Oct 03, 2016

Goodbye, Grit

Velocity control baffle developments for forced vortex grit removal augment performance

Goodbye, Grit

An emerging emphasis at domestic water resource recovery facilities (WRRFs) concerns performance in plant headworks. The escalating application of membrane bioreactors (MBRs), specialized pumping equipment and fine bubble diffusers in downstream plant infrastructure drives investment in protective grit removal equipment. An effective grit removal system prevents grit buildup in unit processes, scouring and plugging in lines, and fouling of diffusers and membranes, while preserving the intended service life of the downstream equipment. 

Because funding is tighter and dollars are scarcer, protecting plant infrastructure is critical. Grit removal performance, proper evaluation of performance and minimizing costs follow accordingly. Now, advancements in hydraulic baffle technology provide retrofit augmentation of existing, older-style vortex grit chambers to improve performance for less total investment, while new forced vortex systems can achieve 95% grit removal across all flows.

Grit Removal Developments

Grit removal system approaches include a basin, channel and/or chamber that reduce flow velocity, allowing inert grit particles to be hydraulically removed or settled out. From a technological standpoint, various approaches range in sophistication, from rudimentary settling basins and channels to aerated chambers to advanced forced vortex systems. Some of these methods continue to evolve because time and experience are the best teachers. Likewise, ongoing research and development by market leaders, the emergence of computation fluid dynamics and intensifying industry competition drive system advancements.

Today, the general “vortex” grit chamber concept dominates the installation landscape, occurring in different forms: sloped-floor settling basins, stacked-tray settling basins and flat-floor grit chambers (also known as hydraulic forced vortex or “PISTA”). There are two primary means by which these grit chambers physically remove grit. Sloped-floor and stacked-tray systems employ settling, while hydraulic, flat-floor chambers rely on hydraulic vortex action. Forced hydraulic vortex systems with flat-floor chambers are most commonly specified based on grit removal efficiency and demonstrated performance over time. To accomplish removal by hydraulic action, these systems combine a specifically inclined inlet flume, circular chamber with a flat-floor upper chamber and lower collection hopper, and an effluent trough that emanates 270 or 360 degrees from the inlet. 

Original forced vortex grit chambers utilized 270-degree layouts and were designed to achieve ranging removal efficiencies for particles from 100 to 300 μ. Over time, chamber designs extended to 360-degree layouts and improved grit capture. Recent hydraulic developments in forced vortex technology provide for the finest of grit removal, including 95% removal efficiency for grit particles sized down to 100 μ across all flows experienced at the plant.

Developments from Smith & Loveless stem from the integration of patented V-FORCE flow control baffles, which ensure proper velocity (in varying flow conditions) and extended flow path around the grit chamber. This hydraulic action sweeps entering grit along the chamber flat floor and into a center core opening to the lower collection hopper. The nearby center rotating paddles above the opening serve to lift and separate lighter organic material in the center, enabling it to continue downstream, while the heavier grit particles retreat to the lower hopper.

The addition of velocity control baffles in 360-degree chamber layouts provides several benefits at the design stage and later operation. First,  the velocity control baffles manage water levels to optimize velocity between 2 and 3.5 ft per second (fps), so that grit can be swept along the floor toward the lower hopper. This eliminates the need to implement downstream control devices. The implementation of the integral baffling enables the chamber to possess a 10:1 turndown, which can reduce the size of the chamber footprint, including number of units required, from conventional vortex approaches and subsequently lower capital costs for a new project. From a performance standpoint, 95% removal efficiencies can now be achieved across all flows for fine grit particles down to 100 μ.

Degrees of Improvement

Another new hydraulic baffle development addresses the numerous 270-degree vortex grit chambers that were installed throughout North America and elsewhere in previous generations. The OPTIFLOW 270 Baffle System can be retrofitted into those systems in order to optimize flow velocity and, ultimately, removal efficiency. Vortex grit removal systems designed to meet previous standards can be upgraded to remove 95% of grit down to 150 μ with the retrofit. This baffle can be applied to both flat-floor vortex 270-degree chambers and as a conversion system for sloped and cone-shaped chambers. It also can be installed for new systems requiring 270-degree layouts. 

The 270-degree baffle system first regulates the proper inlet velocity, then directs the flow toward the center hopper with an additional pass along the flat-floor. This reduces any potential weir effect at the outlet and improves grit capture. It compensates when flows have declined to below 60% of the design rate. 

At a large municipal sewer district in North Texas, the baffle system was retrofitted for extensive testing and potential augmentation of the existing plant’s headworks. The facility processes up to 10,000 gpm through four, 30-mgd flat-floor 270-degree grit chambers installed nearly 20 years ago. Testing measured the incoming grit profile and efficiency testing on two of the 270-degree grit chambers, one with the baffle augmentation and without. The district’s technical staff along with its consulting firms observed the testing, which is being used to inform future improvements. 

The test was conducted using Cross Channel Sampling methodology, per the industry standard for forced vortex grit removal systems. Influent and effluent samples were collected at numerous locations across the entire plant channel, including the left, center and right sides. Each sample was individually marked by channel point to maintain the integrity of the collection. More than 1,000 grams of grit were sampled during the testing period in March 2015. 

Removal efficiency data obtained from the performance testing demonstrated significant improvement of the baffled chamber versus the non-baffle chamber. The upgraded baffled chamber achieved 97% removal of all incoming grit, representing a 7.9 increase in removal over the non-baffled system. It also represented a significant improvement from the original specifications of the existing 270-degree system, which were 95% for 300-μ particles, 85% of 210-μ particles, and 65% for 150-μ particles. The ability to optimize grit removal efficiency for older systems is an economical solution for augmenting older plant infrastructure lacking funding for new capital expenditure. 

In North America and beyond, thousands of vortex grit chambers exist in operation as an approach to grit removal for modern plants and WRRFs. Advancements in velocity control baffle arrangements for hydraulic forced vortex systems offer documented removal efficiencies of 95% for both 270-degree and 360-degree layouts, available via retrofit and new system installations.

About the author

Michael Microbi is technical writer for Smith & Loveless. Microbi can be reached at [email protected].