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Control of activated sludge, including troubleshooting

The what and wherefores of a well-known process, simply explained

August 01, 2014
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Activated sludge isn’t a thing, it’s a process. As a sewage and industrial wastewater treatment, it makes use of air and a biological floc composed of bacteria and protozoa. The idea in controlling activated sludge is to keep the wastewater biomass-to-food ratio in balance.

To control activated sludge, you need to “have a handle” either on aeration, sludge-wasting or return-sludge flow.

Aeration-rate adjustment is probably the simplest way to go. It involves, however, more than measuring aeration-basin dissolved oxygen (DO) concentration once each day. Factors must be considered, for example, as the influence of dissolved-oxygen levels in the aeration basin.

In a diffused air system, aeration balance simply means looking at the basin’s dissolved-oxygen profile and adjusting the air-flow rates. In a mechanically aerated system, consider the number and location of mechanical aerators. By balancing aeration rates, dissolved-oxygen levels can be maintained throughout treatment.

A properly maintained diffuser can also improve aeration efficiency.

In controlling aeration, consider that the aerated oxygen required is based on the direct relationship between the influent bio-chemical oxygen demand (BOD) concentration and the aeration basin dissolved oxygen. As BOD concentration entering the aeration basin rises, the amount of oxygen required to maintain dissolved oxygen levels rises also.

Another factor is how much aeration you need to maintain a given dissolved-oxygen level that is directly proportional to the aeration-basin bacteria amounts.

Control sludge wasting

Sludge wasting is also important to activated-sludge process control. Sludge wasting rates affect the following:

  • Bacteria growth rate
  • Oxygen consumption
  • Mixed liquor settle-ability
  • Occurrence of foaming/frothing
  • The possibility of nitrifying
  • Nutrient quantities needed
  • Final effluent quality

Wasting removes solids buildup in the activated-sludge system, formed when solids amounts in the aeration-tank influent are greater than the solids amounts in the secondary-clarifier effluent. If sludge is not wasted, the secondary clarifier eventually fills up with solids.

Methods of determining the wasting rate include maintaining constant mixed-liquor solids-level control in the aeration tank — as well as constant food-to-microorganism balance — by controlling F/M Ratio or by using Sludge Age (SA) or Mean Cell Residence Time (MCRT) in the overall activated-sludge system to control wasting.

In determining the appropriate sludge wasting target value, consider sludge age, foam, mixed-liquor color & concentration, settle-ability, solids rising and how much sludge is in secondary clarifier.

Finally, to optimize based on sludge-wasting, identify the best sludge age, consider seasonal influences on sludge-wasting values and make small sludge-wasting flow-rate adjustments as needed.

Return-sludge flow control

Return sludge, a mix of water and solids that include live bacteria, is removed from the secondary clarifier bottom and pumped back to the aeration basin. If the sludge isn’t removed, settled-sludge levels will rise. Eventually it will spill over the clarifier effluent weirs and into areas where it is unwanted.

Changes in return-sludge flow-rate affect return-sludge and mixed-liquor concentration in the aeration tank. To control return-sludge flow, monitor and evaluate performance that result from changes in the return-sludge flow rate.

The consequences of incorrect flow should be avoided. These include any possible shut-off of the return-sludge flow, which would cause the secondary clarifier to fill up with solids and bacteria, dissolved oxygen and fresh food in the aeration tank to decrease. Another alternative to avoid is setting return-sludge flow to maximum, causing settled solids in the secondary clarifier to decrease and returned sludge, containing more water, to enter the aeration tank, as well as causing decreased aeration-tank bacteria.

In determining the correct return-sludge flow-rate, monitor settled-sludge depth along with flocculated sludge solid-particle size and sludge-wasting flow-rate changes.

Massive balances

Mass balance refers to the ratio of solids entering the treatment unit in relation to the solids leaving the unit. While using the mass-balance approach, consider both the settled-solids level in the secondary clarifier and the amount of solids leaving the secondary clarifier.

