“Coagulant Change Over from Aluminum Sulfate to Poly AluminumChloride at a Conventional Water Treatment Plant”.

Thomas S Elford Process SpecialistCity of Calgary Waterworks

ABSTRACT:

This paper describes the testing, implementation and interim results of a full-scale coagulantchangeover from Aluminum Sulfate (Alum) to Poly Aluminum Chloride (PACL) at a largeconventional water treatment plant.

Research was conducted at The City of Calgary Water Treatment plant over the last several yearsto determine if a coagulant other than Alum would work better, especially in cold waterconditions. Jar testing and pilot plant work conducted determined that the 18% Al2O3 non-sulfated PACL would perform quite well. A final decision was made to begin a full-scale trial atone of The City of Calgary’s large conventional water treatment plants.

A one-year full-scale PACL trial, using existing infrastructure was initiated in December 2002.Water quality and operational data was used to determine the effectiveness of the PACL over thedifferent seasons. This data and the design lessons and considerations of the changeover will bereported.

INTRODUCTION:

Aluminum Sulfate (Alum) is a well-known coagulant that has been in use at water treatmentplants (WTP’s) since the turn of the century. In the past Alum has worked very well to clarifythe water but as water quality guidelines become more stringent, Alum is proving to be lesseffective. WTP’s in Canada and those in colder climates are finding that Alum just does not givethe clarification needed to produce high quality water. In the last 10 years, new highlymanufactured coagulants called Poly Aluminum Chlorides (PACL’s) have been making theirway onto the markets, and recently they are finding wide acceptance in the water treatmentindustry. PACL’s can be manufactured to produce a wide range of coagulants with differentdegrees of Al2O3 content, basicity, sulfated or non-sulfated and some may include polymers toenhance coagulation to suit site-specific water needs.

Background:

The Glenmore WTP is a conventional sedimentation plant with dual media filtration rated at 450ML/d. Coagulant, chlorine and fluoride are injected into a mechanical in-line rapid mixerfollowed by hydraulic flocculation. The water then flows into one of three 18-ML sedimentationbasins with a retention time between 4 to 8 hours depending on the flow rate. After filtration the

water is post chlorinated and sent to a highly baffled 25-ML clearwell for CT requirements, andfinally pumped out to the distribution system to be enjoyed by the citizens of Calgary.

With water quality guidelines becoming increasingly more stringent The City of CalgaryWaterworks decided after several years of jar testing and pilot work to do a full-scale trial usingPACL as the primary coagulant, in its continual effort to optimize the plant performance. PACLis becoming more widely used as a coagulant and the manufacturer’s claims are that it candeliver better cold water performance, lower particle counts, longer filter runs, and a reduction inthe amount of sludge produced.

Chemistry:

The chemistry of PACL is quite different than liquid Alum. Alum is Aluminum Sulfate bondedto approximately 14 water molecules and has the formula Al2SO4 .14H2O. When Alum is addedto water, hydrolysis occurs forming several monomeric Alumna species including Al3+,Al(OH)2+, Al(OH)4- before precipitating to the solid phase amorphous Aluminum hydroxide(Al[OH]3(am)). The intermediate species formed are highly dependent on water pH, temperature,available alkalinity and the nature of the inorganic and organic particles in the water. The factthat Alum needs to go through the hydrolysis reaction makes it very dependent on water pH,temperature and available alkalinity. The colder the water the more viscous the solution and theslower the reaction path, making Alum less effective in the winter time.

PACL is pre-hydrolyzed meaning that Aluminum and chloride are combined with a solid orsoluble base such as the (OH-) molecule thus improving the performance characteristics of thecoagulant. The intermediate species formed include the monomeric species of Alum andpolymeric high charge Alumna species including Al2(OH)2

4+, Al3(OH)45+ and Al13O4(OH)24

7+.Since PACL is pre-hydrolyzed, the high charged polymeric Alumna species are immediatelyavailable for coagulation and charge neutralization rather than being formed in situ as withAlum. Charge destabilization and floc formation are faster and PACL is less pH and temperaturedependent than Alum.

