AVISTA TECHNOLOGIES (UK) LTD.

Antiscalants, Biocides, Coagulants and Cleaners for Reverse Osmosis Systems

Antiscalants

Without some means of upstream scale inhibition, RO membranes and their flow passages will foul due to scale, the precipitation of sparingly soluble salts. Common examples of scale are calcium carbonate (CaCO3), calcium sulfate (CASO4), barium sulfate (BaSO4), and strontium sulfate (SrSO4). Less common but equally problematic are silica (SiO2) and calcium fluoride (CaF) scales.

Avista Technologies formulates a range of liquid antiscalant/dispersants for membrane separation systems. These formulations are designed to inhibit scale while also dispersing colloidal particles. The inhibition and dispersant properties help extend RO system run times, reduce cleaning frequencies and increase the overall productive life of the RO elements. The application of Vitec® may also eliminate the need for acid addition while maintaining desired recovery rates.

Water analysis data is unique to each treatment site so we have developed the Avista Advisor software program. Cations and anions of a water source as well as general system data are entered into the program which then recommends an appropriate Vitec® product and dosage specific to that facility.

Biocides

Biological fouling of reverse osmosis membranes is one of the most challenging issues faced by operators of membrane separation systems. This is primarily due to the fact that effective biocides must be non-oxidizing in order to avoid permanent destruction of the RO membrane layer.

Avista Technologies provides two types of biocides for use in reverse osmosis systems, RoCide® DB-20 and RoCide® IS2. Both formulations are non-oxidizing and EPA registered. They are safe for use with both polyamide and cellulose acetate membranes. However, they are not for use in systems producing potable water. Both have outstanding environmental properties, degrading rapidly and naturally. This property has made them the chemical of choice, especially for systems operating under strict environmental and discharge regulations.

Shock Treatment and RO Cleaning Applications:

RoCide® DB-20 is a 20% liquid solution of the active ingredient DBNPA. This formulation is used as an intermittent feed for "shock treatment" control or it can be added to a low pH cleaning solution to kill bacterial fungi and algae in membrane systems.

Continuous Injection Biocide:

RoCide® IS-2 is based on a 1.5% isothiazoline mixture. It's a broad-spectrum formulation for use against microbiological contaminants commonly found in RO applications including bacteria, fungi, and algae. RoCide® IS-2 is designed for continuous injection into the RO feedstream to provide constant biological control.

Cleaners

All membrane separations systems require regular cleaning in order to maintain consistent production and optimal membrane life. The symptoms of RO fouling include a decrease in normalized permeate flow, an increase in salt passage, and an increase in differential pressure. Severe element fouling can lead to irreversible membrane damage and can render cleanings difficult or impossible.

Avista Technologies formulates a wide variety of liquid and powder cleaners to address all of the foulants common to RO systems. In addition, our laboratory personnel use an array of equipment and methods to identify the distinct constituents of unique foulants from samples submitted to us for analysis. With these results, we can recommend a specific cleaning regimine or formulate custom cleaners specific to a particular application or geographical region.

Coagulants & Flocculants

Colloid and silt fouling are the major causes of loss in performance of membrane separation systems. For this reason, many system designs include upstream multimedia filtration which involves a variety of mechanisms to achieve particle removal. When added to the feedwater, RoQuest® coagulants neutralize the negative electrostatic charges on both particles and filter media surfaces. Elimination of these charges makes collisions and attachments between particles and media grain surfaces possible.

Avista Technologies formulates a variety of liquid coagulant/flocculant chemicals under the RoQuest® name. RoQuest® 3000 is an organic polymer while the RoQuest® 4000, 5000, and 6000 are a blend of organic coagulants and ferric sulfate.

RoQuest® formulations change the behavior of particles and colloids which normally exhibit negative charges in naturally occurring feed waters. Because like charges repel each other and unlike charges attract each other, the addition of the positively charged RoQuest® polymer allows the negatively charged particles to clump together. This creates larger particles which are more readily retained in the multimedia filter (MMF), resulting in improved MMF filtration.

