environmental controls

WASTEWATER TREATMENT

BY THOMAS J. WEBER

WASTEWATER MANAGEMENT INC., CLEVELAND;

WMI-INC.COM/HOMEPAGE.JHTML

Today, some 15,000 companies in the United States perform electroplating andmetal finishing operations. These firms discharge their spent process wastewaterseither directly to rivers and streams, or indirectly to Publicly OwnedTreatment Works (POTWs). Metal finishing, by far, comprises more individualwastewater discharges than any other industrial category. Typically, pollutantscontained in metal finishing process waters are potentially hazardous, therefore,to comply with Clean Water Act requirements, the wastewaters must be treated,or contamination otherwise removed, before being discharged to waterways orPOTWs. Regulations, in general, require oxidation of cyanides, reduction ofhexavalent chromium, removal of heavy metals, and pH control.Understandably, for companies discharging wastewater directly to waterways(direct discharges), regulations promulgated through the years requireattainment of the more stringent concentration-based limitations for toxicwastewater constituents necessary for protection of aquatic life. These streamstandards were developed from Federal Water Quality Criteria and limit instreampollutant concentrations to levels that will not adversely affect drinkingwater quality and aquatic life. Since the mid 70s, state agencies have continuedto drive direct discharge limitations downward to levels well below waterquality-based stream standards, using antidegration, antibacksliding, andexisting effluent quality (EEQ) policies, and the number of direct dischargershas dropped precipitously. Implementation of biological-based criteria throughbiomonitoring and bioassay testing will continue to force direct dischargingfacility closures and relocation to POTWs.As the overwhelming majority of metal finishing companies are dischargingto POTWs, wastewater treatment systems for these firms are installed for compliancewith federal pretreatment standards, or local pretreatment limitations ifmore stringent than the federal regulations. Federal standards are technologybased,i.e., developed through historical sampling and testing of conventionalwastewater treatment system discharges collected at select, best-operated facilities.The base level technology was called Best Practicable Control TechnologyCurrently Available (BPCTCA), or simply BPT. The more stringent level wastermed the Best AvailableTechnology Economically Achievable (BATEA), andis usually referred to as BAT. The treatment technology of BAT differs mainlyfrom the conventional physical-chemical treatment of BPT in that it includessubsequent polishing filtration, and normally addresses improved methods ofplating bath recovery.The purpose and intent of federal and local pretreatment regulations are toprevent the introduction of pollutants into POTWs that will interfere with theiroperations; to prevent the introduction of pollutants, which will pass throughthe POTW and contaminate receiving waterways; to prevent pollutant concentrationsthat are incompatible with biological processes or otherwise inhibit theprocess; and to reduce the pollutant concentrations of POTW sludges.Since the pretreatment regulations became effective in 1984, the metal finish- ing industry has taken major strides in pollution control through wastewatertreatment system installation and operation, admirably fulfilling the regulatoryintent. Substantial historical reductions for all metals have been demonstratedat many POTWs nationwide.

