CLARIFICATION
There are two type of clarifiers that DMP uses:
Introduction:
The Lamella Gravity Settler, a very compact clarification system, has been applied successfully on numerous processes and wastewater applications throughout the world since its first introduction in early 1970's. One of the more prevalent applications of the Lamella Gravity Settler is associated with metal hydroxide solids separation. The most common method used to remove precipitated metal hydroxides is separation by gravity.
The solubility of dissolved metal ions is a function of the pH. Usually the solubility is very high at low pH (acid conditions). If the pH is increased (increased concentration of hydroxide ions) the metal ions will combine with hydroxide ions to form "insoluble" metal hydroxides which will precipitate out of the solution. If the pH is increased to strongly basic conditions, then some metal hydroxides will transfer into a soluble complex and thus dissolve again (e.g. aluminum and zinc).
Although precipitated metal hydroxides often have a gelatinous appearance they principally consist of very small unordered crystals. As the crystals are small, the absorption capacity is very large and large amounts of water are absorbed into the surfaces of the crystals. The large amounts of absorbed water give the resulting "particle" a specific gravity that is very close to the specific gravity of water, thus slow settling results. Each individual particle in a metal hydroxide suspension is an agglomerate of many crystals, many of which have the shape of a needle. The needles are not arranged orderly but arbitrarily with needles pointing in all different directions. This array of needles combined with the absorbed water generated a voluminous, gelatinous, fluffy snowflake-like particle.
Theory and Design:
The Lamella Gravity Settler (LGS) is an inclined, shallow depth sedimentation device. It performs the same function as conventional clarifier but it occupies only a fraction of the space (approximately 1/10).
For an initial understanding of the Lamella Gravity Settler, it is best to restrict the initial discussion to suspensions in which the particles exhibit "free settling". This phenomenon occurs when the concentration of hydroxide particles is low enough that the individual particles or floc settle independently of one another and follows Stroke's Law, generally less than 500 ppm. At higher concentrations, the settling particles interfere with each other, and "hindered settling" is encountered which is characterized by a clearly defined interface between the suspension and the clarified liquid. The Lamella Gravity Settler can be used to accomplish both free settling and hindered settling; however, since the theory for the hindered settling is considerably more complex, it is expedient to concentrate on free settling.
The basic equations for sizing settling basins were formulated over seventy years ago. Consider for the moment an ideal settling basin. The clarifier feed enters at one end of the basin, flows uniformly along its length at velocity VS, and exists at the other end. The particles settle toward the bottom at velocity VS. If the trajectory takes the particles to the bottom of the basin before they reach the far end, it is assumed that they are removed from the liquid. Therefore, a particle starting at the top must settle through the distance H at velocity Vs in the same time (or less) than the liquid is in the basin.
Thus,
H (ft) L x W x H (ft3)
----------- = ------------------
VS (ft/min) Q (ft3/min)
Simplifying,
Q Q (ft3/min) - VS (ft/min)
---------- = ------------------------------
L x W A (ft2)
Where A is the settling area of the basin and Q/A is known as the overflow rate or surface loading (and is usually expressed in gpm/ft2).
From this relationship, it can be seen that all particles are removed which have a setting rate equal to or greater than the overflow rate, and that the height (or detention time) of the basin is not one of the main parameters affecting the separation efficiency.
The fact that this is true can be illustrated another way, the only difference being that the second basin is half the height of the first. At the same flow rates, the detention time is only half as much and the suspension moves through the basin at twice the velocity. The trajectory of the particles has only half the slope, but since the basin is only half as deep, the particles are still removed.
If the height of the basin is reduced to a few inches and a number of such units are stacked on top of each other, the result is a primitive shallow-depth sedimentation device. A unit containing ten (10) parallel compartments theoretically can handle ten (10) times the flow rate as could the same basin without any plates. The liquid detention time is one-tenth as long. However, the same separation efficiency is achieved since the overflow rate is still the same (10Q/10A=Q/A). Note that the settling area now includes the area of all the plates.
The plate angle is another critical variable. The angle must be steep enough for the solids to flow or slide down the plate. For hydroxide type solids having a low specific gravity, 55 degrees is used because specific gravity difference between the solids and the liquid is very small.
A flash mixer and/or flocculator is generally required ahead of the LGS (as with all settlers on hydroxides) to mix in polyelectrolytes, promote floc growth, and improve the settling characteristics of the solids.
The LGS is a very simple unit to operate. The only item requiring operator attention is the sludge withdrawal rate. The LGS may be constructed of various metals. Tank surfaces are usually mild or 316 stainless while the plates are made of FRP with PVC beams. For some very corrosive applications, rubber lined mild steel tanks are used.
The LGS is sized by laboratory settling tests on representative feed samples (when available) to determine:
· The particle settling velocity (to set the over flow rate and effluent quality)
· The sludge volume (to set the under flow solids concentration)
· The pre-treatment required (polyelectrolyte addition, flash mixing, flocculation, etc.)
Equipment:
The clarifier supplied by DMP Corporation may be one of five types:
· PE construction flat bottomed tank with cone insert.
· FRP construction cone shaped tank with pipe leg supports.
· FRP construction flat bottomed tank with multiple point draw-off.
· PE construction flat bottomed tank with multiple point draw-off.
· Painted steel construction, slanted tank (module 1000)
All DMP clarifiers are sized based on the maximum design flow rate of the treatment system.
Criteria used in determination of clarifier sizing:
· A maximum of 1 gallon per minute per square foot of surface area
· A minimum metal hydroxide settling rate of 8 feet per hour at maximum flow rate
In order to understand the purpose and operation of the clarifier you must become familiar with the terminology used and the principle of operation.