Ancillary Products

In addition to the main product line, Avista offer a small selection of ancillary products which may be required for efficient operation of a membrane system. These include chlorine scavenger, a salt rejection improvement aid and a range of line corrosion inhibitors. For hybrid systems, a thermal antiscalant is also offered. The products are outlined below.

Chlorine Scavenger:

In many cases, chlorine is present in the membrane plant feedwater by either direct or indirect injection. Municipal feedwaters typically contain some level of chlorine residual and water treatment plants inject chlorine in order to reduce biological activity in the pretreatment process. Because polymaide membranes have very little tolerance to chlorine and other oxidising agents, the chlorine must be de-activated to prevent permanent membrane damage. Anti-Chlor 30 is a 30% solution of sodium bisulphite and functions as a chlorine scavenger. The product addition rate may be calculated using Avista Advisor 3.

Membrane Sizing Agents:

Polyamide membranes may lose some of their rejection charerecteristics as a result of oxidation or simply due to age. Phoenix liquid and powder resizing agents are available to coat the surface of partially oxidized/damaged polyamide membranes and improve salt rejection. Elements should always be cleaned prior to sizing.

Corrosion Inhibitors:

Reverse osmosis permeate is often corrosive to downstream piping due to a low level of hardness and alkalinity. In the case of permeate from a seawater plant; the level of chloride will be disproportionately high. Corrosion may be prevented by applying a suitable inhibitor. The ViCoat product range includes formulations based on inorganic pyrophosphate, silicates and silicate/polyphosphate blends.

Thermal Antiscalant:

Vitherm 700 is a blended antiscalant developed for use in thermal distillation systems. Low application rates of Vitherm 700 are effective in preventing scale and metal oxide precipitates over the full range of operating temperatures.

Services for Reverse Osmosis Systems

Avista Technologies offers an array of technical and support services which can be effective tools in improving the performance of reverse osmosis membranes.

Laboratory

Water Analysis: using a minimum of half a gallon (2 liters) of RO feedwater, this test provides the cation and anion data necessary to obtain an accurate and site specific antiscalant recommendation.

Avista Advisor Program: using a complete mineral analysis, this program supplies the user with detail ion saturation data, product selection, dosage, and usage information.

Foulant Identification: requires a membrane leaf which is examined to determine the nature of any foulants present and their effect on the membrane. Potential foulants can include scale, biological matter, metals, etc.

Filtration Study: requires a minimum of five gallons (20 liters) of feedwater to the multi-media or sand filter sent to Avista. From this sample, jar test procedures allow for optimum RoQuest coagulant /filter aid recommendation. Dosage determinations provide the most efficient and cost effective coagulation within the filter.

Particle Counts: using a minimum of a pint (0.5 liters) of filter feedwater, this test provides total particle data ranging from 2-50 µm.

Laboratory Cleaning Study

Laboratory cleaning studies have become an integral part of the Avista membrane autopsies and a useful tool in validating the effectiveness of a cleaning regime against a site-specific foulant.

While Autopsies identify foulants and recommend solutions to minimize future fouling, cleaning studies identify the best cleaner(s) and procedure to address fouling on the membranes already affected.

This approach can also provide an opportunity for us to custom blend and test chemistries specifically for your application or site-specific foulant.

Samples of a dissected membrane are placed on a RoVision cell tester and baseline flow and rejection performance are established under crossflow conditions. A cleaning solution is then recirculated through the tester at optimum temperature. Upon completion, post-clean data is recorded to determine the effectiveness of that particular chemistry. If necessary, this process continues until we’ve identified the cleaner that provides the best result.

Membrane Autopsy

The physical dissection of an RO membrane and the laboratory analysis of its foulants can be one of the most definitive methods of troubleshooting an RO system. An Avista autopsy includes a wide range of procedures, as shown below, and concludes with our written report and recommendations.

