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Energy costs represent a considerable

portion of a treatment facility's annual oper-ation and maintenance expenses, with theactivated sludge process accounting for 50 to60 percent of them. In the backdrop of esca-lating energy costs, a high energy use patternis anticipated to strain operating budgets ofwastewater treatment plants.

Threatened by this eventuality, manyutilities are looking for opportunities forreducing energy use. This article reviewsenergy-saving strategies that could be imple-mented during the design and operation ofan activated sludge process.

Oxygen Demand and CreditIn nitrifying facilities, portions of influ-

ent organic and nitrogen compounds are oxi-dized. The associated oxygen requirementsare called the carbonaceous biochemical oxy-gen demand (CBOD) and nitrogenous oxy-gen demand (NOD). Included in the CBODare the oxidation of the influent substrateand endogenous decay.

If an anoxic zone is incorporated intothe bioreactor for total nitrogen removal orimproving sludge settleability, the resulting

denitrification will provide an oxygen cred-

it. Depending on the extent of denitrifica-tion, it is possible to recover up to 63 percentof the oxygen consumed for ammonia oxida-tion.

The sources of oxygen demand andcredit in an activated sludge process are sum-marized in Table 1. The net oxygen demand isexpressed as follows:

Net Oxygen Demand

= CBOD + NOD - Denitrification Credit

Net Oxygen Demand Distribution

Over-aeration represents an energywaste and may be minimized by bettermatching of air supply to the required oxy-gen demand.

Based on full-scale experience, a plug-flow aeration basin may be divided into equalzones and the net oxygen demand profiledeveloped as outlined in Table 2.Denitrification credit is typically realized inthe anoxic zone but is applied equally acrossall three zones of the aeration basins.

To meet the decreasing oxygen require-ment along the length of the tank, air supply

may be tapered by varying the number of dif-fusers in proportion to the net demand in thezone. If a uniform diffuser arrangement isused, droplet valves must be adjusted and theair pressure increased to achieve the desiredoxygen distribution.

A tapered diffuser arrangement is thefirst step in providing oxygen where it isneeded without increasing blower pressure,which represents higher aeration cost. Careshould be exercised in designing tapered aer-ation systems to ensure that the reducedenergy input provided at the end of a plug-flow basin to maintain low dissolved oxygen(DO) is sufficient for mixing.

Factors Affecting Oxygen DemandNitrification. Some treatment plants are

not required to nitrify, so unwanted nitrifica-tion is a significant source of energy waste.Typically, nitrification represents about 30 to50 percent more oxygen than required forsatisfying CBOD alone.

Unwanted nitrification can be mini-mized by lowering solids retention time(SRT). This can be accomplished by reducingaeration basin mixed liquid suspended solids(MLSS) concentration or by using fewer aer-ation tanks.

Nitrification may also be reduced bymaintaining low mixed liquor DO, approxi-mately 1 mg/L, since low DO conditionsinhibit nitrifiers but not heterotrophs (BOD-removing organisms); however, this strategycould promote filamentous growth. Theresulting poor settleability would requireincreased RAS pumping energy, as discussedlater.

Denitrification. Part of the oxygen usedduring nitrification may be recovered byincorporating an anoxic zone at the begin-ning of the aeration tank for denitrifying thereturn activated sludge (RAS). Additionaldenitrification credit is achieved in a process

Wastewater Process Design

with Energy Savings in Mind

Sam Jeyanayagam and Ifetayo Venner

Sam Jeyanayagam, Ph.D., P.E., BCEE, isa senior associate in the Columbus,

Ohio, office of the environmental engi-

neering firm Malcolm Pirnie. Ifetayo

Venner, P.E., is a process engineer in the

firms Tampa office.