STATUS OF WASTEWATER REGULATIONS

The federal regulations listed in Tables I and II have now been in existence inexcess of 10 years since the 1984 compliance dates. For those metal finishingcompanies still fortunate to be limited by these regulations, each limit and theapplicability of the regulations are of intimate familiarity and compliance is beingachieved on a day-to-day basis. Increasingly, POTWs are imposing, or are beingforced to impose, local pretreatment limitations that are much more stringentthan the federal regulations. Often, these local limits are 10-25% of the Table Iand II concentrations.Properly selecting wastewater treatment technology, modifying production operations and processes, and improving wasteminimization and resource recoverytechniques have become prerequisite to achieving compliance. Implementationof the basic BPT and BAT technologies is often inadequate to meet frequentlyunreasonable, and usually unnecessary, local limits set far below the technologybasedstandards. Increasingly, local limitations are being based on mathematicalmodels using faulty software programs and arbitrary POTW effluent standards,rather than good science and environmental ncessity.Although federal regulations have remained unchanged since their 1984 effectivedate, the U.S. EPA proposes to get back into the act of tightening pretreatmentstandards for metalfinishers. In late 1994, the U.S. EPA proposed drafting MetalProducts and Machinery (MP&M) Effluent Guidelines, which would imposespecific concentration limitations on many metal fabricating and machine shopspresently not covered under any federal industrial pretreatment category. U.S. EPAestimates the regulation would bring another 20,000 companies nationwide underthe pretreatment requirement umbrella. The proposal, however, includes the prospectof shifting all metal finishers and electroplaters to the MP&M Guidelines,thus eliminating the current regulations. The MP&M limits are expected to bedeveloped from reassessing technology-based pollutant concentrations. Thiscould effectively reduce federal pretreatment limitations by 50-90%, depending onthe pollutant, as current effluent quality among metal finishers is much lower, formany reasons, than in the 1970s when the original BPTs/BATs were established.Although metal finishing and POTW effluent quality have continued toimprove annually, the incidence of enforcement actions and amounts of theresultant penalties have increased. Many municipalities have adopted “automatic”penalties for any discharge violation, and have modified pretreatmentordinances to make it easier to collect penalties.The U.S. EPA was required to draft the MP&M Guidelines in March, 1995. Asof the date of this writing, the regulation has not been published. If the regulationis drafted per the original proposal, future regulatory enforcement will bemore likely to increase. Improved treatment system operation and performancewill become an even greater economic necessity of the metal finisher.Furthermore, the treatment focus will further shift from conventionalphysical-chemical treatment to the more advanced, more expensive treatmentmethods of microfiltration and ion exchange polishing, and closed-loop, zerodischargemethods of reverse osmosis and evaporation.

SYSTEM SELECTION CRITERIAF

our major factors contribute to the size, complexity, and cost of conventionalwastewater treatment systems.

Pollutant Type

The complexity of the treatment system needed to effectively remove pollutantsfrom a wastewater is determined by the type and nature of the pollutantsencountered. A basic system will only require simple neutralization and chemicalprecipitation prior to solids separation for certain, although few, metal finishers.The process use of complexing or chelating agents in production baths wouldincrease system complexity, often requiring two-stage treatment or neutralizationand the need to apply chemical coagulants or specialty metal precipitantsto reduce metal solubility.Other pretreatment processes, including hexavalent chromium reduction andcyanide oxidation, are only required when the plating operation utilizes these commonchemicals. Oil separation on a segregative basis may be necessary in facilitieswhere oil and grease concentrations in the combined raw wastewater exceed 200mg/L.Increasingly, today’s metal finishers are modifying processes and gettingrid of certain finishes to eliminate problem pollutants and the resultantsystem complexity, or simply to reduce discharge violations. Over the years,there has been a major industry shift to noncyanide bath finishes. Curbingor modifying the use of complexing chemicals and conversion to trivalentchromium finishes has further reduced system complexity through changesin pollutant type.

Pollutant Loading

Treatment chemical costs and solids handling equipment sizes/costs increaseproportionally to pollutant loading to the wastewater treatment system.Clarification, sludge storage, filter presses, and sludge dryers are sized in accordanceto projected loads and solids generation. Increased size requirements resultin higher capital equipment costs and higher disposal costs for waste residuals.Proper selection of plating baths with reduced metal maintenance levels andprecise control of bath concentrations will reduce loadings. Other commonloading minimization practices include implementing a rigorous housekeepingprogram to locate and repair leaks around process baths, replacing faultyinsulation on plating racks to prevent excessive solution drag-out, installingdrip trays where needed, etc.; using spray rinses or air knives to minimizesolution drag-out from plating baths; recycling rinsewater to plating baths tocompensate for surface evaporation losses; using spent process solutions aswastewater treatment reagents (acid and alkaline cleaning baths are obviousexamples); using minimum process bath chemical concentrations; installingrecovery processes to reclaim plating chemicals from rinsewaters for recycleto the plating bath; and using process bath purification to control the level ofimpurities and prolong the bath’s service life.

Hydraulic Flow Rates

The size and capital costs for wastewater treatment are largely dependent onthe instantaneous flow rate of wastewater requiring treatment. The major contributorto the volume of wastewater requiring treatment is rinsewater usedin the production processes coming in direct contact with the workpiece. Theconversion to air-cooled rectifiers from water-cooled rectifiers, and installationof chillers and cooling towers for reuse ofbath and rectifier cooling water, havelargely eliminated noncontact hydraulic loadings.Other common practices used to reduce wastewater volume include implementingrigorous housekeeping practices to locate and repair water leaksquickly; employing multiple counterflow rinse tanks to reduce rinsewater usesubstantially; employing spray rinses to minimize rinsewater use; using conductivitycells to avoid excess dilution in the rinse tanks; installing flow regulatorsto minimize water use; and reusing contaminated rinsewater and treatedwastewater where feasible.Negative results impacting treatment system operation, however, have resultedfrom zealous water-reduction programs. Rinsewater reductions invariablyresult in increased contaminant concentrations undergoing treatment, andoccasionally to problem levels. Increases in alkaline cleaner and chelating chemicalconcentrations, in particular, commonly impede conventional treatment,resulting in poor coagulation and floccuation.

