Environmental Science & Engineering - www.esemag.com - March 2004
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Multi-stage filtration for North Haven,Maine

By Robert Abernethy, P.Eng.


Clean and healthy water has always been considered a basic human right for citizens in developed countries but there have been some notable outbreaks of water-borne diseases in several small Canadian communities in recent years. These have sharply focused the attention of community leaders on the otherwise mundane topic of water treatment. One positive outcome has been the realization by community leaders of their responsibilities in the delivery of water, including the protection of source waters, the provision of adequate water treatment, and the delivery and monitoring of safe water by trained water professionals.

Meeting these responsibilities has been challenging for many communities. The challenge for small communities is compounded for several reasons, mostly related to the problem of economies of scale and lack of resources. Small communities often do not have the power or the resources to protect watersheds and frequently rely on poor water sources. Small communities usually do not have local engineering resources to design or manage their water systems and must rely on outside expertise. Local operators often have multiple community responsibilities and may not have the time or training to operate complex water treatment plants. Finally, the ongoing cost to operate a water system can be crippling to a small community.

While the monitoring and reporting requirements for a community of 1,000 are not much different from a community of 1,000,000, the financial bases of the two communities are vastly different and therefore the per capita charges for water will also be vastly different.

All of these problems escalate for isolated communities typical of northern Canada. Technicians, spare parts, and treatment chemicals all need to be flown in to many northern communities. Unit costs for commodities such as power, labour or treatment chemicals are many times higher than in southern communities. Isolated northern communities have the doubleedged sword of higher costs and smaller resource bases.

For all of these reasons, small communities are structurally different than large communities and unique solutions to water treatment are required. It is not satisfactory to simply downscale the water treatment technologies used in large communities to smaller sizes. The requirements are different and therefore the technology selection criteria are different.

The treatment technology must be simple to operate. Operators do not have an in-house engineering staff to turn to when there is a chemical, electrical or mechanical upset. A passive treatment system (a system that does not require changes to the process to react to changing raw water conditions) is often preferred over dynamic systems. The level of complexity of instrumentation, controls and/or chemistry of the treatment chemicals in the plant should reflect the level of training and sophistication of the operator. In many small communities, operators have obtained only the bare minimum level of training and experience. Finally, operators cannot baby-sit their water treatment plant, since they often have numerous other responsibilities in the community, such as grading roads, maintaining parks, cleaning the ice at hockey rinks, etc.

Such treatment systems should minimize on-going operating costs. While construction and capital costs are often paid for by senior levels of government, it is the local community that must pay operations costs and therefore operating costs should be the paramount criteria in selecting a treatment technology. Technologies that use large amounts of power, chemicals, labour or proprietary spare parts should be avoided.

Slow sand filtration
These issues are not new or unique to Canada. Agencies such as the United States Environmental Protection Agency (USEPA), the World Health Organization (WHO) and the United Nations Environmental Program (UNEP) have all recognized the special problems associated with water treatment for small communities. Each of these agencies has recommended slow sand filtration as a “Best Available Technology” for small communities, based on the technology’s simplicity, low costs and high efficiency for removing water-borne pathogens without the use of pre-treatment chemicals. Over 200 slow sand filters are installed across North America, mostly for small and mid-sized communities.

Conventional slow sand filter
However, conventional slow sand filtration has several disadvantages that have limited its application in recent years. Limitations of conventional slow sand filters include poor removal efficiencies for organics such as algae, colour and naturally occurring organic matter, which may lead to downstream disinfection by-product formation, as well as the operational problems involved with cleaning the surface of a slow sand filter.


Multi-stage filtration
The Multi-Stage Filter has been designed to provide all of the benefits of slow sand filtration but with enhancements to overcome the limitations of conventional slow sand filtration. Enhancements include pre-ozonation, roughing filtration and a non-destructive filter cleaning technique. Granular activated carbon or limestone steps can also be added if required for unique waters. The benefits of multi-stage filtration include:

North Haven, Maine
The Town of North Haven is an island community, approximately 20 kilometres off the coast of Maine. The island has approximately 350 permanent residents, but the population increases to over 2000 during the summer. The peak day water demand is 950 m3/day. The island has one fresh water source with generally good raw water quality.


