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Introduction

Steel industry wastewater t reatment requirements areseemingly in a constant state of change, making i t di ff i-cult to predict how changes in plant processes and efflu-ent l imit guidel ines wil l affect exist ing steel mil l waste-wa ter trea tment plant s. Meeting stringent effluent l imitshas become challenging, especially for aging facilities. Aseffluent limits are revised and become tighter, post-treat-ment or completely new equipment systems may berequired. However, the first objective should be to opti-mize operation of the existing equipment.

Many s teel mi l l wastewaters are contaminated wi thsoluble metals that must be removed prior to dischargeinto a municipal sewer or receiving strea m. Aging trea t-ment plant s tha t w ere not designed a nd bui lt w i th the l a t-est technologies and equipment may now be faced withinadequa te tr eatm ent capa bil it ies. Adding t o this problema re plant process modifica tions an d revised effluent limitsthat are too stringent for the exist ing systems to meet.Approaches to meet these new requirements includeincreasing chemical dosages a nd/or a dding new trea tmentchemicals or equipment. Although there may be merit tochemical treatment and equipment modificat ions, many

times these methods are not required a nd problems ca n beaddressed wi th basic engineering and less complexupgrades.

Some common wastewater treatment problems thatmay occur are discussed here, along with methods forupgrading a s teel mi l l wastewater t reatment plant tomeet effluent limits in a practical and cost-effective man-ner. Wast ewa ter segregat ion, minor mechanical upgra des,modificat ions to the chemical treatment program or acombination of these can often differentiate betweenmeeting or fai l ing effluent l imits. Identi fying a nd imple-menting proper solutions in a quick and cost-effectivema nner is essential .

Wastewater segregation

One method that can improve wastewater t reatment i ssegregat ion. Ident i fy ing the va rious wast ewa ter sources ,f lowra tes and conta minan t const i tuents can be a benef i -c ia l f i rs t s tep in developing pract ical upgra des . A quickand inexpensive survey of the manufacturing processescan ident i fy the various sources of wastew at er and thei rapproximate f lowrates . Each wastewater source shouldbe sampled and evaluat ed for t he para meters of concern.The resul ts w i l l ident i fy t he ma jor contamina nt origins .At this point , an ini t i a l upgrade may be to segregate ahigh-st rength, low-f low wastewater that i s contaminat-i ng the en t i re w as tew a ter s t ream and a l l ow i t t o b e

metered into the exis t ing t rea tment system. Control ledmetering of high-strength wastes can improve the over-al l performance of a wastewater t reatment plant . As an

al ternat ive, high-st rength wastes may be t reated v ia aside-str eam tr eat ment or disposed of off si te i f cost effec-tive.

Equalization

Wa stew a ter equa lizat ion is employed to smooth out fluc-tuations in influent characterist ics, al lowing the waste-water treatment plant to run more uniformly. Influentwastewater characterist ics can be impacted by fluctua-tions in temperature and pH, as well as concentrateddumps and flow surges. Control l ing influent wastewaterchara cterist ics so there is minimal va riance can be cri t ica lto meeting tigh ter effluent limits. The goal of equa lizat ionis to provide a dequa te da mpening of influent consti tuentsfor s ta b le opera t ion of physical/chemica l t reat mentprocesses.

One problem with steel mil l wastewater treatmentplants is ha ndling of concentra ted process wa ste dumps.Over the years, addit ional process l ines may have beenadded to the production facility. The resultant short-termflow a nd concentra t ion va riat ions may be greater tha n theplant wa s designed to ha ndle. Although i t ma y be tempt-

ing to pursue other wastewater treatment methods (e.g. ,new chemical t reat ment programs, ad dit iona l equipment,or post-treatment to remove residual contaminants),insta l l ing a properly designed front-end equa l izat ion sys-tem ma y be the most pra ctical upgra de.

Equalization system design should be performed on acase-by-case basis using sampling data and stat ist icalprinciples or simpler methods based on experience andpractical judgment. H owever, any a mount of equa l izat ionenhances efficient wastewater treatment.

Many exis t ing wastewa ter plants may a l ready ha ve anappropriately sized equal izat ion system, but continue toexperience wide swings in influent composition. If thiscondition exists, the problem may be poor mixing. If the

equalization basin does not have a mixer, one should beinstal led to avoid waste strat i f icat ion. I f the basin doeshave a mixer, the mixer should be inspected to be suretha t i t is providing adequat e mixing. A general rule ofthumb for adequate equal izat ion mixer sizing is onehorsepower per 7500 gallons of wastewater, assuming theproper shaft length and mixer impeller(s) are used.Proper mixing is as important to equal izat ion as basincapaci ty. Air lancing and tank recirculat ion mixing areother methods tha t a re better tha n not mixing at a l l .

