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The Town of Gillam Water Treatment System Upgrading

Background

The original water treatment plant serving the community of Gillam, Manitoba was constructed in 1966. It was designed to treat approximately 1,800 m3/d of Kettle River water using a Graver Reactivator solids contact clarifier and two Graver Monovalve filters. A raw water supply from the Nelson River and a third filter were added in 1976. The intent was that Nelson River water would be treated by direct filtration, with Kettle water being pretreated in the clarifier as before. This scheme was never implemented, and direct filtration has not been used at the plant.

Because of problems with the filters, the media and some strainers were replaced in 1993. However, an examination in the fall of 1994 indicated that the filters were again badly fouled. Over the years, treated water demand had also increased, to the point where consumption was vastly in excess of normal expectations for a community the size of Gillam. This very high demand was largely the result of consumers running their services to waste continuously to avoid freezing problems during cold weather.

On-site evaluations of the plant were carried out by Reid Crowther & Partners (RCPL) in April 1995. Many serious problems with the existing system were evident, and it was very clear that remedial action was essential. Subsequently, RCPL was retained by the Town to provide engineering services for the required upgrading work.

Preliminary work on the water treatment plant upgrade and water distribution upgrade began in the summer of 1996. During the initial work, it was necessary to bypass the water treatment plant and feed chlorinated raw river water directly to the distribution system. A boil water advisory for drinking water distributed in the Town was issued in the summer of 1996. A separate filtration system was provided for drinking water consumption as an interim measure during the upgrade work.

During the conceptual design stage, conventional treatment including flocculation, gravity settling and filtration was initially considered. However, following pilot testing and cost comparisons of the available options, the Town of Gillam decided to proceed with the design and construction of a new treatment plant system, based on Dissolved Air Flotation technology. Due to the raw water characteristics (in particular, excessive amounts of dissolved or suspended gases which would require degasification using conventional treatment), dissolved air flotation (DAF) was selected as the preferred treatment option. The higher loading rates and smaller retention time required by the DAF system also allowed the existing building to be modified and reused whereas conventional gravity settling would have required an expansion of the existing facility, increasing the cost of the upgrade considerably. A brief description of the DAF process is outlined in the following paragraph.

DAF Process

In the DAF process, raw water is first dosed with a chemical coagulant and the water flocculated in a conventional manner. After flocculation, the water flows evenly into the flotation cell at a low level, then upward past a manifold of nozzles through which a recycled water flow containing microscopic air bubbles is injected. The air bubbles adhere to the floc particles, causing them to become buoyant and rise to the surface of the cell where they form a light, floating sludge. The clarified water leaves the unit at a low level. The surface sludge is removed by a traveling scraper mechanism.

Water Treatment Plant Upgrade

Using the flow data from 1992, the design of the Gillam water treatment plant was based upon the following criteria:

Average Day Demand: 2500 m3/d

Maximum Day Demand: 3750 m3/d

Maximum Hour Demand: 5000 m3/d

The footprint of the two DAF trains required to handle the maximum day demand is 31m2. Had a conventional gravity system been used, the size of the settlers alone would have been approximately the size of the existing building (204 m2).

The original raw water source was the Kettle River, with the Nelson River pumping station and forcemain being added later. The Nelson River was a better water quality source and it was decided to upgrade and increase the supply from the Nelson River to handle the total demand. The Kettle River source will remain as a backup.

Water is pumped from the Nelson River to the water treatment plant and is directed to one or both treatment trains, as dictated by the system demands. The water treatment system consists of two DAF trains and four filters. Each DAF train is comprised of one rapid mixing cell for the injection of alum, two flocculation cells and one DAF tank. Water flows by gravity from the DAF train to a common filter inlet channel. Water is split between the four filters. After filtration, fluoride, chlorine and caustic soda, are dosed as required, just before the water enters the reservoir. From the reservoir, water is pumped into the distribution system by new distribution pumps. As part of the water distribution system upgrade, approximately 24 l/s of water is continuously recirculated back to the water treatment plant and mixed with the filter effluent just before it enters the reservoir. In addition, in the winter a portion of the flow leaving the water treatment plant is heated to ensure that water entering the distribution system is at least 4° C. This temperature ensures that water will not freeze in the distribution system.

Retrofitting of the water treatment plant began in June 1997. The water treatment plant has been in operation and producing good quality water since December, 1997. A one-year old boil-water advisory was lifted in December, 1997, only 4 weeks after commissioning of the water treatment plant began.

Water from the Nelson River is typical of surface water sources with high suspended solids, algae, bacteria, turbidity and colour being the water quality characteristics which required treatment by this system. The removal of turbidity is one of the water quality parameters being treated by this system and will be discussed further. Surface raw water typically has a turbidity range between 30-50 NTU. DAF treatment alone reduces this value to between 0.4 and 2 NTU. However, when a DAF train comes on line, up to one hour may be required before the DAF effluent is below 1 NTU. Thus, the filters are used for polishing and to ensure that final effluent quality remains consistent even after the start-up of a DAF train. Filter effluent turbidity values range from 0.2 to 0.8 NTU. Since highly turbid water had been introduced into the distribution system over the last several years, it is interesting to observe how the quality of the water in the distribution system has improved over the last year. It is important to note that distribution turbidity is measured just before water enters the system, i.e. after the filtered water and recirculated water have been mixed in the reservoir. From Figure 1, it is evident that several months were required before the system was entirely flushed. It is only in the last several months that distribution water turbidity is equal to that of the filter effluent i.e. less than 0.2 NTU.

