WT - Pilot Study of the City of Baltimore's Drinking Water Treatment

City of Baltimore

Information Courtesy of:

Rummel, Klepper & Kahl, LLP

All photgraphs and material used with permission.

The City of Baltimore water utilities are capable of providing 465 million gallons of drinking water per day for a population of 1.5 million people. The water is produced at three conventional water treatment plants: Montebello Filtration Plants 1 and 2 Montebello Filtration Plant 2

and the Ashburton Filtration Plant. The source water originates from the Gunpowder and Susquehana Rivers and undergoes a conventional treatment process including chlorination, alum coagulation, sedimentation, sand filtration, and chemical adjustment of pH and fluoride concentration. The original Montebello Filtration Plant has been in operation since 1915 and has under gone various modifications. The City of Baltimore is developing a Comprehensive Plan for Water Facilities to provide the city with direction for it's water utilities well into the 21st century.

A pilot study was done, as a part of this plan, to identify feasible short term and long term water treatment solutions. The study was conducted from October 1996 through August 1997 and was broken down into four major areas of interest:

TASK A: short term, low-capital improvements that could be made to optimize the existing treatment processes, identify "performance limiting factors"
TASK B: long-term improvements to improve water quality and plant operations;
TASK C: direct filtration as an alternative to conventional filtration for future expansions with spatial constraints
TASK D: microfiltration as a long-term alternative to a conventional water treatment

All of these tasks were evaluated with respect to technical feasibility, cost, public health, and other issues. This site outlines some of the key objectives, findings and recommendations of the study.

Pilot water quality goals were established as a benchmark for improvements in the treatment processes, maximizing the removal of particulate and natural organic matter, and compliance with regulations. The goal for Cryptosporidium inactivation was based on filtration and disinfection performance. The filter productivity goals for conventional and direct filtration were based on the relationship between Unit Filter Run Volume (UFRV, the volume of water processed by a filter between two backwashes normalized by the surface area of the filter). The terminal headloss for Task A was set based on the hydraulic limitations of the current filters at the Montebello I filtration plant. The terminal headloss for Tasks B and C was set based on the assumption that new filters would be designed and constructed if these processes were adopted.

Water Quality Goals
Turbidity	< 0.1 NTU
Total Organic Carbon	> 35 percent removal
Specific UV Absorbance (SUVA)	< 2.0 L/mg-m
Total Trihalomethanes (TTHM)	< 80 µg/L
Haloacetic Acids (HAA5)	< 60 µg/L
Manganese	< 0.05 mg/L
Bromate	< 10 µg/L

Operational Goals
Ozonation Cryptosporidium inactivation	1-log
Filtration Conventional UFRV Direct Filtration UFRV Task A Terminal Headloss Tasks B and C Terminal Headloss	10,000 gal/sf 5,000 gal/sf 48 inches water 96 inches water

The work for this study was performed through pilot and bench scale testing. The pilot tests were done in an onsite trailer Mobile Pilot Plant which was plumbed into the city’s water sources. The treatment trains in the pilot plant included conventional treatment, with the ability to isolate each unit operation, and also membrane microfiltration.

A fly-through animation of the treatment trains in the pilot plant is provided for ease of viewing.

Windows: avi format = (3.2 MB) Mac: QuickTime format = (3.0 MB)

Task A: Immediate Improvements
Objective 1: Determine the most appropriate doses of alum and coagulant aids.
Findings: Through jar testing and pilot testing an alum dose of 15 mg/L was most appropriate for all seasons. A dose of 13 mg/L is commonly used at the Montebello Filtration Plants. It was also found that coagulant aids provided no observable benefit. Treatment Train A

Objective 2: Identify the most appropriate filters media design and operational parameters.
Findings: It was found that the average filter sand depth was 14 inches at the Montebello filtration. The original filters were designed for 20 inches of sand, but had incurred some media losses over the years. A filter with 14 inches of Montebello I media was pilot tested alongside a filter with 14 inches of Montebello I media and a 6 inch cap of virgin sand. It was determined that the capped filter never reached turbidity breakthrough after 65 hours of use at 2 gpm/sf while the uncapped filter reached a turbidity of 0.1 NTU after approximately 30 hours. Particle breakthrough of the capped filter occurred in approximately 45 hours whereas it occurred in the uncapped filter after 15-20 hours. The backwash rates of the filters were also tested and it was found the equipment at Montebello I and II did not achieve backwash rates necessary to completely clean the filters. Particle Breakthrough Graph

