Clean Water Safe Water
Outbreaks of E-coli and Cryptosporidium across Canada have sparked public concerns over the safety of drinking water supplies. Regulatory agencies have reacted by imposing more stringent standards, and communities are upgrading their treatment pla...
Outbreaks of E-coli and Cryptosporidium across Canada have sparked public concerns over the safety of drinking water supplies. Regulatory agencies have reacted by imposing more stringent standards, and communities are upgrading their treatment plants to meet those standards.
In this environment, membrane technology is rapidly gaining popularity for plant upgrades. It provides a consistently high quality output regardless of the feed-water variability, and it is a near absolute barrier against waterborne pathogens. The technology also has a high production capacity but occupies a small footprint, and it can be expanded in modules.
Before consulting engineers and their clients jump on the membrane bandwagon, however, it is important to conduct a thorough review of an existing water treatment system to evaluate the viability of membrane technology. Following are key areas that should be addressed.
For many surface water treatment plants that use conventional clarification and granular filtration, membrane filters may be an attractive alternative for an upgrade. Identifying the client’s objectives, however, are paramount to deciding whether to use membrane filtration. The driving factors behind the project may include the need to increase the plant’s production or improve water quality, as well as issues relating to chemical consumption, power consumption, operations and residuals handling.
When the utility wants to increase production, membranes should definitely be considered. In a recent project in Alberta, Associated Engineering’s design for a water treatment plant that serves over 20,000 people doubled its capacity within the existing plant footprint. The membrane filtration cassettes were fitted into existing tankage, thereby avoiding the cost of expanding the building.
If the need for increased production is coupled with a need to meet more stringent treated water quality standards, membranes are an excellent fit. For example, membranes are one of the few technologies available for removing Cryptosporidium-sized matter.
However, in a scenario where the utility wants only to improve the water quality, membrane filtration may not be the best solution. It will provide excellent turbidity removal and a near absolute barrier against Giardia and Cryptosporidium, but may not be cost effective in every application. For a well operated, conventional facility, it may be more economical to integrate an ultraviolet (UV) disinfection system for inactivating Giardia and Cryptosporidium.
Source water characteristics
It is important to conduct a thorough historical analysis of the source water characteristics to ascertain their impacts on the membrane performance, membrane longevity and fouling.
Turbidity. Membrane filtration systems provide excellent turbidity removal. However, in applications with high raw water turbidity, measures should be taken to protect the membranes from damage that could be caused by high solids concentrations or particularly abrasive raw water particles. In low turbidity applications damage is normally less of a concern.
High turbidity can also affect the membrane performance by increasing the trans-membrane pressure for a given flux. Trans-membrane pressure is the pressure required to push or pull clean water through the membrane. If the trans-membrane pressure climbs outside the design operating range, the production rates will need to be lowered to drop the trans-membrane pressure to mitigate fouling.
Total and Dissolved Organic Carbon (TOC and DOC). The microfiltration and ultrafiltration membrane systems typically used for surface water treatment can only remove the non-dissolved portion of organic content. Dissolved organics are typically removed through chemical pre-treatment. Pretreatment chemicals and the organic carbon itself can contribute to membrane fouling so these processes need to be evaluated carefully.
Iron and Manganese. At high pH or in the presence of oxygen, iron and manganese form precipitates that could lead to membrane fouling. If the chemicals are kept in their reduced state and dissolved in water they should not present a problem.
Temperature. Water temperature affects membrane systems in a number of ways. Temperature alters membrane material characteristics and membrane life, and lowers the solubility of certain compounds, which can lead to chemical precipitation and membrane fouling. Most importantly for Canada, the viscosity and density of water increases at lower temperatures. At lower temperatures a greater membrane area is required to produce a specified flow rate at a given pressure. It’s important that there is enough membrane surface area to meet demands at low water temperatures.
Alkalinity. Alkalinity is not necessarily an important parameter in relation to membrane selection or membrane performance. It is important, however, when evaluating pH depression for enhanced coagulation and dissolved organics removal upstream of membranes. High alkalinity water will require greater amounts of acid or CO2 to lower the pH.
Colour. Colour is expressed as true colour or apparent colour. Apparent colour is determined on the original sample without filtration. True colour refers to colour measured after filtering the sample through a 0.45 micron filter. Membrane filtration will remove the colour related to suspended solids, thereby reducing apparent colour. True colour, which is generally DOC, will pass through microfiltration and ultrafiltration membrane fibres. As was noted with DOC, some form of pretreatment, typically chemical, would be required to remove true colour.