The “sludge-quality” approach to return-sludge flow control relies on monitoring of loading, process balance and sludge-quality characteristics to reveal the clarifier-sludge flow rate that best satisfies the interacting variables’ net requirements. The calculated demand satisfies the coordinated requirements imposed by changing mixed-liquor sludge concentrations and quality, sludge-solids distribution throughout the aeration and clarification tanks and the wastewater flow rates. This approach may yield results that entail either settling too fast, settling too slow or normal settling conditions.

In summary, for return-sludge control, always keep return-sludge flowing, make small adjustments, and repeat adjustments after adequate time passes to evaluate prior adjustment.

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[SIDEBAR 1]

Troubleshooting an activated-sludge process

To identify an activated-sludge process problem, visual observation of the treatment process is essential and settle-ability tests are vital. Settling-test observations lead to appropriate remedies and corrective measures.

Clues that there may be a problem include a cloudy effluent, pin floc or stragglers in effluent, ash on clarifier surfaces or floating solids after extended settling time.

For conventional activated sludge, probable causes of cloudy effluent include the following: aeration-tank mixed-liquor suspended solids are low due to process start-up; organic-loading increase; toxic-shock loading or over-aeration — causing mixed-liquor floc to shear or improper dissolved-oxygen levels in the aeration tank.

Excess organic load can be remedied by reducing the waste-activated sludge rate by an amount less than 10 percent per day, to return to proper loading parameters and increase the returned activated sludge rates. About a 30 percent level of settled solids in the clarifier should be established and maintained.

The probable causes of pin floc or stragglers in effluent include the onset of de-nitrification or excessive grease amounts in the mixed-liquor suspended solid. An industrial waste monitoring and enforcement program can at least minimize grease going into the aeration system.

The cause of ash on clarifier surfaces may be the aeration tank approaching under-loaded conditions by operating with high mixed-liquor suspended solids because of old sludge in treatment process.

Remedies for ash on clarifier surfaces, caused by aeration tank foaming, include adjusting return-activated sludge rates to maintain settled-solids levels in the clarifier of approximately 30 percent; or adjusting mixed-liquor suspended solids and returned-activated sludge concentrations, as well as dissolved oxygen.

The probable causes for any floating solids evident after an extended settling time of one or two hours include that de-nitrification or septicity is happening in the clarifier.

Floating-sludge clumping due to septicity may be remedied by maintaining dissolved oxygen at a minimum level of 1.0 Mg/L, along with making sure adequate mixing is occurring in the aeration tank. Alternatively, adjust returned-activated sludge rate to maintain a level of settled solids depth in the clarifier of approximately 30 percent.

Extended aeration

The below is particular to extended-aeration activated-sludge processes.

1. Some plants, due to clarifier and sludge-return system design, won’t allow maintenance at 30 percent of settled solids.

2. A small amount of shiny, dark-tan foam is acceptable on extended-aeration plant aeration basins. This is primarily true in the winter, when increased solids concentrations are often required.

3. Nitrification, which is the conversion of influent ammonia to nitrites and nitrates, is normal and expected in extended aeration plants, most especially in summer. If sludge is rising and clumping in the secondary clarifier, increase return sludge rates and or lower aeration rates a little. Lowering sludge age can also help, if it is a little too high.

4. Don't forget that slow-settling sludge can be caused by both old and young sludge. Perform a diluted settle-ability test of 50 percent mixed-liquor suspended solids and 50 percent clarifier supernatant to determine whether more or less wasting is appropriate.

5. A small amount of pin floc on the final clarifier surface often accompanies the old sludge that is normal for the extended-aeration process. If amounts become excessive and cover more than 25 percent of the clarifier surface, an increase in wasting of a small percentage may be helpful.

Equipment issues

The probable causes of equipment issues often include leaks in the aeration-system piping, plugged diffusers or air discharging from diffuser-header blow-off pipes, causing local boiling to occur on surfaces near the diffuser-header pipe or insufficient or inadequate oxygen transfer.

Aeration-system piping leaks may be remedied by tightening flange bolts or replacing the flange gaskets. Insufficient or inadequate oxygen transfer may be remedied by improving aeration-system performance by adding or replacing more effective diffusers or mechanical aerators.

