Understanding the Triad of Effective Grit Management

With the introduction of enhanced wastewater treatment processes including MBRs, MBBRs, Advanced Oxidation systems, and a variety of specialized fine bubble diffusers, valving, and pumping equipment, the need for effective grit management is no longer an option – it’s an essential requirement. Grit removal is a crucial factor in the primary treatment phase of any WWTP. Historically, grit characterization was focused on grit capture of 212 μm(micron) and larger, but with increasing scrutiny on treatment plant performance, many owners/operators are looking towards managing grit as small as 75 μm. Modern performance initiatives for treatment plants are at a 95%+ removal efficiency while targeting grit particles 75-150 μm of the total grit load entering the plant.

Below are three distinct grit management processes discussed, and when implemented effectively, can lessen an owner/operator’s grit management costs, equipment failure incidents, maintenance expenditures, and exposure to non-compliance due to plant reductions. Each of these three stages of grit management must ensure that we’re not reintroducing this nuisance grit back into the WWTP but are providing the critical infrastructure between these three processes to ensure that even the finest grit is separated, washed, and dewatered effectively. Foregoing these critical steps will ensure that more advanced, delicate treatment systems are stressed to, and beyond, their limits.

Separation

The goal of any grit removal system should be to (a) prevent abnormal wear to downstream pumps and mechanical equipment due to the abrasive and destructive effects of the grit particulate and (b) to prevent the build-up of ‘process robbing’ inorganic materials in key processing areas of the treatment plant.

Separation is the primary step in capturing the grit component from the influent wastewater stream and should be a 'site-specific match’ to the particular grit that is being encountered at the plant. Just as not all grit characteristics are considered equal from region to region, the same is true that not all grit separation equipment is designed to manage all types of grit equally.

Another challenge for effective grit separation and settling can be attributed to the ‘hitchhiker’ FOGs (fats, oils, and greases) that can be found attached to grit particles across the grit spectrum of the raw wastewater. These attached constituents can have a significant impact on the settleability of grit and can often result in a phenomenon called ‘light grit’, where attached greases actually reduce the specific gravity of grit particles causing them to behave like smaller particles(1).

Preliminary grit testing has proven beneficial to plant owners in developing grit characterization curves to better help select the proper grit management equipment and takes into account the variables associated with region-specific, real-world grit.  In characterizing the grit to be managed at a specific site, the Wet Sieve test method is preferred over the Dry Sieve method to determine the Sand Equivalent Size (SES) of the grit load as this accounts for the variances in grit physical size and any attached organics.

In general, depending on the headworks design of the plant, the grit separation may be accomplished by several means, including vortex separators, aerated grit chambers, hydro-cyclones, and velocity-controlled grit chambers. Whereas some larger and older plants continue to use aerated grit basins and lengthy grit settling chambers, the benefits of designing around a more conscious, performance-based separation design philosophy is proving that certain, high-performance equipment can often be better at ‘fine-tuning’ the separation aspects of the plant’s influent flows. Vortex type separators have been shown to offer several benefits when compared to other traditional liquid/solid separation processes. The HeadCell® separator by Hydro International is an example of this performance-based, fine-tuning approach to the influent grit flows to the WWTP. This stacked, hydraulically independent, multiple vortex tray unit effectively captures and removes 95% of all grit 75 μm and larger at the designed flowrate and operates at less than one foot(12”) headloss. Couple that performance to a substantially smaller operating footprint than an aeration grit basin or voluminous grit chamber, and you begin to see the advantages of a technology-based approach.

Of course, simply separating the grit from the wastewater stream doesn’t do much good if that grit isn’t managed outside the headworks of the plant and disposed of properly. Washing of the captured grit is the next step in providing an owner/operator with an effective grit management system.

Washing

So, you may ask, why wash the grit? During this second stage of a grit management process, the captured grit will inherently have organic, or volatile solids attached to it. While not all these organics pose a serious problem, they can yield some unfavorable conditions worth noting, mainly odors and vector attraction when stored in containers or dumpsters awaiting disposal. Vector attraction describes the occurrence where the volatile solids attract other organisms such as flies, rats, rodents, mosquitos, etc... that come into contact with the grit material and can easily spread disease. Also, I’m sure many of us have experienced that familiar odor that often rides the winds coming from our local neighborhood wastewater plants. These odors can be directly contributed to this extracted grit load in many cases.