Environmental Regulations

The stringency of the concentration-based discharge limitations affecting ametal finisher is often the leading criterion in selecting treatment processes andsystems. Generally, conventional chemical precipitation systems, perhaps withpolishing filtration, are suitable to attain compliance with federal regulationsor reasonable local standards.For those firms residing in communities that have adopted local standardswith metals limitations ranging from 0.1 to 1.0 mg/L, cost and complexity ofthe system can be substantial. Multiple conventional treatment trains in seriesoperations are relatively simple, but effective. Advanced microfiltration, cationexchange polishing, reverse osmosis, and complete evaporation may be necessaryto meet stringent standards or totally eliminate the discharge.

CONVENTIONAL METHOD OF WASTEWATER TREATMENT

To this day, the majority of metal finishers are meeting, or attempting to meet,effluent limitations by treating wastewater by conventional physical-chemicaltreatment. The process basically involves the use of chemicals to react with solublepollutants to produce insoluble byproduct precipitants, which are removedby physical separation via clarification and/or filtration.Conventional treatment systems often include hexavalent chromium reduction,cyanide oxidation, and chemical precipitation in a neutralization tank. Typically,these steps are followed by clarification. As clarification is not a 100% solids separationdevice, additional polishing is often required using one of many filtrationdevices. Increasingly, it is becoming common to eliminate the clarification stagetotally, and its polymer flocculation step, in favor of direct microfiltration. Thesludge from either separation stage is stored/thickened in a sludge tank, thendewatered via a filter press.

Chromium Reduction

Chromium in metal finishing is normally used in the hexavalent ion form (Cr6+) inplating or chromating. As it soluble at all pH values, the chemical reduction step toits trivalent (Cr3+) form is necessary to ensure removal by precipitation. Commonly,trivalent chromium replacement processes are being employed for safety considerationsand the elimination of the reduction wastewater step. Exercise care in selectingtrichromium replacements that may contain ammonia and other chemicals,which can cause complexing of other metals in waste treatment.The reduction of hexavalent chromium is achieved by reaction with sulfurdioxide gas (SO2), or more commonly sodium metabisulfite (MBS). The speed ofthe reaction is pH dependent. At pH 2.5-3, the reaction is virtually instantaneous.Above pH 4, the reaction slows to a point where it becomes impractical for use incontinuous flow systems.The use of pH and oxidation-reduction potential (ORP) controllers is common.Without automatic pH controllers, care must be exercised to ensure completereaction, particularly in batch reactors where the pH is manually adjustedto pH 2.5 prior to MBS addition. MBS addition raises the pH of the solution,often to ranges where reduction times are lengthy. As batch processes are usuallycontrolled visually by color change, a significant MBS overfeed often results.Although MBS and SO2 are the most common chemical reducers used inhexavalent chromium reduction, any strong reducing agent will suffice. Ferrousiron in many forms, including ferrous sulfate, ferrous chloride, ferrous hydrosulfide,or electrochemical ferrous production from iron electrodes, is used.The primary benefit of ferrous reduction is that Fe2+ will reduce hexavalentchromium at near neutral pH values. For low concentration applications (moderatechromating use processes), ferrous addition can eliminate the completechromium reduction stage. The ferric ion formed in the process becomes anexcellent coagulant in the precipitation stage.The only drawback to ferrous reduction is the additional sludge generated its use, as three parts Fe2+ is required to reduce one part Cr6+

.

Chromium Reduction Process Precautions

1. SO2 and MBS form noxious acidic vapors. Avoid excess formation andinhalation of the vapors.

2. pH control is very important. Allowing pH to drift below 2 increases SO2gassing vapors. Allowing pH drift upward to 4 increases reaction times toimpractical levels.

