Sludge happens, now what do we do with it? Here we discuss what sludge is, how we currently remove it, and what we may be doing in the future.
When most people think about wastewater treatment, they think about cleaning the water that goes down their drains and toilets. However, an important part of the treatment process happens after solids are separated from the wastewater.
These solids, commonly referred to as sludge, need to be collected, managed, and ultimately removed from the treatment facility. How we manage this material has a significant impact on the operation, maintenance, and long-term costs of a wastewater treatment plant.
At the Waterville Valley Wastewater Treatment Plant, we currently use a lagoon-based system to store sludge before transferring it to drying beds. Approximately once a year, sludge is pumped from the lagoons into the drying beds, where it remains for roughly another year before being removed from the facility by a third-party contractor.
While this approach has served the facility, our existing lagoons are aging and require repairs. As part of our ongoing plant upgrade planning, we are evaluating whether it makes more sense to rehabilitate the existing lagoons and continue with our current process or transition to a mechanical thickening system that would allow us to manage and remove sludge more frequently.
Both approaches have advantages and disadvantages, and our goal is to determine which option provides the most reliable and cost-effective solution for Waterville Valley over the long term.
What Are We Considering?
We are currently evaluating three potential approaches to sludge management. Each has different advantages, challenges, and impacts on the design and operation of the future facility.
1. Rehabilitating Our Existing Lagoons
One option is to repair and rehabilitate our existing sludge lagoons so we can continue using the same general process we have today.
Our current lagoons are not lined, meaning rehabilitation would require installing liners as part of the improvements. Under this approach, we would continue storing sludge in the lagoons, periodically pumping it into our existing sand drying beds, and having a contractor remove the dried material.
The benefit of this approach is that it would allow us to maintain a familiar sludge management process rather than introducing an entirely new mechanical system. This approach would also free the operators time as managing a lagoon is far less labor intensive than thickening sludge 1-2 times a week.
There are also several additional engineering considerations associated with rehabilitating the lagoons that are still being evaluated as part of our planning process.
2. Mechanical Thickening: Disc Thickener
Another option is to move away from lagoon storage and install mechanical sludge thickening equipment.
A disc thickener uses a series of rotating discs and filtration surfaces to separate water from sludge, increasing the concentration of solids and reducing the volume of material that needs to be hauled away.
One major advantage of disc thickening is its relatively small footprint. Space is an important consideration when designing a new wastewater treatment facility, and this technology could provide a more compact solution than other mechanical thickening options.
However, there is an important consideration: disc thickening is not currently approved by the New Hampshire Department of Environmental Services (NHDES). Its regulatory approval would need to be addressed before it could be selected for the project.
3. Mechanical Thickening: Rotary Drum Thickener
A rotary drum thickener is another mechanical option being evaluated. Similar to a disc thickener, it works by separating water from sludge to produce a thicker material that can be hauled away for further management.
Rotary drum thickeners are commonly used in wastewater treatment facilities and are approved by NHDES, making them an established option from a regulatory standpoint.
The primary drawback is space. A rotary drum thickener requires a larger footprint than a disc thickener, which has implications for how equipment and other treatment processes would fit into the design of the new facility.
Although the equipment takes up more room, it offers an established approach to mechanical sludge thickening and would eliminate the need for our existing sludge lagoons and sand drying beds.
Why Are We Evaluating These Different Options?
The decision is about more than simply choosing a piece of equipment. We need to consider how each alternative will affect the facility for decades to come.
Rehabilitating the lagoons may allow us to continue using a familiar process, but we need to understand the costs associated with repairs, lining, and future maintenance.
Mechanical thickening would represent a different approach. Instead of storing sludge in lagoons and allowing it to dry in sand beds, we would mechanically thicken the material and have it hauled away from the facility on a more frequent basis. This would eliminate the need for both the lagoons and drying beds, but would introduce new equipment, operating requirements, and maintenance considerations.
We are also considering the space required for each technology, the regulatory requirements, and the long-term costs associated with operating and maintaining each system.
Planning for the Future
As we continue planning for the wastewater treatment plant upgrade, evaluating sludge management is an important part of making sure the new facility is reliable, efficient, and sustainable over the long term.
There is no single solution that is automatically the best fit. Our goal is to work with our engineering team to understand the advantages and disadvantages of each alternative and determine which approach makes the most sense for Waterville Valley.
While much of the attention surrounding wastewater treatment focuses on the quality of the water leaving the plant, managing the solids removed during treatment is just as important. Making the right decisions today will help ensure we have a dependable sludge management process for years to come.