ENHANCED TREATMENT WITH THE ADDITION OF ACTIVATED SILICA POLYMER

Vicky Sidorkiewicz, National Silicates Ltd.*

Thomas Huang, Toronto Main Treatment Plant

*National Silicates Ltd., 429 Kipling Ave., Toronto, Ontario M8Z 5C7

INTRODUCTION

Municipal sewage treatment plants serving combined sewer collection systems are typically operating beyond their design treatment capacity. As the volume of combined waste streams increases with population growth, primary clarifiers often have to be expanded or upgraded due to throughput increases and/or the need for solids removal efficiency improvement. Similarly, the handling, processing and disposal of sludge can become increasingly challenging and costly in the face of digester and dewatering capacity limitations. Budgetary constraints and environmental performance are two issues that must be dealt with simultaneously. In order to improve treatment plant performance and to avoid or defer heavy capital investment, alternative treatment enhancement technologies need to be identified and successfully implemented. One practical approach towards addressing this issue, without major capital investment, is to use inorganic or organic polymer treatments. Chemically enhanced treatment is intended to improve the removal of inorganic and organic particulates from the wastewater stream and is based on the addition of a primary coagulant and a coagulating aid. A recently completed study at Toronto MTP has identified a low cost approach to address the issue of hydraulic over-loading of primary clarifiers and improve overall treatment process performance. The sewage influent flow averaging 300,000 m3 per day was treated with ferrous chloride at a 15 mg Fe/ L dosage added to aerated grit tanks, and activated silica polymer at 1.0 to 3.0 mg SiO2/L was added to the primary clarifier feed. The addition of activated silica polymer yielded an average of 50% increase in effluent suspended solids (SS) removal from the entire primary treatment and improved compaction of primary solids by increasing solids content by approximately 30%. Treatment with activated silica has been identified as a simple, low–cost technology for improving settling and compaction of settleable solids in clarification processes of wastewater treatment plant operations.

keywords Activated silica (AS), enhanced coagulation, solids concentration, sludge compaction, ACTIVATED SILICA POLYMER Activated Silica (AS) is an anionic, inorganic coagulant aid which improves the entire flocculation process to a similar degree as commonly used synthetic acrylamide based long chain organic polymers [15]. The terms “activated silica” or “activated silicic acid” can be used interchangeably in describing stable silica sols. Activated silica is made by partially or completely neutralizing the alkali of a dilute sodium silicate to initiate the formation of silica micelles, aging to permit growth of these micelles and finally diluting to stop the further increase in size which leads to formation of gel. Activated silica sol concentrations and/or dosages are most often expressed in terms of % or mg/L SiO2, respectively. The silica sols are usually made starting with a commercial solution of sodium silicate which contains about 8.9% Na2O, 28.7% SiO2 and has specific gravity of approximately 1.4. Many different methods are possible to activate the silicate and various dilution recipes and activating agents can be used in the preparation of the activated silica polymer. The most common technology is the Baylis Process, which uses sulphuric acid as the activator. Ammonium Sulphate, Sodium Aluminate, Sodium Bicarbonate, Ammonium Sulphate, Silicofluoride, Chlorine and Carbon Dioxide have also been used to activate silica solutions. Most recently, carbon dioxide is becoming a preferred activating agent because of the potentially hazardous aspects of using more acidic chemicals like H2SO4. REVIEW OF ACTIVATED SILICA APPLICATIONS IN TREATMENT OF MUNICIPAL AND INDUSTRIAL WASTEWATERS The effectiveness of AS polymer in the coagulation of suspended matter in aqueous systems has been known for decades. The AS polymer usually was used in conjunction with primary coagulants, such as aluminum sulphate, lime, or ferric salts. AS provides denser, larger and stronger flocs than organic polymer. Widespread application of activated silica polymer technology in potable water clarification processes has been documented. Various flocculant surveys have shown AS treatment as superior or comparable to treatment with high molecular weight organic polymers [15]. Overall increases in coagulation process efficiency and reduction of the chemical consumption have resulted from AS use.