3. Underfeed of SO2/MBS causes chrome carryover. Overfeed of MBS/SO2causes increased metal solubilities in neutralization, and reverses the particlecharge and, consequently, results in poor flocculation.

Cyanide Oxidation

Treatment of cyanide (CN) in metal finishing wastewaters is most commonlyperformed by oxidation in an alkaline chlorination process using sodium hypochlorite(NaOCl) or chlorine gas (Cl2). Because of the toxic danger of Cl2 gas,NaOCl processes are considerably more common.The alkaline chlorination process either involves only first-stage CN oxidation,whereby simple cyanides are converted to cyanates (OCN), or the addition of asecond-stage reactor to convert cyanates to carbon dioxide (CO2) and nitrogen (N2).First-stage CN oxidation is carried out at a pH of 10.5 or higher. Thereaction slows greatly at pH values below 10 and virtually ceases at pH values below 9. The process only oxidizes simple cyanides, such as NaCN,KCN, Zn(CN)2, CdCN, CuCN, etc. Complexed cyanides, commonly found inmetal finishing wastewater as iron complexes, are not destroyed in alkalinechlorination processes. In fact, complexed cyanides are not destroyed efficientlyby any common cyanide oxidation process, including ozone. The useof high-pressure/high-temperature thermal processes will, however, destroycomplexes. Also, lengthy exposure to sunlight will convert complexed cyanidesto simple cyanides, to a small extent.As federal and local regulations are generally written for total cyanide monitoringand limiting, complex cyanides are often the species causing violations.Complexed cyanides are most commonly formed by poor housekeeping,control, and rinsing. Drag-out or drippage of CN from baths or bath rinses intoacids and chromates is very common. Steel electrode use in plating baths causesa significant amount of complexed cyanide input to the bath from constantdecomposition. Clean steel parts allowed to fall and accumulate in CN bathsare another major source of complexed CN formation.Although complexed cyanide formation cannot be totally eliminated, reducedformation through housekeeping and improved rinsing can reduce the concentrationto nonproblem levels.Complexed cyanides are generated in both soluble and insoluble forms. Theinsoluble form is removed via mass settling in the clarifier. Conversion of solublecomplexes to insoluble complexes can be achieved to some extent by the additionof MBS to the neutralization tank. The efficiency is improved in the presenceof copper ion. Permanganate addition also has been reported to accomplishimproved precipitation of complexed cyanides.The second-stage CN oxidation process is carried out at a pH of 8.0-8.5. Anamount of Cl2 comparable to that required infirst-stage oxidation (3.5 lb Cl2:1lb CN) is necessary to complete the conversion of OCN to CO2 and N2.Most sewer use ordinances do not require cyanate oxidation or limit cyanatein the discharge. Consequently, many treatment systems only employ first-stageprocesses. A common problem associated with first-stage-only systems is thepropensity to gassing in the neutralization tank, with resultant clarifier floatingproblems. This is caused by an uncontrollable cyanate breakdown, particularlywhen excess residual Cl2 is present in the first-stage dischare.Although reaction times for most simple cyanides and cyanates are 10-15minutes, it is advisable to size reaction tanks at 1 hour and longer if affordable/practical. Certain simple cyanides, including cadmium and copper,only start breaking down after the sodium, potassium, and zinc cyanides aredestroyed, thus requiring longer contact periods. Furthermore, the longerthe reaction, the more efficient the gas venting becomes, reducing the incidenceof clarifier floating.Because precise control of pH and Cl2 is important, pH and ORP controllersare recommended in all continuous control reaction tanks.

Summary of Cyanide Process Precautions

1. First-stage oxidation must be controlled at pH 10.5 or higher. (The higherthe pH, the faster the reaction.)

2. Control the formation of complexed cyanides, as treatment processes donot destroy them. Add MBS to the neutralization tank if soluble complexescause effluent violations.