Washing systems employ different technologies and processes to wash away these organics and send them back to the WWTP treatment processes.  Since these volatile solids are the main ‘food source’ for the essential microorganisms found in a WWTP, they are critical for treatment plant operation and are reintroduced into the plant to aid in the biological treatment.

Grit washing systems come in a variety of shapes, sizes, and washing processes. These include horizontal flow collectors, vortex washer, fluidized bed washer, rotary washer, and aerated grit chamber to name a few. Some of the more noteworthy grit washing systems employ a high-energy vortex action where a grit slurry is introduced tangentially into an enclosed vessel to form a very thin, laminar boundary layer against the wall and base of the vessel where centrifugal forces are much higher than gravitational forces. This extends the grit particle’s residence time in the vessel and allows the accelerating velocity of the vortex flow to effectively break the organics from the grit particles just above this boundary layer. Due to the organic’s lower density, they are easily swept up with the vortex flow and reintroduced to the treatment process via an overflow port. The remaining grit in the boundary layer are moved to the center of the vessel’s lower section by the sweeping vortex motion where the grit slurry can be discharged to the final stage - dewatering.

The advantages of this type of system are that they have no moving parts and operate completely off the hydraulic flow through the unit and can wash fine grit as small as 75 μm very well. Examples of these vortex type grit washing units are the Hydro International SlurryCup™ and TeaCup®, which have been successfully washing grit for 40+ years.   

Dewatering

After the effort has been made to separate and wash the grit, the final stage is to dewater the captured grit and discharge it for disposal. Dewatering the grit load is especially important to lessen the weight, and overall volume of the grit for transportation, typically to a landfill. Therefore, the goal of any grit dewatering system is to produce as dry a product(discharge) as possible. Some of the more common dewatering systems incorporate either an inclined auger(screw) mechanism or an inclined traveling belt, often referred to as an ‘escalator’ dewatering system. The advantages of using a properly designed grit washing unit in conjunction with a grit dewatering system, is that there should be little to no offensive odors associated with the system and the discharged grit will be free of most water, as well producing a very low organics (volatile solids) content of less than 5%.

There are many types of grit dewater systems used in today’s modern WWTPs. These include filter-lined dewatering tanks, bagging systems, detritus tanks, filter presses, belt presses, centrifuges of all types, inclined screw augers, and ‘escalator’ type belt dewatering units, to name a few. The key attributes for selecting the right equipment to dewater your grit is likely dependent on several factors, including reducing disposal costs, reduced maintenance, ease of operation, small footprint, and the final dryness aspect of the discharged material.  Having a drier, high solids material discharge allows for cost-effective disposal as you’re not transporting additional water weight with the grit.

High performance systems such as the Hydro International’s GritSnail® and GritCleanse™ provide effective conveyance of the washed grit via an inclined screw auger or dewatering belt to produce an acceptable dried material for landfill, with very little moisture carry-over. These systems are also effective in reducing odors that are typically associated with ‘wetter’ discharged grit material from other types of systems.

It’s Your Grit, Own It!

Grit management is a problem that every WWTP has to own, and likewise, be cognizant of the consequences of not managing the incoming grit that your WWTP receives on a daily basis. As mentioned above, to adequately protect your treatment plant and save potentially 10's to 100's of thousands of dollars annually in off-line maintenance, the specific grit conditions at your site should be recognized and characterized. Characterizing your specific grit loading will provide you with a broader spectrum understanding of the real-world implications of ‘your’ grit on ‘your’ treatment works. There are many products available to manage the unavoidable grit load into your treatment plant, and having a good foundational understanding of the grit itself is of utmost importance in making the proper equipment selection.

Understanding the three phases of grit management is essential in incorporating the proper equipment that will provide you effective and trouble-free grit management for your plant. Traditional ‘best practices’ for grit management is antiquated and doesn’t take into account the real-world implications of managing finer grit and FOG impregnated grit. At its core, true grit management must be based in science, not on assumptions. Having knowledge of the behavioral characteristics of your ‘region-specific’ grit is essential in ‘scoping in’ and operating an effective grit management system. Thorough preliminary grit testing during peak wet seasons as well as nominal operating times can provide informative grit characteristic curves that can be more closely aligned with suitable grit products for your plant. The evolution of grit management, testing, and characterization will surely drive near-future innovations in the industry and continue to be front and center of effective wastewater infrastructure design.

References:

1) Grit Characterization in Florida WWTPs; Moss Kelly Inc., 08-29-2006