[SIDEBAR 2]

What microorganisms are in activated sludge?

In activated sludge, microorganisms come in contact with wastewater biodegradable materials and consume them as food. In addition, the bacteria develop a sticky layer of slime around the cell wall that enables them to clump together to form bio-solids or sludge, which can then be separated from the liquid phase.

Successful removal of wastes from the water depends on how efficiently the bacteria consume the organic material and on how well the bacteria stick together, form floc, and settle out of the bulk fluid. The flocculation (clumping) characteristics of the micro-organisms in activated sludge let them form solid masses large enough to land at the bottom of the settling basin. As the sludge flocculation characteristics improve, so do settling and wastewater treatment.

After the aeration basin, the mixed liquor flows into a settling basin or clarifier where the sludge is allowed to settle. Some sludge volume is continuously re-circulated from the clarifier, as returned activated sludge back to the aeration basin to ensure adequate microorganisms are maintained. The microorganisms are again mixed with incoming wastewater where they are reactivated to consume organic nutrients. Then the process starts again.

Starting over

Activated sludge, under proper conditions, is very efficient. It usually removes 85 percent to 95 percent of solids and reduces biochemical oxygen demand (BOD) about the same amount. Efficiency depends on many factors, including wastewater climate and characteristics. Toxic wastes that enter the treatment system can disrupt the biological activity. Wastes heavy in soaps or detergents can cause excessive frothing and thereby create aesthetic or nuisance problems. In areas where industrial and sanitary wastes are combined, industrial wastewater must often be pretreated to remove the toxic chemical components before it is discharged into the activated sludge treatment process. Nevertheless, microbiological treatment of wastewater is by far the most natural and effective process for removing wastes from water.

Five major groups of microorganisms are generally found in aeration basins of the activated sludge process:

Bacteriaare primarily responsible for removing organic nutrients from the wastewater.

Protozoaplay a critical role in the treatment process by removing and digesting free swimming dispersed bacteria and other suspended particles. This improves the clarity of the wastewater effluent. Like bacteria, some protozoa need oxygen, some require very little oxygen and a few can survive without oxygen.

Types of protozoa present give some indication of treatment-system performance, as follows:

  • Amoebae have little effect on treatment & die off as amount of food decreases
  • Flagellates feed primarily on soluble organic nutrients
  • Ciliates clarify water by removing suspended bacteria; free-swimming ciliates remove free-dispersed bacteria; crawling or “grazing” ciliates dominate the activated-sludge process; and stalked or “sessile” ciliates dominate at the process end.

Multi-cellular metazoa

Metazoaare multi-cellular organisms larger than most protozoa and which have little to do with removing wastewater organic material. Although they do eat bacteria, they also feed on algae and protozoa. Metazoa dominance is usually found in older systems; namely, lagoon treatment systems. Although their contribution in the activated sludge treatment system is small, their presence does indicate treatment system conditions.

Three most common metazoa found in the activated sludge treatment system include rotifers, which clarify effluent and are first affected by toxic loads; nematodes, which feed on bacteria, fungi, small protozoa and other nematodes; and Tardigrades, also known as water bears, which survive environmental extremes and toxicity.

Filamentous bacteria are present when operating conditions change. These bacteria, which grow in long filaments, begin to take advantage. Changes in temperature, pH, DO and sludge age, or even amounts of available nutrients such as nitrogen, phosphorus, oils and grease, can affect these bacteria. Dominance of filamentous bacteria in activated sludge can cause problems with sludge settling. At times excessive numbers of filamentous microorganisms interfere with floc settling and the sludge becomes bulky. This bulking sludge settles poorly and leaves behind a turbid effluent. Some filamentous microorganisms may cause foaming in the aeration basin and clarifiers.

Algae and fungiare photosynthetic organisms and generally do not cause problems in activated sludge, their presence in the treatment system usually indicate the kinds of problems associated with pH changes and older sludge.

Microorganisms in activated sludge has been described above, however if you have specific issues controlling microorganisms or other wastewater queries, please submit a question.

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