Data Review from Full-Scale Installations for Water Treatment Plant

Copyright © 2009 American Water Works Association and Dr. Margaret Walsh, Principal Investigator, Dalhousie University. All Rights Reserved.

DATA REVIEW FROM FULL-SCALE INSTALLATIONS FOR WATER TREATMENT PLANT RESIDUALS TREATMENT PROCESSES

Submitted to Project Subcommittee:

Damon Roth, Chair; Susan Baez-Cazull; Mike Elliott; Mark Gross; Yann LeGouellec

Project funded by:

The AWWA Technical & Education Council

Project sponsored jointly by:

The WSRD Residuals Management Research Committee and the WQTD Water Treatment Plant Residuals Committee

Submitted by:

N. McCormick, J. Younker, A. Mackie and M. Walsh Department of Civil and Resource Engineering

Dalhousie University Halifax, Nova Scotia, Canada

December 18, 2009


TABLE OF CONTENTS

EXECUTIVE SUMMARY...................................................................................................... 2 1.0 INTRODUCTION............................................................................................................. 3 1.1 Background.................................................................................................................... 3 1.2 Project Objectives.......................................................................................................... 4 2.0 PROJECT METHODOLOGY....................................................................................... 4 2.1 Water Treatment Plant Audits....................................................................................... 4 2.2 Regional Site Visit........................................................................................................ 5 3.0 ANALYSIS OF COLLECTED SURVEY DATA......................................................... 6 3.1 Water Treatment Plant Information.............................................................................. 6 3.2 Spent Filter Backwash Water (SFBW) Treatment........................................................ 9 3.3 Solids Thickening.......................................................................................................... 11 3.4 Dewatering Processes.................................................................................................... 12 3.5 Final Disposal of Waste Streams................................................................................... 154.0 CASE STUDIES................................................................................................................ 165.0 CONCLUSIONS AND RECOMMENDATIONS.......................................................... 186.0 REFERENCES.................................................................................................................. 21


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EXECUTIVE SUMMARY

The overall objective of this project was to collect and compare full-scale operating data for various residuals treatment systems that are currently operating at water treatment plants (WTPs) across North America. Overall, the data available from published sources on performance of residuals treatment processes tends to be limited in scope, and is often based upon pilot-test data collected during the design of residual treatment facilities. Data from full-scale operations is often unavailable, although many WTPs have implemented residuals treatment over the past 20 years.

A general survey form was developed to include general plant information in addition to residuals treatment process characterization and performance data. For the 65 WTPs audited through telephone and email resources, the results of 46 surveys were returned and used for analysis. A regional visit was also conducted at the facilities of several utility participants in Ohio and Kentucky, U.S. The majority of WTPs in the study are conventional filtration plants and use surface water, primarily from river sources. Alum was found to be the most commonly used primary coagulant, with most plants also employing a low dose of polymer as an aid in the clarification and/or filtration processes.

The majority of the 19 plants using clarification processes (defined as sedimentation or plate and tube settlers in the survey) for the treatment of spent filter backwash water (SFBW) or combined SFBW and solids from primary treatment train sedimentation units were found to recycle the clarified water from residuals processing to the front of the plant. None of the five plants using gravity thickening (GT) units for SFBW and main treatment train sedimentation solids processing reported the return of overflows to the front of the plant. However, 50 % of the plants that use GT units after SFBW clarification processes return the thickeners overflow to the front of the plant. The information collected in this survey indicates that the additional solids removal achieved in a two-stage treatment of SFBW and sedimentation solids (e.g., SED-GT) versus a single-stage treatment (e.g., GT) may dictate in-plant recycling versus source water discharge practices of GT overflow streams, and may be firmly entrenched in water quality and risk analysis metrics.

Mechanical and non-mechanical dewatering technology was found to be equally employed amongst the survey plants. The filtrate/centrate generated in the mechanical dewatering units is primarily returned to the front of the residuals treatment train in the majority of the reporting plants. For plants using non-mechanical dewatering processes, the filtrate is primarily returned to the source water. More than half of the plants reported the disposal of the final solid waste residuals generated in their facilities in off- or on-site landfills. However, a slightly more than a quarter of the plants have established reuse pathways for their generated WTP solids, primarily through topsoil manufacturing and agricultural land application practices.

Information provided by the participating utilities regarding unit operation overflow/loading rates and process stream solids analysis was limited in many cases for the clarification, thickening and dewatering processes used at these facilities. The operational and monitoring practices of several plants have been highlighted to capture potential plant models that could be used for implementation at other facilities to achieve system optimization goals.


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1.0 INTRODUCTION

1.1 Background In 2006, the American Water Works Association Research Foundation (AwwaRF formerly; now Water Research Foundation [the Foundation]) and the American Water Works Association (AWWA) published a report titled Water Treatment Residuals Engineering (Cornwell, 2006). This updated version of the 1987 Handbook of Water Treatment Plant Waste Management, also published by AWWA and the Foundation, includes information from many of the Water Research Foundation projects completed in the intervening years on water treatment residuals. The report presents information on five types of water treatment processes that generate residuals, the characteristics of these residuals, and treatment options. Included in the report is information on various studies looking at phys-chem properties of residuals and bench- and pilot-scale residuals treatment testing.

