By Dr. Niwagaba B. Charles*, Bouman Lukas, Meierhofer Regula, Ouma Henry, Wanyama Kennedy
Dr. Charles is an Associate Professor of Civil and Environmental Engineering at Makerere University. He holds a Bachelor’s and Master’s in Civil Engineering from Makerere University and a PhD in Environmental Technology from the Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden. He has 22 years of experience in research, teaching and consultancy in Civil and Environmental Engineering. He has been involved in various research and consultancies funded locally by the Government of Uganda and by various multi-lateral agencies.
Corresponding Author: Dr. Niwagaba B. Charles
The supply of safely managed drinking water to rural areas of Uganda is challenging and needs innovative technologies. Gravity-driven membrane (GDM) filtration offers a viable technological option to an alternative use of surface water in areas where groundwater is challenged by chemical contamination or is not available. In Eastern Uganda, the GDM water kiosks achieved filtration rates of ~ 200-800 L/h with flat-sheet ultrafiltration membrane modules of 75-80m2 membrane surface. The membranes retained particles: bacteria, protozoa, most viruses as well as a fraction of the dissolved organic carbon. Residual chlorination was achieved using a passive in-line T-chlorinator. The cost of the used membrane modules imported from Germany was US$75 per m2. Digital tracking of water consumption and revenue between February 2021 to May 2022, revealed that on average, including the rainy season, 2 m3 of water was dispensed per day and revenue of UGX260,000 was collected per month. With this revenue, the GDM kiosk committee covered operation and minor maintenance expenses and realised some savings. Significant infrastructure renewal, such as new membranes, tanks and appurtenances might need alternative additional funds. The installation of Lugala’s GDM system, supplying safe drinking water to about 100 households cost an estimated UGX17,500,000. With the household size of five persons in Namayingo District, the per capita CAPEX was UGX35,000 (US$ 9.56) for the GDM system, which is 7.6 times smaller than the per capita cost of US$72.6 for rural water supplies, according to the 2020 Water and Environment Sector Performance Report. As a result, GDM was accredited by the Uganda Appropriate Technology Centre (ATC) in 2021. Consequently, the Ministry of Water and Environment (MWE) recommended its promotion in 2022 as one of the technologies for safe water supply in Uganda. A user guide for GDM is publicly available free of costs (Bouman et al., 2022) and there is no patent on the GDM technology.
The Government of Uganda (GoU) aims at increasing safe water coverage in rural and urban areas from the 2020/21 baseline values of 75.9% and 79.2% to 85% and 100% in rural and urban areas, respectively, by the year 2024/25 (NPA, 2020). The Joint Monitoring Program (JMP) reported that in 2017, the national access to safely managed drinking water in Uganda was 7%, derived from 4% for rural and 16% for urban areas (WHO/UNICEF, 2019). A safely managed drinking water service is one that is located on the premises, available when needed and free from faecal and chemical contamination (WHO/UNICEF, 2017) and puts more stringent requirements than just the supply of basic drinking water. In 2020, the Water and Environment Sector Performance Report (WESPR) had no data on the access to safely managed water in rural areas of Uganda. The WESPR of 2020 reported that the access to safely managed water (available on premises) in urban areas was 57.1% (MWE, 2020). Both WESPR and WHO/UNICEF paint a grisly picture on access to safely managed drinking water in Ugandan rural areas.
The predominant improved point sources for the supply of water in rural areas comprise protected springs, boreholes, protected wells, and gravity-flow schemes, which in some cases deliver water to the yard of a dwelling or to public standpipes supplying water to a number of dwellings. Populations which do not have access to improved water sources rely on unsafe sources such as rivers, lakes, and unprotected wells. Waterborne diseases include dysentery, typhoid and cholera. These diseases are responsible for a high mortality rate, especially among the under-five children. To solve these problems, there is a need to improve technologies for the delivery of safely managed water to rural populations.
2.0 PURPOSE OF THIS PAPER
The purpose of this paper is to present a new technology of water treatment using the Gravity Driven Membrane (GDM) technology. The paper presents practical experiences from a pilot scale GDM water treatment system, which has been implemented in a rural area in the Ugandan Eastern District of Namayingo for a period of six years.
