Retrofitting an existing headworks can solve many WWTP issues

By Christa Renouard

When a new wastewater facility is being designed and built, the engineers, contractors and plant operators look for the most effective and cutting-edge solutions 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 making them obsolete.

The treatment objectives of a modern wastewater treatment plant can change faster than the lifespan of a particular treatment technology used in the process. Straightforward technologies, such as mechanical aeration, clarification and digestion, have proved susceptible to bypassed debris and sedimentation. As more material accumulates, it inhibits performance, and in some cases, shuts down a process entirely. All of these issues point to the need for increased performance in headworks processes to protect what happens downstream.

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 is done 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 and 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 for this scenario.

Similarly, the addition of a fine enough screen to protect a membrane bioreactor (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 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.

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 StepScreen® can go from a 6 mm slot opening to a 3 mm opening with only a negligible hydraulic headloss.

The recently introduced centre 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.

Hydraulic effect

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. Also, can the system handle an increased hydraulic elevation upstream without causing flooding? Finally, 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 wash press 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.

Christa Renouard is with Huber Technology Inc. This article appears in ES&E Magazine’s December 2019 issue.