TESTING AND SELECTION:

Jar Tests:

When considering a coagulant or any chemical change or addition to the treatment plant, testingmust be first carried out in the jar test to determine the suitability of the chemical to the plantsspecific water conditions and the varying operating seasons. The jar test must completely mimicplant conditions in order to evaluate the intended chemical properly. It is also a good idea tocompare Alum to PACL in the jar tester, so that the evaluation can be a relative comparison ofthe effectiveness of the new chemical.

Jar testing at the Glenmore Plant was modified to mimic a large sedimentation plant, a shorthigh-speed rapid mix, followed by flocculation and settling. After settling, samples werecollected from the middle of the jar approximately one centimeter below the surface. This

allowed comparison with the plant settled water turbidities to ensure the jar test closely followedwhat the main plant was doing. Comparing Alum with the various kinds of PACL’s available onthe market will help select which coagulant is best suited to your plant. The following graphs arerepresentative of many tests illustrating the reduction in top of jar turbidities of Alum vs PACLduring cold and warm water conditions. The coagulant dose that first gives a top of jar turbidityof around 1.0 ntu is considered the optimum dose.

In Graph 1 the PACL best dose was at 30 mg/L which is half of the dose required if Alum wasused at 60 mg/L. This is very typical of the results for 18% non-sulfated PACL in the cold waterconditions.

Graph 1:

When using the 18% PACL it is cost comparative when the dose of PACL is half of what the dryweight equivalent (DWE) dose of Alum would be. As the water temperature warms up > 4oC,Alum becomes more effective in treating the water. The dose gap between Alum and PACLnarrows.

Graph 2 shows Alum vs PACL jar test in warm water conditions > 4oC. Alum is more effectivein warmer water but PACL still requires a lower dose.

Graph 2:

Pilot Work:

Pilot work at The City of Calgary Water Treatment Plants consisted of testing on three differentpilot plants over several years. One was a conventional sedimentation plant to mimic the mainplant, one was a conventional sedimentation plant with tube settlers and deep bed filters and thelast was a Dissolved Air Flotation plant with Ozone and deep bed filters. Also in 2002 a 2-monthpilot evaluation of a ballasted flocculation process was also performed testing PACL as thecoagulant.

Throughout these tests, pilot plant testing was ongoing to determine the suitability through thevarious seasons. Pilot work is important to take the next step from jar testing. Where jar testingcan help narrow the range of suitable coagulants, pilot work can give a plant confidence in thehandling of the ongoing changes that can occur during a year.

At the Glenmore water plant PACL out performed Alum, especially in the winter months.PACL gave lower settled water turbidity and with a decrease in the amount of chemical used,proved to be cost comparative to Alum. PACL performed very well in the summer monthsgiving longer filter runs then Alum and allowed more flexibility in determining the optimumdose during changing raw water quality periods. PACL at slightly underdose or overdosesituations still provided good filter water quality whereas Alum, the effects of an underdose oroverdose situation were more prevalent. Results like this were important in making the decisionto do a full-scale trial.

Table 1 shows the main coagulants that were used in the pilot trials along with the costs of eachchemical, the chemical strengths and technical specifications.

Table 1-Summary of Coagulants used in Pilot Trials

PACl Cost /kg

Al3+ EquivalentMolar

Concentration ascompared to Alum

Cost/kg ofMolar

EquivalentTechnical Specs

Alum ( 8% Al2O3sulfated)

$0.15/kg 1, Al 3+ $0.15/kg S.G. 1.33pH 1.9 – 2.3Basicity 28%

PACL (18%Al2O3 non-

sulfated)

$0.65/kg 2.17, Al 3+ $0.30/kg S.G. 1.35 – 1.39pH <1.0

Basicity 42%

PACL (10%Al2O3 sulfated)

$0.49/kg 1.24, Al 3+ $0.44/kg S.G. 1.20 – 1.25pH <1.0

Basicity 55%

Water Quality Goals:

The main driver behind the coagulant switch was to increase plant performance to better meetour water quality goals. The primary set of water quality goals we were looking for in coagulantperformance was enhanced settling to give lower top of filter turbidities, longer filter runs withlower finished and filtered water particle counts. With the ability to run the filters longer, fewerbackwashes would be needed thus savings in water and power could be realized.