3. Allow 1 hour or more reaction time to ensure completion of the reactions,and for problem gas venting.

4. Underfeed of chemical allows CN pass through; overfeeds cause increasedgassing and reoxidation of trichrome.

Coagulation/Neutralization Process Considerations

Effluents from hexavalent chromium reduction and cyanide oxidation stagescombine with other alkaline and acid wastewater streams in a neutralizationtank. The express purpose of the neutralization tank is to create a suitable environmentand retention time for soluble pollutants to react and form insolubleprecipitates for eventual physical separation. The principal precipitation processemployed in conventional wastewater treatment systems is that of hydroxideprecipitation. Heavy metals, the prime targets of neutralization-precipitation,have varying solubilities depending on pH. In common mixed-metal wastewaterstreams, control of the neutralization tank at pH 9.2-9.5 is generally suitable tolower metal solubilities, as hydroxides, to concentration ranges where complianceis achievable.In many cases, it is necessary to add chemical coagulants to the wastewater inorder to achieve minimum solubilities and superior flocculation/solids separationin the clarifier. A proper coagulant will effectively tie up anionic surfactants,wetters, and species such as phosphates, which interfere with polymer flocculation;and also add bulk density for improved solids separation.Where coagulants are required for good process performance, it is recommendedthat two-stage neutralization reaction tanks be employed, as coagulantsperform better when reacted with the wastewater at pH values in the 5.5-6.5range.Common chemical coagulants include calcium chloride, ferrous salts, ferricsalts, and alum.For improved coagulation, certain specialty coagulants are available fromchemical suppliers. These chemicals usually contain one of the above base salts,which are sometimes blended with polymers, generally of a cationic nature.Although these specialty products are expensive, with costs ranging from $400to $1,000 per drum, their use is often necessary to achieve compliance.Neutralization is generally achieved using caustic soda (NaOH) and sometimespotassium hydroxide (KOH). Hydrated lime and magnesium hydroxidealso have wide utilization. Although these neutralization chemicals presentcertain handling and feeding problems associated with their solids content,lower metals solubilities are achieved at maintenance of lower neutralizationtank pH (8.0-8.5).The introduction of strong chemical complexers used in production processescommonly impedes the pollutant precipitation process. Common complexers/chelators include ethylene diamine tetra acetic acid (EDTA), nitrilotriacetic acid(NTA), quadrol, glucconates, glutamates, ammonia, and various amies.Complexing agents are commonly used in electroless baths, electroplatingbath brighteners, alkaline cleaners, parts strippers, and numerous other applications.Eliminating their use, where practicable, is the simplest means of mitigatingtheir adverse wastewater treatment effects. Where critical to the process,special means and practices must be employed, which vary with the type andstrength of the complexer, as well as the metal(s) being complexed.Often off-line pretreatment is necessary, as in the case of high volume electro-less bath use. In other cases, the use of specialty chemical precipitants, meteredinto the complexed waste stream or into the neutralization tank, is suitableand effective. Specialty chemical precipitants include dithiocarbamates, dithiocarbonates,starch and cellulose xanthates, poly quaternary amines, and ozonedestruction/hydrosulfite reduction.As complexing chemicals are primary reasons for noncompliance in conventionalsystems, much care and time are necessary to solve the problems createdby them. Often significant trial testing in bench scale treatability tests and closework with chemical suppliers are necessary to resolve complexing problems.In some cases involving simple complexed wastewaters, conversion fromhydroxide precipitation to sulfide or carbonate precipitation in the neutralizationprocess will achieve necessary reductions in metal solubility. Mostmetallic sulfides and metallic carbonates have lower solubilities than theirhydroxide counterparts.Reaction times required for effective coagulation-neutralization-precipitationvary among wastewater types and complexity. We recommend minimum retentiontimes of 30 minutes, 15 minutes in first-stage reactors. As metal hydroxidestend to reduce in volume the longer they are mixed, the longest practical reactiontimes are most desirable.Common problems associated with neutralization/reaction tanks, whichimpede clarifier separation of solids, include soluble complexes caused bychelating agents; charge reversal caused by anionic surfactants, phosphates,and MBS overfeed; solids buoyancy or flotation problems caused by excess oiland grease or gas formation including chemical gassing caused by peroxides,acetates, and carbonates or physical-induced gassing caused by suction leakson transfer pumps, or significant mixer vortex action; overfeed of dump solutions,particularly alkaline cleaners; and high total dissolved solids (TDS),7,000 ppm and higher, from overly zealous water conservation practices, orhigh percentage reuse of treated water.

