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Primary Clarifiers- How they often impact the process negatively when they are overlooked.

Primary wastewater treatment systems may include both clarification and physical-chemical treatment equipment, depending on the components in the wastewater. Clarification, through the process of sedimentation is the separation of suspended particles by gravitational settling. This operation can be used for grit and solids removal in the primary settling basin, removal of oil and grease, removal of chemically treated solids when the chemical coagulation process is used or solids concentration in sludge thickeners.

Many times operators often underestimate how much impact primary treatment can have on the secondary biological portion of the system. The whole purpose of primary treatment is to physically remove as much loading from the system as quickly and as cheaply as possibly without high tech equipment or excessive monitor and control. Often times, though small adjustment to the equipment can significantly improve solids removal as well as BOD and TSS removal or even prevent the growth of filamentous bacteria in the biological portion.

Primary systems are designed to be able to remove a significant portion of the BOD and TSS loading on a plant thus making it easier on the secondary biological portion of the system. Sometimes the addition of chemicals will improve efficiency. Coagulation and flocculation of fine suspended solids will convert some or all of the colloidal solids to settleable solids.

The purpose of a clarifier is to remove solids, produce a cleaner effluent and concentrate solids. Concentration of solids removed from the wastewater reduces the volume of sludge for dewatering and/or disposal. The smaller the volume of sludge removed results in lower capital and operating costs for dewatering equipment and/or sludge disposal. Sometimes existing dewatering equipment may not have enough capacity if the sludge is not concentrated.

Flux or solids loading, Surface Overflow Rate (SOR), and Effluent weir loading are operational parameters to consider when optimizing a clarifier.

Effluent Weir Loading -Expressed as gallons per day per linear foot of weir. Typical values are 10,000 to 20,000 gallons per day per foot of weir

Dry Solids Loading (Flux)-Influent dry pounds of suspended solids per hour divided by the clarifier area

Basin Depth (Retention Time)

Hydraulic Overflow Rate- Determines particle removal efficiency for discrete & flocculant sedimentation. Hydraulic Overflow Rate equals the influent flow rate divided by the clarifier horizontal area

Many parts of the equipment often times can be optimized or adjusted but are usually set when the plant is first designed and started up and never adjusted again in spite of the fact that the plant might have changed loading or design flows. Feedwell, Sludge Rakes, Skimmer Arm and Support, Baffles, Grease Ring, Scum box are just to name a few.

Clarifier Solids Capture-Commonly Determined by Measuring Influent and Effluent Total Suspended Solids by Weight

Chemical Treatment- Alleviates problems associated with too high solids or hydraulic loading, Increases settling rate of solids, increases settling rate of sludge bed, increases solids captured by the clarifier or can increase sludge underflow concentration.

Concentration of Solids- The larger and heavier the suspended solids, the faster they will settle. The more particles there are (within design), the better the settling.

Temperature- At higher temperatures, the water is less dense, therefore, the higher the temperature, the more rapid the

settling.

Detention Time- About 50% of a municipal type suspended solids will settle out in 30 minutes, about 60% after 1 hour,

and about 70% after 2 hours. Usually, clarifier design allows for detention times ranging between 2 to 3 hours, however, it may be as long as 4 to 5 hours. If necessary use a settleometer to check how long solids can be in the clarifier without floating to the surface.

Condition of Wastewater- Wastewater strength, freshness, temperature, and the density, shapes and sizes of particles all impact the efficiency of the unit. Septic wastewater settles slower because of smaller particle size or gas bubbles on particles that can cause floating.

Short Circuiting- Can be caused by uneven weirs, inadequate baffles, wave action or currents.

The following should be routinely checked; Settleable solids in and out of clarifier, Sludge moisture, Sludge pumping cycle and Sludge blanket depth. Weirs, skimmers, draw down tubes and rakes all need regular maintenance.

Causes of Low Solids Removal Efficiencies- Hydraulic Short Circuiting can be caused by currents induced by inlets, effluent weir plates that are not level, a difference in influent and clarifier water temperature, or wind causing problems on large tanks.

Sludge may be scoured & re-suspended by water forward velocity, there could be to low an underflow on sludge pumping rate or schedule, there could be high drive torque that may indicate a high bed. Light organic solids may not increase torque significantly with a high bed as compared to inorganic solids.

Manual measurement of bed by "Sludge Judge" or automatic bed indicators is more reliable than torque measurements.

Causes of Low Solids Removal Efficiencies- Increase in Influent Suspended Solids can cause particles to settle as a mass rather than discretely. Increase in Influent Suspended Solids can cause sudden increase in sludge bed height. If chemicals are used, a decrease in chemical/pound of solids is needed.