A secondary set of water quality goals was determined as a possibility in a reduction in thefinished water Aluminum residual content and a reduction in the sludge generation. In the waterquality goal section, concerns about what changes a new product will bring to the process shouldalso be raised. An example would be what effects the coagulant is going to have on pH.

Once the water quality goals have been established, tracking the full-scale performance willallow for monthly determinations of the ability of the new product. Monthly reports thatcompare water quality data from using Alum of the previous year to PACL will allow the planttimely information about how the coagulant is really performing. At the end of the full-scaletrial it will be known whether or not the coagulant had performed up to its claims. In the full-scale results section is an example of the monthly reports used at the Glenmore WTP.

IMPLEMENTATION:

When the final coagulant product was decided on the next phase was a one-year full-scale trail.In order to implement the change from Alum to PACL using existing infrastructure many factorsmust be considered.

Material Compatibility:

The compatibility of the new product and the existing infrastructure needs to be closely lookedat. Material compatibility sheets can easily be obtained from the supplier. In our case severalstainless steel fittings and pressure gauges had to be replaced due to the corrosiveness of the newproduct.

Along with infrastructure material compatibility one must look at chemical compatibility orincompatibility of the new product. When PACL and Alum are mixed a solid white gelatinousmaterial forms. This gelatinous material could easily plug pipes and pumps causing seriousproblems. See Figure 1 in the section Cleaning and Flushing for illustration.

Tank Cleaning:

The Glenmore plant has two 330 m3 concrete tanks lined with a soft PVC liner. Given thechemical incompatibility between Alum and PACL as described above, a good cleaning wasrecommended for the Glenmore coagulant storage tanks..

After draining the Alum from the tanks and sending in an inspection crew, it was found thatinsoluble material in the Alum solution had settled to the bottom of the tank and after 18 years ofservice it had formed a solid cement like crystalline material about 4 inches thick. This solidmaterial could not be easily removed from the tank and it took two people with 8000-psi waterwands a full day to break up the material. When the material was broken up small enough to beremoved by a vacuum truck, it was found that the solid jagged Alum material had rippednumerous small holes in the liner. A new liner was ordered for replacement at a cost of$100,000.00. It was a good thing that the liner was near its life expectancy and scheduled to bereplaced soon. The obvious recommendation from this experience is that the best plan to dealwith the storage tanks is to actually plan and budget to replace the liner, rather than trying to cutcosts by avoiding this important step.

The Alum crystalline solid material that had been broken up by high pressure water wands andremoved by a vacuum truck was now a very acidic solution with solid material. The materialwas disposed of in holding lagoons at the City landfill and partially neutralized with calciumcarbonate.

Pumps:

When switching from liquid Alum to liquid PACL you will see a drop in the amount ofcoagulant used so it is necessary to ensure that the pump curves will match the expected doserange. The expected drop can be as much as half the amount of liquid Alum required to give thesame dose.

For example if Alum (expressed as 49% active ingredient) is being used and the flow rate is 100ML/d and the required dose is 10 mg/L, what is the pump flow rate in L/min?

Qpump = Dose x DWE factor x Qplant Specific Gravity

Dose = 10mg/L =0.010 g/L

DWE factor = Dry Weight Equivalent factor = 100/49 = 2.04

Qplant = Plant flow rate = 100 ML/day = 100,000,000 L/day = 6944.44 L/minute

Specific Gravity = 1.33 g/cm3 = 1.33 g/mL = 1330 g/L

Qpump = 0.010 g/L x 2.04 x 69444.44 L/minute = 1.065 L/minute1330 g/L

If we wanted to find the same pump flow rate using PACL that is expressed as 100% activeingredient with a Specific Gravity of 1.37, we see that the volume used will be approximatelyhalf of Alum.

