The Lime Delivery SystemFrom Dry Storage to Slakers to ClariCones

Powdered lime is pneumatically transferred into building-enclosed hoppers from a hopper-bottomed

truck. The lime inventory is monitored by a senior operator who maintains control over all bulk chemical supplies.

Blowers (yellow) are used to transfer powdered lime from bulk storage to a feed hopper atop slakers.

Lime Slaking - Batch Process

A weighed quantity of powdered lime is fed successively to each slaker from the feed hopper. This is followed by a weighed quantity of water. Thereafter the slaker contents are mixed and the temperature of the slurry is monitored until an optimum temperature is achieved.

Upon completion of the slaking reaction, the contents of the batch slaker are discharged to lime slurry storage tanks located under the floor of the slakers. The progress of the slaking process can be followed on the operator’s SCADA system.

Lime Recirculation System

Occasionally, lime blockages may occur at the lime feed into the slaker or at the exit pipe to the slurry storage tanks (stainless steel).

Once in storage, the hot lime slurry must be kept mixed to maintain consistency. It is constantly recirculated by pumps through flexible (black) hose in a continuous loop that enters and circles the ClariCone building. Lime slurry is withdrawn, as needed, into each of the four ClariCones.

The quantity of slaked lime slurry in storage and the operation of the slurry recirculating pumps can

be observed and controlled on the SCADA system

Lime Recirculation Loop

The lime recirculation loop (flexible hose and PVC pipe) delivers slaked lime slurry to each of the four ClariCones. The slurry flow is controlled by pinch valves (lower left, red) and metered by a controller (lower center, blue). Lime is delivered by a pipe directly descending into each ClariCone to a depth just above the tangential inflow of the water to be softened. Most of the slaked lime dissolves rapidly, precipitating calcium carbonate and the more flocculant, magnesium hydroxide, thereby forming a stable sludge blanket.

Grit Removal

The heaviest portion (5 to 8%) of the lime slurry, consisting of sandy grit and unreactive (dead-burned) lime, settles into the bottom cylinder of the ClariCone (approximately 1,200 gallon capacity) where it is periodically removed by grit pumps (left) beneath the ClariCones. However, before the grit pumps are activated, a tangential flow of water, called “jetting”, is used to loosen and resuspend the settled solids for more complete removal.

If an excessive amount of heavy material (unreactive lime, calcium carbonate, sand, silt, chert, debris from influent water) is allowed to accumulate in the bottom of the tank, it can disrupt the helical flow of influent water, directing the inflow upward and causing short-circuiting. This may cause upwellings in the blanket.

Grit is generally pumped from each tank in operation every eight hours for a maximum period of five minutes. It is withdrawn through an 8” pipe using a 1000 gpm grit pump. At this flow rate, prolonged gritting may result in the removal of a large portion of the ClariCone blanket.

Lime overfeed, as indicated by transient increases in pH of the softened and settled water (ClariCone effluent), is observed primarily during periods when gritting takes place. Since the lime slurry feed continues uniformly, even while a portion of the ClariCone influent water is withdrawn from the drain as part of the gritting process, the effective lime softening dosage increases.

Concentrator Cone~1,400 gallons

Grit Chamber~1,200 gallons

0’

9’

23.5’

30.5’

34.5’

aupward flow@ 5 MGD

(gpm/ft2 / ft/min)

a1.15 / 0.15

a1.52 / 0.20

a2.76 / 0.37

a36.5 / 4.88