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Improving high rate clarification systems in water treatment

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water-clarification
High rate clarification at Bearspaw water treatment plant.

By Sarah Laidlaw

The City of Calgary operates two conventional surface water treatment plants with a combined production capacity of 950 ML/d. Glenmore water treatment plant was originally constructed in the 1930s and draws its source water from the Elbow River Watershed. Bearspaw water treatment plant was constructed in the 1970s and draws from the Bow River Watershed.

aerial view of treatment plant
Aerial view of the Bearspaw water treatment plant.

In both watersheds, surface water turbidity ranges from 1 NTU to well over 1000 NTU during spring runoff events. In order to handle the large variation in source water turbidity, provide reliable year round capacity and ensure consistent, high quality treated water, both plants were upgraded in 2007, with high rate ballasted flocculation (Actiflo®) clarification systems for pretreatment.

This process consists of coagulation, polymer and microsand injection, flocculation and high rate settling. The settling tanks are equipped with lamella tube settlers, designed for a nominal 40 m/h rise rate, covering a surface area of approximately 104 m2.

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There are four process trains at Glenmore and six trains at Bearspaw, each designed to produce nominally 100 ML/d.

Operating high rate clarification systems provides a number of production and performance benefits. The units have a much smaller footprint than conventional clarifiers (up to 50 times smaller) and the process is able to handle a large variation of incoming turbidity levels. At both Calgary plants, the process has proven to be resilient during high turbidity events. In addition, there is a high chemical (alum) turndown capability, wide nominal flow range (+/- 50%) and solids are removed from the process in real time. Although there are many advantages, the system does require a higher level of attention during variable water conditions.

Lamella tube settlers require periodic cleaning to prevent buildup of floc particles. When operating under high rate loadings with the combination of polymer, microsand and higher source water turbidity, the risk of material buildup increases. This leads to an increased risk of overloading the lamella support systems and results in higher frequency cleaning requirements.

A common practice in Calgary was to wash down the tube settlers from the top, using spray water. However, with higher frequency cleanings and limited access to the larger surface area units, it becomes difficult to achieve uniform coverage using spray wash methods and the surface openings of the tube settlers are more susceptible to damage, further increasing the risk of material buildup.

Challenges faced from 2011 to 2013

In 2011, after the first four years of operation, the City started experiencing surface deterioration of the lamella tube settlers and increased solids buildup within the tubes. Cleaning typically involved fully draining down the tanks to manually clean all areas of the tube settlers. The City began to experiment with an integrated air scour cleaning system to reduce the use of spray water and the frequency of manual maintenance procedures.

Tube settlers before and after cleaning process.
Tube settlers before and after cleaning process.

In the spring of 2013, the lamella support system for one of the clarifiers collapsed due to an increased buildup of solids within the tubes. In June 2013, Calgary experienced one of the most extreme runoff and flood events recorded. The water treatment plants saw source water turbidity rise to 3800 NTU at Bearspaw, and over 4200 NTU at Glenmore, before the turbidity instrument range maxed out.

The City managed to keep the clarified water turbidity coming out of the pretreatment process below 5 NTU throughout the flood event, and maintained consistent high quality production of drinking water. However, these events emphasized the need for improved resiliency of the clarification system and accelerated the development of a permanent and efficient air scour cleaning system.

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