SOLIDS CONTACT CLARIFIER OPERATIONS

Basic operating principles

In operation, fresh chemicals are injected into raw water being pumped at a constant rate into the clarifier center mixing well where it is contacted and mixed with recycled chemicals and solids pumped from the sludge blanket by a recirculation pump. The coagulated water is distributed to a larger flocculation well where the chemical reactions forming insoluble floc particles are completed. The particles increase in size as they contact and adhere to each other.

As the flocculation process proceeds and the particles grow, they become heavy and sink toward the bottom of the clarifier forming a sludge blanket. At this time the floc will capture small impurities in the raw water by electromotive attraction and simple entrapment. The positive ions formed by the chemical flocculent neutralize the natural negative potential of particulate material in surface waters allowing the particles to agglomerate.

Some of this sludge is recirculated to act as a flocculent aid, providing solids to initiate floc formation, and to reduce the amount of new chemical required by recycling.

Settled water rises to the surface outside the flocculation well where it is collected by submerged orifices in effluent troughs and is transferred to the filter feed piping.

GENERAL OPERATIONS

The Enviroquip Solids Contact Clarifier is a high rate, vertical flow treating unit combining chemical mixing, flocculation, coagulation, settling and clarification in a circular tank. Applications of this unit for water treating are lime softening, color removal and turbidity removal.

MIXING

Fresh chemicals are introduced into the rapid mix tank with the raw water. This flash mixing initiates coagulation of the chemicals and is very important in obtaining good results.

The chemical mixture enters the clarifier mixing well near the outlet of the solids recirculation pump. This propeller pump pulls sludge from below the flocculation well mixing it with the incoming flow. This sludge recycle reduces the amount of new chemicals required and promotes the formation of floc particles by seeding the incoming flow. This device is especially important when the turbidity of the raw water is very low having too few particles for good floc formation. The recirculation pumps are variable mechanical type allowing a variable return rate between design flow and up to 4 times design flow. The operator should experiment with different sludge return rates to obtain the best results and most efficient use of chemicals.

Chlorine solution for pre chlorination can be injected into the feed pipe ahead of the mix well also. This chlorine addition serves as an aid to coagulation and settling, oxidizes iron and manganese, destroys tastes and odors, and prevents biological growth in the clarifier and filter tanks. However, adding chlorine solution with a polymer usually does not produce good results as the chlorine tends to hydrolyze the polymer solution, causing the polymer to lose it's charge in a short time. If polymer is the coagulant used, chlorine solution can be injected ahead of the filter feed piping without adverse affects. It should be noted, adding chlorine at this point in the treatment train may increase the formation of trihalomethanes.

The inner mixing chamber is fitted with four or more adjustable diffuser gates. The operator should close these gates sufficiently to create a small level differential between the water surface inside the well and the water surface in the flocculation well. This will provide energy for a mild turbulence, and mixing, to the flow discharging into the flocculation well. The velocity of the flow and orientation of the gates provide rotation to the liquid mass in the flocculation zone, which will enhance flocculation and agglomeration of the floc into large clumps which settle easily reducing the load on the filters.

This action also promotes even distribution into the flocculation zone, avoiding any dominant currents, called short circuiting.

The head differential created by the diffusion ports and their adjustable gates should be at least 1/2 inch but not more than 2 inches. Experimentation will be necessary to optimize performance.

FLOCCULATION

The flocculation well detains the flow sufficiently to allow the chemical reactions to mature before dispersing the mass along the clarifier floor below the well. It also provides the barrier between the downward flow of floc laden water and the upward flow of clarified water. A high energy zone is created in the upper portion with the energy slowly dissipating as the flow reaches the lower portion of the zone, so a fully formed floc has developed as the mixture enters the sludge blanket zone.

SETTLING

The homogenous floc-bearing mass flows out radially and under the bottom of the flocculation chamber. Since the mass is heavier, it separates from the clear water and settles to the tank bottom, causing the clarified water to be displaced upward. The rising clear water is collected by submerged orifices cut in the side of the launder trough located near the surface of the clarifier.

The clarification zone is based on a surface rise rate which is approximately 1.0 gpm per square foot of surface area, and the clarifier depth. These factors combined determine the clarifier detention time which is the volume of the clarification zone divided by the flow rate. The detention time varies from project to project but should be not less than two hours and thirty minutes at design flow.

SLUDGE REMOVAL

The precipitated sludge settles to the tank bottom. A mechanical scraper at the bottom of the unit moves the sludge toward the sludge hopper and prevents the formation of shoals on the tank bottom. Surplus sludge is removed by opening the sludge blow-down valve, located just outside the clarifier tank. The surplus sludge should be removed periodically while the mechanical scraper blades are in operation. Sludge removal reduces the opportunity of the sludge going septic in that there is some small amount of BOD in the raw water that is removed as turbidity. A concentrated sludge bed depth of approximately two feet above the cone bottom is recommended for operating as a solids contact unit. Regular intervals of sludge blow-down should be established based on the influent flow rate and raw water turbidity.

CHECKING TREATMENT

Although there are very few set rules in establishing the ideal treatment, there are basic considerations, which will serve as a guide to the operator in checking the efficiency of the treatment. Basically, the chemicals must form a distinguishable floc particle, which is clearly visible at the top of the flocculation well. The floc should grow with slight mixing and settle in a jar within fifteen minutes. The operator should take a sample from the flocculation zone and observe the settling characteristics.

The operator should check the pH of the water in the treatment zones. If alum is used, it is most efficient in a pH range between 6.0 to 8.0, and this pH range should be maintained in the mixing well. The pH should also be adjusted so that the treated water has a pH at least 7.0. One should keep in mind, that alum lowers the pH and lime increases pH, the dosage should be adjusted to maintain the desired condition. Changes in raw water temperature and pH are quiet common following rain showers, pH has a great deal of influence on the treatment and, as a result, a rain may temporarily upset the process.

INTERMITTENT OPERATION

During periods when water demand dictates running the plant at flows less than the design rate or less than continuous operation, it will be difficult if not impossible to maintain a blanket in the clarifier deep enough to operate the clarifier as a solids contact unit. During this time, the clarifier can be operated as a settling basin with a minimum concentrated sludge bed depth of approximately two feet being adequate for most conditions. The recycle rate should be reduced also, with the exact rate of return determined by experimentation during varying flow conditions. Regular intervals of sludge blow-down should be established based on the influent flow rate and raw water turbidity but as a minimum, once every three to four hours of operation.