Monitoring reverse osmosis

Greg Wainhouse discusses how to improve the quality control of process water and extend reliability

Food and beverage manufacturers use reverse osmosis to purify process water, which may originally come from either the mains supply or a borehole. In either case, the objective is to produce a consistently high-quality water supply for the manufacturing process. Sampling and testing of the water is crucial to achieving this, so improving accuracy and consistency has major benefits.

Reverse osmosis (RO) uses pressure to force water through a membrane, leaving salts and dissolved solids behind. The process delivers consistently high-quality water, provided the membranes do not become blocked or breached, allowing impurities to pass through.

Conventional monitoring of a reverse osmosis system

RO on an industrial scale uses banks of membranes connected in parallel to maximise the volume of water being purified. The water quality is monitored using various methods, depending on the needs to the manufacturing process. The most common arrangement is the use of differential pressure sensors on either side of the membranes as well as conductivity measurements being taken on the ‘pure’ water side.

The difference between inlet and outlet pressures will change as the filter becomes blocked with impurities. System alarms can be configured to alert maintenance staff that the filter needs to be cleaned.

Water that has passed through the RO system, known as the permeate, should have a very low conductivity reading and this can be checked by manually removing samples and testing them in the on-site lab. However, depending on the number of samples being taken each day, this can be a time-consuming exercise.

Regular sampling and testing will clearly deliver a more accurate picture of the water quality and pick up an anomaly more quickly. Better still, an automated process that is capable of sampling multiple lines consistently and providing alarm signals as soon as a membrane breach is detected can be used.

Reverse osmosis system monitoring case study

Bürkert has developed a specialist permeate monitoring system for just this purpose. The advantages are best explained by examining a real-world example.

A beer and cider manufacturer that relies on boreholes for its source of process water was using a conventional RO system with 45m3 of water required per hour. Most RO systems will have a conductivity sensor on the common outlet, but these can struggle to detect small impurities that may arise from damage to a single membrane.

With only a very limited storage capacity, the volume of production water from the RO system needed to keep up with the manufacturing processes. This meant that downtime had to be kept to an absolute minimum. The brewery had found that certain membranes could foul up quicker than others. Conductivity levels could be seen to slowly increase as one filter became blocked and this was quickly followed by several others causing the conductivity level to increase rapidly.

As soon as several filters became blocked, the RO system had to be stopped and repaired as quickly as possible. If this could be achieved while the buffer of stored water is used, then no production capacity was lost. However, if the repair was likely to take more than a few hours, the company needed to hire in a mobile RO system to enable production to continue.

Improved reverse osmosis system monitoring

Mobile RO systems can cost tens of thousands of pounds per week to hire, as well as the additional costs of lost production while the backup system is connected. The brewery approached Bürkert looking for an improvement to its existing system and to find out more information about how they could improve the monitoring of water quality.

The brewery wanted to measure the conductivity of the water from each filter to determine when it either became blocked or started to leach impurities. This would allow the maintenance team to remove the filter from the system without impacting the production process.

Bürkert’s permeate monitoring solution connects to each pressure pipe, regularly samples water from each membrane and tests it for conductivity. This enables a central display to provide the current status of each membrane, with information also available to remote monitoring points if necessary.

Reduced costs for reverse osmosis 

The automation of the sampling and testing processes offers several advantages. Tests can be performed more frequently without increasing the workload of the staff. This also improves the consistency of results, which can be variable when conducted manually and by different personnel.

Data is also recorded by the automated system, which applies trend analysis, allowing maintenance activities to be planned ahead and reducing any downtime even further. In addition to the improved reliability, the time saved in automating the sampling and testing process reduces the payback period; for a typical 40 pressure pipe system, this can be equated to a saving of 108 hours per year, that can be better spent elsewhere in the plant.

The brewery has decided that, with increasing levels of impurities in the borehole water, the flexible, automated monitoring solution from Bürkert will deliver a range of benefits including increased productivity and process reliability.

Designed for easy integration to existing control frameworks, the on-board PLC and software with a choice of communication protocols, the Bürkert permeate monitoring system can be easily connected to onsite SCADA and process control devices.

Irrespective of the application, RO systems need to be regularly monitored to ensure water quality standards are maintained and to identify potential issues at the earliest opportunity. In this way, running costs can be kept to a minimum and product quality can be assured.

Greg Wainhouse is with Bürkert

Recent Issues