Taste and Odour. Taste and odour compounds are generally DOC and, therefore, will pass through microfiltration and ultrafiltration fibres. Integrating adsorption with activated carbon, upstream or downstream of the membranes, is one means of control. Oxidation coupled with or without adsorption, is another alternative. If oxidation with potassium permanganate is employed upstream of the membranes, increased fouling is possible. Also, if adsorption with powder activated carbon is employed upstream of the membranes, you will need to evaluate the abrasion characteristics of the powder activated carbon.
Treated water quality
Provincial water quality regulators such as Alberta Environment are considering more stringent turbidity standards and new particle count standards exclusively for membrane systems. It is anticipated the revised Alberta turbidity standard for membrane filtration will be combined filtered turbidity of less than 0.1 NTU in at least 95% of the measurements in a month and not to exceed 0.3 NTU at any time. Health Canada guidelines that are referenced by some provinces are also moving in that direction.
Giardia and Cryptosporidium cyst-sized particles in the 2 to 15 micron range, generally expressed as counts (cts) per mL of water, pose concerns. Alberta Environment is currently applying a 50 cts/mL maximum limit for membrane systems. Online particle count instruments are used as a continuous indication of membrane integrity. If permeate particle counts jump above 10 cts/mL for an extended period, the membrane fibre may have been breached and should be checked. In addition, particle count data can be used to substantiate log removal of particles in the Giardia and Cryptosporidium cyst-size range.
Integration with the existing system
The operation of a membrane system can be adjusted to accommodate quite a variation in seasonal flow and to meet future demands. However, there is a point where further adjustment becomes too costly. Therefore, it is prudent to install a number of membrane filter trains to cover the range of seasonal flow variations. Complete trains can be brought on or taken off line as required.
The modular nature of membrane systems makes them cost effective when upgrading a plant to meet future demands. The piping, valves, tankage, and pumping for future trains can be installed, but the membrane purchase itself can be deferred. This approach can s
ignificantly lower the life-cycle cost of a project. In fact, it’s not advisable to install more membranes than are required because the longevity of an installed, but not operational, membrane train is similar to that of a train in operation.
Space is often an issue in plant upgrades, so the ability to integrate membranes into existing unit processes makes them an attractive option. However, space for ancillary equipment must be considered as well. Ancillary equipment includes blowers, permeate pumps, backpulse tanks, chemical feed equipment, and air separation equipment in the case of immersed membrane systems.
Determining the best location for integrating a membrane system within an existing treatment facility depends on the circumstances. If the goal is enhanced performance, installing the membranes further downstream is prudent. If the need is for increased production, one should evaluate replacing the primary treatment process.
Often membranes can be retrofitted into existing tanks. This is particularly true for immersed membrane systems integrated into conventional treatment plants. Existing filters, clarifiers, and even portions of clearwells may be re-used.
It is important to recognize that space is required to move membranes in and around the plant. Doorways must be provided to move membranes into the plant for initial installation and replacement. You also need to provide adequate headroom for removing cassettes, particularly immersed membranes, from their installed trains.
The pretreatment unit processes that need to be evaluated for the integration of membranes are: screening, adsorption, oxidation and coagulation. Clarification upstream, and recovery (final product volume vs. feed flow) are also factors.
Integrating membranes into an existing plant will invariably alter the hydraulic profile. In the case of pressure-driven membranes, a pumping step will need to be provided upstream. With vacuum-driven membranes, the pumping unit process would be added downstream of the membranes. Other points in the hydraulic profile will need to be evaluated to ensure that additional pumping is not required. For example, in a pressure-driven system, the system should be reviewed to ensure adequate residual pressure is available to push permeate into the clearwell. In a vacuum-driven system, the membrane process tanks should be evaluated to see if they could be gravity fed from the pretreatment unit process.
Being able to turn the recovery rate down during high turbidity events will reduce solids loading and improve the membrane performance. However, the raw water feed system and waste handling system must be sized to handle the extra flow.
Membrane filtration systems have a waste or reject stream that is similar to a backwash waste stream to dispose of the solids that are rejected by the membranes. For typical surface water applications, the waste stream can vary from 5 to15 per cent of the total feed water flow. Installing a smaller second-stage membrane train will further concentrate the reject stream.
The concentrate waste streams can either be directly discharged to a receiving stream, decant lagoon or sanitary sewer, or may be further dewatered through mechanical processes. Clean-in-place chemical solutions must also be neutralized and disposed of, or recycled. It is advisable to consult with regulatory agencies during the early stages of the project to ensure they concur with the proposed design and disposal methods.
