Improving Drinking Water Plant Performance and Regulatory Compliance

via Chemical Control OptimizationGregg A. McLeod

Most conventional and membranewater treatment facilities are de-pendent upon chemical treat-

ment, including coagulants and polymers,to operate effectively. Misapplication ofthese products can diminish the potentialperformance of these systems. This per-formance includes clarifier operation, filterefficiency, total organic carbon (TOC) re-moval, disinfection byproduct (DBP) com-pliance, lead and copper compliance, andcost. Providing proper chemical control op-timization can not only improve the effi-ciency of the system and regulatorycompliance but also provide a rapid poten-tial pay back.

Coagulant Selection and Performance

There are a variety of different coagu-lants in the marketplace, including alu-minum sulfate (alum), ferric chloride, ferricsulfate, polyaluminum chloride (PACl), andaluminum chlorhydrate (ACH). Each ofthese products possesses varying acidity,performance, and cost. It is difficult, if notimpossible, to predict the performance ofany coagulant on a specific water source;therefore, jar testing is recommended.

The photo with four jars demonstratesresults after 20 parts per mil (ppm) dose offour different coagulants. Although it ap-

F W R J

Figure 1. Alum – ACH – Ferric Chloride – Polyaluminum Chloride

Figure 2. Shown are pH Precipitation Points for Alum (blue), PACl (red), ACH(green), and Ferric Chloride (orange – 2 targets)

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Gregg A. McLeod is sales manager withClearLogx™ in Denver.

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pears the jar second from the left (ACH)provided the best results in terms of flocprecipitation and settling, it is only becausethe optimum pH precipitation point forACH aligns best on this particular watersample. Floc precipitation can be improvedon the other samples by adjusting pH to thepoint of least solubility for that particularcoagulant. Keep in mind that all of thesecoagulant samples possess different opti-mum pH precipitation points.

Floc Precipitation Affected by Temperature

Figure 2 shows established optimumpoints of floc precipitation for various co-agulants.

Temperature Effects on Flocculation

Although it is important to maintainan optimum pH value, there are two addi-tional points to consider: water temperaturechanges and floc particle charge. Raw watertemperature changes can affect the precipi-tation of floc particulates. The lower thewater temperature, the higher the optimumpH value (Figure 3). This temperature de-crease will also affect particle charge.

Temperature Effects on Particle Charge

All of the coagulant samples describedare “acidic” and therefore precipitate as a“cationic” particle charge. This particlecharge is offset by natural ion charges in theraw water (example: turbidity particlescarry an “anionic” charge). Therefore, tur-bidity offset by coagulant in theory pro-duces a “net zero zeta potential” or “neutral”charge. When coagulant doses exceed thatwhich is required to neutralize turbidity, theprecipitated particle charges possess astronger and stronger “cationic” charge.This occurs when employing enhanced co-agulation. Coagulant dose exceeding tur-bidity charges will result in higher removalof soluble organic carbon ions. Particlecharge moves from cationic towards anionicas the water temperature decreases.

Raw Water pH and Alkalinity Effects on Coagulation Selection

Raw water sources around the countryvary widely in terms of pH and alkalinity.

Figure 3. Optimum pH Precipitation Point for ACH While Compensating forTemperature

Table 1. Coagulant Selection

Figure 4. Floc Precipitate (red) Attracts Turbidity (green) and Soluble Organic Ions(blue)

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Generally, softer waters with low alkalinitywill possess a low pH value as well. Higheralkalinity waters will generally possess ahigh pH value. This is important when se-lecting the proper coagulant. A range of co-agulants vary in terms of acidity, TOCremoval, and price (Table 1).

Coagulant performance is dependenton water quality. It is difficult to predictperformance for any source and it is rec-ommended to perform jar testing before se-lecting a coagulant. Although ACH andPACl are more expensive than commodity

coagulants, such as alum and ferric chlo-ride, they are becoming more popular dueto lower coagulant dose, lower sludge gen-eration, and higher TOC removal. and lessdependent on alkalinity adjustment as theyare prehydrolized with an alkalinity base.

Particle Charge Neutralization:Net Zero Zeta Potential

Traditionally, coagulants have been uti-lized primarily to mitigate incoming turbid-ity. Unchecked, turbidity, which does notpossess the “weight” to settle, will pass

through a sedimentation unit or clarifier, ac-cumulate in the filter, and ultimately breakthrough. A primary coagulant can “attract”and “grab” these particles via particle chargeneutralization. Turbidity particles carry an“anionic (-)” or negative charge. An acidic orcoagulant floc particle carries an opposing“cationic (+)” or positive charge. As oppositesattract, the precipitated coagulant floc parti-cle can accumulate turbidity particles viacharge neutralization.

Turbidity mitigation via coagulation cannormally be achieved with a low coagulantdose; however it is important to keep in mindthat all coagulants provide better performanceat lower doses when operating near the opti-mum point of pH “insolubility,” which fluctu-ates depending on water temperature (seeFigures 2 and 3).

For example, a raw water supply possessesa pH of 8.2; dosing ferric chloride at 10 ppmdepresses the pH to 7.6. The ideal point of pHinsolubility is 6.3 at 20ºC. Unless pH is de-pressed from 7.6 to 6.3, precipitation, turbid-ity mitigation, and settling will be poor. It ispossible to overfeed ferric to the point wheresaturation will eventually precipitate enoughfloc to provide mitigation; however, solubleiron will elevate and coagulant cost will in-crease.

