Water Management
Pre-Sedimentation Solutions for Water Treatment Facilities
Custom pre-sedimentation systems can be tailored for unique water quality conditions
Pre-sedimentation solutions for water treatment facilities are often utilized with source waters with high solids, turbidity, and/or organics during certain times of the year, such as spring runoff. This process helps reduce these materials from the raw water to more acceptable levels. As a result, other primary treatment processes, such as high-rate settling and filtration, are greatly improved and optimized. Pre-sedimentation (pre-sed) systems can be designed for site-specific needs to address unique raw water quality in lakes, rivers, and streams.
The pre-sed process can involve fairly simple methods, such as settling ponds where hydraulic detention time is the primary variable used to settle large solids, to more sophisticated processes that can reduce larger amounts of turbidity and organics from volatile source waters. Whatever solution you choose, the end goal is always the same: reduce source water solids, turbidity, and/or organics as economically and sustainably as possible before the water is introduced to the main treatment plant processes.
The 3 Benefits of Pre-Sed Process Solutions
A well-designed pre-sed process solution typically has three benefits: operational flexibility, reduced operating costs, and high performance under extreme events.
1. Operational Flexibility
Implementing a pre-sed process in your water treatment plant will increase flexibility in overall treatment operations. Flexibility will help provide process efficiencies and improve finished water quality. For example, our work at a plant in North Central Montana used a pre-sed process primarily utilized for high turbidity during spring runoff, but can be taken “off-line” in the winter.
However, the above-grade concrete basin was designed and constructed with a cover, exterior insulation, and a small pumped heated water loop to allow winter operation even during extremely cold air temperatures. By adding a pre-oxidant and allowing detention time in the pre-sed basin, operators have benefited from improved treatment, dealing with organics and color during cold water conditions.
2. Reduced Operating Costs
Everyone wants lower costs. With the correct pre-sed solution designed to meet your needs, you can reduce operating costs associated with the main treatment processes. Pre-sed can improve filter run times, reduce waste/recycle water, and reduce chemical addition. All these equal improved plant efficiency and lower operating costs.
We observed these benefits on a recent project when the pre-sed process was brought online. The filter run times increased significantly since the pre-sed basins captured the majority of raw water solids and dampened the peaks in turbidity. Increasing filter run times directly affects the number of filter backwash cycles, which results in a reduced volume of backwash waste and less recycled water to return to the head of the plant. Less backwash pumping, less pumping of recycled water, and less volume of water wasted directly impact cost.
3. High Performance Under Extreme Events
An existing treatment plant upgraded with a pre-sed process is more robust even under extreme source water conditions caused by spring runoff, and other events that lead to water quality degradation.
For example, we designed a pre-sed solution for the Milk River in Montana, which experiences very high turbidity peaks during runoff events and sustained organic loads even in the winter months. This solution uses a year-round pre-sed process for large turbidity spikes in the unpredictable spring and for organic oxidation on a year-round basis.
Another project that needed this kind of attention realized similar benefits when treating water from the Yellowstone River. In both these projects, turbidity removal and chemical conditioning occur in the pre-sed basin, which allows the high-rate clarifiers in the conventional treatment process to act as a polishing process to protect the filters from high turbidity loads. This results in optimized filtration even during events due to spring runoff.
Morrison-Maierle’s pre-sed project at the Fort Peck Water Treatment Plant is an extremely flexible solution that uses concrete basins with serpentine channels.
The Fort Peck pre-sed solution can be taken off-line in the winter when turbidity is low. It incorporates baffled concrete basins to provide pre-oxidation detention time.
Many Solutions to Consider
As with any treatment process, there are many pre-sed options to consider. What is right for one water treatment plant may not work for another. Understanding local conditions and working with the right design team are critical steps to take when considering whether a pre-sed process would work for your needs. Earthen basins, concrete basins with baffles, and concrete basin clarifiers are some pre-sed designs we have implemented in our water treatment design projects.
1. Earthen Basins
Earthen pre-sed basins allow long detention times (days) for a plant to “wait out” peak turbidity events if needed. Earthen basins can be economically advantageous because they require less construction time, materials, and design cost. These can be as effective as other options if the site has enough land to accommodate a larger facility. This is also the main disadvantage of earthen basins; they require additional space that some plants do not have. They also may require additional pumping to fit into an existing plant hydraulic profile.
Other things to consider are short-circuiting through the basin, thus shorter detention times, and spring and fall solids turnover for larger basins, potentially causing treatment upsets.
Finally, earth basins may require more labor-intensive cleaning procedures, usually annually or every several years. Cleaning earthen pre-sed basins typically requires de-watering and drying the basins over many months, so accumulated solids can be removed. For this reason, two or more earthen basin systems are often built in a parallel configuration so one basin can be operational while the other is de-watered for cleaning. Earthen pre-sed basins allow maintenance staff to dredge them while remaining online and managing the material simultaneously.
2. Concrete Basins with Baffles
These pre-sed basins readily accommodate chemical additions with short detention times (15 to 60 minutes), typically for pre-oxidation. We deployed this option at a plant with high organics in the raw water source. Sodium permanganate is added to oxidize organics and improve filtration performance. If needed, the baffled concrete pre-oxidation basin can be converted to a stronger oxidant, such as ozone.
The disadvantage of baffled concrete basins is that accumulated sludge must be removed manually. Typically, this is accomplished by flushing and draining or flushing and pumping out.
3. Concrete Basin Clarifiers
Depending on specific raw water quality and your goals for the pre-sed process, these systems can remove 50 to 90 percent of turbidity. Our team designed a project to manage changing source water conditions that included chemical coagulant addition with rapid mixing, flocculation, and high-rate settling with inclined plate settlers. Solids captured in the sedimentation basin are conveyed to on-site earthen settling ponds used for filter backwash waste.
Another variety of concrete clarifiers utilizes chain and flight sludge collectors in a rectangular basin. These can be advantageous for heavy sediment removal. Organic reduction or turbidity removal is less important than larger material, such as grit and sand.
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An overhead view shows the construction of a pre-sedimentation construction project.
Morrison-Maierle’s Lockwood pre-sed solution uses a serpentine basin with a chemical addition to remove turbidity.
This image shows an example of a large earthern pre-sed basin
Take Aways
Ultimately, a well-designed pre-sed system will be responsive to local conditions and treatment goals. Still, as you can see, many variables come into play when designing a solution and selecting options that meet your needs. Our team at Morrison-Maierle has provided information and design services for various pre-sed systems.
Let’s discuss a solution for your water systemAbout The Author
As the Water Treatment Technical Leader, Jeff Ashley enjoys helping his team deliver high-quality projects to help our clients meet their project goals.
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