Task B: Long Term Improvements to Conventional Filtration
Objective 1: Evaluate alum and an alternative coagulant, ferric chloride, with ozonation

Findings: The appropriate doses for alum and ferric chloride were both determined to be 15 mg/L through jar testing. Tests were then performed with each coagulant using ozone and biofiltration. Filtered water turbidity and particle concentrations for both coagulants were similar, but the filter runs with ferric chloride were twice as long. The ferric chloride also provided better removal of TOC and UV-254.

Objective 2: Evaluate pre- and intermediate ozonation. Ozone Column

Findings: Preozonation and intermediate ozonation were both determined to be feasible for treatment with biological filtration. The ozone demand with ferric chloride was determined to be 0.4 mg/L as opposed the 1.0 mg/L with alum. The ozone demand of the raw water was 1.2 mg/L. All three of these numbers are considered low in comparison with other raw water sources.

Objective 3: Identify the most appropriate design of a biological filter.
Findings: A deep bed GAC-sand dual media filter should be considered for future designs. This is due to a longer period until particle and turbidity breakthrough, lower rate of headloss build up, lower TOC in filtered water, and greater biological activity in comparison to filters with an anthracite layer instead of GAC.

Treatment Train B

Task C: Direct Filtration
Objective 1: Evaluate the feasibility of direct filtration.
Findings: Direct filtration was able to achieve turbidities below 0.1 NTU for filter runs with UFRVs greater than 5,000 gal/sf. The following conditions were necessary for a successful filter run:

Preoxidation with ozone, chlorine, or potassium permanganate
Alum at a dose of 3 - 5 mg/L
Cationic polymer at a dose of 0.5 - 1.5 mg/L
1-stage flocculation at G = 80 s-1 or in-line flocculation

Task D: Membrane Filtration
Objective 1: Evaluate the feasibility of microfiltration as an alternative to conventional treatment.
Findings: The microfiltration process was determined to be capable of meeting all the water quality goals established for this study including a turbidity consistently below 0.1 NTU, an average SDSTHM (Simulated Distribution System THM) concentration of 52 µg/L, and an average SDSHAA(5) concentration of 35 µg/L. Membrane Filter Skid

The microfiltration test period did not include the part of the year during which the highest levels of DBPs were observed; however, annual averages are still expected to be below 60 µg/L for both THMs and HAA(5). Treatment Train D

The following recommendations are based on the results of the pilot study and may be considered for potential changes in the water treatment operations and processes. These recommendations are directly from the pilot testing report submitted to the City of Baltimore by the consulting engineers.

Short term, low capital improvements (Task A): Sand Addition

Refurbish the existing filter backwash systems at Montebello I and II and change the setpoint at Ashburton to achieve the appropriate filter backwash rates of approximately 25 gpm/sf.
Add additional media to the current filters to achieve a total media depth of 20 inches, which will provide better water quality and improve filter run time.

Long term improvements (Task B):

Consider the following treatment train for future plant modifications: coagulation using ferric chloride, flocculation, sedimentation, intermediate ozonation, biological filtration using a GAC-sand dual media filter design, and post-filtration chlorination.

Direct filtration (Task C):

Consider the potential benefits of using direct filtration at the Baltimore water treatment plants.
Consider the following treatment train for direct filtration: preozonation, coagulation using alum and cationic polymer, flocculation, biological filtration, and post-filtration chlorination.

Membrane filtration (Task D):

Consider the potential benefits of using membrane filtration for water treatment in the City of Baltimore.

Montgomery Watson: intern experience, photos (non-digital), diagrams, and raw data.
Rummel, Klepper & Kahl, LLP: photos (non-digital).
City of Baltimore: photos (non-digital), Comprehensive Plan for Water Facilities.

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