Additional applications include treatment of municipal wastewater and various types of industrial waste streams including those from petroleum refineries, pulp and paper and deinking operations [3,12]. The positive effect of AS addition for phosphorus removal in both primary and secondary wastewater treatments plants has also been previously documented [21,22]. Effective application of AS with lime in clarification processes of low alkalinity combined sewers, stormwater, sewer runoffs and typical raw wastewater was reported[ 1,4]. It was found, at an AS dosage range of 2 to 4mg/LSiO2, up to10 fold lime dosage reduction could be achieved and sludge volumes and pH requirements could also be significantly reduced. Industrial wastewater recorded a number of case studies with activated silica polymer used in conjunction with primary coagulant [ 3,6,8,12,14]. It was found, that AS was superior to polyacrylamide technology in removal of inorganic contaminants from the wastewater containing Fe, Cr, Ni, F and hydrocarbons.[3,12,14] In a treatment of petroleum containing wastewater, AS treatment can be used instead of polyacrylamides. High surfactant, fatty acids and detergent condensation products removal efficiencies were recorded with AS added alone in synthetic fiber and detergent plant effluent treatment process. [8,9] Also, better clarification resulted from AS application before alum and lime treatment in the sewage plant effluent treatment for industrial reuse.[13] AS application in wastewater treatment plant is not limited to the clarification processes only. It was found, that improved dewaterability of the centrifuge sludge feed and reduced chemical consumption was obtained when dual chemical conditioning system with AS polymer and cationic organic polymer was applied. AS combined with Na polymethacrylate resulted in substantial organic polymer consumption reduction at an increased filtration rate, in both, municipal and industrial processes.[17] Activated silica technology has not been implemented on full scale in any Ontario Sewage Treatment Plants yet. Over the past several years, the general lack of full plant trials and experimentation can be attributed to significant limitations in the capabilities and availability of suitable preparation systems for activated silica. With the development of modern and reliable preparation technology, it is anticipated that activated silica products will gain widespread acceptance and recognition as an effective and economical chemical enhancement for wastewater treatment.

ENVIRONMENTAL ASPECTS Application of both AS and synthetic organic polymers to wastewater streams provides significant environmental pollution control in terms of the removal of solids and reduction of many other contaminants. Ecological assessment of individual polymer technology need to include risk characterization in the event of potential releases to the surface waters and the polymer fate in natural aquatic environments. AS technology does not present any concerns with respect to polymer fate and aquatic toxicity even in case of overdosing. The produced polymer is usually of 0.5 to 1.0 % SiO2 concentration at neutral pH and does not pose any risk in its final form or during polymer manufacturing process. The potential for respiratory toxicity or irritation being associated with polymer dust exposure for dry organic polymer make up systems [10,11] are non-existent for AS polymer systems. With the development of CO2 AS technology, the issue of using hazardous raw chemicals like H2SO4 have been eliminated. DESCRIPTION OF THE PLANT PROCESS The Main Treatment Plant (MTP) is a conventional activated sludge wastewater treatment plant. The plant’s current rated capacity is 818 ML/d for secondary treatment and 2,532 ML/ day for the primary treatment. The treatment train consists of screening, grit removal, primary clarification and activated sludge (aeration tanks and secondary clarifiers) wastewater treatment processes. Following secondary treatment and phosphorus removal, effluent from the MTP is disinfected using chlorine. The solids or sludge removed during the treatment undergo conventional anaerobic digestion, dewatering, and incineration. Primary clarification consists of twelve rectangular sedimentation tanks. The clarifiers are arranged in three groups (1-6, 7-9, and 10-12). Primary clarification is used for the gravity settling and removal of suspended solids. The primary clarifiers treat raw wastewater after screening and grit removal. Need for increased MTP treatment capacity and improved performance MTP is rapidly approaching its treatment capacity limits based on population growth estimates. The MTP wastewater and Combined Sewer Overflow (CSO) treatment needs as well as ways of improving the effectiveness of the MTP at reducing environmental impacts, were reviewed under the Ontario’s Environmental Assessment Act. MTP Environmental Assessment (EA) has identified a potential future need for additional wastewater treatment capacity. Additional capacity is

also required to prevent CSO from discharging into receiving waters under wet weather conditions. During 1997 alone there were 19 occurrences of primary effluent bypassing secondary treatment into Lake Ontario [20]. Total bypassed flows were estimated to be 3,216.4 ML and underwent disinfection process. The EA Study completed in December 1997[12] identified the following needs: additional capacity to treat up to 102 ML/d of wastewater and 33 t/d of solids to year 2011, CSO treatment of up to 213 ML/d and 26 t/d of solids. The potential to meet near future wastewater/CSO treatment needs was examined and various alternatives assessed. The need for improvement of primary tank sedimentation efficiency was identified in MTP Optimization alternatives included in this study. The option of maximizing the existing primary settling capacity versus the upgrade and expansion alternative seemed the most preferred choice considering that no capital expenses are required. As concluded in numerous previous studies, chemically enhanced primary treatment can prove to be a very effective and inexpensive solution in maximizing primary treatment capacity [2,18 ]. The chemical primary treatment will result in considerable savings in capital costs which in turn, may offset part or all of the operating cost of chemical addition. In the process of continuing CSO management efforts, the MTP EA study recommended CSO primary treatment and disinfection processes for secondary bypasses before discharging into receiving waters. Additional cost savings can be achieved in disinfection costs when chemically treated primary effluent is disinfected with chlorine or UV light. Chemical primary treatment will also improve the effectiveness of the MTP at reducing environmental impacts. METHODOLOGY AND RESULTS The purpose of AS evaluation in MTP was to examine potential improvement of the primary treatment as well as to investigate impact on the plant downstream processes. Both, bench test and field test trials of the addition of AS polymer to the primary clarifier influent feed at the City of Toronto MTP were completed in 1996 and 1997. Evaluation of Activated Silica Addition in Toronto Main Plant (MPT) Laboratory tests Jar tests on the effects of AS treatment were completed during the period from September to December 1996. Representative primary influent samples were collected from the influent distribution channel to primary clarifiers #7-9. The objective was to examine primary effluent and sludge quality with activated silica polymer addition. Jar tests were completed on primary influent samples treated with three types of AS polymer at dosages up to 10 mg/L expressed as SiO2. All of the jar tests also included primary coagulant, ferrous chloride at a dosage