Vandermeyden and Cornwell (1998) prepared a report for the Foundation (formerly AwwaRF) entitled Non-mechanical Dewatering of Water Plant Residuals. This report was intended to be a guide for the proper design of non-mechanical dewatering systems and includes characterization of residuals properties from 60 utilities, which were found to have no correlation to full-scale drainage characteristics. In other words, they found that there were no laboratory tests that could be used as a predictor of full-scale dewatering efficiency. However, it was found that pilot tests did show a strong correlation (R2 = 0.94) to full scale dewatering performance at the 13 utilities selected for pilot testing in that study.

Another study done for the Foundation was conducted by Hsieh and Raghu (1997) and titled Criteria Development for Water Treatment Plant Residual Monofills. The purpose of that project was to determine the properties of WTP residuals to develop design criteria for monofills. Both raw and dewatered residuals samples from 10 utilities were evaluated. The utilities were selected in order to have varied source water qualities and dewatering technologies represented. Average raw residuals solids content were found to be 1.5 %, and dewatered solids contents were found to range from 15 to 82 %. These results were from a single sampling event at each utility.

Several other studies have been performed at bench- and pilot-scale, including work on freeze-thaw beds (Mangat et al. 2008, Martel and Diener 1991, Parker et al. 1998), polymer addition to residuals to aid in dewatering (MacPhee et al. 2001, Dharmappa et al. 1997, Ayol et al. 2005), and treatment and reuse of spent filter backwash water (SFBW) (Bourgeois et al. 2004, Cornwell et al. 2001; Edzwald et al. 2001; Arora et al. 2001; Reissmann and Uhl 2006). However, no follow up studies exist to determine if designs based on bench- and pilot-scale studies have performed effectively or been optimized at the full scale, or if findings from these studies have influenced residuals treatment plant design.

Overall, the data available from published sources on performance of residuals treatment processes tends to be limited in scope, and is often based upon pilot-test data collected during the


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design of residual treatment facilities. Data from full-scale operations is often unavailable, although many water treatment plants have implemented residuals treatment over the past 20 years. The data from these installations could provide new information to the drinking water industry that can be used for design purposes. Therefore, the goal of this project was to compile an analysis of residuals processing data from full-scale operating facilities as to provide insight and reduce uncertainty regarding process performance when planning and designing residuals treatment facilities.

1.2 Project Objectives The overall objective of this project was to collect and compare full-scale operating data for various residuals treatment systems that are currently operating at water treatment plants (WTPs) across North America. The following specific objectives of the project were identified in order to meet the overall goal of this project:

1. To contact water utilities identified by the Project Volunteer Unit to obtain operations data on the main treatment train and residuals treatment systems as specified in the information collection items list.

2. To conduct preliminary analysis of collected data to generate a working database including QA/QC to ensure reported data is standardized by units, categorization of data by treatment process, water quality and geographical location and statistical analysis, including summary statistics.

3. To prepare findings report to be submitted to Project Volunteer Unit that summarizes the preliminary analysis of collected data, including analysis methodology and results.

2.0 PROJECT METHODOLOGY

2.1 Water Treatment Plant Audits This part of the project involved information gathering on full-scale residuals treatment from participating drinking water utilities identified by the Residuals Management Research Committee and Water Plant Residuals Management Committee. A list of potential participating utilities was developed by the Project Volunteer Unit and provided to the working group at Dalhousie University in May 2009. This list was expanded throughout the first four months of the project to include 74 water treatment plants in Canada and the United States. A survey form (see Appendix A) was developed based on the information collection items developed by the Project Volunteer Unit to include (i) plant information (e.g., design capacity, source water, main treatment train, residuals treatment type), (ii) residuals treatment process characterization (e.g., equalization, chemical addition, dewatering technology) and (iii) operational control and performance data (e.g., flowrates, chemical dosages, solids concentration). Successful initial telephone contact with each of the 74 water treatment plants was completed by the end of August 2009. Of the 74 plants contacted, 9 utilities were unable or unwilling to participate in the audit. Appendix B contains the master list of all of the plants contacted by the


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project team at Dalhousie University, and indicates utility involvement in the project. Appendix B also includes information on individual plant production capacity, and main treatment train design and residuals management systems. The 65 plants willing to participate in the study were emailed the survey form developed for the study. Follow-up calls were conducted with each of the plants utilities from two weeks after the survey form was sent to confirm completion of the survey form. Information from the returned survey forms were collated into a central database (see Appendix C). Any deficiencies in the information requested were noted at this point, and follow-up calls were scheduled via email in an attempt to finalize missing information from the submitted surveys. In many cases, more than one follow-up call was required.

In summary, for the 65 water treatment plants audited: • 46 survey forms were completed and returned to Dalhousie University after the initial

contact was established. Follow-up calls were conducted with these plants to finalize the information collected and fill information gaps that were present in some cases.

• 3 survey forms were returned to Dalhousie University; however were not near complete enough to use in the final report. Several unsuccessful attempts were made to re-establish contact with these utilities by telephone and/or email in order to fill the information gaps and complete these surveys.

• 16 survey forms were not returned to Dalhousie University by the final deadline of October 23, 2009. Multiple unsuccessful attempts were made between May to October to re-establish contact with these utilities by telephone and/or email in order to complete these surveys.