3.0 WHY GRAVITY-DRIVEN MEMBRANE (GDM) FILTRATION?
Point water sources in Uganda are, to a large extent, supplied by groundwater. However, the utilisation of these sources is compromised where the groundwater is salty, contaminated or not available. Gravity-driven membrane filtration offers a viable technological option to an alternative use of surface water in these regions. The technology has been tested in water treatment at the Lake Victoria for the last six years by the Swiss Federal Institute of Aquatic Science and Technology, together with local a partner organisation in Eastern Uganda and described in detail in a user guide and a supplementary video series (Bouman et al., 2022) that is publicly available free. There is no patent on the GDM technology. GDM can treat microbiologically polluted, turbid raw water in one step. The heart of the treatment system contains ultrafiltration (UF) modules with nominal pore sizes (“holes”) of 20-40 nm (Bouman et al., 2022). The membranes in combination with a biofilm that grows on the membranes retain particles, bacteria, protozoa, most viruses as well as a fraction of the dissolved organic carbon. While in the biofilm predation of pathogens by other organisms and metabolisation of organic material are mechanisms that purify the water, the UF membranes filter all particles larger than 20-40 nm. However, GDM cannot filter dissolved chemical contaminants of natural or anthropogenic origin (mining, agriculture or industry) and does not remove salts. In conventional ultrafiltration, the operation and maintenance (O&M) procedures are complex. The membranes need to be operated with high pressure, backwashed several times a day and cleaned with chemicals. Compared to conventional ultrafiltration, where mechanical and chemical cleaning is implemented to keep fluxes around 60 L/m²/h, the water production per membrane area is lower for GDM set-ups (4 – 20 L/m²/h). In contrast to conventional ultrafiltration, GDM uses the low pressure (30 – 100 mbar = 30 – 100 cm) of the water column and needs minimal O&M because of the biofilm that is formed on the membranes. Biological activity in the biofilm causes the formation of cavities and keeps the biofilm porous, which leads to the long-term stabilisation of the flux and prevents membrane fouling and clogging. Various parameters influence the biofilm properties. For example, feed water with higher amounts of organic matter or small clay particles forms biofilms with higher resistances and consequently lower fluxes through the membrane (Lee et al., 2019; Chomiak et al., 2014). Relaxation periods and warm temperatures (~ 25-35°C) also have positive effects on the flux. Interestingly, changing the pressure (i.e. the water head) on the membranes from 0.4-5 m did not result in a higher flux. The only recommended regular maintenance tasks for the GDM filter are flushing the membrane tank once per month and a check-up once per year. The flushing procedure removes all the sediments and suspended particles that have been retained by the membranes and removes parts of the biofilm. During the yearly check-up, the membrane and connections are controlled to avoid leakages.
4.0 PRACTICAL EXPERIENCE FROM GDM KIOSKS IN EASTERN UGANDA
In Eastern Uganda, GDM has successfully been operated by the local communities during the last six years with very minor maintenance costs for the GDM filter. The schema of the GDM water kiosks, as they were constructed in Eastern Uganda is presented in Figure 1. The raw water source is Lake Victoria. The water is abstracted from an infiltration well close to the shores of Lake Victoria by a solar water pump, which conveys the water through a 1-2 km pipeline to the treatment site. There, the water is filtered with UF membranes and thereafter stored in a clean water tank. In some kiosks, optional raw water tanks may be installed. After treatment with GDM, the water is safe for drinking. However, as in all water supply systems, the water may be re-contaminated during distribution and transport. Therefore, a chlorination step at the treatment site is recommended to provide residual disinfection and protect the water from recontamination until the point of consumption. Several options have been evaluated at the GDM kiosks in Eastern Uganda (Dössegger et al., 2021). An in-line T-chlorinator resulted in the best option for GDM kiosks. Finally, the water is sold at the taps of a water kiosk. To increase transparency in accountability and give customers 24/7 access to the water, automatic water vending machines were installed at some GDM kiosks. Additionally, they allow easy monitoring of water consumption and water revenues.
Since the operation started in 2016, data of 57 flux measurements is available for the flat-sheet UF membrane modules of 75-80 m2. The median flux was 4 L/m2/h and the median flow rate 300 L/h. The 25% – 75% interquartile range for the flow is ~ 200 – 800 L/h.