Reaction mixing

Mixing wastewaters to complete chemical reactions is

an other area tha t i s of ten overlooked. Man y a ssume thati f a mixer i s ci rculat ing th e wast ewa ter inside a t an k ina v is ib ly acceptable manner, that mixing i s adequate .

46   AISE S t e e l Te c h n o l o g y September 2001

By applying sound engineering practices to several common wastewater treat-

ment problems, simple but effective solutions may allow more stringent effluent

limits to be met while saving significant capital and operating dollars.

Practical Solutions for Optimizing Steel Mill

Wastewater Treatment PlantsThomas W. Woodrow, Senior Project Manager, Killam Associates, Warrendale, Pa.

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This is often not the case. Mixing of wastewater is cri t i-cal for s pecific chemical r ea ctions t o occur. Thus , mixingaf fects overal l performance and must be evalua ted th or-oughly.

Ina dequa te mixing is often blamed for poor wa stewa tertrea tment plant performance. As a result , engineers maybe quick to instal l larger rea ction ta nks with more reten-tion time; la rger, more powerful mixers; or new motors toexisting mixer shafts to improve mixing performance.These options can be quite expensive. Rather, carefulan a lysis of the exist ing mixing equipment can often lead

to more practical improvements.Assuming axial mixing, mixing energy is mainly afunct ion of the pumping ra te wi thin t he react ion ta nk a ssupplied by t he mixer impeller; i t is not s olely a functionof how much horsepower is a pplied to th e sha ft . As pre-viously stat ed, a rule of thu mb for genera l mixing is onehorsepower per 7500 gal lons of wastewater, assumingthe proper shaft length and impeller are used. However,vigorous mixing may be necessary to complete certainchemica l reactions; in this case more mixing energy ma ybe required. U p to one-ha lf horsepower per 1000 gal lonsof wastewater can be required to complete some chemi-cal react ions . Insta l l ing a l arger motor on an exis t ingmixer sha f t wi th a n inadequa te impel ler wi l l not a lwa ysyield better mixing. Replacing the impeller and motorand keeping the gearbox (i f possible) may be an appro-pr i a t e a l t e rna t i ve to rep l ac i ng the en t i re m i xer .Upgrading the mixer may al so el iminate the need forlarger ta nks, which can save a s igni fican t a mount of cap-i t a l .

Another importa nt a spect of mixing is ta nk ba ffl ing. I fta nks a re not properly ba ffled, mixing w ill suffer. Withoutbaffles, swirl ing a nd vortexing results a nd very l i t t le mix-ing takes place, al though i t may appear so visual ly.Upgrading a tank by instal l ing equal ly spaced baffles tothe ta nk wa ll is inexpensive and can improve mixing in amuch more practical manner than replacing tanks orad ding larger mixers.

Understanding the react ions performed in a mixed

react ion tank i s a l so important . Changing the chemicalt rea tm ent program m ay b e a b e t t er so l u t i on thanreplacing the mixers or tanks. For example, hydratedlime is a common chemical used for n eutr a l izat ion/pre-cipi ta t ion of soluble metals in w a stewa ter . As l ime i s notreadily soluble in water, i t requires 20 to 30 minutes ofretent ion t ime to react when properly mixed. By con-trast , sodium hydroxide can be equal ly effective forw a s tew a ter neu t r a l i z a t i on/preci p i t a t i on a nd i s 1 00times more soluble than l ime, therefore completing therequired reaction in five to 10 minutes. Although sodi-um hyd roxide typical ly costs more tha n l ime, the changecould be a pract ical a l ternat ive to replacing react iont a nks and m i xers.

Chemical treatment

The chemical treatment program for a steel mil l waste-wa ter t reatment plant can be simple and s t ra ight forwa rd,or it can be quite complex an d difficult to contr ol. In g en-eral , i f the t reatment plant has adequate equal izat ion,mixing and reaction t imes, and the other equipment isproperly designed and in good working order but effluentl imits ar e not being met, the chemical t reat ment programmay be the problem.

One of the most common wastewater treatment meth-ods employed for heavy metals removal is the use of pri-ma ry coagula nts such a s a luminum sulfate (alum), ca lci-

um chloride and ferric chloride (iron salt). If these con-ventional treatment chemicals are being used and treat-ment is not adequate for parameters such as nickel ,

chrome and zin c, the problem may be the system’s operat -ing pH. Before exploring expensive solutions (such asincreasing the dose of coagulants and flocculants), thepara meters of concern a nd th e neutral izat ion step pH set-point should be investigated. In general, chrome hydrox-ide is least soluble at a pH between 8.0 and 9.0; zinchydroxide is least soluble at a pH between 9.0 and 10.0;a nd nickel hydroxide is least soluble a t a pH betw een 10.0a nd 11.0. Depending on the pa ra meter(s) of most concern,selecting and ma inta ining the optimum pH set point couldbe the most effective treat ment plant upgrade.