2.2 Regional Site Visit A regional visit where the facilities of several utility participants were toured by two of the students at Dalhousie University (Neil McCormick and Jessica Younker) was coordinated by Damon Roth of the Project Volunteer Unit. The site visits to four water treatment plants led by Damon Roth took place from August 11-13, 2009 in Ohio and Kentucky, U.S.

Two conventional filtration water treatment plants in Northern Kentucky Water District were visited by the project team. Both the Taylor Mill and Fort Thomas WTPs treat a high turbidity source water from the Ohio River and the Licking River (tributary of Ohio River), respectively. Spent filter backwash (SFBW) water generated in the Fort Thomas WTP is directed into a backwash recovery tank before being recycled to the head of the raw water reservoir. Sludge solids generated in the four clarifier basins of the main treatment train and solids from the SFBW recovery tank are treated in two gravity thickeners (GT). The Taylor Mill WTP uses two lamella plate settlers to treat SFBW, with the generated solids equalized with sludge solids from the two clarifier basins of the main treatment train before thickening in two GT units. Both plants use belt filter presses (B/F Press) for dewatering the thickened solids from the GT units, dosed with


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10 mg/ L of polymer. Although not routinely monitored, both plants reported achieving a 2 to 5 % solids concentration after GT treatment and a 25 to 35 % solids concentration after dewatering. The final sludge generated in the residuals treatment facilities of both plants is recovered for beneficial reuse in topsoil manufacture.

The Ottawa WTP in Dayton, OH is a groundwater lime softening plant, and the project team was given a tour of the kiln used to recalcify lime solids generated in the main treatment train. The students were able to see the kiln in operation, observe the lime at different stages of the process and discuss with the supervising engineer the cost-benefits of this process with respect to materials and cost savings versus energy use. The Bolton WTP in Cincinnati, OH is a groundwater lime softening plant where residuals are sent to solids lagoons. Lagoons are excavated about every other year and dewatered solids are disposed in a monofill located just outside of the plant boundaries. 3.0 ANALYSIS OF COLLECTED SURVEY DATA A large amount of data was collected from the utilities contacted for this study. The following sections will attempt to distil this information into useful comparisons/information.

3.1 Water Treatment Plant Information Treatment plants submitted information on both their water production design capacity and actual average raw water flowrate. These statistics are summarized in Figure 1. In terms of design capacity, the majority of plants (27 plants or 60 %) fall in the medium size range, between 10 and 100 million gallons per day (mgd). However, the majority of plants have actual raw water flowrates of less than 10 mgd (24 plants or 52 %). Only 2 plants have actual raw water flowrates above 100 mgd (4 %), while 7 have design capacities above 100 mgd (16 %). The mean plant size was 50 mgd for the 45 plants in total that reported their design capacity, and 27 mgd for the 46 plants that reported their actual flowrate. The smallest plant has a design capacity of 2.3 mgd (1.7 mgd for actual flowrate), and the largest 220 mgd (125 mgd for actual flowrate).


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Figure 1: Design capacity and actual average raw water flowrate of surveyed plants.

The majority of treatment plants in the study (36 plants or 78 %) use conventional filtration in their main treatment train. Five of the plants (11 %) employ direct filtration with granular media filters, and two (4 %) are membrane plants that use ultrafiltration (UF) technology. Three plants (7 %) defined themselves solely as lime softening plants. These are included in the total of 8 plants in the study that reported the use of lime and/or soda ash in the main treatment train (6 using lime, 2 using lime and soda ash). Polymer aids are used in 33 of the plants surveyed (e.g., 72 %).

Of the 46 plants used in the study analysis, 91 % of the utilities (42 plants) use surface water as a feed to their treatment plants, and only 9 % (4 plants) use groundwater. The water sources used by the plants surveyed in this study are further defined in Figure 2.


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Figure 2: Treatment plant raw water sources.

Aluminum sulfate (e.g., alum) was reported to be the most widely used primary coagulant, at exactly half of the 44 plants surveyed that responded with primary coagulant data. Polyaluminum chloride (PACl) was reported to be the primary coagulant at six of the plants surveyed (14 %). Ferric based inorganic coagulants were also reported to be a primary coagulant, with five of the plants (11 %) using ferric chloride (FeCl3) and two of the plants (4.5 %) using ferric sulfate (Fe2(SO4)3). One conventional and one membrane filtration plant reported aluminum chlorohydrate (ACH) as the primary coagulant used in the main treatment train (4.5 %). The remaining seven plants (16 %) use a combination of coagulants, either concurrently or depending on the season (e.g. alum in summer, PACl in winter). As presented in Figure 3, most of the alum (82 %) and ferric chloride (80 %) plants use a polymer to aid in flocculation and/or filtration. One plant (WTP10) uses a cationic polymer (4.8 mg/L) as the main coagulant, with capabilities to also use alum. This plant was not included in the summary above or in Figure 3.


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Figure 3: Coagulant and polymer usage by plant.

A main indicator of raw water quality and a potential indicator of coagulant usage is turbidity. Eleven plants (24 %) reported low source water turbidity (< 2 NTU), 22 plants (43 %) reported medium turbidity (between 2 and 10 NTU), and 13 plants (35 %) reported high turbidity (> 10 NTU) source water. The preceding data reflect the average turbidity reported.

Forty-three plants provided either an average coagulant dose or a range of coagulant doses (min-max). WTP35 was removed from this data set, since an extremely wide range was provided (8-150 mg/L). The wide coagulant dose range was due to large source water turbidity spikes in the spring.