Electrostatic Particle AttractionOnto Filter Surface With

Membrane or Conventional Filter

Even the most efficient sedimentationor clarifier systems will allow floc particlesto pass to the filter. Most drinking waterFigure 5. Uncontrolled pH Figure 6. Controlled pH

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Figure 8. Sedimentation, Controlled pHFigure 7. Sedimentation, Uncontrolled pH

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treatment plants are categorized as either“conventional floc–sed–filter,” with the fil-ter comprising of various grades of media(multimedia) or ultrafiltration membranes.Both conventional and membrane filterscan operate with or without sedimentationas pretreatment. In either case, whether in-corporating presedimentation or not, pre-

cipitated floc particles that pass through tothe filter can decrease performance. Forconventional filters, the issue is filter runtime versus particle breakthrough. Formembrane systems, the performance issueis “fouling.” As coagulant particles precipi-tate as a cationic or (+) charge, the corre-sponding filter media or membrane

element possesses an opposing negative (-)charge. Similar to charge neutralization forturbidity mitigation, these precipitated flocparticles will attract or stick to the filter viaelectrostatic attraction. This reaction willdecrease the performance of either filter.

Filter Performance With andWithout Particle Charge Control

By controlling particle charge and neu-tralizing the charge attraction, filter per-formance increase can be dramatic.

Depicted in Figures 13 and 14, a ultra-violet (UF) membrane plant with two sep-arate filter skids conducted a test todemonstrate the effectiveness of particlecharge control. Coagulant was dosed in adirect feed mode (no clarification). Polya-luminum chloride (PACl) coagulant wasdosed upstream into a common line. BothUF filter skids UF Filter 1 (Figure 13) andUF Filter 2 (Figure 14) received this samedose. A controlled dose of liquid causticsoda was dosed ahead of UF Filter 2 skid(Figure 14) and there is a dramatic differ-ence. The red trend lines depict trans mem-brane pressure (TMP) rise and the bluelines depict permeability decline. There is asignificant difference between Skid 1 andSkid 2.

Chemical Control Effects Regarding Regulatory Compliance

Major regulatory issues that relate to chem-ical treatment in a drinking water plant in-clude TOC, DBPs, haloacetic acids (HAA),total trihalomethanes (TTHM), lead andcopper, and arsenic. The TOC and DBPs arein many instances intertwined. As DBPsform when soluble organics, which passthrough a filter, react with chlorine, they in-crease with detention and are compoundedby temperature. The higher the water tem-perature, the faster the formation; there-fore, by reducing soluble organic load, thereis a twofold effect on reduction: 1) lowersoluble organic content will reduce foma-tion when reactive with chorine, and 2)lower soluble organic content will requireless chlorine. Lower organic content withlower chlorine dose will further decreaseDBP formation. So in these cases, solubleorganic removal potential will direct oper-ations to consider superior performing co-agulant options when regulatorycompliance is at issue.

Additionally, operations must consideradditional parameters that require attention,

Figure 9. Precipitated Floc Particle Accumulation via “Electrostatic Attraction” With 20 ppm ACH

Figure 10. Precipitated Floc Can beRinsed With Low Water Pressure

Figure 11. 20 ppm ACH With pH andParticle Charge Control With no Particulate Accumulation

Figure 12. Electrostatic Particle Accumulation (ferric) on UF Elements

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Figure 16. Chloride-to-Sulfate Mass Ratio (CSMR) Calculator

Figure 15. Langelier Saturation Index (LSI) Calculator

Figure 13. UF 20 ppm ACH, no Charge Control Figure 14. UF 20 ppm ACH, no Charge Control

Figure 17. Drinking Water Facility Return on Investment (ROI) Calculator

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including iron and manganese, color, tasteand odor, and corrosion control.

Corrosion Control: Langelier Saturation Index Versus

Chloride-to-Sulfite Mass Ratio

Although corrosion control is not aregulatory compliance issue in itself, leadand copper compliance is, and is directly re-lated to the corrosivity of the water that en-ters the distribution system. Over the years,the standard measurement regarding thecorrosivity of a water supply is the LSIIndex (Langelier Saturation Index). Thisindex takes into consideration a water sam-ple’s pH, calcium hardness, alkalinity, totaldissolved solids value, and temperature.

Another measurement has recentlybeen introduced, which relates water’s cor-rositity to its chloride-to-sulfate mass bal-ance ratio (CSMR). The thought is whenthe chloride level exceeds sulfate by a cer-

tain ratio, there is an appreciable accelera-tion in corrosion. This is important regard-ing coagulant selection as it would assumethat chloride-based coagulants, such as fer-ric chloride, PACl, and even ACH, wouldexceed the optimum ratio.

Before making this assumption, valuesshould be entered into a CSMR calculator.It is feasible that chloride-based coagulants,if they outperform other options in termsof organic removal, could still be utilized. Achemical control logic could monitor doseversus the effect on CSMR.

For example, an operator at a largedrinking water plant wants to dose ferricchloride as it has proven to achieve thehighest soluble organic removal when com-pared to other coagulant via jar testing.There may be concern that this coagulantwill exceed the recommended CSMR. Whenthe operator inputs the raw water chlorideand sulfate levels, as well as coagulant dose,it is confirmed that the ratio is exceeded.However, when the operator inputs a “co-

dose” of aluminum sulfate at a certain dose,the desired ratio is achieved. Additionally,in this case, the less expensive alum maxi-mizes soluble organic removal with a loweroverall ferric chloride demand.

Chemical Control RegardingOverall Plant Operating Cost:

Return on Investment

One of the first issues arising regardinginstallation of an automated control systemfor chemical feed will be the initial capitalcost. Based on the issues raised in this arti-cle, it is very feasible that there can be arapid return on investment (ROI) in as lit-tle as one year or less according to the fol-lowing:� Reduced overall chemical demand� Reduced chemical sludge generation and

disposal� Power savings� Workforce savings� Water conservation ��

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