of 15 mg/L (as Fe). The three types of activated silica polymer included polymer produced by activation with sulfuric acid (Baylis AS), ferric chloride and ferric sulphate. Baylis AS produced the best results at all dosages examined. Baylis AS at a dosage of 1-3 mg/L (as SiO2) with a ferrous chloride dosage of 15 mg/L (as Fe) reduced total phosphorus to less than 1 mg/L and removed TSS to less than 25mg/L. In addition, total solids concentration in primary sludge increased to 1.4 %. Baylis AS also produced a compact and dense sludge. However, rates of compaction were not determined for the individual treatments. The jar test results indicated that AS addition improved the efficiency of solids removal from primary clarifier influent wastewater and increased sludge solids concentration. AS Field Trial Evaluation The results obtained in laboratory evaluations were used to establish conditions for the field trial conducted in November 1997 . The trial set up is shown on Figure 1. Raw sewage influent flow of 300,000 m3 was pretreated with primary coagulant, ferrous chloride prior initial degritting. Baylis AS was injected into the distribution channel that carried initially coagulated sewage after screening and grit removal and prior to the primary settling tanks #7,8 and 9. Mixing was provided with air diffusers located along the distribution channel. In order to improve mixing and provide better polymer distribution into treated sewage for the future trials, it was recommended to relocate polymer injection points from the main distribution channel to each individual collection channel coming out from the grit tanks. AS polymer dosages were estimated based on an average flow of 300,000 m3 per day. The flow-paced polymer injection method was not accessible. In the events of the significant sewage flow changes, pump flow rate adjustment was made. Over 1500 AS polymer batches were made during the trial. AS was applied at 1.0, 2.0, and 3.0 mg/L SiO2 dosages. Observations were made with respect to the effect of activated silica polymer activity on the process performance. The polymer activity is measured by its aging time before final dilution with water, which halts the polymerization process. The aging time is expressed as the % of actual gel time of the polymer if the final dilution did not occur. The range of aging times for optimum polymer activity is 30-70 % of gel time. AS polymer aged at 35%, or greater, of gel time produced the highest solids concentrations. It is anticipated, that better results can be consistently achieved with full process optimization. The primary clarifier operation data before, during and after the activated silica field trial period was collected from MTP. These data included influent flows, primary clarifier flow capacities, raw primary sludge generation rates, total and volatile solids concentrations for the complete set of clarifiers. There were no parallel control treatment trains run

concurrently with the chemical addition trials due to limitations of the plant structural design. A local engineering consulting firm was contracted for MTP operating data analysis.

Grit tanks

7-12

9

8

7

Total solids concentration An average sludge total solids concentration during the trial was monitored and compared to that before and after the trial. An average sludge solids concentration of 3.2 % was achieved during the trial which compared to 2.1 and 2.0 % for pre-trial and post–trial conditions.(Fig.2) Suspended solids(SS) removal The effluent suspended solids (SS) concentration and SS removal from the entire primary treatment system was improved by approximately 50 % during the activated silica trial. Effluent SS concentrations and SS removals for Primary Tanks #7-9 (New PT), which received the activated silica treatment were estimated based on: • total influent SS values for both Old and New PT, • primary clarifier sizes, • flow proportioning method • and effluent SS values from New &Old PT.

Screen Building P

D i s t r i b u t i o n C h a n n e l

N e w P r i m

a r y S e t t l i n g T a n k s

Cond #3

Cond #4

Cond #5

Digester

Digester Digester

Digester

Activated Silica Polymer FeedSystemPrimary

Sludge

3

4

5

6

2

1

O l d P r i m

a r y S e t t l i n g T a n k s

Figure 1-AS field trial schematic

Incoming Raw Sewage

Primary coagulant

1

2

3

4

5TS(%,w/w)

Figure 2-Primary sludge Total solids content( %, w/w)vs. time

0 ppm SiO2 1 ppm SiO2 2 ppm SiO2 3 ppm SiO2

Also, an average primary effluent BOD5 concentration of 176mg/L with no chemical treatment was reduced to 124mg/L with chemical treatment. Sludge generation rate MTP has the capability of returning recycle streams to various locations upstream of the primary clarifiers in both, “P” and “D” sections of the primary treatment. The effect of the quality and quantity of recycle streams on primary clarifier performance and primary sludge characteristics can be significant. Prior to, during and after the chemical trial, all recycle streams were returned to primary clarifiers section “D” of the plant, which was not tested for the AS effects of enhanced coagulation on clarifiers performance. As a result of directing recycle streams to primary treatment “D” section, the raw primary sludge generation rates in tested primary clarifiers #7-9 have changed prior to the activated silica field trial, as shown in Table 1. The redirection of the recycle streams significantly reduced the sludge generation rate prior to the trial period. Similar sludge volumes were generated during and after the activated silica field trial, however, the solids mass generated was 30% higher during the field trial, compared to after the trial. The activated silica trial improved suspended solids removal and compaction of the solids. The assessment of the performance benefits of activated silica polymer addition on the primary clarifiers performance has been completed based on the available MTP operating data.

