In-depth analysis of changes restores system optimization - Case Study

Introduction
A reverse osmosis plant (BWRO) had been operating effectively applying Avista antiscalant and cleaners for about a year. Increased product water demand led to changes in the feed water quality and an increase in plant recovery resulting in an increase in stress on the pre-treatment system and more rapid membrane fouling. The existing membrane cleaning regime was reviewed to try to increase the time between cleans. Working with the site operator Avista Technologies assisted in re-optimising the cleaning.

This case study shows the benefits realized through investigation and use of the most appropriate formulated cleaners.

Background Information
The RO system replaced a de-alkalization plant. As the RO started up, the feedwater was passed through the un-regenerated de-alkalizers. An unexpected consequence resulting from the removal of a pseudo filtration step from the pretreatment was experienced a few months later from bypassing the equipment. At the same time, another borehole source was added doubling the feed conductivity and introducing a higher level of iron to the feed.

EXD of Cartridge filter fouling showing CO and Fe as predominant components suggesting organic material and iron.

The first sign of the change was rapid clogging of guard cartridge filters on the feed to the RO’s. On inspection, the filters were coated heavily in reddish slimy material.

As the site team struggled to improve the feed water quality to the RO units the first stage differential pressure increased to the point where it required cleaning.

SEM photos of the fouling confirmed the biological nature of the foulant

The condition of the cartridge filter, together with the changes in normalized data, were used as an initial guide to the cleaning procedure. As the material was thought to be biological in nature the standard cleaning procedure was carried out using:

• RoClean P111 – good biological and colloidal fouling removal product.
• Citric acid – generic chemical used for sterilization and iron removal

While the clean was in progress analysis of the cartridge filter fouling revealed the material to be a mixture of iron and organic matter, thought to be from bacterial growth.

CIP Analysis
The Clean in Place (CIP) performed with the RoClean P111 and citric acid improved the RO system differential pressure and the normalized flow.

Samples of the CIP solutions were collected and analysed to determine the makeup of the foulants removed from the system, revealing that both the high and low pH solutions contained significant amounts of iron. With the highest iron levels found in the high pH first stage spent cleaning fluid, suggesting that biological material is associated with a portion of the iron.

As the system was seen to be fouling again quite rapidly a review of alternative cleaning regimes was made using the new information.

Since no membranes were available for offsite testing, the following comparison regimes were suggested on-site for effectiveness:
RoClean P111 followed by RoClean P703
RoClean P703 followed by RoClean P111

RoClean P703 is an iron/manganese removal formulation.
The suggested trial was taken up as the cleaning frequency had increased from one clean per 10-12 weeks to one clean per 5-7 weeks.

Results
The results of the first RoClean P111/P703 cleaning combination were very impressive. (See table below)

SEM photos of the fouling confirmed the biological nature of the foulant

The substitution of RoClean P703 significantly improved the reduction in first stage differential pressure achieved by cleaning and this improved fouling removal also resulted in better product quality.

The re-fouling rate was also improved with the cleaning frequency reducing back to once per 10-12 weeks. The re-introduction of the de-alkalizers as a filtration step also helped. The RoClean P703 continued to maintain a healthier average first stage differential pressure and a lower differential pressure after cleaning than was seen prior to the cleaning regime change.

The reverse cleaning regime was not tested as the site team were happy with the improvement seen with the RoClean P703 and did not want to change their procedure further.

Conclusion
The client was very satisfied with the results of the cleaning trial and has continued to apply RoClean P111 and RoClean P703 cleaning products to remove the iron bacterial fouling that builds up on the system.

Significant changes to feedwater quality or pretreatment systems will have an effect on a membrane system.

A detailed analysis of the changes can lead to a new optimized operating regime that in this case led to a restoration of pre-change cleaning frequencies and a reduction in average differential pressure.

Membrane system fouling analysis is not limited to the autopsy of a membrane, analysis of cartridge filter fouling and used CIP solutions can also provide an indication of what is happening on the membranes.