Water. Desalination + reuse
Issue link: https://fhpublishing.uberflip.com/i/459793
TECHNOLOGY | 30 | Desalination & Water Reuse | February-March 2015 Three cleaning regimes The study comprised comparisons of three cleaning procedures starting with conventional reagents and subsequent deployment of effervescing agents and finally microbubble agitation was added to the mix. Conventional reagents Alkaline cleaning is used first to remove clay, organics and biofilm followed by an acidic clean to remove calcium carbonate and phosphate scale and metal oxides and hydroxides. The alkaline cleaning solution is made up in the cleaning tank with permeate water and the solution heated to 40°C before being circulated through the first stage pressure vessels for 30 minutes. The solution is circulated at a flow of 3 – 4.5 m³/hr and at low pressure (2-2.5 bar), at which permeate is not produced. Should the solution show significant discolouration then the solution is discarded and a fresh solution is prepared. Once the pH, temperature and colour has stabilised the solution is allowed to soak for as long as possible, ideally 4-6 hours. For difficult fouling an extended soaking period may be required – overnight for example. The alkaline cleaning solution is then fed at a higher flow and pressure – 9 m³/hr at 3.5-4.0 bar – for 30-60 minutes. This flushes out foulants removed from the membrane surface by the cleaning. Finally the vessels are flushed with good-quality permeate to achieve natural pH levels. The entire process is repeated with the acidic clean-in-place (CIP) solution prepared using 3% acidic cleaner to stabilize the pH at 3.5- 3.7 and heating the cleaning solution to 25-30ºC. At higher temperatures calcium carbonate saturation will decrease and so increase the potential for deposition. The whole alkaline and acid wash procedure typically takes 6-8 hours. Genesol 704 cleaning Genesol 704 is a high ionic strength agent containing detergents, surfactants and chelants combined with effervescing agents to give multiple mechanisms for cleaning. The major fouling at the salad processing plant was in the first stage of the treatment process and due to organics, biofilm and clay. Genesol 704 was used from May 2013 when it reduced the cleaning time to 4-5 hours and improved on the result from conventional cleaning. But rapid refouling persisted. Genesol 704 plus microbubbles An air induction device developed by Genesys was installed which in combination with Genesol 704 produces a suspension of microbubbles between 5 and 500µm in diameter. The microbubble generator – now marketed by Genesys as a Genairator – is installed on a bypass loop of the CIP system after the recirculation pump and cartridge filters on the inlet to the pressure vessels. Following experiments and test results from our laboratory and pilot plant the cleaning procedure adopted was very different to that with conventional cleaners. The procedure is now: • a 20-minute, warm-water flush; • a 20-minute recirculation of 1% Genesol 704 solution warmed to 35-40°C; • a 20-minute soaking period during which permeate flows back across the membrane due to normal osmosis lifting deposits from the feed side membrane surface; • a 20-minute recirculation using the Genairator to produce microbubbles in the cleaning solution which agitate and disrupt deposits on the membrane surface and feed spacer; • repeat twice the recirculation, soaking and microbubble stages maintaining the cleaning solution temperature at 35- 40 °C and pH at 11.5 - 12.0; and • flush with permeate. This procedure is carried out using Genesol 704 only and the whole cleaning process takes 2 hours and 40 minutes. Multiple-mechanism cleaning using microbubbles can be applied to any RO or nanofiltration system. replaced. The ndP was initially recorded at 1.5 bar but by the next day it had risen to 2.5 bar and then to over 4 bar on 22 March when a clean was conducted. The clean was not fully successful as the plant had not been cleaned for 20 days. So cleaning was repeated on 26 March and 31 March. Regular ten-day cleans were reinstated but after only three months the ndP was again above 4 bar. A new set of membrane elements was installed and Genesol 704 was used in subsequent cleans. The first of these cleans was conducted seven days after installation and because ndP was low the period between cleans was extended to 13 days. After one month of operation with new membranes and Genesol 704 the ndP had increased but stabilised at 2.5 bar. While this was a significant improvement on results with conventional cleaners, fouling was still rapid and cleaning had to be done every 11-13 days. On 3 October the Genairclean microbubble system was deployed along with Genesol 704. After three cleans the ndP improved significantly with an average cleaning frequency of 12.5 days. After three months and six cleans the ndP had been reduced from 4.5 bar at the beginning of the year to consistently less than 1 bar on the first stage and on the whole plant (see box Pressure peaks). Between January and April 2014 the period between cleans was extended to 31.6 days and latterly it has been further increased to over 50 days. Added to this significant improvement in the duration of cleaning rapidly fouling membranes is the significantly shorter timescale of the cleaning process using the microbubble system. And no loss of salt rejection has been detected while permeate flow has improved from 15m³/hr at the beginning of 2013 to 24 m³/hr from January to April 2014. The study demonstrates how Genairclean's multiple cleaning mechanisms can prolong membrane life and improve operational efficiency in RO. And the concept can be easily and cost effectively applied to any RO or nanofiltration cleaning system. Furthermore the reduction in feed pressure and differential pressure produced by the system can give considerable operational cost savings through the reduction in energy consumption. Further savings can be anticipated from reductions in chemical consumption, plant downtime and operator input. l