Table 1: Raw Primary Sludge Generation Rates for Primary Clarifiers # 7- 9 October – December 1997

Raw Primary Sludge Generation Rate Total Solids Volatile

Solids

Period (1997)

Volume (m3/d)

(%, w/w) ton/d (% of TS) Oct. 1 – Nov. 10

3,243 2.1 67.6 69

Nov. 10 – Dec101

1,050 3.2 29.7 74

Dec. 10 – Dec. 31

1,035 2.0 20.5 76

Oct. 1 – Dec. 31

2,001 2.4 44.3 72

1 AS field trial period Considering that higher Volatile Solids (VS) were generated in the primary sludge during the trial, less of VS ended up in primary effluent stream to the aeration tanks. More compacted sludge also suggests lowering WAS quantity in the future. The Hydraulic Retention Time (HRT) in anaerobic digestion process was increased from an average 8 days to 13 days during the AS trial. Consequently, increased HRT resulted in more sewer gas produced [20]. As previously demonstrated, longer HRT could increase volatile solids destruction, reduce pathogen levels and overall improve dewatering process[19]. Future work will also examine impact of AS addition in primary treatment on activated sludge processes. One study has demonstrated, that addition of inorganic solids Fe (OH)3, Al (OH)3 and SiO2 enhanced activated sludge process by accelerating bacterial metabolism.[7] A longer trial period is required to confirm the received data and to draw more conclusions as to further potential performance benefits of AS addition downstream from primary treatment processes. COST BENEFIT ANALYSIS Application of the enhanced coagulation process with addition of AS polymer in primary treatment stage could yield significant financial benefits, particularly in the area of capital cost savings due to deferred upgrades. The potential operational costs were also estimated in this study. However, the savings accrued from avoidance of the construction costs were of much greater significance compared to the operational cost

savings. The economic attractiveness of applying AS technology in treatment of municipal wastes would be plant specific. Capital and operational cost savings for MTP include savings generated by 30% sludge compaction increase, 50% SS removal increase and total solids concentration to an average 3.2% increase. Also, longer HRT in anaerobic digestion resulted in more sewer gas produced, increased oxygen transfer efficiency and therefore lower secondary treatment operating costs. Sludge digestion Based on an average increase of primary sludge solids concentration from 2.0% to 3.2%, and HRT increase from 8 to 13 days, the savings obtained in sludge digestion process would be due to deferred digester upgrades. Estimated MTP capital cost savings would be in an order of $20 millions. Operational cost savings would be obtained as a result of increased sewer gas production which could be used for digester heating processes instead of natural gas. Aeration MTP like any other typical wastewater plant, spends about 50% of total energy costs for aeration processes. The BioWin statistical biological process model was used to estimate aeration cost savings. Model input included an average daily flow of 800,000m3/d and an average primary effluent BOD5 concentrations of 176mg/L and 124mg/L for no chemical treatment and chemical treatment scenarios, respectively. Model configuration and operation assumptions included influent TKN=37mg/L, with no nitrification, temperature of 20C, and solids retention time(SRT) of 8 days. The coarse bubble system with oxygen transfer efficiency (OTE) of 6.0% and MTP estimated blower power requirements were applied in this model. Based on this preliminary analysis, the estimated operational cost savings in aeration would be in an order of $300K to $500K per year. More detailed analysis of this saving is required. All of these estimated savings must be balanced against the costs of AS polymer and AS make up equipment. The AS manufacturing equipment cost for an 800,000 m3/day size plant was estimated to be in a range of $ 50 –80 K. As an example, chemical costs for CO2 AS polymer applied at 1.0 mg/L were estimated at $200 K for 800,000 m3/day flow size plant. CONCLUSIONS The addition of activated silica polymer yielded an average of 50% increase in effluent suspended solids (SS) removal from the entire primary treatment and improved compaction of primary solids by increasing solids content by approximately 30% compared to after the trial. An average sludge solids concentration of 3.2 % was achieved during the trial which compared to 2.1 and 2.0 % for pre-trial and post–trial conditions. As a result, HRT in anaerobic digestion was increased from 8 days to 13 days.