Causes of Low Solids Removal Efficiencies- If Sludge is Held Too Long In Clarifier it can create gasification by anaerobic decomposition. Gas bubbles can be seen breaking water surface. Re-suspension can occur of sludge solids. Floating black sludge can be seen. A strong hydrogen sulfide odor can be present in severe cases. This is a common problem in pulp & paper and food industries.

• Short-circuiting - Short-circuiting in a rectangular clarifier is usually evident by surface currents observed over the length of the basin. Circular basins, however, are harder to observe short-circuiting. These problems may be observable as pin floc in certain places around the periphery of the tank and an uneven buildup of sludge at the bottom of the tank. The degree of short-circuiting in circular units can vary considerably, depending on the type of inlet used. Inlet conditions have been shown to be more critical than outlet conditions. The most important factors to consider in controlling short-circuiting are dissipation of inlet velocity, protection of tanks from wind sweep and uneven heating, and reduction of density currents associated with high inlet suspended solids concentrations.

• Turbulence - Turbulence levels in a settling basin are normally difficult to estimate. Usually the designer attempts to minimize sources of turbulence such as inlet, outlet, wind, and density currents by baffles or flow distribution channels. These sources produce unpredictable levels of turbulence and may increase short-circuiting.

• Bottom scour - Where high forward velocities are used, the possibility of scouring previously deposited sludge can occur. Forward velocities should be from 9 to 15 times the settling velocity of critical size solids in order not to cause scour.

Keep clarifier solids in balance . Maximize solids capture by influent flow control to minimize hydraulic surges. Control sludge bed depth by proper sludge pumping rate to minimize solids carry over. Maximum sludge concentration dependent on bed height and solids characteristics. Minimize sludge retention time. Improper sludge pumping rate is a common cause of clarifier failure. Check periodically.

Some primary clarifiers are covered. Open them up and check to see if there are solids floating on the top, and check for gassing. If present, make adjustments. Out of site, out of mind will not help with operation of the treatment plant.

Many times, large rags or debris can be seen in the primary clarifier. This can indicate that preliminary screening is having a problem. Check upstream to see what can be done about optimizing removal of very large pieces of debris and garbage.

There can be many different sizes and shapes for primary treatment. It can be a chemical plant with high solids, a food plant with high grease, a municipal plant with solids and debris or a papermill with tons of fiber. All that matters is how your piece of equipment works, what parameters can you monitor and control and how efficient you can make your system with what you have.

Ashing and gassing can occur in primary clarifiers just as well as secondary clarifiers.

Are you holding the solids too long in the clarifier? This is a very common practice that causes septicity and promotes growth of filaments.

Check for pin floc, air bubbles rising or clumps of floc floating- these all indicate solids held too long.

Solids can float to the top and cause problems

Are algae and scum building up on the weirs? Are there visible signs of floating solids?

Here large amounts of floating solids and scum are building up

Here solids are building up in the scum           Floating islands on the surface of the clarifier

collection box

Ice during wintertime can block up weirs

Flights that are too slow on the top or bottom do not remove the solids fast enough and can generate septic conditions.

Check your operations manual or if your plant is old, contact the manufacturer of the equipment. They should have copies of parameters that can be adjusted and how to manuals in their possession that they can send to you. Many times they will come out and help you optimize their pieces of equipment or provide training and onsite consulting. If not, the internet and the EPA are good sources to find information on every type of piece of equipment available.

Ask how Environmental Leverage Inc. can help you with training and optimization of your plant.

A program with bioaugmentation upstream may help with grease removal and filamentous control if that is a large problem at your plant. Your local sales rep can inform you of numerous programs, training material, lab work or products that we can offer you to help you optimize your wastewater treatment plant.

This is a secondary clarifier, but the principles apply to any primary or secondary piece of equipment. If the skimmers and rakes are not at the proper height , they will not sufficiently remove the grease, scum or floating solids properly.

Here is a primary clarifier at a municipal plant. Solids are being held too long in the system and dead anaerobic floc is floating up to the surface. Adjust the amount of solids that are being drawn off your primary if you see this happening.

Septic conditions can promote filamentous growth, low DO on the influent to the aeration basin, can cause septic conditions to a digestor or make it harder to dewater primary sludge and increase polymer demand.

Here small bits of grease are floating in the water. Large clumps of dead floc have floated to the top and large amounts of grease scum have built up on the sides of the tank. Weir maintenance is also critical in a primary clarifier. The more grease removed from a primary, especially a municipal plant where too much grease can cause significant levels of Nocardia and Microthrix to grow in the secondary treatment plant need to be removed.