A Mobile Wastewater UV Treatment System - by Montana Aquionics Representative, Able Wright

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Aquionics UV technology allows big reduction in capital costs

Most UV installations for water and wastewater treatment, whether they are closed-vessel, medium pressure systems or open-channel, low pressure systems, are situated in permanent structures. For that reason, the mobile UV disinfection system we designed and constructed for the Salish Kootenai Housing Authority is slightly unusual.

The Flathead Reservation of the Confederated Salish and Kootenai Tribes is situated in northwestern Montana. The Salish Kootenai Housing Authority (SKHA) needed to treat secondary wastewater at two remote treatment facility sites. As is commonplace in the western states, where land is plentiful, the Tribes use aerated lagoons to provide secondary treatment of their wastewater. Each of the sites, situated approximately 15 miles apart, consists of a two-cell lagoon structure - the first an aeration lagoon and the second lagoon for storage prior to final disinfection and discharge into local watercourses.

Each site needs to be discharged every three to four months – whenever the secondary lagoon is full. Prior to discharge the wastewater must be disinfected so the BOD (Biological Oxygen Demand) value is no higher than the discharge permit of 30 mg/L and the fecal coliform count is below 200 units/100ml. The SKHA had originally considered installing fixed, open channel UV disinfection systems at each location for the final treatment stage, but it was concerned about the huge expense.

Able Wright, Inc., the Montana representative for Aquionics UV, worked closely with the Tribes´ in-house engineering staff to develop an alternative approach. The solution was a turn-key mobile disinfection unit capable of serving both wastewater facilities. Housed in an 8´ X 12´ enclosed trailer is a pair of Aquionics closed vessel disinfection chambers. The stainless steel UV chambers are installed in series and fitted with medium pressure UV lamps & automated wiper systems. In addition there are the associated electrical controls, an end-suction centrifugal pump, PVC piping system, interior lights, ventilation provisions and cam-lock hose connections located on the exterior of the trailer.

The only additional site work required at the two lagoons was the installation of single phase 230V buried power lines and electrical outlet pedestals at the discharge locations, which are situated on the dikes surrounding the lagoons. The suction hose is connected to a pipe that extends into the lagoon, with the opening 3 ft above the lagoon´s floor, so as not to extract sediment from the bottom. The total cost of the mobile disinfection unit, along with the site work at both facilities, was less than 40% of the cost of two conventional open channel UV systems – a significant cost saving.

When one of the lagoons is almost full the mobile unit is driven to the discharge point and the electrical power cord & hoses are connected. The UV system and end-suction pump are activated and left running continuously, at a rate of about 90 gpm, for approximately four weeks or until a third of the contents of the lagoon has been removed. This process can happen at any time of the year, even in the middle of winter, when outdoor temperatures can be well below freezing. "This is when the medium pressure UV lamps really shine," comments Doug Mitch. "Below +45F, the disinfection effectiveness (as measured by relative UV output) of low pressure UV drops off dramatically, while the effectiveness of medium pressure UV is virtually unchanged, even below freezing."

Obviously, frozen water will not flow, but discharging effluent from an ice covered lagoon is a common necessity in Montana. Here, the temperature of the effluent from the wastewater lagoons is often well below +40F during the winter months. Medium pressure UV systems perform well in this environment – an application Aquionics knows something about. Their medium pressure UV systems are frequently employed for meat brine rapid cycle UV disinfection in the food industry, often at temperatures as low as -10C.

It is noteworthy that the two SKHA installations both have relatively low UV transmissivity* wastewater. The average T10 transmittance value** of the effluent at the two sites is 20%, compared to a typical municipal secondary lagoon effluent transmittance value of 40 to 65%. What these values mean in practice is a lower transmittance value wastewater is more "cloudy" and passes less light, requiring significantly more UV energy to provide the necessary disinfection. During design of the system the effluent transmissivity was tested and the UV equipment carefully selected to provide an applied UV dose which was adequate to insure compliance with the Tribes´ discharge permits.

One drawback to the mobile system is some additional operator attention. "Because the access roads to the lagoons are quite bumpy, care needs to be taken when transporting the mobile unit from one site to the other. This requires removing and re-installing the UV lamps each trip" reports Mitch. "While this is a minor inconvenience, it is more than offset by the huge cost savings." The initial estimate for fixed, open channel UV disinfection systems was approximately $100,000 for each site. The mobile unit, including all the UV equipment and site improvements, cost less than $75,000. "We are very proud of the project as it addressed the technical disinfection needs of the community, with proper attention paid to their cost and value concerns. It also provides the Tribes with a highly versatile disinfection system," he concluded.

* Transmissivity is a measure of the wastewater´s ability to transmit UV light at the germicidal wavelengths

** T10 is the percent of UV radiation flux power remaining after passing through a layer of 10 mm in thickness, measured at a wavelength of 253.7nm

Able Wright, Inc. is a provider of water treatment equipment and technical services, specializing in disinfection and chemical feed for Montana, Wyoming, Idaho, and Washington.

www.ablewright.com