 1-2 days and can be carried out by a local operator that has received one-time training by the technology provider. Thus, the implementation of the disinfection process per se is straight-forward and quick.

However, in real-life applications, source water often shows contamination like the above mentioned (see Fig. 2 below), requiring an in-depth site evaluation and design of an adequate treatment process - which increases complexity and costs of the total treatment system.

Fig. 2. Results that determine the presence of micro-organisms in raw water. Source: SANKAPAL (2022)

The only consumable for the long-term operation of the SuMeWa|OCG unit is salt in form of NaCl. Compared to the purchase of chlorine stock solution, NaCl is easily available, very cost efficient, simple and safe to handle as well as indefinitely storable. The use of NaCl in form of tablets is advised.

As the station works fully automatic, maintenance steps are limited to the refilling of salt tablets. Depending on the size of the saline tank, this may be required only once per year. The station indicates the necessity for refilling via the SCADA System, as illustrated in Fig. 3 below.

Fig. 3. Dashboard of SuMeWa|OCG SCADA system. Source: AUTARCON (2022)

Additional maintenance steps are the check and (re-)calibration of chlorine sensors (as shown in Fig. 4) in the chlorine reactors and the removal of dust from the solar panels.

Fig. 4. Frequent maintenance at Lavale pilot station. Source: OTTER (2022)

In the village of Lavale in the Indian state of Maharashtra, a SuMeWa|OCG station is in operation since December 2021. The plant has a capacity of 300 m³/d to disinfect water for 6.000 inhabitants and was installed as part of the PAVITR project in cooperation with Symbiosis International University in Pune.

Similar pilot stations are in operation in Haridwar (state of Uttarakhand in India), Julo (Uttarakhand), Amarkantak (Madhya Pradesh), and Kharagpur (West Bengal).

A challenge that typically has to be tackled when setting up a pilot like this, is the limited availability of water demand data especially for small scale rural water supply systems in India.

Fig. 5: Example of complete treatment station. Source: OTTER (2018)

The installation of water meters is still uncommon and has led to undersizing or oversizing of the units in the past. For the construction of the operation room costs between Euro 1.500 and 3.000 should be concidered. The costs for a 300 m³/d disinfection station amount to about Euro 25.000 including piping and solar power supply.

SuMeWa|OCG units are installed in 12 developing countries. Most of them are operational in Tanzania, Egypt, India, and Nepal.

0In Tanzania, several stations are installed to safeguard drinking water in rural hospitals. Other stations are supplying entire villages with drinking water via prepaid water taps (OTTER et.al. 2020).

In Egypt, 20 stations are treating groundwater for the supply of villages in the oases of the Western Desert. The main task of these stations is – next to the assurance of residual disinfection – the removal of substantial amounts of dissolved iron and manganese. Therefore, the SuMeWa|OCG was combined with a SuMeWa|IRON unit. Whereas previously existing much larger stations have failed to cope with such iron concentrations, the SuMeWa|IRON units are in operation since 2014 and are constantly supplying safe and iron-free water (JASKOLSKI et.al. 2019). Here, iron removal is based on oxidation via aeration and disinfectant dosing followed by a green sand filtration.

In Nepal, several stations are operating in places where groundwater is at risk due to the infiltration of wastewater from pit latrines and fertigation of agricultural land. Elevated levels of ammonia and iron pose a challenge to the supply of safe water. AUTARCON, together with local partners, has developed a biological nitrification plant that converts ammonia into nitrate, thereby enabling very efficient disinfection of water treated in this way (OTTER et.al. 2020).

A key factor in keeping the stations in Egypt, Tanzania and Nepal operational was the successful introduction of a prepaid payment scheme. With this payment scheme, access to water is only possible after consumers have purchased water credits, which in turn are used to cover operating and maintenance costs.