Water Distribution System Upgrade

In conjunction with the Water Treatment Plant upgrade, a water distribution analysis was conducted. There were four main objectives of the water distribution analysis:

  • Identify potential water distribution supply and return loops from the Water Treatment Plant to reduce the bleeding of water to prevent freezing within the system
  • Identify water distribution system upgrades to provide complete water distribution recirculation system and fire protection
  • Identify minimum required recirculation flow and temperature from the Water Treatment Plant to prevent system freezing
  • Planning of the required upgrades to the water distribution system infrastructure within the Trailer Park area.

The Cybernet water distribution model by Haestad Methods was used to complete the analysis. Heat loss calculations were completed utilizing the Cold Climites Utilities Manual by the Canadian Society of Civil Engineers.

The results of the project work provided a template upon which the Town of Gillam could proceed with upgrades to the existing infrastructure.

The detailed design of the water distribution system infrastructure within the Trailer Park area was based upon the concepts presented in the water distribution analysis project work. The Trailer Park project included the installation of a 4,300 m of shallow bury insulated ductile iron watermains, appurtenances and 200 individually recirculating watermain services. 

Implementation

The first two phases of the water distribution upgrades began in July, 1996 and were completed in September, 1997. This work is approximately equal to 90% of the planned upgrades. Within days of completing the work, a noticeable decrease in demand was observed. The average day demand based on available data from 1998 is approximately 1,100 m3/d. It estimated that demand has been reduced by approximately 55% compared to the average day demand of 2,500 m3/d. The decrease in demand is due to the upgrades as a significant amount of bleeding in the system has stopped.

Dissolved Air Flotation

Flotation was first developed in the mining industry for the separation of minerals, and in the 1930’s the paper industry adapted the process for fibre separation. Since that time, the process has been continually refined; in the early 1960’s, it was first used for the clarification of water for potable use. In the mid 1970’s, new plants using DAF were constructed in the U.K. and Holland. Now, hundreds of DAF plants are in operation, primarily in Europe.

In the DAF process, raw water is first dosed with a chemical coagulant and the water flocculated in a conventional manner. After flocculation, the water flows evenly into the flotation cell at a low level, then upward past a manifold of nozzles through which a recycled water flow containing microscopic air bubbles is injected. The air bubbles adhere to the floc particles, causing them to become buoyant and rise to the surface of the cell where they form a light, floating sludge. The clarified water leaves the unit at a low level. The surface sludge is removed by a traveling scraper mechanism.

It is very important to the process that the air bubbles be extremely small (typically 30 to 80 microns in diameter). Larger bubbles rise too rapidly, and shear the floc rather than adhere to it. Unfortunately, bubbles of this size cannot be formed simply by the direct injection of air into the main flow stream. Therefore, a technique is used that depends on the controlled release of dissolved air from a pressurized, air-saturated stream of recycled water.

Clarified water is taken from the outlet of the flotation cell and pumped through to a saturator vessel. Air at a pressure of 350 to 600 kPa (50 to 85 psi) is also injected into the saturator. The recycle stream, saturated with air and under pressure, is injected into the flotation cell at the inlet end. As the pressure is released, the dissolved air comes out of solution and forms the microscopic air bubbles, which then attach themselves to the floc.

The DAF process has a number of significant advantages over conventional gravity settling:

  • The process does not depend on a large, settlable floc, so flocculation times can often be shorter and flocculation equipment smaller and less expensive.
  • DAF is particularly suited to cold water, where the very small density difference between the floc particles and the water is an advantage rather than a disadvantage.
  • The effect of floc breakup is not nearly as significant, because small particles can be floated and removed just as easily as larger ones.
  • Because the process does not depend on quiescent settling, uneven flow distribution is much less significant.
  • Low turbidity water, or water containing significant amounts of algae, are particularly suited to DAF because the low density of the resultant floc is not a problem. Typically, DAF will easily remove over 95% of algae from raw water, compared the 60 to 75% removals commonly achieved in gravity settlers.
  • The solids content of DAF float sludge is usually significantly higher than that of settled sludge, so the handling and disposal problems and costs are reduced.
  • The system is very responsive to changes in operating conditions, and will usually produce an acceptable quality water within minutes of startup. The floc separation process does not rely on the prior establishment of a sludge blanket, and the process is therefore very tolerant of rapid changes in flow rate.
  • In some cases, coagulant chemical requirements can be reduced because a large, settlable floc is not required. In other cases, polymer may not be necessary, so there are capital and operating cost savings and the potential problem of fouling the filter media by improper use of polymer is eliminated.
  • Solids retention time in a DAF unit is very short - in the order of minutes. Therefore, there is no chance for organic matter to decay and possibly result in taste and odour problems in the treated water. The problem of sludge gassing and floating is eliminated completely.
  • DAF is a high-rate process, and a DAF cell is much smaller than a gravity settler of equivalent flow capacity (typically between one-fifth and one-tenth the size). The cost of the equipment itself, as well as associated costs such as building, land, heating, and others, are correspondingly lower. Operating costs, however, may be slightly higher because of the power requirements of the recycle flow system (offset to some degree by savings in chemical costs).

As may be expected, DAF is not totally without its’ disadvantages. Because the process relies on flotation of solids, it is not particularly suited to treating waters containing large amounts of heavy, dense particles (often indicated by high turbidities). A DAF unit may require somewhat more skilled operators than would a conventional cross-flow type gravity settler, because the process can be slightly more dependent on chemical types and dosages, and there are additional variables such as recycle flow rate and saturator pressure to consider.

Authors:

John McDonald – Water and Wastewater Supervisor - Town of Gillam

Karen Sutherland – Environmental Engineer – Reid Crowther & Partners Ltd.