10 Years and Counting
13 May 2008, News release from Eimco Water Technologies Limited
The oldest MBR in Europe celebrates its 10th year of continuous operation.
Eimco Water Technologies - UK Licensee for Kubota flat sheet membranes, proudly report on the oldest full scale submerged MBR plant for municipal sewage treatment in Europe. The site at Porlock STW, operated by Wessex Water, has now been continuously operating for 10 years. Porlock STW serves a population of 3,800 people. Being a coastal site, the proximity to bathing waters led to the requirement for disinfection, a process achieved through membrane filtration which can be designed by Eimco Water Technologies.
Specialising in MBR technology, Steve Kennedy, Eimco Water Technologies' Integrated Solutions Group Manager, comments "while MBR's have become the norm in the industrial sector an air of scepticism still exists in the municipal market. Price, maintenance, reliability are all common questions. Porlock provides the perfect reference for any water company faced with achieving tight controls on water quality discharges. Porlock's 10 year of history fulfils all expectations proving that MBR's are a robust and efficient process.
Mr Colin Skellett, Wessex Water's Chairman observes "Flat sheet membranes gave answers where other technologies could not respond. Because the plant is situated in Exmoor National Park, an area of outstanding natural beauty, we had to build something sympathetic that blended with the surroundings, a rural farm building look-alike. A conventional activated sludge plant with sand filter and UV disinfection process would imply larger footprint and getting permission from authorities and farmers would be tougher."
After 10 year's operation the Wessex Water's technical team perception is that "overall the plant has performed well with membranes proving exceptionally robust". Effluent quality data remains good (table 1) with the average reduction in faecal coli form levels exceeding 5.8 log over the entire period. Moreover, SDI tests carried out in February 2007 on permeate from the plant showed an average SDI of 1.50 over 3 sample runs, which is well within the guidelines for use as a feed to a Reverse Osmosis system for re-use applications. In the Chairman's words, "we see membranes providing a physical barrier and a physical warranty of high quality effluent".
Simplicity. The works has no primary treatment or grit removal and screening is to 3 mm prior to entering the 4 MBR treatment tanks. At Porlock the treatment capacity is 1,907 m3/d and the total surface area of the flat sheet membranes is 2,880 m2. Microfiltration is achieved by gravity at a maximum head of 0.12 bar. The plant is unmanned and monitored via telemetry. Membrane cleans happen on average every 8 months, consisting of 6 hours off-line per tank, always using sodium hypochlorite at 0.5-0.7% w/v as free chlorine. MLSS ranges from 8,000 to 27,000 mg/l, in occasions exceeding 30,000 mg/l. Long sludge ages (30-90 days) and low F:M ratios (0.02-0.07 kgBOD/kg MLVSS) reduce sludge production (0.35 - 0.50 kgds/kgBOD). The decline in average permeability since 1998 represents a mere 1% increase in TMP over a 9.5 year period. Full membrane access and external cleaning has only been carried out once in year 9.
Seawater ingress during high tides created problems with foaming and fouling until the sewerage network was improved. Following the first removal and overhaul of the membrane units in year 9, all membrane panels were checked and damaged ones replaced. Despite the long term exposure to sand and grit, macroscopic abrasion on membranes was remarkably slight, and no microscopic effect was detected. GRP membrane unit housings were reinforced with steel brackets to compensate for the abrasion.
Durability. To date, a total of 230 panels have been replaced out of the 3,600 installed, giving a failure rate of less than 6.4% after 10 years of operation. The majority of the failures were from potentially preventable causes: 127 had holes from debris and grit, 55 had internal sludge or contamination inside (probably smaller holes), 25 were damaged in handling, 14 had seals torn or split and 9 were removed in error (cleanable staining). Given the remarkably low failure rates, it is not anticipated that the plant will need any further overhaul until year 12 or later.
The keys to the longevity and reliability of the Porlock plant are good competent well trained operational staff, good pre-screening, maintaining uniform even aeration and low maximum TMP due to hydraulically limited gravity flow. According to Mr Skellett, "one of the big dangers is that contractors install plants with a new technology and soon after that they hand it over and disappear. A longer commissioning and training period is required, especially when users are not familiar with this technology. Once staff are trained, MBR plants are relatively straightforward to operate. They need to have the right maintenance systems in place, plus the influent and the sludge conditions shall ensure the membranes are not prematurely fouled."
As Chairman Skellett states, "tighter standards imposed by both the Water Framework Directive and the Bathing Water Directive will drive the requirements for future plants".
"Indeed, if Porlock were designed today it may have to meet nutrient limits. Eimco Water Technologies has designed plants to satisfy some of the tightest discharge limits in the world. This experience, together with the development of Kubota's flat sheet membrane panels has made our MBR more competitive than ever in terms of both CAPEX and OPEX when compared to more conventional technologies" comments Stephen Kennedy.
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