Figure 4 presents the average coagulant dose by coagulant and turbidity level. In general, the plants using both alum and PACl reported a wide range of coagulant dose. For the plants using alum, a general trend of increased coagulant dosage with increasing turbidity levels in the source water was found. No trend between coagulant dose and raw water turbidity was found with the plants using ferric chloride or PACl.


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Figure 4. Coagulant dose by raw water turbidity level. (Error bars represent one standard deviation; n represents the number of data points (i.e., responding plants).

Table 1 shows the average coagulant and polymer doses by coagulant type. In general, polymer usage in the main treatment train was less than 2 mg/ L for all plants reporting the use of dual chemical coagulation programs.

Table 1: Average coagulant and polymer doses of responding plants. (Bracketed values are the median coagulant and polymer doses)

Alum PACl Ferric Chloride Coagulant Dose (mg/ L) 27 ± 20 (25) 11 ± 7 (10) 20 ± 9 (17) Polymer Dose (mg/ L) 1.2 ± 2.5 (0.4) 0.2 ± 0.15 (0.2) 2.0 ± 1.4 (2.1)

Of the 8 plants that reported the use of lime and/or soda ash, chemical doses were provided from six of the plants. These plants reported a mean chemical dosage of 145 mg/ L, with a standard deviation of 58 mg/ L, and a median value of 148 mg/L.

3.2 Spent Filter Backwash Water (SFBW) Treatment 96 % of the utilities surveyed (44 plants) reported spent filter backwash water (SFBW) treatment or recovery units that encompasses the following unit operations:

• Clarifiers (15 plants, 34 %) • Lagoon (13 plants, 29 %) • Gravity Thickener (5 plants, 11 %) • Plate and Tube Settling Basins (4 plants, 9 %) • Wash water recovery basins (3 plants, 7 %) • Lamella plate settlers (1 plant, 2 %) • Dissolved Air Flotation (DAF) (1 plant, 2 %)


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For the remaining two plants, one utility discharges SFBW direct to the sewer and the other discharges dechlorinated SFBW to the plant’s source river water. Due to the low number of plants and reporting details for the plants using washwater recovery basins, DAF and lamella plate settlers, these facilities are not included in the analysis presented below. The information gained from the plants that use lagoons for treatment of SFBW and/or sedimentation solids from the main treatment train is presented in Section 3.4 under Non-Mechanical Dewatering Processes. Clarifiers and Plate/Tube Settlers In total, 19 plants reported the use of clarifiers (e.g., sedimentation processes (SED) as listed in the database presented in Appendix C or plate and tube settlers (P/T SET) for the treatment of SFBW or combined SFBW and solids from primary treatment train sedimentation processes. Of these plants, 79 % (15 plants) recycle the clarified water from residuals processing to the front of the plant. Nine of the plants add polymer to the clarifiers or P/T SET processes at dosage ranging from 0.03 to 1 mg/ L. Six of the clarifiers use polymer. Of these six plants, four reported doses that averaged 0.50 ± 0.36 mg/ L. The two P/T settlers that use polymer have an average dose of 0.31 ± 0.29 mg/ L. Six of the plants that do not add polymer prior to settling treatment processes also do not add polymer in the main treatment train, and collectively have a median raw water turbidity level of 1.2 NTU. Seven of the plants that use clarifiers or plate and tube settlers for the treatment of SFBW reported surface overflow rates (SOFR) for these processes in the survey, ranging from 444 to 1,703 gpd/ ft2. Due to the limited information provided by the utilities, no correlation could be found between polymer addition and sedimentation basin operational design in terms of SOFR.

Only five of the 14 plants using clarifiers for SFBW treatment reported measuring underflow solids concentrations. Two plants reported an underflow solids concentration of < 1 % from their SFBW sedimentation basins, with one plant reporting analysis results on samples taken on a daily basis and the other plant reporting analysis results on samples taken on a bi-monthly basis. The remaining three plants reported an underflow solids concentration of 2 to 3 %; however do not routinely monitor this process stream. With the limited dataset, no correlation was found between the solids concentration data collected and polymer addition or overflow rate.

The four plants using P/T settling basins claimed underflow solids concentrations ranging from 0.1 to 5 %. However, only one plant reported solids analysis results (e.g., 0.1 % solids) on samples taken on a bi-annual basis. The remaining three plants reported that monitoring of this process stream is infrequent.


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Gravity Thickeners Five plants reported co-management of SFBW streams with solids generated in sedimentation processes of the main treatment train in gravity thickening (GT) units. For the plants using these units, none return the GT overflows to the front of the plant. Three of these plants direct the overflow to the source water (e.g., river) and two plants return the overflow to equalization tanks at the front of the residuals treatment train. Four of these plants add polymer prior to the GT units, and three reported the polymer dosage (3.5, 4.7, and 15 mg/L). Four of the five plants using GT units to treat SFBW and sedimentation solids from the main treatment train reported overflow rates, ranging from 357 to 543 gpd/ ft2. Reported underflow solids concentrations achieved at this stage of the residuals treatment train ranged from 1.4 to 10 %.