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designed to achieve total nitrogen removal,such as the Modified Ludzack-Ettinger(MLE) process, where an internal mixedliquor recycle (IMLR) is used. A review of theimpact of RAS and IMLR flows on the deni-trification credit, shown in Figure 1, revealsthe following: In a nitrifying activated sludge process with

no internal recycle (IMLR=0), RAS denitri-fication is the only source of denitrificationcredit. For example, a 50-percent RAS flowwould provide approximately 30 percent ofthe maximum available denitrificationcredit, while a 100-percent RAS rate wouldprovide 50 percent denitrification credit.

As the IMLR flow increases, the differencein the denitrification credits provided bythe various RAS rates decreases.

In theory, an IMLR rate of approximatelythree times Q (influent flow) providesapproximately 80 percent of the denitrifi-cation credit. IMLR rates greater than 3Qto 4Q provide marginal increase in denitri-fication credit.

While IMLR provides increased aerationcredit in nitrogen removal systems, it requiresinternal pumping, which represents an addi-tional capital and energy cost.

Solids Retention Time. The solids reten-tion time (SRT) dictates the extent of thefundamental processes that consume oxygen,which includes microbial growth and decay.For a specific wastewater, selecting a designSRT establishes the process oxygen require-ment. Organic (CBOD) removal can beachieved at very low SRT values (two to fourdays).

Because nitrifiers grow more slowly thanheterotrophs, nitrification requires longerSRTs. Once nitrification is initiated, the NOD

will increase very rapidly with the increase inSRT. At very long SRTs, both CBOD andNOD increase marginally.

One significant oxygen demand isendogenous respiration, or cell decay, whichis a characteristic of long SRT systems. Atvery high SRTs, the aeration basin begins tooperate as an aerobic digester, and addition-al oxygen is required to support endogenousdecay. Some facilities intentionally operateat very long SRTs in order to reduce sludgeproduction to achieve lower overall operat-ing cost. At these facilities, the resultingsludge disposal cost savings is greater thanthe increased cost of aeration to satisfyendogenous demand, but this will notalways be the case.

Since shorter SRTs are generally associ-ated with lower oxygen demand, effortshould be made to operate the system at theminimum SRT required to meet effluentlimits.

Primary Treatment. Enhancing primarytreatment can reduce oxygen demand. Low-cost strategies for improving primary clarifi-er performance, such as flow balancing, baf-fles, and inlet energy dissipation, can result in

increased CBOD removal with an associateddecrease in aeration demand in the bioreac-tor; however, in biological nutrient removalplants, enhanced CBOD removal in the pri-mary clarifiers may lower the denitrificationand enhanced biological removal capabilities.

Factors Affecting Oxygen TransferThe driving force for oxygen transfer is

the DO differential, which is the differencebetween the saturation DO (Csat) and mixedliquor DO (Cmlss):

Oxygen Transfer Efficiency a (Csat Cmlss)

The saturation DO depends on waste-water temperature and mixed liquor DO.Due to reduced oxygen solubility, Csat valuesare lower in the summer, resulting in reducedDO differential (transfer efficiency) for agiven Cmlss. For example, in order to maintain2.0 mg/L DO, approximately 25 percent moreair is required at 23oC than at 12oC.

At many facilities, the lack of automa-tion or the practice of safeoperation causesover-aeration. The resulting high Cmlss lowersthe DO differential, which causes lower oxy-gen transfer rates and increased energy use.For example, a system operating at a DO of4.0 mg/L will require approximately 35 per-cent more air than a system operated at a DOof 2.0 mg/L.

A good DO control strategy can provideconsiderable energy savings, either manuallyor automatically. In manual DO control,operators measure basin DO levels and adjustair supply a few times a day. As a result,over/under-aeration can occur between man-ual sampling episodes. Less energy is wastedwhen air supply is automatically controlled tomaintain a preset DO level in the aerationbasin.

