It is often advantageous to employ a zone of high solids contact to achieve a better quality effluent. This is accomplished in an up-flow clarifier, so called because the water flows upward through the clarifier as the solids settle to the bottom. Most up-flow clarifiers are either solids-contact or sludge-blanket type clarifiers, which differ somewhat in theory of operation. Cross-sections of these two types of units are shown in Figures 1 and 2. Both units have an inverted cone within the clarifier. Inside the cone is a zone of rapid mixing and a zone of high solids concentration. The coagulant is added either in the rapid mix zone or somewhere upstream of the clarifier.
In the solids-contact clarifier, raw water is drawn into the primary mixing zone, where initial coagulation and flocculation take place. The secondary mixing zone is used to produce a large number of particle collisions so that smaller particles are entrained in the larger floc. Water passes out of the inverted cone into the settling zone, where solids settle to the bottom and clarified water flows over the weir. Solids are drawn back into the primary mixing zone, causing recirculation of the large floc. The concentration of solids in the mixing zones is controlled by occasional or continuous blow down of sludge.
The sludge-blanket clarifier goes one step further, by passing the water up from the bottom of the clarifier through a blanket of suspended solids that acts as a filter. The inverted cone within the clarifier produces an increasing cross-sectional area from the bottom of the clarifier to the top. Thus, the upward velocity of the water decreases as it approaches the top. At some point, the upward velocity of the water exactly balances the downward velocity of a solid particle and the particle is suspended, with heavier particles suspended closer to the bottom. As the water containing flocculated solids passes up through this blanket, the particles are absorbed onto the larger floc, which increases the floc size and drops it down to a lower level. It eventually falls to the bottom of the clarifier to be re-circulated or drawn off.
Solids Contact Clarifier
In the solids contact clarifier, the liquid stream enters into a central detention zone where chemicals can be added. Within this zone, there is a re-circulator paddle which is activated by a variable speed drive. This paddle creates a pressure differential within this zone and essentially pumps previously settled material from a central settling cone into the re-circulation zone and positively contacts it with the incoming waste. In so doing, the incoming waste can be flocculated with chemicals added at that point. The incoming solids create a thermodynamically favorable environment to bring a chemical reaction to completion, conserve chemicals and provide a more favorable settling characteristic to the solid. These contacted materials are then brought into a detention zone where true flocculation can occur. At this point, the materials fall out and settle into the clarifier. It is not the intent of a solids contact clarifier to maintain a sludge blanket, although they are periodically operated in that fashion
In addition, to the enhanced settling characteristic imparted by the solids contact clarifer, chemical reactions can occur and take place in a more ideal environment. Not only can we achieve simple sedimentation in an enhanced manner, but reactions such as the removal of phosphates by precipitating calcium phosphate, or the removal of arsenic by the addition of iron in a complexation reaction, can be maintained as well as others. Activated carbon could also be added for absorption.
In summary, the solids contact clarifier can enhance sedimentation by improving the physical characteristics of the material to be removed, can remove materials by utilization of chemical reactions because of ideal reacting environment, can maximize the use of chemicals as previously settled material can be used as a source of these reactants and chemicals, and chemicals can be added in both the re-circulation and flocculating zones.
One other consideration should be made for use of a solids contact clarifier over a conventional clarifier. Because of the efficiencies described above, a solids contact clarifier might often use a smaller space than a conventional clarifier. However, it is not uncommon to have relatively low solids concentration applications where conventional clarification simply will not do the job. Typically, depending upon the nature of the liquid to be treated, do not expect a clarifier to put out less than 10 PPM suspended solids — often the range is 10 to 30 PPM. When treating a flow that has less than 200 PPM suspended solids, that’s demanding a lot in terms of percentage reduction with a conventional clarifier. In these cases, chemical enhancement may be required — thus, a solids contact clarifier can do a much better job. When considering applications with several hundred or thousands of PPM suspended solids, the conventional clarifier may be the better choice. In the case of low influent suspended solids, create an environment where sufficient particles contact each other so that sedimentation can occur. Of course, if the
particles are sufficiently large and discrete that they settle rapidly, this is not a problem — but it is the exception, not the rule. When re-circulated sludge contacts with an influent stream, a crystallization effect occurs where settleable material is created. This parallels the concept behind adding a chemical additive such as lime to an influent waste just for the matter of adding “weight” to the settleable material. Improving the settling efficiency is the result.
In water treatment applications, the solids contact clarifer can be easily applied because without the re-circulation and contacting of the solids, the high rates required for economy are not available for settling nor will the necessary chemical reactions be carried out very effectively. There are a host of applications in water and wastewater treatment that employ a variety of clarification and sedimentation techniques. However, in many cases, the solids contact clarifier is by far the more efficient unit process available.
Raw water and treatment chemicals are intimately mixed in the draft tube in the presence of previously formed slurry recirculated from the reaction flocculation zone and precipitated solids from the bottom of the basin. A high efficiency marine propeller recirculates this flow into the reaction-flocculation zone. Propeller speed is adjustable to attain the optimum recirculation rate and slurry concentration for the treatment involved.
The reaction-flocculation zone under the cone receives the total mixed flow from the mixing zone. Flocculation is accelerated here by the intimate contact between reacting chemicals and recirculating precipitated solids on which the newly-forming material is deposited. Part of the flow, equal to the raw water rate, is then discharged into the separation zone, and the remaining flow is recirculated into the mixing zone.
The large area under the edge of the cone insures even distribution of low velocity entrance flow to the clarification zone. The upward velocity of the water maintains a zone of suspended, reacted slurry. This acts as a filter and catalyst, collecting small particles of sludge and forcing the chemical reactions to completion. At some level, the decreasing velocity is no longer great enough to carry the fine slurry particles and the clarified water escapes toward the effluent launders.
Sludge Removal and Recirculation
A portion of the sludge in the clarification zone becomes too heavy to maintain in suspension and settles to the bottom of the basin. This material is moved to the center of the basin by the sludge collector, where a portion of the solids fall into the sludge hopper and is automatically removed. The remainder of the slurry is recirculated through the draft tube and used to increase the solids content in the flocculation zone and enhance floc formation.