By Dr. Lyle Henson and Isaac Deluna
Hydroelectric generating stations are typically remote in nature and run by a small number of operation and maintenance personnel. The current staff of Manitoba’s Lower Nelson River Stations (LNRS) fluctuates between 20 – 25 people during the day shift and two or three people during the night shift. During heavy maintenance periods (spring and fall) there can be 25 – 30 people during the day shift and 15 – 20 during the night shift.
The original water treatment systems at the LNRS were fed from the station service and cooling water system carrying Nelson River water from the forebay through the turbine unit main strainer to a hydropneumatic tank. Historically, prior to entering the tank, raw water was dosed with chlorine. From the hydropneumatic store tank, “treated” water was then distributed throughout the generating station.
For years, this process was the water treatment system at Limestone Generating Station and Long Spruce Generating Station for production of potable water. A multimedia filtration system was also in operation at Kettle Generating Station during this time. The old water treatment systems at the three stations were not able to meet Manitoba Regulation and Guidelines for Canadian Drinking Water Quality (GCDWQ). So, a decision was made to seek viable replacement options.
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Consideration for the remote nature of these facilities and the limited availability of operations personnel had to be considered when choosing appropriate technology.
Water treatment alternatives considered
Water quality data gathered by Manitoba Hydro indicates that the Nelson River contains high levels of turbidity (up to 60 NTU) and dissolved components. Research indicates that turbidity levels at 30 NTU or higher significantly decrease the effectiveness of chlorination in the treated water, which creates a high risk of microbiological contaminants in the drinking water. Surface water (Nelson River water) typically contains high levels of dissolved organic matter. When mixed with chlorine, it creates byproducts such as trihalomethanes (THMs) and haloacetic acids (HAA) which are considered carcinogenic.
Various water treatment technologies were considered for the upgrade of the water treatment systems at the LNRS. The four most viable were water hauling, microfiltration, ultrafiltration and nanofiltration (tubular membranes).
The first option considered was hauling treated water by truck to the generating station. Depending on the water usage at the generating station, water could be delivered as required. There are advantages with this option regarding permitting, operator classification and operation and maintenance requirements. Classification and permitting would only be required for the distribution system (level 1) and minimal chemicals would be required.
On the other hand, there are disadvantages. The potential of cross contamination when handling water from the water plant to the point of use is a serious concern. The cost of transporting water is high when considering the purchasing/leasing of trucks, maintenance, and garaging. Finally, the process is vulnerable to delays in water delivery due to weather conditions, truck breakdowns, and truck operator availability. The Town of Gillam is a remote location where the closest city/town is 300 km away and it is the only source of water when considering this option.
Preliminary engineering draft assessment conducted by a consultant indicated that the Town of Gillam water treatment plant could not handle any additional load unless upgrades were made to the water treatment process and infrastructure. In addition, its pressure filters were not meeting turbidity standards.
Microfiltration produces higher levels of water quality than more traditional methods of coagulation and filtration and will typically meet or exceed current water quality regulations. One of the major drawbacks of this system is that the raw water requires pretreatment to reduce dissolved organics. If the system is installed without pretreatment, there is a high chance that turbidity and particulate content of the raw water will cause membrane fouling, frequent cleaning cycles, higher operator involvement and increased pathogens.
Because of the pretreatment required, this alternative presented huge disadvantages, such as high capital cost, a larger footprint because of the two-stage treatment, associated chemical costs, and a higher operator classification level.
As part of the potable water treatment plant upgrades at Lower Nelson River Stations, a study was conducted at Henday Converter Station to prove the viability of installing a microfiltration water treatment system without pretreatment.
Two water quality tests were performed at Henday, and in one of the tests, the treated water showed non-compliance with the total trihalomethanes parameter mandated by MR 41/2007. Also, it was determined that dissolved organic carbon removal was poor with this technology.
Pilot plant (ultrafiltration/hollow fibre)
With this technology, raw water is pumped through a coarse filter (50 micron and 100 micron), then through an ultrafiltration (UF) membrane unit and then to a post UF water storage tank. Ultrafiltration membranes are backwashed periodically to avoid differential pressure increase. A chlorination system is provided for injection into the backwash water entering the UF membranes. Backwash water coming out of the membranes is collected in a wash water tank and then disposed of slowly.
Filtered water is then injected with antiscalant solution to control scale on the surface of the membranes. A booster pump feeds the water into two housings of ultrafiltration membranes. While passing through the membranes, water divides into three streams: product, reject, and recycle water going back to the ultrafiltration pump.