Operation and maintenance
Due to the process characteristics and high level of automation, membrane filtration systems generally require less attention from operators compared to traditional systems with respect to maintaining the quality of the plant’s output. But membrane systems do need the operators to focus more on the ancillary systems, on maintaining the mechanical components and calibrating the instruments. If a membrane filtration system fouls or fails, production, not necessarily water quality, is compromised.
It is important to involve administrative, operations and maintenance staff during the piloting and design phase of the project so that they understand the system and provide input. Selecting equipment and instrumentation that plant staff are familiar with is a benefit in any plant upgrade. In selecting ancillary equipment, one should consider what local support is available, the equipment’s serviceability, and whether it has the membrane supplier’s approval.
The membrane equipment supplier provides and programs the automation system to ensure the system is operated within its design parameters. This programmable logic controller (PLC)-based controls system can then be linked back to the overall plant control system for remote monitoring. It’s important to realize that the membrane supplier’s control system is focused solely on their equipment. Considerable effort and coordination by the consulting engineer and client is necessary to integrate that system into the overall plant controls.
Costs and selection
It is essential to review the full life cycle cost analysis and net present value of a membrane system when comparing systems. As a minimum, the operation and maintenance costs should include the plant’s power consumption, chemical usage, and cleaning frequencies. Accrual for replacing the membrane can make up a significant component of the cost analysis. It can be considered either as an annual maintenance accrual or as a capital cost at defined intervals, depending on the utility’s finance structure and preferences.
With membrane manufacturer warranties, it is important to compare extended warranty costs versus risk. Most manufacturers offer warranties on a pro-rated basis, meaning that the warranty value declines based on the useful life of the membrane consumed. Although the installation base of operating membrane facilities is only now reaching five to six years, some utilities have replaced membranes. Most of these replacements, however, were with newer models to increase capacity, rather than due to failures or wear.
Membrane equipment suppliers can and will — for a price — provide remote monitoring and service contracts for the first few years of operation. To reduce risks, utilities can consider negotiating a 10 to 12 year service contract that includes membrane replacement. Selecting a membrane manufacturer that has adopted the philosophy of “universal configurations” is important to accommodate future upgrades and products.
As the technology advances, the size and number of installations increases and the number of competing membrane manufacturers grows. As a result, the cost of membrane manufacturing and system packaging is likely to continue to decrease.
A significant amount of piloting work has been done on membrane systems over the last seven years. It has reached the point where the piloting objective is not so much to see whether a membrane system will provide a consistently low turbidity or low particle count water, but how it will operate as an integrated process and react to various levels of pretreatment.
Selecting the membrane manufacturer at the early stages of the project is very important. Also the engineer must work closely with the supplier through the design phase to fully integrate the system.
Once the membrane supplier has been selected. The owner has the option to maintain a direct supply contract with the supplier or establish a novation agreement for the supply contract with a contractor. In either case, the engineer must have a coordination role to ensure that the utility receives a fully integrated system. And the contract documents and specifications must be clear about lines of responsibility between the membrane supplier and the contractor.
Finally, when considering the use of performance guarantees, be careful with the wording of the guarantee and what the responsibility limitations are for each party. It is imperative to keep good records of operating data to substantiate or enforce a performance guarantee. Ideally, a performance guarantee will be tied into a service and monitoring
contract with the membrane supplier.
Tom Reinders, P.Eng. is an environmental process engineeer and Doug Olson, P.Eng. is manager of the water group with Associated Engineering in Calgary.This paper was one of several given by members of the firm at a water treatment conference the company held in March 2003.
WHAT IS MEMBRANE FILTRATION?
Membrane filtration encompasses a variety of technologies that use pressure or electricity as the “driving force” to separate contaminants from drinking water. Pressure driven membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Electrically driven membrane processes include electrodialysis (ED) and electrodialysis reversal (EDR).
Typically used for surface water applications, MF and UF units are constructed of porous hollow fibre membrane material. This type operates under relatively low pressure (or vacuum). It is typically used to remove particles and suspended solids, turbidity, pathogens, and inorganic precipitates (including iron and manganese). Membrane fibres are assembled into modules. These modules are either housed in a vessel operated under pressure, or assembled into racks, which are submerged into the feedwater and operated under vacuum pressure.
NF and RO units are generally constructed of spiral-wound, semi-permeable membrane material, operated under higher pressures. They are used to separate dissolved substances from the feedwater. NF and RO membrane processes are typically applied to ground water sources where feedwaters are very low in particulate matter, but high in hardness or dissolved organics and metals.