Table 2 presents a summary of the operating data provided by plants discussed in Section 3.2. Table 2. Summary of processes used for treatment of SFBW and main treatment train

sedimentation solids. Clarifiers P/T Settlers Gravity Thickener SFBW SFBW &

Solids SFBW SFBW & Solids SFBW SFBW &

Solids # Plants Reporting Process Type 8 7 3* 1 0 5

# Plants Reporting Equalization 6 6 3 1 - 3

# Plants Reporting Polymer Addition 5 2 1 1 - 4

Polymer Dose (mg/ L)

0.2 – 1 (N=3)

0.3 (N=1)

0.1 (N=1)

0.03 – 1 (N=1) - 3.5 - 15

(N=3) Overflow Rate (gpd/ft2)

292 – 1,584 (N=4)

151 (N=1)

832–1,739 (N=1)

589 (N=1) - 357 - 543

(N=4) Underflow Solids %

1.4 ± 1.3 (N=3)

<1 (N=1)

0.9 ± 1 (N=3)

5 (N=1) - 3.5 ± 2.4

(N=4) *One of these plants treats ultrafiltration membrane concentrate N = # plants with reported data

3.3 Solids Thickening Participants with gravity thickeners (GT) were asked to provide operating values for polymer dosage, overflow rate, and unthickened and thickened solids percentage. The survey identified 13 plants that use gravity thickeners to treat solids from either the main treatment train sedimentation units and/or SFBW clarification process. Of these plants, only 38 % (5 plants) equalize the feed stream(s) prior to treatment. Nine of these plants reported the addition of polymer, at an average dose of 3.5 ± 4.5 mg/ L. Six of the plants using GT units reported overflow rates ranging from 102 to 1,739 gpd/ ft2, with the remaining plants either reporting a flowrate or responding that this operating parameter was unknown. For the six plants that provided both polymer dosages and overflow rate data, a solid correlation between these two


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parameters could not be established. However, it is interesting to note that the one plant reporting the highest GT overflow rate range (e.g., 832 to 1,739 gpd/ ft2) uses polymer at the highest reported dosage of 6 mg/ L. Of the 13 plants using GT units for solids thickening, only four plants reported regular monitoring on a daily or weekly basis of the feedstream (e.g., unthickened % solids), within the range of 0.5 to 3 % solids. In contrast, all of the 13 plants using GT units reported solids analysis of thickened sludge stream. Collectively, the GT thickened sludge data from the reporting utilities ranged from 1 to 20 % solids, with an average of 7.1 ± 5.9 % solids. The wide range of the reported % solids of the GT thickened sludge suggests variability exists in the feed solids concentration at the different plants. Seven of the plants reported that solids analysis on the thickened sludge stream is conducted on a daily basis, one plant reported analysis on a weekly basis and four plants reported infrequent monitoring and analysis on this process stream. No correlation could be found between polymer dose or overflow rate and the solids concentration of the thickened sludge, as shown in Figure 5.

Figure 5. Scatterplots depicting lack of correlation between thickened solids concentration and either polymer dose or overflow rate.

3.4 Dewatering Processes Mechanical Dewatering Processes Of the 46 plants in the study, 21 reported the use of mechanical dewatering processes in their residuals treatment facilities. Seven plants use belt filter presses, six plants use plate and frame presses and eight plants reported using centrifuges. The filtrate/centrate generated in the mechanical dewatering units is returned to the front of the residuals treatment train in 62 % (13 plants) of the reporting plants. For the remaining plants, 14 % (3 plants) return the filtrate to the front of the plant, 14 % (3 plants) discharge to sewer and 10 % (2 plants) reported surface discharge (e.g., wetlands).


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Participants with mechanical dewatering units were asked to provide operating values for polymer dosage, unit loading rate, dewatered cake solids concentration, and filtrate solids concentration. The survey did not include a request for additional information on the use of alternative dewatering aids (e.g., lime or fly ash) in these units or specific information on the type of polymer used in this part of the plant. Information on both polymer dosages and unit loading rate was limited. However, all of the 21 plants were able to provide data related to the percent solids of the dewatered cake from their processing units. One third of the responding plants reported monitoring the dewatered cake solids on a daily basis, one plant on a weekly basis, and 14 % (3 plants) monitor on a monthly or annual basis. Ten plants (48 %) reported infrequent solids measurement practices or did not provide specific information as to their monitoring frequency on this process stream. A summary of the operating data provided by these plants is presented in Table 3. Table 3. Summary of mechanical dewatering unit operating information. Belt Filter Press Plate & Frame Press Centrifuge # Plants Reporting Process Type 7 6 8 # Plants Reporting Day Tanks 5 6 6 # Plants Reporting Polymer Addition 7 2 8 Polymer Dose, mg/ L

0.2 – 10 (N=5)

3.3 (N=1)

0.1 – 1.2 (N=3)

Unit Loading Rate (lbs/ hr)

876 - 2,244 (N=6)

563 - 3,131 (N=2)

750 – 3,200(N=6)

Avg. Dewatered Cake, % Solids 24.6 ± 10.0 (N=7)

41.4 ± 13.0 (N=6)

25.2 ± 5.5 (N=8)

N = # plants with reported data

Less than half of the 21 plants that use mechanical dewatering processes for residuals treatment reported percent solids of the filtrate stream, and collectively reported < 1 % solids. Only four of these plants reported daily or weekly monitoring of this process stream, and all of these plants reported returning the dewatering filtrate stream to front of the residuals treatment train of their respective plant. Of the remaining 17 plants that reported infrequent monitoring or no testing of the dewatered filtrate stream, 65 % of these plants (11 plants) return this process stream to the front of the residuals treatment train. Three of the plants that reported infrequent monitoring discharge the dewatering filtrate to sewer and two plants direct this filtrate stream to either sludge beds or onsite wetland facilities. Only one plant reported returning dewatering filtrate to the front of the plant.