The improved performance of hydraulically overloaded clarifiers could result in reduced primary treatment, digestion and aeration upgrade requirements. Potential further impacts of AS could be reduced recycle loads on primary and secondary treatment processes and reduced flows to dewatering process. In addition, substantial capital and operational cost savings can be achieved. Treatment with AS has been identified as a simple, low–cost technology for improving settling and compaction of settleable solids in clarification processes of wastewater treatment plant operation. ACKNOWLEDGMENTS The authors would like to acknowledge the contributions of Toronto Main Treatment Plant staff especially of Peter Wimmer, Plant Manager, Tony Byrne, JH&SC Supervisor and Operations. The assistance of NSL staff, Tim Evans, Technical Director and Barbara Lempka, Senior Sales Representative was greatly appreciated. REFERENCES

1. Weber, W.J., Ketchum, L.H., (1974) Coagulation of stormwaters and

low alkalinity wastewaters. Journal WPCF, vol. 46, No. 1 2. Heinke, G.W., Tay, A.J.H., and Qazi, M (1980), Effects of chemical

addition on the performance of settling tanks. Journal WPCF, Vol 52, 3. Arishkevich, A.M., Barkalov, V. S., Chinchaceva, V. and others (1989),

Activated silica as an effective flocculant for the removal of inorganic contaminants from industrial wastewaters. Metall. Inst., Dnepropetrovsk, USSR, 4, 72

4. Weber, W.J., Ketchum, L.H., (1974) Activated silica in wastewater coagulation. US Govt Rep. Announce. 74(15), 134

5. Hamilton, J.D., and Sutcliffe, R. (1997) Ecological Assessment of polymers: Strategies for Product Stewardship and Regulatory Programs. Van Nostrand Reinhold, Chpt. 7

6. McCuaig W., Bruce, Atkins, Peter F. (1974), Physical–chemical treatment of pulp and paper waters. Eng. Bull. Purdue Univ., Pt. 2. 716-24

7. Bowen, R. B. (1993), Enhancement of the activated sludge process through addition of inorganic biomedia. Hazard. Ind. Wastes,25,248-54

8. Kaminski, S., (1973), Removal of detergents from waste waters from Elana synthetic fiber plant. Politech. Gliwice, Pol. No 64A, 31-5

9. Ivkina, T. M., Barabash, N.D., Shaskova, T.N., (1990), Removal of nonionic surfactants from rayon-manufacturing wastewater. Tekhnol. Fiz.-Khim. Ochistki Prom. Stochnykh Vod, Moscov, USSR, 35-8

10. Assessment of Health Risks from Exposure to Acrylamide.(1980) U.S. EPA, Office of Toxic Substances, Existing Chemical Assessment Division.

11. Assessment of Airborne Exposure and Dermal Contact to Acrylamide during chemical grouting operations. (1987) Office of Toxic Substances, Exposure Evaluation Division, Field Studies Branch. EPA 5 60/5-87-009.

12. CG&S, Waterloo, Ontario (1997), Executive Summary, Main Treatment Plant, Environmental Assessment, Final Report.

13. Gandurina, L..V., Gervitis, E.I.( 1987), Treatment of petroleum –containing wastewater using activated silica as a flocculant. Khim. Tekhnol. Topl. Masel USSR, (9), 40-1

14. Keating, R.J., Calise, V.J.,(1954), Treatment of sewage plant effluent for industrial re-use. Pres. at 27 th Annual Meeting, Federation of Sewage and Industrial Wastes.

15. Vershininam V., Topilina, O.R., ( 1979), Removal of chromium from wastewaters. Tovarnye Znaki, USSR, (47), 99

16. Brodeur, T.P., Bauer, D. (1974 ), Picking the best coagulant for the job. Water and Wastes Engineering. 11(5) pg. 52-56.

17. Glazunova, G.A., Komogorcev, B. V., Rafienko, A.I. and Shokhin, V.N. (1981), Suspension dehydration. Tovarnye Znaki, USSR (30), 102

18. Briggs, T., Ross, D., Nutt, B., Kuslikis, Mcgregor, B.,(1998), Maximizing Capacity of the Duffin Creek WPC by rerating. Paper WEAO Tech. Symposium, pg.7

19. Butler, R., Clark, D., Cleveland, C., Newlands, D., and G. Newman (1998), An evaluation of the effect of digestion time on biosolids quality. WEFTEC proceedings, pg201-212

20. MTP 1997 Annual Report 21. City of Guelph Eng. Dept. (1971-1972), Phosphorus removal study

report 22. Lempka, B, (1995 and 1996) NSL Int. Rep., Evaluation of AS addition

on Phosphorus removal in Keswick and San Diego wastewater plants

A SIMPLE METHOD OF REMOVING DISINFECTION BYPRODUCT PRECURSORS IN WATER

Tony Robles Town of Kirkland Lake WTP roblest@ntl.sympatico.ca Overall Responsible Operator 705-642-5626 Operator-In-Charge