The cost of the used membrane modules was US$75 per m2 and they were imported from Germany. The lifespan of the membranes is 10 years in conventional configuration, but is likely to be higher in the GDM configuration as they are operated with less pressure and no back-flushing is applied nor chemicals are used. The capacity of the system can be adapted according to the water needs of a community and the exact configuration should always be adapted to the local context. Since February 2021, a water ATM is operational and tracks the water consumption and revenues from the GDM water kiosk in Lugala, Namayingo District. Figure 2 shows the community members in Namayingo collecting water at the water kiosk in Lugala. Until May 2022, approximately 900,000 litres of water were dispensed and UGX4,200,000 revenues collected. On average, this is 2m3/d of water dispensed and revenues of UGX260,000 collected per month, including the rainy seasons when collections decrease. With the revenues, the GDM kiosk committee can cover operation and minor maintenance expenses and generated some savings. Significant infrastructure renewal, such as new membranes, tanks and appurtenances might need alternative additional funds. The Lugala GDM system, supplying safe drinking water to about 100 households cost an estimated UGX17,500,000. The household size of five persons in Namayingo district (UBOS, 2017) translates to a per capita CAPEX of UGX35,000 (US$9.56) for the GDM system, which is 7.6 times smaller than the per capita cost of US$72.6 for rural water supplies according to the WESPR (MWE, 2020).
Membrane integrity tests were conducted after installing the membranes that showed log removal values (LRV) of more than four. From November 2015 to December 2016 microbial water quality tests were conducted at least monthly in the first three constructed GDM kiosks. Up to 97.3% of all samples collected after the membrane and 95% of the samples collected directly from the tap corresponded to the low risk category according to WHO (1997) with most of the samples showing 0 or 1 E. coli/100 mL (Peter-Varbanets, 2017). From September 2019 to October 2019, samples were taken at all the five GDM kiosks. After the membrane, 100% of the 21 samples contained 0 E. coli/100 mL. At the tap, 84% of the 19 samples contained 0 or 1 E. coli/100 mL, 11% contained 2 – 10 E. coli/100 mL and only one sample contained over 10 E. coli/100 mL. Membrane integrity tests showed LRVs of over 4.5 in all the systems except for one (LRV = 2.9). In a recent water quality monitoring phase, we collected 22 biweekly samples from July 2021 until April 2022 at one of the GDM kiosks. From the 22 samples taken at the tap, 91% of the samples corresponded to the low risk category according to WHO with 19 samples being completely free of E. coli contamination. A membrane integrity test conducted in April 2022 revealed LRV of more than five after more than six years of continuous operation. Additionally, water quality tests were performed by the Appropriate Technology Centre (ATC) of Uganda in autumn 2021. In the accreditation report, ATC states that no faecal nor total coliforms were detected in the treated water. The report concludes that “from the water quality results, it was apparent that the GDM filtration system is effective for improving the quality of contaminated surface water to acceptable drinking water standards” (ATC, 2021). ATC showed interest to construct a pilot plant where government engineers and water officers can be trained. In January 2022, the Ministry of Water and Environment (MWE) in Uganda issued a certificate stating that the GDM technology is suitable as an alternative of safe water supply and that the MWE recommends the promotion of the GDM technology.
GDM filtration offers a viable low-cost technological option to an alternative use of surface water in areas where groundwater is contaminated by chemicals or not available. The technology is not patented and a user guide is publicly available free of cost (Bouman et al., 2022). Furthermore, the technology is accredited by the ATC and the MWE recommends to promote GDM in Uganda. In Eastern Uganda, the GDM water kiosks achieved filtration rates of ~ 200-800 L/h with flat-sheet ultrafiltration membrane modules of 75-80 m2 membrane surface and delivered safe drinking water according to WHO. Digital tracking of water consumption and revenues over a period of 16 months revealed that on average, including the rainy season, 2m3 of water were dispensed per day and revenues of UGX260,000 were collected per month. The GDM is low-cost in that the system constructed in Namayingo delivered safe drinking water to 100 households at a per capita CAPEX of UGX35,000 (US$9.56) which is 7.6 times smaller than the per capita cost of US$72.6 reported in 2020 for rural water supplies. Consequently, the GDM technology should be promoted to supply safely managed drinking water to rural areas of Uganda, where fresh surface water sources such as wetlands, rivers or lakes are available. Source protection measures should be developed and implemented for such sources since GDM cannot filter dissolved chemical contaminants of natural or anthropogenic origin (mining, agriculture or industry).
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