Nanofiltration for Safe Drinking Water in Underdeveloped Regions A Feasibility Study

Transcription

1 Nanofiltration for Safe Drinking Water in Underdeveloped Regions A Feasibility Study Sreenivasan Ramaswami, Zafar Navid Ahmad, Maximilian Slesina, Joachim Behrendt, Ralf Otterpohl Institute of Wastewater Management and Water Protection Hamburg University of Technology, Germany UNICEF & WHO (2015) 1 15 September 2016

2 Is improved DW safe? Improved but not necessarily safe Bain et al Improved DW as an indicator Improved: Protected dugwell, public tap, borehole, piped supply, etc. Unimproved sources: surface water, tanker truck, unprotected dugwell, bottled water, etc. Worldwide needs for safe drinking water are underestimated: billions of people are impacted (Payen, 2011) How safe are the global water coverage figures? Case study from Madhya Pradesh, India (Godfrey et al., 2011) More than 1.8 billion worldwide! 2

3 Other challenges 100% % 60% 40% 20% 0% 1,84 Papua New Guinea 0,5 0,45 0,09 0,07 Madagascar Ghana Mozambique UK High costs in developing countries Typical low daily salary (in GBP) and the cost for 50L improved or safe water (in GBP) in some countries [ based on WaterAid (2016) ] Challenges and risks for access Chemical pollutants in water Nearly 4 billion! (Payen, 2011) [ adapted from WaterAid (2016) ] 3

4 Membrane processes for DW Requirement for decentralised solution Ultrafiltration cannot reject viruses, dissolved organics (insecticides, humics, etc.), heavy metals Reverse osmosis requires high investment and operating costs Nanofiltration: better rejection than UF, 200 Da, lower costs than RO Van der Bruggen et al. (2003) NF is used in industrialised countries for production of high quality DW 4

5 NF for high quality DW Pollutant / [Sources] Bacter-, fung-, herb- and pesticides [Van der Bruggen et al., 2001; Košutić et al., 2005; Ogutverici et al., 2016; Pang et al., 2010; Saitúa et al., 2012; Sanches et al., 2012] Emerging micro-pollutants (pharmaceutical residues, hormones, endocrine disruptors, etc.) and pathogens [Lopes et al., 2013; Radjenović, et al., 2008; Sanches et al., 2012; García- Vaquero et al., 2014; Yoon et al., 2007] Harmful monovalent anions (nitrate, fluoride) [Van der Bruggen et al., 2001; Garcia et al., 2006; Shen and Schäfer, 2015] Heavy metal ions (As, Ni, Pb, U, etc.) [Harisha et al., 2010; Košutić et al., 2005; Maher et al., 2014; Favre-Réguillon et al., 2008] Natural organic matter [Costa and de Pinho, 2006; Ericsson et al., 1997] Findings Several NF membranes can remove many of these compounds effectively. To pinpoint some, rejection percentages up to 95, 94 and 92.5% have been reported for triclosan, dichlorodiphenyl-trichloroethane and glyphosate by Ogutverici et al. (2016), Pang et al. (2010) and Saitúa et al. (2012) respectively. Studies (including full scale in DWT plants) confirm that a wide spectrum of emerging pollutants can be retained by NF, better than conventional treatment powered with activated carbon adsorption. Depending on the membrane properties and the chemical characteristics of individual compounds, the rejection capacities can range from about 30% to almost 100%. Some NF membranes can effectively reject nitrate as well as fluoride ions. The main criteria for membrane selection would be the pore diameter, besides the surface charge of the membrane. Numerous studies (lab and pilot scale) report the ability of NF to reject heavy metals from drinking water. Harisha et al. (2010) and Košutić et al. (2005) report rejection% of more than 85% for As using NF, which is not much different from the rejection capacity of RO. Almost all NF membranes can remove humic substances effectively without compromising on permeate flux unlike RO membranes. 5

