Retrofitting Existing Headworks - Getting It Right
A municipal wastewater plant in the United States has a typical design life of 30 years. However, within the treatment system itself, a common design life expectation is 15 to 20 years for individual pieces of equipment. In fact, there are installations where pieces of equipment are still faithfully operating well beyond their twenty-year life expectancy.
The next time you are in London, you can visit a steam-powered wastewater lift station at Crossness originally part of Bazalgette’s redevelopment of the London sewerage system. Originally put into service in 1865, it was decommissioned in the 1950s. Similarly, an operational example in the United States can be seen in Phillipsburg, NJ. The steam powered water pumphouse was commissioned in 1913 and used in service for 56 years. This machine was restored in 2018, and it can still be seen in operation today.
Speed of Change
In the article “Practical Challenges Going From Waste to Resource,” we see that the treatment objectives of a modern WWTP can change faster than the lifespan of a particular treatment technology used in the process. When a new wastewater facility is being designed and built, the engineers, contractors, and plant operators look for the most effective and cutting-edge solution available at the time of the install. But while many circumstances can be foreseen and planned for, changing regulations and treatment environments can usurp plant designs, quickly make them obsolete.
As newer and more complex forms of treatment were employed, new processes were added to existing systems. Straight-forward technologies such as mechanical aeration, clarification, and digestion, proved susceptible to bypassed debris and sedimentation. As more material accumulates, it inhibits performance, and in some cases, shuts down a process entirely. All these issues point back to the need for increased performance in headworks processes to protect what happens downstream.
One Thing Affects Another
Attempts at protection affect processes, too. Even a simple change to finer openings in the headworks screen will impact operations both directly at the screen as well as downstream. Discussed in our previous blog, “Don't Flood the Building - Properly Sizing Screenings Handling Equipment,” we saw how improved performance of a finer screen translated to increased material handling. As the screening openings get smaller, the more variables both upstream and downstream need to be considered.
Sometimes a change at the headworks can be as simple as replacing a worn-out screen with a similar system. In that case, the basic considerations of fit and finish are all that is needed. More commonly, though, a retrofit looks to improve performance or accomplish a new treatment objective. The most desirable scenario for a retrofit is the ability to use existing structures to accommodate the new system with only a minimum amount of civil, structural changes. As changes in product happen all the time, it pays to know someone who can advise you about the latest technologies available.
In some instances, finer screening is only required for a smaller portion of the total incoming flow. Here it may be more practical to create a treatment step further in the process, instead of trying to retrofit the primary headworks. An example might be a situation where land application for sludge requires no visible debris. An inline sludge screen would be an effective solution with this scenario.
Similarly, the addition of a fine enough screen to protect an MBR process would typically be placed downstream of a primary headworks screen and grit chamber. This would leave the existing headworks intact, and the MBR screen would be placed closer to the end process. In many cases, this type of screen could be retrofitted with its own self-contained tank.
Retrofitting the Headworks
When the requirement points to changing the screening installation to benefit the entire flow, then it is best to consider modification of the primary headworks. As we mentioned earlier, a retrofit that would minimize any structural changes of the existing layout is most desirable. The most common headworks modification is the use of a finer opening; the primary treatment objective of increased efficiency is to capture a defined size of screenings debris (described as SCR).
For most screen types, the smaller the opening of the screen, the less open area there is relative to screening area. If the channel size remains the same, then the available net screen area needs to increase. Certain screens such as a Step Screen® can go from a 1/4th-inch slot opening to a 1/8th-inch opening with only a negligible hydraulic headloss. The recently introduced center feed multi-rake bar screen can increase the available screen area by extending along the length of the channel. This design allows for retrofitting screens with finer openings or increasing flows within an existing channel, provided a sufficient channel length is available.
By changing to a finer screen, the hydraulics will be affected not only within the headworks itself but also throughout the other processes downstream. If screenings capture and retention are part of the critical criteria for the screen improvements, then the velocity of fluid passing through the screen needs to be within the range that will not cause debris to be forced through the opening of the screen.
With the potential increased headloss across the finer screen, operational hydraulic levels both upstream and downstream need to be considered.
Can the system handle an increased hydraulic elevation upstream without causing flooding?
Is there a set weir downstream that limits hydraulic differential across the screen (reduced capacity)?
Control philosophy can also affect processes downstream. Most current designs of headworks screens have operational control philosophies that allow for intermittent run time on the machine based on the actual loading hitting the screen. Differential measurement is a typical screen activation step. As the screen accumulates captured screenings, water differential increases. The screen can then be set to activate at a specific allowable differential. This is an excellent solution to minimize wear and tear on the machine and reduce maintenance intervals as a result.
It is essential to understand that this also creates a potential pulsing of the incoming flow as the screen cleans itself. This is especially important if there are processes immediately downstream that might be affected by the changing velocities. Many processes have buffering capacities (grit chambers, clarifiers, etc.) that can minimize the effect.
The Little Things
Sometimes a retrofit challenge can be as simple as the spacing between existing slide gate positioning. How the screen is accessed for service and what clearances are required help determine the practical fit. As pointed out earlier, the smaller the screen opening, the greater the screenings debris volume that is captured and extracted. The size and type of the conveyance system and related washpress needs to be considered to handle increased loading.
Technology developments in the industry have kept pace by offering solutions that will accomplish the required treatment goals. Through proper consideration and qualified professional guidance, it is possible to adapt an existing facility to meet new treatment objectives.