Although the treatment chemicals discussed aboveha ve historical ly been effective for steel mil l wa stewa tertrea tment, lower effluent l imits may be di fficult t o meet,even with optimum pH set points. Once this occurs, acommon solution is to use more of these conventionalchemica ls; however, chemical costs w ill begin to rise a longwith sludge production and corresponding disposal costs.A more pra ctical solution in th is ca se may be to switch toa metals scavenging polymer tha t is capa ble of achievingeffluent limits at lower cost and reducing sludge produc-tion. These chemicals are specially developed and avail-able through chemical suppliers and wil l typical ly workwith existing process equipment at low feed rates. Theswitch to a polymer-type chemical treatment programcould be very beneficial to an exist ing wastewater treat-ment plant , but can only be veri fied through treatabi l i tytest ing.

If low (less tha n 0.5 mg/l) meta ls concentra tions a rerequired due to lower effluent limits, the most practicalal ternative may be to instal l a f inal polishing step usingother proprietar y metals scavenging chemicals or sulfide-based polymers. Several types of proprietary chemicalsare available and can be very effective. These chemicalstypica l ly have the abi l i ty to t ie up residual meta ls remain-ing in solution a fter the primary t reat ment step, and pre-cipi tate them in a final clari ficat ion or fi l trat ion step.Again, verification of the appropriate chemical and equip-ment upgra des required can only be determined throughtreatabi l i ty test ing. Despite the extra effort , this method

of treatment can be more practica l an d cost effective tha nlarger mixers and r eaction ta nks, larger cla ri fiers a nd fi l-tra t ion equipment, a nd ion excha nge a nd/or membranefi l trat ion.

Clarification

Wast ewa ter clari ficat ion via precipita ted sol ids sett l ing iscritical to meeting effluent limits. Most effluent permitslimit th e amount of tota l suspended solids (TSS ) a nd met-a l s tha t c an b e d i scharged to a rece i v i ng s t ream .Therefore, effective solids settling in the clarifier is animportant aspect of the treatment process. In general ,

clarifier effectiveness can be determined by its waste-wa ter overflow ra te. A basic rule of thumb for industrialwa stewa ter tr eatment is an overflow ra te of 0.25 to 0.50gpm per square foot of surface area. I f wastewater flowsexceeding the design of the original clarifier cause theoverflow rates to become too high, excessive amounts ofTSS ma y discharge from the syst em (along w ith meta ls),contributing to effluent limit violations.

One cost-effective method for improving the perform-a nce of an existing clarifier is to insta ll tube settlers. Tubesett lers a re modules of an gled sett l ing tubes insta l led inthe clarifier just below the water surface, effectivelyincreasing the surface area. Additionally, crosscurrentsand short-circuiting are significantly reduced. In effect,the clari fier wil l perform better at the higher flow rate

wit hout increasin g the basin size. Tube settlers can ty pi-cally save money and time in improving existing clarifierperformance.

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Final pH adjustment and filtration

Final pH adjustment prior to discharge i s typical lyrequired to bring the wa stewa ter pH wi t hin permi t l im-its—usually between 6.0 and 9.0. Sulfuric acid is typi-cal ly used for pH a djustment , a s i t i s fas t a c t ing and rel-at ively inexpensive. However, i f the wastewater t reat-ment plant has effluent l imits for total dissolved sol ids(TDS), the addit ion of sulfuric acid at the point of dis-char ge could a dd sufficient TDS to fai l effluent l imits. I fTDS l imits are not being met, one option that may beconsidered is addit ion of expensive ion exchange ormembrane f i l t ra t ion equipment to lower the TDS towi thin permi t l imi ts . A bet ter a l terna t ive may be use ofcarbon dioxide for final pH adjustment. Carbon dioxidelowers pH by t he format ion of carbonic acid, which doesnot add TDS to the wa ter a nd i s a s ef fect ive as sul furicacid for pH adjustment. In addit ion, carbon dioxideusage can be as much a s 50%less on a per pound ba sisas compared to sulfuric acid for performing the sameneutra l i zat ion. In tha t car bon dioxide does not a dd min-eral ac idi ty to the wastewater , i f i t i s accidental lyoverfed, i t sel f buffers a t a pH of a pproximat ely 6.5, pro-viding virtual ly fai l-safe protection against violat ing apermit l imit for pH due t o overfeeding.

