Data normalization identifies performance issues - Case Study
The Municipal RO Plant had been in operation for almost 18 years, producing ~290 gpm of permeate water for a city in Iowa, USA. It was one of the first RO plants in the state and the upper mid-west. In May 2008, city operators complained about a loss in system production. In response, the City contracted an RO membrane service provider to perform a cleaning regimen that produced less than satisfactory results.
The City then contacted Avista Technologies who sent an applications services manager to the site.
The Avista application expert evaluated the equipment and performance data and provided on-site support for the first clean to ensure the recommended procedure was followed. RoClean P303 and P111 powder cleaners were selected for the job and the effectiveness of the combination was immediately apparent. The color and turbidity of the spent cleaning solution confirmed that the chemicals were removing foulant left over from prior cleans. The operators were pleased to see the membrane flux and rejection restored after the clean.
Iron Filter Inspection
Based on the amount of foulant removed and the increased cleaning frequency at the site, Avista recommended a complete physical inspection of the iron filters. The examination revealed a catastrophic failure of the iron and manganese filtration system (Figure 1) which forced the city to by-pass the iron filters.
Avista provided technical guidance to convert the plant to an anaerobic process which kept the iron and manganese in solution as it entered the RO skid. Although this was an operational improvement, the Silt Density Index (SDI) values were still high enough to warrant replacing the existing pre-treatment with a new iron and manganese filter system.
Fig 1 photo shows the upper lateral system
Avista participated in the iron filter pilot study, developing a protocol to ensure the technologies were a good match for the RO process. Evaluating filtration efficiency via particle counts proved to be the only reliable method of predicting RO fouling rates. The results confirmed that the new iron and manganese design would provide excellent filtration quality (Table 1).
Ongoing data normalization is key to identifying and troubleshooting performance issues.
In this case, restoring flux and rejection with an improved cleaning procedure was just addressing the symptom. Expert troubleshooting knowledge took the diagnosis one step deeper to determine the root cause of the issue.
Additional plant improvements included switching antiscalants to Avista Vitec 3000. After review, it was found that although the original antiscalant drum price seemed cheaper, it required an injection rate of 3.5 mg/L. At a dosage of 2 mg/L, Vitec 3000 was able to provide cost savings.