Mechanically - Cleaned Plain Sedimentation Tanks...
These tanks may be rectangular, circular or square, but all operate on the same principle of collecting the settled solids by slow-moving scrapers to the point of removal.
Rectangular Tanks...
In the rectangular tanks the wastewater enters at one end and flows horizontally to the other end. The scrapers (flights) are attached by their ends to two parallel chains which pass over sprockets. The flights move slowly along the tank floor, pushing the settled solids to a sludge hopper at the end of the tank. At the same time, the partially submerged flights, in their return path, push the floating solids, grease and oils (scum) to a trough at the end of tank.
Centerfeed Circular Tanks...
In the centerfeed circular tanks the wastewater enters at the center and flows radially and generally horizontally to the periphery. The circular tanks have scraper arms attached to a central motor-driven shaft. The bottom of the tanks are sloped toward the center and the scrapers move the settled solids to a sludge hopper at the center. Skimmer arms, if present, are attached to the central shaft at the surface for the collection of floating solids, grease and oils (scum).
Square Tanks...
In the square tanks, the wastewater enters at the center and flows to the four sides. The scraper mechanism is similar to that in the circular tanks. The major difference is that the rigid arms of the scraper mechanism are equipped with pivoted corner blades which reach out into the four corners of the tank and move the solids in these areas to the path of the circular mechanism.
Solids Accumulation...
Following biological treatment (and), as an integral part of the secondary treatment process, is the secondary clarifier. The primary and secondary clarifier differ as to physical location and to the density of the solids handled, as well as in the quality of the tank effluent. Secondary solids are usually less dense and secondary effluents are normally of a higher quality than those of primary clarifiers.

In both the primary and secondary clarifiers, the sludge (settled solids) is scraped or drawn to a hopper or sump for removal, treatment and disposal. The sludge removal can occur separately and independently in each unit, or the sludge from the secondary clarifier can be pumped to the primary clarifier or to a separate thickener unit and mixed with the primary sludge. This combination of primary clarifier sludge and secondary clarifier sludge is collectively referred to as waste sludge and will usually undergo further treatment for ultimate disposal. Frequency of solids removal is dependent on sludge quality as well as the design of the related sludge wasting equipment devices.

The actual sludge removal and disposal system (for the various types of clarifiers) varies from plant to plant. In addition, the floatable solids must be skimmed and/or pumped to be disposed of separately or with the settled solids.
Clarifier Efficiency...
To calculate the efficiency of any treatment unit it is necessary to measure influent and effluent parameters. Frequently, the determination of unit efficiency is appraised in terms of BOD and suspended solids removal.

The operator should also be aware that recycled loadings from digesters, vacuum filters, etc. can impose severe organic and suspended solids loadings on primary clarifiers. The strength and volume of these recycled waste streams must be added to the influent wastewater loadings in order to properly calculate unit process efficiencies.
Typical Clarifier Efficiencies...
Listed below are some range values for primary clarifier efficiencies :

Parameter	Removal Efficiency
Biochemical Oxygen Demand	25% - 35%
Settleable Solids	90% - 95%
Suspended Solids	40% - 60%
Total Solids	10% - 15%

Clarifier efficiencies vary depending on influent flow rate and wastewater characteristics as well as in-plant conditions. The most important structural elements effecting clarifier efficiency are inlet flow distribution and effluent weir placement. Although effluent weir configurations are often beyond the control of the operator, there are some minor alterations that can be made to improve the influent flow patterns so that the affect of short-circuiting is minimized. These remedial alterations would include the installation of target baffles or the installation or relocation of a major flow baffle.
Weir Overflow Rate...
Outlet weirs vary greatly in design. They provide for uniformed removal of clarifier effluent from the surface of the tank. The term "weir overflow rate" is used to express the number of gallons of wastewater that flow over one linear foot of weir per day. Most designers recommend 10,000 to 20,000 gallons per day per linear foot of weir. Even more important than overflow rate is the proper location of the weir launders so that upflow velocities are minimized.
Detention Time...
This the time, expressed in hours, that wastewater is held in a tank based on wastewater flow and tank volume. (This assumes total displacement and uniform flow through the tank). Detention periods should be in the 2 - 3 hour range.

The "Actual" Detention Time in a clarifier is a better indicator of process capabilities, since it is a direct measure of the short-circuiting flow patterns in that unit. Experience has shown that the use of a fluorescein dye, introduced as an instantaneous slug at the influent to the unit and then measured in the unit effluent over a period of several hours, is the best measure of the actual detention time.
Surface Settling Rate...
This is expressed as gallons per square foot of tank area based on wastewater flow per day. The range value for primary tanks varies from 500 - 750 gpd/sq.ft. This is an important factor directly affecting the settleable solids and removal efficiency.
Operation...
Collection and removal of sludge from the sedimentation tank, as well as correct maintenance procedures, are important factors in successful plant operation. The mechanical collection equipment can be run intermittently but is most often run on a continual basis. This prevents the excess accumulation of solids on the tank bottom and reduces the load on the collector mechanisms and thereby helps to prevent equipment damage. Solids left in the tank bottom too long will gasify and rise to the surface, therefore, sludge should be removed from the tank as often as necessary. This, in turn, is dependent on waste stream characteristics, volume of flow and sludge quality. The sludge removal schedule must be worked out for each plant by observations and tests, keeping in mind that the objective is to remove sludge at the proper rate, at the proper rate, at the proper concentration and with the proper quality for the receiving process unit. Concentrating the sludge reduces the volume of water being pumped and contributes to pumping efficiency. There are, however, pump design limitations to consider. Seasonal revision of this schedule will probably be necessary. In addition, scum and grease should be removed daily from the tank surface.