Qpump = Dose x DWE factor x Qplant Specific Gravity

Dose = 10mg/L =0.010 g/L

DWE factor = Dry Weight Equivalent factor = 100/100 = 1

Qplant = Plant flow rate = 100 ML/day = 100,000,000 L/day = 6944.44 L/minute

Specific Gravity = 1.37 g/cm3 = 1.37 g/mL = 1370 g/L

Qpump = 0.010 g/L x 1 x 69444.44 L/minute = 0.5069 L/minute1370 g/L

Piping:

The size of the piping must also be considered to ensure proper velocity over the expected dosingranges. At The City of Calgary the Alum pumps were located by the Alum tanks, which areapproximately 500 feet from the Rapid Mix Building. The old Alum system used carrier waterfrom the pumps to the Rapid Mixer and so the resulting piping was 2 and 3 inches in diameter.The PACL coagulant was to be used with out carrier water so new 1 inch piping needed to be runto accommodate the change. Included in the new one-inch line at the high points were air reliefvalves to prevent air lock when charging the new line. Cleaning and Flushing:

All lines and pumps must be flushed thoroughly to ensure that no Alum is left in the system.When Alum and PACL are combined together, it will form a white gelatinous solid that will plugoff the lines and the pumps. Flushing of pumps and lines should proceed for two full days to

make sure no Alum was left in the system. If at all possible the coordination of the cleaning ofthe pumps and piping should be done at the same time the plant is taken down for cleaning andor sludge removal. The diluted Alum can be discharged either to sanitary or wasted into theprocess (only if there are no negative effects – in our case, this was acceptable since the plantwas shutdown and the filters were filtering-to-waste).

Figure 1- White solid formed when Alum and PACL mixed together.

Injection Point:

The use of carrier water to transport the PACL from the chemical pumps to the injection point isstrongly discouraged. The use of carrier water over long distances will result in the coagulantbecoming dilute and the coagulation reaction will be taking place in the piping and not in the rawwater. The coagulant will become less effective in the treatment process.

At the Rapid Mixer the neat PACL is mixed with plant service water at the moment it is about tobe injected into the Rapid Mixer to increase the velocity of the coagulant. A three-inch HastaloyB diffuser pipe spans the width of the Raw water Header. The pipe has holes drilled into it alongits length at different points of its circumference. This helps with insuring that the coagulant is

dispersed across the width of the raw water header and is properly mixed. Figure 2 shows thenew injection system designed for this project.

Figure 2-Coagul

Chemical Delive

With the implemprocedures and uwere opportunitieoff-loading. Theparamount consid Delivery trucks aWTP. This allowonto the plant sitto unload they arsafety orientationto be unloaded in

PACL line

ant Injection System into Rapid Mixer.

ry:

entation of a new coagulant, it is a good time to look at the existing deliverypdate them as necessary to current best practices. At Glenmore WTP, theres to improve the procedures to increase quality assurance and control prior to safety and convenience of plant personnel in handling chemicals was also aeration that required updating.

re now required to call Operations one hour before arriving at the Glenmores Operators the necessary time to prepare for a delivery. All personal coming

e must sign in and receive a visitors badge. When the driver is onsite and readye informed of the sites hazards and evacuation procedures along with a short and whereabouts of spill kits and safety showers. When the coagulant is readyto the tank a sample from the truck is taken and pH and specific gravity test are

Plant ServiceWater line

preformed as a quality control step. When the chemical has passed the quality control tests, onlythen is the driver permitted to unload the chemical.

Start Up of the New Coagulant System:

Standard Operating Procedures (SOP’s) for the new coagulant should be revised and updated toreflect the changes to the system. The SOP’s should be reviewed and discussed with plantoperations and all stakeholders for their comments and feedback. Once finalized, all staff willrequire training on the new systems and procedures. In addition, new forms may be required andthose need to be properly rolled-out as well.

When starting up the WTP with a new coagulant, it should be done after the plant or parts of theplant have been taken off line for cleaning. It is best to bring the plant up slowly to allow for thebasins and filters to be turned over slowly to the new coagulant.

