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False high BOD readings can occur if biological material or algae are present in a BOD sample. These will increase the final BOD reading and potentially increase your final effluent values, which, in turn can mean permit violations or surcharge increases.

Small amounts of maintenance or regular cleaning can significantly impact the final effluent quality.

The two easiest tools to optimize and control a clarifier are a settleometer and a sludge judge.

The sludge judge (or core sampler) is used to measure the blanket at the bottom of the clarifier. The settleometer is used for two reasons- to give you a quick and easy indication of how long the solids take to settle in the clarifier. If it takes 3 hours to completely settle in the jar and you only 1-2 hours in your clarifier, many of your solids will carry over the weirs due to insufficient settling.

If it takes 20 minutes to settle, and inversely, you have 6-8 hours in the clarifier, obviously you are holding the solids too long. You will sooner or later float solids or the whole bed to the top and possible over the weirs.

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.

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.

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.

Polymer programs can allow an increase in removal of suspended solids at the existing unit loadings. Some plants add Alum to remove fines and ash particles in a primary also. This is not the best thing to do, since many times alum creates large, fluffy floc.

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 degredation, 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.

If these are measured as residuals in the final effluent, then this ensures that the conditions in the clarifier have met the requirements for the bacteria to continue to grow in the clarifier and continue any final polishing off of BOD. This also ensures that the floc that is sent to dewatering is in the best condition and will minimize the amount of polymer needed. By following these guidelines, the use of polymers in the clarifier should be minimal and the returned sludge is ready now to be returned for more BOD degradation back in the Aeration basin.

Sometimes the use of Biological additives in the aeration basin can optimize a system and product better solids and better BOD or TSS removal if needed. Sludge reduction, especially if in a municipal wastewater treatment plant can significantly cut back costs on solids handling when a small bioaugmentation program is implemented. Due to the wide variations in influent changes of a some plants, a small daily maintenance dose of bacteria can significantly cut back costs on polymer, while increasing the quality of the final effluent.