FLOCCULATION/CLARIFICATION PROCESSES

The precipitates formed by the proper operation of the coagulation-neutralizationstage are commonly removed in conventional wastewater treatment systemsby clarification or sedimentation. This process involves solids removal by theefficient settling of solids. Buoyancy caused by oils or floating caused by theentrainment of gas bubbles will prevent efficient settling. Generally, floatingproblems are controllable in the typical metal finishing wastewater installation.For certain firms, which employ electrolytic/electrochemical pretreatment orozone generation/airdiffusing treatment techniques, dissolved air flotation(DAF) is the preferred unit for solids separation.Solids separation is improved in clarifiers, or DAF units, by polymer (polyelectrolyte)flocculation. As the average charge of metal hydroxides is positive,a negatively charged (anionic) polymer is used in the flocculation process. It isimperative that the wastewater charge remain positive at all times. Coagulantsand/or cationic polymers may be necessary in certain wastewater types wherecharge reversal is common, as in phosphating operations. Nominal flocculationtime of 1 minute is recommended for floc tank size. Variable speed mixers arerecommended to allow some measure of control of floc size.The size of the clarifier generally varies with the type and style. Basic, open/empty sedimentation tanks commonly used in low-flow installations shouldbe sized for a maximum surface loading rate of 500 gal/day/ft2 of tank surface.Most commonly employed clarifiers are of the lamella type or inclined platevariety. These units are sized based on volumetric flow rate per square foot ofplate pack area projected on the plate incline, or cosine of the degree of plateangle; typically 60O. Recommended loading rates are 0.2-0.4 gal/min/ft2 ofprojected plate area, and a total suspended solids (TSS) concentration of 500ppm or less.Units are manufactured in basic hydraulic flow sizes, i.e., 30 gal/min or 75gal/min, etc. In those cases of high TSS loads (500 ppm or higher), it is notadvisable to size a unit based solely on flow. In these high solids load applications,clarifier selection should be based on 1 lb TSS per hour for each 20 ft2 ofprojected clarifier settling area.Manufacturers will supply design and operational information for theirspecific unit. As a general rule, it is important to evacuate sludge as it accumulatesto prevent its buildup into the plate pack area. This creates blockages andincreases the upflow velocity in the open areas and carries TSS with the highflow. Monthly draining is advisable to minimize ratholing and solids concretion.

EFFLUENT POLISHING

At times, clean water that overflows from a clarifier will require further removalof suspended solids or polishing to meet more stringent discharge requirements.This may be for water reuse or simply as insurance in case of a system malfunction.Sand filters, devices consisting of one or more layers of various sizes andtypes of granular media, are typically used. Gravel, sand, anthracite, garnet, andactivated carbon are common media.The size and number of filters is, as with a clarifier, dependent on the volumeof wastewater to be filtered and the surface area of the filter media. Gravityoperatedsand filters usually are loaded at 0.25-0.5 gpm/ft2, whereas pressuresand filters can operate in the 5.0-10.0 gpm/ft2 range, depending on the suspendedsolids of the effluent.Most sand filters need to be periodically cleaned or “backflushed” to removethe solids that have built up. Clean water, process water, or dilute acid solutionsmay be used for this back flushing. Backflush waters are generally returnedto the collection or equalization tank and returned to the treatment system.Pressure sand filters require less backwash water than larger gravity types.Operationally, care must be taken to ensure that pumps feeding or backflushingthe filters are operating at design capacity to ensure proper loading andadequate cleaning of the media. Sand filter media are rarely replaced, exceptwhen a severe system upset causes solids to block the water distribution headers.