All project staff that were involved in the design, planning, construction, and documentation ofthe coagulant changeover need to be on-site and available to the project on a quick-responsebasis during the initial day and weeks of using a new coagulant. Undoubtedly things will arisethat were not considered and qualified staff needs to be on-hand to address all issues to ensurecontinued water production.

FULL SCALE RESULTS:

A brief overview of the full-scale evaluation of a coagulant changeover from Alum to PACL atthe Glenmore WTP is shown in Table 2 below. Table 3 shows an example of a monthly reportused at the Glenmore WTP. The data for these tables were supplied by Laboratory Services.The Alum data is from the calendar year 2002, and the PACL data is from the test period January2003 to August 2003. The data is discussed season-by-season in the following sections. Anyutility considering this type of coagulant changeover should have their measurement criteria inplace before making the changeover, so they are prepared to gauge the difference in operation.

Table 2-Overview of Results from Glenmore Evaluation.

Parameter Winter Spring SummerAverage Alum PACL Alum PACL Alum PACLEffluent Particle Counts(Counts/mL >2µm)

24 7 17 7 114 9

Effluent Turbidity (ntu) 0.06 0.05 0.07 0.07 0.07 0.06

Settled Water Turbidity(ntu)

0.85 0.79 1.62 1.07 2.01 0.74

% TOC Reduction(mg/L)

15 17 26 37 19 14

Effluent AluminumResidual (ppb)

47 61 34 41 84 162

Table 3-Monthly Reports used at Glenmore WTP.

January Data Comparisons – Alum 2002 vs. PACL 2003Alum PACL

ParameterJan 2002 Jan 2003

Data Source

Avg Filter Run Times (hrs) 48 72 PIAvg Filter Effluent Turbidity (ntu) 0.07 0.08 PIAvg Filter Effluent Particle Counts (>2 NP/mL) 16 2 PIAvg Plant Effluent Turbidity (ntu) 0.03 0.03 PIAvg Plant Effluent Particle Counts (>2 NP/mL) 16 5 PIAvg 33 Plant Settled Water Turbidity (ntu) 0.66 0.63 PIAvg 57 Plant Settled Water Turbidity (ntu) 1.01 0.88 PIAvg 65 Plant Settled Water Turbidity (ntu) 0.85 0.99 PIAvg Effluent Aluminum Residuals (ppb) 58 73.5 LIMSAvg Raw TOC (mg/L) 1.03 0.97 LIMSAvg Effluent TOC (mg/L) 0.88 0.89 LIMSPercent TOC Reduction (%) 14.6 8.3 CalculatedAvg Raw pH 8.11 8.18 Monthly ReportsAvg South pH 7.80 8.00 Monthly ReportsAvg Raw Turbidity (ntu) 0.68 0.71 Monthly ReportsAvg Effluent Turbidity (ntu) 0.06 0.07 Monthly ReportsTotal Monthly Raw water volume (ML) 4221 4512 Monthly ReportsAvg Raw water volume Daily (ML) 136 146 Monthly ReportsTotal Effluent Water volume (ML) 3372 3690 Monthly ReportsAvg Effluent Water volume Daily (ML) 109 119 Monthly ReportsTotal Monthly Wash water usage (ML) 172 175.5 Monthly ReportsAvg Daily Wash water volume (ML) 5.5 5.7 Monthly ReportsWash water % of production 5.1 4.8 Monthly ReportsAvg Coagulant Dose (mg/L) 9.4 3.3 Monthly ReportsAvg Coagulant kg used 1281 477 Monthly ReportsTotal Monthly Coagulant kg used 39726 14794 Monthly ReportsTotal Monthly Filter Effluent Volume (ML) 3930 3876 Monthly Reports

Discussion of Winter Results:

Water conditions for the winter season are classified as temperature < 4oC, Turbidity < 1.0 ntu,and Total Organic Carbon < 2.0 mg/L. During this time the plant ran at a PACL dose of 3 mg/Lcompared to last year Alum dose of 6 mg/L (DWE) or 12 mg/L liquid Alum. When the plant cancut the PACL dose in half of the DWE dose of Alum, the PACL costs about the same as Alum.The Aluminum residuals were comparable, with the PACL results slightly higher. With PACLyou will be using less product and so pH depression will be less as compared to Alum. Lowerparticle counts on the filter effluent and finished water was observed. With Alum the countsaveraged around 20 counts/mL >2µm and with PACL we were consistently below 5 counts/mL

