Package Plants: A Solution for Small Water Systems

Small water treatment systems often find it difficult to comply with the water quality regulations established by the U.S. Environmental Protection Agency (EPA). Small communities frequently face financial constraints in purchasing and maintaining conventional treatment systems. Their problems can be further complicated if they do not have the services of full-time, trained operators. The Surface Water Treatment Rule (SWTR) has greatly increased interest in the possible use of package plants in many areas of the country. Package plants can also be applied to treat contaminants such as iron and manganese in groundwater by applying oxidation and filtration.

What is a Package Plant?
Package technology offers an alternative to in-ground conventional treatment systems. Package processes are not altogether different from other treatment processes, although several package plant models contain innovative treatment elements, for instance adsorptive clarifiers. The primary distinction, however, between package plants and custom-designed plants is that package plants are treatment units assembled in a factory, skid mounted, and transported to the site.

These units are most widely used to treat surface water supplies for removal of turbidity, color, and coliform organisms with filtration processes. However, many other treatment technologies are available to small systems as package plants. These technologies or a combination of them can be incorporated into a package plant to provide comprehensive water treatment, including:

• disinfection (chlorination, ozonation, ultra-violet radiation)
• filtration (bag and cartridge filters, membrane filtration including reverse osmosis or ultra filtration, slow sand filtration, pressure filtration, diatomaceous earth filtration)
• aeration
• ion exchange
• adsorption (using powdered activated carbon or granular activated carbon)
• softening.

How To Select a Package Plant
Package plants can differ widely with regard to design criteria, and operating and maintenance considerations. Package plant systems are most appropriate for plant sizes that treat from 25,000 to 6,000,000 gallons per day (gpd) (approx. 95.6 to 22,700 cubic meters per day). Influent water quality is the most important consideration in determining the suitability of a package plant application. Complete nfluent water quality records need to be examined to establish turbidity levels, seasonal temperature fluctuations, and color level expectations. Both high turbidity and color may require coagulant dosages beyond many package plants design specifications. Pilot tests (tests that evaluate treatment processes and operations on a small scale to obtain performance criteria) may be necessary to select a package plant for more innovative designs using high flow rates and shorter detention time unit processes. Package treatment equipment manufacturers often perform these tests.

Package Plant Advantages
Package plants arrive on site virtually ready to operate, and are built to minimize the day-to-day attention required to operate the equipment. Other major advantages are their compact size, cost effectiveness, relative ease of operation, and design for unattended operation.

The main advantages of a packaged factory-finished system are savings in engineering, design and installation costs, and operation and maintenance. These features make package plants attractive to communities that must operate on a tight budget.

These plants can effectively remove turbidity and bacteria from surface water of fairly consistent quality, provided that they are run by competent operators and are properly maintained. Package plants also can be designed to remove dissolved substances from the raw water, including color -causing substances and trihalomethane precursors (which are organic materials often found in source water that can react with chlorine to form what are called disinfection by-products or DBPs).

Package Plant Limitations
Highly variable influent water quality requires a high level of operational skill and attention, and that tends to negate the package plant advantages of low cost and automation.

Despite the relatively large number of package plants in use, many states are reluctant to endorse them completely. The requirements of the Safe Drinking Water Act and its amendments might challenge package capability. Challenges include the possible inability of these systems to treat multiple types of contaminants.

Many communities are currently using package plants to treat water supplies, but little data has been collected to demonstrate long-term performance and operations of these systems. State agencies responsible for reviewing plans for the installation of package systems must review each potential plan on a case-by-case basis, with only their own experience to judge the potential for success or failure. Presently there is no national verification process for package plants.

Types of Package Plant Filtration Systems

Conventional Package Plants: These are manufactured by several firms to a variety of specifications. As their name indicates, they contain the conventional processes of coagulation, flocculation, sedimentation, and filtration. Typical design standards for these units are:

• 20 to 30 minutes flocculation detention time
• 2 hour sedimentation detention time
• rapid sand filters rated at 2 gpm per square foot (1.34 liters/second/square meter)

Tube-Type Clarifier Package Plants: These systems use tube settlers to reduce settling detention time-which is the average time that the water remins in the settling tank or chamber.

A flow diagram of a tube-type package plant is shown in Figure 1. This design has two versions with different capacity ranges. One version can treat from 10 to 100 gpm (0.63 to 6.3 liters/second), and the other, equipped with dual units, can treat from 200 to 1,400 gpm (12.6 to 88.3 liters/second).

In these package systems, the disinfectant, primary coagulant, and polymer coagulant aid are added before the influent enters the flash mixer. Then the water enters the flocculation chamber where mechanical mixers gently agitate the water for 10 to 20 minutes depending on the flow.

The flocculated water then enters the tube settlers which consist of many one-in.-diameter by 39-in.-long split hexagonal shaped passage ways. The large surface area of these tubes achieves an effective clarification overflow rate of less than 150 gpd/ square foot (6.1 cubic meters /day/square meter). Adequate clarification time is attained with less than 15 minutes detention time.