A conscientious operator is aware that mechanical equipment requires attention and maintenance. Inspection, cleaning, lubrication, sampling, testing and record keeping are important aspects of any maintenance program. The proper time schedules for preventive and corrective maintenance must be determined at each wastewater facility. The best rule is to follow the instruction manual provided by the equipment manufacturer.
Physical Observations...
Physical observations at the water surface of each clarifier can provide the operator with valuable information regarding clarifier efficiency and related plant status. The observed data can dictate the required operational control necessary to achieve optimum unit efficiency and subsequent plant performance. These results should be confirmed through laboratory testing.

Observations which should be made daily and the results recorded :

- Floating solids accumulations should be noted, if present, whether existing as clumps or as a dispersed accumulation.
- The formation of gas bubbles.
- The absence or presence of odors.
- Distribution of flow to multiple tanks.
- Distribution of flow over weirs.
- Loss of solids in the effluent.

Other factors that should occasionally be checked are elevation and placement baffles.
Laboratory Control...
Evaluation of primary treatment is dependent on laboratory analysis. The frequency of testing and the "range" of test results will vary from plant to plant. Waste stream characteristics, environmental conditions and in-plant operation will affect the test data at each facility. Laboratory tests commonly associated with primary tanks are shown below.

Parameter	Type of Sample	Location
BOD	Composite	Influent and effluent
Suspended Solids	Composite	Influent and effluent
Settleable Solids	Grab	Influent and effluent
pH	Grab	Influent and effluent
Temperature	Grab	Influent

Comparison of influent versus effluent test results provides information required to calculate and evaluate clarifier efficiency.
Operational Problems...
If visual observations, confirmed by lab analysis indicate poor clarifier operation, then the problem source must be identified so that corrective and/or preventive action can be taken. Some common problems and discussed below.
Floating Sludge...
Cause : Sludge decomposing in tank and floating to surface.
Prevention and Cure : Remove sludge more completely or more frequently by one or more of the following methods.
Contents Black and Odorous...
Cause : Septic wastewater or strong digester recycled supernatant.
Prevention and Cure : For septic wastewater :

- Correct flow obstructions in the collection system.
- Modify, pretreat, reduce, or eliminate organic wastes, such as cannery, milk processing, brewery, tannery, and organic textiles wastes.
- Pre-Chlorinate in the sewer system; or ahead of sedimentation tank to reduce decomposition of wastewater; this is particularly effective when time of travel in sewer is unusually long or wastewater temperatures are high, or when certain industrial wastes are present.
- Preaerate wastes of types listed in No. 2 above.
- Improve flow of wastewater system to reduce accumulations of solids.

Prevention and Cure : For strong recycled digester supernatant :

- Correct or improve sludge digestion to produce improved quality supernatant.
- Reduce rate of withdrawal of supernatant to sedimentation tank.
- Select better quality supernatant from another zone or delay withdrawal until quality improves, if that is possible.
- Until quality improves, discharge part or all of supernatant to other points such as lagoon, sludge drying bed, or aeration tank until quality improves.
- Presettle the supernatant.
Excessive Sedimentation in Inlet Channels...
Cause : Velocity too low for cross-sectional area of channel at existing flow rates.
Prevention and Cure :

- Reduce cross-sectional area.
- Agitate with air, water, or other means to prevent deposits.
Excessive Fouling of Surfaces and Weirs with Wastewater Solids or Growths...
Cause : Accumulation of wastewater solids and resultant plant growth.
Prevention and Cure : More frequent and thorough scrubbing of all surfaces in contact with wastewater.
Intermittent Surging of Flow...
Cause : High intermittent pumping rates.
Prevention and Cure : Adjust controls for pump sequence, pump combinations, and automatic cut-in and cutoff water surface levels to maintain pumping rates close to rates of inflow.
Broken Scraper Chains and Frequent Shear Pin Failure...
Cause : Excessive load on mechanical scraper.
Prevention and Cure :

- Periodically empty tank and examine all metal parts for defects and wear. - Replace defective and worn parts, particularly hinge pins, chain links, and wear shoes, and badly worn, broken, or warped scrapers (flights).
- Install grit chamber or otherwise reduce entrance of grit to the primary tanks if grit accumulation is evident.
- Operate collector mechanism for longer period and/or pump sludge more often.
Sludge Hard to Remove from Hopper...
Cause :

- High content of grit, clay, or other heavy compacted material.
- Withdrawal lines too small.

Prevention and Cure :

- Reduce grit content by installing or modifying grit chamber, or locate and correct sources of grit entering the wastewater system.
- Loosen compacted material manually or by pressure jetting with air or water hose.
- Back-flush clogged pipelines.
- Pump sludge more frequently.
- Modify sludge piping.