Treatment technology criteria
Wright-Pierce Engineering of Topsham, Maine, were engaged by the town to select a treatment technology. Several criteria were considered in selecting the best technology, including: Three treatment options were considered and pilot tested, including, a) conventional slow sand filtration, b) UF membrane filtration, and c) multi-stage filtration.

Conventional slow sand filtration was pilot tested in 1998 and was observed to meet all of the filtration requirements of the Surface Water Treatment Rule. However, the TOC removal efficiency was not sufficient to meet the TOC removal requirements of the Disinfection By-Product Rule. Furthermore, since no package plant option exists for a conventional slow sand filter, this option would involve the construction of concrete tanks which would be very costly due to the transportation requirements of concrete.

Ultrafiltration membrane technology was pilot tested in 1999 and was also demonstrated to be effective at removing colour and TOC and meeting the SWTR. However, this technology was ultimately not selected due to concerns about membrane fouling due to the high organic load and concerns about the long-term expense of system maintenance. Additionally, the local operators would need to upgrade their operating certification in order to operate a membrane plant.

The Multi-Stage Filtration process was piloted in 2000 and was found to be effective for reducing TOC and colour as well as satisfying all of the other selection criteria. Finally, the package design of the Multi-Stage Filter was attractive for transportation and construction on the island.

Plant design
The following design parameters for the Multi-Stage Filter were used based on the pilot test results:


The normal design flow rate for slow sand filters is between 0.1 m/hr and 0.4 m/hr. Several large European slow sand filtration plants that practise pretreatment, such as in London and Zurich, have filtration rates as high as 0.6 m/hr to 0.8 m/hr. A conservative filtration rate of 0.29 m/hr was chosen for the North Haven design.

Construction of the plant took place in the winter of 2003. Four Multi- Stage Filter tanks were purchased from MS Filter Inc. of Newmarket, Ontario, and installed, each with a flow capacity of 238 m3/day. Each tank consisted of a roughing filter, slow sand filter, and limestone contactor. The limestone contactor was filled with crushed limestone and is used to raise the pH and add alkalinity that provides corrosion control in a simple, passive design. Water flows by gravity through the entire filter and no chemicals are added to the process.

The ozone system consists of modular ozone generators, venturi injectors, two ozone contactors (3.2m H x 0.36m diameter), and appropriate instrumentation and controls. The ozone generators create ozone by the corona discharge method using dried air. The ozone and air mixture is drawn into the water line using a venturi injection system so that the ozone delivery line is always under vacuum as a fail-safe against ozone leaks. Ozone is distributed in the ozone contactor using a mass transfer multiplier and a perforated plenum. Ozone mixing is assisted by packing the column with 2” plastic saddles.

The filter tanks and media were set in place prior to construction of the filter building, allowing water to flow to the filters during construction by a temporary water line. The filters took approximately one month to ripen under cold water and low flow conditions, but were already meeting turbidity treatment requirements (<1 NTU) during start-up of the rest of the plant.

The plant was commissioned in June of 2003 and was producing the following water quality:


Unlike slow sand filters, Multi- Stage Filters have a non-destructive cleaning technique, which means that the surface is not scraped and sand is not replaced. The roughing filter is backwashed to waste and the slow sand filter is surface washed using a modified wet-harrow process. Wastewater is sent out to a holding pond where it evaporates. Annual wastewater production is less than 0.1% of plant capacity. Sludge production is minimal (< 10 kg/year) since no chemical coagulants are added to the process. The length of the filter runs (time between filter cleanings) is 2 – 3 months. Filter cleaning takes less than one hour per filter and the filters can be placed back on line after several hours. No other maintenance of the filters is required.

The total cost of the entire water treatment facility, including the package plant, filtration and administration building, design, and administration, was $US 2,216,747.00. These costs were likely 25% - 40% higher than normal due to the unusual costs related to building a facility on a remote access island. Annual costs directly related to the Multi-Stage Filter (power, labour, building heat, annualized replacement costs) are < $US 20,000/year or <$0.12/m3 (at average flow rates).
Robert Abernethy is President of MS Filter. Contact, e-mail: rabernethy@msfilter.com.

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