__________________________________________________________________ Abstract The formation of disinfection byproducts (DBPs) during water disinfection has long been a problem. The removal of natural organic matter (NOM), specifically DBP precursors in raw water is the most important step. Once DBP precursors are removed by coagulation, the use of expensive technologies to reduce DBP formation will not be needed. This paper describes a simple and new way of removing NOM before primary disinfection. The procedure involves the use of an acidified solution of activated silica (AS) and alum as a flocculant (AS-AL). Together with the coagulant alum, DBP precursor removal of 70 to 85 percent is achievable. The manufacture, use and effectiveness of AS-AL in reducing DBP formation are discussed. __________________________________________________________________ Key Words: Water treatment, THMs, DBPs, HAAs, Activated Silica, Chlorine Demand, DBP Precursors, NOM, Chlorination, Hach THM Plus

INTRODUCTION

The Kirkland Lake Water Treatment Plant (KLWTP) supplies potable water to a Northern Ontario town of about 9 000 people. The plant has a rated output of 260 L/s or about 6 MGD. The shallow lake that supplies raw water to the plant is fed by rain and snow that accumulates in the watershed surrounding the lake. Like most small communities, the source water has small drainage basins and poor quality compared to the water source of large cities. The KLWTP shown in Figure 1 is a conventional surface-water treatment plant that uses alum as coagulant and activated silica as flocculant. Chlorine is used for disinfection after filtration. The plant had difficulties complying with Total Trihalomethanes (TTHMs) maximum allowable concentration of 100 ug/L, and maintaining stable chlorine residuals in the distribution system. Plant management has to develop a method of reducing DBPs formation to comply with regulations. Developing a method in-house and without using

1

outside resources, e.g., consultants, was challenging because the plant has limited resources of analytical equipment, manpower, and expertise. In addition, regulatory requirements in the Certificate of Approval and other government regulations do not allow modifications to plant processes without going through the costly and time consuming approval process. The only option left was to improve the chemical properties of activated silica using only existing plant chemicals, such as, sodium silicate, sulfuric acid, alum, chlorine and HFS.

Related Work

Most researchers tried to correlate the formation of DBPs to total and dissolved organic carbon, ultraviolet absorbance, and chlorine demand. The author believes that the formation of DBPs correlates well with chlorine demand. Past researches have concluded that some NOM are not DBP precursors shown by the fact that greater removal of dissolved organic carbon did not necessarily indicate greater reduction of the DBP formation potential (DBPFP). Alum, synthetic organic polymers, and resins can remove NOM from waters. No synthetic organic polymer used alone performed as well as alum. Resins can effectively remove charged NOM but not the uncharged NOM. Resin particles have to be very small to provide large surface area for NOM removal. Microfiltration (MF) and ultrafiltration (UF) have been used primarily to remove particles and microorganism from water. They remove less DBP precursors than alum, nanofiltration (NF) or reverse osmosis (RO). There are a number of technologies available to reduce DBP formation, such as, ozone, UV, MIEX, DAF, GAC, and chloramines. Most technologies deal with DBPs after they are formed, fewer deal with removing DBP precursors before disinfection and others replace chlorine as the disinfectant. All these DBP mitigation technologies are expensive add-on technologies to the conventional treatment technologies of coagulation, sedimentation and filtration. Most of them are not well suited to small systems with limited resources that often have poor quality source water. The ideal solution is to remove DBP precursors using conventional treatment before disinfection. The method of removing DBP precursors would then be available to both small and large water treatment plants and would not require large capital expenditures. Preferably, the method would be equally effective in treating the clean water source of cities as well as the poor water sources of rural communities. Ones DBP precursors are removed before disinfection then other DBP mitigating technologies will not be needed.

2

System Model

Many researchers have shown that NOM have a neutral to slightly negative charge. Alum has a positive charge and is effective in removing NOM up to 50%. The activated silica used in water treatment has a negative charge and therefore not as effective as alum in removing DBP precursors. However, activated silica improves the effectiveness of alum in the coagulation process by producing a dense and coherent floc. Enhanced coagulation has demonstrated that using high dosage of alum and low pH improves NOM removal. However, significant lowering of the pH of raw water is expensive. Making the activated silica polymer acidic instead of the raw water is more economic. This requires lowering the activated silica pH from about 9.5 to below 2.5. An acidic activated silica polymer is less stable than a basic polymer. Addition of a polyvalent metal ion such as aluminum or iron, or both to the acidified activated silica sol may be enough to stabilize the sol. The adsorbed polyvalent metal ions increase the acidity of the polymer and should enhance the removal of DBP precursors. This acidic solution of AS and alum is hereby called AS-AL. It has been communicated to the author that the AS-AL carries a small positive charge.

Solution

Changing the plant activated silica manufacturing process from AS-XP (basic) to AS-AL (acidic) required minor plumbing changes. The caustic line that was used to make AS-XP is now used to deliver the alum. The SCADA activated silica batching program also required a minor revision. Table 1a and 1b summarize the steps in making AS-AL.

Table 1a Steps in making AS-AL

Step No.

What to do.

