Mexico, MO WTP Lime Slaker Upgrade

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THE PROJECT:

Location: Mexico, MO Missouri American Water – Mexico District Production Facility

BACKGROUND:

Existing Lime SlakerNestled into the sprawling fertile farmland of Mexico, Missouri American Water’s local water treatment plant uses aeration, lime softening, settling, filtration, and disinfection. The facility has had two lime feeding/slaking units in operation since 2000 producing lime slurry to control the pH level of water, as the result of an overall plant upgrade from 3.0 MGD to 4.5 MGD.   At the facility, raw water from six wells enters the plant through a common line and flows through an ultrasonic flow meter to the top of an induced draft aerator.  After passing through the aerator, the water flows through the rapid mix chamber where: lime is added for softening, chlorine for disinfection, and ferric sulfate & an anionic polymer for coagulation.  Enough lime is added to raise the pH of the water to approximately 9.  The water flows out of the rapid mix and is split into two separate flows to the influent of the flocculation basins.  Here it is gently stirred to assist the creation of floc particles.  Water exiting the flocculation basins travels in two flumes to the inlet of the settling basins, where additional settling occurs.  Settled water exits these basins through a weir trough and into two separate flumes to the confluence of the splitting chamber.  At this location carbon dioxide is applied to stabilize the pH of the water prior to it entering the filters.  The water exits the splitting chamber via two separate pipes and is disinfected with chlorine just prior to being delivered to the three gravity, anthracite, sand, and gravel filters.  Filter effluent flows by gravity through a common pipe and is disinfected with chlorine prior to entering the clear-well.  Water is pumped from the clear-well to the distribution system via horizontal, centrifugal pumps located in a building adjacent to the clear-well.   An ultrasonic meter located on the discharge pipe measures the flow of water to the distribution system.

Treatment residuals consist of filter backwash, filter-to-waste water, solids that are periodically cleaned from the rapid mix chamber, flocculation basins and from settling basins.  Water for back washing the filters is obtained from the clear-well via a vertical turbine pump.  Backwash water drains to a residuals basin along with solids from the settling basins.  The supernatant water is decanted from the residuals basin to the influent of the rapid mix.  Solids from the residuals basin are pump to two separate settling lagoons for dewatering and hauled away via truck for land based application.

When the feeding/slaking equipment was in need of an upgrade Kelly stated, "The slakers were rebuilt once, in approximately 2007.  The units themselves were becoming problematic, and replacement parts were not readily available."  The same was the case for the equipment control system. Velodyne Systems was able to meet the needs of the customer and provided upgraded feeder technology, slaker systems, and a new controls package for the entire silo bin system, as well as onsite project management of the installation.

THE CHALLENGES:

A project of this magnitude came with a few obstacles.  Kelly explained, "Time was the largest obstacle, we required a late season start and for the units to be in service by a hard date.  Velodyne put together a project schedule which they were able to complete by the desired date." 

There were also several physical obstacles.  "The unit’s control systems were in the same room, whose atmosphere was not conducive for electrical controls, moisture from the slakers was vented into the room.  During planning, an adjacent room was chosen to install and house the slaker controls, this required coordination with Plant operations, Velodyne, and our electrical contractor, to move the electrical systems while keeping at least one of the existing units in service while the transfer was accomplished.", commented Kelly.    

He went on to describe, "The largest physical obstacle was the tight space in which the units were housed, given the size/weight of the slakers and elevation differences between the ground and floor levels, as well as internal building obstacles."  The transfer of the slakers from outside to inside was completed with the assistance of mechanical equipment. Kelly said, “Once inside the building, an additional hurdle was physically manhandling the slaker over a containment wall.   Velodyne personnel worked in a timely manner and were able to overcome issues."

THE Velodyne SOLUTION:

Upgraded Feeders and Lime SlakersVelodyne Systems was able to install new feeders with a direct drive motor and gearbox to increase feed rate accuracy and overall system performance.  The new slakers are manufactured of 304 stainless steel to ensure a longer life of the equipment. Velodynes fully automated lime detention slaker consistently produces lime slurry ranging from 5% to 15%.  Lime and water are fed in the slaking chamber where they are mixed by a vertical mixer.  An operator then can select a desired temperature set point; the slaking water modulating valve will automatically adjust to maintain the desired temperature.  Slurry overflows through the slaking chamber to the grit removal trough, where the large particles settle out and are removed from the process by way of an auger.

The new slakers also featured volumetric feeders for accurate additions of quick lime, a replaceable wear liner, double wall insulation for maximum heat efficiency, a temperature alarm, cooling sequence, grit removal trough, and dust/vapor remover.

HMI Silo and Slaker Controls Velodyne also provided a ventilation system that carries exhaust from the slakers outside the facility. The control system upgrade consisted of taking a relay logics base control package with local hand switches and installing a brand new Human Machine Interface (HMI) Touchscreen PLC program, and transferring the remainder of the controls into a new location outside the equipment room. Velodyne’s field service technicians worked closely with plant personnel and electrical contractor to move the controls into a different location while keeping one slaker unit on line during the process.   

 “Velodyne Systems personnel worked in a professional and timely manner, we are very satisfied with the new equipment and its operation, and has improved our equipment’s performance and plant atmosphere.” – Patrick Kelly, Operations Superintendent.