Pre-Sedimentation Solutions and Benefits for Water Treatment Facilities

By Bill Wiegand, PE & Jeff Ashley, PE

Pre-sedimentation (pre-sed) is an effective water treatment method, often utilized with source waters that have high solids, turbidity, and/or organics during certain times of the year, such as spring runoff. The pre-sed 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-sed systems can be built to 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 retention time is the primary variable used to settle large solids; to more sophisticated processes that can reduce larger amounts of turbidity and organics for volatile source waters. Whatever solution you and your design engineering team decide to use, 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.

A well-designed pre-sed process solution typically results in the following benefits:

Operational Flexibility

Implementing a pre-sed process in your water treatment plant will result in increased flexibility to overall treatment operations. Flexibility will help provide process efficiencies and improve finished water quality. For example, our work on the Fort Peck Water Treatment Plant in Northeast Montana used a pre-sed process that is primarily utilized for high turbidity during spring runoff but can be taken “off-line” in the winter. This flexibility provides operators the ability to achieve increased filtration efficiency during high turbidity, and increased economy when removed from service during low turbidity.

Reduced Operating Costs

Everyone wants lower costs. With the correct pre-sed solution that’s designed to meet the needs of your users, it’s possible to reduce operating costs associated with the main treatment processes. How? Pre-sed can improve filter run times, reduce waste/recycle water, and reduce chemical addition. Reduced chemical addition equals lower operating costs.

We observed these benefits on the Fort Peck project when the pre-sed process was brought on-line. The filter run times increased significantly since the pre-sed basins were capturing the majority of raw water solids and reducing the peaks in turbidity. Increasing filter run times directly affected the number of filter backwash cycles which resulted in reduced volume of recycle water wasted. Less backwash pumping, less pumping of recycle water and less volume of water wasted directly impacts cost savings.

High Performance Under Extreme Events

An existing treatment plant upgraded with a pre-sed process is more robust even under extreme source water degradations caused by spring runoff, forest fires, drainage basin ground cover loss, and other natural phenomena.

For example, we designed a pre-sed solution for the City of Chinook, Montana where the Milk River source water experiences very high turbidity peaks during runoff events, and sustained organic loads even in the winter months. This pre-sed solution uses a year-round pre-sed process, not only for large turbidity spikes in the unpredictable Milk River, but also oxidation of organics on a year-round basis.

Another project that needed this same kind of attention was the Lockwood Montana Water Treatment Plant. The treatment plant realized similar benefits when treating water from the Yellowstone River. In both the Lockwood and Chinook projects, turbidity removal and chemical conditioning occur in the pre-sed basin, which allows the main high-rate clarifiers to act as a polishing process to protect the filters from high turbidity loads. This results in optimized filtration even during spring runoff on the Yellowstone and Milk Rivers.

Many Pre-Sedimentation Solutions

As noted above, there are many pre-sed benefits to consider. What is right for one water treatment plant may not work for another. Understanding local conditions and working with an experienced engineer are critical steps to take when considering whether and what pre-sed process would work for your needs. Broadly speaking, however, there are a few approaches to designing a pre-sed system.

Earthen Lagoons

Earthen lagoons allow long detention times (days) to allow a plant to “wait out” peak solids and turbidity events. Earthen pre-sed lagoons can be economically advantageous because they tend to require less construction time and materials and design cost. Earthen lagoons can be just as effective as other options when ample space is available. Some disadvantages of earthen lagoons may include:

• they require more land
• freezing concerns in cold weather conditions
• may require additional pumping to fit into an existing plant hydraulic profile
• short circuiting through the lagoon, thus shorter detention times
• spring and fall solids turnover for larger lagoons, potentially causing treatment upsets
• more labor-intensive cleaning procedures (usually only on an annual basis). Cleaning earthen pre-sed lagoons typically requires de-watering and drying the basins over many months, so accumulated solids can be removed. For this reason, earthen lagoon systems are often built in a parallel footprint so one basin can be operational while the other is de-watered for cleaning.

Concrete Basins with Serpentine Channels

These pre-sed basins readily accommodate chemical additions with short detention times (15 – 60 minutes) typically for oxidation. We deployed this option at the above-mentioned Fort Peck Water Treatment Plant. Project owners and the design team had reserved space at the project site for the concrete basins during the preliminary design phase. Sodium permanganate is added to the raw water to oxidize organics and improve filtration performance. After oxidation, a coagulant is added followed by flocculation and high-rate settling for turbidity removal.

Lockwood also uses a serpentine basin with a chemical addition for turbidity removal. Here, sludge settles in the basin and is removed by pumping to waste lagoons twice per year.

Concrete Basin Clarifiers

These solutions offer intermediate detention times (2 to 4 hours) to settle large solids and grit to protect downstream treatment processes. The City of Chinook’s WTP was constructed as a rectangular clarifier to remove solids from the nearby Milk River and also to provide detention time for organics oxidation. Mechanical sludge removal provides an easy and consistent method to remove solids. Two parallel basins are operated for summer time demand, and only one basin is operated in the winter typically for oxidation purposes.

Concrete Basins with Conventional Flocculation / High-Rate Settling

These systems can remove 50 to 90 percent of turbidity depending on specific raw water quality. To manage changing source water conditions, the Fort Peck pre-sed basins also included coagulant chemical addition and rapid mixing, followed by flocculation and high-rate settling with inclined plate settlers. Solids captured in the sedimentation basin are conveyed to on-site earthen setting lagoons where the waste water can be evaporated or ultimately land applied.

Ultimately, a well-designed pre-sed system will be responsive to local conditions and needs, but as you can see, many variables come into play when designing a solution and selecting the correction options. Our team at Morrison-Maierle has provided information and design services for a wide variety of pre-sed systems in Montana.