1 Open water line and fill batching tank with water until total volume is 2 000 liters.

2 Add 100 liters of Sodium Silicate with mixer operating at full speed. 3 Open water supply line and add 11 liters of 93% sulfuric acid into

water line while water is added to the tank. Continue water addition until total volume in the tank is 3 000 liters. Mixer is at high speed. The pH of the batch is about 7.

4 Pause (optional) for about 7 minutes for activated silica to form. This step allows different degrees of polymerization to take place. A longer pause favors the formation of higher molecular weight AS.

5 Open water supply line and add another 9 liters of 93% sulfuric acid into water line while water is added to the tank. Continue water

3

Step No.

What to do.

addition until total volume in the tank is 4 000 liters. Mixer is at high speed. The pH of the batch should be about 1.5 to 2.

6 Add 50 liters of alum to the activated silica batch. Mixer at high speed.

7 Switch mixer speed from high to low. AS-AL batch is ready for use. Table 1b shows the typical timeline for the batching process. Not included in the estimated time is the extra 5 minutes mixing time after each step to make sure the mixture is homogenous before proceeding to the next step. The volume of ingredients added to the batch is measured using “Milltronics” liquid level sensor. Water, alum, and sodium silicate volumes are measured using the AS batch tank sensor. The acid volume is measured using the acid “day” tank sensor.

Table 1b Timeline in making AS-AL

Step No.

What to do.

1 Estimated time: 13 minutes at 150 L/min water addition rate. The water is added at the bottom of the tank and mixed with leftover activated silica from the previous batch. It is advisable to keep the volume of leftover activated silica to a minimum (<50 L).

2 Estimated time: 15 to 35 minutes. The time required to add 100 liters of sodium silicate, depends on pump capacity and viscosity of the silicate. The silicate is added through the top of the tank to prevent it from contacting acidic solution that will cause gelling. The silicate should be stored at all times above 23 degrees Centigrade to keep the silicate fluid enough to pump. After silicate addition, the batch pH is greater than 11 and SiO2 concentration of about 2%.

3 Estimated time: 5 minutes The water supply is first opened and after 1 minute the concentrated acid is injected into the water line for dilution and mixing before it comes in contact with the silicate solution inside the batch tank. The agitator provides vigorous mixing of the acid and silicate solution. After acid addition the pH of the batch is about 7, SiO2 concentration about 1.5%. Batch volume about 3 cubic meters.

4 Estimated time 7 minutes. A pause (optional) or aging time for the formation of activated silica at about pH 7.

5 Estimated time: 5 minutes The water supply is first opened and after 1 minute the concentrated acid is injected into the water line for dilution and mixing before it comes in contact with the silicate solution inside the batch tank. The agitator provides vigorous mixing of the acid and AS solution. Add water until the total volume in the tank is

4

Step No.

What to do.

4000 liters. SiO2 concentration about 1%. After acid addition the pH of the batch is about 1.5 to 2.

6 Estimated time: 10 minutes. Like the silicate, the alum is added through the top of the tank.

7 Switch mixer from high to low speed. Batch is ready for use. DBPs may be produced in the treatment plant and the distribution system as long as the water is in contact with free chlorine residual. The formation of the DBPs is influenced by chlorine contact time, chlorine dose and residual, temperature, pH, precursor concentration, and bromide concentration. The DBPFP is used to calculate the percent removal of DBP precursors. Because DBPFP depends on experimental conditions, data obtained using this method must only be compared to data obtained from methods using the same conditions. DBPFP will be defined here under specific conditions of temperature (20°C), incubation time (24h), darkness, and residual free chlorine (1-3 ppm), and then analyzing the resulting DBPs by the Hach THM Plus method. The Hach THM Plus analytical method provides an inexpensive, fast and operator-friendly method of finding the concentration of DBPs in water samples. The THM Plus method reacts with trihalogenated disinfection byproducts formed as the result of the disinfection of drinking water with chlorine in the presence of NOM. The concentration is reported as ug/L chloroform. Monitoring the performance of the plant using AS-AL requires the use of unconventional methods due to limited resources available. Emphasis is given to improving plant performance economically and in a short time period. There are three visual (qualitative) indicators to help monitor the performance of the new flocculant. The first are the glass windows at the side of the three clarifiers below the waterline as shown in Figure 2, the second is the Particle Index Monitor for the filter effluent as shown in Figure 3, , and the third is a white porcelain water fountain located near the plant control room as shown in Figure 4. The three equipment provide important clues on the direction of the test; going better (+) or going bad (-). The Degremont upflow clarifier is more sensitive to the quality of the sludge in the clarifier than a conventional clarifier. It requires a dense and coherent sludge to run properly. The glass window provides information on how well the sludge blanket handle designed rise rate and shows the sharpness of the sludge/clarified water interface.