Non-Mechanical Dewatering Processes Plants with non-mechanical dewatering units (e.g., lagoons, sand drying beds, solar drying beds, freeze-thaw beds) were asked to provide information on the number of processing units, size of units, duration of bed loading, dewatered solids % at cleaning, decant/filtrate recycle practices and final solids disposal location.


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In total, 23 out of the 46 plants audited reported using non-mechanical dewatering methods for residuals management. More than half of these plants (13 WTPs) use 1 to 3 lagoons as a single clarification, thickening and dewatering unit operation. These lagoons receive SFBW and/or solids from clarification operations in the main treatment train, and have been classified as Clarification Lagoons for reporting purposes in this report. Two of the 23 plants also reported the use of lagoons for dewatering after clarification of the SFBW with sedimentation processes, and have been classified as Dewatering Lagoons for reporting purposes in this report.

Seven plants reported the use of drying beds for dewatering solids from clarification, thickening or mechanical dewatering processes. Four of these plants classified these units as general drying beds (3 to 10 units per WTP), one reported as solar drying beds (4 sand beds and 2 earthen lagoons) and two reported as freeze-thaw drying beds (2 units at each WTP).

The final plant in this group reported the use of geotextile tubes (Geotube®) for dewatering solids from a sedimentation pond. Geotube® technology uses large-diameter porous tubes made from heavy-duty synthetic fabric that trap solids and allow water to escape. Due to the limited information obtained from this final plant audit, and the fact that the survey was not designed to specifically request information related to geotextile dewatering technology, results of this plant audit are not presented in this section. A summary of the operating data provided by the plants employing non-mechanical dewatering processes is presented in Table 4. Table 4. Summary of non-mechanical dewatering unit operating information. Clarification

Lagoon Dewatering

Lagoon Drying Bed

Sand Solar F/T # Plants Reporting Process Type 13 2 4 1 1 Duration of Bed Loading, months 3 - 36

(N=12) 4 - 180 (N=2)

6 - 12 (N=2)

2 - 5 (N=1)

30 (N=1)

Dewatered Solids at Cleaning, % 12.2 ± 7.2 (N=12)

16 (N=1)

60 (N=1)

40 (N=1)

N/A (N=1)

# Plants that Recycle Decant 1 0 2 1 0 # Plants that Discharge Decant to Source# Plants with Other Option

10 2

1 1

0 2

0 0

1 0

N = # plants with reported data N/A: Not available


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3.5 Final Disposal of Waste Streams The surveyed plants were asked where their waste streams, both solid and liquid, are disposed. All plants reported both their final liquids disposal location and final solids disposal location. These locations are reported in Figure 6 and 7, respectively. Almost half of the plants audited (46 % or 21 plants) reported disposal of liquid waste streams to the source water, with the other two primary management options being recycling liquid waste residual streams in the plant (36 % or 15 plants) and routing to a sanitary sewer (9 % or 4 plants). The remaining plants reported directing their liquid waste streams to groundwater (2 plants), lagoons (2 plants), or constructed wetlands (2 plants). More than half of the plants (52 % or 24 plants) reported the disposal of the final solid waste residuals generated in their facilities in on- or off-site landfills. However, 17 of the plants (37 %) have established reuse pathways for their generated WTP solids, primarily through topsoil manufacturing and agricultural land application practices. Note that two of the plants reported both reuse and landfills as disposal locations for their solid waste streams, and are therefore each included twice in Figure 7.

Figure 6. Final disposal locations of liquid waste

Figure 7. Final disposal locations of solid waste.


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4.0 CASE STUDIES

WTP10 is a unique plant in this study because it measures almost every parameter of interest within its residuals treatment facilities on a regular basis. The conventional filtration plant treats a high turbidity (e.g., 53 NTU) and hard (e.g., 300 mg/ L as CaCO3) river water. The plant has a design capacity of 30 mgd, with typical production at 11 mgd. The plant uses lime and soda ash for softening and a cationic polymer (PHI2356) at an average dose of 4.8 mg/ L for coagulation. Six filters are operated at the plant with an estimated filter run time of 3 days prior to backwashing. Approximately 215,000 gallons of water are used for each backwash, resulting in an average daily SFBW production rate of 0.2 mgd. SFBW generated in this plant is stored in an underground washwater recovery basin (~500,000 gal) and returned to the plant influent at a controlled rate and blended with incoming raw water. The main treatment train (pre-sedimentation and softening basin solids) are dosed with approximately 0.03 mg/ L of polymer and directed into a 220,000 gal gravity thickener (GT) unit located in the residuals plant. The GT is operated at an overflow rate of 102 gpd/ ft2 to increase solids from 1 to 3 % (influent) to 15 – 20 % (thickened). The GT overflow is returned to the front of the plant, and thickened solids are further processed in four belt filter presses (Andritz CPF) operated at a combined 8,977 lbs/ hr loading rate to achieve a 35 to 55 % solids cake that is landfilled. Filtrate generated in the B/F Presses is directed to sewer. In comparison, WTP44 is a lime softening plant that also treats a high turbidity (e.g., 30 NTU) and hard (e.g., 235 mg/ L as CaCO3) river water. The plant has a design capacity of 14 mgd, with typical production at 8 mgd. The plant uses lime at an average dose of 80 mg/ L and ferric chloride coagulant at an average dose of 17 mg/ L. Similar to WTP10, this plant operates 6 filters with an estimated filter run time of 7 ½ days prior to backwashing. Approximately 75,000 gallons of water is used for each backwash, resulting in an average daily SFBW production rate of 0.3 mgd.