• External visual examination
• Internal visual examination
• Wet test of the element
• Chemical identification of surface foulants using: 
    - FTIR spectrometer
    - Energy Dispersive X-Ray Spectrometer (EDX)
    - Scanning electron microscope (SEM)
• Loss on ignition test
• Cell and dye test
• Fujiwara Test
• Summary
• Recommendations

Off-Site Cleaning and Restoration (OSCAR™)

OSCAR is Avista Technologies proprietary Off-Site Cleaning and Restoration service. This intensive testing and cleaning procedure was developed to restore membrane performance to at or near original specifications. Special cleaning techniques combined with our own RoClean formulations allow us to obtain significantly better results than can generally be achieved on-site.

The OSCAR Advantage:

The excellent results typically achieved during OSCAR are possible because the equipment was designed specifically to clean membrane elements. This cleaning and restoration process is unlike conventional on-site clean-in-place (CIP) systems.

Among the specialized OSCAR features is a novel sealing arrangement which allows the membranes to be cleaned in a reverse direction (brine to feed). This significantly improves the removal of particulate matter from the element feed spacer.

Ideal solution temperatures are maintained throughout the cleaning sequence with the use of a dedicated heating system.

Procedure:

Membranes submitted for OSCAR undergo the following procedures at Avista:

• Physical inspection to identify any signs of telescoping, etc.
• Wet pre-test at standard operating conditions to compare permeate flow, salt rejection, and differential pressure against original performance specifications.
• Thorough cleaning using OSCAR and Avista Technologies own RoClean formulations.
• Post-test and normalization of the data to identify the improvements achieved during OSCAR.
• Upon completion, element are preserved, heat-sealed in a poly bag, and packed in a new box.
• A full report outlining the pre and post-test data is included with the return shipment.
• Each box is marked with the element serial number to assist operators in determining the re-load sequence of the membranes into the system.

On-Site

Avista personnel are available to perform a variety of on-site support functions including; clean-in-place (CIP) supervision, chemical dosing, troubleshooting, system audits, and training. They can also oversee the commissioning of chemical dosing systems, set up chemical calibrations, and assist with demonstration CIP operations.

Avista Advice Training

Avista personnel believe that the experience gained over decades in the membrane industry should be shared with as wide an audience as possible in the interest of promoting best practice. It is mutually beneficial for our customers to have detailed knowledge of our product line, design tools, and technical services to ensure our formulations are applied in a safe and efficient manner.

The Avista Advice Training Program offers a series of short presentations on the theory, application and design aspects of our products and/or services. We also have a full range of presentations on wider membrane related topics including; system design, operational monitoring, and troubleshooting common problems. Presentations have also been prepared for OEM’s and end users to assist with employee training.

Avista encourages the customization of presentations using the list below and discussing specific requirements with our technical staff.

Sessions Available:

The Avista Advice Training Program covers the following topics:

• Membrane fundamentals and construction
• How antiscalants, coagulants and cleaners work
• Chemical selection and dosing unit design
• Cleaning plant design and recommended cleaning procedures
• Types of pre-treatment and chemical application
• Membrane plant monitoring and troubleshooting
• Membrane fouling and cleaning
• LSI or CCPP
• OSCAR and the membrane autopsy
• Avista Advisor

Software to Calculate Reverse Osmosis Chemical Requirements

Avista Advisor - Version 3

Since its earliest conception, Avista Advisor has been a user friendly and highly effective tool for reverse osmosis (RO) plant designers and operators. Its primary function has been the recommendation and dosing of Vitec antiscalants.

However, the radical redesign of Version 3 now makes Advisor a complete chemical design tool for RO systems. Additional functions include biocide and coagulant recommendations and dechlorination calculations, allowing designers to correctly size chemical feed pumps and storage tanks.

The new version also includes an improved cleaner selection page and the ability to directly input a membrane concentrate analysis, allowing a more accurate design for nanofiltration systems.