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The Particle Index Monitor gives two indications, numbers going up (negative results) and numbers going down (positive results). The particle index monitor provides an early warning and prevents process upset. It is more sensitive to particles above one micron than a turbidimeter. Staining on the surface of a water fountain is an indicator of the presence of organic matter in the treated water. The more often the water fountain requires cleaning the higher the organic content of the water. Cleaning frequencies are measured in days, weeks, or months. Quantitative means of monitoring the process are: DBPFP using Hach THM Plus, and the plant online and lab chlorine analyzers. The daily plant chlorine dose or consumption also provides quantitative indication on the progress of the test.

Implementation

On October 14, 2005, all plant operating parameters were kept unchanged except for the addition of AS-AL (6 ppm as silica) after the alum. The first indicator of the removal of DBP precursors would be a decrease in chlorine consumption. A significant drop of chlorine daily dosage was observed the days following the AS-AL addition. Based on three years of data (2003-2005) for the same period, about 50% savings in chlorine is realized as shown in Figure 5. A DBPFP procedure was used to measure percent removal of DBP precursors. DBPFP may vary from day to day. For consistency, the samples of raw and treated water were taken the same hour of the day. The percent removal of the DBP precursors may be calculated from the data. For example, samples taken November 12, 2005 of the raw and treated water were subjected to a 1-day and 2-day DBPFP test as shown in Figure 6. The amount of DBP precursor’s removed on the 1st and 2nd day of incubation was 78 and 79 percent respectively. It appears that the longer the incubation period the higher the percent removal. On the same day, the DBPs of the treated and distribution water samples were 46 ppb and 88 ppb respectively. The 2-day chlorine demand of the raw water was 8 ppm. The author prefers to use 1-day incubation period to closely simulate what is happening in the distribution system. The historical level of DBPs in the distribution samples are shown in Figure 7. The data shows that the plant could pass the present Canadian MAC for TTHM by just using AS-XP. The US EPA standards for DBPs are more stringent than the Canadian standards and include haloacetic acids. Only by using AS-AL can the KLWTP easily pass the USEPA standards which the Canadian regulatory body may tend to adopt in the future. The following visual observations can be made after one month of using AS-AL at the plant:

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• Sludge characteristics are about the same as using AS-XP and way better than using standard AS;

• The particle index is about the same as using AS-XP. The particle count of the treated water remains very good and may be equal or better than ultrafiltration;

• Even without prechlorination, the clarifier settling tubes are cleaner compared to using standard AS, shown in Figures 8 and 9 respectively;

• The floating organic scum after a filter backwash, shown in Figure 10, disappeared; and

• The water fountain test results are shown in Table 2.

Table 2 Frequency of cleaning the water fountain

Flocculant used Frequency of cleaning

Standard Activated Silica (AS) Every other day AS-XP (Basic) Once a week AS-AL (Acidic) Still clean after one month and waiting

CONCLUSIONS AND RECOMENDATIONS

The flocculant AS-AL can remove 70 to 85% DBP precursors in raw water. At this level of DBP removal, any plant can easily pass the USEPA limits for DBPs. There are other benefits of having low chlorine demand water at the plant and distribution system. Some of these benefits are: (1) more stable plant operation, (2) more stable chlorine residual in the distribution system, (3) less instrument maintenance, and (4) lower chemical consumption. This process of removing DBP precursors can be used by small and large water treatment plants regardless of the concentration of NOM in the source water. The flocculant AS-AL will help bring the conventional water treatment process into the 21st century. The results after one month of plant operations are very encouraging. There is a need for further research to investigate the mechanism of DBP precursor removal using AS-AL. Plant optimization is still on going to find the optimum dosage, the ratio of alum to silica in AS-AL, and the optimum dosage of alum.

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ACKNOWLEDGEMENT

The author is grateful for the cooperation and support provided by the Superintendent of Works, Frank Spinato, and plant operations staff members. The author would also like to acknowledge Brian Whitehead, Drinking Water Unit, Water Monitoring Section, Environmental Monitoring & Reporting Branch, of the Ministry of the Environment for providing analytical services during the test work.

REFERENCES

K. R. Lange; R. W. Spencer; Environmental Science and Technology, v. 2, n. 3, pp. 212-216, (1968); ON THE MECHANISM OF ACTIVATED SILICA SOL FORMAION R. K. Iler; THE CHEMISTRY OF SILICA (1979), John Wiley & Sons, Inc. A. D. Nikolaou, et. al.; ORGANIC BY-PRODUCTS OF DRINKING WATER CHLORINATION, Gloval Nest: the Int. J. v. 1, n.3, pp 143-156, 1999. D. Bursill; DRINGKING WATER TREATMENT – UNDERSTANDING THE PROCESSES AND MEETING THE CHALLENGES, Water Science and Technology Supply; v.1, n. 1, pp 1-7.

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Figure 1. Schematics of the Kirkland Lake WTP

Figure 2 Clarifier sludge viewing window

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Figure 3 Particle Index Monitor

Figure 4. Water fountain

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Fig. 8 - Clarifier settling tubes using AS-AL

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Fig. 9 - Clarifier settling tubes using standard AS

Fig. 10 - Floating organic matter after a filter backwash

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