In contrast to WTP10, this utility uses non-mechanical residuals dewatering technology with the operation of three lagoons. Decant from the lagoons is permitted to a NPDES permitted outfall, and the dewatered solids are reused in land applications. The plant reported that the lagoons are operated for three years between cleaning, and 23 % dewatered solids content is achieved. The plant also reported that complete testing of the lagoon solids is conducted on a yearly basis.

As outlined in Figure 8, these two plants demonstrate parallels with regards to source water quality, plant capacity and SFBW production, but two very different approaches in residuals treatment, monitoring and final disposal. WTP10 has an aggressive mechanical residuals treatment train that achieves partial water recovery and reuse from thickening operations. The high solids sludge that is produced is landfilled. In contrast, WTP44 uses a simpler design with non-mechanical dewatering through lagoon operations. Although water recovery of the lagoon supernatant for in-plant recycling is not achieved, the solids are recovered for land application.


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Figure 8. Comparison of mechanical and non-mechanical approaches to residuals management

In terms of process monitoring, WTP10 measures all of the parameters for solids and liquids generated in the residuals processing facility. The overflow rate of the gravity thickener is monitored continuously, and unthickened and thickened residual streams are sampled for percent solids measurements daily. Similarly, for its belt filter press, loading rate and dewatered cake percent solids are monitored daily. A summary of monitoring data collected daily and residuals generation tracking related to the belt filter presses during the month of August, 2009 is presented in Table 5. Table 5. WTP10 average data from dewatering process

August 2009

Sludge Flow (gpm)

Feed Solids (%)

Cake Solids (%)

Solids Loading (lbs/hr)

Polymer Consumption (lbs/ton ds)

Hours Run per

day

Loads to Landfill per day

Average 64.7±3.5 25.7±1.9 50.5±2.3 8977±499 1.9±0.2 10.6±3.2 6 ± 2

Final disposal needs/constraints of each plant could possibly explain the differences in residuals management and process design with regards to available final disposal or reuse options for the


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lime solids and/or water resources and recovery strategies. For WTP10, monitoring of solids levels in the dewatered sludge would be of economical benefit in terms of reducing overall handling and disposal costs associated with landfilling. For WTP44, final solids monitoring and testing is only conducted on an annual basis, and would be more closely linked with regulations related to solid waste reuse policies in land applications.

5.0 CONCLUSIONS AND RECOMMENDATIONS The overall objective of this project was to collect and compare full-scale operating data for various residuals treatment systems that are currently operating at water treatment plants (WTPs) across North America. The majority of WTPs in the study are conventional filtration plants (78 % of reporting facilities) and use surface water (91 % of reporting facilities), primarily from river sources. Alum was found to be the most commonly used primary coagulant, with most plants also employing a low dose of polymer as an aid in the clarification and/or filtration processes.

Liquid Residuals Recycle Practices and Solid Waste Management • The majority of plants using clarifiers for the treatment of spent filter backwash water

(SFBW) or combined SFBW and solids from primary treatment train sedimentation basins were found to recycle the clarified water from residuals processing to the front of the plant.

• None of the plants using gravity thickening (GT) units for SFBW processing reported the return of overflows to the front of the plant.

• 50 % of the plants that use GT units after SFBW clarification processes return the thickeners overflow and blend with raw water flows to augment plant production.

• Filtrate generated in the mechanical dewatering units is primarily returned to the front of the residuals treatment train in the majority of the reporting plants.

• For plants using non-mechanical dewatering processes, the filtrate is primarily returned to the source water. Specific point of discharge (e.g., downstream or intake) was not part of the information gathering of this study.

• 40 % of the plants reported the disposal of the final solid waste residuals generated in their facilities in landfills.

• 35 % of the plants have established reuse pathways for their generated WTP solids, primarily through topsoil manufacturing and agricultural land application practices.

Spent Filter Backwash Water Treatment • 96 % of the utilities (44 plants) surveyed reported SFBW treatment or recovery units, of

these, 43 % (19 plants) were primarily comprised of clarifiers (e.g., sedimentation and plate/tube settlers.

• Thirteen of these are lagoons, which are analyzed in the dewatering section. Sixteen of the remaining plants recycle treated SFBW to the head of the plant.


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• Sedimentation basins were found to have an average polymer dose of 0.50 ± 0.36 mg/ L and P/T settlers a lower average dose of 0.31 ± 0.29 mg/ L.

• Only 5 of the plants that use SED or P/T SET for the treatment of SFBW reported surface overflow rates (SOFR) for these processes in the survey, ranging from 444 to 1,703 gpd/ ft2.

