One of seawater desalination processes is by utilizing reverse osmosis (RO) membrane technology. Proper design and control of RO feed water quality is the most important factor to deliver trouble-free and maintain high reliability of the RO units operation.
This paper examines the importance and useful of monitoring RO pre-treatment units in preven ting inorganic scale, fouling, bio-fouling, RO membrane hydrolysis/degradation, even membranes replacement. It also shares the writer experiences in monitoring and controlling the RO Pre-Treatment plant in supporting high reliability of the power generation operation, especially related to water usage.
INTRODUCTION
PT. IPMOMI is an IPP, which has steam turbine power generations, with capacity of 2 x 615 MW. Steam is produced by 2 units of high-pressure boiler, with working pressure of 180 Kg/cm2. Water is one of important materials to support the power generation reliability of operation. This power generation facility is located in the most populated island in Indonesia, with population density of 732 people/km2.
Water utilization and consumption for public purposes is critical issue. Due to this condition, this power generation is located near seashore and utilizes seawater as raw water for all water utilization. Reverse osmosis (RO) units have been utilized to produce required water, with much lower dissolved solids (salt) content, from seawater.
The RO membranes are spiral wound thin film composite (TFC) type. These consist of a polyamide salt-rejecting film on a polysulphone base. The very thin surface layer of polyamide (up to 3 _m) provides the semi-permeable and salt-rejecting properties of the membrane. Pre-Treatment systems for RO units are designed to produce RO feed water with a reduced inorganic scale, fouling potential by removing particulates, micro-pollutants and micro-organisms. The tropical climate of this area enhances bio-fouling potential problem.
FEED WATER QUALITY IS AN IMPORTANT FACTOR
Eliminating or reducing micro-pollutants in the RO feed water will minimize membrane fouling and maintaining a good production rate, a low salt passage, and an extended membrane lifetime.
Type of the pre-treatment units varies depending on source and analysis of raw water, which will be treated, and size of the membrane system. This can vary from a simple micron cartridge filter to a sophisticated system that may include Dissolved Air Flotation (DAF), clarifier, media filtration.
Although cost consideration may dictate the final choice of system, cost alone should not be the overriding factor. It is essential to understand that a plat designed with a good pre-treatment system will have less downtime, require less cleaning, be easier and cheaper to maintain and have a significantly longer lifetime.
In addition to the capital equipment, there are ranges of on-going chemical treatments that may be used. These include coagulant, flocculants (polymer), mineral acid, sodium hypochlorite, sodium bisulfite, antiscalant and non-oxidizing biocide (for RO membrane sanitation).
Feed water quality dictates performance of the RO membrane units, which in turn limits the RO %Recovery (production rate), pressure drop, %Salt Rejection (water quality). It is very important to monitor RO feed water quality and RO Pre-Treatment performance.
RO feed water quality is characterized by the following parameters: pH, FRC, Turbidity, Iron, Bacteria, Silt Density Index (SDI15). RO feed water pH control will influence setting of maximum level of %Recovery and dosage of antiscalant chemical to prevent inorganic scale at reject stream of the RO units.
For, sea water with high total dissolved solids (TDS) of 46,000 – 50,000 _S/cm, operation of the RO units is limited by maximum %Recovery of 35%, and RO feed water pH less than 7.4, and applying antiscalant chemical to prevent inorganic scale. RO feed water free residual chlorine should be maintained 0 (nil), or by practice at our power generation facility, RP feed water ORP level is less than 350 mV.
The purpose is to prevent RO membrane hydrolysis due to oxidants attack. Free residual chlorine in RO feed water is eliminated by addition of sodium bisulfite. Turbidity & SDI15 of RO feed water indicates amount of suspended solids contents, which has big potential to cause RO membrane fouling.
In our practice, RO feed water turbidity is maintained less than 0.25 NTU, and SDI15 less than 3, will prevent significantly RO membrane fouling. It is indicated by stable trend of pressure drop across the membrane units, though they have been operated for 3 months after previous cleaning. RO membrane chemical cleaning is conducted after 3 months of operation after last cleaning, although pressure drop increase of the RO membrane units are still less than 10%.
RO membrane autopsy is one of the most powerful methods to analyze kind & level of unwanted materials accumulation on the RO membrane surface during the unit operation. It is also able to define what kind of cleaning chemicals required, their dosage, and correct chemical cleaning procedure. The presence of both soluble iron (as Fe2+) and insoluble iron (as Fe3+) should be measured. They often cause a fouling problem if not adequately removed by the Pre-Treatment system. Although ferrous iron ions are soluble, there is risk of oxidation to insoluble ferric salts. For this reason, ferrous iron are often deliberately oxidized by chlorine and removed by filtration. In our practice, iron level in the RO feed water is maintained in the level of 12 –25 ppb, and it has not caused fouling problem potential significantly.
Bacteria analysis is needed to monitor their activity and bio-fouling potential throughout the pre-treatment and membrane systems. As a rule of thumb, maximum bacteria level is maintained less than 10,000 colonies forming unit per-ml (cfu/ml). RO membrane sanitization by applying non-oxidizing biocide, once a week is required to control bacteria population & growth.
MONITORING PRE-TREATMENT PERFORMANCE
An important aspect for trouble-free operation is the regular monitoring of feed water condition throughout the pre-treatment system and regular inspection of equipment. Analyzing samples of feed water at each treatment stage enables a good assessment to be made of plant performance.
A careful inspection of pipe work, dosing tanks and cartridge filters can identify bio-film formation at an early stage, prompting sanitization procedure and in some cases preventing widespread bio-fouling. Where an ineffective antiscalant program is not being used of the pre-treatment has failed, visual inspection may show colloidal particles, iron, or inorganic scale on the cartridge filter fabric.
Monitoring procedures should be routine with reading plotted. There are a number of monitoring systems available to record the important parameters, such as pH, ORP, turbidity, SDI15, %Recovery, pressure drop, %Salt Rejection etc. Many of these systems incorporate an alarm to alert the operator when parameters exceed fixed limits.
CONSEQUENCES OF POOR PRE-TREATMENT
There are many reasons why the pre-treatment system may fail during the lifetime of the plant. It should be assumed that failures will happen and design features and monitoring procedures should detect these occurrences early so as to minimize their effects.
Many of foulant can be cleaned by utilizing chemical cleaning and sanitization process if the foulant build up is not too severe. However, some pre-treatment failures cause irreversible fouling or permanent RO membrane damage. The most common of RO membrane damage is caused by chlorine/oxidant attack, which requires RO membrane replacement to recover their function.
CONCLUSIONS
Proper RO feed water quality control & management is important factor to deliver trouble-free RO units operation, and maintaining high reliability of their operation.
Other important factors to support problem-free of RO units operation are :
* Design of proper required RO pre-treatment units & process
* Do any improvements (if necessary) to deliver the best RO feed water quality
* Select a proper & effective scale & biofouling control program tailored for each plant facility and feed water source. Selected program should be based on proven efficacy not the lowest price
* Train all on-site staffs in the key parameters and warning sign, which indicate occurring RO plant failure
* Do RO membrane cleaning regularly (at least once per-6 months), and clean fouled plant early in the fouling stage and flush all equipment thoroughly after shutdown and on start up
* Do monitoring daily of RO Pre-Treatment and RO units performance
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