Air Lance - A length of pipe with an air line and a valve that is used to agitate the sludge bed.
Curding - Large particles floating freely in the clarifier.
Clarifier - Equipment supplied to separate the insoluble solids from water by gravity. They have also been referred to as settlers or stilling tanks.
Capillary Action - This occurs when water flows fast enough to upset the sludge bed and draws solids along with the water flow. Example: When wind picks up spray off the water.
Cone - The tank can be supplied by manufacturer either with a cone shaped bottom, or can have cone inserted into tank. The purpose of the cone is to assist the downward movement of the precipitated metal hydroxide sludge toward a central draw-off port. Most cones are shaped to achieve a 45-60 degree angle.
Disperser - Usually a perforated pipe located directly over the cone to allow for uniform dispersion of the treated solution into the clarifier from the flocculation tank.
Flocculation - A term used to express the formation of the metal hydroxides into large particles to increase their weight and size, and increase their settling rate.
FRP construction - Fiberglass resin filament wrapped.
Gravity - A term used to express the downward movement of the metal hydroxides separating from water.
Maximum hydraulic settling rate - The maximum flow design of clarifier based on surface area.
Minimum Settling Rate - The gravity settling rate of metal hydroxide sludge expressed in feet per hour.
Overflow decant weir - Located at the top of the clarifier to allow the clear water that has separated from the metal hydroxides to decant continuously.
PE construction - Polyethylene material.
"Rat-Holing" - Water taking the course of least resistance through the sludge bed. A hole is created on the bed and a large percent of the flow goes through this hole to the draw off.
Sludge draw-off - An access point at the bottom of the tank or cone in the clarifier to allow removal of the precipitated sludge to equipment provided for sludge dewatering.
Sludge bed - The volume of precipitated metal hydroxides that have accumulated in the clarifier.
Sludge blanket - The volume and height of the metal hydroxides settled above the disperser. A good sludge blanket is important to the operation of a Low Flow Clarifier. A good sludge blanket will help to "filter" the new incoming sludge of the fine solids that are a problem on some types of wastewater. It helps to eliminate "snowing".
Snowing - A term used to describe a light particle of metal hydroxide floc particle that has not gained in sufficient size and weight to settle quickly.
Velocity - The measurement in feet per second at which the solution is moving.
Weir length - The linear length of area where water passes over for discharge.
Weir head - The measurement of the height of solution over the weir.
Operation:
The waste water being introduced to the clarifier must be:
· Properly treated.
· Adjusted to optimum pH range to insure insolubility of metal.
· Proper dosages of polymer to insure large particle formation of metal hydroxide.
The water enters the clarifier, by gravity, from the flocculation tank through a disperser pipe located above the cone, to uniformly disperse the solution in the clarifier. Upon entering the clarifier, the velocity of the solution is reduced sufficiently to allow for the gravity separation of the metal hydroxides from the water. The metal hydroxides move downward and the clear effluent moves upward being displaced by the new solution entering the clarifier. Clear effluent overflows the weir for discharge or further filtration.
The linear length of the weir is designed to minimize the water head over the weir. This also reduces the velocity of the water and restricts the capillary action of the decanting solution.
Snowing in the clear water may be attributed to the following:
· Improper dosages of polymer.
· Improper pH range.
· Insufficient height of the sludge blanket above the disperser to trap these particles.
· Air bubbles entrapped in the particles along with the floc may also be attributed to this condition.
It is important that the height of the sludge blanket be maintained over the disperser when drawing off sludge for dewatering. We recommend that this height be maintained at approximately 10-12 inches above the disperser. Do not allow the sludge blanket to rise too close to the decant weir. Capillary action of the decanting water may track some of the sludge particles over the weir.
Sludge draw-off for dewatering may be done continually or periodically to maintain the sludge blanket maximum and minimum levels discussed. It is important that the sludge be removed slowly to eliminate a rat-holing effect.
Rat-holing also restricts the downward movement of sludge along the sides of the cone. Natural compaction of sludge continues to take place, possibly placing the sludge in a gelatinous condition which is harder to move.
This problem in effect reduces the usable clarifier volume to a size that is much smaller and the effectiveness is greatly reduced. This condition is evident when there are generally upsets around the center of the clarifier. This is when sludge mushrooms up around the center pipe.
To correct this problem, the operator needs to use an air lance that is provided with the system and agitate the sludge bed.
Agitation of Clarifier:
· DMP reccomends the clarifier be air lanced once a shift.
· Temporarily take the DMP waste treatment system off line (turn off the main feed pumps).
· pre-mix one gallon of DMP 73-111 polymer diluted to five gallons of water.
· Using an air lance, vigorously agitate the sludge bed, especially along the cone sides. Control the agitation rate to insure that the sludge is not allowed to overflow the decant weir.
· Add the premixed polymer to the clarifier.
· Agitate the clarifier slowly to allow good floc formation without tearing the floc particles apart.
· Allow the clarifier to settle for about 10 minutes.
· Place the clarifier back into service.
Should improperly treated water be detected in the clarifier, and the sludge bed becomes contaminated, it would become necessary to treat the clarifier. Use of the air lance technique described may be used for mixing the treatment chemicals added to the clarifier.
Curding is another condition that may require the air lancing of the clarifier. Curding takes place when gas pockets form in the sludge bed, especially in sludge that has been allowed to compact. Gas development impregnates the sludge particles and eventually breaks a portion loose and floats to the top. Gas can be formed naturally in sludge that has become septic, or from an excess of treatment chemicals used earlier in the treatment.