We believe users will find it to be an extremely useful design tool.

Chemical Dosing

Avista Advisor 3 allows you to calculate the following chemical dosing:

• Antiscalant
• Biocide
• Coagulant
• Cleaners
• Dechlorination
• pH adjustment

It will also allow you to calculate:

• Dosing pump flow rates
• Dosing tank sizes
• Chemical consumption
• Chemical usage cost

The software will allow you to produce datasheet printouts for each chemical that is calculated and a summary sheet for dosing equipment sizing and chemical consumption. The calculations can be performed in any order.

Below is a description of the antiscalant function of the Advisor. Contact Avista or its representatives for a complete copy of the Version 3 User Guide.

Antiscalant Projection

The antiscalant calculation in Advisor is based on the requirement of an inhibitor to control scale in the membrane system concentrate. This is normally accomplished by inputting raw water analysis, temperature and system recovery, and allowing the software to calculate the concentrate analysis. The concentrate analysis is then used to compute the scaling potential, which is then related to a requirement for antiscalant.

While this is generally accurate enough for reverse osmosis systems, the analysis of the concentrate from a nanofiltration plant is highly dependent on the selected membrane. Therefore, nanofiltration systems require the additional ability to input a concentrate analysis. This information is available on the projection obtained from the membrane manufacturer.

Raw Water

The raw water analysis may be input as mg/l ion, mg/l calcium carbonate or as meq/l. As each input is made, the corresponding values for that ion are automatically calculated and displayed. The ionic strength and total dissolved solids are also displayed, as are the levels of scale in the feed water and the carbonate and carbonic acid levels. A running total of anions and cations are calculated. If these fall short, the program adds the required level of sodium or calcium (in the event of a shortage of cations) or chloride or sulphate (in the event of a shortage of anions).

If acid or sodium hydroxide is used to alter the feed water pH, then the required chemical may be selected from the pH correction drop-down box. Hydrochloric or sulfuric acid are the two options for selection. If the feedwater pH is increased, sodium hydroxide may be selected. The revised level of carbon dioxide and carbonate is displayed as the pH calculation is made. pH adjustment is not normally required for scaling control.

It is necessary to input the feedwater analysis for all projections, including those cases where a brine analysis is input. This screen provides pH and temperature data used in other calculations.

Calculation

The software calculates the concentrate analysis based on typical membrane rejections. It calculates the scaling potential of the concentrate and the required level of antiscalant to protect the membrane system. This is then converted to a feed dose rate.

By adjusting the system recovery, the concentrate analysis will change, thus changing the scaling potential. The scaling tendencies for the salts that are a concern in an RO plant are displayed in graphical form, and in a table beneath the graph. If the user wishes to see the actual values of the salts in the concentrate, then these may be displayed.

The Vitec antiscalant may be selected by using the drop down list. The dose rate of antiscalant is displayed. In the event that the scaling potential is outside the displayed antiscalants limits, “NA” (not applicable) is displayed. A flow rate for the dosing pump is also displayed. In the event that the flow rate is below the limits of the available pump, then a dilution may be applied to the antiscalant.

Should the projected concentrate analysis be significantly different from the projected (or actual) analysis, and then the concentrate analysis may be input directly by pressing the “Input Reject Analysis” button.

Reject Analysis

For systems using nanofiltration type elements, the conventional membrane rejection calculations are not sufficiently accurate for a scaling projection to be made. For that reason, a concentrate analysis may be entered by clicking the concentrate button from the raw water analysis screen. If this option is taken, then the system recovery is included to calculate the feed flow rate only.

Antiscalant Selection

There may be a variety of Vitec antiscalants that can control scaling in the system. Please consult the antiscalant selection guide or your Avista representative for advice on the most appropriate product for your application. Your representative will consider relevant product approvals, reliability of the water source, and usage cost when advising the most appropriate product.

Click Here To View More Information