6 Aim of this work Requirement for decentralised solution Ultrafiltration cannot reject viruses, dissolved organics (insecticides, humics, etc.), heavy metals Reverse osmosis requires high investment and operating costs Nanofiltration: better rejection than UF, 200 Da, lower costs than RO NF is used in industrialised countries for production of high quality DW Van der Bruggen et al. (2003) Research question: Can a micro-enterprise using nanofiltration produce safe drinking water at reasonable prices for a rural area in a developing country? 6

7 Ghana as reference country Ghana Several NGOs are working there already Availability of literature 7

8 Materials & Methods Experiments with model groundwater Feed 15 mg TOC/L; 275 µs/cm 750 W rotary vane pump 800 L/h Experimental setup Disc tube module with Dow NF270 with 1 m 2 membrane area Temperature controlled at 14 o C Seven concentration trials at 7 bar 120 L feed water recovery ~88% Cleaning: 0.1% NaOH and 0.2% HCl ET-System RTS Rochem Technical Services GmbH Fouling experiment at 5 bar for 28 d 8

9 Water flux - concentration trials Flux at 25 o C (Lm -2 h -1 ) Operation (h) Seven consecutive concentration trials without membrane cleaning Marginal difference in filtration trend Initial rapid decline membrane compaction Low fouling longer operation possible Concentration polarisation insignificant 25% flux decline during the trial Average flux of about 52 Lm -2 h -1 at 7 bar Temp. corrected flux (Lm -2 h -1 ) Water recovery (%) 9

10 Rejections concentration trials TOC (mg/l) Feed Retentate Permeate d Trial TOC (mg/l) R P Conductivity (µs/cm) Feed Retentate Permeate d Trial Water recovery (%) All permeate samples had < 2 mg TOC/L conductivities between µs/cm ph between 7.2 and 8.2 Poor rejection of nitrate ions by NF270 10

11 Flux decline - fouling Permeate flux at 25 o C (Lm -2 h -1 ) Time (d) Initially about 29% flux decline, thereafter about 40 Lm -2 h -1 at 5 bar Water permeabilities of about 8 Lm -2 h -1 bar -1 possible for long durations TOC in permeate samples were about 1.5 mg/l Possibility to provide clean water during long continuous operation 11

12 The micro-enterprise concept Micro-enterprise: <10 employees; annual turnover < 2 million 1. Water extraction 2. Pre-filtration (if needed) 3. Nanofiltration 4. (Re-)filling 5. Door-to-door delivery Schematic of operations in a micro-enterprise Ahmad (2015) 12

13 Economic evaluation Existence of a well or bore-hole Life of ET-System: 4 years (27000 hours) 20 hours of operation per day and CIP once in two weeks 6720 operating hours per year Water flux of 60 Lm -2 h -1 at 8 bar about 403 m 3 clean water per year Investments for land, mechanical, electrical, etc. / Misc. expenses & taxes One-time investment Fixed costs - for first 4 years Cost (in ) 1 - personal communication, Ghana Statistical Service (2014) Variable costs For 4 yrs. (in ) ET-System (trade discount possible) For electricity ( 0.3 per kwh) L water containers (250 nos.) For chemicals ( 2.6 per month) 125 Delivery vehicle (tricycle cart) Personnel cost (one employee) Initial investment for 4 yrs. (total) 4000 Total variable costs 9175 Fixed costs (for every 4 yrs.) after first 4 yrs. Revenue for 4 yrs. (in ) Motor plus pump (aft-shop.de) 500 Water cost ( 0.01 per L) 16,125 Membrane (replacement, RTS) Total fixed cost after first 4 yrs

14 Conclusion Nanofiltration can produce safe drinking water at low prices ( < 0.01 per litre ) With a 1m 2 unit, production capacities of 1200 litres clean water per day NF permeate for drinking and cooking needs Micro-enterprise employing NF can be a solution for economic water scarcity 14

15 Sreenivasan Ramaswami, M.Sc. Doctoral researcher Hamburg University of Technology Institute of Wastewater Management and Water Protection Hamburg, Germany Website: 15