A pilot plant was installed at Limestone Generating Station in 2016 to determine if this technology was suitable for use at the LNRS. The pilot plant resulted in the following findings:
The 50-micron filters constantly plugged up (every second day), requiring operator intervention. These filters were located downstream from the raw water supply. Note, new filters would need to be purchased and installed. This negatively impacted the operation and maintenance cost of the proposed system.
The UF system frequently stopped due to high differential pressure at the 50-micron filters. The pilot plant could not run for more than half a day without operator intervention.
This technology required the use of antiscalant (chemical product) prior to UF, which adds another cost to the operation. This antiscalant has been approved but it may have a negative impact on the environment.
Pilot plant (nanofiltration/Fyne process)
A small nanofiltration pilot plant was installed at the Limestone Generating Station and the Radisson Converter Station in 2014. A small Mini-Fyne pilot unit was used for the pilot tests, fitted with three full-scale 4m C10 modules, each containing a different membrane to be tested (AFC30, AFC31 & ES404).
The semi-permeable nanofiltration membrane is coated on the inside of the membrane tubes. Membrane tubes are connected by “U”-shaped connectors in a series flow path within each module.
There is only one inlet and one outlet connection on each module for raw water. A 12 mm diameter foam ball is fitted in one of the raw water connections on each module. A screened “foam ball catcher” at each end of the flow path keeps the foam ball from leaving the system. During operation, flow reversal causes the foam ball to pass through all the tubes in the module before being caught in the foam ball catcher at the other end of the module. This provides cleaning of the inside wall of the membrane tube.
A pressurized feed of raw water was supplied to the unit for the pilot. The Mini-Fyne unit has a higher–pressure recirculation pump, driven by a variable frequency drive, to provide the correct flow conditions at the membrane surface for process and foam ball clean.
When the unit is filtering, raw water is circulated at pressure by a recirculation pump through the inside of the membrane tubes. Additional raw water is drawn into the recirculation loop. Clean filtered water passes through the membrane tube wall and is collected in the module shroud.
The concentration of organics and other contaminants slowly builds up in the recirculation loop and this water is discharged periodically to waste in a flush cycle. The recirculation pump is slowed down and the reject by-pass valve is opened. The frequency of opening of the valve controls the recovery (percentage for raw water converted to filtered water).
During the filtration process, the inside walls of the membrane tubes slowly become coated with contaminants from the raw water. To maintain cleanliness of the membrane surface and to discharge the concentrated raw water contained in the recirculation loop, the unit periodically and automatically performed a “foam ball clean”. During the foam ball clean, the direction of the flow of the raw water in the module is reversed.
This causes the foam ball to pass through each of the 72 membrane tubes inside the module, cleaning the inside surface of the membrane tube. At the same time the reject valve opens, discharging the concentrated raw water and drawing fresh water into the recirculation loop. The foam ball clean occurred every six flush cycles.
For the pilots, the C10 modules were fitted with PCI type AFC30, AFC31 & ES404 membranes. These membranes are manufactured from polyamide (AFC30 & AFC31 75% retention of CaCl2) and polyethersulphone (ES404 4000 MWCO) material.
The unit is automatically controlled by a programmable relay controller to undergo automated foam ball cleaning cycles on a pre-set frequency. A VFD driven recirculation pump draws water into the recirculation loop and filtrate leaves the system through a permeate flow meter. The recirculation loop water becomes more concentrated over time and is purged to drain each foam ball cycle. The foam ball interval setting controls the operating recovery.
The pilot plant resulted in the following:
- High quality potable water that exceeds Manitoba Regulation and Guidelines for Canadian Drinking Water Quality.
- Low operation and maintenance requirements, essentially “hands off” operation.
- No chemical required for treatment.
- Chemical cleaning extended to three to four months due to mechanical foam ball clean.
- Superb colour removal.
- Outstanding turbidity removal.
- Smaller footprint.
- Easy installation.
The results conclusively showed that the AFC30 membrane exceeded the GCDWQ and would meet the needs of the Limestone, Long Spruce, Kettle and Radisson facilities. Additionally, the unit proved to be largely hands off and required a minimum amount of manpower to operate. There was only a single clean performed during the pilot and this was prior to start-up at the Radisson facility.
As anticipated, the pilot confirmed the need to perform a chemical clean every three or four months due to the efficacy of the foam ball clean. The results of both pilots supported the use of the Fyne process for all of Manitoba Hydro’s generating stations and HVDC facilities due to performance, limited manpower requirements and cost.
Dr. Lyle Henson is with Membrane Specialists. Isaac Deluna is with Manitoba Hydro. This article appears in ES&E Magazine’s June 2018 issue.