As noted earlier, a tapered diffuserarrangement is the first step in providingoxygen where it is needed without increasingblower pressure. A more refined strategyinvolves the use of motorized valves to inde-pendently control airflow to the differentzones to maintain DO set points. The bloweroutput is controlled based on a dischargepressure setpoint, which is the lowest pres-sure commensurate with the most-open valveposition. On-line DO meters are typicallyused with this setup.

More sophisticated instrumentation,

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such as on-line nutrient analyzers, oxidation-reduction potential (ORP) probes, andnicotinamide adenine dinucleotide reduced(NADH) probes, have also been used quitesuccessfully for achieving tight DO control,particularly in nitrification/denitrificationprocesses.

Air can be delivered to the aeration basinusing surface aerators or diffusers. The

amount of oxygen that can be transferred bya surface aerator is a function of its design aswell as the operating conditions. On-site oxy-gen transfer tests and aerator power drawmeasurements would allow surface aeratoroxygen transfer rates under actual operatingconditions to be determined.

Diffused aeration systems are classifiedas fine (porous) or coarse (nonporous) bub-ble systems. Fine bubble diffusers producebubbles in the range of two to five millime-ters in diameter when new. The resulting highsurface-area-to-volume ratio of the bubbles

allows relatively high oxygen transfer effi-ciencies to be attained.

In contrast, coarse bubble diffusers pro-duce significantly larger bubbles (six to 10millimeters) and are associated with lowertransfer efficiencies. Many facilities have real-ized significant energy cost savings by replac-ing coarse bubble diffusers with fine bubbledevices.

Aeration basin configuration affects thealpha factor, which is a measure of oxygenmass transfer in wastewater relative to that inclean water. Increased alpha could result in

energy savings. The oxygen transfer rate andalpha factor typically increase along thelength of the aeration tank in plug-flow sys-tems.

Activated sludge systems are often under-loaded during initial years of use or during cer-

tain seasons. Consideration should be given totaking aeration basins off-line to minimize aer-ation costs, but process parameters such asfood-to-microorganism (F:M) ratio, SRT, andhydraulic retention time should be checked toensure that they are within the facility's recom-mended design range.

Mixing RequirementsThe activated sludge is a suspended

growth process, so sufficient power must beprovided for mixing. The objective is to keep

the solids in suspension without causing sig-nificant floc shear, which would impact set-tling.

In an activated sludge basin, in order tosimplify operations and to minimize cost, thesame equipment is used to keep the solids insuspension and to transfer the required oxy-gen. As stated before, the process oxygenrequirement is determined by the SRT, as isthe power needed to transfer this oxygen.

As shown in Figure 2 (Adopted fromGrady et al. 1999), for a given SRT and oxy-gen transfer device, the volumetric power

input (hp/1,000 ft3

in mechanical aerators) orvolumetric air input (m3/min. 1,000 m3 indiffused aeration) decreases with increasingbasin volume. This value should not be solow as to cause the solids to settle (mixinglimited) or so high as to cause the solids to

shear (shearing limited). The approximatelimiting values for an activated sludge systemare provided in Table 3.

As illustrated in Figure 3, good engineer-ing design is an iterative process involving theuse of several interrelated factors in order tosize the activated sludge basin to achieveoptimum energy savings. Once the SRT hasbeen selected and the necessary solids inven-tory determined, the size and cost of the acti-vated sludge basin may be minimized byusing the highest possible MLSS concentra-

tion. Doing so, however, will require the useof larger clarifiers at a higher cost, so thedesigner should consider an MLSS concen-tration that achieves a trade-off between thevolume of the activated sludge basin and thesize of the clarifier to minimize the total sys-tem cost.

The next step is to use the activatedsludge basin volume to determine the volu-metric power/air input. If the basin volume istoo large, the volumetric power/air input maybe less than the limit of mixing, so thepower/air input will have to be increased

beyond the input required for oxygen trans-fer in order to keep the solids in suspen-siona waste of power. Conversely, if thebasin volume is too small, the volumetricpower/air input may exceed the limit ofshear, causing floc breakup.

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