• Nine of the 19 plants using SED or P/T SET for SFBW clarification reported measuring underflow solids concentrations, with the data ranging from 0.1 to 5 % solids. Measurement frequency was widely reported as infrequent for these process streams.

• Four of the five plants using GT units to process SFBW and clarifier solids from the main treatment train reported overflow rates from 357-543 gpd/ft2, and underflow solids concentration achieved at this stage of the residuals treatment train ranged from 1.4 to 10 %.

Solids Thickening • Nine plants (69 %) using GT units to treat solids from either the main treatment train and/or

SFBW clarification processes reported the addition of polymer, at an average dosage of 3.5 ± 4.5 mg/ L.

• Six plants (46 %) using GT units reported overflow rates ranging from 102 to 1,739 GPD/ ft2, with the remaining plants either reporting a flowrate or responding that this operating parameter was unknown.

• Only 4 plants reported regular monitoring on a daily or weekly basis of the GT feedstream (e.g., unthickened % solids), within the range of 0.5 to 3 % solids.

• All 12 GT plants reported on analysis of GT thickened sludge stream that collectively ranged from 1 to 20 % solids. More than 50 % of the plants monitor this residual stream quality on a daily basis.

Dewatering Processes • Mechanical and non-mechanical dewatering technology was found to be equally employed

amongst the survey plants. • Mechanical dewatering technologies encompassed belt press filters, plate and frame presses,

and centrifuges equally distributed amongst the reporting plants. • Information on polymer dosages and unit loading rate was limited, with many utilities

reporting that this data was not known or unavailable. • All 21 mechanical dewatering plants were able to provide data related to the percent solids of

the dewatered cake from their processing units, ranging from 15 to 70 % solids. • More than 50 % (10 plants) of these plants reported infrequent solids measurement practices

or did not provide specific information as to their monitoring frequency on this process stream.

• 70 % (16 plants) of the plants reporting the use of non-mechanical dewatering for residuals treatment use 1 to 3 lagoons as a single clarification, thickening and dewatering unit operation.


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• Seven plants reported the use of drying beds for dewatering solids from clarification, thickening or mechanical dewatering processes.

Overall, the results of this study demonstrate that there is limited data being collected on the operational and process performance of full-scale residual treatment systems. Unit operations and water quality sampling/analysis in the main treatment train of a drinking water plant have been, and continue to be, extensively monitored and studied to meet continuous improvement and optimization targets to protect public health. However, the results of this study suggest that the benefits of basic process monitoring of the liquid and solids residual streams being managed in adjoining waste handling facilities is not being realized. System optimization for current facilities and continuous improvement in the design of future residuals treatment facilities will need to rely on this type of information being generated in the full-scale plants currently operating.

6.0 REFERENCES


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Ayol, A., Dentel, S., & Filibeli, A. (2005). Dual Polymer Conditioning of Water Treatment Residuals. Journal of Environmental Engineering , 131 (8), 1132-1138.

Bourgeois, J., Walsh, M., & Gagnon, G. (2004). Comparison of process options for treatment of water treatment residual streams. J. Environ. Eng. Sci. , 3, 477-484.

Cornwell, D. A. (1987). Handbook of Practice-Water Treatment Plant Waste Management, Denver, CO: AwwaRF (now the Water Research Foundation) and AWWA.

Cornwell, D. A. (2006). Water Treatment Residuals Engineering. Denver, CO: AwwaRF (now the Water Research Foundation) and AWWA.

Cornwell, D., MacPhee, M., & McTigue, N. (2001). Treatment Options for Giardia, Cryptosporidium, and Other Contaminants in Recycled Backwash Water. Denver: AwwaRF (now the Water Research Foundation) and AWWA.

Dharmappa, H., Hasia, A., & Hagare, P. (1997). Water Treatment Plant Residuals Management. Wat. Sci. Tech. , 35 (8), 45-56.

Hsieh, H. N., & Raghu, D. (1997). Criteria Development for Water Treatment Plant Monofills. Denver, CO: AwwaRF (now the Water Research Foundation) and AWWA.

MacPhee, M., Charles, G., & Cornwell, D. (2001). Treatment of Arsenic Residuals From Drinking Water Removal Processes. Cincinnati, OH: USEPA.

Mangat, S., Bilevicius, R., Griffin, D., Kjartansen, K., & Wobma, P. (2008). Freeze Thaw Pond Design. Proceedings of the Water Environment Federation, Residuals and Biosolids 2008 (pp. 765-773). WEF.

Martel, C., & Diener, C. (1991). A Pilot-Scale Study of Alum Sludge Dewatering in a Freezing Bed. Jour. AWWA , 83 (12), 51-55.

Parker, P., Collins, A., & Dempsey, J. (1998). Effects of Freezing Rate, Solids Content, and Curing Time on Freeze/Thaw Conditioning of Water Treatment Residuals. Environ. Sci. Technol. , 32 (3), 383-387.

Reissmann, F., & Uhl, W. (2006). Ultrafiltration for the reuse of spent filter backwash water from drinking water treatment. Desalination , 198, 225–235.

Vandermeyden, C., & Cornwell, D. A. (1998). Nonmechanical Dewatering of Water Plant Residuals. Denver, CO: AwwaRF (now the Water Research Foundation) and AWWA.

Documents

Data Review from Full-Scale Installations for Water Treatment Plant