What does your clarifier look like?
Wastewater training information below on clarifier optimization.

Did you know that algae on the clarifier weirs could increase your BOD?

Small amounts of maintenance or regular cleaning can significantly impact the final effluent quality.
Do you know if you have short circuiting or solids build-up in your clarifier?

Well, if you have islands floating around in your clarifier, or plant growth, that is a sure sign that some changes need to be made.

Sludge Blanket Depth

Sludge blanket depth in the clarifier should be measured at the same time each day. The best time is during the period of maximum daily flow, because the clarifier is operating under the highest solids loading rate. Adjustments in the RAS flow rate should be adjusted according to the influent changes and age of the biomass.

An additional advantage of monitoring the sludge blanket depth is that problems, such as improperly operating sludge collection equipment, will be observed due to irregularities in the blanket depth. A plugged pick-up on a clarifier sludge collection system would cause sludge depth to increase in the area of the pick-up, and decrease in the areas where the properly operating pick-ups are located. These irregularities in sludge blanket depth are easily monitored by measuring profiles of blanket depth across the clarifier.

Holding primary influent or effluent and associated solids too long generally causes not only septic conditions, which can generates odors, but also generates low D.O. conditions. Low D.O. conditions cause the bacteria to generate specific compounds with sulfides and/or organic acids. Holding times of influent should always be kept to a minimum. Generation of septic conditions can also contribute to ashing in the clarifier, since the solids are turning anaerobic in the bottom of the clarifier and generate gases that cause clumps of solids to float to the top. This increases solids carryover.

 These conditions usually lead to the growth of excessive levels of filaments later on in the aerobic biological portion of the system. Sometimes, anaerobic sludge is sent to the primary clarifier via the belt press supernatant. This can also lead to an increase in septic conditions in a primary clarifier, since you are seeding the system with bacteria that like to grow in anaerobic or facultative conditions.

One way to tell if the solids are being held too long in the clarifier is if there is ashing on the clarifier or gassing.

Ashing occurs when little pieces of floc float up to the top of the clarifier due to trapped air bubbles in the floc. This is usually caused by the biological formation of H₂S or N₂ gas when the floc is held too long in the clarifier and runs out of O₂.

The bacteria do not stop growing in a clarifier unless there is no more food. If the conditions are not right, many problems are caused.

Gassing can be visible, and usually precludes ashing. Generation of H₂S gassing can cause major safety issues.

How do I make sure that the conditions are right in the clarifier?

Primary clarifiers are a bit different that secondary clarifiers, but the concepts are still the same. Neither clarifier is sterile. There are bacteria in both places. Both usually have BOD present and bacteria present. Both are places where biological activity occurs and this is often overlooked. A primary clarifier is supposed to pull out as many solids as possible to alleviate some of the BOD loading on the secondary system. Primary Clarifiers are typically upstream from additional water treatment unit processes such as filters or Activated sludge systems. Any actions that can be taken at the clarifier which reduce the suspended solids loading and BOD loading on the Activated sludge can help to improve the efficiency of the downstream unit. Primary clarifiers always have high BOD loadings.

Table 2 - Typical design parameters for primary clarifiers in municipal treatment

The best way to assess the operational performance of a primary clarifier is to review the treatment efficiency for suspended solids removal. If the clarifier shows erratic or inconsistent results, look for hydraulic loading increases (calculated as gallons per day per square foot; m3/day/m2). If the efficiency of removal for suspended solids does not average 40-50%, or BOD averages of 20-30%, over an extended period of time, look for turbulence in the basin or other operational deficiencies.

Secondary Clarifiers are a bit easier to optimize
A secondary clarifier is meant to not only settle the solids and allow clear water to flow over the weirs, but it is also meant to be a thickener. It’s purpose is to thicken the biological solids for two reasons. To be able to return some of the solids back to the aerated portion of the system for more BOD degradation, but to also thicken the solids so that dewatering is easier.
By thinking of the clarifier as an extension of the Aeration Basin, where continued biological degradation and final polishing of the water and bacterial floc occurs, the system can be optimized easier. Polymer usage in a secondary clarifier can be cut back significantly or eliminated with the right optimization.

The performance of secondary wastewater treatment systems is determined by comparing the quality of the overflow from secondary clarifiers to that of the incoming wastewater. The biological treatment unit converts some of the soluble and insoluble organics to suspended organic solids. However, the treatment process is successful only if these organic solids are removed in the secondary clarifiers. Secondary clarifier operational variables have the most critical effect on overall plant performance.

One way is to optimize what is occurring in the activated sludge portion of the system and correlate those parameters to what is going on in both portions of the system.

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