Final pH adjustment in combination with final f i l tra-

t ion i s a nother problemat ic area . Aw astewa ter t reat mentplant ma y uti l ize pH a djustment and fi l tra t ion for controlof residua l meta l hydroxide suspended sol ids an d st i l l fa i leffluent l imits for meta ls. Understa nding the chemistry ofinsoluble metal hydroxides may provide a practical solu-tion. Some wastewater treatment plants place the finalpH a djustment step ahead of f inal f i ltra t ion. By doing so,am photeric metal h ydroxides can dissolve at lower pH lev-els and pass through the filter, resulting in exceedance ofthe effluent l imits for t otal meta ls. A more pra ctical solu-tion to this problem is to simply reverse the treatmentstep order (i .e. , perform final filtration first and pHadjustment last). These upgrades can usually be complet-ed with minor piping modificat ions that are much less

expensive than addi t ional chemical t reatment , ionexcha nge or membrane fi l tra t ion.

Flow reduction and water conservation

Recently, regulatory agencies have been encouragingapplying pressure for flow reductions and water conser-va tion. Although t here can be ma ny benefits to flow reduc-tion and wa ter conservat ion, such as lower opera ting costs(i.e., chemica ls, sludge disposal, city/well w a ter u sa ge a ndPOTW discharge fees), such improvements have imple-mentation costs and secondary impacts. Costs includeexpensive polishing or side-stream treatment equipment,chemical treatment, cooling towers and recirculat ion

wa ter piping systems. Secondary impacts can include dis-posal of concentrated wastes or the discharge of lowerflows w ith higher tota l dissolved solids (TDS ) and/orincreased toxicity potential.

Al though not encoura ged a s a means of meet ing ef f lu-ent l imi ts , f lows from various sources may be di lut ingthe ef f luent s t ream to a point w here TDS concentra t ionsare being met . (Many discharge ef f luent permi ts now contain TDS concentrat ion l imits.) Therefore, i f the

remaining discharges are more concentrated, reducingthe f low could resul t in increased TDS violat ions . I f th isoccurs i t is di ff icult to reintroduce di lution water backinto the discharge i f regulatory agencies have beenma de aw ar e of the f low reduct ion. At t his point , meet ingTDS l imits can become difficult and expensive, as TDSremoval can only be accompl ished v ia d ra ma t ic processchanges, or f inal t reatment wi th ion exchange, mem-brane f i l t ra t ion or evaporat ion. In the f inal analysis ,reducing f lows could resul t in increased capi ta l andoperat ing expenses . Ath orough eva luat ion of the impa ct

on TDS result ing from flow reductions should be per-formed before implementation.Another parameter of concern is effluent toxici ty.

Man y s teel mi l l wast ewa ter ef f luents ha ve ef f luent t ox-ic i ty l imi ts—a measurement of the wastewater ’ s toxiceffect on selected aquatic species. As with TDS, flow reduct ions could resul t in increasing the concentra t ionand hence the toxic i ty of the wastewater ef f luent ,resul t ing in toxici ty test fa i lures . Meet ing toxici ty l im-i ts can become dif f i cul t a nd expensive, and ma y requiredramat ic process changes or other complex t reatmentschemes. A thorough evalua t ion of the impact of f low reductions on toxici ty should also be performed beforeimplementat ion.

Other process improvements

Other process improvements not discussed that should beconsidered for cost-effective upgr a des of a n existin g st eelmi ll wa stewater t r eatment plant ma y include:

• Improving react ion ta nk inlet/out let conf igura-tions to increase effective retention time, reduceshort-circuiting and enhance chemical reactions.

• Opt imum locat ion of pH probes and chemical feedlines.

• S l u d ge r e cy cl in g , h i gh -d e n si t y s l u dg e (H D S )a nd/or iron co-precipita tion t o enha nce syst em per-

formance.• Intermi t tent use of ef f luent bag f i lt ra t ion in place

of media filters.

Conclusion

A number of operat iona l deficiencies or equipmentshortcomings were ident i f ied and discussed that mayprevent a s t ee l m i l l w as tew a ter t rea tm ent p l an t f romm eet i ng ex i s t i ng or proposed e f f l uen t l i m i t a t i ons .Correction of such condit ions should be evalua ted beforeconsiderin g th e add it ion of ma jor fa ci l it ies. Typicalproblems that may occur in exis t ing s teel mi l l waste-

wa ter t rea tment plant s not bui l t to meet s t r ingent eff lu-ent l imi ts a nd/or han dle cha nges in plant f low an dwa stewa ter const i tuent s were a ddressed. Applica t ion ofsound engineering pra ct ices to several common w ast e-wa ter t r eat ment problems ca n break t he cycle of myopicf ixes tha t a f f l i ct ma ny plant s . Implement ing s imple, yeteffective solutions could be beneficial in meeting morestr ingent ef f luent l imi ts , whi le saving s igni f icant capi-t a l a nd opera t i ng do l la rs .