>2µm. The PACL allowed for longer filter runs so less backwash water was used during thewinter season. Operational requirements prevented the implementation of longer filter runs inthe summer months, more so due to predictable washing requirements rather than the quality ofthe filter effluents. It is strongly suspected that longer runs could be handled in the summer, andthis will be investigated further next year if the PACL coagulant remains in usage.

Discussion of Spring Results:

Spring conditions are when the snow starts to melt at the end of March to the end of April orMiddle of May. During this time the water temperature is slowly warming up but is still lessthan 4oC with turbidity less than 5 ntu. At this time the plant usually experiences a sharpincrease in TOC up to approximately 5 mg/L.

PACL performed very well with lower particle counts < 5 counts/mL > 2µm in the filtered andfinished water. Settled water turbidities remained just below 1.0 ntu as compared to last yearwith Alum the turbidities were 1.8 ntu.

Discussion of Summer Results:

Summer is the time the plant generally experiences runoff from the mountains. The snow in themountains finally all melts and a large amount of water with silt and clay come into the plant.This is the time when we were looking for the PACL to really excel and demonstrate it’susefulness to the Glenmore WTP. Unfortunately, the Glenmore WTP did not see any runoff thisyear and we did not see turbidities above 4.0 ntu. Operations were glad this was the case, but thecoagulant changeover research team was very disappointed by the missed opportunity todemonstrate the suspected usefulness of PACL in summer operation under heavier turbidityconditions.

CONCLUSION:

When a proposed plan for a coagulant study is first brought forward, the project can only besuccessful with a thorough plan that involves all the proper stakeholders. The followingsuggestions are offered to ensure success in the implementation of a coagulant changeover.

• First, an objective for water quality goals should be determined so that the specific goals ofthe coagulants can be judged.

• A testing plan must be formalized to determine which chemical is best suited to the specificwater. With the aid of jar testing and pilot plant work (if possible), the most suitable product canbe determined.

• Once the chemical has been chosen, a detailed plan should be made to determine thesuitability of the existing infrastructure.

• Material compatibility (very important),• the size of the pumps and pipes,

• suitability of the existing coagulant tanks,• cleaning and flushing of the pumps, pipes and tanks, • disposal of waste materials,• chemical delivery methods (injection and mixing into the process stream)• project documentation including drawings, operation philosophies, and standard

operating procedures,• development of chemical delivery programs that reflect best practices,• and staff training and communication (also very important)

• With the aid of detailed monthly reports the performance of the coagulant can be monitoredclosely.

In summary there are likely a number of coagulant products available to any WTP that willwork, the challenge for the staff at any given facility is to find the one which best suits the waterbeing treated and the production restraints that exist at that WTP (infrastructure and price).

References:

American Water Works Association. (1984) “Basic Science Concepts and Applications”,Principles and Practices of Water Supply Operations, pp 557-559.

DeWolfe, James R. (Correspondance, email), McGuire Environmental Consultants.

Exall, Kirsten N, Vanloon. Gary W, (Nov. 2000) “Using Coagulants to Remove OrganicMatter”, AWWA J., Vol. 92, No.11, pp. 93-102

MacLeod, B.W, Simpson, M.R, Zimmerman, J.A. (1993) “Developing Performance-Based BidSpecifications for Selected Water Treatment Chemicals”. Presented at the Florida WaterResources Conference.

Pernitsky, David J. (2003). “Optimizing Coagulation Chemistry for Turbidity and OrganicCarbon Removal”. Associated Engineering conference, Calgary Alberta.

Ravina, Louis. (1993) “Every thing you want to know about Coagulation and Flocculation”, ZetaMeter Inc.

Kawamura, Susumu. (2000) “Integrated Design and Operation of Water Treatment Facilities”,Sec. Ed. John Wiley and Sons, Inc.