Extreme Cold-Climate Surface Water Treatment
Treatment technology considerations for remote plant sites
Selecting appropriate treatment technologies for a surface water treatment plant in extreme cold-climate regions presents many design challenges and options. This article shares findings from the 2007 CFS Alert Water Treatment Options Analysis study commissioned by the Department of National Defence with the support of Defence Construction Canada. Hatch Mott MacDonald acted as the consultant.
In extreme cold-climate regions, temperatures may drop as low as -50°C and snow stays on the ground continuously for three to six months. Maintaining adequate temperatures in water supply, distribution and raw water piping is critical to ensure that water remains above freezing temperature at all times and that intakes are free of ice. In order to achieve this, one return pump at the treatment plant is run continuously, delivering water through a heat exchanger and back through the return line to the intakes that are not in service.
About 50% to 60% of the treated heated water must return back to the intakes for temperature control. Raw water flow to the treatment plant must be more than twice as high as design year maximum day demand. In order to avoid stagnation and freezing in the distribution main, one highlift pump operates continuously with a capacity of more than the peak hour flow to return water back to the reservoir at the treatment plant.
The capacity of the treatment plant and overall size of the process units, including the onsite reservoir, must be more than twice the size required for design year maximum day demand. Additional required equipment comprises a heat exchanger, return pipe to the intake and return pumps with insulation for all outside pipes and mains.
The site for the treatment plant is called a remote location because direct land transportation is not possible from other parts of the country. Marine or air transportation from the nearest town to the remote treatment plant site are possible options. Process options with continuing consumables that would require frequent deliveries—chemical treatment, for instance—are not desirable.
Water Quality & Treatment
Typical raw water can be characterized as having very low turbidity (less than 0.5 NTU), with moderate hardness, pH and TDS. Turbidity follows an annual pattern in which the first five months of the year exhibit very low values (less than 0.3 NTU). Increases in turbidity associated with ice breakup and the beginning of overland recharge by snowmelt occur in June (turbidity spikes greater than 1.5 NTU), followed by a gradual diminution of turbidity through the summer and fall seasons. Microbiological quality of the source water normally is good, but runoff water to surface sources may be contaminated by pathogens from wildlife in the area.
To comply with the requirements of Guidelines for Canadian Drinking Water Quality (GCDWQ), it is necessary to provide filtration, primary disinfection, secondary (residual) disinfection, 3-log reduction of Cryptosporidium and Giardia Lamblia, and 4-log reduction of viruses. Treated water turbidity should be less than 0.1 NTU, and specific water quality parameters must be below the GCDWQ maximum acceptable concentrations.
The preferred treatment technology must be compatible with the remote location conditions. To define specific treatment technologies, three filtration processes—cartridge filtration, membrane filtration and reverse osmosis (RO)—are considered to meet the multibarrier protection defined by Health Canada. Alternative disinfectants, such as ozone and ultraviolet (UV) treatment, can be considered an additional barrier. Chlorine (as potassium hypochlorite) can be considered to provide a free chlorine residual in the distribution system.
In remote cold-climate regions where in-stock delivery, overnight courier service and fast response by manufacturers’ service personnel are not available, the degree of automated systems reliability is an important consideration in process design. Typically, package water treatment units come with a fairly complete automation system, permitting unattended operation and automated failovers to backup equipment. The final design of the selected option should be based on robust units, with a minimum instrument package and little automation, relying instead on manual valving and motor starters. In the event of a system fault, the failed unit may be taken offline manually and a standby unit brought into service.
Costs common to all alternatives are not presented. For example, the operation and maintenance of the raw water supply system and heat exchanger will be unaffected by the choice of the treatment process.
Equipment supply, delivery and installation costs. Equipment costs will be solicited from manufacturers and suppliers with freight-onboard pricing at the town nearest the plant site, packaged suitably for marine or air transportation to the remote plant site. Delivery costs are estimated equivalent to dollars per cubic meter of package. Installation labor and material costs can be estimated on a case-specific basis with input from both the equipment supplier and a specialist general/mechanical contractor.
Operating and maintenance labor costs. Costs are estimated for additional labor, such as inspection, operation, calibration, maintenance, end-of-life component replacement and the like. In addition to these costs associated with discrete alternatives, observation and calibration labor would be required if the recommended instrumentation upgrades were implemented.
Consumables and replacement maintenance costs. Costs are estimated for routine parts and auxiliary equipment replacement. Replacement maintenance reflects the reality that some process components may not last the typical 20-year life cycle.
Power costs. At the remote location, all power required by the treatment processes is generated on- site from diesel generators. For the purposes of the analysis, it is assumed that the power needs for any process can be met without having to provide any additional power for other purposes.
Training costs. Training should be delivered before personnel are deployed to the remote location. There should always be two persons trained in operation and maintenance.
The plant operator would be fully trained in process operations. A member of the client’s staff also would receive this training in order to provide initial remote support or as an emergency short-term replacement.
It is assumed that the process alternatives for solids removal share a similar degree of complexity and all would require the same length of training time. Because UV irradiation equipment is substantially less complex than ozone disinfection equipment, it is assumed that UV training would require less time than ozone training.
All three filtration options and two disinfection options can meet or exceed GCDWQ guidelines and site-specific design considerations. The selection of treatment options is dependent on capital cost, ease of operation and maintenance of the processes systems, and overall life-cycle cost analysis.
The RO system is more effective than membrane and cartridge filtration systems in terms of particle removal, but it also is the most complex process and has the highest capital and life-cycle cost values. The membrane filtration system is more effective in particle removal and has a lower life-cycle cost than cartridge filtration, but it is a more complex process with a higher capital cost. UV irradiation is a significantly less complex process with a lower capital cost, but it has a higher life-cycle cost than an ozonation system.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/Membrane%20Filtration%20Process.jpgA membrane filtration system is more effective in particle removal with a lower life-cycle cost than cartridge filtration.