SLUDGE THICKENING AND DEWATERING

Sludge (settled solids) produced from treatment of metal finishing wastes generallycontains between 1.0 and 2.0% total solids. Disposal of such a watery sludgeis very expensive. Most medium and large generators of wastewater choose tothicken and dewater sludge, thus reducing the volume of waste to be disposed.A sludge thickener, although not always necessary prior to dewatering, servesseveral worthwhile functions. First, it creates storage volume for the sludge inthe event that the dewatering equipment is not in operation. Second, it allowsfor aconsistent sludge blanket level in the clarifier. Sludge can be intermittentlyremoved from the clarifier by means of a timer on the sludge pump. This reducesthe possibility of solids drafting over the clarifier weir(s) because of a high sludgeblanket. Finally, sludge stored in a thickener may increase in solids content to asmuch 3-4%.Increased solids content does two things: it decreases cycle time required bythe dewatering equipment (filter press, centrifuge, belt press) and, as a rule ofthumb, regardless of the type of dewatering equipment, the thicker the feedsludge, the drier the sludge cake. The objective is to reduce the volume to bedisposed of by removing as much water as possible.The filter press is most often used in the dewatering of metal finishingsludges because generally it is made to handle smaller volumes of sludge, issimple to operate, and produces a dry, easily disposable filter cake. Sludge fromthe thickener, or directly from the bottom of the clarifier, is usually pumpedvia an air diaphragm pump to the filter press. The polypropylene filter mediaretains the solids while the liquid portion or filtrate flows through the mediaand discharges. Filtrate usually returns to the collection/equalization tank forretreatment. After a certain length of time (2-4 hours), the chambers of the pressare completely full and a filter cake of 25-35% solids has formed. The hydraulicpressure that had been holding the plates together is now released and the filtercake is discharged.Filter press operation requires little operator attention except at the beginningand end of a press cycle. Presses without an automatic plate shifter often requiretwo people to separate the plates to discharge the cake, one on either side of thepress. Cake that has had enough time to sufficiently dewater will literally fallout of the press upon opening.The highest operational cost involved with a filter press is the replacement ofthe filter cloths. Cloth life is directly dependent on the number of presscycles peryear. The metal hydroxide sludges produced from treatment of metal finishingwastes are generally of moderate pH and nonabrasive. Cloth life of 1-2 years iscommon. Replacement of cloths is labor intensive, especially the caulked, gasketedvariety, but all the cloths, even in a large press (10 ft3), can be changed in3-4 hours. Because plates and cloths are usually of polypropylene construction,they can be routinely cleaned by immersion in an acid without damage.

SYSTEM OPERATION AND PERFORMANCE

The best system design may result in inadequate results unless operators andmanagement devote the necessary resources. These resources include time,talent, and training. Sufficient time is required for normal operation and routinepreventive maintenance. The talent of motivated operators is necessary toanticipate problems and take preventive steps to assure continuous compliance.Training is critical for operators to understand how system performance isaffected by changes in production, chemicals, or regulatory limits.The operator needs to keep a daily log listing volumes treated, chemicalsconsumed, sludge produced, and effluent results. Either the operator or managementshould review these results to evaluate trends so costs can be controlledand results improved. For instance, increases in sludge production withoutcorresponding increases in production may indicate increased drag-out losses,failure of recovery equipment, or changes in treatment chemistry.Regulatory authorities require timely and accurate analytical data to confirmcompliance with effluent limitations. Operators need daily analytical data tocontrol system performance and to make needed adjustments to treatmentchemistry. This is often accomplished using inexpensive troubleshooting analyticaltools including pH papers in lieu of a hand-held pH meter, and potassiumiodide-starch papers for cyanide oxidation process control. Quick and easytests for CN and metals used in the process are important. A number of testkit suppliers are available to choose from. It is not always necessary to have thesophistication of a spectrophotometer or atomic absorption unit for in-housetroubleshooting and quality control. It is important, however, to have this serviceand complete analytical services available from a competent outside laboratory.All regulatory agencies will require data submission based on approved testmethods and procedures with report submittals.It is imperative to know your regulator and communicate with him/herregarding system operations, both good and bad. Most agencies require notificationof system upsets and slug loads. Although the typical metal finisheris reluctant to report problems, it is always better to report problems than forthe regulator to find them. Notification always can be used as mitigation atenforcement proceedings.

COMMON MISCONCEPTIONS AMONG METAL FINISHERS

ABOUT WASTEWATER TREATMENT

• Regulatory agencies only set effluent standards at reasonable levels necessaryfor environmental and POTW protection.

• Consultants and suppliers always know how to solve your problems.

• The use of ion exchange for complete wastewater treatment is a practicalapproach to eliminating discharges.

• Microfiltration is a sure method of compliance because it filters outeverything.

• The cyanide oxidation system is not working well because you have totalcyanide discharge violations.

• When floating in the clarifier occurs, the probable cause is oil and grease.

• A polishing filter will solve all the problems.

• Metal violations are always due to clarifier or polishing filter problems.

• All laboratories generate good data.

• pH and ORP electrodes only have to be cleaned weekly.

• If poor floc formation is observed, the polymer is bad or you’re not addingenough.

• In most cases, sludge dryers will save you money.

• Clarifiers and filter press cloths do not need to be periodically cleaned.

• The pH reading on the controller is always correct.