The clarified water then enters a gravity flow mixed-media filter (a filter with a coarse-to-fine gradation of filter media or several types of such media). A constant filtration rate is maintained by a low-head filter effluent pump discharging through a float-operated level control valve. After a preset headloss value is reached, backwashing of the filter is initiated automatically. The headloss here is the pressure or energy loss resulting from turbulence caused by the flow conditions and from the roughness of the channel walls. A manual backwash cycle can be initiated at any time if desired. Settled sludge from the tube settlers is flushed during the backwash cycle. Combining backwashing and tube settler flushing simplifies operation and reduces operator skill requirements.

Adsorption Clarifier Package Plant: Adsorption clarifier package plants use an upflow filter-an adsorption clarifier-with low density plastic bead media to replace the flocculation and sedimentation basin, thereby combining these two steps into one. A mixed-media filter completes the treatment. Figure 2 shows a typical example of this type of unit.

While passing through the adsorption media, the coagulant and water are mixed, contact-flocculated, and clarified. The mixing intensity, as measured by the mean velocity or gradient, ranges from 150 to 300 feet per second. Flocculation is accomplished through turbulence as water passes through the adsorption media. In addition, flocculation is enhanced by contact between the flocculated materials and the floc-coated media. Turbidity is reduced through adsorption of the coagulated and flocculated solids on to the adsorption media and the previously adsorbed materials. The adsorption clarifier can achieve 95 percent or greater removal at 10 gpm/square foot (6.8 liters/ second/square meter). This highly efficient clarification method results in extremely compact designs.

Adsorption clarifiers are cleaned by a combination of air scouring followed by water flushing. The air scouring starts the cleaning process for the plastic media used in the clarifier. Adsorption clarifier cleaning is initiated more frequently than filter backwashing because more solids are removed by the clarifier. The cleaning process is automatically initiated either by a timer or a pressure switch that continuously monitors headless across the adsorption media.

The air introduced under the adsorption media causes a vigorous scrubbing action. This action dislodges solids, which are washed away by the flow of the influent water. Flushing is generally timed to occur between every fourth and eighth hour. Complete cleaning of the adsorption media is not desired because performance is enhanced by some residual solids.

Operation and Maintenance
Package plant operation is simplified by automated features such as effluent turbidimeters connected to chemical feed controls and other operating functions, such as backwashtng. Chemical feed controls are especially important for plants without full-time operators or with variable influent characteristics. Maintenance requirements are well documented in manuals. However, the operator needs to be well acquainted with water treatment principles and the plant manual, and should have attended a comprehensive training session,

The effluent turbidimeters and fail-safe controls are built into many plants to ensure that the finished water does not exceed set turbidity levels. Automated chemical feed systems are especially appropriate for plants without full-time operators or with highly variable influent characteristics.

Typical operator and maintenance manuals contain operating principles, methods of establishing proper chemical dosages, operating instructions, and trouble shooting guides.

Periodic visits by the manufacturer to make adjustments to the plant and inspect the equipment operation and performance are recommended. The first visit should be no more than six months after initial operation; the next should follow in another six months. Subsequently, annual visits should be sufficient.

Operators are the critical factor in overall success of any package plant, particularly in situations where raw water quality varies. When the automation fails, the operator needs to turn off the automatic control instrumentation and operate the plant manually.

Where Can I Find More Information?
The information presented in this article was obtained primarily from two sources:

• Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities, EPA/625/4-89/023 · "Package Plants for Small Systems: A Field Study" by Susan Campbell, Benjamin W. Lykins Jr., James Goodrich, Dallas Post, and Trudle Lay. AWWA Journal, Nov. 1995 pages 39-47.

Technologies for Upgrading Existing or Designing New Drinking Water Treatment Facilities can be ordered free from the U.S. Environmental Protection Agency Office of Research and Development at 513-569-7562.

The National Drinking Water Clearinghouse (NDWC) offers this publication also, but at a cost to help recover photocopying expenses. The 209-page book costs $30.05. To order call the NDWC at 800-624-8301 or 304-293-4191 and request Item #DWBKDMO4. NDWC products also may be ordered via e-mail at ndwc_orders@ ndwc.wvu.edu.

Also, the NDWC's Registry of Equipment Suppliers of Treatment Technologies for Small Systems (RESULTS) is a public reference database that contains information about technologies in use at small water systems around the country. For further information, call the NDWC at one of the above numbers.

Editor's Note: This article appeared as a Tech Brief Fact Sheet in the National Drinking Water Clearinghouse publication On Tap in November, 1997. It has been edited to the journalistic style of Water Online and Public Works Online. Free copies of Tech Brief Fact Sheets can be obtained by calling the above NDWC telephone numbers, or you may download them from the NDWC web site at http://www.ndwc.wvu.edu.